Networking notes

DATA COMMUNICATION AND
COMPUTER NETWORKS

DATA COMMUNICATIONS
Data communications are the exchange of data between two devices via some form of
transmission medium such as a wire cable. For data communications to occur, the
communicating devices must be part of a communication system made up of a combination of
hardware (physical equipment) and software (programs). The effectiveness of a data
communications system depends on four fundamental characteristics: delivery, accuracy,
timeliness, and jitter.
1. Delivery. The system must deliver data to the correct destination. Data must be received by
the intended device or user and only by that device or user.
2. Accuracy. The system must deliver the data accurately. Data that have been altered in
transmission and left uncorrected are unusable.
3. Timeliness. The system must deliver data in a timely manner. Data delivered late are useless.
In the case of video and audio, timely delivery means delivering data as they are produced, in the
same order that they are produced, and without significant delay. This kind of delivery is called
real-time transmission.
4. Jitter. Jitter refers to the variation in the packet arrival time. It is the uneven delay in the
delivery of audio or video packets.
Components:
A data communications system has five components.
Data Communication and Computer Networks Page 2

1. Message. The message is the information (data) to be communicated. Popular forms
of information include text, numbers, pictures, audio, and video.
2. Sender. The sender is the device that sends the data message. It can be a computer,
workstation, telephone handset, video camera, and so on.
3. Receiver. The receiver is the device that receives the message. It can be a
computer, workstation, telephone handset, television, and so on.
4. Transmission medium. The transmission medium is the physical path by which a message
travels from sender to receiver. Some examples of transmission media include twisted-pair
wire, coaxial cable, fiber-optic cable, and radio waves
5. Protocol. A protocol is a set of rules that govern data communications. It represents an
agreement between the communicating devices.
DATA TRANSMISSION MODES
Communication between two devices can be simplex, half-duplex, or full-duplex.
Simplex:
In simplex mode, the communication is unidirectional, as on a one-way street. Only one of the
two devices on a link can transmit; the other can only receive. Keyboards and traditional
monitors are examples of simplex devices. The keyboard can only introduce input; the monitor
can only accept output. The simplex mode can use the entire capacity of the channel to send data
in one direction.
Half-Duplex:
In half-duplex mode, each station can both transmit and receive, but not at the same time. When
one device is sending, the other can only receive, and vice versa. In a half-duplex transmission,
the entire capacity of a channel is taken over by whichever of the two devices is transmitting at
the time. Walkie-talkies and CB (citizens band) radios are both half-duplex systems. The half-
duplex mode is used in cases where there is no need for communication in both directions at the
same time; the entire capacity of the channel can be utilized for each direction.

Full-Duplex:
In full-duplex both stations can transmit and receive simultaneously. The full-duplex mode is
like a two way street with traffic flowing in both directions at the same time. In full-duplex
mode, signals going in one direction share the capacity of the link: with signals going in the other
direction. One common example of full-duplex communication is the telephone network. When
two people are communicating by a telephone line, both can talk and listen at the same time. The
full-duplex mode is used when communication in both directions is required all the time. The
capacity of the channel, however, must be divided between the two directions.

NETWORKS
A network is a set of devices (often referred to as nodes) connected by communication links. A
node can be a computer, printer, or any other device capable of sending and/or receiving data
generated by other nodes on the network.
Network Criteria
A network must be able to meet a certain number of criteria. The most important of these are
performance, reliability, and security.
Performance:
Performance can be measured in many ways, including transit time and response time. Transit
time is the amount of time required for a message to travel from one device to another. Response
time is the elapsed time between an inquiry and a response. The performance of a network
depends on a number of factors, including the number of users, the type of transmission medium,
the capabilities of the connected hardware, and the efficiency of the software.
Reliability:
Network reliability is measured by the frequency of failure, the time it takes a link to recover
from a failure, and the network's robustness in a catastrophe.
Security:
Network security issues include protecting data from unauthorized access, protecting data from
damage and development, and implementing policies and procedures for recovery from breaches
and data losses.
PHYSICAL STRUCTURES
TYPES OF CONNECTIONS: A network is two or more devices connected through
links. A link is a communications pathway that transfers data from one device to another.
There are two possible types of connections: point-to-point and multipoint.

Point-to-Point
A point-to-point connection provides a dedicated link between two devices. The entire capacity
of the link is reserved for transmission between those two devices. Most point-to-point
connections use an actual length of wire or cable to connect the two ends, but other options, such
as microwave or satellite links, are also possible. When you change television channels by
infrared remote control, you are establishing a point-to-point connection between the remote
control and the television's control system.
Multipoint
A multipoint (also called multidrop) connection is one in which more than two specific devices
share a single link. In a multipoint environment, the capacity of the channel is shared, either
spatially or temporally. If several devices can use the link simultaneously, it is a spatially shared
connection. If users must take turns, it is a timeshared connection.

PHYSICAL TOPOLOGY:
The term physical topology refers to the way in which a network is laid out physically. One or
more devices connect to a link; two or more links form a topology. The topology of a network is
the geometric representation of the relationship of all the links and linking devices (usually
called nodes) to one another. There are four basic topologies possible: mesh, star, bus, and ring.
1. Mesh: In a mesh topology, every device has a dedicated point-to-point link to every other
device. The term dedicated means that the link carries traffic only between the two devices it
connects. To find the number of physical links in a fully connected mesh network with n nodes,
we first consider that each node must be connected to every other node. Node 1 must be
connected to n - I nodes, node 2 must be connected to n – 1 nodes, and finally node n must be
connected to n - 1 nodes. We need n(n - 1) physical links. However, if each physical link allows
communication in both directions (duplex mode), we can divide the number of links by 2. In
other words, we can say that in a mesh topology, we need n(n -1) /2 duplex-mode links. To
accommodate that many links, every device on the network must have n – 1 input/output ports to
be connected to the other n - 1 stations.
Advantages:
1. The use of dedicated links guarantees that each connection can carry its own data load, thus
eliminating the traffic problems that can occur when links must be shared by multiple devices.
2. A mesh topology is robust. If one link becomes unusable, it does not incapacitate the entire
system.

3. There is the advantage of privacy or security. When every message travels along a dedicated
line, only the intended recipient sees it. Physical boundaries prevent other users from gaining
access to messages.
4. Point-to-point links make fault identification and fault isolation easy. Traffic can be routed to
avoid links with suspected problems. This facility enables the network manager to discover the
precise location of the fault and aids in finding its cause and solution.
Disadvantages:
1. Disadvantage of a mesh are related to the amount of cabling because every device must be
connected to every other device.
2. Installation and reconnection are difficult.
3. The sheer bulk of the wiring can be greater than the available space (in walls, ceilings, or
floors) can accommodate.
4. The hardware required to connect each link (I/O ports and cable) can be prohibitively
expensive.

2. Star Topology: In a star topology, each device has a dedicated point-to-point link only to a
central controller, usually called a hub. The devices are not directly linked to one another. Unlike
a mesh topology, a star topology does not allow direct traffic between devices. The controller
acts as an exchange: If one device wants to send data to another, it sends the data to the
controller, which then relays the data to the other connected device .
Advantages:
1. A star topology is less expensive than a mesh topology. In a star, each device needs only one
link and one I/O port to connect it to any number of others.
2. Easy to install and reconfigure.
3. Far less cabling needs to be housed, and additions, moves, and deletions involve only one
connection: between that device and the hub.
4. Other advantage include robustness. If one link fails, only that link is affected. All other links
remain active. This factor also lends itself to easy fault identification and fault isolation. As long
as the hub is working, it can be used to monitor link problems and bypass defective links.
Disadvantages:
One big disadvantage of a star topology is the dependency of the whole topology on one single point,
the hub. If the hub goes down, the whole system is dead. Although a star requires far less cable than a
mesh, each node must be linked to a central hub. For this reason, often more cabling is required in a
star than in some other topologies (such as ring or bus).

3. BUS: A bus topology is multipoint. One long cable acts as a backbone to link all the devices
in a network. Nodes are connected to the bus cable by drop lines and taps. A drop line is a
connection running between the device and the main cable. A tap is a connector that either
splices into the main cable or punctures the sheathing of a cable to create a contact with the
metallic core. As a signal travels along the backbone, some of its energy is transformed into heat.
Therefore, it becomes weaker and weaker as it travels farther and farther. For this reason there is
a limit on the number of taps a bus can support and on the distance between those taps.
Advantages:
Advantages of a bus topology include ease of installation. Backbone cable can be laid along the
most efficient path, then connected to the nodes by drop lines of various lengths. In this way, a
bus uses less cabling than mesh or star topologies. In a star, for example, four network devices in
the same room require four lengths of cable reaching all the way to the hub. In a bus, this
redundancy is eliminated. Only the backbone cable stretches through the entire facility. Each
drop line has to reach only as far as the nearest point on the backbone.
Disadvantages:
Disadvantages include difficult reconnection and fault isolation. A bus is usually designed to be
optimally efficient at installation. It can therefore be difficult to add new devices. Signal
reflection at the taps can cause degradation in quality. This degradation can be controlled by
limiting the number and spacing of devices connected to a given length of cable. Adding new
devices may therefore require modification or replacement of the backbone. In addition, a fault
or break in the bus cable stops all transmission, even between devices on the same side of the
problem. The damaged area reflects signals back in the direction of origin, creating noise in both
directions.

4. RING: In a ring topology, each device has a dedicated point-to-point connection with only the
two devices on either side of it. A signal is passed along the ring in one direction, from device to
device, until it reaches its destination. Each device in the ring incorporates a repeater. When a
device receives a signal intended for another device, its repeater regenerates the bits and passes
them along.
Advantages:
A ring is relatively easy to install and reconfigure. Each device is linked to only its immediate
neighbors (either physically or logically). To add or delete a device requires changing only two
connections. The only constraints are media and traffic considerations (maximum ring length and
number of devices). In addition, fault isolation is simplified. Generally in a ring, a signal is
circulating at all times. If one device does not receive a signal within a specified period, it can
issue an alarm. The alarm alerts the network operator to the problem and its location.
Disadvantages:
Unidirectional traffic can be a disadvantage. In a simple ring, a break in the ring (such as a
disabled station) can disable the entire network. This weakness can be solved by using a dual
ring or a switch capable of closing off the break. Ring topology was prevalent when IBM
introduced its local-area network Token Ring. Today, the need for higher-speed LANs has made
this topology less popular.

NETWORK CATEGORIES
Local Area Networks (LAN):
Local area networks, generally called LANs, are privately-owned networks within a single
building or campus of up to a few kilometers in size. They are widely used to connect personal
computers and workstations in company offices and factories to share resources (e.g., printers)
and exchange information. LANs are distinguished from other kinds of networks by three
characteristics:
(1) Their size,
(2) Their transmission technology, and
(3) Their topology.
LANs are restricted in size, which means that the worst-case transmission time is bounded and
known in advance. Knowing this bound makes it possible to use certain kinds of designs that
would not otherwise be possible. It also simplifies network management. LANs may use a
transmission technology consisting of a cable to which all the machines are attached, like the
telephone company party lines once used in rural areas. Traditional LANs run at speeds of 10
Mbps to 100 Mbps, have low delay (microseconds or nanoseconds), and make very few errors.
Newer LANs operate at up to 10 Gbps.
Metropolitan Area Network (MAN):
A metropolitan area network, or MAN, covers a city. The best-known example of a MAN is the
cable television network available in many cities. This system grew from earlier community
antenna systems used in areas with poor over-the-air television reception. In these early systems,
a large antenna was placed on top of a nearby hill and signal was then piped to the subscribers'
houses. At first, these were locally-designed, ad hoc systems. Then companies began jumping
into the business, getting contracts from city governments to wire up an entire city. The next step
was television programming and even entire channels designed for cable only. Often these
channels were highly specialized, such as all news, all sports, all cooking, all gardening, and so
on. But from their inception until the late 1990s, they were intended for television reception only.
Cable television is not the only MAN. Recent developments in high-speed wireless Internet
access resulted in another MAN, which has been standardized as IEEE 802.16.
Wide Area Network (WAN):
A wide area network, or WAN, spans a large geographical area, often a country or continent. It
contains a collection of machines intended for running user (i.e., application) programs. These

machines are called as hosts. The hosts are connected by a communication subnet, or just subnet
for short. The hosts are owned by the customers (e.g., people's personal computers), whereas the
communication subnet is typically owned and operated by a telephone company or Internet
service provider. The job of the subnet is to carry messages from host to host, just as the
telephone system carries words from speaker to listener. Separation of the pure communication
aspects of the network (the subnet) from the application aspects (the hosts), greatly simplifies the
complete network design. In most wide area networks, the subnet consists of two distinct
components: transmission lines and switching elements. Transmission lines move bits between
machines. They can be made of copper wire, optical fiber, or even radio links. In most WANs,
the network contains numerous transmission lines, each one connecting a pair of routers. If two
routers that do not share a transmission line wish to communicate, they must do this indirectly,
via other routers. When a packet is sent from one router to another via one or more intermediate
routers, the packet is received at each intermediate router in its entirety, stored there until the
required output line is free, and then forwarded. A subnet organized according to this principle is
called a store-and-forward or packet-switched subnet. Nearly all wide area networks (except
those using satellites) have store-and-forward subnets. When the packets are small and all the
same size, they are often called cells. The principle of a packet-switched WAN is so important.
Generally, when a process on some host has a message to be sent to a process on some other
host, the sending host first cuts the message into packets, each one bearing its number in the
sequence. These packets are then injected into the network one at a time in quick succession. The
packets are transported individually over the network and deposited at the receiving host, where
they are reassembled into the original message and delivered to the receiving process. Not all
WANs are packet switched. A second possibility for a WAN is a satellite system. Each router
has an antenna through which it can send and receive. All routers can hear the output from the
satellite, and in some cases they can also hear the upward transmissions of their fellow routers to
the satellite as well. Sometimes the routers are connected to a substantial point-to-point subnet,
with only some of them having a satellite antenna. Satellite networks are inherently broadcast
and are most useful when the broadcast property is important.
ANALOG AND DIGITAL
Analog Data: The term analog data refers to information that is continuous; For example, an
analog clock that has hour, minute, and second hands gives information in a continuous form; the

movements of the hands are continuous. Analog data, such as the sounds made by a human
voice, take on continuous values. When someone speaks, an analog wave is created in the air.
This can be captured by a microphone and converted to an analog signal or sampled and
converted to a digital signal.
Digital Data:
Digital data refers to information that has discrete states. For example, a digital clock that
reports the hours and the minutes will change suddenly from 8:05 to 8:06. Digital data takes on
discrete values. For example, data are stored in computer memory in the form of Os and 1s. They
can be converted to a digital signal or modulated into an analog signal for transmission across a
medium.
Analog and Digital Signals:
Like the data they represent, signals can be either analog or digital. An analog signal has
infinitely many levels of intensity over a period of time. As the wave moves from value A to
value B, it passes through and includes an infinite number of values along its path. A digital
signal, on the other hand, can have only a limited number of defined values. Although each value
can be any number, it is often as simple as 1 and O. The simplest way to show signals is by
plotting them on a pair of perpendicular axes. The vertical axis represents the value or strength of
a signal. The horizontal axis represents time. Figure below illustrates an analog signal and a
digital signal. The curve representing the analog signal passes through an infinite number of
points. The vertical lines of the digital signal, however, demonstrate the sudden jump that the
signal makes from value to value.

Periodic and Nonperiodic Signals:
A periodic signal completes a pattern within a measurable time frame, called a period, and
repeats that pattern over subsequent identical periods. The completion of one full pattern is
called a cycle. A nonperiodic signal changes without exhibiting a pattern or cycle that repeats
over time.
PERIODIC ANALOG SIGNALS:
Periodic analog signals can be classified as simple or composite. A simple periodic analog
signal, a sine wave, cannot be decomposed into simpler signals. A composite periodic analog
signal is composed of multiple sine waves.
Sine Wave
The sine wave is the most fundamental form of a periodic analog signal. When we visualize it as
a simple oscillating curve, its change over the course of a cycle is smooth and consistent, a
continuous, rolling flow. Figure below shows a sine wave. Each cycle consists of a single arc
above the time axis followed by a single arc below it.
Characteristics of Signals:
1. Peak Amplitude
The peak amplitude of a signal is the absolute value of its highest intensity, proportional to the
energy it carries. For electric signals, peak amplitude is normally measured in volts. Figure
below shows two signals and their peak amplitudes.

2. Period and Frequency
Period refers to the amount of time, in seconds, a signal needs to complete 1 cycle.
Frequency refers to the number of periods in I s. Note that period and frequency are just one
characteristic defined in two ways. Period is the inverse of frequency, and frequency is the
inverse of period, as the following formulas show.
f=1/T and T=1/f
Period is formally expressed in seconds. Frequency is formally expressed in Hertz (Hz), which is
cycle per second.

3. Phase
The term phase describes the position of the waveform relative to time O. If we think of the
wave as something that can be shifted backward or forward along the time axis, phase describes
the amount of that shift. It indicates the status of the first cycle. Phase is measured in degrees or
radians [360° is 2n rad; 1° is 2n/360 rad, and 1 rad is 360/(2n)]. A phase shift of 360°
corresponds to a shift of a complete period; a phase shift of 180° corresponds to a shift of one-
half of a period; and a phase shift of 90° corresponds to a shift of one-quarter of a period.
I. A sine wave with a phase of 0° starts at time 0 with a zero amplitude. The amplitude is
increasing.
II. A sine wave with a phase of 90° starts at time 0 with a peak amplitude. The amplitude is
decreasing.
III. A sine wave with a phase of 180° starts at time 0 with a zero amplitude. The amplitude is
decreasing.
Page 17Data Communication and Computer Networks

4. Wavelength
Wavelength is another characteristic of a signal traveling through a transmission medium.
Wavelength binds the period or the frequency of a simple sine wave to the propagation speed of
the medium. While the frequency of a signal is independent of the medium, the wavelength
depends on both the frequency and the medium. Wavelength is a property of any type of signal.
In data communications, we often use wavelength to describe the transmission of light in an
optical fiber. The wavelength is the distance a simple signal can travel in one period. Wavelength
can be calculated if one is given the propagation speed (the speed of light) and the period of the
signal. However, since period and frequency are related to each other, if we represent
wavelength by λ, propagation speed by c (speed of light), and frequency by f, we get
Wavelength=Propagation speed * Period = propagation speed/frequency
λ=c/f
The wavelength is normally measured in micrometers (microns) instead of meters.
Bandwidth
The range of frequencies contained in a composite signal is its bandwidth. The bandwidth is
normally a difference between two numbers. For example, if a composite signal contains
frequencies between 1000 and 5000, its bandwidth is 5000 - 1000, or 4000. Figure 3.12 shows
the concept of bandwidth. The figure depicts two composite signals, one periodic and the other
nonperiodic. The bandwidth of the periodic signal contains all integer frequencies between 1000
and 5000 (1000, 100 I, 1002, ...). The bandwidth of the nonperiodic signals has the same range,
but the frequencies are continuous.

DIGITAL SIGNALS
In addition to being represented by an analog signal, information can also be represented by a
digital signal. For example, a 1 can be encoded as a positive voltage and a 0 as zero voltage. A
digital signal can have more than two levels. In this case, we can send more than 1 bit for each
level. Figure 3.16 shows two signals, one with two levels and the other with four.
We send 1 bit per level in part a of the figure and 2 bits per level in part b of the figure. In
general, if a signal has L levels, each level needs log2L bits.
Bit Rate
Most digital signals are nonperiodic, and thus period and frequency are not appropriate
characteristics. Another term-bit rate is used to describe digital signals. The bit rate is the
number of bits sent in 1s, expressed in bits per second (bps). Figure 3.16 shows the bit rate for
two signals.

TRANSMISSION IMPAIRMENT
Signals travel through transmission media, which are not perfect. The imperfection causes
signal impairment. This means that the signal at the beginning of the medium is not the
same as the signal at the end of the medium. What is sent is not what is received. Three
causes of impairment are attenuation, distortion, and noise.
1. Attenuation
Attenuation means a loss of energy. When a signal, simple or composite, travels through a
medium, it loses some of its energy in overcoming the resistance of the medium. That is why a
wire carrying electric signals gets warm, if not hot, after a while. Some of the electrical energy in
the signal is converted to heat. To compensate for this loss, amplifiers are used to amplify the
signal. Attenuation is measured in terms of Decibels.
The decibel (dB) measures the relative strengths of two signals or one signal at two different
points. Note that the decibel is negative if a signal is attenuated and positive if a signal is
amplified.
dB=10log10 P2/P1
Variables PI and P2 are the powers of a signal at points 1 and 2, respectively.
2. Distortion:
Distortion means that the signal changes its form or shape. Distortion can occur in a composite
signal made of different frequencies. Each signal component has its own propagation speed
through a medium and, therefore, its own delay in arriving at the final destination. Differences in
delay may create a difference in phase if the delay is not exactly the same as the period duration.
In other words, signal components at the receiver have phases different from what they had at the

sender. The shape of the composite signal is therefore not the same. Figure 3.28 shows the effect
of distortion on a composite signal.
3.Noise
Noise is another cause of impairment. Several types of noise, such as thermal noise, induced
noise, crosstalk, and impulse noise, may corrupt the signal. Thermal noise is the random motion
of electrons in a wire which creates an extra signal not originally sent by the transmitter. Induced
noise comes from sources such as motors and appliances. These devices act as a sending
antenna, and the transmission medium acts as the receiving antenna. Crosstalk is the effect of
one wire on the other. One wire acts as a sending antenna and the other as the receiving antenna.
Impulse noise is a spike (a signal with high energy in a very short time) that comes from power
lines, lightning, and so on.
Signal-to-Noise Ratio (SNR)
The signal-to-noise ratio is defined as
SNR= Average Signal power / Average Noise Power
SNR is actually the ratio of what is wanted (signal) to what is not wanted (noise). A high SNR
means the signal is less corrupted by noise; a low SNR means the signal is more corrupted by
noise. Because SNR is the ratio of two powers, it is often described in decibel units, SNR dB,
defined as
SNRdB = l0logl0 SNR

DATA RATE LIMITS
A very important consideration in data communications is how fast we can send data, in bits per
second. over a channel. Data rate depends on three factors:
1. The bandwidth available
2. The level of the signals we use
3. The quality of the channel (the level of noise)
Two theoretical formulas were developed to calculate the data rate: one by Nyquist for a
noiseless channel. another by Shannon for a noisy channel.
Noiseless Channel: Nyquist Bit Rate
For a noiseless channel, the Nyquist bit rate formula defines the theoretical maximum bit
rate BitRate = 2 x bandwidth x 10g2 L
In this formula, bandwidth is the bandwidth of the channel, L is the number of signal levels used
to represent data, and BitRate is the bit rate in bits per second. According to the formula, we
might think that, given a specific bandwidth, we can have any bit rate we want by increasing the
number of signa11eve1s. Although the idea is theoretically correct, practically there is a limit.
When we increase the number of signal1eve1s, we impose a burden on the receiver. If the
number of levels in a signal is just 2, the receiver can easily distinguish between a 0 and a 1. If
the level of a signal is 64, the receiver must be very sophisticated to distinguish between 64
different levels. In other words, increasing the levels of a signal reduces the reliability of the
system.
Noisy Channel: Shannon Capacity
In reality, we cannot have a noiseless channel; the channel is always noisy. In 1944, Claude
Shannon introduced a formula, called the Shannon capacity, to determine the theoretical highest
data rate for a noisy channel:
Capacity =bandwidth X log2 (1 +SNR)
In this formula, bandwidth is the bandwidth of the channel, SNR is the signal-to-noise ratio, and
capacity is the capacity of the channel in bits per second. Note that in the Shannon formula there
is no indication of the signal level, which means that no matter how many levels we have, we
cannot achieve a data rate higher than the capacity of the channel. In other words, the formula
defines a characteristic of the channel, not the method of transmission.

Bandwidth in Bits per Seconds
The term bandwidth can also refer to the number of bits per second that a channel, a link, or even
a network can transmit. For example, one can say the bandwidth of a Fast Ethernet network is a
maximum of 100 Mbps. This means that this network can send 100 Mbps.
DIGITAL-TO-ANALOG CONVERSION
Digital-to-analog conversion is the process of changing one of the characteristics of an analog
signal based on the information in digital data. Figure 5.1 shows the relationship between the
digital information, the digital-to-analog modulating process, and the resultant analog signal.
A sine wave is defined by three characteristics: amplitude, frequency, and phase. When we vary
anyone of these characteristics, we create a different version of that wave. So, by changing one
characteristic of a simple electric signal, we can use it to represent digital data. Any of the three
characteristics can be altered in this way, giving us at least three mechanisms for modulating
digital data into an analog signal: amplitude shift keying (ASK), frequency shift keying (FSK),
and phase shift keying (PSK).
Aspects of Digital-to-Analog Conversion
Before we discuss specific methods of digital-to-analog modulation, two basic
issues must be reviewed: bit and baud rates and the carrier signal.
Data Element Versus Signal Element
Data element as the smallest piece of information to be exchanged, the bit. We also defined a
signal element as the smallest unit of a signal that is constant.

Data Rate Versus Signal Rate
We can define the data rate (bit rate) and the signal rate (baud rate). The relationship between
them is
S= N/r baud
where N is the data rate (bps) and r is the number of data elements carried in one signal
element. The value of r in analog transmission is r =log2 L, where L is the type of signal
element, not the level.
Carrier Signal
In analog transmission, the sending device produces a high-frequency signal that acts as a base
for the information signal. This base signal is called the carrier signal or carrier frequency. The
receiving device is tuned to the frequency of the carrier signal that it expects from the sender.
Digital information then changes the carrier signal by modifying one or more of its
characteristics (amplitude, frequency, or phase). This kind of modification is called modulation
(shift keying).
1. Amplitude Shift Keying (ASK)
In amplitude shift keying, the amplitude of the carrier signal is varied to create signal elements.
Both frequency and phase remain constant while the amplitude changes. Although we can have
several levels (kinds) of signal elements, each with a different amplitude, ASK is normally
implemented using only two levels. This is referred to as binary amplitude shift keying or on-off
keying (OOK). The peak amplitude of one signal level is 0; the other is the same as the amplitude
of the carrier frequency. Figure 5.3 gives a conceptual view of binary ASK.

2.Frequency Shift Keying (FSK)
In frequency shift keying, the frequency of the carrier signal is varied to represent data. The
frequency of the modulated signal is constant for the duration of one signal element, but changes
for the next signal element if the data element changes. Both peak amplitude and phase remain
constant for all signal elements.
One way to think about binary FSK (or BFSK) is to consider two carrier frequencies. In Figure
5.6, we have selected two carrier frequencies,f1 andf2. We use the first carrier if the data element
is 0; we use the second if the data element is 1. However, note that this is an unrealistic example
used only for demonstration purposes. Normally the carrier frequencies are very high, and the
difference between them is very small.
3. Phase Shift Keying (PSK)
In phase shift keying, the phase of the carrier is varied to represent two or more different signal
elements. Both peak amplitude and frequency remain constant as the phase changes. Today, PSK
is more common than ASK or FSK. The simplest PSK is binary PSK, in which we have only two
signal elements, one with a phase of 0°, and the other with a phase of 180°. Figure 5.9 gives a
conceptual view of PSK.

MULTIPLEXING
Whenever the bandwidth of a medium linking two devices is greater than the bandwidth needs of
the devices, the link can be shared. Multiplexing is the set of techniques that allows the
simultaneous transmission of multiple signals across a single data link.
In a multiplexed system, n lines share the bandwidth of one link. Figure 6.1 shows the basic
format of a multiplexed system. The lines on the left direct their transmission streams to a
multiplexer (MUX), which combines them into a single stream (many-to-one). At the receiving
end, that stream is fed into a demultiplexer (DEMUX), which separates the stream back into its
component transmissions (one-to-many) and directs them to their corresponding lines. In the
figure, the word link refers to the physical path. The word channel refers to the portion of a link
that carries a transmission between a given pair of lines. One link can have many (n) channels.
There are three basic multiplexing techniques: frequency-division multiplexing, wavelength-
division multiplexing, and time-division multiplexing. The first two are techniques designed for
analog signals, the third, for digital signals.
1. Frequency-Division Multiplexing
Frequency-division multiplexing (FDM) is an analog technique that can be applied when the
bandwidth of a link (in hertz) is greater than the combined bandwidths of the signals to be
transmitted. In FDM, signals generated by each sending device modulate different carrier
frequencies. These modulated signals are then combined into a single composite signal that can
be transported by the link. Carrier frequencies are separated by sufficient bandwidth to
accommodate the modulated signal. These bandwidth ranges are the channels through which the
various signals travel. Channels can be separated by strips of unused bandwidth-guard bands-to

prevent signals from overlapping. In addition, carrier frequencies must not interfere with the
original data frequencies.
Multiplexing Process
Figure 6.4 is a conceptual illustration of the multiplexing process. Each source generates a signal
of a similar frequency range. Inside the multiplexer, these similar signals modulates different
carrier frequencies. The resulting modulated signals are then combined into a single composite
signal that is sent out over a media link that has enough bandwidth to accommodate it.
Demultiplexing Process
The demultiplexer uses a series of filters to decompose the multiplexed signal into its constituent
component signals. The individual signals are then passed to a demodulator that separates them
from their carriers and passes them to the output lines. Figure 6.5 is a conceptual illustration of
demultiplexing process.

2. Wavelength-Division Multiplexing
Wavelength-division multiplexing (WDM) is designed to use the high-data-rate capability of
fiber-optic cable. The optical fiber data rate is higher than the data rate of metallic transmission
cable. Using a fiber-optic cable for one single line wastes the available bandwidth. Multiplexing
allows us to combine several lines into one. WDM is conceptually the same as FDM, except that
the multiplexing and demultiplexing involve optical signals transmitted through fiber-optic
channels. The idea is the same: We are combining different signals of different frequencies. The
difference is that the frequencies are very high. Figure 6.10 gives a conceptual view of a WDM
multiplexer and demultiplexer. Very narrow bands of light from different sources are combined
to make a wider band of light. At the receiver, the signals are separated by the demultiplexer.
Although WDM technology is very complex, the basic idea is very simple. We want to combine
multiple light sources into one single light at the multiplexer and do the reverse at the
demultiplexer. The combining and splitting of light sources are easily handled by a prism.

One application of WDM is the SONET network in which multiple optical fiber lines are
multiplexed and demultiplexed.
3. Time-Division Multiplexing
Time-division multiplexing (TDM) is a digital process that allows several connections to share
the high bandwidth of a linle Instead of sharing a portion of the bandwidth as in FDM, time is
shared. Each connection occupies a portion of time in the link. Figure 6.12 gives a conceptual
view of TDM. Note that the same link is used as in FDM; here, however, the link is shown
sectioned by time rather than by frequency. In the figure, portions of signals 1,2,3, and 4 occupy
the link sequentially.
Note that in Figure 6.12 we are concerned with only multiplexing, not switching. This means that
all the data in a message from source 1 always go to one specific destination, be it 1, 2, 3, or 4.
The delivery is fixed and unvarying, unlike switching.
TRANSMISSION MEDIA
A transmission medium can be broadly defined as anything that can carry information from a
source to a destination. For example, the transmission medium for two people having a dinner
conversation is the air. The air can also be used to convey the message in a smoke signal or
semaphore. For a written message, the transmission medium might be a mail carrier, a truck, or
an airplane.
In data communications the definition of the information and the transmission medium is more
specific. The transmission medium is usually free space, metallic cable, or fiber-optic cable. The
DatainformationCommunicationisusuallyandasignalComputerthat isNetworkstheresult of a conversion of data from another form.Page 29

Guided Media
Guided media, which are those that provide a conduit from one device to another, include
twisted-pair cable, coaxial cable, and fiber-optic cable. A signal traveling along any of these
media is directed and contained by the physical limits of the medium. Twisted-pair and coaxial
cable use metallic (copper) conductors that accept and transport signals in the form of electric
current. Optical fiber is a cable that accepts and transports signals in the form of light.
1.Twisted-Pair Cable
A twisted pair consists of two conductors (normally copper), each with its own plastic insulation,
twisted together, as shown in Figure 7.3.
One of the wires is used to carry signals to the receiver, and the other is used only as a ground
reference. The receiver uses the difference between the two. In addition to the signal sent by the
sender on one of the wires, interference (noise) and crosstalk may affect both wires and create
unwanted signals. If the two wires are parallel, the effect of these unwanted signals is not the
same in both wires because they are at different locations relative to the noise or crosstalk
sources (e,g., one is closer and the other is farther). This results in a difference at the receiver. By
twisting the pairs, a balance is maintained. For example, suppose in one twist, one wire is closer
to the noise source and the other is farther; in the next twist, the reverse is true. Twisting makes it
probable that both wires are equally affected by external influences (noise or crosstalk). This
means that the receiver, which calculates the difference between the two, receives no unwanted
signals. The unwanted signals are mostly canceled out. From the above discussion, it is clear that
the number of twists per unit of length (e.g., inch) has some effect on the quality of the cable.
Applications
Twisted-pair cables are used in telephone lines to provide voice and data channels. The local
loop-the line that connects subscribers to the central telephone office-commonly consists of
unshielded twisted-pair cables. The DSL lines that are used by the telephone companies to Data
Communication and Computer Networks Page 30

provide high-data-rate connections also use the high-bandwidth capability of unshielded twisted-
pair cables. Local-area networks, such as lOBase-T and lOOBase-T, also use twisted-pair cables.
2.Coaxial Cable
Coaxial cable (or coax) carries signals of higher frequency ranges than those in twisted pair
cable, in part because the two media are constructed quite differently. Instead of having two
wires, coax has a central core conductor of solid or stranded wire (usually copper) enclosed in an
insulating sheath, which is, in turn, encased in an outer conductor of metal foil, braid, or a
combination of the two. The outer metallic wrapping serves both as a shield against noise and as
the second conductor, which completes the circuit. This outer conductor is also enclosed in an
insulating sheath, and the whole cable is protected by a plastic cover (see Figure 7.7).
Applications
Coaxial cable was widely used in analog telephone networks where a single coaxial network
could carry 10,000 voice signals. Later it was used in digital telephone networks where a single
coaxial cable could carry digital data up to 600 Mbps. However, coaxial cable in telephone
networks has largely been replaced today with fiber-optic cable. Cable TV networks also use
coaxial cables. In the traditional cable TV network, the entire network used coaxial cable. Later,
however, cable TV providers replaced most of the media with fiber-optic cable; hybrid networks
use coaxial cable only at the network boundaries, near the consumer premises. Cable TV uses
RG-59 coaxial cable. Another common application of coaxial cable is in traditional Ethernet
LANs. Because of its high bandwidth, and consequently high data rate, coaxial cable was chosen
for digital transmission in early Ethernet LANs.

3. Fiber Optic Cable: A fiber-optic cable is made of glass or plastic and transmits signals in
the form of light. To understand optical fiber, we first need to explore several aspects of the
nature of light. Light travels in a straight line as long as it is moving through a single uniform If a
ray of light traveling through one substance suddenly enters another substance (of a different
density), the ray changes direction. Figure 7.10 shows how a ray of light changes direction when
going from a more dense to a less dense substance.
As the figure shows, if the angle of incidence I (the angle the ray makes with the line
perpendicular to the interface between the two substances) is less than the critical angle, the ray
refracts and moves closer to the surface. If the angle of incidence is equal to the critical angle,
the light bends along the interface. If the angle is greater than the critical angle, the ray reflects
(makes a turn) and travels again in the denser substance. Note that the critical angle is a property
of the substance, and its value differs from one substance to another.
Optical fibers use reflection to guide light through a channel. A glass or plastic core is
surrounded by a cladding of less dense glass or plastic. The difference in density of the two
materials must be such that a beam of light moving through the core is reflected off the cladding
instead of being refracted into it. See Figure 7.11.

Cable Composition
Figure 7.14 shows the composition of a typical fiber-optic cable. The outer jacket is made of
either PVC or Teflon. Inside the jacket are Kevlar strands to strengthen the cable. Kevlar is a
strong material used in the fabrication of bulletproof vests. Below the Kevlar is another plastic
coating to cushion the fiber. The fiber is at the center of the cable, and it consists of cladding and
core.
Applications
Fiber-optic cable is often found in backbone networks because its wide bandwidth is cost-
effective. Today, with wavelength-division multiplexing (WDM), we can transfer data at a rate
of 1600 Gbps. The SONET network provides such a backbone. Some cable TV companies use a
combination of optical fiber and coaxial cable, thus creating a hybrid network. Optical fiber
provides the backbone structure while coaxial cable provides the connection to the user premises.
This is a cost-effective configuration since the narrow bandwidth requirement at the user end
does not justify the use of optical fiber. Local-area networks such as 100Base-FX network (Fast
Ethernet) and 1000Base-X also use fiber-optic cable.
Advantages and Disadvantages of Optical
Fiber Advantages
Fiber-optic cable has several advantages over metallic cable (twisted pair or coaxial).
a. Higher bandwidth. Fiber-optic cable can support dramatically higher bandwidths (and hence
data rates) than either twisted-pair or coaxial cable. Currently, data rates and bandwidth

utilization over fiber-optic cable are limited not by the medium but by the signal generation and
reception technology available.
b. Less signal attenuation. Fiber-optic transmission distance is significantly greater than that of
other guided media. A signal can run for 50 km without requiring regeneration. We need
repeaters every 5 km for coaxial or twisted-pair cable.
c. Immunity to electromagnetic interference. Electromagnetic noise cannot affect fiber-optic
cables.
d. Resistance to corrosive materials. Glass is more resistant to corrosive materials than copper.
e. Light weight. Fiber-optic cables are much lighter than copper cables.
f. Greater immunity to tapping. Fiber-optic cables are more immune to tapping than copper
cables. Copper cables create antenna effects that can easily be tapped.
Disadvantages
There are some disadvantages in the use of optical fiber.
a. Installation and maintenance. Fiber-optic cable is a relatively new technology. Its
installation and maintenance require expertise that is not yet available everywhere.
b. Unidirectional light propagation. Propagation of light is unidirectional. If we need
bidirectional communication, two fibers are needed.
c. Cost. The cable and the interfaces are relatively more expensive than those of other guided
media. If the demand for bandwidth is not high, often the use of optical fiber cannot be justified.
UNGUIDED MEDIA: WIRELESS
Unguided media transport electromagnetic waves without using a physical conductor. This type
of communication is often referred to as wireless communication. Signals are normally broadcast
through free space and thus are available to anyone who has a device capable of receiving them.
Unguided signals can travel from the source to destination in several ways: ground propagation,
sky propagation, and line-of-sight propagation, as shown in Figure 7.18. In ground propagation,
radio waves travel through the lowest portion of the atmosphere, hugging the earth. These low-
frequency signals emanate in all directions from the transmitting antenna and follow the
curvature of the planet. Distance depends on the amount of power in the signal: The greater the
power, the greater the distance. In sky propagation, higher-frequency radio waves radiate upward
into the ionosphere where they are reflected back to earth. This type of transmission allows for
greater distances with lower output power. In line-or-sight propagation, very high-frequency

signals are transmitted in straight lines directly from antenna to antenna. Antennas must be
directional, facing each other, and either tall enough or close enough together not to be affected
by the curvature of the earth. Line-of-sight propagation is tricky because radio transmissions
cannot be completely focused.
1. Radio Waves
Waves ranging in frequencies between 3 kHz and 1 GHz are called radio waves. Radio waves,
for the most part, are omnidirectional. When an antenna transmits radio waves, they are
propagated in all directions. This means that the sending and receiving antennas do not have to
be aligned. A sending antenna sends waves that can be received by any receiving antenna. The
omnidirectional property has a disadvantage, too. The radio waves transmitted by one antenna
are susceptible to interference by another antenna that may send signals using the same
frequency or band. Radio waves, particularly those waves that propagate in the sky mode, can
travel long distances. This makes radio waves a good candidate for long-distance broadcasting
such as AM radio. Radio waves, particularly those of low and medium frequencies, can penetrate
walls. This characteristic can be both an advantage and a disadvantage. It is an advantage
because, for example, an AM radio can receive signals inside a building. It is a disadvantage
because we cannot isolate a communication to just inside or outside a building. The radio wave
band is relatively narrow, just under 1 GHz, compared to the microwave band. When this band is
divided into sub bands, the sub bands are also narrow, leading to a low data rate for digital
communications.

Omnidirectional Antenna
Radio waves use omnidirectional antennas that send out signals in all directions. Based on the
wavelength, strength, and the purpose of transmission, we can have several types of antennas.
Figure 7.20 shows an omnidirectional antenna.
Applications
The omnidirectional characteristics of radio waves make them useful for multicasting, in which
there is one sender but many receivers. AM and FM radio, television, maritime radio, cordless
phones, and paging are examples of multicasting.
2. Microwaves
Electromagnetic waves having frequencies between I and 300 GHz are called microwaves.
Microwaves are unidirectional. When an antenna transmits microwave waves, they can be
narrowly focused. This means that the sending and receiving antennas need to be aligned. The
unidirectional property has an obvious advantage. A pair of antennas can be aligned without
interfering with another pair of aligned antennas. The following describes some characteristics of
microwave propagation:
a. Microwave propagation is line-of-sight. Since the towers with the mounted antennas need to
be in direct sight of each other, towers that are far apart need to be very tall. The curvature of the
earth as well as other blocking obstacles do not allow two short towers to communicate by using
microwaves. Repeaters are often needed for long distance communication.
b. Very high-frequency microwaves cannot penetrate walls. This characteristic can be a
disadvantage if receivers are inside buildings.

c. The microwave band is relatively wide, almost 299 GHz. Therefore wider sub bands can be
assigned, and a high data rate is possible
d. Use of certain portions of the band requires permission from authorities.
Unidirectional Antenna
Microwaves need unidirectional antennas that send out signals in one direction. Two types of
antennas are used for microwave communications: the parabolic dish and the horn (see Figure
7.21). A parabolic dish antenna is based on the geometry of a parabola: Every line parallel to the
line of symmetry (line of sight) reflects off the curve at angles such that all the lines intersect in a
common point called the focus. The parabolic dish works as a funnel, catching a wide range of
waves and directing them to a common point. In this way, more of the signal is recovered than
would be possible with a single-point receiver. Outgoing transmissions are broadcast through a
horn aimed at the dish. The microwaves hit the dish and are deflected outward in a reversal of
the receipt path. A horn antenna looks like a gigantic scoop. Outgoing transmissions are
broadcast up a stem (resembling a handle) and deflected outward in a series of narrow parallel
beams by the curved head. Received transmissions are collected by the scooped shape of the
horn, in a manner similar to the parabolic dish, and are deflected down into the stem.

3. Infrared
Infrared waves, with frequencies from 300 GHz to 400 THz (wavelengths from 1 mm to 770
nm), can be used for short-range communication. Infrared waves, having high frequencies,
cannot penetrate walls. This advantageous characteristic prevents interference between one
system and another; a short-range communication system in one room cannot be affected by
another system in the next room. When we use our infrared remote control, we do not interfere
with the use of the remote by our neighbors. However, this same characteristic makes infrared
signals useless for long-range communication. In addition, we cannot use infrared waves outside
a building because the sun's rays contain infrared waves that can interfere with the
communication.
Applications
The infrared band, almost 400 THz, has an excellent potential for data transmission. Such a wide
bandwidth can be used to transmit digital data with a very high data rate. The Infrared Data
Association (IrDA), an association for sponsoring the use of infrared waves, has established
standards for using these signals for communication between devices such as keyboards, mice,
PCs, and printers. For example, some manufacturers provide a special port called the IrDA port
that allows a wireless keyboard to communicate with a PC. The standard originally defined a
data rate of 75 kbps for a distance up to 8 m. The recent standard defines a data rate of 4 Mbps.
Infrared signals defined by IrDA transmit through line of sight; the IrDA port on the keyboard
needs to point to the PC for transmission to occur.
SWITCHING
A network is a set of connected devices. Whenever we have multiple devices, we have the
problem of how to connect them to make one-to-one communication possible. One solution is to
make a point-to-point connection between each pair of devices (a mesh topology) or between a
central device and every other device (a star topology). These methods, however, are impractical
and wasteful when applied to very large networks. The number and length of the links require
too much infrastructure to be cost-efficient, and the majority of those links would be idle most of
the time. Other topologies employing multipoint connections, such as a bus, are ruled out
because the distances between devices and the total number of devices increase beyond the
capacities of the media and equipment.

A better solution is switching. A switched network consists of a series of interlinked nodes,
called switches. Switches are devices capable of creating temporary connections between two or
more devices linked to the switch. In a switched network, some of these nodes are connected to
the end systems (computers or telephones, for example). Others are used only for routing. Figure
8.1 shows a switched network.
The end systems (communicating devices) are labeled A, B, C, D, and so on, and the switches
are labeled I, II, III, IV, and V. Each switch is connected to multiple links. Traditionally, three
methods of switching have been important: circuit switching, packet switching, and message
switching. The first two are commonly used today. The third has been phased out in general
communications but still has networking applications. We can then divide today's networks into
three broad categories: circuit-switched networks, packet-switched networks, and message-
switched.

1. CIRCUIT-SWITCHED NETWORKS
A circuit-switched network consists of a set of switches connected by physical links. A
connection between two stations is a dedicated path made of one or more links. However, each
connection uses only one dedicated channel on each link. Each link is normally divided into n
channels by using FDM or TDM. Figure 8.3 shows a trivial circuit-switched network with four
switches and four links. Each link is divided into n (n is 3 in the figure) channels by using FDM
or TDM.
The end systems, such as computers or telephones, are directly connected to a switch. We have
shown only two end systems for simplicity. When end system A needs to communicate with end
system M, system A needs to request a connection to M that must be accepted by all switches as
well as by M itself. This is called the setup phase; a circuit (channel) is reserved on each link,
and the combination of circuits or channels defines the dedicated path. After the dedicated path
made of connected circuits (channels) is established, data transfer can take place. After all data
have been transferred, the circuits are torn down.
Three Phases
The actual communication in a circuit-switched network requires three phases: connection setup,
data transfer, and connection teardown.

Setup Phase
Before the two parties (or multiple parties in a conference call) can communicate, a dedicated
circuit (combination of channels in links) needs to be established. The end systems are normally
connected through dedicated lines to the switches, so connection setup means creating dedicated
channels between the switches. For example, in Figure 8.3, when system A needs to connect to
system M, it sends a setup request that includes the address of system M, to switch I. Switch I
finds a channel between itself and switch IV that can be dedicated for this purpose. Switch I then
sends the request to switch IV, which finds a dedicated channel between itself and switch III.
Switch III informs system M of system A's intention at this time. In the next step to making a
connection, an acknowledgment from system M needs to be sent in the opposite direction to
system A. Only after system A receives this acknowledgment is the connection established.
Data Transfer Phase
After the establishment of the dedicated circuit (channels), the two parties can transfer
data. Teardown Phase
When one of the parties needs to disconnect, a signal is sent to each switch to release the
resources.
2.PACKET SWITCHED NETWORK
In a Computer Network, the communication between two ends is done in blocks of data called
packets. So instead of continuous communication the exchange takes place in the form of
individual packets between the two computers. This allows us to make the switches function for
both storing and forwarding because a packet is an independent entity that can be stored and
sent later.
a. DATAGRAM NETWORKS
In data communications, we need to send messages from one end system to another. If the
message is going to pass through a packet-switched network, it needs to be divided into packets
of fixed or variable size. The size of the packet is determined by the network and the governing
protocol. In packet switching, there is no resource allocation for a packet. This means that there
is no reserved bandwidth on the links, and there is no scheduled processing time for each packet.
Resources are allocated on demand. The allocation is done on a first come, first-served basis.
When a switch receives a packet, no matter what is the source or destination, the packet must
wait if there are other packets being processed. As with other systems in our daily life, this lack

of reservation may create delay. For example, if we do not have a reservation at a restaurant, we
might have to wait. In a datagram network, each packet is treated independently of all others.
Even if a packet is part of a multipacket transmission, the network treats it as though it existed
alone. Packets in this approach are referred to as datagrams. Datagram switching is normally
done at the network layer. The switches in a datagram network are traditionally referred to as
routers.
In this example, all four packets (or datagrams) belong to the same message, but may travel
different paths to reach their destination. This is so because the links may be involved in carrying
packets from other sources and do not have the necessary bandwidth available to carry all the
packets from A to X. This approach can cause the datagrams of a transmission to arrive at their
destination out of order with different delays between them packets. Packets may also be lost or
dropped because of a lack of resources. In most protocols, it is the responsibility of an upper-
layer protocol to reorder the datagrams or ask for lost datagrams before passing them on to the
application. The datagram networks are sometimes referred to as connectionless networks. The
term connectionless here means that the switch (packet switch) does not keep information about
the connection state. There are no setup or teardown phases. Each packet is treated the same by a
switch regardless of its source or destination.
b. VIRTUAL-CIRCUIT NETWORKS
A virtual-circuit network is a cross between a circuit-switched network and a datagram network.
It has some characteristics of both.

1. As in a circuit-switched network, there are setup and teardown phases in addition to the data
transfer phase.
2. Resources can be allocated during the setup phase, as in a circuit-switched network, or on
demand, as in a datagram network.
3. As in a datagram network, data are packetized and each packet carries an address in the
header. However, the address in the header has local jurisdiction , not end-to-end jurisdiction.
The reader may ask how the intermediate switches know where to send the packet if there is no
final destination address carried by a packet.
4. As in a circuit-switched network, all packets follow the same path established during the
connection.
5. A virtual-circuit network is normally implemented in the data link layer, while a circuit-
switched network is implemented in the physical layer and a datagram network in the network
layer. But this may change in the future.
Figure 8.10 is an example of a virtual-circuit network. The network has switches that allow
traffic from sources to destinations. A source or destination can be a computer, packet switch,
bridge, or any other device that connects other networks.

DATA TRANSMISSION AND MODES
The transmission of binary data across a link can be accomplished in either parallel or serial
mode. In parallel mode, multiple bits are sent with each clock tick. In serial mode, 1 bit is sent
with each clock tick. While there is only one way to send parallel data, there are three subclasses
of serial transmission: asynchronous, synchronous, and isochronous.
1.Parallel Transmission
Binary data, consisting of 1s and 0s, may be organized into groups of n bits each. Computers
produce and consume data in groups of bits much as we conceive of and use spoken language in
the form of words rather than letters. By grouping, we can send data n bits at a time instead of 1.
This is called parallel transmission. The mechanism for parallel transmission is a conceptually
simple one: Use n wires to send n bits at one time. That way each bit has its own wire, and all n
bits of one group can be transmitted with each clock tick from one device to another. Figure 4.32
shows how parallel transmission works for n =8. Typically, the eight wires are bundled in a cable
with a connector at each end. The advantage of parallel transmission is speed. All else being
equal, parallel transmission can increase the transfer speed by a factor of n over serial
transmission.
But there is a significant disadvantage: cost. Parallel transmission requires n communication
lines just to transmit the data stream. Because this is expensive, parallel transmission is usually
limited to short distances.

2.Serial Transmission
In serial transmission one bit follows another, so we need only one communication channel
rather than n to transmit data between two communicating devices (see Figure 4.33).
The advantage of serial over parallel transmission is that with only one communication channel,
serial transmission reduces the cost of transmission over parallel by roughly a factor of n. Since
communication within devices is parallel, conversion devices are required at the interface
between the sender and the line (parallel-to-serial) and between the line and the receiver (serial-
to-parallel).
Serial transmission occurs in one of three ways: asynchronous, synchronous, and isochronous.

a. Asynchronous Transmission
Asynchronous transmission is so named because the timing of a signal is unimportant. Instead,
information is received and translated by agreed upon patterns. As long as those patterns are
followed, the receiving device can retrieve the information without regard to the rhythm in which
it is sent. Patterns are based on grouping the bit stream into bytes. Each group, usually 8 bits, is
sent along the link as a unit. The sending system handles each group independently, relaying it to
the link whenever ready, without regard to a timer. Without synchronization, the receiver cannot
use timing to predict when the next group will arrive. To alert the receiver to the arrival of a new
group, therefore, an extra bit is added to the beginning of each byte. This bit, usually a 0, is
called the start bit. To let the receiver know that the byte is finished, 1 or more additional bits are
appended to the end of the byte. These bits, usually I s, are called stop bits. By this method, each
byte is increased in size to at least 10 bits, of which 8 bits is information and 2 bits or more are
signals to the receiver. In addition, the transmission of each byte may then be followed by a gap
of varying duration. This gap can be represented either by an idle channel or by a stream of
additional stop bits. The start and stop bits and the gap alert the receiver to the beginning and end
of each byte and allow it to synchronize with the data stream. This mechanism is called
asynchronous because, at the byte level, the sender and receiver do not have to be synchronized.
But within each byte, the receiver must still be synchronized with the incoming bit stream. That
is, some synchronization is required, but only for the duration of a single byte. The receiving
device resynchronizes at the onset of each new byte. When the receiver detects a start bit, it sets
a timer and begins counting bits as they come in. After n bits, the receiver looks for a stop bit. As
soon as it detects the stop bit, it waits until it detects the next start bit. Figure 4.34 is a schematic
illustration of asynchronous transmission. In this example, the start bits are as, the stop bits are
1s, and the gap is represented by an idle line rather than by additional stop bits.
The addition of stop and start bits and the insertion of gaps into the bit stream make
asynchronous transmission slower than forms of transmission that can operate without the
addition of control information. But it is cheap and effective, two advantages that make it an
attractive choice for situations such as low-speed communication. For example, the connection
of a keyboard to a computer is a natural application for asynchronous transmission. A user types
slowly in data processing terms, and leaves unpredictable gaps of time between each character.

b. Synchronous Transmission
In synchronous transmission, the bit stream is combined into longer "frames," which may
contain multiple bytes. Each byte, however, is introduced onto the transmission link without a
gap between it and the next one. It is left to the receiver to separate the bit stream into bytes for
decoding purposes. In other words, data are transmitted as an unbroken string of 1s and 0s, and
the receiver separates that string into the bytes, or characters, it needs to reconstruct the
information. Figure 4.35 gives a schematic illustration of synchronous transmission. We have
drawn in the divisions between bytes. In reality, those divisions do not exist; the sender puts its
data onto the line as one long string. If the sender wishes to send data in separate bursts, the gaps
between bursts must be filled with a special sequence of Os and Is that means idle. The receiver
counts the bits as they arrive and groups them in 8-bit units. Without gaps and start and stop bits,
there is no built-in mechanism to help the receiving device adjust its bit synchronization
midstream. Timing becomes very important, therefore, because the accuracy of the received
information is completely dependent on the ability of the receiving device to keep an accurate
count of the bits as they come in. The advantage of synchronous transmission is speed. With no
extra bits or gaps to introduce at the sending end and remove at the receiving end, and, by
extension, with fewer bits to move across the link, synchronous transmission is faster than
asynchronous transmission. For this reason, it is more useful for high-speed applications such as
the transmission of data from one computer to another. Byte synchronization is accomplished in
the data link layer.

c. Isochronous
In real-time audio and video, in which uneven delays between frames are not acceptable,
synchronous transmission fails. For example, TV images are broadcast at the rate of 30 images
per second; they must be viewed at the same rate. If each image is sent by using one or more
frames, there should be no delays between frames. For this type of application, synchronization
between characters is not enough; the entire stream of bits must be synchronized. The
isochronous transmission guarantees that the data arrive at a fixed rate.
CYCLIC REDUNDANCY CHECK (CRC)
Cyclic codes are special linear block codes with one extra property. In a cyclic code, if a
codeword is cyclically shifted (rotated), the result is another codeword. For example, if 1011000
is a codeword and we cyclically left-shift, then 0110001 is also a codeword. In this case, if we
call the bits in the first word ao to a6' and the bits in the second word bo to b6.

In the encoder, the dataword has k bits (4 here); the codeword has n bits (7 here). The size of the
dataword is augmented by adding n - k (3 here) Os to the right-hand side of the word. The n-bit
result is fed into the generator. The generator uses a divisor of size n - k + I (4 here), predefined
and agreed upon. The generator divides the augmented dataword by the divisor (modulo-2
division). The quotient of the division is discarded; the remainder (r2r1r0) is appended to the
dataword to create the codeword. The decoder receives the possibly corrupted codeword. A copy
of all n bits is fed to the checker which is a replica of the generator. The remainder produced by
the checker is a syndrome of n - k (3 here) bits, which is fed to the decision logic analyzer. The
analyzer has a simple function. If the syndrome bits are all as, the 4 leftmost bits of the codeword
are accepted as the dataword (interpreted as no error); otherwise, the 4 bits are discarded (error).
Encoder
Let us take a closer look at the encoder. The encoder takes the dataword and augments it with n -
k number of 0's. It then divides the augmented dataword by the divisor, as shown in Figure
10.15.

As in decimal division, the process is done step by step. In each step, a copy of the divisor is
XORed with the 4 bits of the dividend. The result of the XOR operation (remainder) is 3 bits (in
this case), which is used for the next step after 1 extra bit is pulled down to make it 4 bits long.
There is one important point we need to remember in this type of division. If the leftmost bit of
the dividend (or the part used in each step) is 0, the step cannot use the regular divisor; we need
to use an all-Os divisor. When there are no bits left to pull down, we have a result. The 3-bit
remainder forms the check bits (r2' r1 and r0). They are appended to the dataword to create the
codeword.
Decoder
The codeword can change during transmission. The decoder does the same division process as
the encoder. The remainder of the division is the syndrome. If the syndrome is all Os, there is no
error; the dataword is separated from the received codeword and accepted. Otherwise, everything
is discarded. Figure 10.16 shows two cases: The left hand figure shows the value of syndrome
when no error has occurred; the syndrome is 000. The right-hand part of the figure shows the
case in which there is one single error. The syndrome is not all 0s .

Divisor
Let us first consider the divisor. We need to note the following points:
1. The divisor is repeatedly XORed with part of the dividend.
2. The divisor has n - k + 1 bits which either are predefined or are all Os. In other words, the bits
do not change from one dataword to another. In our previous example, the divisor bits were
either 1011 or 0000. The choice was based on the leftmost bit of the part of the augmented data
bits that are active in the XOR operation.
3. A close look shows that only n - k bits of the divisor is needed in the XOR operation. The
leftmost bit is not needed because the result of the operation is always 0, no matter what the
value of this bit. The reason is that the inputs to this XOR operation are either both Os or both 1s.
In our previous example, only 3 bits, not 4, is actually used in the XOR operation.
Polynomials
A better way to understand cyclic codes and how they can be analyzed is to represent them as
polynomials.
A pattern of Os and 1s can be represented as a polynomial with coefficients of 0 and 1. The
power of each term shows the position of the bit; the coefficient shows the value of the bit.
Figure 10.21 shows a binary pattern and its polynomial representation. In Figure 10.21a we show
how to translate a binary pattern to a polynomial; in Figure 1O.21b we show how the polynomial
can be shortened by removing all terms with zero coefficients.
Figure 10.21 shows one immediate benefit; a 7-bit pattern can be replaced by three terms. The
benefit is even more conspicuous when we have a polynomial such as x23 + X3 + 1. Here the bit
pattern is 24 bits in length (three Is and twenty-one Os) while the polynomial is just three terms.

PARITY CHECK CODE
The most familiar error-detecting code is the simple parity-check code. In this code, a k-bit
dataword is changed to an n-bit codeword where n = k + 1. The extra bit, called the parity bit, is
selected to make the total number of Is in the codeword even. Although some implementations
specify an odd number of 1's. The minimum Hamming distance for this category is dmin =2,
which means that the code is a single-bit error-detecting code; it cannot correct any error. Our
first code (Table 10.1) is a parity-check code with k -= 4 and n =5.
Figure 10.10 shows a possible structure of an encoder (at the sender) and a decoder
(at the receiver). The encoder uses a generator that takes a copy of a 4-bit dataword (ao, aI' a2'
and a3) and generates a parity bit roo The dataword bits and the parity bit create the 5-bit
codeword. The parity bit that is added makes the number of Is in the codeword even.

This is normally done by adding the 4 bits of the dataword (modulo-2); the result is the parity bit.
In other words,
If the number of 1s is even, the result is 0; if the number of 1s is odd, the result is 1. In both
cases, the total number of 1s in the codeword is even. The sender sends the codeword which may
be corrupted during transmission. The receiver receives a 5-bit word. The checker at the receiver
does the same thing as the generator in the sender with one exception: The addition is done over
all 5 bits. The result, which is called the syndrome, is just 1 bit. The syndrome is 0 when the
number of Is in the received codeword is even; otherwise, it is 1.
The syndrome is passed to the decision logic analyzer. If the syndrome is 0, there is no error in
the received codeword; the data portion of the received codeword is accepted as the dataword; if
the syndrome is 1, the data portion of the received codeword is discarded. The dataword is not
created.
HDLC
High-level Data Link Control (HDLC) is a bit-oriented protocol for communication over point-
to-point and multipoint links.
Configurations and Transfer Modes
HDLC provides two common transfer modes that can be used in different configurations: normal
response mode (NRM) and asynchronous balanced mode (ABM).
Normal Response Mode
In normal response mode (NRM), the station configuration is unbalanced. We have one primary
station and multiple secondary stations. A primary station can send commands; a secondary
station can only respond. The NRM is used for both point-to-point and multiple-point links, as
shown in Figure 11.25.
Asynchronous Balanced Mode
In asynchronous balanced mode (ABM), the configuration is balanced. The link is point-to-point,
and each station can function as a primary and a secondary (acting as peers), as shown in Figure
11.26. This is the common mode today.

Frames
To provide the flexibility necessary to support all the options possible in the modes and
configurations just described, HDLC defines three types of frames: information frames (I-
frames), supervisory frames (S-frames), and unnumbered frames (V-frames). Each type of frame
serves as an envelope for the transmission of a different type of message. I-frames are used to
transport user data and control information relating to user data (piggybacking). S-frames are
used only to transport control information. V-frames are reserved for system management.
Information carried by V-frames is intended for managing the link itself.
Frame Format
Each frame in HDLC may contain up to six fields, as shown in Figure 11.27: a beginning flag
field, an address field, a control field, an information field, a frame check sequence (FCS) field,

and an ending flag field. In multiple-frame transmissions, the ending flag of one frame can serve
as the beginning flag of the next frame.
Fields
Let us now discuss the fields and their use in different frame types.
o Flag field. The flag field of an HDLC frame is an 8-bit sequence with the bit pattern 01111110
that identifies both the beginning and the end of a frame and serves as a synchronization pattern
for the receiver.
o Address field. The second field of an HDLC frame contains the address of the secondary
station. If a primary station created the frame, it contains a to address. If a secondary creates the
frame, it contains a from address. An address field can be 1 byte or several bytes long, depending
on the needs of the network. One byte can identify up to 128 stations (l bit is used for another
purpose). Larger networks require multiple-byte address fields. If the address field is only 1 byte,
the last bit is always a 1. If the address is more than 1 byte, all bytes but the last one will end
with 0; only the last will end with 1. Ending each intermediate byte with 0 indicates to the
receiver that there are more address bytes to come.
o Control field. The control field is a 1- or 2-byte segment of the frame used for flow and error
control. The interpretation of bits in this field depends on the frame type.
o Information field. The information field contains the user's data from the network layer or
management information. Its length can vary from one network to another.
o FCS field. The frame check sequence (FCS) is the HDLC error detection field.
Page 55Data Communication and Computer Networks

Control Field
The control field determines the type of frame and defines its functionality. The format is
specific for the type of frame, as shown in Figure 11.28.
Control Field for I-Frames
I-frames are designed to carry user data from the network layer. In addition, they can include
flow and error control information (piggybacking). The subfields in the control field are used to
the control field is 0, this means the frame is an I-frame. The next 3 bits, called N(S), define the
sequence number of the frame. Note that with 3 bits, we can define a sequence number between
0 and 7; but in the extension format, in which the control field is 2 bytes, this field is larger. The
last 3 bits, called N(R), correspond to the acknowledgment number when piggybacking is used.
The single bit between N(S) and N(R) is called the PIF bit. The PIP field is a single bit with a
dual purpose. It has meaning only when it is set (bit = 1) and can mean poll or final. It means
poll when the frame is sent by a primary station to a secondary (when the address field contains
the address of the receiver). It means final when the frame is sent by a secondary to a primary.
Control Field for S-Frames
Supervisory frames are used for flow and error control whenever piggybacking is either
impossible or inappropriate. S-frames do not have information fields. If the first 2 bits of the
control field is 10, this means the frame is an S-frame. The last 3 bits, called N(R), corresponds
to the acknowledgment number (ACK) or negative acknowledgment number (NAK) depending
on the type of S-frame. The 2 bits called code is used to define the type of S-frame itself. With 2
bits, we can have four types of S-frames, as described below:

o Receive ready (RR). If the value of the code subfield is 00, it is an RR S-frame. This kind of
frame acknowledges the receipt of a safe and sound frame or group of frames. In this case, the
value N(R) field defines the acknowledgment number.
o Receive not ready (RNR). If the value of the code subfield is 10, it is an RNR S-frame. This
kind of frame is an RR frame with additional functions. It acknowledges the receipt of a frame or
group of frames, and it announces that the receiver is busy and cannot receive more frames. It
acts as a kind of congestion control mechanism by asking the sender to slow down. The value of
NCR is the acknowledgment number.
o Reject (REJ). If the value of the code subfield is 01, it is a REJ S-frame. This is a NAK frame,
but not like the one used for Selective Repeat ARQ. It is a NAK that can be used in Go-Back-N
ARQ to improve the efficiency of the process by informing the sender, before the sender time
expires, that the last frame is lost or damaged. The value of NCR) is the negative
acknowledgment number.
o Selective reject (SREJ). If the value of the code subfield is 11, it is an SREJ S-frame. This is a
NAK frame used in Selective Repeat ARQ. Note that the HDLC Protocol uses the term selective
reject instead of selective repeat. The value of N(R) is the negative acknowledgment number.
Control Field for V-Frames
Unnumbered frames are used to exchange session management and control information between
connected devices. Unlike S-frames, U-frames contain an information field, but one used for
system management information, not user data. As with S-frames, however, much of the
information carried by U-frames is contained in codes included in the control field. U-frame
codes are divided into two sections: a 2-bit prefix before the PtF bit and a 3-bit suffix after the
PtF bit. Together, these two segments (5 bits) can be used to create up to 32 different types of U-
frames.
THE OSI MODEL
The OSI model is based on a proposal developed by the International Standards Organization
(ISO) as a first step toward international standardization of the protocols used in the various
layers (Day and Zimmermann, 1983). It was revised in 1995(Day, 1995). The model is called the
ISO-OSI (Open Systems Interconnection) Reference Model because it deals with connecting
open systems—that is, systems that are open for communication with other systems.

The OSI model is a layered framework for the design of network systems that allows
communication between all types of computer systems. It consists of seven separate but related
layers, each of which defines a part of the process of moving information across a network.
The OSI model is composed of seven ordered layers: physical (layer 1), data link (layer 2),
network (layer 3), transport (layer 4), session (layer 5), presentation (layer 6), and application
(layer 7). Figure below shows the layers involved when a message is sent from device A to
device B. As the message travels from A to B, it may pass through many intermediate nodes.
These intermediate nodes usually involve only the first three layers of the OSI model.
LAYERS IN THE OSI MODEL
1. Physical Layer
The physical layer coordinates the functions required to carry a bit stream over a physical
medium. It deals with the mechanical and electrical specifications of the interface and
transmission medium. It also defines the procedures and functions that physical devices and
interfaces have to perform for transmission to occur.

The physical layer is also concerned with the following:
o Physical characteristics of interfaces and medium. The physical layer defines the
characteristics of the interface between the devices and the transmission medium. It also defines
the type of transmission medium.
o Representation of bits. The physical layer data consists of a stream of bits (sequence of Os or
1s) with no interpretation. To be transmitted, bits must be encoded into signals--electrical or
optical. The physical layer defines the type of encoding .
o Data rate. The transmission rate-the number of bits sent each second-is also defined by the
physical layer. In other words, the physical layer defines the duration of a bit, which is how long
it lasts.
o Synchronization of bits. The sender and receiver not only must use the same bit rate but also
must be synchronized at the bit level. In other words, the sender and the receiver clocks must be
synchronized.

o Line configuration. The physical layer is concerned with the connection of devices to the
media. In a point-to-point configuration, two devices are connected through a dedicated link. In a
multipoint configuration, a link is shared among several devices.
o Physical topology. The physical topology defines how devices are connected to make a
network. Devices can be connected by using a mesh topology (every device is connected to
every other device), a star topology (devices are connected through a central device), a ring
topology (each device is connected to the next, forming a ring), a bus topology (every device is
on a common link), or a hybrid topology (this is a combination of two or more topologies).
o Transmission mode. The physical layer also defines the direction of transmission between two
devices: simplex, half-duplex, or full-duplex. In simplex mode, only one device can send; the
other can only receive. The simplex mode is a one-way communication. In the half-duplex mode,
two devices can send and receive, but not at the same time. In a full-duplex (or simply duplex)
mode, two devices can send and receive at the same time.
2. Data Link Layer
The data link layer transforms the physical layer, a raw transmission facility, to a reliable link. It
makes the physical layer appear error-free to the upper layer (network layer). Other
responsibilities of the data link layer include the following:
o Framing. The data link layer divides the stream of bits received from the network layer into
manageable data units called frames.
o Physical addressing. If frames are to be distributed to different systems on the network, the
data link layer adds a header to the frame to define the sender and/or receiver of the frame. If the
frame is intended for a system outside the sender's network, the receiver address is the address of
the device that connects the network to the next one.
o Flow control. If the rate at which the data are absorbed by the receiver is less than the rate at
which data are produced in the sender, the data link layer imposes a flow control mechanism to
avoid overwhelming the receiver.
o Error control. The data link layer adds reliability to the physical layer by adding mechanisms
to detect and retransmit damaged or lost frames. It also uses a mechanism to recognize duplicate
frames. Error control is normally achieved through a trailer added to the end of the frame.
o Access control. When two or more devices are connected to the same link, data link layer
protocols are necessary to determine which device has control over the link at any given time.

3. Network Layer
The network layer is responsible for the source-to-destination delivery of a packet, possibly
across multiple networks (links). Whereas the data link layer oversees the delivery of the packet
between two systems on the same network (links), the network layer ensures that each packet
gets from its point of origin to its final destination. If two systems are connected to the same link,
there is usually no need for a network layer. However, if the two systems are attached to
different networks (links) with connecting devices between the networks (links), there is often a
need for the network layer to accomplish source-to-destination delivery. Other responsibilities of
the network layer include the following:
o Logical addressing. The physical addressing implemented by the data link layer handles the
addressing problem locally. If a packet passes the network boundary, we need another addressing
system to help distinguish the source and destination systems. The network layer adds a header
to the packet coming from the upper layer that, among other things, includes the logical
addresses of the sender and receiver.
o Routing. When independent networks or links are connected to create intemetworks (network
of networks) or a large network, the connecting devices (called routers or switches) route or
switch the packets to their final destination. One of the functions of the network layer is to
provide this mechanism.
4. Transport Layer
The transport layer is responsible for process-to-process delivery of the entire message. A
process is an application program running on a host. Whereas the network layer oversees source-
to-destination delivery of individual packets, it does not recognize any relationship between
those packets. It treats each one independently, as though each piece belonged to a separate
message, whether or not it does. The transport layer, on the other hand, ensures that the whole
message arrives intact and in order, overseeing both error control and flow control at the source-
to-destination level.
Other responsibilities of the transport layer include the following:
o Service-point addressing. Computers often run several programs at the same time. For this
reason, source-to-destination delivery means delivery not only from one computer to the next but
also from a specific process (running program) on one computer to a specific process (running
program) on the other. The transport layer header must therefore include a type of address called

a service-point address (or port address). The network layer gets each packet to the correct
computer; the transport layer gets the entire message to the correct process on that computer.
o Segmentation and reassembly. A message is divided into transmittable segments, with each
segment containing a sequence number. These numbers enable the transport layer to reassemble
the message correctly upon arriving at the destination and to identify and replace packets that
were lost in transmission.
o Connection control. The transport layer can be either connectionless or connection oriented.
A connectionless transport layer treats each segment as an independent packet and delivers it to
the transport layer at the destination machine. A connection oriented transport layer makes a
connection with the transport layer at the destination machine first before delivering the packets.
After all the data are transferred, the connection is terminated.
o Flow control. Like the data link layer, the transport layer is responsible for flow control.
However, flow control at this layer is performed end to end rather than across a single link.
o Error control. Like the data link layer, the transport layer is responsible for error control.
However, error control at this layer is performed process-to-process rather than across a single
link. The sending transport layer makes sure that the entire message arrives at the receiving
transport layer without error (damage, loss, or duplication). Error correction is usually achieved
through retransmission.
5. Session Layer
The services provided by the first three layers (physical, data link, and network) are not
sufficient for some processes. The session layer is the network dialog controller. It establishes,
maintains, and synchronizes the interaction among communicating systems. Specific
responsibilities of the session layer include the following:
o Dialog control. The session layer allows two systems to enter into a dialog. It allows the
communication between two processes to take place in either half duplex (one way at a time) or
full-duplex (two ways at a time) mode.
o Synchronization. The session layer allows a process to add checkpoints, or synchronization
points, to a stream of data.
6. Presentation Layer
The presentation layer is concerned with the syntax and semantics of the information exchanged
between two systems.

Specific responsibilities of the presentation layer include the following:
o Translation. The processes (running programs) in two systems are usually exchanging
information in the form of character strings, numbers, and so on. The information must be
changed to bit streams before being transmitted. Because different computers use different
encoding systems, the presentation layer is responsible for interoperability between these
different encoding methods. The presentation layer at the sender changes the information from
its sender-dependent format into a common format. The presentation layer at the receiving
machine changes the common format into its receiver-dependent format.
o Encryption. To carry sensitive information, a system must be able to ensure privacy.
Encryption means that the sender transforms the original information to another form and sends
the resulting message out over the network. Decryption reverses the original process to transform
the message back to its original form.
o Compression. Data compression reduces the number of bits contained in the information. Data
compression becomes particularly important in the transmission of multimedia such as text,
audio, and video.
7. Application Layer
The application layer enables the user, whether human or software, to access the network. It
provides user interfaces and support for services such as electronic mail, remote file access and
transfer, shared database management, and other types of distributed information services.
Specific services provided by the application layer include the following:
o Network virtual terminal. A network virtual terminal is a software version of a physical
terminal, and it allows a user to log on to a remote host.
o File transfer, access, and management. This application allows a user to access files in a
remote host (to make changes or read data), to retrieve files from a remote computer for use in
the local computer, and to manage or control files in a remote computer locally.
o Mail services. This application provides the basis for e-mail forwarding and storage.
o Directory services. This application provides distributed database sources and access for
global information about various objects and services.

TCP/IP PROTOCOL SUITE
The TCPIIP protocol suite was developed prior to the OSI model. Therefore, the layers in the
TCP/IP protocol suite do not exactly match those in the OSI model. The original TCP/IP
protocol suite was defined as having four layers: host-to-network, internet, transport, and
application. However, when TCP/IP is compared to OSI, we can say that the host-to-network
layer is equivalent to the combination of the physical and data link layers. The internet layer is
equivalent to the network layer, and the application layer is roughly doing the job of the session,
presentation, and application layers with the transport layer in TCP/IP taking care of part of the
duties of the session layer.
TCP/IP is a hierarchical protocol made up of interactive modules, each of which provides a
specific functionality; however, the modules are not necessarily interdependent. Whereas the
OSI model specifies which functions belong to each of its layers, the layers of the TCP/IP
protocol suite contain relatively independent protocols that can be mixed and matched depending
on the needs of the system. The term hierarchical means that each upper-level protocol is
supported by one or more lower-level protocols.
At the transport layer, TCP/IP defines three protocols: Transmission Control Protocol (TCP),
User Datagram Protocol (UDP), and Stream Control Transmission Protocol (SCTP). At the
network layer, the main protocol defined by TCP/IP is the Internetworking Protocol (IP); there
are also some other protocols that support data movement in this layer.

1. Host-to-Network Layer:
The TCP/IP reference model does not really say much about what happens here, except to point
out that the host has to connect to the network using some protocol so it can send IP packets to it.
This protocol is not defined and varies from host to host and network to network.
2. Internet Layer:
Its job is to permit hosts to inject packets into any network and have they travel independently to
the destination (potentially on a different network). They may even arrive in a different order
than they were sent, in which case it is the job of higher layers to rearrange them, if in-order
delivery is desired.
The internet layer defines an official packet format and protocol called IP (Internet Protocol).
The job of the internet layer is to deliver IP packets where they are supposed to go. Packet
routing is clearly the major issue here, as is avoiding congestion.
3. The Transport Layer:
The layer above the internet layer in the TCP/IP model is now usually called the transport layer.
It is designed to allow peer entities on the source and destination hosts to carry on a
conversation, just as in the OSI transport layer. Two end-to-end transport protocols have been
defined here. The first one, TCP (Transmission Control Protocol), is a reliable connection-
oriented protocol that allows a byte stream originating on one machine to be delivered without
error on any other machine in the internet. It fragments the incoming byte stream into discrete
messages and passes each one on to the internet layer. At the destination, the receiving TCP
process reassembles the received messages into the output stream. TCP also handles flow control
to make sure a fast sender cannot swamp a slow receiver with more messages than it can handle.
The second protocol in this layer, UDP (User Datagram Protocol), is an unreliable,
connectionless protocol for applications that do not want TCP's sequencing or flow control and
wish to provide their own. It is also widely used for one-shot, client-server-type request-reply
queries and applications in which prompt delivery is more important than accurate delivery, such
as transmitting speech or video.
4. The Application Layer:
The TCP/IP model does not have session or presentation layers. On top of the transport layer is
the application layer. It contains all the higher-level protocols. The early ones included virtual
terminal (TELNET), file transfer (FTP), and electronic mail (SMTP). The virtual terminal

protocol allows a user on one machine to log onto a distant machine and work there. The file
transfer protocol provides a way to move data efficiently from one machine to another.
Electronic mail was originally just a kind of file transfer, but later a specialized protocol (SMTP)
was developed for it. Many other protocols have been added to these over the years: the Domain
Name System (DNS) for mapping host names onto their network addresses, NNTP, the protocol
for moving USENET news articles around, and HTTP, the protocol for fetching pages on the
World Wide Web, and many others.
Comparison of the OSI and TCP/IP Reference Models:
The OSI and TCP/IP reference models have much in common. Both are based on the concept of
a stack of independent protocols. Also, the functionality of the layers is roughly similar. For
example, in both models the layers up through and including the transport layer are there to
provide an end-to-end, network-independent transport service to processes wishing to
communicate. These layers form the transport provider. Again in both models, the layers above
transport are application-oriented users of the transport service. Despite these fundamental
similarities, the two models also have many differences Three concepts are central to the OSI
model:
1. Services.
2. Interfaces.
3. Protocols.
Probably the biggest contribution of the OSI model is to make the distinction between these three
concepts explicit. Each layer performs some services for the layer above it. The service
definition tells what the layer does, not how entities above it access it or how the layer works. It
defines the layer's semantics.
A layer's interface tells the processes above it how to access it. It specifies what the parameters
are and what results to expect. It, too, says nothing about how the layer works inside.
Finally, the peer protocols used in a layer are the layer's own business. It can use any protocols it
wants to, as long as it gets the job done (i.e., provides the offered services). It can also change
them at will without affecting software in higher layers.
The TCP/IP model did not originally clearly distinguish between service, interface, and protocol,
although people have tried to retrofit it after the fact to make it more OSI-like. For example, the

only real services offered by the internet layer are SEND IP PACKET and RECEIVE IP
PACKET.
As a consequence, the protocols in the OSI model are better hidden than in the TCP/IP model
and can be replaced relatively easily as the technology changes. Being able to make such
changes is one of the main purposes of having layered protocols in the first place. The OSI
reference model was devised before the corresponding protocols were invented. This ordering
means that the model was not biased toward one particular set of protocols, a fact that made it
quite general. The downside of this ordering is that the designers did not have much experience
with the subject and did not have a good idea of which functionality to put in which layer.
Another difference is in the area of connectionless versus connection-oriented communication.
The OSI model supports both connectionless and connection-oriented communication in the
network layer, but only connection-oriented communication in the transport layer, where it
counts (because the transport service is visible to the users). The TCP/IP model has only one
mode in the network layer (connectionless) but supports both modes in the transport layer, giving
the users a choice. This choice is especially important for simple request-response protocols.
CONNECTING DEVICES
1. Repeaters
A repeater is a device that operates only in the physical layer. Signals that carry information
within a network can travel a fixed distance before attenuation endangers the integrity of the
data. A repeater receives a signal and, before it becomes too weak or corrupted, regenerates the
original bit pattern. The repeater then sends the refreshed signal. A repeater can extend the
physical length of a LAN, as shown in Figure 15.2.

A repeater does not actually connect two LANs; it connects two segments of the same LAN. The
segments connected are still part of one single LAN. A repeater is not a device that can connect
two LANs of different protocols.
It is tempting to compare a repeater to an amplifier, but the comparison is inaccurate. An
amplifier cannot discriminate between the intended signal and noise; it amplifies equally
everything fed into it. A repeater does not amplify the signal; it regenerates the signal. When it
receives a weakened or corrupted signal, it creates a copy, bit for bit, at the original strength.
2. Bridges or Link Layer Switches
A bridge or Link layer switch (or simply Switch) operates in both the physical and the data link
layer. As a physical layer device, it regenerates the signal it receives. As a data link layer device,
the bridge can check the physical (MAC) addresses (source and destination) contained in the
frame. A bridge has filtering capability. It can check the destination address of a frame and
decide if the frame should be forwarded or dropped. If the frame is to be forwarded, the decision
must specify the port. A bridge has a table that maps addresses to ports.
Let us give an example. In Figure 15.5, two LANs are connected by a bridge. If a frame destined
for station 712B13456142 arrives at port 1, the bridge consults its table to find the departing port.
According to its table, frames for 7l2B13456142 leave through port 1; therefore, there is no need
for forwarding, and the frame is dropped. On the other hand, if a frame for 712B13456141
arrives at port 2, the departing port is port 1 and the frame is forwarded. In the first case, LAN 2
remains free of traffic; in the second case, both LANs have traffic. In our example, we show a
two-port bridge; in reality a bridge usually has more ports.

3. Hubs
A Hub is a device that operates only in the physical layer. Signals that carry information within a
network can travel a fixed distance before attenuation endangers the integrity of the data. A
repeater receives a signal and, before it becomes too weak or corrupted, regenerates and retimes
the original bit pattern. The repeater then sends the refreshed signal. In a star topology , a
repeater is a multiport device , often called as hub, that can be used to serve as the connecting
point and at the same time function as a repeater. Figure below shows that when a packet from
station A to station B arrives at the hub , the signal representing the frame is regenerated to
remove any possible corrupting noise , but the hub forwards the packet from all outgoing ports
except the one from which the signal was received. In other words, the frame is broadcast. All
the stations in the LAN receive the frame , but only station B keeps it. The rest of the stations
discard it .
A hub is a physical layer device. They do not have a link layer address and they do not check the
link layer address of the received frame. They just regenerate the corrupted bits and send them
out from every port.
4. Routers
A router is a three-layer device that routes packets based on their logical addresses (host-to-host
addressing). A router normally connects LANs and WANs in the Internet and has a routing table
that is used for making decisions about the route. The routing tables are normally dynamic and
are updated using routing protocols. Figure 15.11 shows a part of the Internet that uses routers to
connect LANs and WANs.

A router is a three layer device. It operates in the physical, data link and network layers. As a
physical layer device, it regenerates the signal it receives. As a link layer device the router
checks the physical addresses contained in the packet. As a network layer device , a router
checks the network layer addresses. A router can connect networks. In other words, a router is an
internetworking device; it connects independent networks to form an internetwork. There are
three major differences between a router and a repeater or switch.
a. A router has a physical and logical address for each of its interfaces.
b. A router acts only on those packets in which the link layer destination address matches the
address of the interface at which the packet arrives.
c. A router changes the link layer address of the packet when it forwards the packet.
5. Gateways
A gateway is normally a computer that operates in all five layers of the Internet or seven layers
of OSI model. A gateway takes an application message, reads it, and interprets it. This means
that it can be used as a connecting device between two internetworks that use different models.
For example, a network designed to use the OSI model can be connected to another network
using the Internet model. The gateway connecting the two systems can take a frame as it arrives
from the first system, move it up to the OSI application layer, and remove the message.

ETHERNET ADDRESS
Ethernet or physical or MAC address
Each station on an Ethernet network (such as a PC, workstation, or printer) has its own network
interface card (NIC). The NIC fits inside the station and provides the station with a 6-byte
physical address. As shown in Figure 13.6, the Ethernet address is 6 bytes (48 bits), normally
written in hexadecimal notation, with a colon between the bytes.
Unicast, Multicast, and Broadcast Addresses: A source address is always a unicast address-the
frame comes from only one station. The destination address, however, can be unicast, multicast,
or broadcast. If the least significant bit of the first byte in a destination address is 0, the address is
unicast; otherwise, it is multicast. A unicast destination address defines only one recipient; the
relationship between the sender and the receiver is one-to-one. A multicast destination address
defines a group of addresses; the relationship between the sender and the receivers is one-to-
many. The broadcast address is a special case of the multicast address; the recipients are all the
stations on the LAN. A broadcast destination address is forty-eight 1's. For Example:
a)- 4A:30:10:21:1O:1A - This is a unicast address because A in binary is 1010 (even).
b)- 47:20:1B:2E:08:EE - This is a multicast address because 7 in binary is 0111 (odd).
c)- FF:FF:FF:FF:FF:FF - This is a broadcast address because all digits are F's.
The way the addresses are sent out on line is different from the way they are written in
hexadecimal notation. The transmission is left-to-right, byte by byte; however, for each byte, the
least significant bit is sent first and the most significant bit is sent last. This means that the bit
that defines an address as unicast or multicast arrives first at the receiver.

INTERNET PROTOCOL ADDRESS
IP Address or Network address or logical address
An IPv4 address is a 32-bit address that uniquely and universally defines the connection of a
device (for example, a computer or a router) to the Internet. IPv4 addresses are unique. They are
unique in the sense that each address defines one, and only one, connection to the Internet. Two
devices on the Internet can never have the same address at the same time. The IPv4 addresses are
universal in the sense that the addressing system must be accepted by any host that wants to be
connected to the Internet.
Address Space
A protocol such as IPv4 that defines addresses has an address space. An address space is the total
number of addresses used by the protocol. If a protocol uses N bits to define an address, the
address space is 2
N
because each bit can have two different values (0 or 1) and N bits can have
2
N
values. IPv4 uses 32-bit addresses, which means that the address space is 2
32
or
4,294,967,296 (more than 4 billion). This means that, theoretically, if there were no restrictions,
more than 4 billion devices could be connected to the Internet.
Notations
There are two prevalent notations to show an IPv4 address: binary notation and dotted decimal
notation.
Binary Notation
In binary notation, the IPv4 address is displayed as 32 bits. Each octet is often referred to as a
byte. So it is common to hear an IPv4 address referred to as a 32-bit address or a 4-byte address.
The following is an example of an IPv4 address in binary notation:
01110101 10010101 00011101 00000010
Dotted-Decimal Notation
To make the IPv4 address more compact and easier to read, Internet addresses are usually written
in decimal form with a decimal point (dot) separating the bytes. The following is the dotted
decimal notation of the above address:
117.149.29.2
Figure 19.1 shows an IPv4 address in both binary and dotted-decimal notation. Note that because
each byte (octet) is 8 bits, each number in dotted-decimal notation is a value ranging from 0 to
255.

IP Address Classes:
IPv4 addressing, at its inception, used the concept of classes. This architecture is called classful
addressing. In classful addressing, the address space is divided into five classes: A, B, C, D, and
E. Each class occupies some part of the address space. We can find the class of an address when
given the address in binary notation or dotted-decimal notation. If the address is given in binary
notation, the first few bits can immediately tell us the class of the address. If the address is given
in decimal-dotted notation, the first byte defines the class. Both methods are shown in Figure
19.2.
Classes and Blocks
One problem with classful addressing is that each class is divided into a fixed number of blocks
with each block having a fixed size as shown in Table 19.1.

Previously, when an organization requested a block of addresses, it was granted one in class A,
B, or C. Class A addresses were designed for large organizations with a large number of attached
hosts or routers. Class B addresses were designed for midsize organizations with tens of
thousands of attached hosts or routers. Class C addresses were designed for small organizations
with a small number of attached hosts or routers.
We can see the flaw in this design. A block in class A address is too large for almost any
organization. This means most of the addresses in class A were wasted and were not used. A
block in class B is also very large, probably too large for many of the organizations that received
a class B block. A block in class C is probably too small for many organizations. Class D
addresses were designed for multicasting. Each address in this class is used to define one group
of hosts on the Internet. The Internet authorities wrongly predicted a need for 268,435,456
groups. This never happened and many addresses were wasted here too. And lastly, the class E
addresses were reserved for future use; only a few were used, resulting in another waste of
addresses.
COMMON NETWORK APPLICATIONS
1. TELNET
TELNET is an abbreviation for TErminaL NETwork. It is the standard TCP/IP protocol for
virtual terminal service as proposed by the International Organization for Standards (ISO).
TELNET enables the establishment of a connection to a remote system in such a way that the
local terminal appears to be a terminal at the remote system. TELNET was designed at a time
when most operating systems, such as UNIX, were operating in a timesharing environment. In
such an environment, a large computer supports multiple users. The interaction between a user
and the computer occurs through a terminal, which is usually a combination of keyboard,
monitor, and mouse. Even a microcomputer can simulate a terminal with a terminal emulator.
Logging
In a timesharing environment, users are part of the system with some right to access resources.
Each authorized user has an identification and probably, a password. The user identification
defines the user as part of the system. To access the system, the user logs into the system with a
user id or log-in name. The system also includes password checking to prevent an unauthorized
user from accessing the resources. Figure 26.1 shows the logging process.

When a user logs into a local timesharing system, it is called local log-in. As a user types at a
terminal or at a workstation running a terminal emulator, the keystrokes are accepted by the
terminal driver. The terminal driver passes the characters to the operating system. The operating
system, in turn, interprets the combination of characters and invokes the desired application
program or utility. When a user wants to access an application program or utility located on a
remote machine, she performs remote log-in. Here the TELNET client and server programs
come into use. The user sends the keystrokes to the terminal driver, where the local operating
system accepts the characters but does not interpret them. The characters are sent to the TELNET
client, which transforms the characters to a universal character set called network virtual
terminal (NVT) characters and delivers them to the local TCP/IP protocol stack. The commands
or text, in NVT form, travel through the Internet and arrive at the TCP/IP stack at the remote
machine. Here the characters are delivered to the operating system and passed to the TELNET

server, which changes the characters to the corresponding characters understandable by the
remote computer. However, the characters cannot be passed directly to the operating system
because the remote operating system is not designed to receive characters from a TELNET
server: It is designed to receive characters from a terminal driver. The solution is to add a piece
of software called a pseudoterminal driver which pretends that the characters are coming from a
terminal. The operating system then passes the characters to the appropriate application program.
2. ELECTRONIC MAIL
One of the most popular Internet services is electronic mail (e-mail). The designers of the
Internet probably never imagined the popularity of this application program. At the beginning of
the Internet era, the messages sent by electronic mail were short and consisted of text only; they
let people exchange quick memos. Today, electronic mail is much more complex. It allows a
message to include text, audio, and video. It also allows one message to be sent to one or more
recipients.
Architecture
User Agent
The first component of an electronic mail system is the user agent. It provides service to the user
to make the process of sending and receiving a message easier.
Services Provided by a User Agent
A user agent is a software package (program) that composes, reads, replies to, and forwards
messages. It also handles mailboxes. Figure 26.11 shows the services of a typical user agent.
Composing Messages
A user agent helps the user compose the e-mail message to be sent out. Most user agents provide
a template on the screen to be filled in by the user. Some even have a built-in editor that can do

spell checking, grammar checking, and other tasks expected from a sophisticated word
processor. A user, of course, could alternatively use his or her favorite text editor or word
processor to create the message and import it, or cut and paste it, into the user agent template.
Reading Messages
The second duty of the user agent is to read the incoming messages. When a user invokes a user
agent, it first checks the mail in the incoming mailbox. Most user agents show a one-line
summary of each received mail. Each e-mail contains the following fields.
1. A number field.
2. A flag field that shows the status of the mail such as new, already read but not replied to, or
read and replied to.
3. The size of the message.
4. The sender.
5. The optional subject field.
Replying to Messages
After reading a message, a user can use the user agent to reply to a message. A user agent usually
allows the user to reply to the original sender or to reply to all recipients of the message. The
reply message may contain the original message and the new message.
Forwarding Messages
Replying is defined as sending a message to the sender or recipients of the copy. Forwarding is
defined as sending the message to a third party. A user agent allows the receiver to forward the
message, with or without extra comments, to a third party.
Handling Mailboxes
A user agent normally creates two mailboxes: an inbox and an outbox. Each box is a file with a
special format that can be handled by the user agent. The inbox keeps all the received e-mails
until they are deleted by the user. The outbox keeps all the sent e-mails until the user deletes
them. Most user agents today are capable of creating customized mailboxes.
User Agent Types
There are two types of user agents: command-driven and GUI-based. Command-Driven:
Command-driven user agents belong to the early days of electronic mail. They are still present as
the underlying user agents in servers. A command-driven user agent normally accepts a one-
character command from the keyboard to perform its task. For example,

a user can type the character r, at the command prompt, to reply to the sender of the message, or
type the character R to reply to the sender and all recipients. Some examples of command-driven
user agents are mail, pine, and elm.
GUI-Based: Modern user agents are GUI-based. They contain graphical-user interface (GUI)
components that allow the user to interact with the software by using both the keyboard and the
mouse. They have graphical components such as icons, menu bars, and windows that make the
services easy to access. Some examples of GUI-based user agents are Eudora, Microsoft's
Outlook, and Netscape.
Addresses
To deliver mail, a mail handling system must use an addressing system with unique addresses. In
the Internet, the address consists of two parts: a local part and a domain name, separated by an @
sign
Local Part: The local part defines the name of a special file, called the user mailbox, where all
the mail received for a user is stored for retrieval by the message access agent.
Domain Name: The second part of the address is the domain name. An organization usually
selects one or more hosts to receive and send e-mail; the hosts are sometimes called mail servers
or exchangers. The domain name assigned to each mail exchanger either comes from the DNS
database or is a logical name (for example, the name of the organization).
3. SMTP
The actual mail transfer is done through message transfer agents. To send mail, a system must
have the client MTA, and to receive mail, a system must have a server MTA. The formal
protocol that defines the MTA client and server in the Internet is called the Simple Mail Transfer
Protocol (SMTP).

SMTP is used two times, between the sender and the sender's mail server and between the two
mail servers. SMTP simply defines how commands and responses must be sent back and forth.
Commands and Responses
SMTP uses commands and responses to transfer messages between an MTA client and an MTA
server (see Figure 26.17).
Commands: Commands are sent from the client to the server. The format of a command is
shown in Figure 26.18. It consists of a keyword followed by zero or more arguments. SMTP
defines 14 commands. The first five are mandatory; every implementation must support these
five commands. The next three are often used and highly recommended. The last six are seldom
used.

Responses: Responses are sent from the server to the client. A response is a three digit code that
may be followed by additional textual information. Table 26.8 lists some of the responses.
Mail Transfer Phases
The process of transferring a mail message occurs in three phases: connection establishment,
mail transfer, and connection termination.
4. POP3
Post Office Protocol, version 3 (POP3) is simple and limited in functionality. The client POP3
software is installed on the recipient computer; the server POP3 software is installed on the mail
server. Mail access starts with the client when the user needs to download e-mail from the
mailbox on the mail server. The client opens a connection to the server on TCP port 110. It then
sends its user name and password to access the mailbox. The user can then list and retrieve the
mail messages, one by one.
POP3 has two modes: the delete mode and the keep mode. In the delete mode, the mail is deleted
from the mailbox after each retrieval. In the keep mode, the mail remains in the mailbox after
retrieval. The delete mode is normally used when the user is working at her permanent computer
and can save and organize the received mail after reading or replying. The keep mode is
normally used when the user accesses her mail away from her primary computer (e.g., a laptop).
The mail is read but kept in the system for later retrieval and organizing.

5. IMAP
Another mail access protocol is Internet Mail Access Protocol, version 4 (IMAP4). IMAP4 is
similar to POP3, but it has more features; IMAP4 is more powerful and more complex.
POP3 is deficient in several ways. It does not allow the user to organize her mail on the server;
the user cannot have different folders on the server. In addition, POP3 does not allow the user to
partially check the contents of the mail before downloading. IMAP4 provides the following extra
functions:
o A user can check the e-mail header prior to downloading.
o A user can search the contents of the e-mail for a specific string of characters prior to
downloading.
o A user can partially download e-mail. This is especially useful if bandwidth is limited and the
e-mail contains multimedia with high bandwidth requirements.
o A user can create, delete, or rename mailboxes on the mail server.
o A user can create a hierarchy of mailboxes in a folder for e-mail storage.
6. Web-Based Mail
E-mail is such a common application that some websites today provide this service to anyone
who accesses the site. Two common sites are Hotmail and Yahoo. The idea is very simple. Mail
transfer from Alice's browser to her mail server is done through HTTP. The transfer of the
message from the sending mail server to the receiving mail server is still through SMTP. Finally,
the message from the receiving server (the Web server) to Bob's browser is done through HTTP.
The last phase is very interesting. Instead of POP3 or IMAP4, HTTP is normally used. When
Bob needs to retrieve his e-mails, he sends a message to the website (Hotmail, for example). The
website sends a form to be filled in by Bob, which includes the log-in name and the password. If
the log-in name and password match, the e-mail is transferred from the Web server to Bob's
browser in HTML format.
7. FTP
File Transfer Protocol (FTP) is the standard mechanism provided by TCP/IP for copying a file
from one host to another. Although transferring files from one system to another seems simple
and straightforward, some problems must be dealt with first. For example, two systems may use
different file name conventions. Two systems may have different ways to represent text and data.

Two systems may have different directory structures. All these problems have been solved by
FTP in a very simple and elegant approach.
FTP differs from other client/server applications in that it establishes two connections between
the hosts. One connection is used for data transfer, the other for control information (commands
and responses). Separation of commands and data transfer makes FTP more efficient. The
control connection uses very simple rules of communication. We need to transfer only a line of
command or a line of response at a time. The data connection, on the other hand, needs more
complex rules due to the variety of data types transferred. However, the difference in complexity
is at the FTP level, not TCP. For TCP, both connections are treated the same. FTP uses two well-
known TCP ports: Port 21 is used for the control connection, and port 20 is used for the data
connection.
Figure 26.21 shows the basic model of FTP. The client has three components: user interface,
client control process, and the client data transfer process. The server has two components: the
server control process and the server data transfer process. The control connection is made
between the control processes. The data connection is made between the data transfer processes.
The control connection remains connected during the entire interactive FTP session. The data
connection is opened and then closed for each file transferred. It opens each time commands that

involve transferring files are used, and it closes when the file is transferred. In other words, when
a user starts an FTP session, the control connection opens. While the control connection is open,
the data connection can be opened and closed multiple times if several files are transferred.
Transmission Mode: FTP can transfer a file across the data connection by using one of the
following three transmission modes: stream mode, block mode, and compressed mode. The
stream mode is the default mode. Data are delivered from FTP to TCP as a continuous stream of
bytes. TCP is responsible for chopping data into segments of appropriate size. If the data are
simply a stream of bytes (file structure), no end-of-file is needed. End-of-file in this case is the
closing of the data connection by the sender. If the data are divided into records (record
structure), each record will have a 1-byte end-of-record (EOR) character and the end of the file
will have a 1-byte end-of-file (EOF) character. In block mode, data can be delivered from FTP to
TCP in blocks. In this case, each block is preceded by a 3-byte header. The first byte is called the
block descriptor; the next 2 bytes define the size of the block in bytes. In the compressed mode,
if the file is big, the data can be compressed. The compression method normally used is run-
length encoding. In this method, consecutive appearances of a data unit are replaced by one
occurrence and the number of repetitions. In a text file, this is usually spaces (blanks). In a binary
file, null characters are usually compressed.
8. HTTP
The Hypertext Transfer Protocol (HTTP) is a protocol used mainly to access data on the World
Wide Web. HTTP functions as a combination of FTP and SMTP. It is similar to FTP because it
transfers files and uses the services of TCP. However, it is much simpler than FTP because it
uses only one TCP connection. There is no separate control connection; only data are transferred
between the client and the server. HTTP is like SMTP because the data transferred between the
client and the server look like SMTP messages. In addition, the format of the messages is
controlled by MIME-like headers. Unlike SMTP, the HTTP messages are not destined to be read
by humans; they are read and interpreted by the HTTP server and HTTP client (browser). SMTP
messages are stored and forwarded, but HTTP messages are delivered immediately. The
commands from the client to the server are embedded in a request message. The contents of the
requested file or other information are embedded in a response message. HTTP uses the services
of TCP on well-known port 80.

HTTP Transaction
Figure 27.12 illustrates the HTTP transaction between the client and server. Although HTTP
uses the services of TCP, HTTP itself is a stateless protocol. The client initializes the transaction
by sending a request message. The server replies by sending a response.
Messages
The formats of the request and response messages are similar; both are shown in Figure 27.13. A
request message consists of a request line, a header, and sometimes a body. A response message
consists of a status line, a header, and sometimes a body.

Request and Status Lines: The first line in a request message is called a request line; the first
line in the response message is called the status line. There is one common field, as shown in
Figure 27.14.
a. Request type. This field is used in the request message. In version 1.1 of HTTP, several
request types are defined. The request type is categorized into methods as defined in Table 27.1.
b. URL. URL means Uniform Resource Locator
c. Version. The current version of HTTP
d. Status code. This field is used in the response message. The status code field is similar to
those in the FTP and the SMTP protocols. It consists of three digits. Whereas the codes in the
100 range are only informational, the codes in the 200 range indicate a successful request. The
codes in the 300 range redirect the client to another URL, and the codes in the 400 range indicate

an error at the client site. Finally, the codes in the 500 range indicate an error at the server site.
We list the most common codes in Table 27.2.
e. Status phrase. This field is used in the response message. It explains the status code in text
form. Table 27.2 also gives the status phrase.

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