Unit 1 introduction to computer networks

Components of a data communication system
The word data refers to information presented in whatever form
is agreed upon by the parties creating and using the data.
Data communications are the exchange of data between
two devices via some form of transmission medium such as a
wire cable.

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, telephone handset, video camera,
and so on.
3. Receiver. The receiver is the device that receives the message.
---It can be a computer, 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. Without a
protocol, two devices may be connected but not communicating.
---just as a person speaking French cannot be understood
by a person who speaks only Japanese.

PROTOCOLS
A protocol is a set of rules that govern data
communications. It determines what is communicated,
how it is communicated and when it is communicated. The
key elements of a protocol are syntax, semantics and timing

Data flow (simplex, half-duplex, and 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.
Examples:- 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.
Examples:-When cars are traveling in one direction, cars
going the other way must wait. 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 is half-duplex
systems.

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.
Example:- 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.

COMPUTER NETWORKS
Computer network consists of two or more computers
that are linked in order to share resources, exchange data files or to
allow electronic communication. The computers on a network may
be linked through cables, telephone lines, radio waves, satellites or
infrared light beams.
There are two aspects of computer networks – hardware
and software.
Hardware includes physical connection between
two machines by using adaptors, cables, routers, bridges etc.
software includes a set of protocols. Protocols
define a formal language among various components. It makes
hardware usable by applications.

Categories of 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.

A fully connected mesh topology (five devices)
•In mesh topology every device has a dedicated point-to-point link to
every other device.
•The link carries traffic only between the two devices it connects.
•Duplex-mode
•Advantages:
guaranteed dedicated links
eliminates traffic problems
privacy and security
this makes fault identification easy
•Disadvantages:
cabling and number of IO ports required
wiring is greater than available space
hardware is required for each link – expensive

A star topology connecting four stations
•In star topology, each device has a dedicated point-to-point link only to a central
controller called hub.
•The controller acts as an exchange: if one device wants to send data to another , it
sends the data to controller, which then relays the data to the another connected
device.
•Advantages:
less expensive
robustness – if one link fails, only that link is affected, other links remain
active.
•Disadvantages:
dependency of the whole topology on one single point.
star requires less than mesh, each node is linked to the hub. So more
cabling is required .

A bus topology connecting three stations
•A bus topology is a multipoint .
•One long cable acts as a backbone to link all the devices in the
network.
•Nodes are connected by bus cable by drop line and taps.
a drop line is a connection running between the device and
the main cable
a tap is a connector that either splices or punctures.
•Advantages:
easy of installation
•Disadvantages:
difficult reconnection
addition of new devices require modification or replacement
of the backbone.

A ring topology connecting six stations
•In ring topology each device has a point-to-point connection with only the two devices
on either side of it.
•A signal is passed along a ring in one direction, from device to device until it reaches
its destination.
•Advantages:
easy to install and reconfigure
to add or delete a device requires changing only two connections. The only
constraints are media and traffic.
•Disadvantages;
unidirectional
a break in a ring can disable the entire network

A hybrid topology: a star backbone with three bus networks

Uses of Computer Networks
• Business Applications
• Home Applications
• Mobile Users

Business Applications of Networks
•Resource
sharing
•High reliability
•Saving money
•scalability
Client-server model

Business Applications of Networks (2)
• The client-server model involves requests and
replies.

Home Network Applications
1. Access to remote information
• Many people pay their bills, manage bank accounts, handle their investments
electronically.
• Home shopping.
• On-line newspaper which can be personalized.
• Access to information system like world wide web, which contains information
about
arts,business,cooking,government,health,history,science,sports,travel,……..
• All the above applications involve in interactions between a person and remote
database.
2. Person-to-person communication
• Electronic mail or email which allow users to communicate with no delay
• Videoconference- which makes possible
3. Interactive entertainment
• Huge and growing industry.
• Video
• Game playing
4. Electronic commerce
Cont.

Some forms of e-commerce
Electronic Commerce, commonly known as E-
commerce or eCommerce, is trading in products or services using
computer networks, such as the Internet
E-commerce facilitates home shopping, catalogue of company
products.
It is also popularly employed for bill payments , banking,
investments, ………

Mobile Network Users
Many professionals uses desktop computers at office and
want to be connected to the office network while travelling and at
home also. This is possible by wireless networks, hence use of
laptop, notebook computers and personal digital assistants(PDAs)
is increased.
Wireless networks are used in:- taxis, military applications,
airports, banking, weather reporting
• Combinations of wireless networks and mobile
computing.

Network Hardware
• Local Area Networks
• Metropolitan Area Networks
• Wide Area Networks

There are two types of transmission technology:
1. Broadcast Networks
This has a single communication channel that is shared by all the
machines on the network.
The data transmitted is converted in small packets form. Each
packet contains address field of the destination station.
Ex:- a person standing at corridor “watson, come here. I want
you”
sending same packets to all the stations within a network is
called as broadcasting.
When data packets are sent to a specific group of stations it is
called as multicasting. This is a selective process

2. Point-to-point Network
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.
This network provides a dedicated link between any two
stations . Such a transmission is called unicasting.

• Classification of
interconnected
processors by
scale.
•Personal area network is sending a message over a very short
distance
•Computers that communicate by exchanging messages over
longer cables. LAN MAN WAN
•The connection of two or more networks is called an
internetwork.

The Local Network (LAN)
Client
ClientClient
Client Client
Client

Metropolitan Area Network (MAN)

Local Area Networks(LANs)
• LANS are privately-owned networks within a single
building or campus of up to few kilometers in size.
• LANS are distinguished based on
– Their size
– Their transmission technology
– Their topology
• LANS are restricted in size
• LANS use a transmission technology consisting of a
single cable to which all machines are attached like
telephone company lines once used in rural areas.
• LANS run at speeds of 10 to 100 Mbps, have low
delay and make very few errors.

Local Area Networks
Two broadcast networks
• (a) Bus
• (b) Ring

Bus topology
1.In bus network, at any instant one machine is the master and is
allowed to transmit. All other machines are required to refrain.
2. when two or more machines want to transmit simultaneously?
The mechanism is IEEE 802.3 (ETHERNET) is a bus-based broadcast
network with decentralized control operation at 10 or 100 Mbps.
3.Computers on ETHERNET can transmit whenever they want to, if
two or more packets collide, each computer waits for random time and
tries again later.
Ring topology
1.In this each bit propagates around on its own, not waiting for the rest
of the packets .
2. In this each bit circumnavigates the entire ring.
3. IEEE 802.5( IBM token ring) is a popular ring-based LAN operated
at 4 and 16 Mbps.

Metropolitan Area Networks
• MAN is basically a bigger version of a LAN and
normally uses similar technology.
• It might cover a group of near by offices, may be
either private or public
• A MAN just has one or two cables and does mot
have any switching elements
• The main reason for even distinguishing MAN’s is
that a standard has been adopted and this standard is
now being implemented
• It is called DQDB (distributed queue dual bus,
802.6)

DQDB consists of two unidirectional buses to which all computers
are connected .
Each bus has head end. A device that initiates transmission
activity.
The key aspect of MAN is a broadcast medium to which all
computers are attached.

Wide Area Networks
• Relation between hosts on LANs and the
subnet.

Wide Area Networks
• WANs spans a large geographical area, often a
country or continent.
• It contains collection of machines for running
user applications, called hosts or end user.
• The hosts are connected by communication
subnet or subnet. The subnet carries message
from host to host.
• For communication aspect – subnet
application aspect - hosts

• In WAN the subnets consists of two distinct
components: transmission lines and switching
elements.
• Transmission lines are circuits or channels
• Switching elements are specialized computers
used to connect two or more transmission lines.
These are called routers
• Each host is connected to LAN on which a router
is present, or in some cases host can be connected
directly connected to router.
• The collection of communication lines and routers
form the subnet.

• If two routers do not share a cable or not wish to communicate
, they must do this indirectly ie., via other router.
• When the packet is send from one router to another via one or
more intermediate routers , the packet is received at each
router and stores until required output line is free and then
forward.
• A subnet using this principle is called point-to-point , store-
and-forward, or packet-switching subnet.

NetworkSoftware
The first computer designed with the
hardware as the major concern and the software as
an afterthought. This no longer works. Network
software is now highly structured.
• Protocol Hierarchies
• Design Issues for the Layers
• Connection-Oriented and Connectionless Services
• Service Primitives
• The Relationship of Services to Protocols

Protocol Hierarchies
• To reduce the design complexity, most networks
are organized as a series of layers or levels. Each
one built upon the one below it.
• The number of layers, name of each layer,
contents of each layer and the function of each
layer differ from network to network.
• Layer n on one machine carries on a conversation
with layer n on another machine. The rules and
conventions used in this conversation are
collectively known as the layer n protocol.

Protocol Hierarchies
• Between each pair of adjacent layers there is an interface.
• A set of layers and protocols is called a network architecture.
• A list of protocols used by a certain system , one protocol per layer, is called a
protocol stack.

Protocol Hierarchies (2)
• The philosopher-translator-secretary
architecture.

Protocol Hierarchies (3)
• Example information flow supporting virtual
communication in layer 5.

Tasks involved in sending a letter
We use
the concept of
layers in our
daily life. As an
example, let us
consider two
friends who
communicate
through postal
mail.

Design Issues for the Layers
• Addressing – each layer needs a mechanism for
identifying senders and receivers.
• The rules of data transfer – simplex, half-duplex, full-
duplex
• Error Control – error-correction and error-detection
• Flow Control - The communication channels must
preserve the order of messages sent on them –
disassembling, transmitting, and then reassembling.
• Multiplexing – inconvenient or expensive to set up a
connection for each pair of communication process.
• Routing – multiple paths between source and destination
, a route must be chosen

Connection-Oriented and Connectionless
Services
• Connection-oriented is modeled after the telephone system.
• To talk to someone, you pick up the phone, dial the number,
talk, and then hang up.
• To use a connection-oriented network service, the service user
first establish a connection, uses the connection, and then
releases the connection.
• Connectionless service is modeled after postal system.
• Each message carries the full destination address, and each one
routed through the system independent of all the routers.
• When two messages sent to the same destination, the first one
sent will be first one to arrive. If first one is delayed the second
one arrives first.
• With connection-oriented service this is not possible.

Service Primitives
• Five service primitives for implementing a
simple connection-oriented service.

Services to Protocols Relationship
• The relationship between a service and a protocol.
• A service is a set of primitives(operations)that a layer provides to
the layer above it
• A protocol is a set of rules governing the format and meaning of
the frames, packets, or messages that are exchanged by the peer
entities within the layer

Reference Models
• The OSI Reference Model
• The TCP/IP Reference Model

The OSI Reference Model
• In 1947, the international standards organization(ISO)
is a multinational body dedicated to worldwide
agreement on international standards.
• An ISO standard that covers all aspects of network
communications is the open systems interconnection
model.
• In late 1970s an open system is a set of protocols that
allow any two different systems to communicate
• It divides the communications processes into seven
layers.

• The main concept of OSI is that the process of
communication between two endpoints in a
telecommunication network can be divided into seven distinct
groups of related functions, or layers.
• Each communicating user or program is at a computer that can
provide those seven layers of function.
• The seven layers of function are provided by a combination of
applications, operating systems, network card device drivers
and networking hardware that enable a system to put a signal
on a network cable or out over Wi-Fi or other wireless
protocol).

Unit 1 introduction to computer networks

The OSI model has seven layers. The principles that
were applied to arrive at the seven layers are:
1. A layer should be created where a different level of
abstraction is needed.
2. Each layer should perform a well defined function.
3. The function of each layer should be chosen with an
eye toward defining internationally standardized
protocols.
4. The layers boundaries should be chosen to minimize
the information flow across the interfaces.
5. The number of layers large enough that distinct
functions need not be thrown together in the same
layer out of necessity, and small enough that the
architecture does not become unwieldy.

7 Layers
7. Application Layer
6. Presentation Layer
5. Session Layer
4. Transport Layer
3. Network Layer
2. Data Link Layer
1. Physical Layer
All
People
Seem
To
Need
Data
Processing

Physical layer
• Converts bits into electronic signals for outgoing messages
• Converts electronic signals into bits for incoming messages
• The physical layer is concerned with transmitting raw bits over a communication
channel. The design issues have to do with making sure that when one side sends a 1
bit, it is received by the other side as a 1 bit, not as a 0 bit.
• The design issues are
• Transmission medium
• Synchronization of bits
• Physical topology
• Transmission mode
• The bottom layer of the OSI model

Data link layer
• The main task of the data link layer is to transform a raw
transmission facility into a line that appears free of undetected
transmission errors to the network layer.
• It accomplishes this task by having the sender break up the input
data into data frames (typically a few hundred or a few
thousand bytes) and transmits the frames sequentially.

• At the receiving end, this layer packages raw data from the
physical layer into data frames for delivery to the Network layer
• At the sending end this layer handles conversion of data into
raw formats that can be handled by the Physical Layer
• If the service is reliable, the receiver confirms correct receipt
of each frame by sending back an acknowledgement frame
• The physical layer accepts and transmits stream of bits, the
data link layer should create and recognize frame boundaries.
This can be accomplished by attaching special bit patterns to
the beginning and ending of frame.
• A duplicate frame could be sent if the acknowledgement frame
from receiver back to the sender were lost.

Network layer
•The network layer controls the operation of the subnet.
•The network layer is responsible for the delivery of individual
packets from the source host to the destination host.
•The network layer controls the operation of the subnet. A key design
issue is determining how packets are routed from source to
destination.

• Routes can be based on static tables that are ''wired into'' the
network and rarely changed. They can also be determined at the
start of each conversation.
• If too many packets are present in the subnet at the same time,
they will get in one another's way, forming bottlenecks. The
control of such congestion also belongs to the network layer.
• When a packet has to travel from one network to another to get
to its destination, many problems can arise. The addressing used
by the second network may be different from the first one. The
second one may not accept the packet at all because it is too
large. The protocols may differ, and so on. It is up to the
network layer to overcome all these problems

Transport layer
• Manages the transmission of data across a network
• Manages the flow of data between parties by segmenting long data streams
into smaller data chunks (based on allowed “packet” size for a given
transmission medium)
• Reassembles chunks into their original sequence at the receiving end
• Provides acknowledgements of successful transmissions and requests
resends for packets which arrive with errors

•The basic function of the transport layer is to accept data from above, split it
up into smaller units if need be, pass these to the network layer, and ensure
that the pieces all arrive correctly at the other end.
•The transport layer is responsible for the delivery of a message from one
process to another.
•If transport connection requires a high throughput, the transport layer might
create multiple network connections.(if expensive multiple several transport
connections onto the same network connection).
•The transport layer also determines what type of service to provide to the
session layer, and, ultimately, to the users of the network. The most popular
type of transport connection is an error-free point-to-point channel that
delivers messages or bytes in the order in which they were sent. The type of
service is determined when the connection is established.
•The transport layer is a true end-to-end layer, all the way from the source to
the destination.
•The difference between layer 1 through 3 , which are chained, and layer 4
through 7, which are end-to-end

Reliable process-to-process delivery of a message

Session layer
•The session layer allows users on different machines to establish
sessions between them.
•Sessions offer various services, including dialog control (keeping
track of whose turn it is to transmit), token management
(preventing two parties from attempting the same critical operation
at the same time), and synchronization (check pointing long
transmissions to allow them to continue from where they were
after a crash).

Presentation layer
•The presentation layer is concerned with the syntax and semantics
of the information transmitted.
•In order to make it possible for computers with different data
representations to communicate, the data structures to be exchanged
can be defined in an abstract way, along with a standard encoding to
be used ''on the wire.'' The presentation layer manages these abstract
data structures and allows higher-level data structures (e.g., banking
records), to be defined and exchanged.

Application layer
•The application layer is responsible for providing services to the user.
•The application layer contains a variety of protocols that are commonly
needed by users. One widely-used application protocol is HTTP
(Hypertext Transfer Protocol), which is the basis for the World Wide Web.
When a browser wants a Web page, it sends the name of the page it wants
to the server using HTTP. The server then sends the page back. Other
application protocols are used for file transfer, electronic mail, and
network news.
•Network virtual terminal

An exchange using the OSI model

THE SEVEN OSI REFERENCE
MODEL LAYERS

The interaction between layers in the OSI model

The TCP/IP reference model
The TCP/IP reference model was developed prior to OSI model. The
major design goals of this model were,
1. To connect multiple networks together so that they appear as a
single network.
2. To survive after partial subnet hardware failures.
3. To provide a flexible architecture.
Transmission control protocol/ information protocol
Unlike OSI reference model, TCP/IP reference model has only 4
layers. They are,
1. Host-to-Network Layer
2. Internet Layer
3. Transport Layer
4. Application Layer

• The TCP/IP reference model.

• Protocols and networks in the TCP/IP model
initially.

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.

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.
• 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
• 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(transmitting speech or video.)

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, and HTTP, the protocol
for fetching pages on the World Wide Web, and many others.

• Telnet is a network protocol used on the Internet or local area
networks to provide a bidirectional interactive text-oriented
communication facility using a virtual terminal connection.
• The term telnet may also refer to the software that implements the
client part of the protocol. Telnet client applications are available for
virtually all computer platforms. To telnet means to establish a
connection with the Telnet protocol, either with command line client
or with a programmatic interface.
• The Telnet program runs on your computer and connects your PC to
a server on the network. You can then enter commands through the
Telnet program and they will be executed as if you were entering
them directly on the server .This enables you to control the server
and communicate with other servers on the network.
• To start a Telnet session, you must log in to a server by entering a
valid username and password. Telnet is a common way to remotely
control Web servers.

• The File Transfer Protocol (FTP) is a
standard network protocol used to transfer computer
files from one host to another host over a TCP-based
network, such as the Internet.
• FTP is built on a client-server architecture and uses
separate control and data connections between the client
and the server.
• The first FTP client applications were command-line
applications developed before operating
systems had graphical user interfaces, and are still
shipped with most Windows, Unix, and Linux operating
systems. Many FTP clients and automation utilities
have since been developed for desktops, servers, mobile
devices, and hardware, and FTP has been incorporated
into productivity applications, such as Web page editors

• Simple Mail Transfer Protocol (SMTP) is an Internet
standard for electronic mail (e-mail) transmission
• Short for Simple Mail Transfer Protocol, a protocol for
sending e-mail messages between servers.
• Most e-mail systems that send mail over the Internet use
SMTP to send messages from one server to another; the
messages can then be retrieved with an e-mail client using
either POP or IMAP.
• In computing, the Post Office Protocol (POP) is
an application-layer Internet standard protocol used by local e-
mail clients to retrieve e-mail from a remote server over
a TCP/IP connection.
• Virtually all modern e-mail clients and servers support POP and
IMAP (Internet Message Access Protocol) are the two most
prevalent Internet standard protocols for e-mail retrieval ,with
many webmail service providers such as Gmail, Outlook.com
and Yahoo! Mail also providing support for either IMAP or
POP3 to allow mail to be downloaded.

• The Domain Name System (DNS) is
a hierarchical distributed naming system for
computers, services, or any resource connected to
the Internet or a private network. It associates
various information with domain names assigned to
each of the participating entities.
• The Domain Name System distributes the
responsibility of assigning domain names and
mapping those names to IP addresses by
designating authoritative name servers for each
domain. Authoritative name servers are assigned to
be responsible for their supported domains. This
mechanism provides distributed and fault tolerant
service and was designed to avoid the need for a
single central database.

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.

• The Advanced Research Projects Agency
Network (ARPANET) was one of the world's first
operational packet switching networks, the first
network to implement TCP/IP, and was the main
progenitor of what was to become the global Internet.
• The network was initially funded by the Advanced
Research Projects Agency (ARPA, later DARPA)
within the U.S. Department of Defense for use by its
projects at universities and research laboratories in the
US.
• The packet switching of the ARPANET, together with
TCP/IP, would form the backbone of how the Internet
works.

• SATNet - Satellites Network
• The SATNet network is providing the first
network for sharing ground stations in
between the members of the community of
CubeSat developers.

• Packet radio is a form of packet
switching technology used to transmit digital
data via radio or wireless communications link
s. It uses the same concepts of data
transmission via Datagram .
• A datagram is a basic transfer unit associated
with a packet-switched network. The delivery,
arrival time, and order of arrival need not be
guaranteed by the network.

Comparing OSI and TCP/IP Models
Concepts central to the OSI model
• Services
• Interfaces
• Protocols

A Critique of the OSI Model and Protocols
• Bad timing
• Bad technology
• Bad implementations
• Bad politics

Bad Timing
• The apocalypse of the two elephants.

A Critique of the TCP/IP Reference Model
• Problems:
• Service, interface, and protocol not distinguished
• Not a general model
• Host-to-network “layer” not really a layer
• No mention of physical and data link layers
• Minor protocols deeply entrenched, hard to replace

Hybrid Model
• The hybrid reference model to be used in this
book.

Example networks
•Novell NetWare
•ARPANET
•INTERNET

Novell NetWare
•The most popular network system in the PC world is Novell
NetWare.
•It is designed to be used by the companies downsizing from a
mainframe to a network of PCs.
•This is based on client-server model – PCs operate as servers,
providing file services, database service, and other services to
clients.
• The physical and data link layers can be chosen from among
various industry standards, including Ethernet, IBM token ring ,and
ARC net.

• The network layer runs an unreliable connectionless internetwork protocol called
IPX(internet packet exchange)
– It passes packets transparently from source to destination, even if the source and
destination are on different networks.
– IPX is similar to IP, expect that uses 10-byte addresses instead of 4-byte addresses.
– NCP (Network core protocol) is a connection-oriented transport protocol.
– SPX(sequenced packet exchange) is also available, but provides only transport.
• The format of an IPX packet is:
• SAP ( service advertising protocol)
– The packets are collected by special agent processes running on the router machine. The
agents use the information contained in them to construct database of which servers are
running where.
– When a client machine is booted, it broadcast a request asking where the nearest server is,
the agent on local router machine sees the request looks in database for best server.
– The choice of server is send back to the client. The client establishes a NCP connection
with server. Using this the client and server negotiates the maximum packet size.

• Ethernet is a family of computer
networking technologies for local area
networks (LANs) and metropolitan area
networks(MANs).
• Systems communicating over Ethernet divide
a stream of data into shorter pieces
called frames.
• Each frame contains source and destination
addresses and error-checking data so that
damaged data can be detected and re-
transmitted.

• Token ring local area network (LAN) technology is a protocol which
resides at the data link layer (DLL) of the OSI model. It uses a special
three-byte frame called a token that travels around the ring. Token-
possession grants the possessor permission to transmit on the
medium. Token ring frames travel completely around the loop.
• Initially used only in IBM computers, it was eventually standardized
with protocol IEEE 802.5.
• The data transmission process goes as follows:
• Empty information frames are continuously circulated on the ring.
When a computer has a message to send, it seizes the token. The
computer will then be able to send the frame.
• The frame is then examined by each successive workstation. The
workstation that identifies itself to be the destination for the
message copies it from the frame and changes the token back to 0.
• The frame continues to circulate as an "empty" frame, ready to be
taken by a workstation when it has a message to send.
• The token scheme can also be used with bus topology LANs

• ARCNET is a widely-installed local area network (LAN)
technology that uses a token-bus scheme for managing line
sharing among the workstations and other devices
connected on the LAN.
• The LAN server continuously circulates empty message
frames on a bus (a line in which every message goes
through every device on the line and a device uses only
those with its address). When a device wants to send a
message, it inserts a "token" (this can be as simple as setting
a token bit to 1) in an empty frame in which it also inserts
the message.
• When the destination device or LAN server reads the
message, it resets the token to 0 so that the frame can be
reused by any other device. The scheme is very efficient
when traffic increases since all devices are afforded the
same opportunity to use the shared network

The ARPANET
• Advanced research projects agency(ARPA) had a
mission of advancing technology that might be useful
to the military.
• ARPA decided that the network should be packet-
switched network, consisting of subnet and host
computers.
• The subnet would consists of minicomputers called
IMPs( interface message processors) connected by
the transmission lines.
• The subnet was to be datagram subnet, so if some
lines and IMPs were destroyed, messages could be
automatically rerouted along alternative paths.

The ARPANET
• The original ARPANET design.

• The software was split into two parts: subnet and host.
• The subnet software consisted of the IMP end of the
host-IMP connection, the IMP-IMP protocol, and a
source IMP to destination IMP protocol designed to
improve reliability.
• outside the subnet, software was also needed namely ,
the host end of the host-IMP connection, the host-host
protocol, and the application software.
• Later the IMP software was changed to allow terminals
to connect directly to a special IMP, called a
TIP(terminal interface processor).

The ARPANET
• Growth of the ARPANET (a) December 1969. (b) July 1970.
• (c) March 1971. (d) April 1972. (e) September 1972.

Internet Usage
• Internet refers to collection of different
physical networks like LAN, MAN,WAN in
order to transmit data from one computer to
another computer. (or)
• Internet is network of networks
• E-mail
• News
• Remote login
• File transfer

Architecture of the Internet
• Overview of the Internet.

Unit 1 introduction to computer networks

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Unit 1 introduction to computer networks