Switch networking

Unit-IV
Network Switching Techniques: Circuit, Message,
Packet and Hybrid Switching Techniques.X.25, ISDN.
Logical Addressing, Ipv4, Ipv6, Address Mapping, ARP,
RARP, BOOTP and DHCP, User Datagram Protocol,
Transmission Control Protocol, SCTP.
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Introduction
• 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.
• A better solution is switching.

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). Others are used
only for routing.
• Figure shows a switched network.

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.}

Key Points
• Switching is the generic method for establishing a path for
point-to-point.
• It involves the nodes in the network utilizing their direct
communication lines to other nodes.
• Each node has the capability to switch to a neighboring node.
• One of the most important functions of the network layer is to
employ the switching capability of the nodes in order to route
messages across the network.
• There are three basic methods of switching: circuit switching,
packet switching and message switching.

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.
• Circuit switching takes place at the physical layer.
• Before starting communication, the stations must make a
reservation for the resources to be used during the
communication. These resources, such as channels, switch
buffers, switch processing time, and switch input/output ports,
must remain dedicated during the entire duration of data
transfer until the teardown phase.

Example
• Let us use a circuit-switched network to connect eight
telephones in a small area. Communication is through 4-kHz
voice channels. We assume that each link uses FDM to
connect a maximum of two voice channels. The bandwidth of
each link is then 8 kHz. Figure shows the situation.
• Telephone 1 is connected to telephone 7; 2 to 5; 3 to 8; and 4
to 6. Of course the situation may change when new
connections are made. The switch controls the connections.

Circuit-Switched Network used in
Example

Communication Process Steps
• 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.
• 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.

Advantages of Circuit Switching
• The main advantage of circuit switching is that a committed
transmission channel is established between the computers
which gives a guaranteed data rate.
• In circuit switching there is no delay in data flow because of
the dedicated transmission path.

Disadvantages of Circuit Switching
• It takes long time to establish connection.
• More bandwidth is required in setting up of dedicated
channels.
• It cannot be used to transmit any other data even if the channel
is free as the connection is dedicated in circuit switching.

Packet Switching
• 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 a packet-switched network, there is no resource reservation;
resources are allocated on demand.

Advantage of Packet Switching over
Circuit Switching
• More efficient in terms of bandwidth, since the concept of
reserving circuit is not there.
• Minimal transmission latency.
• More reliable as destination can detect the missing packet.
• More fault tolerant because packets may follow different path
in case any link is down, Unlike Circuit Switching.
• Cost effective and comparatively cheaper to implement.

Disadvantage of Packet Switching
over Circuit Switching
• Packet Switching don’t give packets in order, whereas Circuit
Switching provides ordered delivery of packets because all the
packets follow the same path.
• Since the packets are unordered, we need to provide sequence
numbers to each packet.
• Complexity is more at each node because of the facility to
follow multiple path.
• Transmission delay is more because of rerouting.
• Packet Switching is beneficial only for small messages, but for
bursty data (large messages) Circuit Switching is better.
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Delays in Packet switching
• Transmission Delay
• Propagation Delay
• Queuing Delay
• Processing Delay

DATAGRAM NETWORKS
• In data communications, we need to send messages from one
end system to another.
• 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 datagram's.
• Datagram switching is normally done at the network layer.
• The switches in a datagram network are traditionally referred
to as routers.

Example
•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.
•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.

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 circuit-switched network, all packets follow the same
path established during the connection.
4. 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.

Message Switching
• Message switching was a technique developed as an alternate
to circuit switching, before packet switching was introduced.
• In message switching, end users communicate by sending and
receiving messages that included the entire data to be shared.
• Messages are the smallest individual unit.
• The sender and receiver are not directly connected. There are a
number of intermediate nodes transfer data and ensure that the
message reaches its destination.
• Message switched data networks are hence called hop-by-hop
systems.

Message Switching Characteristics
• They provide 2 distinct and important characteristics:
1. Store and forward – The intermediate nodes have the
responsibility of transferring the entire message to the next
node. Hence, each node must have storage capacity. A
message will only be delivered if the next hop and the link
connecting it are both available, otherwise it’ll be stored
indefinitely. A store-and-forward switch forwards a message
only if sufficient resources are available and the next hop is
accepting data. This is called the store-and-forward property.
2. Message delivery – This implies wrapping the entire
information in a single message and transferring it from the
source to the destination node. Each message must have a
header that contains the message routing information,
including the source and destination.

Message Switched Network
• Message switching network consists of transmission links
(channels), store-and-forward switch nodes and end stations as
shown in the following figure:

Advantages of Message Switching
• As message switching is able to store the message for which
communication channel is not available, it helps in reducing
the traffic congestion in network.
• In message switching, the data channels are shared by the
network devices.
• It makes the traffic management efficient by assigning
priorities to the messages.

Disadvantages of Message Switching
• Message switching cannot be used for real time applications as
storing of messages causes delay.
• In message switching, message has to be stored for which
every intermediate devices in the network requires a large
storing capacity.

X.25
• X.25 is a protocol suite defined by ITU-T for packet switched
communications over WAN (Wide Area Network).
• It was originally designed for use in the 1970s and became
very popular in 1980s.
• Presently, it is used for networks for ATMs and credit card
verification.
• It allows multiple logical channels to use the same physical
line. It also permits data exchange between terminals with
different communication speeds.

X.25 has three protocol layers
• Physical Layer: It lays out the physical, electrical and
functional characteristics that interface between the computer
terminal and the link to the packet switched node. X.21
physical implementer is commonly used for the linking.
• Data Link Layer: It comprises the link access procedures for
exchanging data over the link. Here, control information for
transmission over the link is attached to the packets from the
packet layer to form the LAPB frame (Link Access Procedure
Balanced). This service ensures a bit-oriented, error-free, and
ordered delivery of frames.

X.25 has three protocol layers
• Packet Layer: This layer defines the format of data packets
and the procedures for control and transmission of the data
packets.
• It provides external virtual circuit service. Virtual circuits may
be of two types: virtual call and permanent virtual circuit.
• The virtual call is established dynamically when needed
through call set up procedure, and the circuit is relinquished
through call clearing procedure.
• Permanent virtual circuit, on the other hand, is fixed and
network assigned.

ISDN: (Integrated Services Digital Network)
• These are a set of communication standards for simultaneous digital
transmission of voice, video, data, and other network services over
the public switched telephone network.
• The main feature of ISDN is that it can integrate speech and data on
the same lines, which were not available in the classic telephone
system.
• ISDN is a circuit-switched telephone network system but it also
provides access to packet switched networks that allows digital
transmission of voice and data.
• This results in potentially better voice or data quality than an analog
phone can provide.
• It provides a packet-switched connection for data in increments of
64 Kbit/s. It provided a maximum of 128 Kbit/s bandwidth in both
upstream and downstream directions..
• The ISDN works based on the standards defined by ITU-T (formerly
CCITT). The Telecommunication Standardization Sector (ITU-T)
coordinates standards for telecommunications on behalf of the
International Telecommunication Union (ITU) and is based in
Geneva, Switzerland.

Principles of ISDN
The various principles of ISDN as per ITU-T recommendation
are:
• To support switched and non-switched applications
• To support voice and non-voice applications
• Intelligence in the network
• Layered protocol architecture
• Variety of configurations

ISDN Services
ISDN provides a fully integrated digital service to users. These
services fall into 3 categories- bearer services, teleservices and
supplementary services.
Bearer Services:
• Transfer of information (voice, data and video) between users
without the network manipulating the content of that information is
provided by the bearer network.
• There is no need for the network to process the information and
therefore does not change the content.
• Bearer services belong to the first three layers of the OSI model.
They are well defined in the ISDN standard.
• They can be provided using circuit-switched, packet-switched ,etc

ISDN Services
Teleservices:
• In this the network may change or process the contents of the data.
• These services corresponds to layers 4-7 of the OSI model.
• Teleservices relay on the facilities of the bearer services and are
designed to accommodate complex user needs.
• The user need not to be aware of the details of the process.
• Teleservices include telephony, teletex, telefax, videotex and
teleconferencing.
Supplementary Service:
• Additional functionality to the bearer services and teleservices are
provided by supplementary services.
• Reverse charging, call waiting, and message handling are
examples of supplementary services which are all familiar with
today’s telephone company services.

Addressing
Four levels of addresses are used in an internet employing the
TCP/IP protocols:
• Physical (link) addresses,
• Logical (IP) addresses
• Port addresses
• Specific addresses

Physical Addresses
• The physical address, also known as the link address, is the
address of a node as defined by its LAN or WAN. It is
included in the frame used by the data link layer. It is the
lowest-level address.
• The size and format of these addresses vary depending on the
network.
• For example, Ethernet uses a 6-byte (48-bit) physical address
that is imprinted on the network interface card (NIC).

Logical Addresses
• Logical addresses are necessary for universal communications
that are independent of underlying physical networks.
• A logical address in the Internet is currently a 32-bit address
that can uniquely define a host connected to the Internet.

Port Addresses
• The IP address and the physical address are necessary for a
quantity of data to travel from a source to the destination host.
• arrival at the destination host is not the final objective of data
communications .
• computers are devices that can run multiple processes at the
same time. For example, computer A can communicate with
computer C by using TELNET. At the same time, computer A
communicates with computer B by using the File Transfer
Protocol (FTP). For these processes to receive data
simultaneously, we need a method to label the different
processes.
• In the TCP/IP architecture, the label assigned to a process is
called a port address.
• A port address in TCP/IP is 16 bits in length.

Specific Addresses
• Some applications have user-friendly addresses that are
designed for that specific address.
• Examples include the e-mail address (for example,
forouzan@fhda.edu) and the Universal Resource Locator
(URL).

Logical Addresses
• we use the term IP address to mean a logical address in the
network layer of the TCP/IP protocol suite.
• The Internet addresses are 32 bits in length; this gives us a
maximum of 232 (4,294,967,296 (more than 4 billion))
addresses.
• These addresses are referred to as IPv4 (IP version 4)
addresses or simply IP addresses .

IPv4 ADDRESSES
• An IPv4 address is a 32-bit address.
• IPv4 addresses are unique(Two devices on the Internet can
never have the same address at the same time) and universal.
Notations:-
There are two prevalent notations to show an IPv4 address:
• binary notation
• 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.

Example
• Change the following IPv4 addresses from binary notation to
dotted-decimal notation.
a. 10000001 00001011 00001011 11101111
b. 11000001 10000011 00011011 11111111

Solution
• We replace each group of 8 bits with its equivalent decimal
number and add dots for separation.
a. 129.11.11.239
b. 193.131.27.255

Example
• Change the following IPv4 addresses from dotted-decimal
notation to binary notation.
a. 111.56.45.78
b. 221.34.7.82

Solution
• We replace each decimal number with its binary equivalent.
a. 01101111 00111000 00101101 01001110
b. 11011101 00100010 00000111 01010010

Classful Addressing
• IPv4 addressing, used the concept of classes.
• 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.

Example
• Find the class of each address.
a. 00000001 00001011 00001011 11101111
b. 11000001 10000011 00011011 11111111
c. 14.23.120.8
d. 252.5.15.111

Solution
a. The first bit is 0. This is a class A address.
b. The first 2 bits are 1; the third bit is 0. This is a class C address.
c. The first byte is 14 (between 0 and 127); the class is A.
d. The first byte is 252 (between 240 and 255); the class is E.
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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.
In c1assfnl addressing, a large part of the available addresses
were wasted.

Netid and Hostid
• In classful addressing, an IP address in class A, B, or C is
divided into netid and hostid.
• These parts are of varying lengths, depending on the class of
the address.
• Figure shows some netid and hostid bytes.
• Note that the concept does not apply to classes D and E.
• In class A, one byte defines the netid and three bytes define the
hostid.
• In class B, two bytes define the netid and two bytes define the
hostid.
• In class C, three bytes define the netid and one byte defines the
hostid.

Mask
• The masks for classes A, B, and C are shown In above table.
• The concept does not apply to classes D and E.
• The mask can help us to find the netid and the hostid.
• For example, the mask for a class A address has eight 1s,
which means the first 8 bits of any address in class A define
the netid; the next 24 bits define the hostid.
• The last column of Table shows the mask in the form n where
n can be 8, 16, or 24 in classful addressing. This notation is
also called slash notation or Classless Inter domain Routing
(CIDR) notation.

Subnetting
• During the era of classful addressing, subnetting was
introduced.
• If an organization was granted a large block in class A or B, it
could divide the addresses into several groups and assign each
group to smaller networks (called subnets).

Advantages of classful addressing
• Although classful addressing had several problems and
become obsolete, it had one advantage,
• Given an address, we can easily find the class since the prefix
length of each class is fixed, we can find prefix length
immediately. i.e. no extra information needed to extract prefix
and suffix.

Switch networking

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