3.6 Switching: Circuit-switched networks, Packet switched networks —Datagram approach, virtual circuit approach.

1. Switching
Pranoti R. Doke
ME (EnTC), BE(EnTC)
Circuit Switching and Packet Switching

2. Switching
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 two basic methods of
switching Circuit Switching and Packet Switching.
Packet-switched and circuit-switched networks use two
different technologies for sending messages and data from one
point to another.
Each has its advantages and disadvantages depending on
what you are trying to do.

3. Circuit Switching Technique
Circuit switching was designed
in 1878 in order to send
telephone calls down a
dedicated channel.
This channel remains open and in
use throughout the whole call and
cannot be used by any other data
or phone calls.

4. Circuit Switching
Figure: The switched path
In circuit switching, two
communicating stations are
connected by a dedicated
communication path which
consists of intermediate
nodes in the network and
the links that connect these
nodes.

5. Circuit Switching
When the two hosts shown in the figure initiate a connection, the
network determines a path through the intermediate switches and
establishes a circuit which is maintained for the duration of the
connection.
When the hosts disconnect, the network releases the circuit.
Communication via circuit switching implies that there is a
dedicated communication path between the two stations. The path
is a connected through a sequence of links between network
nodes. On each physical link, a logical channel is dedicated to the
connection.

6. Circuit Switching
There are three phases in circuit switching:
Establish
Transfer
Disconnect
The telephone message is sent all together; it is not broken up.
The message arrives in the same order that it was originally sent.

7. Circuit Switching
Circuit switching is commonly used technique in telephony, where the
caller sends a special message with the address of the callee (i.e. by
dialing a number) to state its destination. It involved the following
three distinct steps, as shown in Figure shows Circuit Establishment:
To establish an end-to-end connection before any transfer of data.
Some segments of the circuit may be a dedicated link, while some
other segments may be shared. Data transfer: Transfer data is from
the source to the destination. The data may be analog or digital,
depending on the nature of the network. The connection is generally
full-duplex. Circuit disconnect: Terminate connection at the end of
data transfer. Signals must be propagated to deallocate the dedicated
resources.

8. Circuit Switching
 In modern circuit-switched networks, electronic signals pass through
several switches before a connection is established.
 During a call no other network traffic can use those switches.
 The resources remain dedicated to the circuit during the entire data
transfer and the entire message follows the same path.
 Circuit switching can be analog or digital.
 With the expanded use of the Internet for voice and video, analysts
predict a gradual shift away from circuit-switched networks.
 A circuit-switched network is excellent for data that needs a constant
link from end-to-end, for example, real-time video.

9. Advantages: Circuit Switching
 Circuit is dedicated to the call – nointerference, no sharing
 Guaranteed the full bandwidth for the durationof the call
 Guaranteed quality ofservice

10. Disadvantages: Circuit Switching
 Inefficient – the equipment may be unused for a lot of the call; if no data is
being sent, the dedicated line still remains open.
 It takes a relatively long time to set up thecircuit.
 During a crisis or disaster, the network may become unstable or
unavailable.
 It was primarily developed for voice traffic ratherthan data traffic.

11. Packet Switching
 In packet-based networks, the message gets broken into
small data packets.
 These packets are sent out from the computer and they travel
around the network seeking out the most efficient route to
travel as circuits become available.
 This does not necessarily mean that they seek out the
shortest route. Each packet may go a different route from the
others.

12. Packet Switching

13. Packet Switching
Packet switching was designed to address the shortcomings of
circuit switching in dealing with data communication. Unlike circuit
switching where communication is continuous along a dedicated
circuit, in packet switching, communication is discrete in form of
packets.
Each packet is of a limited size and can hold up to a certain number
of octets of user data. Larger messages are broken into smaller
chunks so that they can be fitted into packets. In addition to user
data, each packet carries additional information (in form of a
header) to enable the network to route it to its final destination

14. Packet Switching
A packet is handed over from node to node across the network.
Each receiving node temporarily stores the packet, until the next
node is ready to receive it, and then passes it onto the next node.
This technique is called store-and-forward and overcomes one of
the limitations of circuit switching.
A packet-switched network has a much higher capacity for
accepting further connections. Additional connections are usually
not blocked but, simply slow down existing connections, because
they increase the overall number of packets in the network and
hence increase the delivery time of each packet.

15. Packet Switching
 Each packet is sent with a ‘header address’ which tells it
where its final destination is, so it knows where to go.
 The header address also describes the sequence for
reassembly at the destination computer so that the packets
are put back into the correct order.
 One packet also contains details of how many packets should
be arriving so that the recipient computer knows if one packet
has failed to turn up.
 If a packet fails to arrive, the recipient computer sends a
message back to the computer which originally sent the
data, asking for the missing packet to be resent.

16. Advantages of Packet Switching
 Bandwidth used to full potential
 Devices of different speeds can communicate
 Not affected by line failure (redirects signal)
 Availability – no waiting for a direct connection to become
available
 During a crisis or disaster, when the public telephone
network might stop working, e-mails and texts can still be
sent via packet switching

17. Dis Advantages of Packet Switching
 Under heavy use there can be a delay
 Data packets can get lost or become corrupted
 Protocols are needed for a reliable transfer
 Not so good for some types data streams (e.g.real-time video streams can
lose frames due to the way packets arrive out of sequence)

18. Circuit Switching Vs Packet Switching
 It is easier and less expensive to double the capacity of a packet switched
network—a circuit network is heavily dependent on the number of channels
available.
 Circuit-switched technologies, which take four times as long to double their
performance/cost, force ISPs to buy that many more boxes to keep up.
 This is why everyone is looking for ways to get Internet traffic off the
telephone network.
 The alternative of building up the telephone network to satisfy the demand
growth is economically out of the question

19. Circuit Switching Vs Packet Switching
The battle between circuit and packet technologies has been around a long
time, and it is starting to be like the old story of the tortoise and the hare.
▪ In this case, the hare is circuit switching—fast, reliable and smart. Hare
starts out fast and keeps a steady pace, while the tortoise starts slow but
manages to double his speed every 100 meters.
▪ If the race is longer than 2 km (1.2 miles), the power of compounding favors
the tortoise (packet switching).