Circuit Switching

Introduction

The concept of circuit switching was introduced in Chapter 2 during the description of how a public switched telephone network (PSTN) was used to make a phone call.
Circuit switching uses a dedicated channel/circuit which lasts throughout the connection: the communication line is effectively ‘tied up’.
When sending data across a network, there are three stages:

1. First, a circuit/channel between sender and receiver must be established.

2. Data transfer then takes place (which can be analogue or digital); transmission is usually bi-directional.

3. After the data transfer is complete, the connection is terminated.

Pros	Cons
the circuit used is dedicated to the single transmission only	it is not very flexible (for example, it will send empty frames and it has to use a single, dedicated line)
the whole of the bandwidth is available	nobody else can use the circuit/channel even when it is idle
the data transfer rate is faster than with packet switching	the circuit is always there whether or not it is used
the packets of data (frames) arrive at the destination in the same order as they were sent	if there is a failure /fault on the dedicated line, there is no alternative routing available
a packet of data cannot get lost since all packets follow on in sequence along the same single route	dedicated channels require a greater bandwidth
it works better than packet switching in real-time applications	prior to actual transmission, the time required to establish a link can be long

The dedicated route from ‘A’ to ‘B’ is first of all established (shown in orange on the diagram).
The following connections are then partially implemented: A–R2, R2–R5, R5–R8, R8–R7, R7–R10 and finally R10–B.
All packets (frames) follow this single route and communication will take place, provided ‘B’ is not busy.
The main uses of circuit switching include public telephone networks, private telephone networks and private data networks.

Packet switching was introduced in Chapter 2 when describing VoIP, together with a diagram to show how the individual packets are routed from client to client.

Packet switching is a method of transmission in which a message is broken up into a number of packets that can be sent independently to each other from start point to end point.
The data packets will need to be reassembled into their correct order at the destination.

each packet follows its own path
routing selection depends on the number of datagram packets waiting to be processed at each node (router)
the shortest path available is selected
packets can reach the destination in a different order to that in which they are sent

As figure shows, the message sent by computer ‘A’ was split into four packets.
The original packet order was:
and they arrived in the order:
which means they need to be reassembled in the correct order at the destination.

Pros	Cons
no need to tie up a communication line	the protocols for packet switching can be more complex than those for circuit switching
it is possible to overcome failed or faulty lines by simply re-routing packages	if a packet is lost, the sender must re-send the packet (which wastes time)
it is easy to expand the traffic usage	does not work well with real-time data streams
circuit switching charges the user on the distance and duration of a connection, but packet switching charges users only for the duration of the connectivity	the circuit/channel has to share its bandwidth with other packets
high data transmission is possible with packet switching	there is a delay at the destination while packets are reassembled
packet switching always uses digital networks which means digital data is transmitted directly to the destination	needs large amounts of RAM to handle the large amounts of data

Feature	circuit switching	packet switching
actual route used needs to be set up before transmission can begin	Yes	No
a dedicated transmission path is required	Yes	No
each packet uses the same route	Yes	No
packets arrive at destination in the correct order	Yes	No
all the bandwidth of the channel is required	Yes	No
is bandwidth wasted?	Yes	No

Sometimes it is possible for packets to get lost and keep ‘bouncing’ around from router to router and never actually get to their destination. Eventually, the network could grind to a halt as the number of ‘lost’ packets mounts up and clogs up the system. To overcome this, a method called hopping is used.

A hop number is added to the header of each packet.
Each packet is only allowed to hop a finite number of times (this number is determined by the network protocol and routing table being used).
Each time a packet passes through a router, the hop number is decreased by 1.
If the packet has not reached its destination and the hop number = 0, then it will be deleted when it reaches the next router.

Routing tables contain the information necessary to forward a package along the shortest/best route to allow it to reach its destination.
As soon as the packet reaches a router, the packet header is examined and compared with the routing table.
The table supplies the router with instructions to send the packet (hop) to the next available router.

Routing tables include

number of hops
MAC address of the next router where the packet is to be forwarded to (hopped)
metrics (a cost is assigned to each available route so that the most efficient route/path is found)
network destination (network ID) or pathway
gateway (the same information as the next hop; it points to the gateway through which target network can be reached)
netmask (used to generate network ID)
interface (indicates which locally available interface is responsible for reaching the gateway).