Switching Methods in Computer Network

For data transfer, different types of switching methods are available.

Following figure shows the types of switching:

type switching

Circuit Switching

Circuit switched network consists of a set of switches connected by physical links.
In circuit switched network, two nodes communicate with each other over a dedicated communication path.
There is a need of pre-specified route from which data will travel and no other data is permitted.
Before starting communication, the nodes must make a reservation for the resources to be used during the communication.
In this type of switching, once a connection is established, a dedicated path exists between both ends until the connection is terminated.

The end systems, such as telephones or computers are directly connected to a switch.
When system A needs to communicate with system B, system A needs to request a connection to system B that must be accepted by all switches as well as by B itself.
This is called as setup phase in which a circuit is reserved on each link, and the combination of circuits or channels defines a dedicated path.
After the establishment of the dedicated circuit, the data transfer can take place.
After all data has been transferred, the circuit is torn down.

Packet Switching

In packet switching, messages are divided into packets of fixed or variable size.
The size of packet is decided by the network and the governing protocol.
Resource allocation for a packet is not done in packet switching.
Resources are allocated on demand.
The resource allocation is done on first-come, first-served basis.
Each switching node has a small amount of buffer space to hold packets temporarily.
If the outgoing line is busy, the packet stays in queue until the line becomes available.

Packet switching method uses two routing methods:

1. Datagram Packet Switching

Datagram packet switching is normally implemented in the network layer.
In datagram network, each packet is routed independently through the network.
Each packet carries a header that contains the full information about the destination.
When the switch receives the packet, the destination address in the header of the packet is examined; the routing table is consulted to find the corresponding port through which the packet should be forwarded.

2. Virtual Circuit Packet Switching

Virtual circuit packet switching is normally done at the data link layer.
Virtual circuit packet switching establishes a fixed path between a source and a destination to transfer the packets.
It is also called as connection oriented network.

A source and destination have to go through three phases in a virtual circuit packet switching:
     I. Setup phase
     ii. Data transfer phase
     iii. Connection release phase

A logical connection is established when a sender sends a setup request to the receiver and the receiver sends back an acknowledgement to the sender if the receiver agree.
All packets belonging to the same source and destination travel the same path.
The information is delivered to the receiver in the same order as transmitted by the sender.

Message Switching

In message switching, it is not necessary to establish a dedicated path between transmitter and receiver.
In this, each message is routed independently through the network.
Each message carries a header that contains the full information about the destination.
Each intermediate device receives the whole message and buffers it until there are resources available to transfer it to the next hop.
If the next hop does not have enough resources to accommodate large size message, the message is stored and switch waits.
For this reason a message switching is sometimes called as Store and Forward Switching.
Message switching is very slow because of store-and-forward technique.
Message switching is not recommended for real time applications like voice and video.