Packet Switching

Packet switching is similar to message switching using short messages. Any message exceeding a network-defined maximum length is broken up into shorter units, known as packets, for transmission. The packets, each with an associated header are transmitted individually through the network.

Figure 10: A Connection Between Two Systems A and D using Circuits that are Shared using Packet Switching.

Figure 11: Packet-Switched Communication between A and D

Source: http://www.erg.abdn.ac.uk/users/gory/course/intro-pages/me.html

The fundamental difference in packet communication is that the data is formed into packets with a predefined header format and well-known idle patterns which are used to occupy the link when there is no data to be communicated.

A packet network equipment discards the idle patterns between packets and processes the entire packets as one piece of data.

The equipment examines the packet header information and then either removes the header (in an end system) or forwards the packet to another system. If the out-going link is not available, then the packet is placed in a queue until the link becomes free. A packet network is formed by links which connect packet network equipment

There are two important benefits from packet switching.

The first and most important benefit is that since packets are short, the communication links between the nodes are only allocated to transferring a single message for a short period of time while transmitting each packet. Longer messages require a series of packets to be sent, but do not require the link to be dedicated between the transmission of each packet. The implication is that packets belonging to other messages may be sent between the packets of the message being sent from A to D. this provides a much fairer sharing of the resources of each of the links.

Another benefit of packet switching is known as pipelining. Pipelining is visible in the figure below. At the time packet 1 is sent from B to C, packet 2 is sent from A to B; Packet 1 is sent from C to D while packet 2 is sent from B to C. And, packet 3 is sent from A to B and so forth. This simultaneous use of communications link represents a gain in efficiency; the total delay for transmission across a packet network may be considerably less than for message switching, despite the inclusion of a header in each packet rather than in each message

Comparison of the three switching methods (see Figure 12)

Figure 12: Three Switching Methods