Packet Switching Definition

Packet switching is the dividing of messages into packets before they are sent, transmitting each packet individually, and then reassembling them into the original message once all of them have arrived at the intended destination.

Packets are the fundamental unit of information transport in all modern computer networks, and increasingly in other communications networks as well. Each packet, which can be of fixed or variable size depending on the protocol, consists of a header, body (also called a payload) and a trailer. The body contains a segment of the message being transmitted.

The header contains a set of instructions regarding the packet's data, including the sender's IP address, the intended receiver's IP address, the number of packets into which the message has been divided, the identification number of the particular packet, the protocol (on networks that carry multiple types of information, such as the Internet), packet length (on networks that have variable length packets) and synchronization (several bits that help the packet match up to the network).

Packets are switched to various network segtments by routers located at various points throughout the network. Routers are specialized computers that forward packets through the best paths, as determined by the routing algorithm being used on the network, to the destinations indicated by destination IP addresses in the packet headers. During transport from one host to another, packets may be routed out of order and across a variety of paths to get to the desired end point.

This contrasts with circuit switching, in which a dedicated, but temporary, circuit is established for the duration of the transmission of each message. The most familiar circuit-switching network is the telephone system when used for voice communications. Circuit-switching is ideal when data must be transmitted quickly and must arrive in the same order in which it is sent, as is the case with most real-time data, such as live audio and video.

Packet switching is used to optimize the use of the bandwidth available in a network, to minimize the transmission latency (i.e. the time it takes for data to pass across the network) and to increase robustness of communication. It is more efficient and robust for data that can withstand some delays in transmission, such as web pages and e-mail messages.

Most modern data communications protocols, including TCP/IP, X.25 and frame relay, are based on packet switching technologies. Moreover, packet switching is increasingly being used for voice communications as well, such as with VoIP (voice over Internet protocol), GPRS (general packet radio service) and i-mode.

The concept of packet switching was first developed in the early 1960s, apparently independently, by researchers at the Massachusetts Institute of Technology (MIT) and Rand Corporation.

Created November 4, 2005.
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