packet switching in The Network Encyclopedia
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`packet switching
`
`Definition of packet switching in The Network Encyclopedia.
`packet switching
`
`The process by which a networking or telecommunications device accepts a packet and
`switches it to a telecommunications device that will take it closer to its destination. Packet
`switching allows data to be sent over the telecommunications network in short bursts or
`“packets” that contain sequence numbers so that they can be reassembled at the destination.
`Wide area network (WAN) devices called switches route packets from one point on a packet-
`switched network to another. Data within the same communication session might be routed
`over several different paths, depending on factors such as traffic congestion and switch
`availability.
`
`Packet switching is the transmission method used for most computer networks because the
`data transported by these networks is fundamentally bursty in character and can tolerate
`latency (due to lost or dropped packets). In other words, the transmission bandwidth needed
`varies greatly in time, from relatively low traffic because of background services such as name
`resolution services, to periods of high bandwidth usage during activities such as file transfer.
`This contrasts with voice or video communication, in which a steady stream of information
`must be transmitted in order to maintain transmission quality and in which latency must
`remain minimized to preserve intelligibility.
`
`The Internet is the prime example of a packet-switched network based on the TCP/IP protocol
`suite. A series of routers located at various points on the Internet’s backbone forward each
`packet received on the basis of destination address until the packet reaches its ultimate
`destination. TCP/IP is considered a connectionless packet-switching service because
`Transmission Control Protocol (TCP) connections are not kept open after data transmission is
`complete.
`
`X.25 public data networks are another form of packet-switching service, in which packets (or
`more properly, frames) formatted with the High-level Data Link Control (HDLC) protocol are
`routed between different X.25 end stations using packet switches maintained by X.25 service
`providers. Unlike TCP/IP, X.25 is considered a connection-oriented packet-switching protocol
`because it is possible to establish permanent virtual circuits (PVCs) that keep the logical
`connection open even when no data is being sent. However, X.25 can be configured for
`connectionless communication by using switched virtual circuits (SVCs). An X.25 packet-
`switched network typically has a higher and more predictable latency (about 0.6 seconds
`between end stations) than a TCP/IP internetwork. This is primarily because X.25 packet
`switches use a store-and-forward mechanism to buffer data for transmission bursts, which
`introduces additional latency in communication. In addition, X.25 uses error checking between
`each node on the transmission path, while TCP/IP uses only end-to-end error checking.
`
`Frame relay (also called fast packet switching) is another connection-oriented packet-switching
`service that gives better performance than X.25. It does this by switching packets immediately
`instead of using the store-and-forward mechanism of X.25 networks. Frame relay also
`eliminates flow control and error checking to speed up transmission. This is possible because
`frame relay networks use modern digital telephone lines, which are intrinsically much more
`reliable than the older analog phone lines on which much of the X.25 public network still
`depends. Frame relay supports only connection-oriented PVCs for its underlying switching
`architecture.
`
`Finally, Asynchronous Transfer Mode (ATM) is another packet-switching service in which small
`fixed-length packets called cells are switched between points on a network.
`
`NOTE
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`http://www.thenetworkencyclopedia.com/entry/packet-switching/[7/26/2017 6:35:51 AM]
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`packet switching in The Network Encyclopedia
`
`Packet switching is different from circuit switching, in which switches are configured in a fixed
`state for the duration of the session so that the route the data takes is fixed. A network that is
`circuit-switched requires a dedicated switched communication path for each communication
`even if its full bandwidth is not being used. In packet switching, bandwidth can be used when
`available for more efficient transmission. Circuit switching is generally used in telephone
`systems, while packet switching is used for computer networks. Digital cellular phone services
`are generally also circuit-switched, but Personal Communications Services (PCS) cellular
`systems are gradually being migrated to packet-switched networks for greater efficiency in
`data transmission.
`
`Another difference between packet switching and circuit switching is that circuits must first be
`established before any data is sent, and this generally involves a certain amount of setup time.
`During this process, the request for a circuit connection must pass through the circuit-switched
`network, resources must be reserved for the connection, and a signal must be returned to the
`initiating station when the circuit is established and data transmission can begin. Circuit-
`switched networks are thus useful only when the duration of the data transmission is much
`longer than the setup time involved in establishing the circuit. With packet switching, data can
`be sent at the start of transmission, which is better suited to the bursty, irregular nature of
`short network transmissions over a computer network or WAN link.
`
`See also
`packet-switching services
`
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