X.25is an analog, packet-switched technology designed for long-distance data transmissionand standardized by the ITU in the mid-1970s. The original standard for X.25 specified amaximum of 64-Kbps throughput, but by 1992 the standard was updated to includemaximum throughput of 2.048 Mbps. It was originally developed as a more reliable alternative to the voice telephone system for connecting mainframe computers and remote terminals.Later, it was adopted as a method of connecting clients and servers over WANs.The X.25 standard specifies protocols at the Physical, Data Link, and Network layers of theOSI model. It provides excellent flow control and ensures data reliability over long distances byverifying the transmission at every node. Unfortunately, this verification also renders X.25 comparatively slow and unsuitable for time-sensitive applications, such as audio or video. On theother hand, X.25 benefits from being a long-established, well-known, and low-cost technology.X.25 was never widely adopted in the United States, but was accepted by other countries andwas for a long time the dominant packet-switching technology used on WANs around the world.Recall that, in packet switching, packets belonging to the same datastream may follow different, optimal paths to their destination. As aresult, packet switching uses bandwidth more efficiently and allowsfor faster transmission than if each packet in the data stream had tofollow the same path, as in circuit switching. Packet switching is alsomore flexible than circuit switching because packet sizes may vary.Frame relayis a digital version of X.25 that also relies on packet switching. ITU and ANSIstandardized frame relay in 1984. However, because of a lack of compatibility with otherWAN technologies at the time, frame relay did not become popular in the United States andCanada until the late 1980s. Frame relay protocols operate at the Data Link layer of the OSImodel and can support multiple different Network and Transport layer protocols. The nameis derived from the fact that data is separated into frames, which are then relayed from onenode to another without any verification or processing.An important difference between frame relay and X.25 is that frame relay does not guaranteereliable delivery of data. X.25 checks for errors and, in the case of an error, either corrects thedamaged data or retransmits the original data. Frame relay, on the other hand, simply checksfor errors. It leaves the error correction up to higher-layer protocols. Partly because it doesn’tperform the same level of error correction that X.25 performs (and, thus, has less overhead),frame relay supports higher throughput than X.25. It offers throughputs between 64 Kbpsand 45 Mbps. A frame relay customer chooses the amount of bandwidth he requires andpays for only that amount.Both X.25 and frame relay rely on virtual circuits.Virtual circuitsare connections betweennetwork nodes that, although based on potentially disparate physical links, logically appearto be direct, dedicated links between those nodes. One advantage to virtual circuits is theirconfigurable use of limited bandwidth, which can make them more efficient. Several virtualcircuits can be assigned to one length of cable or even to one channel on that cable. A virtualcircuit uses the channel only when it needs to transmit data. Meanwhile, the channel isavailable for use by other virtual circuits.X.25 and frame relay may be configured asSVCs (switched virtual circuits) or PVCs(permanent virtual circuits). SVCs are connections that are established when parties need totransmit, then terminated after the transmission is complete. PVCs are connections that areestablished before data need to be transmitted and maintained after the transmission iscomplete. Note that in a PVC, the connection is established only between the two points(the sender and receiver); the connection does not specify the exact route the data will travel
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