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2.2 | |
WAN Technologies | |
| 2.2.4 | |
X.25 |
In response to the expense of leased
lines, telecommunications providers introduced packet-switched
networks using shared lines to reduce costs. The first of these
packet-switched networks was standardized as the X.25 group of
protocols. X.25 provides a low bit rate shared variable capacity that
may be either switched or permanent.
 X.25 is a network-layer protocol and subscribers are provided with a network address. Virtual circuits can be established through the network with call request packets to the target address. The resulting SVC is identified by a channel number. Data packets labeled with the channel number are delivered to the corresponding address. Multiple channels can be active on a single connection. Subscribers connect to the X.25 network with either leased lines or dialup connections. X.25 networks can also have pre-established channels between subscribers that provide a PVC. X.25 can be very cost effective because tariffs are based on the amount of data delivered rather than connection time or distance. Data can be delivered at any rate up to the connection capacity. This provides some flexibility. X.25 networks are usually low capacity, with a maximum of 48 kbps. In addition, the data packets are subject to the delays typical of shared networks. X.25 technology is no longer widely available as a WAN technology in the US. Frame Relay has replaced X.25 at many service provider locations. Typical X.25 applications are point-of-sale card readers. These readers use X.25 in dialup mode to validate transactions on a central computer. Some enterprises also use X.25 based value-added networks (VAN) to transfer Electronic Data Interchange (EDI) invoices, bills of lading, and other commercial documents. For these applications, the low bandwidth and high latency are not a concern, because the low cost makes the use of X.25 affordable.
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