X.25 is a legacy Network layer protocol that provides subscribers 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.
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. For these applications, the low bandwidth and high latency are not a concern, and the low cost makes X.25 affordable.
X.25 link speeds vary from 2400 b/s up to 2 Mb/s. However, public networks are usually low capacity with speeds rarely exceeding above 64 kb/s.
X.25 networks are now in dramatic decline being replaced by newer Layer 2 technologies such as Frame Relay, ATM, and ADSL. However, they are still in use in many portions of the developing world, where there is limited access to newer technologies.
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[1] Douglas E. Comer. Internetworking with TCP/IP. Principles, Protocols, and Architectures (4th Edition), volume 1. Prentice Hall, 2000.
[2] James F. Kurose and Keith W. Ross. Computer Networking: A Top-Down Approach Featuring the Internet (2nd Edition). Addison Wesley, 2003.
[3] Gary R. Wright and W. Richard Stevens. TCP/IP Illustrated. Addison-Wesley, 1995.
