Packet Switching: X.25 & Frame Relay

An Introductory Overview



X.25

X.25 is a protocol that the CCITT developed to provide reliable data communications on public data networks. It uses packet switching and virtual circuits, and provides a data rate up to 64kbps. It provides very robust error checking features, which makes it a good choice for older networks. Because of its extensive error checking, it not only works well on these older networks that are more susceptible to physical interference, but this also has made it a widely accepted protocol that is used in many parts of the world. (I understand that it is not used in Hawaii anymore though). Although most packet switching technology does not use a dedicated physical or virtual circuit and is generally connectionless in nature, X.25 establishes virtual circuits that allow it to be connection oriented. The connection is established, the data is transferred, and then the connection is terminated.

Frame Relay

Frame Relay is based on X.25 packet switching technology; however, it uses a fast packet switching technology that provides no error checking. This is made possible because of todays modern digital and optical networks that generally transfer data with a very low rate of errors. So frame relay leaves it up to the end stations to handle any errors. Because it does not provide any error correction, frame relay reduces the amount of overhead associated with the data transmission, and this allows for the fast packet switching.

Frame relay is generally purchased as an alternative to a dedicated leased line, such as T1. The bandwidth for frame relay is provided in the form of a Commited Information Rate (CIR), generally a percentage of a T1 line. So, for example, a company may purchase a CIR that is half of the 1.544 Mbps available on a T1 line, but because frame relay can handle data bursts, the customer may actually use more than the CIR, if available. However, my understanding is that if the customer consistently and regularly bursts above their CIR then the telephone company may decide to back charge them and increase their usage rate. So, depending on a company's bandwidth needs and what type of traffic they will be generating, frame relay can offer a very good alternative to a full dedicated leased line. Other alternatives to leased lines include the many flavors of DSL (digital subscriber line), or xDSL. More on this will be published in the near future!

ComTest Technologies can assist you with your Frame Relay needs. Just visit some of the Additional Resources below, or call (808) 831-0601. We represent a number of Frame Relay access devices, including Verilink (www.verilink.com).


Additional Resources

Frame Relay Forum - detailed white papers!
Securing Mission Critical Frame Relay Networks
Get a FREE Frame Relay Security Guide

References

This page was written by Will Twiggs, an associate with ComTest Technologies, Inc. For more information or if you have questions about this material, you can contact the author at william@comtest.com

Data and Telecommunications Systems and Applications by Charles N. Thurwachter, Jr.

Telecommunications, 3rd Edition, by Warren Hioki


This page created by Will Twiggs
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Last updated on 02/13/2001