What are the fundamentals of ATM and ISDN?

Lisa Phifer explains the fundamentals of ATM and ISDN and why they are used.

Where can I learn about the fundamentals of ATM and ISDN and can you explain why they are used?

Integrated Services Digital Network (ISDN) is a 90's network technology that transmits digital signals over the copper wire traditionally used to carry voice traffic. There are two levels of ISDN service: Basic Rate Interface (BRI) and Primary Rate Interface (PRI). A residential user who purchases an ISDN BRI from a telco gets two 64 Kbps channels that can be used for digital voice or data. In the US, a business that purchases an ISDN PRI gets 23 of those "B" channels. Each ISDN BRI or PRI also has a "D" channel for signaling between the service provider and the remote data terminal. That terminal may be an ISDN modem or router that connects a home PC or office network to the Internet. To learn more, consult this Cisco ISDN overview, or browse ISDN specifications available from the ITU.

Asynchronous Transfer Mode (ATM) is a switching technology that transmits short 53-byte cells across a network composed of ATM edge and core switches, typically interconnected by fiber. ATM is designed to switch those cells at very high speeds over virtual connections (PVCs or SVCs) that can be provisioned or established as needed. ATM can be used to transport any type of traffic across long distances at high rates -- for example, letting a service provider transport video between cities at 155 or 622 Mbps. To learn more, consult these SearchNetworking white papers on ATM, or browse specifications posted at the ATM Forum.

Broadband ISDN (B-ISDN) is the high rate, fiber-based evolution of the older ISDN specifications. B-ISDN uses ATM and SDH (synchronous digital hierarchy) instead of copper wire to deliver integrated digital services. To learn more about the history of B-ISDN and role of ATM, visit this MFA Forum page.

This was first published in December 2006

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