ISDN: The old warhorse is still kicking

Think ISDN is being supplanted by DSL and cable modems? Think again. Fast, reliable ISDN continues to hang in there. If you're seeking a high-speed solution, catch up on ISDN basics.

Integrated Services Digital Network (ISDN) is an ITU-T (International Telecommunication Union) standard with roots dating back as far as 1968. While ISDN has never had the following that the ITU-T had hoped for, it's still a good solution for building a cost-effective, high-speed voice and data network.

Think DSL and cable modems are the way to go? Sometimes such fat-pipe options have advantages; other times, they don’t. It often depends upon factors such as your location, service provider, the number of other subscribers on the node servicing your location, and where the central office is located. ISDN is proven.

ISDN overview
Need a refresher? ISDN is a digital switching technology used to transport voice and data. In North America, there are two types of ISDN services available.

ISDN Basic Rate Interface (BRI) provides a data transfer rate of 128 Kbps (2 x 64 Kbps bonded) over existing telephone wire. ISDN-BRI is actually made up of three channels. Two of these channels, known as the B channels, operate at 64 Kbps and are responsible for carrying data. They can be bonded for a total data throughput of 128 Kbps. The third channel, known as the D channel, operates at 16 Kbps and is responsible for signaling. Framing and synchronization, totaling 48 Kbps, are added to the B and D channels, bringing the total bandwidth to 192 Kbps.

ISDN Primary Rate Interface (PRI) offers connectivity at T-1 (1.544 Mbps) speeds. ISDN-PRI consists of 24 channels. Twenty-three of these channels, known as the B channels, function at 64 Kbps per channel and can be bonded for a total data throughput of 1.472 Mbps. The remaining channel, known as the D channel, operates at 64 Kbps and is used for signaling. Eight Kbps of framing and synchronization are added, bringing the total bandwidth up to 1.544 Mbps.

Why is ISDN cost-effective?
ISDN channels can be split to provide separate functions. This means that voice services, video services, and data services can be provided over the same medium. Additionally, unlike expensive leased lines, ISDN service is billed based on usage. Furthermore, most telephone companies charge less for ISDN service than for fractional T-1 or frame relay lines.

Want more?
ISDN can provide high-speed voice, video, and data transmission for a fraction of the cost of leased lines. Moreover, using Dial-on-demand (DDR) routing, ISDN can provide a cost-effective backup for existing T1 services. (For more on this method, see "Back up your WAN with Cisco's DDR .") To learn about designing, implementing and supporting ISDN networks, check out "Cisco IOS 12.0 Dial Solutions" by Cisco Systems, Inc.

Warren Heaton CCDA, CCNA, MCSE+I is the Cisco Program Manager for A Technological Advantage in Louisville, KY.

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