What is an Integrated Services Digital Network?

An integrated services digital network (ISDN) is a network that uses existing copper wiring to carry digital signals. ISDNs can transmit voice traffic as well as data packets. These networks are based on a dual channel architecture, with a "B" or bearer channel and a "D" or delta channel.

The "B" channel is used for voice, data, multimedia, and video calls, and operates at 64 kbps. "B" channels can be pooled together for applications that require higher bandwidth. The "D" channel, which may be 16 or 64 kbps, is mostly used by switching equipment to allow communication between the in the integrated services digital network and the user's site. These channels are available in different interfaces.

The basic rate interface (BRI) is the most common interface used by people for Internet connections. An ISDN BRI has two "B" channels and a single 16 kbps "D" channel running over a single copper phone line, earning it the name "2B+D." A BRI can support two voice, fax, or data conversations and a packet switched data conversation at the same time. Multiple BRIs can be combined for additional bandwidth, depending on the integrated services digital network hardware being used.

The second option is known as primary rate interface (PRI). PRI is normally used by corporations or businesses that need intensive bandwidth. Integrated services digital network PRI supports as many as 23 "B" channels and a single 64 kbps "D" channel. These configurations are known as "23B+D." In Europe, the PRI supports 30 "B" channels and is known as "30B+D."

Proper configuration of integrated services digital network equipment requires defining how the ISDN line will be used. Customer needs or the phone company may require each "B" channel to have a separate phone number. Customers will also need to know their service profile identifier (SPID). SPIDs look like a phone number with additional digits and are used to identify the ISDN equipment to the phone company network. Like phone numbers, each "B" channel can be assigned its own SPID.

An integrated services digital network does require extra hardware and has some practical limitations. The hardware requirements include an ISDN terminal adapter and router. Customers must be physically located no more than 18,000 feet (5,486.4 m) or approximately 3.4 miles (5.471 km) from a telephone company central office in order to receive ISDN service. Expensive "repeater" hardware can sometimes, but not always, be used to avoid the distance limitation.

Although ISDN has been around for decades, it wasn't until the 1990s that the regional Bell operating companies (RBOCs) in the United States agreed to a national ISDN standard, NI-1. With the adoption of this standard, customers could purchase ISDN equipment from any manufacturer and be confident it would be compatible with their particular provider's central office switch. In recent years, ISDNs usage has declined due to the surging popularity of more cost effective broadband services and cable modems. ISDN is still used in more remote areas where broadband service is not yet available and as dedicated backup lines.