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Cover Story
Get Fluent in ISDN

-- by Cynthia Morgan

The long wait for ISDN to become a reality bred snide definitions of the acronym, such as "it still doesn't network" and "I smell dollars now." It actually stands for Integrated Services Digital Network, and it's basically the digital version of an ordinary analog phone call. Like every other communications technology, ISDN comes with its own lingo. Here are some terms you'll likely encounter while setting up ISDN service:

Basic Rate Interface (BRI): Also referred to as "2B+D," this ISDN service is generally sold to consumers and small businesses. Installed over ordinary phone lines, it offers two 64Kbps B channels, with a 16Kbps D channel to manage call traffic. The B channels can be combined into a single 128Kbps connection.

Central Office (CO) Switches: CO switches connect telephones, ISDN TAs and other devices at the phone company offices.

Channel Rate-Adaptation Protocols: These protocols tell the ISDN terminal adapter (TA) how to change its transmission/reception speeds to match those of the connecting device. Europe and Japan primarily use the V.110 protocol; the U.S. uses V.120.

Clear Channel: A 64Kbps or 56Kbps channel used for remote access to large systems (such as mainframes) or videoconferencing systems.

Directory Number (DN): The DN is the actual number(s) for an ISDN line, in the standard XXX-XXXX format. You'll usually receive one for each B channel.

Multilink PPP (MPPP or MLPPP): This technique combines two B channels into a single, high-speed data path.

National ISDN-1(NI-1): The CO switch standard in the U.S. that allows different manufacturers' devices to talk to each other.

Network Termination-1 (NT-1): NT-1 devices condition signals from the phone company wires for the TA. Older TAs require a separate NT-1; newer models have built-in NT-1 adapters.

Point-to-Point Protocol (PPP): ISPs often use the PPP connection scheme to link your ISDN TA to the Internet.

Primary Rate Interface (PRI): Also known as "23B+D," this is a very high-speed ISDN connection. In the U.S., it combines up to 23 64Kbps channels, managed with one 64Kbps D channel, to make a T1 line capable of 1.544Mbps transmission.

SPID (Service Profile IDentifier) numbers: SPIDs are used in North America to identify your TA to the phone company network. A SPID generally consists of the directory number surrounded by other alphanumeric characters.

Terminal Adapter (TA): Often inaccurately referred to as a modem, a TA is the ISDN device used to connect a computer to the phone company's network. It replaces your modem.

Terminal Endpoint Identifier (TEI): The TEI identifies your specific connection to the CO switch.

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Copyright 1997 CMP Media Inc.


(From Windows Magazine, January 1997, page 198.)