Tuesday, October 11, 2005

What is ISDN?

ISDN stands for Integrated Serviced Digital Network. It's an internationally standardized telecommunications service that was originally designed to support voice, data and video services for home and business users. Today the low bandwidth consumer version of ISDN has all but disappeared. Business telecom services over high bandwidth ISDN are alive and well, and are particularly advantageous to dial-up ISPs and call centers.

ISDN is based on the concept of 64 Kbps digital channels. That number is not arbitrary. It is the original channel size for a digitized telephone call of 8 bits sampled 8,000 times per second. Sound familiar? That's the standard chosen for T1 telephone lines. A T1 voice line has 24 channels or 24 individual telephone lines all digitized an transmitted on two pair of copper wires.

In ISDN lingo, those same channels are called "B channels." The B stands for Bearer. What the ISDN service is bearing can be traditional voice telephony or Internet and other digital data.

ISDN also defines another type of channel called "D channels." The D stands for Delta, although it is sometimes referred to as a data channel. A Delta channel is used for signaling and control, but is sometimes pressed into service to actually carry data communications.

The low bandwidth version of ISDN is called Basic Rate Interface or BRI. It consists of 2 Bearer channels and 1 Delta channel. The B channels are 64 Kbps each. The BRI D channel is 16 Kbps. This is sometimes referred to as 2B+D.

ISDN BRI runs at a combined bandwidth of 144 Kbps, which is low enough to be easily transported on a single copper wire pair telephone line. It was envisioned as a way to improve upon dial-up Internet access, which has to convert from digital computer data to analog tones that can travel on a voice telephone line. You're lucky to get 48 Kbps on a phone line, and often it's more like 33 Kbps on a noisy line. With ISDN, one Bearer channel can be used for Internet access at a full 64 Kbps and the other Bearer can be used for telephone calls. The Delta channel controls everything. If you don't need a voice line, the two Bearer channels can be bonded to give you a 144 Kbps Internet service. For ISDN BRI you need a special ISDN digital modem.

The reason you don't hear much about ISDN BRI is that another standard, DSL or Digital Subscriber Line, was developed to provide much higher bandwidths of always-on Internet access while sharing an existing voice telephone line. It's not uncommon to get 10 times the bandwidth on DSL that you would on ISDN BRI, and for less cost.

ISDN PRI is another matter. The PRI stands for Primary Rate Interface. This is the high bandwidth version of ISDN that consists of 23 Bearer channels of 64 Kbps each and 1 Delta channel that is also 64 Kbps. Those 24 channels are delivered on a specially configured T1 line called T1 PRI.

Like a regular T1 telephone line, the Bearer channels can be used to replace individual analog telephone lines. On a regular T1 line, all 24 channels are Bearer channels and signaling is done by "robbing" bits from the datastream to provide on-hook / off-hook signaling. Phone numbers are conveyed by DTMF touch-tones on the voice channels themselves. There is no room for other services such as Caller ID.

T1 PRI changes all that. The 64 Kbps Delta channel has plenty of capacity to handle all the signaling for the other 23 channels, plus Caller ID. A common use for a T1 PRI line is to consolidate up to 23 separate telephone lines plus Caller ID for PBX telephone systems used by business offices and call centers. A T1 PRI can be cost effective in replacing as few as 10 or 12 analog phone lines. A PRI interface board in the PBX system provides the digital interface.

Another popular use of ISDN over T1 is for dial-up Internet Service Providers. The 23 digital lines can each interface to one 56 Kbps analog modem in a modem bank located near the customers. The combined modem signals are then conveyed to the distant ISP over the T1 PRI line at full digital speed.

If your business has an application that is appropriate for ISDN PRI service, our technical consulting team is happy to assist you in finding the best value in T1 PRI service for your location. There is no cost or obligation for this service. Visit T1 Rex to request a complimentary T1 PRI quote now.

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