What: ISDN Telephone Hybrid
When: May 15, 1999 8:00 AM

Our guest speaker in May was Greg Shay of Telos Systems. He discussed the new Telos line of ISDN equipment, which was recently introduced at the NAB in Las Vegas.

Here is some background information about ISDN. A PRI is basically a T-1 with 24 - 64k channels of data. 23 channels are bearer channels, and one is for signaling. It is a 1.536 MBPS channel, and provides more options than a simple BRI (2B + D). Caller ID is available, and calls may even be transferred back to the CO under the right conditions.

Europe is almost 100% ISDN now. A European PRI is 2 MBPS carrier with 30 bearer channels, 1 data channel, and 1 Sync channel. When ordering equipment, be sure to keep in mind where the equipment will be used, because the European (S) interface is different than the American (U) interface.

When installing an ISDN PRI, if at all possible, have your circuit connected through the local SONET (Synchronous Optical Network). Having a Sonet terminal on premises in your building is most desirable. A SONET is just a huge dual redundant drop and insert ring for data. You only pay for the bandwidth you use, and the redundancy of the ring allows the system to be self-healing to a large extent. As long as only one of the redundant rings is broken at a time, the data will make it to its destination safely. Typically, the "last mile" - the connection from your equipment to the Sonet ring - is the most at risk of failure.

An ISDN line as designed by Bell Labs, is essentially an 8 bit, 8 kHz sample rate AES/EBU signal. While not totally accurate, that is the easiest way to approach audio via ISDN. The near end works out as a 4-wire analog circuit, but the far end is still typically an analog phone, where most of the crosstalk takes place. Eliminating the near end crosstalk improves the null by 6 dB simply because the last mile is ISDN.

The equipment introduced by Telos comes in various packages. The larger system, called the 2101, features a router at its heart (256 X 256 matrix), which will allow a great deal of flexibility in setting up and maintaining the system. Up to 256 connections can be made, but care must be used in counting. For example, the screener telephone requires a single connection, while a hybrid requires two connections. With current hardware, a full system envisions up to 96 lines (4 T-1s), at least 24 channels of hybrids, and screener phones.

A middle-sized system is the 2X12. It incorporates 2 hybrids, and interfaces up to 6 BRI lines (12 calls). It is still in development, but should be available by the end of the year.

The smallest system uses a single BRI . It is simply called the ISDN hybrid, and is available now.

The capabilities and features are very similar to those found in the present analog equipment. "Mash conferencing" is still possible, although not recommended because each additional conferenced caller reduces the overall quality of the caller audio.

New and improved features include call screen manager software (System Producer), which allows control of the caller interface via TCP/IP from anywhere on the network, including the Internet.