ISDN Information:

Solve Your ISDN Problems

(This article first appeared in Radio World magazine)

Problem: Multiple Disconnects
ISDN equipment is digital and susceptible to power fluctuations, spikes and dips — just like personal computers. Power hits or dips can cause terminal adapters to disconnect an ISDN call. Power hits may also cause some codecs with built-in TAs to lose their parameters, requiring SPIDS and LD numbers to be reentered before ISDN will work.

All CPE should be connected to AC through a surge protector. Where available, a regulated power supply such as that used in a computer room or electronics facility can be used. A small UPS and battery also are recommended to attain clean power. Some UPS devices can regenerate 110V/60Hz that is helpful when overseas.

Problem: Call Not Completing
When provisioning the service, providers frequently configure one B-Channel for voice and data, and one for voice only. If both channels are not configured for data, the number will ring but will not pick up. Ensure that both B-Channels are provisioned for data, or voice and data.

Problem: Multiple ISDN Hang-ups
A faulty or intermittent ISDN card in the equipment can cause the problem. The local company can do diagnostics on the local ISDN card and replace the card to clear the problem. Checking SPIDS and doing a quick dial-up will not find the problem as hang-ups occur intermittently and the call will complete and work for a while. Many vendors now subscribe to a new technical reference from AT&T and the Vendors ISDN Association. The reference allows long-distance providers and customers to dial in to remote equipment from a central site and "read" several days' worth of ISDN call history. As ISDN is an "intelligent" service, the conversations that occur between the local switch and the remote equipment often can reveal the cause of intermittent problems.

Problem: Cannot Dial Long Distance
If the long-distance carrier PIC is not specified during installation, ISDN may not be optioned for long distance. The installer usually checks the continuity of a newly installed ISDN only locally. Until the switch is reprogrammed, customers can use the long-distance service provider's seven-digit dial code (10-10-288 for AT&T).

Problem: Audio in the United States
In the United States, do not forget the NT-1. Audio via ISDN is a three-step setup: codec, terminal adapter and NT-1. External TAs come with or without NT-1 built-in. NT-1 is not required in Europe because it must be supplied by the local telephone company.

Problem: Audio Dropouts
Most codecs have internal jumpers for in/out levels (bridge/terminating). If jumpers are not set for the application, overdrive can occur in the front end of the CODEC. This can cause audio dropouts on peaks. In some cases this can cause termination of the ISDN call. Peak limiting prior to codec input is a good idea for sports remotes and other applications where audio levels change rapidly.

Problem: Show Sounds Like a Tape on Slow Speed
When using Layer III, check the sample rate. Layer II and III work at different sample rates (24 kHz, 32 kHz, 48 kHz and others). In Layer II and III processing, the higher the sample rate, the lower the processing/delay time. If sample rate settings on send/receive codecs are not the same, codecs often will not frame properly in Layer III mode. Protocol for Layer II avoids this because, unlike Layer III, the sample rate is part of the header.

Problem: Codec is not framing
If a G.722 codec is fed with a tone prior to establishing an ISDN connection, the call will complete. However, the far-end codec will not frame for a while, or until the tone is taken down. G.722 uses ADPCM coding and only sends out the difference of the previous sample. Since the tone has no change in waveform, the far end codec has nothing to which to reference; hence, it will not frame. When the tone is terminated, the codec will frame.

Problem: Central Office not Recognizing Terminal Equipment
Usually, equipment requires a four-digit SPID. However, local companies may only provide a two-digit code. Adding two digits to the end of the SPID (typically "00" or "01") can resolve the problem. Also, remember to program the directory numbers in addition to the SPID.

ABC Satellite Services can be reached at (212) 456-5801