Black Box MWU2000-X21, MWU2000-G703 manual 127, Potential Problems in Fractional E1/T1 Applications

2.048Mbps Wireless Modem

Echo cancellers circuits are used in long distance and some wireless links to solve the echo problem. The operation of echo cancellers is based on estimation of the interfering echo signal that is added to the desired signal and subtraction of a similar signal, thus cancelling or significantly reducing the interfering echo signal.

Regular voice channels are used in many applications for modem and fax communications. These devices have a built-in special echo canceller, as the echo canceller designed for regular voice signal is inadequate for cancellation of modem/fax signals. Thus, the echo canceller used in a voice circuit should be disabled when the channel is used for transmission of voice or fax signals. To support identification of modem and fax signals by echo canceller circuits, these devices transmit a signal at 2100Hz. Upon detection of such a signal the echo canceller will be disabled for the duration of the presence of this signal. Some modem/fax devices use a 2100Hz signal with phase reversal, reducing the probability of identifying a voice signal as a 2100Hz tone. Another method for disabling echo cancellation is VPA – Voice Path Assurance, based on identification of a continuous tone in the 2000-2100Hz range.

Echo cancellation should not be used in non-voice channels, as it may interfere with proper reception of data or signalling information.

Potential Problems in Fractional E1/T1 Applications

Echo cancellation should not be used in channels carrying data or signalling. This includes channel 0 in Fractional E1, channel 16 in Fractional E1 systems using CAS (Channel Associated Signalling) or any other channel(s) that may be used to transfer signalling information between both sides of the link. This may also be true for some channels that are used by voice-mail, auto-attendant or

other PBX systems.

In systems where fax equipment is being used, it should be verified that the tone disabling method selected in the modem matches the capabilities of the fax equipment.

Fax machines and some protocols used by specific PBX auxiliary systems (Voice-mail, auto- attendant, etc.) might be highly sensitive to errors. It is recommended to achieve a good link with a LIU BUFFER ERROR RATE of 10-5or better.

Fax machines and some protocols used by specific PBX auxiliary systems (Voice-mail, auto- attendant, etc.) might be highly sensitive to link delay. It is recommended to use a one-way delay that does not exceed 1 second.

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