127
2.048Mb
p
s Wireless Modem
TECHNICAL: 0118 931 2233
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.
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 10-5 or 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.