This was from a response to someone in the USA who was reporting that V.34
was failing (in fact his USR Courier was failing to connect), yet V.32bis
worked.

If your return loss is too low, which on voice calls sound like a "hollow
pipe" effect, V.32bis may work and V.34 can fail.

Return Loss     Protocol Supported
6-8 dB poor     V.22 or V.22bis
10 dB           V.32
15 dB           V.32bis
20 dB           V.34
25 dB excellent

The crazy think is that the USR Courier V.34 can fail to train on such lines.

Also, V.32bis modems generally have a higher transmit level by a couple of
dB than V.34 modems which make them slightly more immune to low return loss
problems.

The solution is to get the hybrids at the exchange or SLC [RIM, RCM] balanced.

This information is from Frank Nitzsche of Telstra who can be contacted
by voice at +61 2 9396 3053 or fax +61 8 9264 2296.

[end quote]

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