To test the protocol, use two sound cards, line out on one to line in of the other. Use symbol rate between 2400 and 3200 baud to approximate telephone line conditions.
Modulation protocols:
Bell 103 (300bps)
v.21 (300bps)
v.22 (1200bps)
v.22bis (2400bps) – Patents on v.21/v.22/v.22bis expire 2008
v.23 (1200bps/75bps half duplex)
v.32 (9600bps)
v.32bis (14.4kbps)
HST (9600bps, 14.4kbps, 16.8kbps, 21kbps, 24kbps, proprietary)
v.32terbo (19.2kbps/12.0kbps, proprietary), 19.2ZYX (19.2kbps, proprietary)
v.FC (28.8kbps, proprietary)
v.34 (28.8kbps)
v.34bis/v.34plus (33.6kbps)
K56Flex (56kbps PCM downstream, v.34+ upstream)
X2 (56kbps PCM downstream, v.34+ upstream)
v.90 (56kbps PCM downstream, v.34+ upstream)
v.92 (56kbps, PCM downstream/upstream)
Error correction protocols:
MNP (2-4, 10)
v.42 (LAPM)
Data compression protocols:
MNP5 (RLE, Huffman), MNP7
v.42bis (BTLZ)
v.44 (LZJH)
Handshaking protocols:
MNP6,MNP9 (Universal Link Detection)
http://en.wikipedia.org/wiki/Fax
http://www.vocal.com/data_sheets/v44.html
http://www.vocal.com/data_sheets/v42.html
http://www.vocal.com/data_sheets/v22.html
http://www.algotron.com/pages/v42ds.html
http://www.floreatinc.com/modem_protocols/modem_protocols.html
http://www.3amsystems.com/wireline/hmo.htm
http://www.ncoretech.com/modem/algorithms.html
http://www-users.cs.york.ac.uk/~fisher/modembook/
http://www.linmodems.org/#linmodems