COURIER HIGH SPEED MODEMS
APPENDIX A. LINK NEGOTIATION (HANDSHAKING) AND ERROR CONTROL
This appendix includes information on how U.S. Robotics modems negotiate with remote modems for the rate and other characteristics of each connection. In addition, you'll find information on error control and, especially useful, some statistics and guidelines on using the modem for the best throughput.
Some of the following text includes the term ARQ, which means Automatic Repeat Request. ARQ is a method used in many error control protocols to ensure that any data that has been corrupted in transit is retransmitted. We use the term in our documentation to designate a connection under error control.
NOTE: High speed calls are highly vulnerable to errors unless the data is protected by error control. The V. protocol opera- tions described below take place even if one of the modems is not set for error control, thereby prohibiting error control for the call. If your modem connects with a modem at high speed but without error control, and if you are not using an error control protocol for your call, you may lose data.
Dual Standard modems in HST mode, in contrast, are unable to connect with other HST modems at higher than 2400 bps, unless error control is negotiated for the call.
V.34 HANDSHAKING
The Courier defaults to V.34 and tries for the highest possible speed when it attempts to connect with another modem, 28.8K bps. The entire V.FC range comprises 28.8K, 26.4K, 24K, 21.6K, 19.2K, 16.8K, 14.4K, 9600, 7200, 4800, and 2400 bps. If the remote modem is not V.34 capable, a connection is made using the highest compatible modulation scheme (V.FC, V.32 terbo, V.32 bis, and so on, down to as low as Bell 103, or 300 bps).
Link Negotiation and Error Control
