3Com Courier Reference

Contents

3Com Courier Reference Manual pdf 101 page image

Error Control 13-5Lower-speedV.ProtocolsOlder, lower-speed "V." protocols do not employ line probing. Insteadthey use predefined answer tones to specify or identify speed capabilities.These protocols define the following maximum speeds: V.32 terbo: 16.8 and 19.2 Kbps, with an additional USdevice-to-USR device speed of 21.6 Kbps. V.32 bis: 14.4 Kbps. V.32, V.22 bis, etc.: 9600 bps and lower.ASL (used in V.32 terbo and V.32 bis modes) is a technique that allows theCourier V.Everything modem’s receivers and transmitters to actindependently of each other. One transceiver may slow down and thenspeed up without affecting the data flow on the other. The result is moreefficient line operation.While most modems on the market now use higher speeds, there may bea problem in answering older, V.32 modems at 9600 bps. Use RegisterS28 to modify the duration of the extra tones used in V.32 negotiations,in the rare instance that this may be necessary. See Appendix A,S-Registers.Error Control Courier V.Everything modems can employ error-control techniques duringdata connections.High-speed data calls are vulnerable to errors unless the data is protecteby error control. If your Courier V.Everything modem connects with aremote device at a high speed, but without error control, and if you arenot using an error control protocol for your call, you may lose data.Automatic Repeat Request (ARQ) is a method used in many error-controlprotocols to ensure that any data that has been corrupted in transit isretransmitted. We use the term to designate a connection under errorcontrol.Error-ControlCommandsYou can use Error-Control commands to enable ARQ (error control) orenable synchronous protocols. The Courier V.Everything modem and theremote device must use the same protocol.