Multi-Tech Systems BA-Series manual Error Correction Commands

MultiModemBA User Guide

DSR/CD Interaction Control &SF

In typical applications, Data Set Ready (DSR) follows Carrier Detect (CD). &SF0 enables DSR to follow CD, and is the factory default. &SF1 enables DSR to operate independent of CD. If this is your application, refer to the &S command for control of the Data Set Ready signal.

4.4.8Error Correction Commands

You can use AT commands to place your modem one of three V.42 (error correction) modes of operation. These are the Normal, Auto-Reliable,and Reliable modes.

Normal Mode &E0

In Normal mode of operation, the modem’s V.42 error correction capabilities are disabled, and the modem functions as a non-error-correcting modem.

Auto-Reliable Mode &E1

In Auto-Reliable mode during the handshaking procedures at the start of the on-line connection, the modem automatically determines whether or not the modem with which it is communicating is using V.42 error correction. If the modem determines that the other modem is using V.42, it switches itself into MNP or LAPM Reliable (V.42) mode and its error-correction capability is enabled. If it is determined that the other modem is not using V.42, the modem remains in Normal mode, without error correction. The default setting is Auto-Reliable mode (&E1).

The method the modem uses to determine if the modem it calls is a V.42 modem involves the use of

a“Link Request”.

When the modem is in Auto-Reliable mode and originates a call, it goes through normal handshaking procedures just like any dial-up modem. After establishing the on-line connection, the modem transmits a Link Request message to the answering modem. (The Link Request message is generally about 33 coded characters.) If the answering modem replies with an appropriate V.42 acknowledgment response, the modem switches into Reliable mode. Otherwise it stays in Normal mode. This V.42 Link Request handshaking procedure generally takes about five seconds.

When operating in V.42 Reliable mode, the modem uses its memory, or buffer, to store data as it is received. During periods of error-caused retransmissions or compression slowdowns, this buffer may fill up. To prevent buffer overflow and subsequent loss of data, the modem uses "flow control" to signal the computer attached to its RS-232C port that the modem buffer is close to being full. This causes the computer to pause in its data transmission until the modem is able to empty its buffer sufficiently to accept more data, at which time the modem signals the computer that it may resume transmission.

Reliable Mode &E2

In Reliable mode, the modem uses its V.42 error correction capabilities during all transmissions, and must be connected to another modem with a similar protocol activated (MNP or LAP-M).

V.42 Mode Select #L

The V.42 standard implements both MNP Class 3 & 4 and LAP-M error correction methods. The V.42 Mode Select command (#L) selects which type of error correction (MNP or LAP-M) your modem uses for transmissions.

The various #L command options are as follows.

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