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Chapter 5 - Command Mode
5.4.8 Error 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. (Flow Control is discussed in Section 5.4.9).
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).