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Chapter 5 - Error Correction, Data Compression, and Speed Conversion
5.6 Modes of Operation
You can configure your MT2834MR6 in one of three different V.42 modes of operation, each with or without
compression. They are the non-error correction, reliable, and autoreliable modes. The modes and data
compression features are selected with commands described later in this chapter in “Compression, Error
Correction, Flow Control, Pass Through, and Pacing Commands.”
5.6.1 Non-Error Correction Mode (
&E0
)
In the non-error correction mode of operation, the MT2834MR6’s error correction capabilities are disabled,
and the modem functions as a non-error-correcting modem.
5.6.2 Reliable Mode (
&E2
)
In reliable mode, the MT2834MR6 uses its V.42 error correction capabilities during all transmissions. When
in reliable mode, the MT2834MR6 must be connected to a modem with a similar V.42 protocol activated
(MNP or LAPM). If the modems cannot establish a V.42 error correction connection, the MT2834MR6 will time
out and hang up.
5.6.3 Autoreliable Mode (
&E1
)
In autoreliable mode, the MT2834MR6 automatically determines, during the handshake at the start of the
online connection, whether or not the other modem is using error correction. If the MT2834MR6 determines
that the other modem is using V.42, it switches itself into the reliable (V.42) mode of operation. If it determines
that the other modem is not using V.42, the MT2834MR6 remains in non-error correction mode.
The MT2834MR6 uses a “Link Request” to determine whether the other modem is using V.42 error
correction. When the MT2834MR6 is in autoreliable mode and originates a call, it goes through normal
handshaking procedures. After establishing the online connection, the MT2834MR6 transmits a Link Request
message to the answering modem. If the answering modem replies with an appropriate V.42
acknowledgment response, the MT2834MR6 switches into reliable mode. Otherwise, it remains in non-error
correction mode. This V.42 handshake procedure generally takes about five seconds.
When operating in V.42 reliable mode, the MT2834MR6 stores data in a buffer as it is received. During
periods of error-caused retranmissions 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-
232 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.