Black Box MD1640A, MD1641A manual Normal Mode &E0, Auto-Reliable Mode &E1

CHAPTER 5: Command Mode

Normal Mode &E0

In Normal mode of operation, the Series II Modem’s V.42 error correc- tion 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 Series II 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 deter- mined 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 Series II 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 Series II 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 RS232C port that the modem buffer is close to being full. This causes the com- puter 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.)

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