Filter
Top Products
71
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.)