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Operation
V.42 bis Data Compression
V.42 bis data compression is only possible when the modem has established a V.42 (LAPM) error
correcting connection. V.42 bis compression generally gives improved throughput results versus
MNP 5 compression, particularly when transferring previously compressed files. V.42 bis only
compresses files where it can improve performance. MNP 5 attempts to compress every file, even
if doing so can't improve the throughput and results in degraded performance.
Many "bulletin boards" use a file compressor, such as PKZip, to store and send files. V.42 bis
provides greater efficiency than MNP 5 for such files. V.42 bis advantages are only available using
the V.42 (LAPM)/MNP reliable (\N5), V.42 (LAPM) reliable (\N4), or V.42 (LAPM)/MNP auto-
reliable (\N3) operating modes.
V.42 (LAPM)/MNP Auto-reliable Mode
When the type and status of the remote modem is not known, the modem can use V.42 (LAPM)/
MNP Auto-reliable Mode (\N3) to make an initial connection and then look at the remote's data
stream for incoming V.42 or MNP characters. If the appropriate characters are detected, the modem
establishes a reliable link. If the modem cannot establish a reliable link, it falls back to Wire Mode
or direct V.14 mode (see
The AT command \Cn determines what mode the modem will fall back to and how it will handle
data it receives before its mode is established. The handshake sequence that occurs when a modem
auto-answers a call in V.42 (LAPM)/MNP Auto-reliable Mode can have one of three outcomes:
•
success, resulting in a negotiated error correcting link
•
failure, resulting in a fallback to the mode specified by \Cn
•
reception of two successive fallback characters, if enabled by \Cn, resulting in an
immediate fallback to the specified mode. The %An command defines the modem's
fallback character. (Use of a fallback character permits the 4-second handshake sequence
to be cut short when there is no chance it will succeed.)
V.42/MNP Reliable Mode
The V.42/MNP reliable mode (
mode (\N3). The difference is that failure to establish an error correcting link will cause the modem
to disconnect rather than to fall back to a non-error correcting mode.
MNP-10 Mode for Cellular Phones
This mode (-Mn) enables or disables cellular phone mode. Mn enables or disables automatic power
levels, and @Mn sets the initial transmit power level for a cellular phone. When using cellular
phones, *Hn establishes the negotiation speed of two MNP-10 modems.
Note
MNP-10 mode for cellular phones will not be available in modems configured for AES Encryption.
Note
Refer to
Table 3-4
sion and error correction.
060R122-000
Issue 15
Non-Error Correcting Modes
) is very similar in performance to the V.42/MNP auto-reliable
\N5
for AT commands used to configure the modem for the different types of data compres-
SpectraComm Dual V.34 Modem
Installation and Operation
Asynchronous Operation
, below).
3-29
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