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Operation
MNP Reliable Mode Error Correction (Levels 2 - 4)
In MNP Reliable Mode (\N2), the modem will only connect with another modem that has an error
correction system capable of establishing an MNP reliable link. When the connection is made, the
modem immediately attempts to establish an MNP reliable link. If the attempt fails, the modem
disconnects. A failed attempt might be caused by:
a. Remote modem does not support an MNP reliable link.
b. Remote modem is an MNP modem, but is not in the proper mode.
c. Telephone connection is so poor and noisy that even an MNP reliable link connection would
not be able to assure dependable communications.
You may not want to use certain software error-correcting protocols with an MNP reliable link
because redundant error checking will reduce the link's file transfer efficiency. Some file transfer
protocols are designed for use with hardware error-correcting modems, such as IMODEM and
YMODEM-G. (ZMODEM has been shown to perform within a few percentage points of the other
protocols.)
When using MNP reliable mode, it is important that you know the remote system's capabilities and
status. If you call a non-MNP or V.42 modem while in MNP reliable mode, your connection will
fail. If you cannot obtain the appropriate information about the remote system, you should probably
use V.42 (LAPM)/MNP auto-reliable mode.
MNP Reliable Mode with Data Compression (Level 5)
MNP Level 5 provides error correction and data compression for improved throughput. Level 5
error correction provides synchronous framing with adaptive frame lengths. The sending modem
performs data compression by detecting redundant data and re-coding it into fewer bits. The
receiving modem decompresses the data before passing it to the receiving computer.
MNP Level 5 throughput is very sensitive to the data structure being transmitted. ASCII text files
will produce the greatest throughput gains, about 70% improvement in comparison to nominal VF
data rate. Files with .EXE, .COM and .ARC extensions may show as little as 60% of the
throughput benefits accorded ASCII files.
V.42 (LAPM) Reliable Mode Error Correction
The V.42 (LAPM) reliable error correction mode (\N4) is very similar to the MNP reliable mode
(\N2), except that, during the handshake, it first looks for the LAPM character. The modem
transmits a Link Access Protocol (LAPM) character as part of the handshake when it attempts to
negotiate a V.42 link. The V.42 protocol provides error correction compliant with the ITU-T V.42
error correction scheme and supports the ITU-T V.42 bis data compression algorithm.
In addition to its own error correction technique, the V.42 specification includes MNP-2 through
MNP-5 capability. It is backward compatible with the current GDC MNP-5 FEM. Since the V.42
protocol also includes the MNP protocols, if the V.42 (LAPM) connection attempt fails, the modem
will drop the call.
Due to increased complexities during the handshaking procedures, the V.42 (LAPM) reliable mode
is recommended for use only with other V.42 modems when data integrity is critical. All V.42/MNP
options use AT commands only for set up.
3-28
SpectraComm Dual V.34 Modem
Installation and Operation
Asynchronous Operation
060R122-000
Issue 15
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