Silicon Laboratories SI2493/57/34/15/04 - Manual (Page 124)

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5.10. Data Compression
The modem can achieve DTE (host-to-ISOmodem) speeds greater than the maximum DCE (modem-to-modem)
speed through the use of a data-compression protocol. The compression protocols available are the ITU-T V.44,
V.42bis, and MNP5 protocols. Data compression attempts to increase throughput by compressing the information
to be sent before actually sending it. The modem is thus able to transmit more data in a given period of time.
Table 83 details the ISOmodem error correction and data compression modes of operation.
5.11. Error Correction
The ISOmodem can employ error correction (reliable) protocols to ensure error-free delivery of data sent between
two modems. The error control methods are based on grouping data into frames with checksums determined by
the contents of each frame. The receiving modem checks the frames and sends acknowledgments to the
transmitting modem. When it detects a faulty frame, the receiving modem requests a retransmission. Frame length
varies according to the amount of data transmitted and the number of retransmissions requested from the opposite
end.
The ISOmodem supports V.42 and MNP2–4 error correction protocols. V.42 (LAPM) is most commonly used and is
enabled in the \N3 and \N4 modes. In the default mode (\N3), the ISOmodem attempts to connect with V.42 error
correction and V.42bis data compression (Si2457/34/15), and falls back to either V.42 only, MNP 2–5, or no error
correction (wire mode) if necessary. In \N4 mode, the ISOmodem hangs up if a V.42 connection cannot be
established. If the ISOmodem hangs up in V.42 mode after all data are successfully sent, the result code is OK. If
the modem hangs up before all data are successfully sent, the result code is NO CARRIER. If the modem connects
without a protocol, NO CARRIER is always sent.
The V.42 specification allows an alternate error correction protocol, MNP2-4. MNP2-4 is enabled in \N2 mode. In
\N2 mode, the ISOmodem hangs up if an MNP2, 3, or 4 connection cannot be established.
5.12. Wire Mode
Wire mode (\N0) is used to communicate with standard, non-error-correcting modems. When optioned with \N3,
the ISOmodem falls back to Wire mode if it fails in an attempt to negotiate a V.42 or MNP2-4 link with the remote
modem. Error correction and data compression are not active in wire mode.
Table 83. Enabling Error Correction/Data Compression
To Enable Use AT Commands
V.44*
V.42bis
V.42 (LAPM)
MNP5
MNP2–4
Wire
+DS44 (argument)
\N3 and %C1 (default)
V.42 and
V.42bis only
\N4 and %C1
V.42 only \N4 and %C0
MNP2-4 only \N2 and %C0
MNP2-5 only \N2 and %C1
No data compression and
no error correction
\N0 and %C0
*Note: V.44 is available only on Si2493.