Silicon Laboratories SI2493/57/34/15/04, SI2494/39 Data Compression, Error Correction, Wire Mode

AN93

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.

Table 83. Enabling Error Correction/Data Compression

To Enable	Use AT Commands
V.44*	+DS44 (argument)
V.42bis	\N3 and %C1 (default)
V.42 (LAPM)
MNP5
MNP2–4
Wire
V.42 and	\N4 and %C1
V.42bis only
V.42 only	\N4 and %C0
MNP2-4 only	\N2 and %C0
MNP2-5 only	\N2 and %C1
No data compression and	\N0 and %C0
no error correction

*Note: V.44 is available only on Si2493.

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.

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