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Chapter 5 - Error Correction, Data Compression, and Speed Conversion
5.13.4 Retransmit Count ($R)
If errors are received during a reliable connection, the modem resends the block of data that contained
errors. If another error occurs, the block is re-sent again. The modem counts the number of times that a data
block is re-sent. If the same block of data is re-sent 12 times and still has not been received properly, the
modem aborts the connection, as dictated by the AT$R0 command.
This retransmit counter can be disabled with the $R1 command. When the retransmit counter is disabled, the
modem keeps trying to send data and will not abort, not matter how many times the same block is re-sent.
Type AT$R0 to disconnect if the retransmit count is exceeded (factory default).
Type AT$R1 to not disconnect because of excessive retransmits.
5.13.5 V.42 Error Correction at 300 bps ($E)
At 300 bps, error correction is not normally used. $E1 lets the MT2834MR6 function in either non-error
correction ( &E0 ), autoreliable ( &E1 ) or reliable ( &E2 ) mode at 300 bps. $E0 , which is the factory default
setting, disables V.42 error correction at 300 bps altogether.
5.14 Summary
V.42 error correction is built into the MT2834MR6's hardware, and detects virtually 100 percent of
transmission errors, which are usually caused by noisy phone lines. When errors are detected, the V.42
protocol causes the modem to retransmit the errant data block.
V.42bis and MNP data compression, also built into the MT2834MR6's hardware, provides an effective
throughput higher than the modem's transmission speed. The amount of increase depends on the type of
data being transmitted.
The MNP protocol is divided into several classes, which for our purposes are numbered 3 through 5. The
MT2834MR6 uses all of these MNP classes, which yields higher throughput and error-free transmissions.
LAP-M provides another type of error correction.
The MT2834MR6 can be set for three different modes of operation. Non-error correction mode disables error
correction, reliable mode enables it, and autoreliable mode allows the modem to automatically turn V.42 on
when it detects its use in another modem.
Flow control methods are used by the MT2834MR6 to prevent data loss from buffer overflow. The modem can
both initiate it (flow control) and respond to it (pacing), using either XON/XOFF commands or RS-232
signaling via the serial port's CTS and RTS signals. Hewlett Packard ENQ/ACK pacing is also supported. If
needed, XON/XOFF commands can be "passed through" the modem to a remote computer or terminal.
Speed conversion allows the MT2834MR6 to operate at one speed over the telephone lines and another
speed at the RS-232 serial port, allowing the computer or terminal to communicate with the modem at a fixed
speed of up to 115,200 bps, while the modem operates at various speeds up to 33,600 bps. This ability is for
data compression, since your terminal or computer must present data to the modem at a higher speed than
the modem sends it over the phone line.
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