Installing and Administering PPP

152 Appendix A
Modem Connections
Dial Up Modems
data transmitted, and MNP 7 offers 3:1 compression. CCITT-sanctioned
V.42bis can compress data 4 to 1 under the best conditions. If your
modem offers a choice between an MNP protocol and V.42bis, choose the
latter. In addition to having a better compression algorithm, it works
better with precompressed data streams.
Run the serial port at its maximum speed (usually 38400 bps) to gain the
most benefit from in-modem data compression. Some modem's data
compression adds significant latency to data throughput, adversely
affecting interactive responsiveness between local and remote users.
Experiment with your modem and the type of data you transmit to find
the best configuration.
PPP supports several other kinds of compression that work at different
layers of the communications stack. For more information on the
following types of compression supported by PPP, see pppd(1:
• HDLC Frame
• Address, Control and Protocol Fields
• Van Jacobson TCP Header
• PPP Link
• Predictor-1
Error Correction
In addition to data compression protocols, Microcom, Inc. developed error
correction protocols with similar names. Its MNP 4 is one of two major
protocols for error correction. The other is Link Access Procedure for
Modems (LAPM). Many modems support either one of the two protocols
or both protocols. Some manufacturers have developed their own
proprietary error control protocols, but the CCITT V.42 standard
requires that a modem support MNP4 and/or LAPM to be compliant or
compatible with the standard. The receiving and transmitting modems
negotiate to discover the highest level of error correction protocol each
can support.
Error correction verifies data that has traveled across the
communications connection. Noisy lines destroy data in transmission,
especially when the transmission is high speed. Depending on the
protocol, bits, bytes or packets are subjected to some form of cyclical
redundancy check to ensure the integrity of the received data. The
receiving end informs the transmitting end when the data has been