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10–1
Chapter 10
ERROR CONTROL AND DATA
COMPRESSION
Error Control
Error control keeps the modem data link error-free by detecting and retransmitting
erroneous data. ZyXEL modems support both MNP and V.42 error control proto-
cols. The MNP protocol was an industry de facto standard developed and licensed
by Microcom, Inc. ZyXEL modems support level 4 and 3 error control protocols,
commonly denoted as MNP4 and MNP3. V.42 is a newly developed standard by
CCITT (now called ITU-T) - the international telecommunications standard mak-
ing organization. V.42 supports both LAPM (Link Access Procedure for Modem)
and MNP4. A V.42 handshake will try an LAPM connection first, and if not suc-
cessful, it will try MNP4.
The error control (MNP4, LAPM) methods in modem to modem connections are
based on techniques utilized by both modems. They are explained below.
1) Asynchronous to Synchronous Conversion
An error control function can be applied only when the DTE (terminal or compu-
ter) to DCE (modem) interface is asynchronous. If the interface is synchronous,
then the hosts on both sides will do the error control function. The modem con-
verts asynchronous characters to a synchronous data stream. The error control pro-
tocol (LAPM or MNP4) removes the start and stop framing bits in the
asynchronous format and forms the data stream into blocks. Every block is framed
by a start flag (01111110) and an end flag (01111110). The maximum data block
size used in the 2864 series is 256 bytes. The maximum number of outstanding
blocks without acknowledgement is 31 for 128-byte blocks. These values are
adjustable according to the modem on the other side.
2) CRC (Cyclical Redundancy Check) Error Detection
At the end of every data block, a 16-bit number CRC, which is calculated through
a polynomial function, is sent. The receiving modem receives the block, calculates
its own CRC through the same polynomial function, then compares the numbers.
If it matches the received CRC, everything is alright. If not, an error(s) has occurred
somewhere in the block. The modem checks every block received for error(s).