Chapter Nine - ERROR CONTROL AND DATA COMPRESSION
ERROR CONTROL
Error Control keeps the modem data link error free by detecting and retransmitting the data in
error. ZyXEL modems support both MNP and V.42 error control protocols. 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 - the international telecommunications standard making
organization. V.42 supports both LAPM (Link Access Procedure for Modem), and MNP4. V.42
handshake will try an LAPM connection first, if not successful, it will try MNP4.
The error control (MNP4, LAPM) methods used on modem to modem connections are based on
techniques utilized by both modems. These techniques are explained below.
1. Asynchronous to Synchronous Conversion
Error control function can be applied only when the DTE (terminal or computer) to DCE (modem)
interface is asynchronous. If the interface is synchronous, then the hosts on both sides will do the
error control function. Modem converts asynchronous characters to synchronous data stream.
Error control protocol (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 U-1496
series is 256 bytes. The maximum number of outstanding blocks without acknowledgements 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 number. If it is the same as 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).
3. Automatic Re-transmission Request (ARQ) Error Correction
Once a data block is received error free, the receiving modem will acknowledge this block
immediately. The sending modem receives the acknowledgement and will check if any block(s)
is(are) not acknowledged. Let us assume 18 blocks were transferred and the first 14 blocks were
acknowledged. If no acknowledge for block 15 is received in a given time, this block must have
been damaged. Acknowledgments of blocks 16 through 18 are ignored. Blocks 15 through 18 are
resent.
4. ARQ Error Correction with Selective Reject
ZyXEL modems support V.42 error control with selective reject. Using this feature, the mode
only resends the data block(s) in error, not all the data blocks starting from the one in error (just
block 15 in above example). This saves retransmission time, greatly improving efficiency in high
error rate conditions.
Error control protocols can detect and recover almost 100% of errors to make an error-free
transmission.
