Chapter 4 - Advanced Data Communications
4-6
error control protocols, commonly denoted as MNP4 and MNP3. V.42 is a
standard developed by ITU-T. 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 successful, 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.
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 all right. If not, an error(s)
has occurred somewhere in the block. The modem checks every block received
for error(s).
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 acknowledgment 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
acknowledgment for block 15 is received in a given time, this block must have
been damaged. Acknowledgments of blocks 16 through 18 are ignored. Blocks
starting from 15 are resent.
ARQ Error Correction with Selective Reject
ZyXEL modems support V.42 error control with selective reject. Using this
feature, the modem only re-sends the erroneous data block(s), not all the data
blocks starting from the one in error (just block 15 in the above example). This
saves retransmission time and greatly improves efficiency in high error rate
situations.
Data Compression
The Omni 288S supports both V.42bis and MNP5 data compression protocols.
Data compression works by representing the original data information in less bits
and transmitting the reduced data bits through the data link. The receiver recovers
the original information by reversing the representing process. The process of
representing original data in less bits is called redundancy removal.
Data compression needs an error-free data link to work correctly. MNP5 is used
with MNP4 error control and V.42bis is used with V.42 error control.
MNP5 data compression utilizes the run-length encoding and adaptive frequency
encoding techniques. V.42bis uses a string coding algorithm.
