Table of Contents
Advertisement
Chapter 5 - Error Correction, Data Compression, and Speed Conversion
5.2
How V.42 Detects and Corrects Errors
Some of the better known software-based error-correction protocols include Xmodem and Kermit (for
asynchronous file transfer software), X.PC (Tymnet's asynchronous software protocol), and SDLC and HDLC,
two popular synchronous protocols common in the IBM mainframe environment. V.42 is functionally similar to
SDLC and HDLC, with some extras.
The main advantage of hardware-based V.42 error correction over these software-based protocols is in
throughput. Throughput is the effect that the use of the protocol has on the overall data rate. For example,
V.42 transmissions using a 2400 bps modem have an effective throughput of about 2600 bps. The same
modem using software-based X.PC would have an effective throughput of less than 2400 bps. Another way
of stating it is that V.42 has an efficiency of about 108%, while X.PC has an efficiency of about 91%.
Simply stated, one modem with V.42 sends coded data to another modem with V.42, and the receiving
modem is able to determine if there are any errors. If there are, the receiving modem tells the sending modem
to re-send the errant data until it's correct.
Technically speaking, when using V.42, the sending modem uses a polynomial function to calculate a 16-bit
number that is a function of all the data sent in a particular message or block. It then sends those sixteen bits
at the end of the block, which can include up to 256 characters. The other V.42 modem, as it is receiving the
block, calculates its own version of the 16-bit number. Then it compares its number with the 16-bit number
sent with the block. If the numbers are the same, the block is free from errors. If the numbers are different, an
error has occurred somewhere in the block. That's how errors are detected .
Once an error is detected, the receiving modem's V.42 error correction is activated.
For all practical purposes, the result of the V.42 error correction protocol is error-free transmission. Using the
16-bit redundancy check, it will detect every error which is 16 bits or smaller with 100% probability. As a
result, the chances of an error occurring are actually so small that you can ignore them.
67
Advertisement
Table of Contents
