Owner`s manual
PN 88300150
4/3/01 46
5.1 Introduction
Your MT2834MR has intelligent features beyond those of the AT command set (described in Chapter 3 of
this manual). This chapter covers these high performance features and commands which provide error
correction, data compression and speed conversion capabilities. Error correction in your modem is via the
ITU-T V.42 standard. Data compression in the MT2834MR is either MNP Class 5 or ITU-T V.42bis. The
remainder of this chapter describes these features, and the commands to operate them, as well as
descriptions of related commands.
Error correction is incorporated via the ITU-T V.42 standard. V.42 actually uses two error correction
protocols, LAP-M and MNP Class 3 & 4. MNP Class 3 & 4 error correction emerged as the industry
standard among modem manufacturers over the past decade. It’s now in the public domain, and has
been implemented in dozens of modem brands that offer error correction with world wide installations in
the hundreds of thousands of units. LAP-M error correction is similar to MNP Class 3 & 4. They both
convert asynchronous data characters to a synchronous data stream.
The MT2834MR provides two types of data compression: V.42bis and MNP Class 5. V.42bis is newer
and it requires concurrent error correction using LAP-M. V.42bis is a very efficient data compression
technique that can provide up to a 4-to-1 compression, depending on the type of files transmitted. MNP
Class 5 data compression requires concurrent error correction using MNP Class 3 & 4. It is an older and
more established standard that offers data compression in the 2-to-1 range (also dependent on the type
of data).
The speed conversion feature allows the modem to operate at one speed over the telephone lines and
another speed at the RS232C serial port. This allows the computer or terminal to communicate with the
modem at a fixed speed of up to 115,200 bps, while the modem operates at various speeds up to 28,800
bps. This is vital if data compression is to be effective (your terminal or computer must present data to the
phone line at a higher speed than which the modem is sending it over the phone line).
This chapter also covers commands that control error correction, turn data compression on and off and
others that are related to high performance operation along with the changes in the result codes.
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 own 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 were any errors. If there were, the receiving modem tells the sending
modem to resend 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 which is a function of all the data sent in a particular “message” or “block”, and then sends
those sixteen bits at the end of the block. (The “block” can include up to 256 characters.) The “receiving”
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.