Modems Revealed
By Grant Kwai
Copyright (c) 1992 Apple Users' Group, Sydney
Republished from Applecations, a publication of the Apple Users' Group, Sydney, Australia.
It has been two years since my last article explaining how and what a modem is. As with the rest of the computer related industry, much has changed in that time. I hope to bring you up to date with current trends in this telecommunication field.
The term 'modem' is short for MODulator/DEModulator. A modem is a device which allows you to transfer programs or messages from one computer to another using your standard telephone line.
Speed
Like all else in our society, the ability to go faster and faster has been carried onto all walks of life. Modems have also been along for this ride.
Way back many, many years ago (in the early 80's) a modem operated at 300 baud. This basically meant that it could transmit and receive data at about 30 characters per second. A few years later, 1200 baud arrived on the scene. This later rose to 2400 baud. Today, 9600 baud is about to become the 'norm'.
If you had ever looked at a modem or perhaps inquired about one to a salesperson, they might give you some mumbo-jumbo about such things as V21, V22bis and V42bis. Here is an explanation as to what all this means.
First of all, it may be best to inform you there are two 'standards'. They are the CCITT (Consultative Committee on International Telegraphy and Telephony) and BELL. The latter plays little part these days. The Bell standard is an American system and only relates to 300 and 1200 speeds.
CCITT is now the recognised standard for high speed telecommunications. This means we can now 'speak' to other computers in any country in the world.
Like car engines, things are normally categorised into things like V6, or V8 engines, twin cam etc. For modems, similar naming applies.
The CCITT standard can be recognised on modems by their Vxx designation (where xx represents a number). Sometimes at the end of these numbers is the word BIS. It is easiest to think of this as yet another protocol.
V21 - You will rarely find a need for this protocol. V21 refers to the 300 baud speed. Only a handful of institutions and bulletin board have this as their maximum speed. Unless you want a collectors item, you are wasting your money buying a 300 baud maximum speed modem.
V22 - Though not as rare as V21, V22 is the 1200 baud rate. Most modems you will find have this transmission protocol. If 1200 baud is the maximum baud rate of the modem, you would be wise to buy one which is faster.
V22bis - The most popular protocol in use today. It is represents 2400 baud, and is currently the 'standard' baud rate of the majority of bulletin board systems around today. However, no doubt in a couple of years time, it too will become outdated. If you want to enter the world of modems, this is still the cheapest and best entry level to start at.
V23 - Though you may think this is faster again, it is actually slower. V23 is 1200/75 baud. That is, it has a split board rate. Very few places use this and is just about defunct.
To understand the following protocol it might be useful to give you this piece of information. The modulating technique employed by 2400 baud modems is to transmit at four bits per baud, at 600 baud, or bits per second (4 x 600 = 2400).
V32 - Is a data transmission protocol. It is a 4800 baud and 9600 baud standard using a method called TCQAM (Trellis Coded Quadrature Amplitude Modulation). To achieve such a high baud rate, TCQAM encodes two or four bits per baud, at 2400 baud (hence 4800 and 9600 baud). This was the standard for high speed transmission before V32bis.
V32bis - This is the latest transmission protocol from CCITT. V32bis is a 14400 baud full duplex protocol, using six bits per baud, at 2400 baud. The main drawback with going so fast over a standard phone line is that line noise can cause severe problems. I'm sure that many times you have had a 'crossed line' while talking on the phone. Imagine what this could do with a modem transmission.
V32bis is more intelligent than other protocols. If line noise is a problem at its maximum speed, it has the ability to slow down its transmission rate to achieve the optimum rate of transferral.
It should be noted that Australia is regarded as having one of the 'cleanest' phone lines in the world.
Error Correction and Compression
The remaining protocols seemed to have cause much confusion amongst those who own modems, let alone those who are thinking of purchasing one. I hope to clear some of this confusion below.
Most modem owners will generally think that Vxx implies the speed of a modem. This WAS true for the above protocols, however it is no longer the case. V42 and V42bis are not related to an actual speed, but rather the way it can speed one of the preceding protocols up.
V42 - This is an ERROR CORRECTION protocol. It uses a technique called LAP-M (Link Access Protocol for Modems). As stated above, this protocol relates to the data being correctly received and sent, not to the actual speed of the transmission. Nevertheless, a speed increase is possible.
A 2400 baud modem can theoretically transmit at 240 characters per second(cps). Practically though, this figure is more like 232 cps. When a modem sends these characters, each consists of an 8 bit character, plus a start and stop bit. Hence, a total of 10 bits per character is sent. What the V42 correction protocol does is strip off these start and stop bits, effectively reducing the transmission size by 20%. Thus in theory a 20% increase in speed is possible, but in reality it is more like 15%. Therefore a speed of about 270 cps at 2400 baud is achievable.
V42bis - This is a data COMPRESSION standard. As this suggests, it 'squeezes' the packet of data into a smaller one via a compression algorithm. By doing so, an increase in speed is naturally achieved. The compression technique used by V42bis is called Limpel-Ziv encoding. A compression ratio of 4:1 is claimed on an uncompressed
ASCII text file (a normal text file, which doesn't contain any formatting commands for programs). However in reality (e.g with program files) it is more like 2 or 3 to 1. Assuming a 4:1 compression ratio, this, in theory means a V32 or 9600 baud modem can effectively transmit at a staggering 19200 baud.
The one thing they fail to mention with V42bis (and MNP 5 -see below) is that most files on BBS systems are already compressed. Long before these new standards were around, the best and quickest way of transmitting a file was to compress whatever you want to send as much as possible before you actually sent it.
These compressed files are normally smaller than what V42bis could achieve anyway. The reason for this is V42bis must compress the data 'on the fly', hence a fast algorithm is required. External compression programs on the other hand aim at creating the smallest file possible.
It should be noted however that V42bis is intelligent in some ways. It can sense if a file is already compressed and turn itself 'off'. What this basically means is if you are transferring a compressed file, V42bis will have no effect. Hence, the only real advantage of V42bis is in transferring straight text (e.g. if your main interest lies in message writing and/or reading) or uncompressed files.
Other Protocols
The only other common protocols for modems are developed by a company called Microcom Inc. Their standards have the identity of MNP (Microcom Network Protocol) followed by a number between 2 and 5 inclusive.
You will most likely find that most high speed modems offer MNP and CCITT protocols.
We can break the MNP protocols into two groups, error correcting and compression. MNP 2-4 are error correcting protocols, whereas MNP 5 is a compression protocol.
A modem with MNP level 4 also has MNP 2-3 protocols built in. Being error correction protocols, they operate much in the same way as V42.
You will find modems that offer V42 and V42bis also offer MNP. MNP is used as a fall-back in case one of the CCITT protocols is not present on both modems. By doing so, at least some decrease in transferral time is attained.
MNP level 5 on the other hand is a compression protocol like V42bis. It uses an encoding technique called Run Length Encoding. It is not quite as good as V42bis, and quoted as having a compression ration of about 2:1.
Again, this is a theoretical result. MNP is used as a fall-back for the V42bis protocol.
It should be noted that MNP level 5 is 'dumb' That is, it will try to compress everything it sends. Thus it would be good if you are transmitting uncompressed files. On the other hand, if the file you are transmitting is already compressed, then it can take longer than normal.
Modems which offer MNP level 5 will also offer MNP 2-4. Hence you will have a modem which has both error correction and compression qualities.
Software Protocols
All of the above protocols have to do with the actual hardware of the modem. Before the advent of these protocols, people came up with ways of checking for errors using software.
You will hear the terms Xmodem, Ymodem, Zmodem, Kermit plus a host of others. Zmodem is by far the best of these, with Kermit mainly being used on mainframes and networks.
Each of these offers some sort of error correction. They use a CRC (Cyclical Redundancy Check).
XMODEM was the first file transfer protocol introduced in 1977 by Ward Christensen.(under the CP/M environment). It used a 1 byte checksum.
YMODEM was Xmodem-CRC(improvement on Xmodem by offering 2 byte CRC) with batch ability. That is, it was capable of sending several files in secession without user input.
ZMODEM uses a 16 or 32 bit CRC, giving better error correction. An additional features is automatic 'crash recovery'. This means if you are suddenly disconnected midway through transferring a file, you can continue where you left off when you try and download (receive) that file again. This was not possible under earlier protocols.
That just about sums up all you will need to know (plus more!) about modems. I hope that at least next time you see these terms in some advertisement or article, you will have an understanding of what it means.

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