The Good, The Bad, The Ugly - Choosing a Modem

Hi there !

Welcome to my new column, where I will be addressing various computing issues that concern the end-user, i.e., you.

This month, I begin with the first of a series of articles dealing with modem communications.

A Tale of Bargains

Sometime back, I went to see a friend who was having a bad time. Let me tell you all about it:

This friend recently went on a trip to the Far East. Being a computer buff, always ready to go in for "the latest," this chappie went in for a modem. "After all, with I-NET and other communication services coming up in India, a modem is a useful thing to have" was his argument.

Unfortunately, knowing nothing about modems, he went the classic route - he bought what seemed a really good bargain. "After all, all modems work the same way" says he.

Famous last words.

When he hit town, he connected everything up as shown in the three and half pages of documentation that accompanied the modem, fired up the pirated communication package that he got with it, and tried to log into my BBS (Bulletin Board Service).

Next morning, I looked at the activity log on my BBS machine, and saw a chaos of failed transfers and junked messages recorded there. I immediately got on the phone to my friend to check what had happened.

"I got all sorts of junk on my screen, and everytime I tried to transfer a file, the transfer got aborted because of too many errors" says my bargain-hunting friend. I told him I'd be right over to see what was the problem.

Reaching the scene of the crime, I flipped through the pages of the manual (which didn't take too much time, considering its size), trying to find out how to enable the modem's error correcting feature. It had to be there somewhere . . .

It wasn't.

My friend had spent his money on a cheap, non error correcting modem. Something akin to buying a bullock cart for the German Autobahn. What had seemed a bargain was in fact a piece of junk.

Cross Connections

If there is one thing that is consistant about India's telephone system, it is the terrible quality of the lines. Often, a call to a local number feels like a long-distance call to the moon - via Pluto. Cross-connections, crosstalk, clicks, buzzes and beeps are part and parcel of any phone conversation.

Now consider how a modem works. What it does is to take your computer's digital signal, and turn it into Audio tones, which can be transmitted over the phone line. The receiving modem turns these tones back into digital signals for the receiving computer's listening pleasure.

Great in theory. Now, just to provide a bit of entertainment to the process, add a heavy dose of "Indian Telephone Line".

Fireworks ! Special Effects ! Chaos ! Now you know where the director of the movie "F/X" went for inspiration.

Since the receiving modem tries to make sense of every blip, beep and squawk it hears on the line, the noise on the line drives it plain batty.

This is what was happening to my friend's first attempt at datacomm. Since his modem lacked a way to distinguish between signal and noise, what he got on his screen was his modem's interpretation of the "symphony" it heard on the phone line.

The fact is - Indian telephone lines are completely unsuitable for unprotected computer conversations. (Actually, I have felt that way about some human phone conversations, too). The line noise caused by outdated crossbar exchanges and rotten trunk cables makes the Indian telephone system a place they send little modems to if they have been bad boys.

So how does one "protect" a datalink from line noise ?

Error Correcting Modems

Picture this :

You are talking to a friend and are giving him a phone number he wanted. Line noise being what it is, he doesn't hear you clearly, meaning that he doesn't get the phone number.

So he says "hold it, I didn't get that clearly, can you repeat that please ?" and you repeat the number until he gets it perfectly.

That is how error correcting modems work. They "packetise" the data being transferred, and add some more data to it that allows the receiving modem to decide whether the data packet is "clean" or has been "hit" by line-noise. If it detects such an error, it asks the sending modem to retransmit the data packet until it gets through clean. This process is completely transparent to the user.

Error detection and correction is a basic requirement if you plan to use a modem on Indian telephone lines. This feature comes in two flavours - MNP and V.42.

MNP is the grand-daddy of error-correcting schemes, and was developed by modem manufacturer Microcom (therefore the acronym MNP - Microcom Network Protocol). Almost every error correcting modem produced today incorporates this scheme.

V.42 is the CCITT's version of almost the same thing. The CCITT is the international body that decides what is a standard in communications, and what is not. It tags these weird names to their recommendations (such as V.42) that in itself tells you nothing except that it is official.

Anyway, MNP and V.42 are two schemes used for error correcting modems, and they ensure that they can communicate with other modems speaking the same lingo without errors. The two are compatible, and will talk to each other.

But error-correcting modems don't stop at that. They have another function - data compression.

Effectively, what this means is that the modem collects a packet of data, compresses it, then sends it on its way. The receiving modem decompresses the packet and passes it on to the computer. Since it takes less time to send a compressed packet of data, the effective throughput of data between the modems is increased.

MNP has the MNP5 compression standard (MNP4 is the error correcting part of the protocol), where as V.42 has V.42bis. Unlike the error correcting parts of the protocols, MNP5 and V.42bis don't talk to each other.

This is also where the old gives way to the new. MNP5 has a maximum compression ratio of 2:1, meaning that the effective throughput of a MNP5 modem running at 2400 bps (bits per second) is at the most 4800 bps.

V.42bis, on the other hand, compresses things down to 4:1, giving an effective throughput of 9600 bps.

The two also differ in how they treat pre-compressed data, such as the contents of a ZIP file. Whereas MNP5 tries to compress every packet of data it gets, thereby wasting time on data that cannot be compressed further, V.42bis recognises the pre-compressed state of the data, and simply sends it on its way without any further waste of time.

Most V.42bis modems also incorporate MNP5, so that if you are calling a MNP5 modem with your V.42bis modem, it will step down to the older protocol. Naturally, the same is not true the other way round.

By now you must have guessed that my friend's modem had neither MNP nor V.42 error correction - to say nothing about compression.

Modem Vendor Jargon - Tricks and Traps

"This modem has dynamic signal balancing circuitry" says Vendor X, trying to convince me that his modem is just what I needed. When I asked him about error correction, he slunk away.

"This modem is rated for 2400 baud - meaning that you can transfer 2400 bytes in one second !" says Vendor Y. Surprising, I said. I have always thought that "bps" stands for "bits per second", and that it takes 10 bits to transfer one 8 bit byte, meaning that 2400 bps equals 240 bytes a second. The vendor quickly became a dot on the horizon, fading out.

"This modem works at 1200 bps. Since Indian telephone lines do not support faster speeds, this saves you money because you don't spend on expensive 2400 bps capabilities that are useless to you" warbles Vendor Z. I just cast a pitying smile on the poor animal, while checking on the progress of the 9600 bps file transfer from Bombay on the V.32/V.42bis modem standing on my desk.

"An internal card modem is much more productive than an external modem" says yet another aspirant. When I eagerly asked him to prove his point by attaching the modem to my Notebook computer (on which I am writing this and which does not have any expansion slots), and then move it to my other machines all within a minute, he politely wished me good day.

"This modem is imported from the USA, and is better than anything available in the Indian market" shimmers the heavily accented salesman. "Great !" I say, "naturally it also comes with the Department of Telecommunications License that allows me to use it on P&T lines without having my phone disconnected . . ."

Where did he go? Probably left before I could ask him about servicing of his modem in India in case of problems.

"This 2400 bps modem actually operates at 9600 bps" gloats another. "Oh Gee !" smiles his tormentor, "let's try that with this heavily compressed ZIP file". Is that a bird ? Is it a plane ? Nope, it is just another jargoning salesman beating a hasty retreat.

I think the savvy reader is beginning to see the point I am trying to make. Modems are new to most people, and many modem vendors and their salesmen will try to confuse you with technical jargon that is designed to cover the obvious flaws of the merchandise, such as the lack of Error Correction.

I think this is highly irresponsible, and whereever possible try to get such vendors blacklisted by my clients. To help my clients decide what is genuine and what is not, here is a set of questions that I ask them to place to the vendor (some vendors call them "Atul's Dirty Dozen") :

1. Does the modem have error correcting facilities ? (The answer better be "YES")

2. If yes, what is the error correction scheme that is used ? (Should be MNP or V.42)

3. Does the modem have compression ? (Ideally, should be "YES")

4. If yes, what scheme does it use (Should be MNP5 or V.42bis. Some vendors use MNP7 - a protocol that was Dead on Arrival and is not used anywhere in the world)

5. What is the modem's true speed ? (should be 2400, 9600 or 14400 bps, with four times that if using compression)

6. Does the modem have the D.O.T. license for use on the Indian P&T lines ? (Must be "YES", or you may have your phone disconnected)

7. Is the modem an internal or external model ? (An eternal model has many advantages over an internal one, such as easy mobility, status lights, a voice/data switch, independant power supply, audible speaker, etc.)

8. Does it come with any licensed communication software ? (The answer must be "YES" - modems are useless without communication software, and the software must be legal - piracy is a crime)

9. Does the modem come with any written warranty of at least one year? (If no, you can expect it to fail two days after the invoice is paid)

10. Is the modem made in India ? (Unlike many other electronic devices, Indian modems made by reputable manuacturers tend to perform better under Indian conditions than imported ones, and have the advantage of local support in case of problems. Besides, I am pretty patriotic)

11. Is a comprehensive manual included with the modem ? (You'll need one to optimise performance and adapt to different situations)

12. Is a serial cable included with the external modem ? (Many vendors try to make this a hidden cost for which they charge you later. Make sure that this is not so)

Get the answers to these questions from the vendor in writing. If possible, ask for a list of references, such as at least five other customers. Check with them about the vendor's claims of performance and support. If more than 20% of them have complaints, you can expect to join their ranks soon.

Also make sure that the support personnel knows their business. Ask someone from the company to show you how to use the modem, and ask him to demonstrate a file transfer between two of his modems, as well as between his modem and that of another make. If he cannot demonstrate a file transfer, or resorts to the Kermit protocol (a very slow file transfer protocol that places the least strain on the communication link - more on this in a later article), then you cannot expect much support from the vendor when you have problems.

Finally, make sure that you test the modem yourself for at least two days, preferably in the presence of a friend who knows something about modem communications. I have often seen situations where someone blames the bad performance of the modem on his lack of knowledge about such things. This is rarely true.

If you expect the modem to act as a Host Modem that receives calls 24 hours a day all round the year, do not buy the modem if no customer can report long-run performance of the modem. Many modems are likely to fail under such conditions because of component degradation due to heat and power fluctuations.

Winding down

Hey ! Don't tell me I am out of space already ? I don't like that evil gleam in my editor's eye as he advances with his dreaded pair of scissors.

Anyway, to close this piece - my friend bit the bullet and bought a modem locally according to my guidelines. I am happy to report that he not only happily communicates all over the world with his new modem, but is also busy advising people on how to buy a modem.

Next month, I am going to get a bit technical by talking about things like Hardware Flow Control and how it plays an important role in high speed communications. So stay tuned !