Classic Computer Magazine Archive COMPUTE! ISSUE 56 / JANUARY 1985 / PAGE 160


Arlan R. Levitan

Smokey & The Modem—Part 8086

I live in the greater Detroit area, a hotbed of muscle cars and micros. In this town it's hard to miss the fact that America's involvement with microcomputers shares a lot of overtones with its longstanding love affair with the automobile.

The image of T-shirted car enthusiasts discussing the displacement and horsepower of their chariot engines while Bruce Springsteen tunes play in the background comes readily to mind when you hear the name Motown. But in this and other towns, you're just as likely to find corporate and casual computer users congregating and speaking in reverent tones about the capacity of their hard disks and the cycle times of their central processing units.

Motor cars and micros. Both encourage a fascination with speed and power. And while General Motors, Ford, Chrysler, and American Motors are still fighting a pitched battle against foreign manufacturers for the hearts and minds of the car-buying public, IBM, Apple, Commodore, Tandy, and Atari are girding themselves for an expected onslaught of Japanese MSX-standard computers.

The marketing type who coined the term "power user" to describe personal computer owners who can't get enough memory or a fast enough CPU had a firm grip on the ego-related realities of the micro market. Reminding an avid power user that faster processors, massive mass storage, and megabyte memories don't necessarily let you write text or enter spreadsheet data twice as fast is about as fruitful as discussing the 55 mph speed limit with the owner of a 1967 Plymouth Road Runner running a Hemi-Head engine.

Souped-Up Modems

All this discussion of speed has a point. I've received a number of inquiries from readers about what kind of modem they should purchase. In particular, there seems to be a lot of interest in the new high-speed 2400 bits per second (bps) modems appearing on the scene.

The major factor which determines the price of a modem is the maximum speed at which it can send and receive data over the phone lines. A low-speed modem's top rate is 300 bps, equivalent to about 30 characters per second. These modems range in price from $50 to $280, depending on what other features are included. They're often referred to as Bell 103-compatible (Bell 103 is a phone company standard).

Bell 212-compatible modems can handle data transmissions at both 300 and 1200 bps. They used to cost $500 to $700, but recent developments in chip technology have allowed several manufacturers—notably Anchor Automation and Qubie Corporation—to break the $300 price barrier with full-featured 300/1200 bps modems. Industry projections indicate that by 1986, these medium-speed modems will dominate the consumer market and typically list for under $200.

The new kids on the block are the 2400 bps modems. Although they are twice as fast as 1200 bps units and operate on standard voice-grade phone lines, they also command a premium price ($800 to $1,500). Sometimes these 2400 bps modems are referred to as CCITT (Consultative Committee on International Telephony and Telegraphy) V.22 units—by those who own the Telecommunications Edition of Trivial Pursuit. Some 2400 bps units are also capable of 1200 and 300 bps transmission.

The terms high, medium, and low speed refer to transmissions over regular (voice-grade) telephone lines, the kind you have in your home. True high-speed transmissions aren't practical on these lines. Instead, specially prepared conditioned lines are required by businesses which transmit data at rates from 9600 to 57,600 bps. Both the conditioned lines and the high-speed modems are expensive and are limited to point-to-point transmissions. The line is permanently installed between two locations and cannot be used to access the regular telephone network. Of course, conditioned lines are out of the question for most of us.

Judging A Modem By Its Baud

You'll often see the term baud when reading about transmission speeds. Modems will be advertised as "1200 baud" or "2400 baud." But strictly speaking, this is an improper use of terminology. Baud (named after Georges Baudot, a telecommunications pioneer) is used to describe the division of each second into tiny, discrete pieces (also called signal modulation) by a modem's electronic circuitry.

A 300 bps modem's signal is indeed modulated at 300 baud. Since each tiny division holds one bit of data, the effective transmission rate is calculated as 300 baud per second times one bit per baud, or 300 bits per second (bps).

Things take a different turn with 1200 bps modems. You might expect each second to be divided into 1200 pieces. This is not the case. A 1200 bps modem actually divides each second into 600 pieces. Using a technique called four-level phase shift keying (psk for short), each piece can represent a string of two bits.

This isn't as complicated as it may seem. All it means is that by using a method that plays with the phase characteristics of the modem's signal, each baud can be in one of four binary phases, namely:

00 or 01 or 10 or 11

There you have it. Each baud can be in one of four phases, with each representing exactly two bits. Multiply 600 baud per second times two bits per baud and voila! You get 1200 bits of information per second (1200 bps).

Even More Bits Per Baud

Knowing this, it may come as no surprise to learn that 2400 bps modems also use a modulation rate of 600 baud. What is different is the method of phase shift keying. A 2400 bps modem uses a method that yields 16-level phase shift keying, so each piece or baud can represent a string of four bits:

0000 0001 0010 0011
0100 0101 0110 0111
1000 1001 1010 1011
1100 1101 1110 1111

So with a 2400 bps modem, each baud can be in one of 16 phases, with each representing exactly four bits. Multiply 600 baud per second times four bits per baud and we get (drumroll, please . . .) 2400 bits of information per second.

That's why you should avoid terms like 1200 baud and 2400 baud when describing modems. Both are actually 600 baud units which use clever schemes to pack more than one bit per baud. Use bits per second (bps) instead.

This information can really come in handy for small talk at user group parties; it's a lot more impressive to computer hobbyists than crushing a dozen aluminum beverage cans into your forehead.

Do You Need The Speed?

Under most transmission schemes in use today, it actually takes ten bits to send one character of data. Therefore, the approximate character transmission speeds of 300, 1200, and 2400 bps modems under optimal conditions are 30, 120, and 240 characters per second, respectively.

Is the extra cost of a medium- or high-speed modem a worthwhile investment for you? That depends on your telecomputing style.

Do you plan to make heavy use of commercial information services such as CompuServe, The Source, Delphi, or Dow Jones News/ Retrieval? Since none of the commercial services offers 2400 bps service yet, spending big bucks on a 2400 bps modem is not a good bet. Why don't they offer 2400 bps service? Because there has to be a 2400 bps modem on both ends of the connection—yours and theirs. Since very few people own 2400 bps modems right now, information services wouldn't get much return on their investment in 2400 bps equipment while the price of the new technology is relatively high.

Besides, medium-speed 1200 bps units offer a very good price/performance value. However, you must balance the shorter connect times made possible by faster modems against any surcharges imposed on the higher transmission rates.

Here's a quick example. Suppose Steven J. is a frequent user of the Just Folks Information Service. Steve calls only during the evening (referred to as non-prime time by the commercial information services) and spends about five hours a month on Just Folks with his 300 bps modem. Assume that Just Folks' hourly charges are $7.75/hour for 300 bps, non-prime time access; plus a $3/hour surcharge for 1200 bps, non-prime time access. Steve's yearly cost for accessing Just Folks at 300 bps is:

$7.75/hour * 5 hours/month * 12 months/year = $465

If Steve upgraded to a 1200 bps modem, he'd reduce his yearly cost to:

$10.75/hour * 1.25 hours/month * 12 months/year = $161.25

The money Steve saves in a year would pay for a brand-new 1200 bps modem!

The Point Of Diminishing Returns

Admittedly, this is an ideal case. It assumes that armed with a 1200 bps modem, Steve will stay on-line only one quarter of the time that he would with his 300 bps unit. Depending on exactly what he's doing, the reduction may not be so dramatic, but under this rate structure a 1200 bps modem looks extremely attractive.

Now let's suppose that Just Folks decides to bite the bullet and support 2400 bps. Assume that to recoup its investment in the new equipment, the service tacks on an $8/hour surcharge for non-prime time 2400 bps access. Steven J.'s yearly bill would be:

$15.75/hour * .625 hours/month * 12 months = $118.13

Although upgrading from 300 to 1200 bps saved Steven about $300, the difference between 1200 and 2400 bps is only a little over $40 for the year! The key in this example is the additional surcharge for 2400 bps.

You can use this method to estimate your operating costs for accessing information services, computer-based bulletin board systems, or school computers. Just plug in the appropriate numbers for your intended use.

Hurry Up And Wait

The cost effectiveness of a medium- or high­speed modem also depends on how quickly the remote system responds to commands typed in from your computer. When the remote system is heavily loaded with users, slow response times are very common. In fact, if the system is very busy, a 1200 bps user can wait just as long as a 300 bps user for requests to be processed, and data may be transmitted to you in spurts rather than a continuous stream, lowering the effective transmission rate.

I've been logged onto some information services during the evening (8:00 p.m. to midnight Eastern Standard Time) at 1200 bps and have clocked effective transfer rates below 300 bps. In these cases, there's no advantage to 1200 bps—it actually costs more than using a low-speed unit for the same amount of data. A 2400 bps modem would be even more expensive overkill.

If you're a night owl, you'll find the best effective transmission speeds on the commercial services between 1:00 a.m. and 7:00 a.m. EST.

Response time is usually no problem on Bulletin Board Systems (BBSs). Since you are typically the only person using a BBS at any one time, the remote system can devote its full attention to you alone, so your transmission rate is preserved.

The Future Of 2400 Bps

Does the lack of support for 2400 bps bode ill for the acceptance of the new high-speed modems? While it certainly doesn't help matters, there is some hope for life in the fast lane.

Many 2400 bps modem manufacturers see the thousands of popular BBS systems run by hobbyists as the key. Since a BBS needs only one modem, the investment is more manageable by the individual or club operating the system.

Several of these manufacturers are reported to be working with the system operators of a number of popular bulletin boards to start a seed program for 2400 bps modems. By special arrangement, 2400 bps modems will be made available to selected system operators at prices very close to that of 1200 bps modems.

Industry-wide support of such a project would be welcome indeed. If significant numbers of bulletin boards support 2400 bps, it will provide a real incentive for everyone else to acquire high-speed modems. Since BBSs typically do not charge for connect time, it would cost users nothing extra to access them at 2400 bps. The only charges are for long-distance phone calls, and those charges are based only on the duration of the call. The additional cost of a 2400 bps modem can be recovered fairly quickly.

As the numbers of 2400 bps users grow, one of the major commercial information services will move to offer 2400 bps service and its competitors will quickly follow. The greater the perceived size of the 2400 bps market, the lower the extra 2400 bps surcharges will be.

Taking The Plunge

So we come to where the rubber meets the road. Should you spend the extra dollars today on a 2400 bps modem?

The economic case is weak at best. The short-term potential savings are low, considering the limited support of 2400 bps at this time.

On the other hand, computing, like cars, is a personal experience for many people. Critics can drone on and on for years about why it's inappropriate for humans to relate to machines. But it doesn't change the fact that driving down the road in a convertible with the wind in your hair and finding the last bug in a program are both kicks. Using a 2400 bps modem on good old regular phone lines is a lot like driving a Shelby AC Cobra with a 289 cubic-inch V-8. There may not be many places you can run flat out, but it can be a heck of a lot of fun when you do.

Two advantages of a 2400 bps modem over a Shelby Cobra: It costs about $60,000 less and you'll never get a speeding ticket.


Arlan R. Levitan
The Source: TCT987
CompuServe: 70675,463
Delphi: ARLANL