Have you heard the one about the traveling salesmen? (Teletalk) Corey Sandler.

Breaker, breaker. This is Big Daddy comin' at ya from northbound I-95. Would y'all please be ready to download today's fiscal report and accept my edit of the production spreadsheet?"

Or, how about: "You say you want a 16-bedroom Greek Revival mansion with a heated swimming pool in the shape of a guitar? No problem. Let me pull over and query the Multiple Listing Service."

Or, "Something's odd about the reading on this gauge. Let me punch it into my briefcase computer here and let the mainframe run a quick 24-hour simulation before we lock up the rig for the night."

Or, have you heard the one about the traveling salesman who filed his orders from his car in the parking lot outside his best customer's factory?

What's wrong with these pictures? Well, we have had the possibility of telecommuing from offices, hotel rooms, and factory floors using transportable and then laptop computers for some time now. But the difference here is the severance of the tie that binds computers to the telephone system. Think of the wireless computer as the ultimate cordless telephone.

In my own case, this past winter I suffered through a soul-testing trial in my business as a writer the day my commuter train took a powder just north of mowhere and sat stranded in a swamp for two hours. I was on deadline, with the final draft of a column sitting in the memory of a portable computer on my lap, and there was no way for me to do anything but grit my teeth and listen to the batteries drain. Og what I would have done for a jungle drummer, a smoke signaler, or a kid with two tin cans and a very long piece of string.

Well, beam me up, Scotty. The next time I travel I just might be able to modulate and demodulate my deathless prose from the middle of the Meadowlands, the top of the chairlift, or an air mattress in the middle of a swimming pool.

We are on the threshold of the widespread use of the wireless modem, the next logical step in the progression from the 12-ton mainframe to the wristwatch sub-microcomputer.

You have already seen private two-way radio modem systems in use if you have had dealings with two of the more ubiquitous business entities in our society: Federal Express and IBM. Federal Express links video terminals in many of its delivery trucks to a dispatching computer. Many IBM repair people in major cities carry small portable computers with radio links for communication and diagnostic assistance.

A Marriage Made in Silicon

A company called Electronic Systems Technology has married an FM transceiver to a modem and a battery pack. You can plug it into any computer, including a laptop portable, or you can buy their model which comes with everything the wireless worker could ever ask for. All you need is another transceiver and a 6" whip antenna, and you're in business.

The EST cordless computer can transmit within a one-mile radius inside a factory or office, or as far as 30 miles across fields and valleys (or streets and alleys) if larger, roof-mounted antennas are used.

AT&T and Bell Atlantic have begun an experiment in the Baltimore and Washington metropolitan area in which they equipped some of their customers with modems that tied portable computers into the cellular telephone system for on-the-go worldwide telecomputing.

Recently, a number of U.S. airlines began flying with the ultimate in cordless telephones: a model that can link the passenger in the air to any telephone on the ground. And, yes it is possible to strap on an acoustic coupler to make Flight 753 into your own personal flying radio modem. (The handpiece is a bit on the boxy side, so you had best travel with some masking tape and rubber bands.)

The AT&T device under test is a black box, blessed with the clever name of CTS 1620. Operating at a zippy 2400 baud, the device works much like any other modem, modulating the digital 0's and 1's of a computer signal into an analog warble for radio transmission.

The technical tricks that had to be solved were related to the nature of the cellular radio system and the transmission and reception problems engendered by the concrete canyons of major cities.

The early radio telephone systems relied on a single large antenna and transceiver for a metropolitan area. the system could accommodate only a limited number of users, and most cities had vast dead spots into which the signal would not travel. The replacement system, in widespread use now for about two years, is called the cellular telephone network. This system has a number of low-powered base stations spread through a metropolitan area. Each station has its own transmission and reception frequencies.

Assuming that the mobile phone user is in a car, as the vehicle moves from one cell and into another a computer-controlled system hands over the call to the next cell and switches the frequencies. The computer takes about 100 milliseconds to make the switch, which is faster than even your Aunt Millie talks. In theory, then, human users will never know the difference. However, your computer or an ordinary modem just might realize that there has been a break in the line or a shift in receiving frequency. You could lose the link, or if the changeover occurred in the middle of transmission of data, the computer could miss a few characters, or drop a few digits from financial statement.

Other problems include "ghosting" of signals in and among the skyscrapers and general distortion caused by environmental factors.

A lot of effort--and a significant amount of cost and transmission overhead--is often devoted to error detection and correction hardware and software. (Consider the fact that simple parity-checking, the lowest level error correction system, adds a 14% load to data with the addition of its eighth bit parity indicator.) Other types of error correction can add as much as 50% overhead to a data signal--another way of stating that is to say that error correction can cut by half the amount of information transmitted in a particular time period.

It is important at some point to consider distinctions among types of data for telecommunication. If the computer is being asked to monitor, the heartbeats of a critically ill patient, or to send telemetry to control the blastoff of the Space Shuttle or convey multimillion dollar buy and sell instructions for an investment banking house, there can be no allowance for error. Better to have slow, totally accurate information than fast and wrong data.

On the other hand, if the data is a word processing file or an interoffice electronic mail memo, the 1-in-100,000 character garble that changes "soup" to "soap" won't make all that much difference, and in any case the real meaning should be evident in the context in which it is presented.

AT&T claims it has perfected a proprietary means of modulation of the signal between the cellular phone and the base station that will help eliminate problems brought about by ghosting and distortion. In the area of error fixes, the system uses an "automatic request for repeat" protocol that has the modem ask for retransmission if a block of data doesn't come through properly. The method is similar in concept to the XModem system used in some microcomputer telecommunications systems. XModem breaks data into blocks that can be checked to the transmitting modem by the receiver.

According to AT&T, the system will work just about anywhere a cellular telephone will operate, although throughput (the net number of corrected characters) will drop in troublesome areas.

AT&T is also making a big point of the encryption capabilities of its test modem. Many government agenices and major corporations worry about sending their secret information over public radio networks. If you have ever picked up an older cordless phone and listened in to your neighbor's kids engaged in the commerce of teendom, you'll understand the innocent half of their concern. The darker side involves industrial governmental electronic espionage.

The AT&T system under test includes an encryption/decryption device in the cellular telephone and a similar device at the central office of the phone company, with the signal proceeding from there in unscrambled form over ordinary telephone company lines. An alternative under study would have the phone system, with the customer having a decryptor at each end. This latter option should allow the user to include full DES or similar encryption with secret keys.

The companies and agenices testing the AT&T/Bell Atlantic system include the federal government, insurance and banking representatives, and real estate agents. In one instance, a real estate company is using the system to link an agent in an automobile to the Multiple Listing Service database in a central mainframe. Laptop computers in use in the test include models from Hewlett-Packard, Data General, Radio Shack, and other manufacturers.

The CTS-1620, part of AT&T Technologies' Cellular Telecommunications System, is not yet available in a retail version. Its price is expected to be in the range of $1500 to $2500. The user will also have to have a cellular telephone and transmitter--at an additional $2000 or so.

Do-it-yourself Network

The EST wireless modem is designed for private networks that do not necessarily tie into common carriers such as the phone company. The ESTeem wireless modem, priced at $995 in its simplest model or $1195 in a CMOS battery-powered version, can link as many as 255 computers or peripherals. The model that includes its own laptop is based on the NEC 8021A portable, and sells for $2295. One interesting application of this package calls for direct warehouse inventorying using a bar code reader.

While the AT&T cellular modem might be used by a traveling salesman on the road, the EST system is more likely to be used within the confines of a large factory, warehouse, or office. Or, it might be installed at a wellhead to transmit data back to a central monitoring site, or carried by forest rangers or security personnel on patrol. In certain circumstances, the company claims, an office might choose to connect computers to each other or to printers and othe r peripherals using a wireless link instead of going to the trouble and expense of installing wiring.

The wireless modem will accept data at any standard speed from the RS-232C communications port of any computer and store it in a buffer where it is formed into "packets" and then transmitted in bursts of information at 2400 baud. A built-in Z80 microprocessor controls the one-watt transceiver and error correction activities. The transceiver uses VHF FM radio at frequencies between 72.040 and 72.960 MHz. The user can select one of 24 available channels and a unique address to each node on the network.

No special software is required--I tried the device with PC-Talk and Cross-talk XVI at a demonstration linking an Apple IIc to an IBM PC.

EST is located at 1301 N. Kellogg St., Kennewick, WA, 99336. The phone number is (509) 735-9092. With luck, I'm located on the beach right now, miles away from the nearest power line and telephone.