Link up! Communications save time, travel and money.

By James O. Scharf

JAKARTA (JP): A few days ago, while I was scrounging around in Glodok, I noticed that not many people were selling modems. I was surprised to learn that many dealers, who were previously doing a good business in communications, had stopped selling them. The general opinion was, "No market!"

In most office environments, the fax machine has taken over as the prime communications device. I admit, I use my fax much more than I use my modem. Let's face it, the fax is very user friendly and gets the job done. Yet, there are many instances when the fax is a poor substitute for high-speed communications and may often create problems in offices where telephone traffic is intense and phone lines are limited. Here's a couple of actual (and typical) cases.

One CEO needed to fax large documents to his home office in London and their other Asian-based offices. His international phone bills were killing him. A friend of his, a CEO at another company with multiple offices in Jakarta and remote sites throughout Indonesia, complained that his telephone lines were so tied up with fax-traffic that he usually had to resort to sending documents by courier. After installing communications, the first CEO is now saving over US$40,000 a year and the second has reduced his number of dedicated fax lines and no longer needs courier services.

Communications can be a real boon if properly set up and can just as easily turn into a nightmare if they aren't. If you're new to computers, you may be wise to stay away from them until you're confident that you know both your machine and its operating system.

A simple communications set-up would be a computer and modem at each end of a telephone line. One modem won't do the job, at least not yet. Although "modem" has become a word that everyone uses, it is really a contraction of the electronics term "modulator/demodulator." Without going into a lot of electronics claptrap and 50-cent techno-jargon, you may think of it simply as an encoder/decoder.

Actually, it works pretty much like your telephone when you dial a number. It takes binary data, changes it into a series of audio tones and sends these over the telephone wire to another modem which receives the tones and turns them back into binary data.

Buying a modem is a bit like buying a computer. They run the gamut from simple and slow to complex and fast, with the price running accordingly.

The first thing to consider is speed. Although you will hear the terms "baud" and "bps" (bits-per-second) used interchangeably, this is not always true. Baud is defined as: a change of signal state per unit of time (seconds) and is a function of the signal being sent. This becomes more confusing as the various transmission standards redefine bit usage and various data compression and error suppression techniques are employed.

To avoid confusion, it may be better for you to refer to bps values. Since, usually, a start bit and a stop bit are appended to the 8-bit character, you may divide bps by 10 to get characters per second. I have included both values in the figure.

A realistic range of modem speeds will use 1200, 2400, 4800, 7200, 9600, 12000, 14400 and 19200 bps. Anything under these speeds is too slow to consider (300 or 600bps) and above these speeds (38400 or 57600bps and up) may be unrealistic.

Telephone lines, especially in Jakarta, are just not that clean. On international lines, higher bps rates are possible and initial data links can be established at 57600bps. (Although downshifting usually occurs.)

Other than speed, communications also have direction. On a two-wire telephone system this is called a "half-duplex" channel and is analogous to a radio operator saying "over" to let the other person know he is ready to receive.

Four-wire telephone systems have "full-duplex" capability since they can use one two-wire pair to send and the other two- wire pair to receive.

Modern modems can also have full-duplex capabilities on a two- wire system by using half of the signaling rate capability to send, and half to receive. Multi-duplex or "multiplex" channels use this technique to carry more than two data signals on the same channel. This forms the basis for networked systems.

Unlike voice communications, data communications need some method of telling the receiver that a new character is being started or stopped. This could be compared to two people sending Morse code with flashlights. They may agree upon a two second "off" between characters and a five second "off" between words.

Asynchronous transmission requires the sender and receiver to agree on a mutual transmission format to accomplish data synchronization. We may both agree to use one start bit, eight data bits, and one stop bit as our transmission format.

This bit sequence is called a "frame." We may also agree to use parity error checking within the frame. If so, another bit will be appended to the seven data bits just before the stop bit. (Parity error checking was covered in this column on Oct. 18, '93.) Most users just set "None" for parity since it's not very reliable for communications.

Synchronous transmission is based on frames being defined by external timing signals that are added to the transmission signal. Start and stop bits are no longer required. This requires added modem circuitry and more advanced error correction.

As you can see, both sender and receiver must agree on just how to set up their modems before successful communications can occur. At minimum, these communication "parameters" include: baud rate, number of data bits, number of stop bits and parity setting. For example, CompuServe requires eight data bits with one stop bit, no parity and full-duplex.

Another confusing issue for users new to communications are "protocols." File transmission protocols provide error detection and define how much data will be sent as a block. Some of the more popular protocols are: Kermit (96 characters), Xmodem (128 bytes), Ymodem (1K) and Zmodem.

Which protocol is being used must also be agreed upon before transmission starts. The problem with Kermit, Xmodem and Ymodem is that they will retransmit the entire block of data should an error occur somewhere in the block. On noisy phone lines, this could slow data through-put considerably.

Zmodem sends large blocks of data but uses "checkpoint restart." Every so often during transmission, Zmodem will issue a checkpoint. Should an error occur, Zmodem will retransmit only the data between the checkpoint and the error. You and a friend may try transmitting a 20K or so file using each protocol and checking transmission times.

Now we get to the really heavy-duty protocols that do not retransmit data but provide error correction "on the fly." TCP/IP stands for Transmission Control Protocol/Internet Protocol and was originally developed by the Defense Advanced Research Projects Agency. ISO/OSI stands for International Standards Organization/Open System Interconnection. This protocol divides communications into a seven layer "protocol stack" and is only used in networked systems. Both of these protocols rely on the Hamming, Reed-Solomon or Bose-Chaudhuri-Hocquenghem error correction codes.

If you're just communicating locally, my advice is to keep it cheap and simple. A 9600 baud modem running Zmodem will work nicely and won't put you in the poor house. By the way, make sure the modem you choose is Hayes compatible. Hayes is the de facto standard for PC to modem communications and uses the AT command set.