Why 300 Baud?
Michael E. Day
Chief Engineer, Edge Technology
Time plays a very important role in telecommunications. Every industry builds up its own group of words and terms to define those things it deals with that cannot be readily described with everyday language. The telecommunications field has more than its share of buzz words. Close analysis shows that many of these words deal with time and time-related things.
Time is important in all aspects of telecommunication, from the transmission of a single bit of information, to the overall system performance. Although there are no exact limits to the use of time as it relates to telecommunication, there are practical limits and deterioration effects which must be considered when designing a system.
There are many factors which must be taken into account when designing a telecommunications system. The purpose of the system generally determines what methods will be used. Of major importance is the amount of information flow that will occur. This will generally determine the minimum acceptable system communication speed.
Short Messages Permit Slow Speeds
If the message to be sent is relatively short, and little or no response is expected, then very slow system speeds can be acceptable. Western Union makes use of this on their TELEX network which operates at 6.7 characters per second. A slow system speed has the advantage of using only a small amount of the communications bandwidth. This means that more systems can be installed on the same communications link and can thus reduce the per-user cost of the communications system. The TELEX system serves users who must be sure that their messages get to the parties to which they were sent in a short period of time, but at low cost. (In some ways it can be cheaper to send a TELEX than it is to send a letter).
Another low cost system structure is the message forwarding system. Here higher communication speeds are allowed, but direct communication with the target party is not allowed. Instead the message is built up at a location close to the sending party and then transmitted to the target party at a time when the cost is at the lowest rate. In the case of packet switchers, it is sent when a communications link becomes available.
When a large amount of data is to be sent, the speed of the communications link becomes important. Transmission at higher speeds requires a larger communications bandwidth. Because of this, there is an associated increase in the cost of the communications link as well as the equipment required.
Between these extremes lie general information communications systems involving a low to medium amount of data transfer. Often this occurs in conjunction with user interaction with the data flow. This is the area where most computer use occurs.
Although the exact system configurations change as technology changes, the general structure of use remains relatively constant. Any change in one area of the telecommunications field tends to affect the other areas in some way.
The Three Common Computer Modems
There are three types of modems that the average computerist encounters: 1. The 103 style modem (the most commonly used). 2. The 202 style modem (far less common, but it has the advantage of higher speed at a reasonable cost, but with an increase in complexity of use). 3. Finally, the newer 212A style modem combines the higher speed of the 202 with the ease of use of the 103 (but costs more).
The 202 modem is sort of an ugly duckling in the telecommunications field. It has the advantage of higher speed than the 103, yet it can be built at a lower cost. The problem comes from its complexity of use.
The 202 is a half duplex modem. This means that it can only transmit in one direction at a time and, thus, requires some amount of computer control over its use. If the intended use requires high speed at lost cost, this is normally considered an acceptable trade off. If the computer control requirement is not acceptable, the higher cost 212A modem is used to achieve high speed without computer control. If cost is important, the 103 is used.
Although the 103 is more expensive to make on a price/performance basis, there is a demand for the low cost 103's. They are mass produced and mass distributed at low profit margins, and are often of lower quality.
Acoustic Versus Direct Connect
The modems come in two major classifications, the acoustic type and the direct connect type. Until recently, the acoustic was the most popular with the general user. This was due largely to regulations which inhibited the direct connection of modems to the phone lines. There are still many regulations which inhibit this, but they have been reduced to the point that the general user at least can consider it as an option. It is actually possible to build a direct connect modem less expensively than the acoustic version, but, until the regulations are further reduced and demand increases, this will probably not happen.
Acoustic versions of the 202 modem are essentially nonexistent. The main reason for this is the telephone. The telephone uses a carbon microphone to convert sound into electricity. The carbon microphone is very rugged, cheap to build, has a high gain factor, and has natural filtering characteristics in the frequency range of the human voice. Unfortunately, it is also very noisy and disruptive to the signal that it converts.
The 202 cannot work within this environment. The 212A modem fares a little better because the part of the signal it uses for data transfer suffers a minimal amount of disruption from the carbon microphone. The 103, however, works reasonably well with the carbon microphone. This is due to the narrow bandwidth used for transmission, as well as the high redundancy in the signal used. Although some 103 modems are capable of operation at speeds of up to 600 baud, the disruption of the carbon microphone tends to limit the speed to 300 baud. If the phone system is in good working order, it is possible to operate as high as 450 baud. Alternatively, if the system is in poor condition, it may require that operation be reduced to 200 baud, 150 baud, or even as low as 110 baud. Generally speeds below 110 baud do not actually achieve any improvement in reliability of operation on the 103.
If problems are encountered, or higher speeds are desired, the direct connect modems are generally required. If only a slight improvement is needed, some degree of improvement can sometimes be obtained by replacing the carbon microphone with a capacitor (or condenser) microphone (sometimes referred to as a supermike). The capacitor microphone is more expensive to make than the carbon, but does not disrupt the signal. There is one disadvantage to most capacitor microphones. It requires power to operate, and it gets this power from the phone line. If another phone comes on the line, it can steal the power away from the capacitor microphone and inhibit it from working. This is particularly true if the other phone has a carbon microphone in it which tends to drop the power well below the capacitor microphone's normal operating level.
The 103 type modem has become a standard for medium speed communications over the telephone network not because someone felt it should be, or because someone made it so. It became the standard because it was the optimal solution to the problem at hand.