A Monthly Column
Telecommunications:
Choosing A Modem
Michael E. Day
Chief Engineer, Edge Technology
This month, columnist Michael Day tackles some of the practical aspects of adding a modem to your computer. A modem will let your computer talk to other computers over the telephone lines. If you're shopping for a modem, you'll need to answer some questions first. Will you want the computer to be able to "pick up the phone" by itself and answer incoming calls automatically? To place its own calls automatically? How can you use a modem with a "Princess" phone? Will you need an extra interface to connect the modem to your computer or will it just plug in?
There are a number of different types of modems on the market. In order to understand each modem's capability in each application, we need to break them down into different groups.
One aspect to consider is the way the modem attaches to the phone line. There are three basic methods:
- Acoustic coupler: a modem that communicates through a telephone by sound.
- Inductive coupler: a modem that communicates through a telephone by electromagnetic induction.
- Direct connect: a device which allows a modem to connect directly to the phone line instead of using the telephone to communicate through.
The acoustic coupler and direct connect are the most commonly used methods. The inductive coupler is actually a modification of the acoustic coupler, and generally still requires that the transmitter portion of the modem be acoustically coupled. The usually stated advantage of the inductive coupler is its avoidance of the distortion and interference caused by the extra translation to sound and back occuring in the acoustic coupler.
This is of little help, however, since the majority of problems with acoustic coupling involve the transmitter portion, which needs to be acoustically coupled anyway. The inductive coupler also has its own set of problems which can often make it more difficult to work with if care has not been exercised in its design.
Each telephone has its own quirks with regard to inductive coupling, and this can sometimes make it difficult to get a modem to work properly. Also, because of the sensitivity involved in properly picking up the weak electromagnetic signals, the inductive coupler is more sensitive to stray magnetic fields, fields generated by a computer or terminal can interfere with the operation of the modem.
Portability Of The Acoustic Coupler
The acoustic coupler modem has been around for some time. The advantage of the acoustic coupler is its portability. It can be used with any telephone that has a standard handset (referred to as a 500 style phone). This means that you are not limited as to the electrical connections to the phone line, only to the physical construction of the phone handset, which must fit into the rubber or plastic "cups" of the acoustic coupler.
This can be a problem if you have a princess or flip phone or some other style which does not use the standard 500 series handset. Other problems with the acoustic coupler are a result of the acoustic coupling itself. Because of the signal's conversion from electrical to acoustic and back to electrical again, some amount of distortion is generated. This can be held to a minimum with proper modem design, but cannot be eliminated entirely. The telephone itself is a major contributor to this problem. A great portion of the distortion can be cured simply by replacing the carbon microphone in the telephone handset with a condensor (capacitor) microphone.
The standard acoustic coupler used to communicate over the telephone is referred to as a 103 compatible modem. This is a full duplex type modem — that is, it can send and receive data at the same time. Because of the method used to encode and decode the data to be transmitted by this type of modem, the maximum communication speed that can be used reliably is 300bps (30 characters per second).
This limitation is the result of many things, including the bandwidth of the phone line, the encoding scheme, and the signal distortion caused by the acoustic coupling. Although it is possible to push the speed a bit higher, reliability will suffer. A very good modem might let you push it to 400 or 450 bps, but, because of acoustic distortion, this is about the maximum.
Another problem with the acoustic coupler is its limited sensitivity. Most are not able to operate at a receive level of less than -30db to -36db. This is equivalent to a whisper. About 90% of the phone calls placed will provide a communications link allowing a signal through stronger than this. Therefore, this is not a great problem. However, the strength of the transmitted signal is of major importance, since the weaker the transmitted signal is, the weaker the received signal at the other end is going to be. Generally, it is desirable to have a transmitted signal strength of between -6db and -10db. This is equivalent to loud talking.
Direct Connect Method
The other method of attaching a modem to the phone line is the direct connect method. Here you attach the modem directly to the phone line, rather than acoustically through a telephone handset. This eliminates the problem of distortion by not going through the translation to sound and back.
There are many types of direct connect modems. The type that most computer users need is the 103 compatible type. It uses the same conversion scheme to communicate as the 103 compatible acoustic coupler, although the translation to sound and back is, of course, not necessary.
The main advantages of the direct connect modem are reduced signal distortion and improved sensitivity, (since the sound translation is eliminated). Some direct connect modems are able to communicate at up to 600bps (60 characters per second). However, some reduction in reliability should be expected at these high speeds. Other modems are designed so that they cannot operate at speeds above 300bps – in order to achieve a higher degree of reliability at the normal speeds.
Another attractive aspect of the direct connect modem is that it allows the computer to access the phone line directly to answer, and even make, calls by itself. This cannot easily be done with the acoustic coupler because it goes through a phone which must be physically lifted off a receiver to start dialing. The disadvantage of a direct connect modem, however, is that it must be physically attached to the phone line. This can be a problem with a business phone setup, pay phone, etc. As a result, the direct connect method is usually used in a fixed location where the phone line is easily attached. Often a phone line is solely for the use of a computer.
Three Types Of Direct Connect Modems
The three main types of direct connect modems are the "auto answer," "auto answer/manual originate," and "auto answer/auto originate."
An auto answer modem can answer calls, but is unable to place any. An auto answer/manual originate modem can answer calls as well as allowing you to manually make a call (after which control is transferred to the modem). Finally, an auto answer/ auto originate modem lets the computer answer calls automatically as well as place them by itself without operator intervention.
While most acoustic couplers are separate devices that attach to a computer or terminal through some sort of interface (usually an RS232 serial interface), a few are built into the equipment. If the acoustic coupler is to be purchased as a separate device, be sure to find out if the separate interface will be needed and include it in the cost of the modem. Often a computer already has an RS232 port available to which to attach the modem. If this is the case, the additional interface may not be needed.
Direct connect modems come as either a separate unit that attaches to the computer, or as a device that can be mounted inside the computer. If it is a separate external unit, the interface must be taken into account. Generally, modems which are designed to be installed inside the computer provide their own interface to the computer, and the extra cost need not be considered.
Although the final decision must be made according to your own needs, a general rule of thumb is that if you have no need for automatic operations, an acoustic coupler will probably serve best. If, however, you do desire automatic functions, then you will need the direct connect type modem.