To connect one computer to another through a conventional telephone system, you need a device known as a modem. A modem must be connected to your computer to send and receive e-mail and computer-transmitted faxes. The modem must be connected to a telephone cable. The telephone cable then plugs directly into a telephone socket.
The word modem stands for MOdulator-DEModulator. Modems can dial the number of another computer or host computer, answer or make a data call, and hang up. When you transmit data from your computer, the modem converts it into a form that can be transmitted over the telephone line. When your computer receives data, the modem also converts incoming signals into a form that can be interpreted by your computer. There are two basic types of modem, internal and external.
Modems are relatively inexpensive. Prices vary greatly from country to country, but tend to be cheaper in industrialized countries. You can expect to pay between US$40 and US$300 for a modem, depending on its specifications.
An internal modem is a card which contains circuits and chips. The card fits into an expansion slot inside your personal computer in such a way that its telephone socket is accessible through an open slot in the back or side of the computer. The telephone cable plugs directly into a telephone socket.
A normal external modem is a small box, usually the size of a paperback book. There are smaller-sized external modems, known as pocket modems. External modems often have a series of little indicator lights on the front panel, which light up or flicker to show what the modem is doing at any given time. These lights can provide useful clues about possible problems if the system is not working properly.
An external modem is connected to a computer by a serial cable to the appropriate serial port. Some external modem models no longer need a modem cable and can be connected directly to the serial port at the rear of the computer. The latest modems are only a few millimeters thick and about the size of a credit card. They are known as PCMCIA Modem Cards and fit directly into a PCMCIA slot on the outside of your computer. They are primarily used with portable or notebook computers.
Which type of modem should I buy external or internal?
Both kinds of modem work well and usually come with communications software. Your choice might depend on the following factors:
An internal modem
An internal modem may be the best choice if you:
An external modem may be the best choice if you:
A modem which can be used to send and receive faxes is known as a fax modem. All fax modems can also be used for computer communications, but the reverse is NOT true. If you want to use a modem to send and receive faxes you must buy a modem which supports the fax function.
What should I look for in a fax modem?
Fax modems have a number of features. They:
If you buy a fax modem, you might want to check whether your purchase includes:
Since data modems are usually cheaper, why buy a fax modem?
A fax modem is particularly useful to communicate with people who have a fax but no e-mail. Today, however, many of the faster modems have a standard fax feature, yet are almost as cheap as data modems. The fax feature does not alter the modem s performance. So if the fax modem has all the other features you want and is reasonably priced, buy it.
How fast are modems and why does it matter?
Modem speed refers to how fast the modem can send and receive data over the telephone line. Most modems used by NGOs have speeds ranging from 300 to 28,800 bits per second (BPS).
Higher speed modems present many advantages, since they:
Is a modem's speed limited by the speed of the modem at the other end?
Yes. In developing countries, e-mail service providers usually offer access at either 2400 or 9600 bps. This is not to say that you should not buy a faster modem, since service providers tend to upgrade service over time. Simply put, in most cases, a 2400 bps modem will do the job.
How do I decide which speed is best for me?
This depends on a number of factors, including:
Buying a faster modem can also help where telephone lines are old, noisy and where local calls are expensive. While they cost more to buy, the lower communications costs would make your investment worthwhile.
On the other hand, a lower speed modem (2400 or 9600 bps) should be adequate if:
To make sending data more efficient, computer scientists have developed methods to compress information. To the computer user, the compressed document looks and is just like the original. It has simply been temporarily converted into a more compact form, which takes less time to send. This is known as data compression.
How does data compression affect my documents and my modem's performance?
Data compression:
Electrical disturbances (also known as line noise ) scramble or corrupt data as it flows through telephone lines. Error checking is a process through which the receiving modem continuously verifies that the data being received is arriving in exactly the same form in which it is being sent. If the receiving modem detects errors in the data, it tells the sending modem to re-send the parts of the data that were corrupted.
Error correction, like data compression, is expressed in terms of protocols. Often these protocols carry numbered codes similar to those of data compression protocols. For example, the V.42 (A CCITT protocol) is the most sophisticated version of error checking in widespread use for personal computer modems.
Do I need permission to use a modem over telephone lines?
This varies from country to country. In some developing countries, the local telegram and telecommunications authorities do not allow users to connect data communications equipment to telephone lines. This sort of prohibition, where applicable, often extends to modems, faxes, and all other data transmission equipment. You may need to check with your local telephone company. Your network or e-mail service provider may be able to help you in this matter.
In many countries, local telecommunications authorities have prepared a list of modem brands approved for use in that country. These are often known as approved modems. Sometimes local computer users expect these lists to include all popular brands of modems, but this is not always the case. To avoid potential problems, you should check with your local service provider or telecommunications authority before buying a modem. Ask for a list of approved modems and, if one exists, choose only from those listed. Where these kinds of PTT rules apply, you are legally bound to use an approved modem.
How do I connect my modem to the telephone line?
Most modems come with a socket which accepts a telephone cable with what is known as an RJ11 plug. The other end of this cable must be connected to the telephone line socket on the wall (the wall socket outlet ). Most developing country telephone line sockets are NOT compatible with this plug. Making these two incompatible pieces work together requires a local, and often creative, solution.
If any local friends or contacts have fitted a computer with a modem, you may want to ask them how they connected their modem to the telephone line. Your solution may be the same. You may want to contact your service provider or your local telephone company for help in connecting your modem to the telephone line.
A precaution worth taking in developing countries is to fit a lightning surge protector to the telephone line attached to your modem; if lightning strikes your telephone line locally, it may severely damage the modem (and in the case of internal modems, the computer as well).
Do I need software to use a modem?
Yes. You need what is known as communications software to use a modem. Basically, these are programs which help you connect your computer to a modem, enable your computer to contact a host computer (or e-mail service), and send and receive data over telephone lines, checking for and informing you of any errors at every stage.
Communications software is dealt with in the next section.
Back to Section IV: Access to Computer Networks
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