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Modems that send random information need start and stop bits to let

them know when one piece of information stops and the next piece of

information begins. This kind of informaiton is referred to as


Other modems are "synchronous"; they send and receive information at a

certain time. The modem sends a signal with its start bit which lets the

receiving modems synchronize their internal clocks. This allows the modems

to send and receive information without having to keep sending start and

stop bits.

The only major difference between the two is the cost. Synchronous modems

will work a little faster than asynchronous modems. But, because of the

difference in the prices of the two, you will find that most BBSes use

asynchronous communications.


Different Modem Speeds

Modems have to "talk" at the same speed so that they can pick up each other's

signals. If they can't operate on the same signal speed, then they won't

be able to understand each other. Because of this, modems operate in


The term "baud" got its name from Emile Baudot, who was an early pioneer

in telegraphy. "Baud" deals with how fast a modem can change its tone to

indicate a bit. This type of bit is not exactly the same thing as bits

transmitted per second (bps), but the two words are interchangeable.

Standard modems can operate at speeds of 300, 1200, 2400, and 9600. Most of

the modems can adjust their speeds to accommodate slower modems. However,

if your modem is able to operate at 1200 bps, you cannot deal with a modem

that operates at 2400 or 9600 bps at a speed greater than 1200 bps. A

modem that operates at 9600 bps can send an average of 960 characters a


Full And Half Duplex

If a computer modem operates in "full-duplex," that means that it can send

and receive information at the same time. Telephones are full-duplex because

you can talk and listen at the same time. Most modems are able to do this.

If you have "half-duplex", then you can only do one thing at a time.

If you are talking, then you can only talk. You can't be listening if you

are talking. The other person has to wait to tell you something back.

CB radio is half-duplex because you can't be talking to someone and

listening to them talk at the same time. Most BBSes are full-duplex so

that they can send and receive information at the same time.

Other Modem Standards


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Several standards have been established for communication by the Consultive

Committee for International Telephone & Telegraph (CCITT). These standards

are used to refer to the way that data is exchanged.

Different speeds will use different standards. For instance, a 2400 bps will

have different standards than a 9600 bps.

Not all of the standards are directly connected with modem speed. The

standard of V.42 is used to detect and correct errors. There are standards

for doing a lot of things before you send them through the modem. You can

use a standard to compress your file before you send it. This will, in

essence, speed up your transmittal. The details surrounding the standards

used with computers are rather lengthy and hard to understand, and needn't

be covered in this section. Fortunately, all you need to know about

them is whether or not the computer you are dealing with matches the same

kind of standards that your computer has. Most modems will work all of the

communications out with themselves when they connect with each other, and

the computers will select the best way to communicate.





































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