This section discusses modems that users use to receive and send their data and information through communications channels. All communications must flow through some types of channels. Most of the computer communications are transmitted via ordinary telephone lines. Therefore, many require devices to convert the digital signals (represented by the presence or absence of an electronic pulse) into analog signals. This converter is called a modem.
The modem is an acronym for "MOdulator- DEModulator." A modem converts the digital signals into continuous analog signals (Modulation), and converts from analog to digital (Demodulation). The modem enables digital microcomputers to communicate across analog telephone lines. Both voice communications and data communications can be carried over the same telephone line.
Microcomputers require modems to send and receive messages over telephone lines. However, not all computer communications should have converters such as modems. Computer systems connected by coaxial or fiber- optic cables can send and receive digital signals directly through these circuits. Most modems today are both data and fax modems in that they can transfer computer data and also acting as fax machines.
Communications Speeds
The modem handles the dialing and answering of the call and controls transmission speed. Communications speeds are expressed in bits per second (bps). A baud is commonly and erroneously used to specify bits per second for modem speed. This is not always true. Modems are classified by the speed at which they operate. The modems used with microcomputers usually operate at 2400, 9600, 14400, and 28800 bits per second.The higher the speed, the faster users can transmit a document and therefore the cheaper your line costs. For example, transmitting a 100-page single-spaced report takes 25 minutes at 2400 bps. It takes 6 1/4 minutes at 9600 bps, about 4 1/6 minutes at 14400 bps, and 2 1/12 minutes at 28800 bps.
- Baud
- This is a switching speed or signaling rate of a line. It refers to the number of transitions (frequency changes) made per second. Baud equals to bits per second at only low speeds. (e.g., 300 baud is the same as 300 bps. However, the V.22 modem can generate 1,200 bps at 600 baud.)
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