Introduction...
3. Modems
Modem is the shorter form for MOdulator/DEModulator. It connects a computer to a phone line. A modem converts digital signals (a stream of bits) to analogue signals ( a continuous wave) which is the format used in telephone lines. That analogue signal is sent through the telephone lines.
Figure 1.4 shows how external modems are used with computers to send and receive data over the telephone network. There are internal modems that fit to an expansion slot on the motherboard. Modems have different transmission speeds, measured in bits per second (bps), or in baud.
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| Figure 1.4: Modems converting digital bit streams to analogue signals and vice versa |
Serial Data Transmission
On most links data is transmitted serially. The rate at which data moves is measured in bits per second (bps). Sometimes bps is considered as same as baud rate.
NOTE
Baud is the number if signal level changes per second in line - regardless of the information content of those signals.
bits per second, is the rate of transfer of data bits.
The ratio of bps to baud depends on the information coding scheme that you are using. For example, each character in asynchronous RS-232 coding includes a start and stop bit that are not counted as data bits, so here the bps rate is actually less than the baud rate.
There is a number of stranded bit rates in common use;
- 300 bps, and multiples (1200, 2400, 9600, 19200 bps, ect) - Low speed carrier (e.g. Telecom) links
- 48, 64, 128, 192, 380, 512, 1024, 2048 kbps and multiples - ISDN and DDS links
- 2, 4, 16, 100, 155, 622, 1000 Mbps - LANs
