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A modem is a modulator/demodulator. It converts digital signals from a computer to analogsignals so they can be sent over a telephone line. The receiving computer will normally have a

modem which will convert the analog signals back to digital signals. A modem may be aninternal modem which is a card inside your computer or it may be external with a connection to a

serial RS-232 line on your computer. The telephone jack that will plug into the modem is called

an RJ-11 jack. Speed of the modem is measured in bits per second (bps). There are a set of V-series standards developed by the International Telecommunications Union which indicate thespeed of the modem.

y V.22bis - 2400bpsy V.32 - 9600bpsy V.32bis -14,400bpsy V.32terbo - 19,200bpsy V.FastClass (V.FC) - 28,800bpsy V.34 - 28,800bpsy V.42 - 57,600bps

These are data compression standards which enable the transmission to operate at a higher speed.

Due to data compression used on modems in recent years baud rates and bps when referring tomodem speed are no longer the same. Now, more than one bit can be sent with each sound wave

oscillation (baud). In the past, only one bit could be sent per sound wave oscillation.

Modem Types

y Asynchronous - The common modem used today. Each byte is placed between a stop anda start bit. Each modem must operate with the same start and stop bit sequence, operate atthe same baud rate and have the same parity settings for the data checking in order to

communicate correctly. Define parity checking.y Synchronous - Synchronous modems can be faster than asynchronous. They depend on

timing to communicate. Data is transmitted in frames with synchronization bits which areused to be sure the timing of the transmission and reception of data is accurate.

Synchronous modems are normally used on dedicated leased lines. Synchronous modemsare one of binary synchronous communications protocol (bisync), high level data link

control (HDLC), or synchronous data link control (SLDC). Three methods can be used tocontrol synchronization:

oAdditional clock signaloGuaranteed state change - Clocking is part of the data signal.oOversampling - The reciever samples the signal much faster than the data is sent.

The extra samples can be used to be sure the clock is synchronized.y Digital Modems - These are used with ISDN services and are not actually modems,

although they are called modems. They can provide connection speeds of 128Kbps.

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Synchronous Modems

Synchronous modems operates in the audio domain, at rates up to 28800 bps in audio lines, used in

telephones systems (using synchronous data). The usual modulation methods are the phase modulation

and integrated phase and amplitude (at higher rates than 4800 bps).

In synchronous modems, equalizers are used, in order to offset the misfit of the telephone lines. These

equalizers are inserted in addition to the equalizers, that sometimes already exist in the telephone lines.

Asynchronous Modems

Most of the modems that operate in slow and moderate rates, up to 1800 bps, are asynchronous (using

asynchronous data). Asynchronous modems operate in FSK modulation and use two frequencies for

transmission and another two for receiving. Asynchronous modems can be connected in different

options to the communication media:

y Using 2-wire or 4-wire interface.y Using switched lines or leased lines.y Using interface to call unit/automatic answer, when dialing-up.