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:
- Using 2-wire or 4-wire interface.
- Using switched lines or leased lines.
- Using interface to call unit/automatic answer, when dialing-up.
In a 2-wire line, full duplex operation can be achieved by splitting the channel into two sub-channels.l
Asynchronous data is not accompanied by any clock, and the transmitting and receiving modems know only the nominal data rate. To prevent slipping of the data relative to the modems' clocks, this data is always grouped in very short blocks (characters) with framing bits (start and stop bits). The most common code used for this is the seven-bit ASCII code with even parity.
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