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Fig. TRAN-1. The communications space environment.

criterion. For most designers, however, the digital interface begins at the digital data transmission side of the modem. There are still several concerns at the modem interface:

1. Data is transmitted on a one-bit at a time (serial) basis.

2. We can have either the equivalent of two simultaneous links -- full duplex, or a link which is either receiving or transmitting, but whose direction can be reversed -- half duplex.

3. The modems and telephone links add noise, hence the data received is not exactly the data transmitted.

4. There is a large number of different types of modems and corresponding interfaces.

Ignoring the error problem and various modem interface signals (e.g., those that indicate whether the unit is turned on or off), the modem can be considered


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