Hardware Reference
In-Depth Information
Although dial-up modems are generally thought of as analog devices, 56K (V.9x) modem connections
are actually digital. The typical PC modem is an asynchronous device, meaning it transmits data in an
intermittent stream of small packets. The receiving system takes the data in the packets and
reassembles it into a form the computer can use.
Asynchronous modems transmit each byte of data individually as a separate packet. One byte equals 8
bits, which, using the standard ASCII codes, is enough data to transmit a single alphanumeric
character. For a modem to transmit asynchronously, it must identify the beginning and end of each byte
to the receiving modem. It does this by adding a start bit before and a stop bit after every byte of data,
thus using 10 bits to transmit each byte (see
Figure 16.5
)
. For this reason, asynchronous
communications have sometimes been referred to as
start-stop
communications. This is in contrast to
synchronous communications, in which a continuous stream of data is transmitted at a steady rate.
Figure 16.5. Asynchronous modems frame each byte of data with a start bit and a stop bit,
whereas synchronous communications use an uninterrupted stream of data.
Note
During high-speed modem communications, the start and stop bits are usually not transmitted
over the telephone line. Instead, the modem's data compression algorithm eliminates them.
However, these bits are part of the data packets generated by the communications software in
the computer, and they exist until they reach the modem hardware. If the ends of a modem
connection use different values for start and stop bits, the connection transmits gibberish
instead of usable data.
The use of a single start bit is required in all forms of asynchronous communication, but some
protocols use more than one stop bit. To accommodate systems with different protocols,
communications software products usually enable you to modify the format of the frame used to
transmit each byte. The standard format that describes an asynchronous communications format is
parity/data bits/stop bits. Almost all asynchronous connections today are therefore abbreviated as N-
8-1 (No parity/8 data bits/1 stop bit). The meanings for each of these parameters and their possible
variations are as follows:
•
Parity
—Before error-correction protocols became standard modem features, a simple parity
mechanism provided basic error checking at the software level. Today, this is almost never
