Hardware Reference
In-Depth Information
typical PC modem is an asynchronous device, meaning it transmits data in an intermit-
tent stream of small packets. The receiving system takes the data in the packets and reas-
sembles 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 communica-
tions, 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 syn-
chronous communications use an uninterrupted stream of data.
Synchronous modems generally are used in leased-line environments and in conjunction
with multiplexers to communicate between terminals to UNIX- or Linux-based servers
and mainframe computers. Therefore, this type of modem is outside the scope of this
book.
Whenever modems (without a qualifier such as “DSL” or “Cable”) are referred to in this
book, I will be discussing the asynchronous, analog variety. (Synchronous modems are
not found in typical computer stores and aren't included in normal computer configura-
tions, so you might not ever see one unless you go into the data center of a corporation
that uses them.)
Note
Duringhigh-speedmodemcommunications,thestartandstopbitsareusuallynottransmitted
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
