Figure 1-2.—Asynchronous character code.
greater than the reference (power gain), while a minus
sign (–) indicates that the power level is less than the
reference (power loss). The value 0 dBm indicates
that the output power is equal to the 1-milliwatt
reference. It is also used to express a definite amount
of power (1 milliwatt). The value 0 dBm equates to
1 milliwatt.
DIGITAL DATA COMMUNICATIONS
TECHNIQUES
Data signals transmitted over communications
channels need to follow specific protocols to ensure
they are synchronized. In normal I/O data exchanges,
this process is accomplished by the system of requests
and acknowledges. In addition, the data signals have
to be properly formatted for the receiving computer to
decode them properly.
ASYNCHRONOUS AND SYNCHRONOUS
COMMUNICATIONS
Two major data-formatting methods are used to
make sure the transmitting computer and the receiving
computer(s) are synchronized:
asynchronous
(character framed) and synchronous (message
framed).
Both methods are used to identify
intelligence transmitted in the form of serial bit
streams.
Asynchronous Transmission
Asynchronous transmission of data is commonly
found in landline communications systems and some
forms of teletype communications. Generally,
asynchronous, or character-framed, transmission is
used to transmit seven- or eight-bit data, usually in
ASCII character format. Each character has a specific
start and end sequence—usually one start bit and one
or two end (stop) bits.
Figure 1-2 illustrates the
transmission format of an asynchronous data stream.
A parity bit (even or odd) maybe included to ensure
the accuracy of the transmitted data. Asynchronous
characters may be transmitted one at a time or as a
string of characters;
however, each character
transmitted will have start and end bits. When data
signals are transmitted in this format, synchronization
occurs on a character-by-character basis between the
transmitting and receiving devices and provides some
allowance for timing inaccuracies. Any inaccuracy in
timing is corrected with the arrival of the next
character.
Synchronous Transmission
Most tactical
digital
information
links
communicate
using
synchronous
messages.
Synchronous transmission is a more sophisticated
method of data transmission. It sends data in long
uninterrupted streams, with a predefine start and stop
sequence. The start sequence is generally referred to
Figure 1-3.—Synchronous message format.
1-5
