time-division multiplexer system. The data from each
user can be in the form of bits, bytes, or blocks. The
data signals from all users are compiled into frames
for transmission on a
single, high-speed
communications channel.
Transmit and receive frames are used for
half-duplex communications. Transmit frames are
sent and a receive time slot is enabled for return
information.
In this manner, a single carrier
frequency and modem may be used to transmit and
receive information at a fairly high rate of speed.
Since time slots are preset and assigned, if a user
has no data to transmit, the time slot is wasted.
Advantages of a TDM system include the following:
its ability to handle devices with varying speeds, its
effectiveness when used with devices that transmit
data almost continuously, and its simple
implementation.
Frequency-Division Multiplexing
Frequency-division multiplexing (FDM) divides
a band of frequencies into several distinct channels or
tones. Each tone carries a portion of the data being
transmitted. FDM devices can be complex because a
separate modulator/demodulator circuit is required for
each tone used.
The composite tones are then
modulated to a single carrier frequency for radio
transmission.
FDM allows for the parallel transmission of data
over a single communications channel. For example,
the Link-11 communications system uses 15 audio
tones to transmit 30 bits of parallel data. Each tone
transmits two bits of differential quadrature
phase-shift keyed data.
SUMMARY—FUNDAMENTALS OF DATA
COMMUNICATIONS
This chapter introduced you to the building blocks
of a data communications system. The following
information summarizes the important points you
should have learned.
COMMUNICATIONS SYSTEMS— Digital
data devices that exchange data over distances are
known as communications systems.
A basic
communications system consists of the following
three components:
a transmitter, a receiver, and a
communications channel. The transmitter converts
digital data into a form (digital or analog) useable by
the communications channel. The receiver accepts
data from the communications channel and converts
the
data
back
to
its
pure
digital
form.
Communications systems that can transmit and
receive data are known as duplex systems, while
communications systems that are limited to transmit
only or receive only are simplex systems. Duplex
systems that transmit data, pause, and then receive
data are half-duplex systems. Full-duplex systems
can transmit and receive data simultaneously.
COMMUNICATION CHANNELS— Several
types of communications channels are in use today.
The most common are landlines and radio
communications. Landlines are physical cables that
connect computers; they are common in local area
networks.
Radio communications use the
radio-frequency bands to exchange information. The
most common bands used in the Navy are the HF and
UHF bands.
DECIBEL MEASUREMENT SYSTEM— The
decibel measurement system is used to measure the
gain or loss of amplifiers, antennas, communications
lines, and other types of communications equipment.
A gain of +3 decibels (dB) indicates that the output
power of the circuit, compared to the input power, has
doubled. Each +3 dB gain indicates a doubling of
power. For example, a signal that has a gain of 6 dB
is twice as strong as a signal that has a gain of 3 dB.
A S Y N C H R O N O U S
T R A N S -
MISSION— Asynchronous transmission refers to
data sent without the use of timing pulses. Data
signals are sent a byte at a time, with start, stop, and
parity bits added to each byte.
S Y N C H R O N O U S
T R A N S -
MISSION— Synchronous transmission refers to the
sending of long, uninterrupted streams of data with a
predefined start and stop sequence.
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