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CHANNEL CHARACTERISTICS
Telephone lines have varying characteristics, some of which,
such as distortion, delay, echo, and noise, are detrimental to
data channel operation.
Noise
Background Noise
A certain amount of noise on the channel is expected, and
IBM modems are designed for efficient operation when it is
present. In voice conversation, background noise can be
heard as a faint hiss or as an
absenc~
of complete silence in
quiet periods. Switched network channels tend to have
higher noise levels than dedicated channels.
Impulse Noise
Impulse noise results from occasional electrical disturbances.
In voice conversation, impulse noise is heard as "static"; in
data communication, impulse noise is destructive, and re-
transmission is necessary. Some impulse noise is generally
present on a communications channel and should be con-
sidered in planning system throughput.
Echo
Single "talker" echo (transmitter to receiver and back) must
have a power level below the receiver threshold of the trans-
mitting modem. Double "listener" echo must have a power
level considerably lower than the level of the non-reflected
signal.
To minimize the impediment to voice conversation caused
by echo, common-carrier channels have devices called echo
suppressors. An echo suppressor allows signals to pass in the
direction of the strongest signal, and it has the ability to
change direction when one speaker stops talking and the -
other starts.
An echo suppressor has the same turnaround capability in
a channel used for data communications, but the turnaround
time (unnoticed in voice conversation) is objectionable in
data communications. Dedicated channels are often specified
without echo suppressors, to eliminate the turnaround time
allowance; thus echo may be present.
Distortion and Delay
Distortions and delays are inherent in common-carrier power-
ing and regenerating equipment. Common carriers offer ser-
vices conditioned for differing specifications with respect to
distortions and delays. The tolerable levels are specified by
the grade of conditioning when arrangements are made for
Modem Environment
the channel. (For example, the 3872 requires a type 3002
channel with C 1 conditioning.)
Equalizers
Modem equalizers can compensate for certain imbalances in
common-carrier equipment, which result in delay and amp-
litude distortion. Equalization in the modem permits satis-
factory operation on a less expensive channel. Normally,
equalization to the characteristics of a dedicated channel is
performed manually at the time of modem installation. In
switched network operation, channel characteristics may
be different for each call; equalization is performed auto-
matically by the 3872 Modem.
Bridges
In a duplex centralized multipoint configuration, communi-
cation between tributary stations is not allowed. In addi-
tion, the tributary stations should not hear each other or
themselves as they transmit. In duplex operation, proper
insertion of split four-wire or individual two-wire bridges
(Figure 1-6) at each drop from the dedicated channel is
necessary in the common carrier equipment. This arrange-
ment prevents false signals on the control station transmit
line caused by interference between control station carrier
and a tributary station transmission.
MODEM-DATA TERMINAL EQUIPMENT INTERFACE
Portions of the modem-DTE interface have been discussed
earlier under "Modem Concepts and Configurations"
(Figure 1-4). This interface is standard in the industry
(see the notice "To The Reader" at the front of this manual.)
The DTE interface of the IBM 3872 Modem is designed to
provide proper operation with DTE devices that comply
with EIA RS-232-C and CCITT Recommendation V.24.
(The DTE interface of the 3872 is specified under "3872-
DTE Interface", following.) The following brief descriptions
of the interface lines (Figure 3-1) are intended to relate the
standards description to the 3872.
Functional Description of DTE Interface Lines
• 'Transmitted data' - The transmitting DTE controls this
line to present serial binary data to the modem.
• 'Transmit signal element timing' - The transmitting modem
raises (turns on) this line at the optimum time for the DTE
to change the binary value (on or 1; off or 0) of the 'trans-
Modem Environment
3-1
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