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same rate, and throughput is half that of normal 2400-bps
operation. Reliable 1200 bps operation, however, may
actually improve throughput, as compared to a degraded
2400 bps operation.
Front panel switches, indicators, and a meter provide
much information and control to the 3872 user ( see "Opera-
tions" in Part 2). In addition, the call progress monitor
allows audible monitoring of automatic call origination.
Testing procedures verify proper transmit and receive
operation of the 3872 Modem at either or both ends of the
communications channel. Manual operation of automatic
switched network functions is provided for by front panel
switches: with automatic answering, a call can be answered
manually at the common-carrier telephone set; with auto-
matic call origination, a call can be made manually-either at
the modem or at the common-carrier telephone set. Refer
to "Operations" in Part 2, for operating and testing pro-
cedures.
Leased Line (Point-to-Point)
T
R [E]
Leased Line (Centralized Multipoint)
Control-Modem
T
R
Public Switched Network
T
R
[AE)
LEGEND
T Transmitter
R Receiver
[E] Equalizer
[AE]Automatic Equalizer
...
-.....
-
. ...
Figure 1-2. Equalizer Location
by
Configuration
1-2
IBM 3872 Modem User's Guide
Duplex
R (E]
T
Ha If-Duplex
Tributary Modems
R [E]
T [E)
~
R [E]
-
!(E]
--
R [AE]
T
R
[AE]
T
SPECIAL FEATURES
The basic 3872 Modem, without special features, is a ded-
icated channel multipoint control station modem. Special
features (Figure 1-3) equip the 3872 for the following con-
figurations and additional capabilities:
• Two modems in one cabinet
(with some exclusions)
• Point-to-point operation
• Multipoint tributary operation
• Switched network operation
(with automatic answering)
• Switched network operation
of a modem configured for
dedicated channel operation
• Automatic answering when
operating on switched
network backup
• Automatic (programmed)
call origination on switched
network
• Direct line attachment
(in some World Trade
countries)
• Voice communication on
dedicated channel facilities
• Multiple terminals using
one modem
Second Modem feature
Point-to-Point feature
Multipoint Tributary
feature
Switched Network
feature
Sw.itched Network Back-
up feature
Switched Network Back-
up, with Automatic
Answering feature
Automatic Call
Origination feature
Direct Line Attachment
feature
Alternate Voice feature
Fan-Out feature
Refer to "Special Feature Descriptions" in Part 3. Refer
to "Operations" in Part 2, for special feature operating and
testing procedures. Refer to Appendix C, in Part 4, for
switched network backup planning and operating considera-
tions.
PLANNING A MODEM INSTALLATION
The specific requirements of a modem installation vary from
system to system and may change within a system as it
expands. Your IBM sales representative can assist in deter-
mining the specific requirements for a specific system. This
section of the manual describes some of the general consider-
ations.
Data Terminal Equipment Considerations
Data terminal equipment (DTE) considerations are part of
data system characteristics. They include the following items,
which bear on the requirements for the modems and the
communications channels:
1. Terminal locations. This characteristic can affect the
choice of point-to-point, multipoint, or switched network
modem and channel configurations. The origins of input
data entering the system and the destinations of processed
data leaving the system may have a wide and irregular
range of geographic locations.
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