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In this manual, the IBM 3872 Modem (Frontispiece) is de-
scribed as an item of equipment in the medium-speed modem
field.
FUNCTION OF A MODEM
The term modem is a contraction of the terms "modulator"
and "demodulator". The modulator in the modem modifies
binary data signals to modulated line signals which can be
transmitted over a data communications channel. The demod-'
ulator in the modem retrieves data from the modulated line
signals, and converts it to binary data.
As described in this manual, a modem is an electronic
device that is used when two machines in a data system are
located too far apart to be directly connected by a multiwire
cable. A communications channel (usually a telephone line)
provides the interconnection, and a modem at each end
adapts the machine signals to and from the communications
channel (Figure 1-1 ). The transmitting modem modifies
binary data for transmission; the receiving modem retrieves
the data and converts it back to binary form.
HIGHLIGHTS
The IBM 3872 Modem is a synchronous, 2400-bps modem
with half-speed (1200 bps) capability. It operates on ded-
Station A (Transmitting)
IBM 3872 Modem
icated voice-grade communications channels and, in the USA
and Canada, it operates on public, switched network, voice-
grade communications channels.
Using differential phase shift keying (DPSK), it converts
business machine serial binary data signals into signals suit·
able for transmission over the communications channel when
transmitting. When receiving, it retrieves data from the chan-
nel signals and converts the data back to serial binary form.
DESIGN FEATURES
The 3872 has an internal pseudo-random bit-pattern gener-
ator that is used to minimize the likelihood of prolonged in-
tervals of sensitive bit patterns on the channel. It is also used
for testing purposes.
Equalization provided in the modem compensates for
distortion, introduced by the communications channel, that
may impair transmission. A 3872 equipped for switched net-
work operation has automatic receiver equalization; a 3872
equipped for point-to-point operation on a dedicated channel
has switch-adjustable receiver equalization; a 3872 equipped
for multipoint tributary operation has switch-adjustable trans-
mitter and receiver equalization (Figure 1-2).
Every 3872 is capable, under operator or prog~ammed
control (and regardless of configuration), of operating at
half-speed (1200 bps). Operation may be able to continue
at the satisfactory lower rate when a 2400 bps throughput
is degraded. All modems on the channel must operate at the
Station B (Receiving)
Data
Terminal
Equipment
QData
Communications Channel
Modem
Data
Terminal
Equipment
Modem
Serial Data Out
Serial Data In
(pulses)
(pulses)
I
I
I
Converts pulses to
Converts modulated
modulated signal
I
I
signal to pulses
I
I
I
I
I
1 ...
4
. _ _ _ _ _ _ Modem-Line-Modem System _ _ _ _ _ ........
.,.!
described in this manual
Note: Data transfer is shown from Station
A
to Station B.
When the data transfer direction 1s reversed, the
roles of the modems and the data terminal equipment
are interchanged.
Figure 1-1. Simplified Teleprocessing System
I
IBM 3872 Modem
1-1
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