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CHANNEL SPECIFICATIONS
The 3872 requires a communications channel having char-
acteristics of FCC Tariff 260-type 3002 channel with C 1
conditioning. In addition, the specifications implied by
Bell System administrative instructions are required. These
specifications are described in Bell System Data Communi-
cations Technical Reference PUB41004, Transmission
Specifications for Voice Grade Private Line Data Channels
(March 1969). CCITT Recommendation M.102 defines a
channel equivalent to a FCC Tariff 260-type 3002 with C2
conditioning. A single M.102 channel or two M.102
channels in tandem provide satisfactory service for 3872
operation. See "Communications Channel Specifications"
under "Installation Planning" in Part 3.
Power level and signal-to-noise ratio may vary among
World Trade countries. The transmit level and receiver
sensitivity threshold of the 3872 are strapped in World
Trade applications, to allow for adjustments to the specific
situation. See "Strapping Options" under "Installation
Planning" in Part 3.
DIRECT LINE ATTACHMENT
For use on dedicated channels in some World Trade countries
(especially the United Kingdom), it may be necessary to pro-
vide de (direct current) isolation from channel access facilities.
The Direct Line Attachment feature provides isolation trans-
formers for this purpose. See "Special Feature Descriptions"
in Part 3.
SWITCHED NETWORK OPERATION IN THE
USA AND CANADA
The use of the 3872 on switched network lines in the USA
and Canada is standard practice, with the proper features.
Switched network operation in World Trade countries, how-
ever, is by special arrangement. See your IBM sales
representative.
CADUCEE ATTACHMENT FEATURE (FRANCE)
The Caducee Attachment feature for the IBM 3872 Modem
equips a basic 3872 (without other features) to operate on
the French PTT Caducee Network.
It
provides an interface
and control functions compatible with the Caducee coffret
(line coupler and control unit); this feature includes the
Talk/Data switch and appropriate equalization. The Caducee
Attachment feature is also available with either or both
modems of a 3872 dual modem.
Appendix A. World Trade Considerations
DTE Interface
The 3872-to-DTE interface is the conventional DTE interface
described in Part 3; however, 'data terminal ready' must be
controlled for switched network mode. The discrete lines
illustrated in Figure A-1 are described as follows:
• 'Calling indicator' (CI) - an incoming call from the
Caducee Network raises this line for 225ms, only. The
signal originates in the coffret and reaches the DTE
interface via the 3872 Caducee feature.
• 'Data terminal ready' (DTR) - this line originates in the
DTE; it must be up to make or hold a Caducee Network
connection, if the 3872 Talk/Data switch is in the DATA
position. (Dropping DTR breaks the connection.) If
the switch is in the TALK position, an incoming call may
be answered manually at the coff ret or the coffret may be
placed in intervention mode for remote end signaling.
• 'Request to send' (RTS) - this line is raised by the DTE
to initiate transmission. Because of the "four-wire,"
duplex facilities ('transmit' line and 'receive' line) pro-
vided by the Caducee Network, 'RTS' may be on con-
tinuously, and the 3872 is allowed to control the trans-
mitting of carrier signals.
If the DTE keeps 'RTS' on, or if the 3872 is strapped
for continuous carrier, carrier starts when the connection
is made and stops when it is broken; the rise of 'clear to
send' is delayed 25ms on the first transmission and then
remains up until the connection is broken.
If the DTE waits for 'data set ready' before raising
RTS, 'clear to send' is delayed 25ms; RTS may then
remain up until the connection is broken ('clear to send'
remains up, also). If the DTE drops 'RTS' after each
transmission (and the 3872 is not strapped for con-
tinuous carrier), the 25ms clear-to-send delay recurs with
each new transmission.
• 'Data set ready' (DSR) - the Caducee feature raises this
line when all of the following conditions are present:
(1) a connection is made, (2) 'RTS' is up (or continuous
carrier is strapped), (3) the Talk/Data switch is set to
DATA, and (4) the communications channel has not
been dedicated to the coffret telephone set. When
'DSR' and 'RTS' (or continuous carrier) are up, the
3872 transmits continuous carrier signals; 'clear to send'
rises 25ms later.
Coffret Interface
The coffret interface provides facilities for controlling a
Caducee Network connection and for transmitting and
Appendix A. World Trade Considerations
A-1
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