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Operation
When a switched network connection has been made
('coupler cut through' and 'data set ready' are up), modem
operation is point-to-point, half-duplex. This section de-
scribes procedures used to establish the switched network
connection, to transfer the use of the line to the modem,
and to disconnect from the switched network.
To manually place a data call on the switched network,
the DTE operator uses the telephone set attached to the
DAA. Assuming that the called station will answer auto-
matically, the calling operator waits for the answer tone to
end and cradles the telephone (thus transferring the line to
the modem).
At the called station, the modem senses the ringing on
the line, goes off-hook, generates the answer tone and
becomes ready, and transfers the channel to the answering
modem. The modem receivers are equalized automatically,
· and the channel then becomes available for use by the DTEs.
After data transfer, both stations "hang up the phone''
(go on-hook) to disconnect from the switched network.
Going on-hook can be automatic at either or both stations.
Conventionally, the calling station transmits first and the
called station receives. Figure 3-10 illustrates the placing of
the call, the transfer of the line to the modems, the auto-
matic equalization of the modems, the transfer of data, and
the automatic disconnecting of the call. Reversals of trans-
Calling (Sending) Station
DTE
Modem
Ready
DAA
Operator dials
number on TALK
Line
mission are the same as point-to-point, half-duplex. (See
"Operating Sequences" in Part 1.)
SWITCHED NETWORK BACKUP FEATURE
This feature equips the 3872 Modem configured for dedi-
cated channel operation with a backup switched network
data communication capability. Calls are made or answered
manually. With this
feature~
switched network service must
be obtained from the common carrier (including an access
arrangement with a telephone set) in addition to the dedi-
cated line service. The data system must provide optional
switched network access at the remote location, and pro-
cedures for standby changeover must be established (see
Appendix C). This feature excludes the Second Modem
feature.
Compatibility with the Switched Network feature on
the remote 3872 requires a 2.S-second delay before
the first transmission of each call.
Operation of the Switched Network Backup feature is identi-
cal to that described in "Switched Network, Manual Call, Man-
ual Answer Sequences". (See "Operating Sequences" in Part 1.)
SWITCHED NETWORK BACKUP WITH
AUTOMATIC ANSWERING FEATURE
This feature equips a modem using the Switched Network
Backup featme to automatically answer incoming calls. De-
pending upon the DTE and the data system, unattended
Answering (Receiving) Station
DAA
Modem
DTE
Ringing signal - - - - -
T
~Ready
Stop ringing - - - - - - G o "off hook,"
+
become ready
Become r e a d y - - - -
t
Become Ready
...___ Operator hears
'4--+4------.....,.t---Generate answer
I
answer tone end, goes
to DAT A, and cradles
handset.
' - -------·
'
r- .,.___
Become ready
Become ready
tone
!
Become ready
Jll
RequesT to Send - - - - Generate equalization signals-------+-1---------1-Equalize,
T
synchronize
Delay/clamp, equalize, synchronize
T
'
~
Clear to send
Generafe equalization .signals
r
Send data
Modulate carrier _ _ _ _ _ _ _ _ _ _ _ ,_,_ _ _ _ _ _ _ _ _ Demodulate
t
carrier
Drop send request
~Finish
sending
t
T
Received data
Drop clear to send,
.,.___ stop carrier
- - - - - - - - - - - - - t M
r
Become not ready ----..Go "on hook"-----1-
•
-i
C:-
~
Become not ready
~
Become not ready,
Become ready again
disconnect
Sense end of data
t
4---Go "on hook" ----Become not ready
r-
Become not ready,
~Become
not
ready~
---,
disconnect
Become ready again
Figure 3-10. Manual Call, Automatic Answering (Switched Network)
Special Feature Descriptions
3-17
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