HP 600 Series, Services And Applications

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Figure 10. Connection Sequence for V.25 bis
Figure 10 shows the sequence of events in a successfully initiated
connection between routers using V.25 bis terminal adapters. To signal that it
is ready to engage in a V.25 bis dialog, the adapter raises its CTS lead (clear
to send). The “CRN” sent by the router to the adapter is a connection request
and the phone number to dial. The adapter responds with “VAL”, which
simply indicates that the request was formatted properly and that the
adapter was in the proper state to receive the request. The adapter lowers
CTS when an answer tone is detected to disable further V.25 bis dialog. The
call is then placed.
On the receiving side, the adapter that previously raised CTS sends the
router an “INC” to indicate that an incoming call has arrived. The originating
phone number and the first part of the data packet is forwarded to the
router; this assumes that ANI (automatic number identification) is available.
Thus, the “ring indicator” control signal is not required for detection of an
incoming call since that is obtained using the V.25 bis “INC” indication. If the
TA
TA
DTR
VAL
CTS
CTS
DSR
DTR
CRN
DSR
CTS
CTS
INC
a
b
ISDN
Network
Events
*** Connection Established ***
Data Transmitted
data
rec’d
call TA "B"
call from "A"
rec’d
A
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p
li
c
a
ti
o
n
N
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/
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3
Improving Network Availability
Switched Circuit Types
3-21