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Cisco VISM Installation and Configuration Guide
Release 3.0, Part Number OL-2521-01 Rev. D0, June 2004
Chapter 3 VISM Functional Description
Signaling Function
Figure 3-4 CAS Signaling in Initiating and Terminating a Call
Note Figure 3-4 shows only the local CAS aspects of call setup. The entire process of call setup involves
many more messages between the local and remote call agents and the local and remote VISMs.
Refer to the “Call Control Function” section on page 3-15 for more information.
The call processing for the VoIP switching and switched AAL2 operating modes is described in the
following list:
1. The call agent requests VISM to look for an off-hook signal when the line is idle.
2. Upon receipt of an on-hook signal, VISM starts a timer and checks later to ensure that the line is
still off-hook.
3. VISM notifies the call agent that the caller has gone off-hook (this timer mechanism is also used
when processing on-hook/off-hook signaling to determine flash-hook events).
4. When the call agent is informed that the user is off-hook, the call agent instructs VISM to generate
dial tone and to look for dialed digits.
D
S0 VISM Call Age
nt
Notification Request, (off hook)
Ack
Ring to Ground
VISM
Start Timer
Loop still closed
Clear Timer
Notify Message, (off hook)
Dial tone
Digits
Notify Message (digit string)
Connect
On-hook
Notify Message, (on-hook)
Notification Request (dial tone,
digit collect, on-hook)
CALL IN PROGRESS
Delete Connection
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