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Cisco VISM Installation and Configuration Guide
Release 3.0, Part Number OL-2521-01 Rev. D0, June 2004
Chapter3 VISM Functional Descriptio n
Signaling Function
5. When the call agent receives the dialed numb er, it uses the dial plan to communicate with th e remote
call agent to set up the call.
6. The call is terminated when either the called or calling party go es on-h ook.
CCS Processing in Switched AAL2 PVC Operating Mode
In the switched AAL2 PVC operating mode, CCS signaling can be configured to pass (backhaul) CCS
signals between the user PBXs and the call agents.
You can configure T1 and E1 lines for CCS signaling. You must specify a particular DS0 as an Integrated
Services Digital Network (ISDN) Dchannel to carry the Primary Rate Interface (PRI) signaling.
Signaling from the private branch exchange (PBX) is received on the D channel as level 3 Q.931
messages encapsulated in the information field of level 2 Q.921 LAPD information frames.
The Q.921 link is terminated at the VISM, and on the call agent side, a Redundant User Datagram
Protocol/User Datagram Protocol/Internet Protocol (RUDP/UDP/IP) connection is used to carry level 3
Q.931 signaling between VISM and the call agent. This link to the call agent flows through an
intermediate router. From VISM to the router, the R UDP/ UDP/IP pack ets are se gmented a nd transport ed
as AAL5 ATM cells.
The function of the VISM PRI/backhaul feature is to pass the Q.931 messages betwe en the PBX and the
call agent.
VISM handles all Q.921 frame types. For information type frames, t he pr oc ess i s d escri bed in the
following list:
1. VISM extracts the Q.931 frame.
2. VISM then places it in an RUDP datagram.
3. The RUDP datagram is encapsulated in UDP and IP packets (using the IP address and a spe cified
port number of the destination call agent).
4. The SAR section of VISM segments the IP message into AAL5 ATM cells for transport to the call
agent via an edge router.
In CCS processing, communication between VISM and the call agent involves both call control
information using xGCP protocols and CCS signaling using Q.931 protocol. Both are t ransported using
the AAL5 ATM connection.
Signaling from the call agent to the PBX is handled in the same manner but in reverse:
1. Signaling from the call agent is stripped of its RUDP/UDP/IP headers and trailers.
2. Signaling is then encapsulated into Q.921 information type frames for transmission to the user’s
PBX.
VISM is not involved with the signaling content but acts as an interface between the PBX and the call
agent.