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VISM and VISM-PR Card Features
Figure 1-6
PVC operating modes.
Figure 1-6
VISM Block Diagram for VoIP Switching and Switched AAL2 PVC Operating Modes
CCS
TDM Line
T1/E1 Lines
Handling
CAS
Voice
The CAS signaling path on the TDM side is embedded in the voice stream but is separated at the bearer
processing function. The CAS signaling then joins the CCS signaling path for CAS/CCS processing and
is backhauled to the call agent. The path between the call agent and bearer processing, via a connection
handling function, is for call setup and teardown.
Voice TDM Network Interface
The voice payload path is shown as a solid line along the bottom of
streams arrive and depart on the T1/E1 lines. Depending on the application, these streams consist of
voice bearer channels (with or without CAS signaling) and separate CCS channels (if CCS signaling is
used). The TDM line handling function provides the physical layer interface, which includes framing,
line codes, clocking, loopbacks, physical alarms, etc. Bearer channels, including CAS, are sent to the
bearer processing function. CCS channels are sent to the CAS/CCS processing function.
Further processing of the bearer channels is performed by the DSPs. This processing provides ECAN,
compression, A/Mu law conversion, silence suppression, and fax/modem handling. If CAS signaling is
present, signaling bits are extracted at the DSP stage and sent to the CAS/CCS processing function.
ATM Network Interface
The ATM processing function receives the processed DS0 voice streams and prepares them for transport
over a packet network. The voice streams are divided into specific sample periods (for example, 5 ms or
10 ms) and formatted into service specific convergence sublayer (SSCS) packets appropriate for the
method of transport over the ATM network. The available transport methods are VoIP (using AAL5) and
voice over AAL2. Processing of the ATM packets further segments the voice payload into ATM cells for
transport over the network using a SONET port on the PXM card.
Cisco VISM Installation and Configuration Guide
1-10
shows the major functional blocks and interfaces for the VoIP switching and switched AAL2
VISM
CCS
CAS/CCS
Processing
CAS
CAS
Bearer
Processing
Voice
Chapter 1
CCS (Q.931)
CAS (xGCP)
xGCP
Connection
Handling
Connection Setup
Teardown
ATM Processing
Release 3.0, Part Number OL-2521-01 Rev. D0, June 2004
Overview of the VISM and VISM-PR Cards
Call
Agent
Voice payload
To ATM
Network
Figure
1-6. All external TDM
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