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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
ATM Voice Data Processing Function
Use the CLI PRI/backhaul commands to do the following:
•Create and delete session sets
•Create, delete, configure, and display sessions and sessions groups
•Create, delete, configure, and display D channels for CCS
•Display PRI/backhaul statistics
Refer to Chapter 5, “CLI Commands,” for more information on CLI commands.

CAS Processing in AAL2 Trunking Operating Mode

CAS signaling is extracted from the voice data and is transported across the packet network in AAL2
trunking operating mode. The signaling is transported across the same trun k (VC) and the same channel
identifier (CID) as its associated voice stream, using AAL2 Type 3 messages in accordan ce wi th ITU- T
I.366.2. The messages are used for CAS (A,B,C, and D) robbed bits, fax/modem tones, and digits and
are transported with triple redundancy.

CCS Processing in AAL2 Trunking Operating Mode

CCS signaling is maintained as Q.931 messages and transported across the packet network using an
AAL5 PVC in AAL2 trunking operating mode. The local and remote ends of the PVC are the same as
those for the AAL2 PVC trunk carrying the associated voice data.
ATM Voice Data Processing Function
The VISM DSPs process incoming voice data (for compression, ECAN, etc.) and then the data is
prepared for transport over the ATM network. Voice samples are processed into ATM packets and then
into ATM cells in preparation for transport. VISM then transports the cells to Voice over ATM (VoATM)
networks with the following supported operating modes:
•VoI P
•Switched AAL2 PVC
•AAL2 trunking
•AAL1 SVC

Transporting Voice Cells with VoIP

The VoIP switching operating mode processes voice cells in the following order to transport them over
ATM networks:
1. Formatted into Real-Time Transport Protocol (RTP) packets.
Note RTP allows time-stamping of the voice samples, which permits dejittering o f the samples
transmitted to the destination TDM line.
2. Encapsulated in a UDP packet.