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Voice, IP, and ATM MPLS Features

Voice Over AAL2 Network

The Voice over AAL2 (VoAAL2) solution with VISM is used to replace trunk lines between voice switches. A separate PVC is needed for voice channels destined for a specific remote endpoint. Signaling information is sent using one of two methods: CAS signaling is sent with voice traffic, and CCS signaling is sent on a separate AAL5 connection. Except for signaling extraction and insertion, the VISM does not participate in the signaling process.

  • As described with the previous VISM features, the VISM supports standards-compliant AAL2 adaptation for the transport of voice over an ATM infrastructure with AAL2 trunking mode.

Here are the major features of AAL2 trunking mode:

  • The voice payload is transported across the packet network using an AAL2 PVC. Multiple voice channels are supported in a single PVC using CIDs (channel identifiers) and subcell multiplexing. A single VISM supports up to 64 AAL2 PVCs.

  • Call setup through a call agent is not supported. All bearer paths are provisioned using either the command-line interface (CLI) or Simple Network Management Protocol (SNMP).

  • If CAS signaling is used, the signaling is transported across the packet network along with the voice payload using AAL2 Type 3 packets.

  • CCS signaling channels are supported and are transported across the packet network using a separate AAL5 PVC.

In AAL2 trunking mode, VISM provides a set of AAL2 trunks carrying the voice payload over the packet network. Figure 22-6 shows a high-level view of VoAAL2 between two voice switches.

Figure 22-6 VoAAL2 with VISM

Voice payloads and CAS signaling are transported across the AAL2 trunk. If a channel is configured for CCS signaling, the signaling is transmitted by extracting HDLC frames and forwarding them over preprovisioned AAL 5 virtual circuits (the voice payload is still transmitted using AAL 2).

In this role, the VISM merely passes signal traffic to and from the trunk.

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