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.
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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.