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VoIP Network

The VoIP solution with VISM requires one or more (up to eight) call agents to manage signaling for the voice calls. The Cisco Virtual Switch Controller (VSC) is one such call agent. Other call agents might be supported. The call agent receives call requests and other signaling messages from the voice switch and converts these to Simple Gateway Control Protocol (SGCP) or MGCP and sends them to the VISM. The VISM uses the signaling messages to set up or break down voice calls. Multiple call agents communicate with each other using Enhanced ISDN User Part (E-ISUP).

Here are the major features of VoIP mode:

  • Calls are set up under the control of one or more (up to eight) call agents using SGCP or MGCP protocol (CLI-configurable).

  • CAS signaling can be extracted from the voice payload and backhauled to the call agent using either SGCP or MGCP.

  • CCS signaling on an ISDN D-channel can be backhauled as Q.931 to the call agent using RUDP protocol.

  • The voice payload is converted to IP packets, which are then tunneled through an AAL5 PVC to the IP network.

  • A single PVC for both signaling and data can be configured with a second (redundant) channel together with automatic switchover in the event of a failure. Alternatively, AAL5 PVCs can be configured to provide a split into control and bearer streams, each on a separate PVC. If the signal and data streams are split, one of the PVCs (but not both) can be provided with a second redundant PVC.

  • In VoIP mode, VISM operates in conjunction with a call agent.

Briefly, the call setup procedure consists of the following steps:

Step 1

The calling party lifts the handset and dials a number. The resulting on/off hook and digit signaling are sent to the call agent either directly or by VISM through CAS backhauling over SGCP.

Step 2

The call agent at the originating end looks up the called number in a table and locates the remote call agent that manages the called number. It forwards the IP address of the calling VISM to the remote call agent using C-ISUP.

Step 3

The remote call agent responds by providing the IP address of the remote VISM to the originating VISM.

Step 4

With the exchange of IP addresses, a bearer circuit is set up across the IP/ATM network between the calling and the called VISMs. This procedure also involves the negotiation of coding, compression, and so on. Endpoints that identify the two ends of the call to the VISM/DS1/DS0 level are identified with the bearer circuit so that a complete end-to-end bearer circuit is established between the calling and called parties. At this point, the voice call can proceed.

Step 5

CAS signaling is sent to the call agent for processing. Any resulting action required of VISM is executed by commands from the call agent.


In VoIP mode, a single AAL5 PVC is set up for communication with the packet-switching network. All calls use this single PVC. It is the responsibility of the network edge router to extract the IP addresses from the voice payloads and route the traffic across the IP network.

Figure 22-7 VoIP Network

The network connecting VISMs is IP. This can be an IP-only network (for example, Cisco 7500 series routers) or an IP+ATM network (for example, MGX RPMs with an ATM backbone network). The VISMs have PVCs between them to carry control and voice (bearer) traffic. These PVCs use AAL5 for segmentation and reassembly (SAR) of the IP packets to ATM. Figure 22-7 shows a high-level view of a VoIP network using VISM. You can use the same PVC or separate PVCs for control and voice traffic.

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