Voice over Frame Relay Voice Bandwidth

In a Voice over Frame Relay (VoFR) network, the frame-relay voice-bandwidth command is used in a Frame Relay map class to set aside the bandwidth required to successfully transport the desired number of calls. This method of bandwidth provisioning operates in much the same way as IP RTP Priority and Low Latency Queuing features that reserve bandwidth for traffic flows. Unlike LLQ or RTP Priority, the frame-relay voice-bandwidth command has the capability to provide CAC. Because VoFR operates at Layer 2, Frame Relay headers can be examined to determine whether a frame is carrying voice payload or data payload. The channel identification (CID) in the voice frames is used to identify which individual frames belong with the current voice conversations in progress. Because the frame-relay voice-bandwidth command sets aside a maximum bandwidth for voice conversation, and tracks the number of conversations in progress, the frame-relay voice-bandwidth command has the capability to deny the admission of an additional conversation if the maximum bandwidth allocated to voice will be exceeded.

This CAC feature only applies when VoFR is used, as defined in Frame Relay Forum Implementation Agreement FRF 11. VoFR does not use IP, UDP, and RTP to encapsulate the voice traffic. By eliminating the need for IP and RTP/UDP headers, VoFR reduces the amount of overhead needed to transport the voice payload, as show in the following formula:

For example, a G.729 call using 50 pps requires 10.4 kbps, as shown in the following calculation:

For example, a G.711 call using 50 pps requires 69.6 kbps, as shown in the following calculation:

Bandwidth per call = (160 + 6) * 8 * 50 = 69.6 kbps

Figure 8-10 shows a host site connected to a remote site via a Frame Relay network. Assume that VoFR was selected to carry the voice payload and 6 simultaneous calls, using G.729 codec with 50 pps, are required to be successfully transported and protected.

Figure 8-10 Voice over Frame Relay (VoFR)

Figure 8-10 Voice over Frame Relay (VoFR)

The bandwidth required to successfully support and protect six simultaneous calls is determined by the following formula:

Bandwidth required = Number of desired calls * Bandwidth per call

In the case of the network in Figure 8-10, the following bandwidth is required:

After the bandwidth requirements have been determined, the requirement can be applied to the VoFR map class to establish voice conversations. If the bandwidth requirements are not applied to the VoFR map class, the Voice-Bandwidth size defaults to 0, resulting in CAC rejects for all call attempts because of insufficient bandwidth.

Example 8-8 demonstrates how CAC for VoFR is configured by provisioning 64 kbps to transport and protect voice conversations across the Frame Relay network.

Example 8-8 Frame Relay Voice Bandwidth interface Serial0/0 encapsulation frame-relay no fair-queue frame-relay traffic-shaping

interface Serial0/0.1 point-to-point frame-relay interface-dlci 100 class vofr

map-class frame vofr frame cir 256000 frame bc 2560 frame fragment 320

frame fair-queue !64 kbps is enough for six G.729 calls at 10.4 kbps each, frame-relay voice-bandwidth 64000

You can implement this CAC method only if VoFR is a viable technology in your network.

Table 8-7 evaluates the VoFR Voice-Bandwidth mechanism against the CAC evaluation criteria described earlier in this chapter.

Table 8-7 VoFR Voice-Bandwidth CAC Evaluation Criteria

Table 8-7 evaluates the VoFR Voice-Bandwidth mechanism against the CAC evaluation criteria described earlier in this chapter.

Table 8-7 VoFR Voice-Bandwidth CAC Evaluation Criteria

Evaluation Criteria

Value

VoX supported

VoFR

Toll bypass or IP telephony

Toll bypass only

Platforms and releases

Cisco 2600s, 3600s, 3810, and 7200 router; Cisco IOS Release 12.0(4)T

PBX trunk types supported

All

End to end, local, or IP cloud

Local

Per call, interface, or endpoint

Per call, per PVC

Topology awareness

None

Guarantees QoS for duration of call

None

Postdial delay

None

Messaging network overhead

None

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