Calculating Trunk Capacity or Bandwidth

The trunk capacity for voice calls can be calculated by the following formula:


Trunk capacity = (number of simultaneous calls to be supported) * (bandwidth required per call)

The first component of this formula, the number of simultaneous calls to be supported, is the number of circuits required for the known amount of traffic, as calculated from the Erlang tables.

NOTE If 100 percent of calls must go through, Erlang tables are not required; instead, the maximum number of simultaneous calls required should be used.

The second component of this formula, the bandwidth required for one call, depends on the codec used and whether cRTP and VAD are used. Earlier in this chapter, the section "Voice Bandwidth Requirements," including Table 8-6, illustrated some bandwidth calculations.

CAUTION Including VAD in bandwidth calculations can result in insufficient bandwidth being provisioned if the calls do not include as much silence as assumed and when features such as music on hold are used.

As an example of calculating the trunk capacity, assume that G.729 compression is used over a PPP connection at 50 pps and cRTP is used. From Table 8-6, each call uses 11 kbps. If five simultaneous calls are to be supported, 5 * 11 = 55 kbps is required for the voice calls.

NOTE The bandwidth for other traffic that will be on the link must also be accounted for.

As another example, based on the Erlang tables, ten circuits are required between two locations to satisfy user demands. VoIP over PPP is used on the link. The G.729 codec, using 50 samples per second, is used. cRTP is not used.

The per-call bandwidth information in Table 8-6 indicates that one voice call without header compression requires 26 kbps of bandwidth. Therefore, 10 * 26 = 260 kbps of bandwidth is required between the two locations, in each direction, to carry ten simultaneous voice calls.

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