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Bandwidth Calculations

As mentioned earlier in this document, voice traffic does not ride for free—on the contrary, it requires a significant investment in bandwidth (no matter the VO technology). For Frame Relay, the measurements are quite easy to calculate.

The first thing that occurs in a voice call is the call setup. For Frame Relay voice calls, this requires 18Kbps for the duration of the call setup (not the call itself, just the setup; you get some of it back). Without proper bandwidth provisioning, it may be difficult to get calls set up as you near saturation on the link. With proper planning, this should not become an issue, right?

Once the call is completed, the amount of bandwidth required can be easily calculated. The voice payload is 30 bytes; the overall voice frame generally is 37 bytes. By default, CSA-CELP 8Kbps compression is utilized. Therefore, the following applies:

Frame size/payload * compression = necessary bandwidth

In this case:

37/30 * 8 = 9.86 (always round up) = 10Kbps per call (This is sometimes 11Kbps per call, depending on the implementation.)

Numerous supported compression algorithms exist, all with varying compression results (8, 16, 32, 40k, etc.). It should also be stated that compression is not mandatory. Fax over Frame Relay is also supported. All of these varying compression methods have different bandwidth requirements. CSA-CELP (8Kbps compression) generally has the lowest bandwidth requirements.

OK, so what does bandwidth per call have to do with anything? It has to do with everything. In a cost-driven world, it is imperative to understand that voice does require additional bandwidth over and above what has been set aside for data. This translates into a significant increase in the CIR requirements. Higher CIR equals higher line charges—these are the recurring line charges that are assessed each month.

If a CIR of 256000bps is negotiated for the data connection, and call capacity planning has determined that 25 phone calls will be active at a given time, an additional 250000bps (10000bps per call) of CIR will be required for the calls to be supported. Suddenly, the requirements demand 456000bps in CIR. 512000bps is probably the closest increment that most carriers will offer.

A final note of interest in calculating bandwidth requirements is that a VoFR call setup is larger than the actual call. As we've seen, there is a 10000bps-per-call requirement. However, VoFR call setup requires 18000bps (or sometimes 19000bps, depending on the implementation) in order to complete. This should not become an issue in your calculations if they are properly done. The idea is that only the very last call will need the 8000bps above what is set aside for voice traffic. With proper capacity planning, this state of "call saturation" should not occur—we all know how that goes, however. With proper monitoring of PVCs, it should be known through trending analysis well ahead of time that PVCs are approaching saturation. After it is known, proper steps can be taken to increase voice bandwidth.

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