VoIP Cookbook: Calculating The Required Bandwidth
To estimate the amount of bandwidth consumed by a Codec, use Asterisk Guru's tools, which is available at http://www.asteriskguru.org/tools/bandwidth_calculator.php.This tool enables us to calculate the amount needed by a variety of Codecs in respect to a certain number of calls taking place simultaneously. The resulting calculated values will be shown as Incoming and Outgoing bandwidth.
An example of calculated required bandwidth for GSM and G.729 Codec is shown in the following table
| Number of calls | GSM | G.729 | ||
|---|---|---|---|---|
| Incoming (Kbps) | Outgoing (Kbps) | Incoming (Kbps) | Outgoing (Kbps) | |
| 1 | 28.63 | 28.63 | 23.63 | 23.63 |
| 2 | 57.25 | 57.25 | 47.25 | 47.25 |
| 3 | 85.88 | 85.88 | 70.88 | 70.88 |
| 4 | 114.50 | 114.50 | 94.50 | 94.50 |
| 5 | 143.13 | 143.13 | 118.13 | 118.13 |
| 6 | 171.75 | 171.75 | 141.75 | 141.75 |
So suppose your bandwidth capacity is 64 Kbps and the Codec you use is G.729. Then the maximum number of VoIP calls you can have for optimal voice quality is two. This is of course assuming that the Internet is not being used for other traffic, such as, e-mail, browsing, chatting or downloading.
A much more detailed calculation of a VoIP packet can be looked at http://www.packetizer.com/voip/diagnostics/bandcalc.html. By using this tool, we can see the bandwidth, packet rate, delay and even performance. The parameters used to derive the calculation are Payload (Codec), type of protocol and whether we want to use Silence Suppression. If you tick Silence Suppression, the average of bandwidth or packet delivered will become smaller.
From all of these calculations, we can deduce at least two things:
- The smaller the bandwidth, the larger the Mega Instruction Per Second (MIPS) required to operate in a processor. Today this might not be a problem, given that high-speed processors are now largely available at affordable price.
- The smaller the bandwidth, the higher the delay process. This is related to the needs to process high voice compression.
There are more interesting things we can conclude from these calculation. We recommend that you spend more time using the tool so that you will understand a variety of effects occurring in VoIP communication session.