VoIP: How much bandwidth?

BANGALORE, INDIA: Now enterprises are looking to deploy VoIP over WAN or VPN to reduce their telephone costs. A common question we get asked is how much bandwidth should be allocated for VoIP? Well there are quite a few factors in play when determining bandwidth for VoIP such as, protocol, codec and compression technology used, andwhether silence suppression is used.

Out of these, type of codec you decide to usemakes a lot of difference. Just in case you don't know already, codecs are used for converting analog voice signal to digital . Every application supports several codecs and usually depending upon the connection speed. They automatically determine which codec to use. Most commonly used codecs include G.723.1 -which is a high quality code, uses about 87.2Kbps in both directions, while for dailup and other similar connections, iLBC (Internet Low Bit Rate Codec) is used, which consumes about 27.7 Kbps. To know more about bandwidth used by the codecs, please refer to the table below.

Another catch here is in applications like Skype, you cannot specify which codec to use. Applications automatically identifies the connection speed and choose the codec accordingly. Another point worth mentioning here is, bandwidth requirements will change according to compression used. Similarly if longer packets are used, bandwidth requirements are also reduced. To reduce bandwidth consumption by applcations, use cRTP and enhanced cRTP, as these reduce the size of header in a packet. If you want to know more about these, you can refer to rfc3545.

Setup for VoIP

For testing the bandwidth consumption of VoIP over VPN, we built the following setup. We first built a VPN server with the help of OpenVPN, and then connected the VPN server's external NIC to the WAN emulator, which emulates the actual internet scenario. Then we connected a machine to the internal interface of the VPN server. We then connected a client machine directly to the WAN emulator, which means it is virtually connected directly to the Internet. Then we connected the client machine to the VPN server and started a VoIP session using software called 'MyPhone'.

Observation

Initially we limited the bandwidth to 512 Kbps using the WAN emulator. And we started a VoIP communication using MyPhone. With 512 Kbps, the conversation was smooth and clear. Next we decreased the bandwidth limit to 256 Kbps and further to 128 Kbps. There was hardly any difference in the voice quality. But, we did observe delay in the voice as we decreased the bandwidth. Then we decreased the speed to what we get over GRPS. Still we could not hear the voice. The voice was cracking a lot and only 20% of the conversation was understandable. Next we increased the bandwidth to what we get on dial-up (54 Kbps ideally). The VoIP quality improved a lot compared to the GPRS, but still not recommended for a smoother VoIP call. Hence for a VoIP call, the minimum desirable bandwidth is 128 Kbps.