VoIP: How Much Bandwidth ?

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'.

BR = Bit rate NEB = Nominal Ethernet Bandwidth (one direction)

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.