VoIP Telephone CallsWhat is VoIP? | VoIP Bandwidth | Skype Bandwidth | ACELP |
In the old days our plain old telephone service (POTS) was analogue. As you spoke on the phone, the microphone would vibrate, and send electrical waves down the wire to cause the receiver to vibrate exactly the same at the other end to reproduce your voice. This worked quite well for many years, since old Alexander Graham Bell invented it, but the voice signal was subject to interference, losses and distortion, and often did not come out quite right at the other end. Only one conversation could be held on one copper line at a time until they learned to multiplex several calls together, but even this took up a lot of resources.
In today's digital world, almost all voice telephone traffic is digitized in some way before it is sent down the line, and then converted back to an audible signal at the distant end.
If enough bandwidth is used, this method can provide crystal clear reproduction of your voice. To make voice traffic more cost effective, the voice is sampled and then digitized and added together with many other voice conversations on the same transmission medium, whether it be copper wire, fiber, satellite or internet.
To be even more cost effective, the digital signal is compressed before it goes down the wire and then decompressed at the other end into good quality voice.
The most common G.729 ACELP voice algorithm samples the voice in 10mS slices and uses 8kbps of bandwidth. When sent over the internet, the voice packets need to be addressed and labeled so that they know where to go and what do with them at the other end. This addressing scheme takes up an additional 16kb of header (or overhead) making each voice circuit 16+8=24kbps. So your typical VoIP call takes up 24kbps of your bandwidth.
It is important that you have that bandwidth available during the call, otherwise you will have distorted sound, drop outs, dropped calls, or one way calls where only one party can hear.
This interesting document states that Skype claims bandwidth usage 24-128 kpbs. During their testing they observed typical bandwidths 40 kbps. They observed reasonable call quality at bandwidths as low as 16kbps and almost unintelligible voice at 12 kbps.