VoIP is dependent on a certain amount of bandwidth and it is only due to the widespread acceptance of broadband modems that (useable) VoIP is possible. So – what exactly is bandwidth and how does it affect VoIP?

Bandwidth refers to the amount of digital data that can be transmitted in a certain time period. Internet connections are usually measured in KiloBits Per Second (kbps). Your connection may be rated at 128/512 kbps. This means that the upload stream (leaving your computer) has a maximum bandwidth of 128 kbps and download stream (coming to your computer) has a maximum bandwidth of 512 kbps.

To give you an idea of what that means in real-world terms consider the size of a web page which is made up of a few paragraphs of text and a few illustrations. The size of this web page might be about 3,000 bytes or 24,000 bits. At a download speed of 512 kbps this page can be viewed in 0.05 seconds – almost instantaneously.

Wait a minute (you may be saying) – web pages don’t load that fast on my computer. You won’t reach your maximum transfer speeds for a couple of reasons. First there is latency – the delay caused by the physical limitations of each piece of hardware the data is passed through. Latency is introduced through the transmission medium, the routers that determine the path the data will take, and storage when the data finally reaches your computer.

Other delays are caused by pathway congestion, error checking, transmission negotiations and extra data which is sent with the web page to determine the type of data being sent and its origin and destination.

What this means for VoIP is that enough bandwidth must be supplied to allow for the transmission of the actual voice data in real time as well as extra bandwidth for the overhead required for any data transmission.

The actual amount of voice data depends on the codec (enCOder/DECoder) used to compress the data, and this can range anywhere from 16 – 64 kbps. Extra overhead totals about 10 – 24 kbps for a total load of about 26 – 88 kbps. When it comes to VoIP it is wise to err on the side of caution – it is better to assume that you will need 88 kbps.

That translates into an Internet connection that can handle at least 128 kbps on the upload side. Extra bandwidth will allow others in the household to surf the net or download files while you are talking on the phone. If you wish to use conference calls even more bandwidth will be required.

There is no easy rule for determining the amount of bandwidth that a particular household will need. Count on 128 kbps as a minimum but consider going up to the maximum available if you have several computers and other devices connected to the Internet. Note that these speed are for uploads – the download speeds are usually quite a bit faster and present no problem for VoIP.