Calculating how much bandwidth a Voice over IP call occupies can feel a bit like
trying to answer the question; “How elastic is a piece of string?” However, armed
with a basic understanding of the parts that make up the whole, the question becomes
easier to understand. This White Paper examines the process that turns voice into
Voice over IP
The amount of bandwidth required to carry voice over an IP
network is dependent upon a number of factors. Among the most important
are:
Codec (coder/decoder) and sample period
IP header
Transmission medium
Silence suppression
The codec determines the actual amount of bandwidth that the voice data will occupy.
It also determines the rate at which the voice is sampled. The IP/UDP/RTP header
can generally be thought of as a fixed overhead of 40 octets per packet, though
on point-to-point links RTP header compression can reduce this to 2 to 4 octets
(RFC 2508). The transmission medium, such as Ethernet, will add its own headers,
checksums and spacers to the packet. Finally, some codecs employ silence suppression,
which can reduce the required bandwidth by as much as 50 percent.