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Does a VoIP call use 64kbps?
64kbps is the bandwidth of the voice codec when using G711 for example. However, this needs to be encapsulated in IP and other headers.
When converting an analog voice signal to digital, various codecs (coder-decoder algorithms) can be used. The two most common codecs in a Cisco IP telephony environment are G711 and G729 - there are many others, but G711 and G729a (a variant of G729) are supported on Cisco's IP telephony handsets.
G711 comes in two forms - G711ulaw which is used in the USA and Canada, and G711alaw which is used in most of the rest of the world. The G711 codec uses 64 kbps for a unidirectional voice stream without headers.
G729 and G729a use 8 kbps without headers.
Normally, G711 is used on the LAN as it gives a better voice quality than G729 and bandwidth is more plentiful on a LAN. Across the WAN G729 is generally used when a bandwidth saving is required.
When calculating the amount of bandwidth a codec requires, all headers need to be taken into account. An RTP header is added to give sequencing and timestamps. UDP is the protocol used at layer 4 and IP at layer 3 of the OSI model. Various layer 2 technologies such as Ethernet, PPP and Frame-relay may be used.
An example voice packet with headers looks as follows:
The amount of bandwidth required for calls without the layer 2 headers would be:
G711 = 80 kbps
G729/G729a = 24 kbps
Working out the actual amount of bandwidth used for a call (including all headers) can be very tedious. One would have to make allowance for the different header sizes of Ethernet, PPP, frame-relay etc and also work it out for any other headers such as IPSec, GRE and MPLS.
This is where the Cisco Codec Calculator can help you. It will calculate exactly how much bandwidth a voice call (or multiple voice calls) will use across a number of layer 2 infrastructures. It can also take into account MPLS, VPNs, header compression and other options.
Visual DEMO:
DEMO of codec calculator
Click here for a demonstration of the codec calculator:
The codec calculator reveals that the actual bandwidth of the call is much higher with all the headers added.
Some examples:
* G729 across Frame Relay uses 28.14 kbps (including all headers and 5% for signalling)
* G711 across Ethernet uses 91.56 kbps (including all headers and 5% for signalling)
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