What bandwidth/codec issues are there?

Q. What bandwidth/codec issues are there?

Expat Email support the following Codecs on their Asterisk based VoIP network:

G711A/law (Expat Email Preferred Codec)
G711U/law
G729A/B

For calls between two SIP devices all codecs are supported provided the devices at each end of the call support them.

Bandwidth calculations

The following table shows the relevant bandwidth usage for these codecs.

Codec

Packet Size

Codec Bandwidth

IP Bandwidth

G.711

120

64Kbps

80Kbps

G.729A

10

8Kbps

24Kbps

Notes:

Figures above are for simplex traffic - normal duplex voice traffic will double these figures

The main cause of the extra IP bandwidth usage is IP and UDP headers.

Because a normal conversation includes a lot of silence, which often means no packets are sent at all, the full bandwidth is not used at all times.

Choice of Codec

The choice of codec that you make can have a large effect on your VoIP communications. To reduce the bandwidth costs of voice transmission, a variety of different codecs are in use.

In Digital PSTN networks the G711 codec is typically used. This is widely thought to be the best quality Codec to use. Expat Email recommend this codec as we use this codec internally and when we pass the voice traffic to our PSTN gateway this also uses the G711 codec. Using this codec reduces the amount of coding and decoding of the voice traffic across multiple networks.

When selecting your Codec you need to take into account the amount of bandwidth each codec uses.

G711 codecs are the most expensive when it comes to bandwidth.

G729 codecs are ideal if you are having bandwidth issues. (in order to use this codec you must request it is enabled on your VoIP account)

If you are using the Single Line account you can use any codec that the originating and destination devices can support.

Mac and Softphones

There are as far as we are aware at the moment, no softphone packages that support the g729-A codec on the MAC

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