Using Codecs Correctly with VoIP
The Importance of VoIP Codecs
A VoIP codec plays a large part in the experience of making an Internet call. Codec stands for coder/decoder and is essentially an algorithm for translating audio into a series of signals. One mustn't think that the codec is meant only for Internet applications. Even the traditional PSTN system implements an algorithm for transmitting voice over wireline. There are many factors affecting the performance of the codec such as the number of samples it uses to re-create the audio signal as well as the compression necessary. One issue of relevance is that the greater the compression the lesser bandwidth that is used by VoIP. However, it could also mean an increase in CPU processing time which may be an issue for slower hardware.
When two VoIP devices communicate, they have to negotiate the codec to use between them. It's important for them to use the same one because otherwise it's like two people speaking different languages to each other with no translator in between. One VoIP device sends the list of codecs available in its SIP header and thereby negotiates which one will be utilized for the duration of the call. The RTP media stream will then be encoded using this codec.
Selecting a Proper VoIP Codec
Hopefully in the future there will be no need for customers utilizing VoIP to have to manually select which codec to have to use. Even today, most SIP clients will not require users to expressly configure the codecs and will make a best effort to negotiate the appropriate ones to use with the relevant SIP provider. However, expert VoIP users might want to take matters into their own hands depending on their specific requirements and trade-offs which they need between quality, bandwidth and CPU power.
It's important to find out which codecs your SIP provider supports because your SIP client will be connecting with them first. You can specify multiple codecs to be used in order of preference so that the negotiation will end up with you having your most preferred codec in the circumstances available. Make sure you input these preferences into your VoIP configuration settings screen in your SIP client. There are many different clients available for both PCs as well as mobile platforms. VoIP for the iPhone is available using a number of free or paid services. Contact your SIP provider to find out whether they can give you a client which they have developed in-house so that it comes preconfigured with all the settings that work best with their system. This would be the optimal solution since parameters like codecs are automatically in place.
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