SIP Trunks are a new VoIP phone line technology which can give you great sound quality and new features not available with analog lines or even PRI service.
What's a Codec?
A codec, which stands for coder-decoder, converts an audio signal (your voice) into compressed digital form for transmission (VoIP) and then back into an uncompressed audio signal for replay. It's the essence of VoIP. Codecs vary in the sound quality, the bandwidth required, the computational requirements, etc. Each service, program, phone, gateway, etc., typically supports several different codecs, and when talking to each other, negotiate which codec they will use.
Little Known Fact - You can assign a different codec to individual phones. Your staff can use medium quality/low bandwidth G.729 codec while the boss and legal department uses the superior quality/heavy bandwidth G.722 codec.
Common VoIP Codec Protocols
G.729 G.729 is a codec that has low bandwidth requirements but provides good audio quality. This is the most commonly used codec in VoIP calling and has a MOS rating of 4.0
G.711 G.711 is a codec that was introduced by ITU in 1972 for use in digital telephony. With only a 1:2 compression and a 64K bitrate for each direction (128K plus some overhead), it is best used where there is a lot of bandwidth available. G.711 has a MOS rating of 4.2
G.722 G.722 is a high bit rate (48/56/64Kbps) ITU standard codec which, because it is of even better quality of the traditional public switched telephone network (PSTN), it can be used for a variety of higher quality speech applications. This standard also requires an adequate amount of bandwidth and usually rates a 5.0 on the MOS scale.
There are many codecs out there, some like the G.711 are royalty free, others require licensing (which is often included in the gateways). Some that we haven't mentioned here drive the wholesale movement of voice traffic among the carriers and are used for specialty applications. And to make it a bit more confusing, they all contain variations within their own specification.
How to Decide?
The codecs that provide the best quality consume the most data bandwidth, thus there is a trade-off that you need to consider. The easiest way is to ascertain, on a phone by phone basis, whether you want the voice conversation to be:
- Slightly less then the quality of an excellent cell phone call (G.729)
- Equal to the quality of an analog land-line today (G.711)
- Better than the public switched telephone network for voice critical applications (G.722)