Codec Configurations

VoIP Codec Configurations

We originate all call invites and UDP traffic from our SuperPOP IP range. For redundancy reasons you should allow SIP and UDP traffic from all of our IPs on your gateways.

We will offer you call media with the following codecs and in this preference order:

G711 alaw

G711 ulaw

G729

GSM

G723

Codec G711

G.711 is a high bit rate (64 Kbps) ITU standard codec. It is the native codec of the digital telephone network.

There are two versions: A-law and U-law. U-law is indigenous to the T1 standard used in North America and Japan. The A-law is indigenous to the E1 standard used in the rest of the world. The difference is in the method the analog signal being sampled. In both schemes, the signal is not sampled linearly, but in a logarithmic fashion. A-law provides more dynamic range as opposed to U-law.

Using G.711 for VoIP will give the best voice quality; since it is the same codec used by the PSTN network and ISDN lines, it sounds just like using a regular or ISDN phone. It also has the lowest latency (lag) because there is little to no need for buffering, which costs processing power. The downside is that it takes more bandwidth than other codecs, up to 84 Kbps including all the UDP and IP overhead. However, with increasing broadband bandwidth, this should not be a problem.

G.711 is supported by most VoIP providers.

Codec G729

Codec G.729 may require a license unless using pass-thru, free version available for use in countries without patents or for educational use only

G.729 is an ITU standard codec. It offers toll quality speech at a reasonably low bit rate of 8Kbps. However, it is a rather "costly" codec in terms of CPU processing time, therefore some VoIP phones and adapters (notably the Linksys/Sipura/Cisco brands) can only handle one G.729 call (channel) at a time. This can cause calls to fail if the user attempts to use three-way calling, or place simultaneous calls on both lines of a two-line device, and G.729 is the only allowed codec.

Codec GSM

GSM (Global System for Mobile communications) is a cellular phone system standard popular outside the USA. The GSM standard includes a codec, often just referred to as the GSM when discussing codecs.

The original 'Full Rate' GSM speech codec is named RPE-LTP (Regular Pulse Excitation Long-Term Prediction). This codec uses the information from previous samples (this information does not change very quickly) in order to predict the current sample. The speech signal is divided into blocks of 20 ms. These blocks are then passed to the speech codec, which has a rate of 13 kbps, in order to obtain blocks of 260 bits.

Codec G723

G.723.1 is an ITU standard codec.

It is reasonably low bit rate (6.3Kbps or 5.3Kbps) and used in the H.323 standard.