The ITU-T has defined a number of different standards for the coding and compression of voice traffic. The table below outlines the main standards that you should be familiar with, including their associated data rates. As a rule, don’t worry about the codec names of the standards – focus on the ITU standards associated with the codecs instead, since this is typically how they are referred to. The codecs listed in the tables are ordered according to their data rates.
As the table above suggests, using the G.729 codec (which uses a data rate of 8 kbps) will obviously result in significant bandwidth savings over using the G.711 codec, with its 64 kbps data rate. The complexity heading in the table is used to define how many voice calls can function over a single digital signal processor (DSP). In general, a single voice network module will include many DSPs. A medium complexity codec can typically support 4 calls per DSP, while a high complexity codec will support only 2.
When codec data rates are provided, the number is associated with transmissions in a single direction only. In other words, the G.729 codec uses a data rate of 8 kbps from the sender to the receiver. A two-way VoIP conversation across the same circuit would require 16 kbps total – 8 kbps in each direction.