TDMA based digital cellular communication systems, like GSM and D-AMPS, have a fixed rate speech encoder delivering data packets of fixed size to a channel encoder that encodes the data packets up to some given (fixed) total gross bit rate (the total bit rate available for information transfer on a given channel if no channel protection is provided). This scheme gives a certain degree of channel protection that is close to optimal for some conditions but not for others. For example, on good channels bandwidth is wasted on unnecessarily strong channel encoding and on bad channels the channel encoding may be too weak. Furthermore, since the same transmission power is used in both cases, unnecessary interference is produced in the first case, while insufficient transmission power may be provided in the second case.
It has also been suggested [1, 2] to use dynamically changing encoding modes with different mixes of speech/channel encoding suitable for different radio environments.