★ Speech coding is all about turning voice into digital form

★ Speech is inherently analog, as GSM is a digital system.

★ The digitization is employed by ISDN, and the current telephone systems that are used for multiplexing voice lines, with high speed trunks, optical fiber lines is done by Pulse Code Modulation(PCM)

★ The PCM output is 64 kbps, which is too high over a radio link in feasibility

★ The 64 kbps signal is redundant

★ The algorithm used in conventional cellular is Vector Sum Excited Linear Predictive speech compression.