This European telecommunication Standard (ETS) is an introduction to GSM 06.20 (ETS 300 581-2) (4), GSM 06.21 (ETS 300 581-3 (5), GSM 02.22 (ETS 300 581-4 (6), GSM 06.41 (ETS 300 581-5) (7) and GSM 06.42 (ETS 300 581-6 (8) ETSs dealing with the speech processing functions in the half-rate channel of the GSM system. A general overview of the speech processing functions is given, with reference to the ETSs where each function is specified in detail.