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 06.22 (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.