GSM yakashandisa mhando dzakasiyana-siyana dzemavoice codecs kuti ipinze 3.1 kHz audio pakati pe7 ne13 kbit / s. Pakutanga, ma codecs maviri, akatumidzwa mazita emhando dzedata chiteshi dzavakagoverwa, akashandiswa, anonzi Half Rate (6.5 kbit / s) uye Full Rate (13 kbit / s). Izvi zvakashandisa hurongwa hwakavakirwa pamutsetse wekufungidzira wekodhi (LPC). Mukuwedzera pakushanda nema bitrates, ma codecs aya akaitawo kuti zvive nyore kuziva zvikamu zvakakosha zve odhiyo, zvichibvumira mhepo interface layer kuisa pamberi uye kuchengetedza zviri nani zvikamu izvi zvechiratidzo. GSM yakawedzerwazve muna 1997 neyakagadziridzwa yakazara chiyero (REF) codec, 12.2 kbit / s codec inoshandisa yakazara-rate chiteshi. Pakupedzisira, nekusimudzira kwakasimba kweEFTS, UMR yakadzoserwa kuita yakasimba-yakagadziriswa codec inonzi yakaderera-band AMR, iyo ichiri kushandiswa zvakanyanya kana ichishandiswa pazvikamu zvepamusoro-band kana yakaderera-band mhando, asi
pakati pe7 ne13