GSM Full Rate Audio (GSM 06.10)
Legacy GSM voice and older Asterisk/voicemail systems still produce `.gsm` files. Distinct from the GSM cellular network spec — this is the audio codec bitstream format.
What It Defines
GSM full-rate speech codec at 13 kbps, stored as raw 33-byte frames (`.gsm` files) or in WAV `fmt` tag 0x0031. Magic often starts with GSM frame sync patterns; libsndfile/tsoaudio conventions prepend a 33-byte frame header. MIME type: `audio/x-gsm`.
Canonical (Normative)
Related References
European standards body for ICT, originally created for GSM. ETSI partners with 3GPP and is the European partner organization. Publishes NFV, MEC (Multi-access Edge Computing), and TETRA (digital trunked radio) standards in addition to mobile.
Related Specs
WAV is the universal file format for call recordings, IVR prompts, voicemail exports, and PSTN debug captures. Every telephony platform (Twilio, Asterisk, FreeSWITCH) reads and writes WAV-wrapped G.711.
AMR-NB is the fallback voice codec for pre-VoLTE mobile networks and many carrier voicemail systems. The `#!AMR` file header and RFC 4867 RTP framing are distinct from AMR-WB.
G.711 is the common denominator of telephony. Every PSTN gateway and legacy SIP trunk speaks G.711. When codec negotiation fails and you're left with 64 kbps PCM, this is why.
GSM underpins SMS delivery, international roaming, and network fallback. Understanding A-interface, MSC, and MSISDN is essential for any telecom or SMS gateway integration.