AMR Narrowband (AMR-NB)
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.
What It Defines
Adaptive Multi-Rate narrowband speech codec (4.75–12.2 kbps) for GSM/2G/3G. `.amr` files use a `#!AMR\n` magic header followed by speech frames with 3-bit mode indices. RTP carriage defined in RFC 4867. MIME type: `audio/amr`.
Canonical (Normative)
Related References
The global collaboration of telecommunications standards bodies that produces the mobile network specifications: GSM/EDGE, UMTS/HSPA, LTE/4G, 5G NR, and IMS. Technical specifications (TS) and reports (TR) are freely available at 3gpp.org. Releases (Rel-8, Rel-15, Rel-17…) provide versioned capability sets.
Related Specs
AMR-WB is mandatory for VoLTE — every LTE voice call uses it. Understanding rate adaptation and the AMR payload format (RFC 4867) is essential for mobile VoIP and operator codec policy.
The AMR file header is a minimal envelope pattern — one magic line then framed payloads. Required when parsing `.amr`/`.awb` attachments from carrier APIs or MMS gateways.
VoLTE and IMS carry AMR over RTP per RFC 4867 — not as `.amr` files. CMR bits and octet-align vs bandwidth-efficient modes cause real interoperability bugs between carriers.
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.
LTE is the dominant global mobile network. VoLTE, QoS bearers, and EPC architecture are directly relevant when building mobile-connected applications, IoT modules, or carrier infrastructure.