Real-Time Transport Protocol (RTP) Payload Format for the Variable-Rate Multimode Wideband (VMR-WB) Extension Audio Codec
updates
- rfc-4348 — Real-Time Transport Protocol (RTP) Payload Format for the Variable-Rate Multimode Wideband (VMR-WB) Audio Codec
Extracted elements (13)
No new media type registration is required for VMR-WB mode 4 because the new mode shares the same characteristics and functionalities as existing modes, and all provisions and restrictions of RFC 4348 apply equally to mode 4.
VMR-WB mode 4 uses previously reserved CMR values (7, 8) and frame type entries (7, 8, 10) from RFC 4348 tables, exploiting the original design's provision for future extension rather than requiring new protocol machinery.
All frame sizes in Table 3 are unique and different, which allows the use of header-free payload format for all modes of operation including the new VMR-WB mode 4 frames.
The existing flexibility in RFC 4348 for future extensions allows addition of VMR-WB mode 4 without impacting interoperability with earlier RFC 4348 implementations. Existing table entries (frame types) remain unchanged and original modes are unaffected.
If a mode-set is specified during session initiation, the encoder MUST NOT use modes outside of the negotiated subset. The allowed mode set is now 0 to 4 (updated from 0 to 3 per RFC 4348).
The mode-set MIME parameter MAY be extended in the future; if not present, all operating modes in the set 0 to 4 are allowed for the session.
The 'mode-set' MIME parameter value 4 is defined to indicate support for VMR-WB mode 4. Active modes of operation are negotiated through the offer/answer model defined in Section 9.3 of RFC 4348.
VMR-WB mode 4 is a new operating mode added to the VMR-WB codec, operating at lower average data rates than existing modes (0-3) to improve system capacity in IP and non-IP networks. It has similar characteristics to existing modes including variable bit rate, wideband processing, and discontinuous transmission support.
IANA updated the VMR-WB media type registration (originally in RFC 4348) to extend the mode-set parameter to include mode 4, changing the allowed range from '0 to 3' to '0 to 4' and adding RFC 4424 as a reference.
Security considerations are identical to those specified in RFC 4348. No new security issues are introduced by the addition of VMR-WB mode 4.
Table 1 is updated to include CDMA2000 rate-set I permissible frame types: Full-Rate at 266 bits (13.3 kbps) and 171 bits (8.55 kbps), Half-Rate at 124 bits (7.2 kbps) and 80 bits (4.0 kbps), Quarter-Rate at 54 bits (2.7 kbps) and 40 bits (2.0 kbps), Eighth-Rate at 20 bits (1.0 kbps) and 16 bits (0.8 kbps), Blank and Erasure at 0 bits.
Table 2 (CMR values) is updated: CMR value 7 maps to VMR-WB mode 4, CMR value 8 maps to VMR-WB mode 4 with maximum half-rate encoding, replacing previously reserved values. CMR values 9-14 remain reserved; CMR 15 indicates No Preference.
Table 3 (frame types for RTP transport) is updated: FT 7 is Full-Rate 8.55 kbps (171 bits), FT 8 is Half-Rate 4.0 kbps (80 bits), and FT 10 is Eighth-Rate 0.8 kbps (16 bits), replacing reserved entries 7, 8, and 10 of RFC 4348. All frame sizes remain unique to allow header-free payload format use.