(registered 2018-11-28, last updated 2018-11-28)

Type name: audio

Subtype name: TETRA_ACELP

Required parameters: In SDP information describing the media type the
   following parameter is required:

   rate: RTP timestamp clock rate, which is equal to the sampling 
   rate. The typical rate is 8000; other rates may be specified.

Optional parameters: In SDP information describing the media type the
   following parameters are optional:

   ptime: packet time in milliseconds for the media represented in 
   a packet made up of one full TETRA speech frame.

   maxptime: maximum amount of media that can be encapsulated in each
   packet – this is the framing rate for the TETRA Inter-System 
   Interface.

   If the values of these optional parameters are omitted then the 
   following values shall be used.

   ptime: 60
   maxptime: 60

Encoding considerations: This media type is framed binary data.

Security considerations: RTP packets using the payload format defined
   in this specification are subject to the general security 
   considerations discussed in RFC 3550 and in any used profile, like
   AVP [RFC 3551] or SAVP [RFC 3711]. As this format transports 
   encoded speech, the main security issues include confidentiality, 
   authentication, and integrity of the speech itself. The payload 
   format itself does not have any built-in security mechanisms, 
   although any audio media and control information carried may be 
   separately ciphered.

   External mechanisms, such as SRTP [RFC 3711], may be used for the 
   security functionality. The appropriate mechanisms to provide 
   security to RTP and the payloads following this memo may vary, 
   depending on the application, the transport, and the signalling 
   protocol employed. 

   Security mechanisms that may be used are TLS [RFC 4346] (RTP over 
   TCP) and IPsec [RFC 4301], but other alternatives may also exist. 

   This payload format, which does not include any executable 
   content, does not exhibit any significant non-uniformity in the 
   receiver side computational complexity for packet processing, and 
   thus is unlikely to pose a denial-of-service threat due to the 
   receipt of pathological data.

   The payload audio part consists of media data that will either 
   decode to a 30ms speech frame as described in ETSI EN 300 395-2 
   or, if indicated, stolen frames which contain a Layer 2 signalling
   message as described in ETSI EN 300 392-2. An incorrectly 
   formatted L2 signalling message will be discarded as invalid. 
   Consequently, even if the payload contents are corrupted, the 
   decoded data size is capped and receipt of pathological data will 
   not cause an overly large decoded data set.

   It is recommended that the payload should be at a minimum 
   integrity protected in order to prevent payload modifications that
   may impact downstream terminal behaviour.

Interoperability considerations:

   This media type carries TETRA ACELP encoded audio framed as per 
   Generic Speech Format specification (TS 100 392-3-8) to allow 
   interoperability between network operators independent of the 
   network manufacturer.

   The encoding of the audio part of the payload is described in ETSI
   EN 300 395-2: "Terrestrial Trunked Radio (TETRA); Speech codec for
   full-rate traffic channel; Part 2: TETRA codec".

   For the purposes of using the TETRA_ACELP media type within the 
   TETRA Inter-System Interface (ISI) the following values of the 
   optional SDP parameter values are mandated or else the remote peer
   will refuse the connection setup.

   ptime: 60
   maxptime: 60

Published specification:

   ETSI TS 100 392-3-8 v1.3.12.7: Terrestrial Trunked Radio (TETRA); 
   Voice plus Data (V+D); Part 3: Interworking at the Inter-System 
   Interface (ISI); Sub-part 8: Generic Speech Format Implementation

   ETSI EN 300 395-2 v1.3.1: "Terrestrial Trunked Radio (TETRA); 
   Speech codec for full-rate traffic channel; Part 2: TETRA codec".

   ETSI EN 300 392-2 v3.8.1: "Terrestrial Trunked Radio (TETRA); 
   Voice Plus Data (V+D); Part 2: Air Interface (AI)".

Applications that use this media type: Devices or applications that 
   send or receive TETRA ACELP encoded audio over RTP/IP.

Person & email address to contact for further information:
   Miguel Angel Reina Ortega <MiguelAngel.ReinaOrtega&etsi.org>

Intended usage: COMMON

Restrictions on usage: This media type depends on RTP framing, and 
   hence is only defined for transfer via RTP (RFC 3550). Transfer 
   within other framing protocols is  not defined at this time.

Author: ETSI TCCE WG3

Change controller: ETSI