AMR rtp 及sdp 相关协议

(RFC 3267 published June 2002, subtype last updated April 2007 per RFC4867)

Media Type name:     audio

Media subtype name:  AMR-WB

Required parameters: none

Optional parameters:

  These parameters apply to RTP transfer only.

  octet-align: Permissible values are 0 and 1.  If 1, octet-aligned
		   operation SHALL be used.  If 0 or if not present,
		   bandwidth-efficient operation is employed.

  mode-set:  Restricts the active codec mode set to a subset of all
		   modes, for example, to be able to support transport
		   channels such as GSM networks in gateway use cases.
		   Possible values are a comma-separated list of modes from
		   the set: 0,...,8 (see Table 1a [4]).  The SID frame type
		   9, SPEECH_LOST (frame type 14), and NO_DATA (frame type
		   15) are never included in the mode set, but can always
		   be used.  If mode-set is specified, it MUST be abided,
		   and frames encoded with modes outside of the subset MUST
		   NOT be sent in any RTP payload or used in codec mode
		   requests.  If not present, all codec modes are allowed
		   for the payload type.

  mode-change-period: Specifies a number of frame-blocks, N (1 or
		   2), that is the frame-block period at which codec mode
		   changes are allowed for the sender.  The initial phase of
		   the interval is arbitrary, but changes must be separated
		   by multiples of N frame-blocks, i.e., a value of 2
		   allows the sender to change mode every second frame-
		   block.  The value of N SHALL be either 1 or 2.  If this
		   parameter is not present, mode changes are allowed at
		   Any time during the session, i.e., N=1.

  mode-change-capability: Specifies if the client is capable to
		   transmit with a restricted mode change period.  The
		   parameter may take value of 1 or 2.  A value of 1
		   indicates that the client is not capable of restricting
		   the mode change period to 2, and that the codec mode may
		   be changed at any point.  A value of 2 indicates that the
		   client has the capability to restrict the mode change
		   period to 2, and thus that the client can correctly
		   interoperate with a receiver requiring a mode-change-
		   period=2.  If this parameter is not present, the mode-
		   change restriction capability is not supported, i.e.
		   mode-change-capability=1.  To be able to interoperate
		   fully with gateways to circuit switched networks (for
		   example, GSM networks), transmissions with restricted
		   mode changes (mode-change-capability=2) are required.
		   Thus, clients are RECOMMENDED to have the capability to
		   support transmission according to
		   mode-change-capability=2.

  mode-change-neighbor: Permissible values are 0 and 1.  If 1, the
		   sender SHOULD only perform mode changes to the
		   neighboring modes in the active codec mode set.
		   Neighboring modes are the ones closest in bit rate to
		   the current mode, either the next higher or next lower
		   rate.  If 0 or if not present, change between any two
		   modes in the active codec mode set is allowed.

  maxptime: The maximum amount of media which can be encapsulated
		   in a payload packet, expressed as time in milliseconds.
		   The time is calculated as the sum of the time that the
		   media present in the packet represents.  The time SHOULD
		   be an integer multiple of the frame size.  If this
		   parameter is not present, the sender MAY encapsulate any
		   number of speech frames into one RTP packet.

  crc: Permissible values are 0 and 1.  If 1, frame CRCs SHALL be
		   included in the payload.  If 0 or not present, CRCs
		   SHALL NOT be used.  If crc=1, this also implies
		   automatically that octet-aligned operation SHALL be used
		   for the session.

  robust-sorting: Permissible values are 0 and 1.  If 1, the
		   payload SHALL employ robust payload sorting.  If 0 or if
		   not present, simple payload sorting SHALL be used.  If
		   robust-sorting=1, this also implies automatically that
		   octet-aligned operation SHALL be used for the session.

  interleaving: Indicates that frame-block level interleaving SHALL
		   be used for the session, and its value defines the
		   maximum number of frame-blocks allowed in an
		   interleaving group (see Section 4.4.1).  If this
		   parameter is not present, interleaving SHALL NOT be
		   used.  The presence of this parameter also implies
		   automatically that octet-aligned operation SHALL be
		   used.

  ptime: see RFC 2327 [11].

  channels: The number of audio channels.  The possible values
		   (1-6) and their respective channel order is specified in
		   Section 4.1 in [12].  If omitted, it has the default
		   value of 1.

  max-red: The maximum duration in milliseconds that elapses between
		   the primary (first) transmission of a frame and any
		   redundant transmission that the sender will use.  This
		   parameter allows a receiver to have a bounded delay when
		   redundancy is used.  Allowed values are between 0 (no
		   redundancy will be used) and 65535.  If the parameter is
		   omitted, no limitation on the use of redundancy is
		   present.

Encoding considerations:
	The Audio data is binary data, and must be encoded for non-
	binary transport; the Base64 encoding is suitable for email.
	When used in RTP context the data is framed as defined in [14].

Security considerations:
	See Section 7 of RFC 4867.

Public specification:
	RFC 4867
	3GPP TS 26.190, 26.192, 26.193, 26.201

Applications that use this media type:
	This media type is used in numerous applications needing
	transport or storage of encoded voice.  Some examples include;
	Voice over IP, streaming media, voice messaging, and voice
	recording on digital cameras.

Additional information:
	The following applies to stored-file transfer methods:

	Magic numbers:
	  single-channel:
	  ASCII character string "#!AMR-WB\n"
	  (or 0x2321414d522d57420a in hexadecimal)
	  multi-channel:
	  ASCII character string "#!AMR-WB_MC1.0\n"
	  (or 0x2321414d522d57425F4D43312E300a in hexadecimal)
	File extensions: awb, AWB
	Macintosh file type code: amrw
	Object identifier or OID: none

	AMR-WB speech frames may also be stored in the file format "3GP"
	defined in 3GPP TS 26.244 [31] and identified using the media
	type "audio/3GPP" or "video/3GPP" as registered by RFC 3839
	[32].

Person & email address to contact for further information:
	Magnus Westerlund 
	Ari Lakaniemi 

Intended usage: COMMON.
	This media type is widely used in streaming, VoIP, and messaging
	applications on many types of devices.

Restrictions on usage:
	When this media type is used in the context of transfer over
	RTP, the RTP payload format specified in Section 4 SHALL be
	used.  In all other contexts, the file format defined in Section
	5 SHALL be used.