Audio/Video Transport Working Group
Internet Draft                        SPIRIT DSP

Intended status: Informational          						    February 25, 2009

RTP Payload Format for IP-MR Speech Codec draft-ietf-avt-rtp-ipmr-02.txt draft-ietf-avt-rtp-ipmr-03.txt

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This document specifies the payload format for packetization of SPIRIT
IP-MR encoded speech signals into the Real-time Transport Protocol
(RTP). The payload format supports transmission of multiple frames per
payload and introduced redundancy for robustness against packet loss.

Table of Contents

 1.	Introduction	2 Introduction......................................................3
 2.	IP-MR P-MR Codec Description	2 Description............................................3
 3. Payload Format	4 Format....................................................4
    3.1. Payload Format Structure	4 Structure.....................................4
    3.2. Payload Header	4 Header...............................................4
    3.3. Speech Table of Contents	5 Contents.....................................5
    3.4. Speech Data	5 Data..................................................6
    3.5. Redundancy Header	5 Header............................................6
    3.6. Redundancy Table of Contents	6 Contents.................................7
    3.7. Redundancy Data	6 Data..............................................7
 4. Payload Examples	6 Examples..................................................7
    4.1. Payload Carrying a Single Frame	6 Frame..............................7
    4.2. Payload Carrying Multiple Frames with Redundancy	7 Redundancy.............8
 5. Media Type Registration	8 Registration...........................................9
    5.1. Registration of media subtype audio/ip-mr_v2.5	8 audio/ip-mr_v2.5...............9
    5.2. Mapping Media Type Parameters into SDP	9 SDP......................10
 6. Security Considerations	9 Considerations..........................................11
 7. IANA Considerations	10 Considerations..............................................11
 8. Normative References	10 References.............................................11
 9. Author's Information	10 ............................................11
 10.	Expiration date	10 Disclaimer......................................................12
 11. Legal Terms	10 Terms.....................................................12

1. Introduction

This document specifies the payload format for packetization of SPIRIT
IP-MR encoded speech signals into the Real-time Transport Protocol
(RTP). The payload format supports transmission of multiple frames per
payload and introduced redundancy for robustness against packet loss.

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
document are to be interpreted as described in RFC 2119. 2119 [RFC 2119].

2. IP-MR Codec Description

The IP-MR codec is scalable adaptive multi-rate wideband speech codec
designed by SPIRIT for use in IP based networks. These codec is suitable
for real time communications such as telephony and videoconferencing.

The codec operates on 20 ms frames at 16 kHz sampling rate and has an
algorithmic delay of 25
ms. 25ms.

The IP-MR supports six wide band speech coding modes with respective bit
rates ranging from about 7.7 to about 34.2 kbps. The coding mode can be
changed at any 20 ms frame boundary making possible to dynamically
adjust the speech encoding rate during a session to adapt to the varying
transmission conditions.

The coded frame consists of multiple coding layers-base (or core) layer
and several enhancement layers which are coded independently. These
enhancement layers can be omitted and remaining base layer can be
meaningfully decoded without artifacts. This making bit stream scalable
and allows reduce bit rate during transmission without re-encoding.

This memo specifies an optional form of redundancy coding within RTP for
protection against packet loss. It is based on commonly known scheme
when previously transmitted frames are aggregated together with new
ones. Each frame is retransmitted once in the following RTP payload
packet. f(n-2)...f(n+4) denote a sequence of speech frames, and
p(n-1)...p(n+4) a sequence of payload packets:

     | f(n-2) | f(n-1) |  f(n)  | f(n+1) | f(n+2) | f(n+3) | f(n+4) |

      <---- p(n-1) ---->
               <----- p(n) ----->
                        <---- p(n+1) ---->
                                 <---- p(n+2) ---->
                                          <---- p(n+3) ---->
                                                   <---- p(n+4) ---->

But because of scalable nature of IP-MR codec there is no need to
duplicate a whole previous frame - only core layer may be retransmitted.
This reduces redundancy overhead while keeping efficiency. Moreover, the
speech bits encoded in core layer are divided on six classes (from A to
F) of perceptual sensitivity to errors. Using these classes as
introduced redundancy make possible to adjust trade-off between overhead
and robustness against packet loss.

The mechanism described does not really require signaling at the session
setup. The sender is responsible for selecting an appropriate amount of
redundancy based on feedback about the channel conditions.

The main codec characteristics can be summarized as follows:


    o Wideband, 16 kHz, speech codec


    o Adaptive multi rate with six modes from about 7.7 to about
      34.2 kbps


    o Bit rate scalable


    o Variable bit rate changing in accordance with actual speech


    o Discontinuous Transmission (DTX), silence suppression and
      comfort noise generation


    o In-band redundancy scheme for protection against packet loss

3. Payload Format

3.1. Payload Format Structure

The IP-MR payload format consists of a payload header with general
information about packet, a speech table of contents (TOC), and speech
data. An optional redundancy section follows after speech data. The
redundancy section consists of redundancy header, redundancy TOC and
redundancy data payload.

T The following diagram shows the standard payload format layout:
  +---------+--------+--------+- - - - - - +- - - - - - +- - - - - - +
  | payload | speech | speech | redundancy | redundancy | redundancy |
  | header  | TOC    | data   | header     | TOC        | data       |
  +---------+--------+--------+- - - - - - +- - - - - - +- - - - - - +

3.2. Payload Header

The payload header has the following format:
   0                   1
   0 1 2 3 4 5 6 7 8 9 0 1
  |T| CR  | BR  |D|A|GR |R|

    o T (1 bit): Reserved compatibility with future extensions. SHOULD
      be set to 0.


    o CR (3 bits): coding rate of frame(s) in this packet, as per the
       following table:
|  CR   | avg. bitrate |
|   0   |   7.7 kbps   |
|   1   |   9.8 kbps   |
|   2   |  14.3 kbps   |
|   3   |  20.8 kbps   |
|   4   |  27.9 kbps   |
|   5   |  34.2 kbps   |
|   6   |  (reserved)  |
|   7   |   NO_DATA    |

The CR value 7 (NO_DATA) indicates that there is no speech data (and
speech TOC accordingly) in the payload. This MAY be used to transmit
redundancy data only. The value 6 is reserved. If receiving this value
the packet SHOULD be discarded.


    o BR (3 bits): base rate for core layer of frame(s) in this packet.
      Values in the range 0-5 indicate bitrates for core layer, same as
      for CR. Values 6 and 7 are reserved. If one of
these values is     received the packet SHOULD be discarded. The base rate is the lowest rate
      for scalability, so speech payload can be scaled down not lower
      than BR value. If a received packet has BR > CR then during
      decoding it will be assumed that BR = CR.


    o D (1 bit): indicates if the DTX mode is allowed or not.


    o A (1 bit): byte-aligned payload. If A=1 then all speech frames
      MUST be byte-aligned. This mode speeds up speech data access.
      The A=0 value specifies bandwidth-efficient mode with no byte alignment (including
      alignment(including end of header).


    o GR (2 bits): number of frames in packet (grouping size). Actual
      grouping size is GR + 1, thus maximum grouping supported is 4.


    o R (1 bit): redundancy presence bit. If R=1 then the packet
      contains redundancy information for lost packets recovery.
      In this case after speech data the redundancy section is present.

3.3. Speech Table of Contents

The speech TOC contains entries for each frame in packet (grouping size
in total). Each entry contains a single field:


    o E (1 bit): frame existence indicator. If set to 0, this indicates
      the corresponding frame is absent and the receiver should set
      special LOST_FRAME flag for decoder. This can be followed by the
      lost frame itself or by empty frames generated by the encoder
      during silence intervals in DTX mode.

Note that if CR flag from payload header is 7 (NO_DATA) then speech TOC
is empty.

3.4. Speech Data

Speech data of a payload contains one or more speech frames or comfort
noise frames, as specified in the speech TOC of the payload.

Each speech frame represents 20 ms of speech encoded with the rate
indicated in the CR and base rate indicated in BR field of the payload
header. The length of the speech frame is variable due to the nature of
the codec and can be calculated after decoding.

3.5. Redundancy Header

If a packet contains redundancy (R field of payload header is 1) the
speech data is followed by redundancy header:

   0 1 2 3 4 5
  | CL1 | CL2 |

Redundancy header consists of two fields. Each field contains class
specifier for amount of redundancy partly taken from the preceding
packet (CL1) and pre-preceding packet (CL2), e.g. distant from the
current packet by 1 and 2 packets accordingly. The values are listed
in the table below:

            |  CL   | amount redundancy |
            |   0   |       NONE        |
            |   1   |      CLASS A      |
            |   2   |      CLASS B      |
            |   3   |      CLASS C      |
            |   4   |      CLASS D      |
            |   5   |      CLASS E      |
            |   6   |      CLASS F      |
            |   7   |     (reserved)    |

Each specifier takes 3 bits, thus the total redundancy header size is 6

3.6. Redundancy Table of Contents

  | Pkt1 Entries| Pkt2 Entries|

The redundancy TOC contains entries for redundancy frames from preceding
and pre-preceding packets. Each entry takes 1 bit like speech TOC entry



    o E (1 bit): frame existence indicator. If set to 0, this indicates
      the corresponding frame is absent.


    o For each preceding and pre-preceding packet the number of entries
      is equal to the grouping size of the current packet. E.g. maximum
      number of entries is 4*2 = 8.


    o If class specifier in the redundancy header is CL=0 (NO_DATA)
      then there is no entries for corresponding packet redundancy.

3.7. Redundancy Data

Redundancy data of a payload contains redundancy information for one or
more speech frames or comfort noise frames that may be lost during
transition, as specified in the redundancy TOC of the payload. Actually
redundancy is the most important part of preceding frames representing
20 ms of speech. This data MAY be used for partial reconstruction of
lost frames. The amount of available redundancy is specified by CL flag
in redundancy header section (3.5). This flag SHOULD be passed to
decoder. The length of redundancy frame is variable and can be 281
calculated after decoding.

4. Payload Examples

A few examples to highlight the payload format follow.

4.1. Payload Carrying a Single Frame

The following diagram shows a standard IP-MR payload carrying a single
speech frame without redundancy:

   0                   1                   2                   3
   0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
  |0|CR=1 |BR=0 |0|0|0 0|0|1|sp(0)                                |
  |                                                               |
  |                                                               |
  |                                                               |
  |                                                               |
  |                                                               |
  |                      sp(193)|P|

In the payload the speech frame is not damaged at the IP origin (E=1),
the coding rate is 9.7 kbps
(CR=1), kbps(CR=1), the base rate is 7.8 kbps (BR=0), and
the DTX mode is off. There is no byte alignment (A=0) and no redundancy
(R=0). The encoded speech bits - s(0) to s(193) - are placed immediately
after TOC. Finally, one zero bit is added at the end as padding to make
the payload byte aligned.

4.2. Payload Carrying Multiple Frames with Redundancy

The following diagram shows a payload that contains three frames, one of
them with no speech data. The coding rate is 7.7 kbps (CR=0), the base
rate is 7.7 kbps (BR=0), and the DTX mode is on. The speech frames are
byte aligned (A=1), so 1 zero bit is added at the end of the header.
Besides the speech frames the payload contains six redundancy frames
(three per each delayed packet).

The first speech frame consists of bits sp1(0) to sp1(92). After that 3
bits are added for byte alignment. The second frame does not contain any
speech information that is represented in the payload by its TOC entry.
The third frame consists of bits sp3(0) to sp3(171).

The redundancy header follows after speech data. The one-packet- delayed
redundancy contains class A+B bits (CL1=2), and two-packet- delayed
redundancy contains class A bits (Cl2=1). The one-packet- delayed
redundancy contains three frames with 20, 39 and 35 bits respectively.
The first frame of two-packet-delayed redundancy is absent, it is
represented in its TOC entry, and two other frames have sizes 15 and 19

Note that all speech frames are padded with zero bits for byte

   0                   1                   2                   3
   0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
  |0|CR=0 |BR=0 |1|1|1 0|1|1 0 1|P|sp1(0)                         |
  |                                                               |
  |                                                               |
  |                  sp1(92)|P|P|P|sp3(0)                         |
  |                                                               |
  |                                                               |
  |                                                               |
  |                                                               |
  |                                               sp3(171)|P|P|P|P|
  |CL1=2|CL2=1|1 1 1|0 1 1|red1_1(0)                    red1_1(19)|
  |red1_2(0)                                                      |
  |   red1_2(38)|red1_3(0)                                        |
  |         red1_3(34)|red2_2(0)          red2_2(14)|red2_3(0)    |
  |             red2_3(18)|P|P|P|P|

5. Media Type Registration

This section describes the media types and names associated with this
payload format.

5.1. Registration of media subtype audio/ip-mr_v2.5

Type name: audio

Subtype name: ip-mr_v2.5

Required parameters: none

Optional parameters:

* ptime: Gives the length of time in milliseconds represented by the
media in a packet. Allowed values are: 20, 40, 60 and 80.

Encoding considerations: This media type is framed binary data (see RFC
4288, Section 4.8).

Security considerations: See RFC 3550 [RFC 3550]

Interoperability considerations: none

Published specification: RFC XXXX

Applications that use this media type: Real-time audio applications like
voice over IP and teleconference, and multi-media streaming.

Additional information: none

Person & email address to contact for further information:
Elena Berlizova

Intended usage: COMMON

Restrictions on usage: This media type depends on RTP framing, and hence
is only defined for transfer via RTP
(RFC 3550). [RFC 3550].

Sergey Ikonin <>

Change controller: IETF Audio/Video Transport working group delegated
from the IESG.

5.2. Mapping Media Type Parameters into SDP

The information carried in the media type specification has a specific
mapping to fields in the Session Description Protocol (SDP) [RFC4566], [RFC 4566],
which is commonly used to describe RTP sessions. When SDP is used to
specify sessions employing the IP-MR codec, the mapping is as follows:


    o The media type ("audio") goes in SDP "m=" as the media name.


    o The media subtype (payload format name) goes in SDP "a=rtpmap"
    as the encoding name. The RTP clock rate in "a=rtpmap" MUST 16000.


    o The parameter "ptime" goes in the SDP "a=ptime" attributes.

Any remaining parameters go in the SDP "a=fmtp" attribute by copying
them directly from the media type parameter string as a semicolon-
separated list of parameter=value pairs.

Note that the payload format (encoding) names are commonly shown in
upper case. Media subtypes are commonly shown in lower case. These
names are case-insensitive in both places.

6. Security Considerations

RTP packets using the payload format defined in this specification are
subject to the security considerations discussed in the RTP
specification [RFC3550], [RFC 3550], and any appropriate RTP profile. This implies
that confidentiality of the media streams is achieved by encryption.
Encryption may be performed after compression so there is no conflict
between the two operations.

This payload format 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.

7. IANA Considerations

One media type has been defined and needs registration in the media
types registry.

8. Normative References


  [RFC 2119] Bradner, S., "Key words for use in RFCs to Indicate
             Requirement Levels", BCP 14, RFC 2119, March 1997.

  [RFC 3550] Schulzrinne, H., Casner, S., Frederick, R., and
             V. Jacobson, "RTP: A Transport Protocol for Real-Time
             Applications", STD 64, RFC 3550, July 2003.

  [RFC 4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
             Description Protocol", RFC 4566, July 2006.

9.	Author's Author(s) Information

Sergey Ikonin email:

Russia 109004 B.Kommunisticheskaya st. 27
Building 27, A. Solgenizyn street
Tel: +7 495 661-2178
Fax: +7 495 912-6786

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