Audio/Video Transport Working Group P. Luthi
Internet Draft PictureTel
Document: <draft-ietf-avt-rtp-g7221-00.txt> June 2000
Category: Standards Track
RTP Payload Format for ITU-T Recommendation G.722.1
Status of this Memo
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1. Abstract
ITU-T Recommendation G.722.1 [2] is a wide-band audio codec, which
operates at one of two selectable bit rates, 24kbit/s or 32kbit/s.
This document describes the payload format for including G.722.1
generated bit streams within an RTP packet [3]. Also included here
are the necessary details for the use of G.722.1 with MIME [4] and
SDP [5].
2. Conventions used in this document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in
this document are to be interpreted as described in RFC-2119 [6].
3. Overview of ITU-T Recommendation G.722.1
G.722.1 is a low complexity coder, it compresses 50Hz - 7kHz audio
signals into one of two bit rates, 24 kbit/s or 32 kbit/s. The
coder may be used for speech, music and other types of audio.
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Some of the applications for which this coder is suitable are:
o Real-time communications such as videoconferencing and
telephony.
o Streaming audio
o Archival and messaging
A fixed frame size of 20ms is used, and for any given bit rate the
number of bits in a frame is a constant.
4. RTP payload format for G.722.1
The RTP timestamp MUST be in units of 1/16000 of a second. The RTP
payload for G.722.1 has the format shown in Figure 1.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| RTP Header [3] |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
| |
+ one or more frames of G.722.1 |
| .... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1: RTP payload for G.722.1
G.722.1 uses 20 ms frames and a sampling rate clock of 16 kHz. The
encoding and decoding algorithm can change the bit rate at any 20ms
frame boundary, but no bit rate change notification is provided in-
band with the bit stream. Therefore, a separate out-of-band method
is REQUIRED to indicate the bit rate (see section 6 for an example
of signaling bit rate information using SDP). For the payload
format specified here, the bit rate MUST remain constant for a
particular payload type value. An application MAY switch bit rates
from packet to packet by defining two payload type values and
switching between them.
The assignment of an RTP payload type for this new packet format is
outside the scope of this document, and will not be specified here.
It is expected that the RTP profile for a particular class of
applications will assign a payload type for this encoding, or if
that is not done then a payload type in the dynamic range shall be
chosen.
When operating at 24 kbit/s, 480 bits (60 octets) are produced per
frame, and when operating at 32 kbit/s, 640 bits (80 octets) are
produced per frame. Thus, both bit rates allow for octet alignment
without the need for padding bits.
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The number of bits within a frame is fixed, and within this fixed
frame G.722.1 uses variable length coding (e.g. Huffman coding) to
represent most of the encoded parameters [2]. All variable length
codes are transmitted in order from the left most (most significant
- MSB) bit to the right most (least significant - LSB) bit, see [2]
for more details.
The use of Huffman coding means that it is not possible to identify
the various encoded parameters/fields contained within the bit
stream without first completely decoding the entire frame.
For the purposes of packetizing the bit stream in RTP, it is only
necessary to consider the sequence of bits as output by the G.722.1
encoder, and present the same sequence to the decoder. The payload
format described here maintains this sequence.
Figure 2 illustrates how the G.722.1 bit stream MUST be mapped into
an octet aligned RTP payload.
An RTP packet SHALL only contain G.722.1 frames of the same bit
rate.
first bit last bit
transmitted transmitted
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ sequence of bits (480 or 640) generated by the |
| G.722.1 encoder for transmission |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | | | |
| | | ... | |
| | | | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ... +-+-+-+-+-+-+-+-+-+-+-+-+
|MSB... LSB|MSB... LSB| |MSB... LSB|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ... +-+-+-+-+-+-+-+-+-+-+-+-+
RTP RTP RTP
octet 1 octet 2 octet
60 or 80
Figure 2: The G.722.1 encoder bit stream is split into
a sequence of octets (60 or 80 depending on
the bit rate), and each octet is in turn
mapped into an RTP octet.
The ITU-T standardized bit rates for G.722.1 are 24 kbit/s and
32kbit/s. However, the coding algorithm itself has the capability
to run at any user specified bit rate (not just 24 and 32kbit/s -
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see section 5 for further details on acceptable non-standard bit
rate values) while maintaining an audio bandwidth of 50 Hz to 7 kHz.
When operating at non-standard rates the payload format SHOULD
follow the guidelines illustrated in Figure 2. It is RECOMMENDED
that values in the range 16000 to 32000 be used, and that any value
MUST be a multiple of 400 (this maintains octet alignment and does
not then require (undefined) padding bits for each frame if not
octet aligned). For example, a bit rate of 16.4 kbit/s will result
in a frame of size 328 bits or 41 octets which are mapped into RTP
per Figure 2.
4.1 Multiple G.722.1 frames in a RTP packet
More than one G.722.1 frame may be included in a single RTP packet
by a sender.
Senders have the following additional restrictions:
o SHOULD not include more G.722.1 frames in a single RTP packet
than will fit in the MTU of the RTP transport protocol.
o All frames contained in a single RTP packet MUST be of the same
length, that is they MUST have the same bit rate (octets per
frame).
o Frames MUST not be split between RTP packets.
It is RECOMMENDED that the number of frames contained within an RTP
packet be consistent with the application. For example, in a
telephony application where delay is important, then the fewer
frames per packet the lower the delay, whereas for a delay
insensitive streaming or messaging application, many frames per
packet would be acceptable.
4.2 Computing the number of G.722.1 frames
Information describing the number of frames contained in an RTP
packet is not transmitted as part of the RTP payload. The only way
to determine the number of G.722.1 frames is to count the total
number of octets within the RTP packet, and divide the octet count
by the number of expected octets per frame (either 60 or 80 per
frame, for 24 kbit/s and 32 kbit/s respectively).
5. MIME registration of G.722.1
MIME media type name: audio
MIME subtype: G7221
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Required parameters: None
Optional parameters:
bitrate: the data rate for the audio bit stream. This parameter
is necessary because the bit rate is not signaled within the
G.722.1 bit stream. At the standard G.722.1 bit rates, the
value MUST be either 24000 or 32000. If using the non-standard
bit rates, then it is RECOMMENDED that values in the range
16000 to 32000 be used, and that any value MUST be a multiple
of 400 (this maintains octet alignment and does not then
require (undefined) padding bits for each frame if not octet
aligned).
ptime: RECOMMENDED duration of each packet in milliseconds.
Encoding considerations:
This type is only defined for transfer via RTP as specified in
"draft-ietf-avt-rtp-g7221-01".
Security considerations: none
Interoperability considerations: none
Published specification:
See ITU-T Recommendation G.722.1 for encoding algorithm
details.
Applications which use this media type:
Audio and video streaming and conferencing tools
Additional information: none
Person & email address to contact for further information:
Patrick Luthi
Luthip@pictel.com
Intended usage: COMMON
Author/Change controller:
Author: Patrick Luthi
Change controller: IETF AVT Working Group
6. SDP usage of G.722.1
When conveying information by SDP [5], the encoding name SHALL be
"G7221" (the same as the MIME subtype). An example of the media
representation in SDP for describing G.722.1 at 24000 bits/sec might
be:
m=audio 49000 RTP/AVP 121
a=rtpmap:121 G7221/16000
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a=fmtp:121 bitrate=24000
where "bitrate" is a variable that may take on values of 24000 or
32000 at the standard rates, or values from 16000 to 32000 (and
SHOULD be an integer multiple of 400) at the non-standard rates.
7. Security Considerations
The registration procedure specified in this memo does not impose
any security considerations on its own.
8. References
1 Bradner, S., "The Internet Standards Process -- Revision 3", BCP
9, RFC 2026, October 1996.
2 ITU-T Recommendation G.722.1, available online from the ITU
bookstore at http://www.itu.int.
3 H. Schulzrinne, S. Casner, R. Frederick, and V. Jacobson, "RTP: A
Transport Protocol for real-time applications", RFC 1889, January
1996, updated by draft-ietf-avt-rtp-new (work in progress).
4 N. Freed & N. Borenstein, "Multipurpose Internet Mail Extensions
(MIME) Part One: Format of Internet Message Bodies", RFC 2045,
November 1996.
5 M. Handley and V. Jacobson, "SDP: Session Description Protocol",
RFC 2327, April 1998.
6 Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997
9. Acknowledgments
The author wishes to thank Tony Crossman for starting this work on
G.722.1 packetization and for authoring the initial draft. The
author also wishes to thank Steve Casner and Colin Perkins for their
helpful comments.
10. Author's Addresses
Patrick Luthi
PictureTel Corporation
100 Minuteman Road
Andover, MA 01810
USA
Phone: +1 (978) 292 4354
Email: luthip@pictel.com
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