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tion to the packet’s sequencing feature:
1. Payload identification. The Payload-Type field of
the RTP header uses the profile identifiers given at
RFC 3551 (Schulzrinne, 2003), as a mean to pro-
vide information to identify the codec, the frame
type and how frames of a given codec (voiced or
SID) are packet. For example, for the G.723.1 three
types of frames are distinguised and marked using
two bits at the beginning of each frame: Active for
5.3 mode, Active for 6.3 mode and SID. Neverthe-
less for the G.729 codec, the frame type is indicated
by the size of the packet.
2. Jitter compensation. A timestamp is inserted in
the RTP header which allows to compensate the
network jitter. The receiver of a packet contain-
ing voice frames can adjust the play-out instant of
the first frame of the packet by using this times-
tamp. Subsequent frames of the packet are time-
consecutive, so they can be played-out on the
proper instant at the receiver with no additional in-
formation other than the audio profile identifier.
Since the RTP header only provides one tim-
ing instant, there would be no way to choose the
right playout time for SID frames of the same
IP packet having any time-slot gap in between
them. As a result, the current packetization scheme,
RFC 3551 (Schulzrinne, 2003) also imposes that only
consecutively-generated SID frames are carried in the
same packet (Zopf, 2002). But the large RTP/UDP/IP
header size when compared to the reduced size of
the SID frames (typically 2 or 4 bytes) makes worth
considering to share the same packet for several SID
frames.
This paper proposes a new packetization scheme
to allow the transmission of non-consecutively-
generated SID frames in the same packet. The
new scheme does not modify the existing packeti-
zation scheme for active frames, and is backward-
compatible with the RFC 3551 packetization profile.
This scheme will save bandwidth in conversations us-
ing SID-capable codecs, specially in scenarios with
a large number of potential users, where the header-
compression technique shows scalability problems.
This is may be the case of a corporate network branch
to central office communication, or the case of the
VoIP Service Provider transport service, since the cost
of the transport can be considered proportional to the
required bandwidth.
The remainder of this paper is organized as fol-
lows: the mechanism is described in section 2. The
analytical expression of the achieved rate reduction is
deduced in section 3. The analytical expressions are
experimentally validated in section 4 and, finally, sec-
tion 5 concludes the paper.
2 PROPOSED PACKETIZATION
SCHEME
To reduce the bandwidth usage of the current IETF
packetization scheme and at the same time solving
the problem of obtaining the right playout time for
every SID frame but the first one of each packet, we
define a new payload type called multi-SID. To main-
tain backward compatibility, we re-define the format
of the existing RTP payload profile identifier number
13 (Schulzrinne, 2003) currently devoted to indicate a
generic non-proprietary SID frame. This generic SID
frame, whose format is defined in (Zopf, 2002), uses
the first byte to indicate the power level of background
noise with a number between 0 and 127 (note that the
most significant bit is always set to 0.)
We propose to extend the aforementioned format to
include also our new multi-SID payload type, which
will be indicated by setting the first bit of the first byte
to one as shown in figure 1. In our format, the re-
maining bits of the first byte of this multi-SID frame
identify the codec (and thus the SID frame size being
used.) Following bytes carry the first SID frame (e.g.
two-bytes size in figure 2.) At the end of the every
SID frame but the last one, an additional field called
NF stands for the number of frame-period gaps to be
inserted before playing the next SID frame. The NF
field has a length of one octet.
A A A
A A S
S S S S
1 COD NFS S
1 byte
S S
A A
A A
...
t
Frames
RFC 3551
multi−SID packet
Proposed
S S
SAAAA S S S SA
A
S
A
S
IP+UDP+RTP Header
ACT frame
SID frame
Figure 1: Proposed packetization scheme.
Finally, we propose to use the multi-SID payload
type only whenever two or more SID frames gener-
ated within N
fpp
frame intervals would be forced to
travel in different packets according to the current
RFC 3551 scheme. This implies that the bandwidth
consumption will always be equal or less than with
the RFC 3551 packetization scheme. The bandwidth
saving achieved by this procedure will be estimated in
next section.
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