RFC 5404 RTP Payload Format for G.719 January 2009
--+--------+--------+--------+--------+--------+--------+--------+--
| 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) ---->
Figure 1: An example of redundant transmission
Here, each frame is retransmitted once in the following RTP payload
packet. f(n-2)...f(n+4) denote a sequence of audio frames, and
p(n-1)...p(n+4) a sequence of payload packets.
The mechanism described does not really require signaling at the
session setup. However, signaling has been defined to allow for the
sender to voluntarily bind the buffering and delay requirements. If
nothing is signaled, the use of this mechanism is allowed and
unbounded. For a certain timestamp, the receiver may receive
multiple copies of a frame containing encoded audio data, even at
different encoding rates. The cost of this scheme is bandwidth and
the receiver delay necessary to allow the redundant copy to arrive.
This redundancy scheme provides a functionality similar to the one
described in RFC 2198, but it works only if both original frames and
redundant representations are G.719 frames. When the use of other
media coding schemes is desirable, one has to resort to RFC 2198.
The sender is responsible for selecting an appropriate amount of
redundancy based on feedback about the channel conditions, e.g., in
the RTP Control Protocol (RTCP) [RFC3550] receiver reports. The
sender is also responsible for avoiding congestion, which may be
exacerbated by redundancy (see Section 9 for more details).
4.3.2. Use of Frame Interleaving
To decrease protocol overhead, the payload design allows several
audio transport frames to be encapsulated into a single RTP packet.
One of the drawbacks of such an approach is that in the case of
packet loss, several consecutive frames are lost. Consecutive frame
loss normally renders error concealment less efficient and usually
causes clearly audible and annoying distortions in the reconstructed
audio. Interleaving of transport frames can improve the audio
quality in such cases by distributing the consecutive losses into a
number of isolated frame losses, which are easier to conceal.
Westerlund & Johansson Standards Track [Page 6]
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