Adaptive Error Resilient Mechanisms for Real-Time Multimedia Streaming over Inter-Vehicle Communication Networks - Digital Signal Processing for In-Vehicle Systems and Safety - page 282

Fig. 21.2 Real-time multimedia streaming by means of the RTP and RTCP protocols
the sampling clock. The RTP receiver uses the sequence number to detect lost packets
and the timestamp field to determine when to play out received data.
The RTP Control Protocol (RTCP) [ 3 ] is used to monitor the quality of service
and to convey information about the participants in an ongoing session. Basically,
RTCP carries long-term statistic information (e.g., mean packet loss rate (PLR),
round trip time, jitter, etc.) related to the RTP session participants. The full real-
time multimedia streaming procedure is shown in Fig. 21.2 .
In this work, we discuss how RTCP reports can support RTP transmission to
track frequent variations of the wireless channel in order to provide the streaming
server with regular feedbacks from the receiver on the suffered packet loss rate.
Timely feedback is used, at the sender, to adapt the transmission policy to the
channel characteristics in order to achieve the best video quality as perceived by the
end user. Error control techniques are introduced to improve communication
reliability against time-varying and bursty packet losses. In fact, IEEE 802.11
link-layer retransmissions are efficient only in a shorter timescale and in the face
of short-term fluctuations (fast fading); more persistent fluctuations (slow fading) in
a high-mobility scenario render these mechanisms inefficient. Application-level
error control techniques may provide additional reliability on a longer timescale
and, as described in the next sections, cross-layer integration can be exploited to
regulate the trade-off between error control aggressiveness and transmission over-
head according to the channel loss trends reported by the RTCP protocol.
21.3 Forward Error Correction
Generic forward error correction is a codec-independent method of protecting the
information conveyed in data packets against packet erasures by adding redundant
data to the transport stream. In this work, we use a common method for generating