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The rapid growth in the use of video streaming over IP networks has outstripped the rate at which new network
infrastructure has been deployed. These bandwidth-hungry applications now comprise a significant part of all Internet
traffic and present major challenges for network service providers. The situation is more acute in mobile networks where
the available bandwidth is often limited. Work towards the standardisation of High Efficiency Video Coding (HEVC),
the next generation video coding scheme, is currently on track for completion in 2013. HEVC offers the prospect of a
50% improvement in compression over the current H.264 Advanced Video Coding standard (H.264/AVC) for the same
quality. However, there has been very little published research on HEVC streaming or the challenges of delivering
HEVC streams in resource-constrained network environments.
In this paper we consider the problem of adapting an HEVC encoded video stream to meet the bandwidth limitation in a
mobile networks environment. Video sequences were encoded using the Test Model under Consideration (TMuC HM6)
for HEVC. Network abstraction layers (NAL) units were packetized, on a one NAL unit per RTP packet basis, and
transmitted over a realistic hybrid wired/wireless testbed configured with dynamically changing network path conditions
and multiple independent network paths from the streamer to the client. Two different schemes for the prioritisation of
RTP packets, based on the NAL units they contain, have been implemented and empirically compared using a range of
video sequences, encoder configurations, bandwidths and network topologies.
In the first prioritisation method the importance of an RTP packet was determined by the type of picture and the temporal
switching point information carried in the NAL unit header. Packets containing parameter set NAL units and video
coding layer (VCL) NAL units of the instantaneous decoder refresh (IDR) and the clean random access (CRA) pictures
were given the highest priority followed by NAL units containing pictures used as reference pictures from which others
can be predicted. The second method assigned a priority to each NAL unit based on the rate-distortion cost of the VCL
coding units contained in the NAL unit. The sum of the rate-distortion costs of each coding unit contained in a NAL unit
was used as the priority weighting.
The preliminary results of extensive experiments have shown that all three schemes offered an improvement in PSNR,
when comparing original and decoded received streams, over uncontrolled packet loss. Using the first method
consistently delivered a significant average improvement of 0.97dB over the uncontrolled scenario while the second
method provided a measurable, but less consistent, improvement across the range of testing conditions and encoder
configurations.
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