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MPEG-4 video QoS at the Wireless Transmitter Ger Cunningham and Liam Murphy Performance Engineering Laboratory, Department of Computer Science, University College Dublin, IRELAND E-mail: {ger.Cunningham, liam.Murphy}@ucd.ie Abstract – This paper considers the adaptation of video for quality-of-service (QoS) in a system containing a video server linked to a mobile client. The video server is linked to the wireless network via a dedicated wire link that is lightly loaded. Locating the adaptation functionality at the transmit-side of the wireless link rather than at the server is proposed. The research work to determine the adaptation algorithm is outlined. I INTRODUCTION
In a heterogeneous network with a wireless link to the client and a congestion-free wireline link between a video server and the wireless link, adaptation of the video signal according to the end-to-end bandwidth fluctuations to achieve the best QoS is best performed at the transmit-side of the wireless link, rather than at the video server. An example of such a network is a mobile phone operator that provides video services from a video server to its customers. The video server is accessed not via the Internet, where congestion is always a potential hazard, but via a dedicated network that is typically lightly loaded. In such a network the end-to-end bandwidth fluctuations are due to the wireless link only. The fluctuations are due to the bandwidth allocation policy of the wireless operator and the instantaneous fluctuations due to radio conditions. Wireless channels are typically noisy and suffer from a number of channel degradations due to fading and interference. This leads to bit and burst errors, and thus a
reduction in throughput. Mobile wireless networks are particularly prone to channel degradations. This paper outlines the reasoning for locating the adaptation at the transmit-side of the wireless link and gives an introduction to the adaptation algorithm. It outlines the proposed research into the adaptation algorithm. The diagram below shows a model of the end-to-end system under consideration. Tx refers to the transmit-side of the wireless link, and Rx refers to the receive-side of the wireless link. The wireline network may contain routers, but these are not shown as they are not prone to congestion in this system and so are not of interest here. Video Server
Tx Wireline
Rx
Decoder
Wireless
Figure 1. The end-to-end system II ADAPTATION BY SERVER
A lot of research has been carried out recently on the adaptation of video in a heterogeneous network such as the Internet. In this research the video server generally makes use of probes [3] or protocols such as RTCP (RTP Control Protocol) [5] to determine the end-to-end bandwidth fluctuations. RTCP is used to periodically convey statistics about the transmission to the server [1], and the server adjusts the bitrate of the video stream accordingly. Such an approach can work in an Internet environment where changes in throughput
are generally less dynamic than in a mobile wireless network. By contrast, the transmitside of the wireless link is continuously aware of the wireless bandwidth. In the system shown in Figure 1 the wireline network is not prone to congestion, so there is no need for adaptation of the video bitrate to be done by the server. The further the server is from the wireless link, there is an increased delay in informing the server of the bandwidth changes, and an increased delay in the adjusted bitrate reaching the wireless link from the server. By the time the adjusted bitrate reaches the wireless link the wireless bandwidth may have changed and the adjusted bitrate may now be inappropriate. By contrast, when the adaptation is done at the transmit-side of the wireless link there is no such delay and the adaptation can follow the bandwidth fluctuations almost immediately. In summary, the transmit-side of the wireless link has a continuous and immediate awareness of the wireless bandwidth, while the server receives only periodic updates and the resulting adaptation takes longer to come into affect. The transmit-side of the wireless link is therefore better suited to adapting the bitrate of the video stream. III ADAPTATION AT TRANSMIT-SIDE OF WIRELESS LINK
Adaptation of the video bitrate can be effectively performed at the transmit-side of the wireless link. The proposed location of the adaptation functionality is shown in Figure 2. The adaptation function receives video packets from the server in a UDP transport layer stream. It adapts the video bitstream according to the wireless bandwidth and injects the packets into the wireless stack in a UDP transport layer stream. The RTP protocol (Real-time Transport Protocol) may also be used. RTP supports payload identification, sequence numbers, timestamps, and delivery monitoring [2].
Wireline Protocol Stack
Video adaptation
Wireless Protocol Stack
Wireline input
Wireless output
Figure 2. Position of Video adaptation
a) Sensing bandwidth fluctuations As mentioned above, the adaptation function injects packets it receives from the server into the wireless stack. When the throughput of the wireless link reduces (due to bit or burst errors on the link or due to the bandwidth allocation by the wireless network), the buffer at the Data Link Layer can fill up. When this happens the adaptation function will be prevented from injecting any further packets into the wireless stack until space becomes available in the Data Link Layer’s buffer. In this way the adaptation function learns of the fluctuations in the available bandwidth. The Data Link Layer will retransmit packets that are received incorrectly at the decoder-side of the wireless link for a limited number of times. It may also apply extra FEC when the error rate is high. The Data Link Layer is not the focus of the research – it is mentioned here to explain why burst or bit errors in the wireless link can cause the buffer at the Data Link Layer to fill up. b) Adaptation of the video bitstream An MPEG-4 video bitstream contains data of different levels of importance to the perceptual quality of the video. This factor has lead to different adaptation mechanisms, or filters. There are many different types of filters [3], such as frame-dropping filter, layer-dropping filter, frequency filter, and re-quantization filter, which are described as follows. A frame-dropping filter can distinguish the frame types (i.e., I, P and B frames) and drop frames according to importance. The
dropping order would be first B, then P, and finally I frames. A layer-dropping filter can distinguish the layers and drop layers according to importance. The dropping order is from the highest enhancement layer down to the base layer. A frequency filter operates in the frequency domain (i.e. DCT coefficients). Frequency filtering mechanisms include low-pass filtering, colour-reduction filtering, and colour-to-monochrome filtering. In lowpass filtering the high frequency DCT coefficients are discarded. A colourreduction filter performs the same operation as a low-pass filter, except that it only operates on the chrominance information. A colour-to-monochrome filter removes all colour information from the video stream. A re-quantization filter dequantizes the DCT coefficients and re-quantizes them using a larger quantization step. In our research the dropping of high frequency DCT components will be considered for small reductions in bandwidth, and the dropping of B frames will be considered for larger reductions in bandwidth. More serious reductions in bandwidth will not be considered initially. Dropping high frequency components makes the picture blurry. Dropping B frames affects the frame rate. Dropping high frequencies and B frames have less of an effect than other video data. Both can be dropped relatively easily. c) Identifying the unimportant video data The video data arrives at the adaptation function in packet form. Each packet contains a tag field that signals the importance of the data to the adaptation algorithm. The packets are stored in the server with the tag field filled in. The adaptation algorithm removes the tag before transmitting a packet.
d) Adapting the bitstream The packets arrive from the server at a steady rate. When the required wireless bandwidth is available, the adaptation function simply injects the packets into the wireless stack without viewing the packets’ tags. When the wireless bandwidth is insufficient, packets for transmission will begin accumulating in the adaptation function, requiring the adaptation algorithm to determine what parts of the video stream to drop. The algorithm uses the tag field, among other things, in making its decision. IV PROPOSED RESEARCH
The proposed research will look at how best to adapt the video when the wireless bandwidth is reduced – what factors to take into account: whether to drop unimportant video immediately, and not allow any buildup of packets? Whether to wait a little before dropping video data to see if the bandwidth reduction is temporary? When dropping high frequency DCT components for a short duration, whether it is best to apply the same to the rest of the frame? Dropping unimportant video data immediately may be too harsh and not give the most effective perceptual quality to the viewer. Waiting a little before dropping video data to see if the bandwidth increases may be unduly optimistic i.e. the bandwidth may not improve in that time and may even get worse. The behaviour of wireless channels has been characterised and this will be used in experiments to determine the appropriate behaviour of the algorithm. The tag field might have multiple levels of importance. It might also convey that packets are from one frame. As the tags are not transmitted over the wireless link and congestion of the wire link is not an issue, there is no need to limit the size of the tag field. The goal is an adaptation function that adapts according to the fluctuation wireless bandwidth but does so in a way that
maximises the perceptual quality to the viewer. It is desirable that the adaptation algorithm be fast and only require minimal processing. It should therefore only require minimal knowledge of video. The tagging of the video packets will be designed to facilitate this. Frame-based MPEG-4 [4] will be used. MPEG-4 is chosen because of its wide acceptance. V CONCLUDING REMARKS
The above approach has no feedback mechanism from the client so is also applicable to multicasting. Only one client is shown in Figure 1, but this is only for simplification purposes. The above adaptation algorithm can also be applied in real-time video conversational services between the mobile clients. ACKNOWLEDGEMENT
The support of the Research Innovation Fund of Enterprise Ireland is gratefully acknowledged.
REFERENCES
[1] N. Cranley, L. Murphy, “Adaptive Quality of Service for Streamed MPEG-4 over the Internet”, UKTTS, May 2001
[2] H. Schulzrinne, S. Casner, R. Frederick, V. Jacobson, RTP: A Transport Protocol for Real-Time Applications, FRC1889. [3] Dapeng Wu, Yiwei Thomas Hou, Wenwu Zhu, Ya-Qin Zhang, Jon M. Peha, “Streaming Video over the Internet: Approaches and Directions”, IEEE Transactions on Circuits and Systems for Video Technology, Vol.11, No.3, March 2001, pp282-300. [4] Moving Pictures Expert Group (MPEG) Official Web Site, http://mpeg.telecomitalialab.com [5] Nick Feamster, Deepak Bansal, Hari Balakrishnan, “On the Interactions Between Layered Quality Adaptation and Congestion Control for Streaming Video”, 2001
