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  • Project Number: 248495 Project acronym: Optiband Project title: Optimization of Bandwidth for IPTV Video Streaming Deliverable reference number: D3.1 Deliverable title: Existing data dropping analysis report Due date of deliverable: M4 Actual submission date: 03-May-2010 Start date of project: 1 January 2010 Duration: 30 months Organisation name of lead contractor for this deliverable: Corrigent Systems Ltd. Project co-funded by the European Commission within the Seventh Framework Programme (2007-2013) Dissemination Level PU Public X PP Restricted to other programme participants (including the Commission Services) RE Restricted to a group specified by the consortium (including the Commission Services) CO Confidential, only for members of the consortium (including the Commission Services) The research leading to these results has received funding from the European Community’s Seventh Framework Programme (FP7/2007-2013) under grant agreement n 248495
  • D3.1 - EXISTING DATA DROPPING ANALYSIS REPORT OPTIBAND 248495 03/05/2010 Project history This page is used to follow the deliverable production from its first version until it is approved by all involved partners. Please give details in the table below about successive releases: Release Date Reason of this release Dissemination of this release (task number and/or validation level, WP/SP level, Project Office Manager, Steering Committee, etc) V1 15-04-2010 Draft version WP level V2 29-04-2010 Edition Management Support Team OptiBand Confidential 2 OptiBand Consortium
  • D3.1 - EXISTING DATA DROPPING ANALYSIS REPORT OPTIBAND 248495 03/05/2010 Table of Contents Table of figures .......................................................................................................................................... 4 Glossary ..................................................................................................................................................... 5 1. Executive Summary ........................................................................................................................... 6 2. Existing Data Dropping Analysis ....................................................................................................... 7 2.1 Smoothing MPEG packets .......................................................................................................... 7 2.2 Requantization ........................................................................................................................... 8 2.3 Selective Frame Dropping .......................................................................................................... 9 2.4 Multi-rate Streaming ................................................................................................................. 10 2.5 Scalable Coding ....................................................................................................................... 11 3. Conclusions...................................................................................................................................... 12 4. Bibliography ..................................................................................................................................... 13 OptiBand Confidential 3 OptiBand Consortium
  • D3.1 - EXISTING DATA DROPPING ANALYSIS REPORT OPTIBAND 248495 03/05/2010 Table of figures Figure 1 - Smoothing of data-packets in MPEG VBR stream ........................................................................ 7 Figure 2 - Shaping multiple channels to DSL user ........................................................................................ 7 Figure 3 – The development of the error through the GOP (error-drifting) ..................................................... 8 Figure 4 – Multirate streaming .................................................................................................................... 10 OptiBand Confidential 4 OptiBand Consortium
  • D3.1 - EXISTING DATA DROPPING ANALYSIS REPORT OPTIBAND 248495 03/05/2010 Glossary Abbreviation / acronym Description ADSL Asymmetric Digital Subscriber Line AVC Advanced Video Coding GOP Group Of Pictures HD High Definition IGMP Internet Group Management Protocol IPTV Internet Protocol Television MPEG Moving Picture Experts Group PON Passive Optical Network QoE Quality of Experience SVC Scalable Video Coding VBR Variable Bit Rate VBV Video Buffering Verifier VDSL Very high bitrate Digital Subscriber Line WFQ Weighted Fair Queuing OptiBand Confidential 5 OptiBand Consortium
  • D3.1 - EXISTING DATA DROPPING ANALYSIS REPORT OPTIBAND 248495 03/05/2010 1. Executive Summary This report lists relevant traffic management and data dropping algorithms to be considered for Optiband project. The focus of this report was on streaming media over the Internet, targeted to address the problem of network bandwidth mismatch, when the bit rate of the pending transmit video data is greater than the current available network bandwidth. The data-dropping device goal was to save bandwidth and ensure that the frames which have been selected to be sent can entirely arrive at the decoder before their playback deadlines. The algorithms that were analyzed were such that the frames which were selected to be dropped shouldn’t impair the correct decoding of any other frames which have been selected to be sent, meaning that the decoding of the sent frames shouldn’t be depend on the decoding of the dropped frames, hence continuous and intact video stream can be observed by the receiver even in the condition that only partial video data were received. This paper is organized as follows. In Section 2, the traffic management methods and data dropping algorithms are introduced. The description, characteristics, advantages and disadvantages of each method is described in each sub-section within Section 2. Section 3 concludes the proposed methods advantages and limitation, and recommends which methods are worth further analysis, and which are recommended to be excluded due to major limitations and conflicts. The methods that were analyzed in this report are:  Smoothing MPEG packets  Requantization  Selective frame dropping  Multi-rate streaming  Scalable coding OptiBand Confidential 6 OptiBand Consortium
  • D3.1 - EXISTING DATA DROPPING ANALYSIS REPORT OPTIBAND 248495 03/05/2010 2. Existing Data Dropping Analysis 2.1 Smoothing MPEG packets A relatively easy way of adapting the video streams to the network bandwidth limitations is to advance and delay MPEG packets in order to equalize the bit-rate level of the total stream; this smoothing technique is effective for VBR encoded streams. Figure 1 - Smoothing of data-packets in MPEG VBR stream shows video stream, encoded using variable bit-rates, and how MPEG packets can be advanced or delayed in the time-domain to decrease the peaks. Figure 1 - Smoothing of data-packets in MPEG VBR stream The smoothing is done by shaper with a predefined MAX_RATE and Max_Burst_Rate (MBR). The shaper uses buffer to accommodate MPEG packets that can’t be transmitted due to network bandwidth constraints. When the video bit rate exceeds the network available bandwidth, MPEG packets starts to accumulate at the buffer and will be sent out at the shaper pace rate. During ―quiet‖ intervals, when the video bit rate is lower than the network allowed rate, the buffer evacuates faster to compensate the delay caused by the peak interval. Figure 2 - Shaping multiple channels to DSL user describes how multiple video channels are scheduled and shaped towards a user with limited access bandwidth. Buffer Shaper W1 CH1 Receive W2 Transmit CH2 W3 CH3 Weighted Fair Queuing Figure 2 - Shaping multiple channels to DSL user Each channel buffer is configured with a weight factor (Wn). The weight can be derived by the following factors:  The channel priority (premium, foreign channels, etc.)  The channel characteristics (Sports, News, Action, etc.)  The channel buffer fillness The shaper limits the total transmitted rate of the three channels to the user’s access rate, and the Weighted Fair Queuing (WFQ) schedules between the channels according to their weights. Advantages of MPEG smoothing: Channels exploit the opportunities of ―quiet‖ intervals of other competing channels to burst and transmit the packets that were delayed at peak intervals. OptiBand Confidential 7 OptiBand Consortium
  • D3.1 - EXISTING DATA DROPPING ANALYSIS REPORT OPTIBAND 248495 03/05/2010 Disadvantages of MPEG smoothing: There are limits as to how much smoothing can reduce the rate-peaks. These limits are defined by the Virtual Buffer Verifier specifications (VBV) that specifies how much data the decoder of the stream able to store in the local memory before it converts the data into base band video. It is based on the fact that data must arrive at the decoder before it has to be removed from the buffer for decoding. The smoothing technique is only effective with VBR streams, and it relies on ―silent‖ intervals that can compensate on peak rates interval. Otherwise, this technique is useless, smoothing only lowers the maximum rate, not the average rate, and it doesn’t guarantee any output bandwidth. 2.2 Requantization Requantization is another technique to achieve rate reduction, it requires partial decode, and then apply increased level of quantization, resulting in higher compression rates and reduction in video quality. The usage of requantization technique needs to avoid picture breakdown from visual artifacts in the video, the deeper the quantizer used, the video quality will get worse and frequent artifacts will occur. The requantization removes information and compromises the picture quality, which introduces errors. Depending on the GOP sequence, there is a difference where this error is introduced. Errors introduced on B frames will have little effect. It will only be shown for 1 frame. If the error is a block-artifact, the visual impact will be a short pulse. The situation is however more serious if errors are applied on the P or I-frames, the error will propagates throughout the GOP-sequence. Figure 3 – The development of the error through the GOP (error-drifting) Simply put, one bad decision at the start of the GOP is compounded by another one. The sum of these errors, ―breathing‖, is perceived as degradation of picture quality throughout the GOP, until a new GOP starts with an I-frame. The cycle may repeat every GOP. To prevent any major introduction of artifacts in the decoded video, the requantizer could be limited to only include B-frames, avoiding quality degradation of I and P frames. It will have a rate-reducing effect since most of the frames in the stream are B-frames. On the other hand, the biggest frames by far in the GOP – the ones carrying most information – are the I and P frames. Only very limited re-quantification can be done on these frames in order to avoid severe block artifacts in the decoded video. The typical rate-reduction made by requantization is app 10-20%, depending on how the GOP previously has been encoded. As an example, better quality encoders have better motion-estimation performance, meaning that the B-frames already will be small in size. In that case, the benefit of requantization is reduced. Advantages of requantization: By limiting requantization to B-frames, rate reduction can be achieved with minimal and quality degradation. Depending on the GOP structure, the more B-frames in a GOP, the requantization become more effective for rate reduction maintaining QoE almost unaffected. Disadvantages of requantization: Requantization needs to be employed at some intermediate network nodes to reduce the video contents to an appropriate target user access rate. However, it also brings extra computational burden, which may not be practical in certain real-time applications. Moreover, video streams transported over the network are encrypted and are not expected to be decrypted at intermediate network nodes. The requantization requires access to the video content, extraction processing and insertion of content to the transported video streams. Therefore, this method seems not practical for IPTV managed network. OptiBand Confidential 8 OptiBand Consortium
  • D3.1 - EXISTING DATA DROPPING ANALYSIS REPORT OPTIBAND 248495 03/05/2010 2.3 Selective Frame Dropping In MPEG IBP sequences, B frames are never used as references so they may be dropped without affecting the ability to decode the rest of the GOP. If P frame is dropped, the following B frames can’t be decoded until the next I or P frame, whichever comes first, but there shouldn't be any problem decoding the P frames since it reference the previous the I or P frame. In a congested network links, when the network infrastructure bandwidth is lower than the transported video stream, the video may be delayed, which will cause the decoder to receive frames with timestamps in the past. In such situation, the received frames are useless, and could be dropped at the network congestion point without overloading the congested links. Algorithms for frame dropping can vary from the extremely simple to very complicate. In a network environment where the available bandwidth changes dynamically, it is desirable for a rate adaptor to control the media quality in an adaptive way according to the dynamics of underlying network resource. The rate adapter should be carefully designed to maintain the synchronization characteristics of real-time streaming; special care should be given to the correct implementation of video and audio synchronization after frame-drop. A generic rate adaptor algorithm drops video frames when the transmission buffer is full because of insufficient bandwidth, to reduce the transmitted bit-rate. The characteristics of the algorithm are:  Number of frames currently in the buffer (not the number of bytes) is indicating buffer fullness,  Less important frames (B-frame) from the buffer are dropped before the more important frames (I- frame and P-frame),  The transmission of I-frames is delayed when conditions are bad, even if they are out-of-date with regards to the display time (they can still be used to decode subsequent interpredicted frames). The size of the transmission buffer should be big enough to hold a number of frames, increasing the transmission buffer size could potentially permit a more elaborate prioritization; however it can cause increased latency and memory usage. The maximum allowed delay is limited by the VBV buffer at the receiving decoder, delaying frames beyond the maximum allowed VBV delay makes the frame useless and irrelevant for decoding. When a packet belonging to next frame is about to be written to the transmission buffer and the buffer is full, the scheduler drops a frame based on the priority assigned earlier. When the network bandwidth is so low that also P-frames need to be dropped, then the GOP is considered as "disturbed" and the rest of the GOP (which depends on the P-frame) is also dropped. If only B-frames are dropped the distortions are relatively low because there are no subsequent frames depending on them. The dropping of frames causes the effect of the video playback being temporarily frozen, the duration of which depends on the amount of frames dropped after which the playback resumes from the next frame which got through. The basic idea is to selectively drop frames with low priority to ensure that other more important frames in this GOP can be reliably delivered to the receiver within the time limitation of the decoder VBV buffer. For each GOP, the frame-dropping policies are determined in advance according to the buffer fill level, and slightly re-adjusted after each frame has been sent according to the current available network bandwidth. The delivery of streaming media has the properties of real-time, continuity, and data dependency. Real-time delivery demands that all data contained in an individual video frame must arrive at the receiver before its playback deadline. Continuity demands that all video frames must be rendered in a pre-defined order. Generally, compressed video has two types of data dependency: inter-frame dependency demands that before decoding a frame, all frames it refers to must have already been decoded; intra-frame dependency demands that it’s preferable to decode a frame after all packets belonging to this frame have already been received, otherwise decoding errors and mosaic-like rendering effects may be resulted. The loss of some packets in an I-frame or P-frame may result in not only mosaic-like rendering effect in the current picture, but also similar effects in all of the following P- and B-frames within the current GOP. Once a frame is decided to be dropped, all frames referencing to this frame within the same GOP must also be dropped, otherwise unpredictable errors may occur during the decoding of these frames. According to the frame-dropping criteria and the inter-frame dependency relationships, it can be derived that:  Every B-frame can be dropped at any time  When P-frame is dropped, all other P- and B-frames follows this frame by decoding order within the same GOP should also be dropped  When I-frame is dropped, all other P- and B-frames within the same GOP should also be dropped. OptiBand Confidential 9 OptiBand Consortium
  • D3.1 - EXISTING DATA DROPPING ANALYSIS REPORT OPTIBAND 248495 03/05/2010 When some frames are needed to be dropped, the frame-dropping policies should ensure that the frames are dropped according to their importance and priority, i.e., first B-frames, then P-frames in backward order, and finally I-frames. Advantages of selective frame dropping: The selective dropping criteria differentiates between frame types, it first randomly drops B-frames that has the minimal impact on the decoding and don’t impact the decoding of other frames in the GOP. The rate adapter device doesn’t need to process the video content and therefore may also operate in encrypted environment, assuming it can identify the frame type of each MPEG packet. The dropped B-frames can significantly reduce the stream bandwidth consumption and can easily achieve the target bandwidth constrains goals. Disadvantages of selective frame dropping: Every dropped frame has an immediate impact on the video quality, as B-frames are not carrying redundant information. The user may experience artifacts and the probability of picture freeze is increasing as more B- frames are dropped. Dropping B-frames as direct impact on user’s QoE which can’t be acceptable by users viewing HD channels. 2.4 Multi-rate Streaming Another method for video adaptation is multirate streaming, in which multi-copies of a single video source with different compression rates are transmitted, and the rate adapter device located at the distribution network selects one of the streams to send per end user according to its access bandwidth limitation. Each end user has different access rate, depend on its access technology (ADSL, ADSL2, VDSL, PON, etc) and the access profile that the user has with its service provider. A user with low access rate shouldn’t dictate low quality for other users with higher access rates, the intention is to maximize each user QoE regardless of other users that share the same network, and hence, each end user may get different stream of a channel that was selected by the rate adapter based on each user access bandwidth limitations. Moreover, each user has different number of STB, and is watching different combination of video channels that have different characteristics (action, news, sports, etc). To maximize the video quality and ensure each user experience the best QoE it can get, the rate adapter device has to take this information into account and perform dynamic rate selection. Figure 4 – Multirate streaming illustrates typical network description of multi-rate streaming architecture: IPTV distribution Edge Multirate network device DSLAM encoders Hub Figure 4 – Multirate streaming The edge device receives multi-rate streams of each channel from the network, it monitors the requests generated by the user (such as IGMP) to conclude which channel each user STB is watching. By knowing OptiBand Confidential 10 OptiBand Consortium
  • D3.1 - EXISTING DATA DROPPING ANALYSIS REPORT OPTIBAND 248495 03/05/2010 each channel characteristics, the edge device able to decide which copy of the channel is best to be forwarded towards each user. Advantages of multi-rate streaming: From the end user STB point of view, the received stream is standard and fluent H.264 AVC coded stream, the STB is not aware to the existence of the multi-stream structure at the distribution network, and it normally decodes the stream as it originally generated by the head-end. Disadvantages of multi-rate streaming: The multi-rate streams inflate the bandwidth consumption at the metro network, about 3 times more than a regular single-rate streaming, assuming 4 different rate streams are generated per channel. Each rate stream is generated by H.264/AVC encoder, to generate the multi-rate streams, the amount of encoders at the head-end site is relatively increase, which make the solution more expensive for the service provider. 2.5 Scalable Coding Scalable coding can be regarded as a potential solution to the problem, allowing the split of the stream into a number of substreams. The substreams can provide reduced quality at lower bit rate, and the full quality can be achieved by suitably combining all layers. SVC is a layered video representation with multiple dependencies with one base layer and multiple enhancement layers, in which enhancement layers can increase the spatio-temporal resolution and improve fidelity upon base layer. The resulting decoded video has lower temporal or spatial resolution or reduced fidelity while retaining a reconstruction quality that is close to that achieved using the existing single-layer H.264/AVC design with the same quantity of data as in the partial bit stream. Due to layered structure of SVC, each user receives only layers that their sum bandwidth is less than the user access rate. Users with low access rate gets the basic layer and experience low quality video while high access rate user gets enhancements layers in addition and therefore experience high quality video. Users with different access rate don’t impact each other; users with high access rate aren’t degraded by low rate users. As for scalable coding, the source is encoded once, and the bit stream can be truncated at almost any point, and still can be decoded with reconstruction quality corresponding to number of bits recovered, thus can be used to satisfy various adaptation needs without transcoding. The computational costs involved are typically much smaller than the transcoding case. However, the expense is the losses in compression efficiency, and nowadays scalable coding algorithms have not been really deployed in practical media streaming applications. Advantages of scalable coding: As for scalable coding, the source is encoded once, and the bit stream can be truncated at almost any point, and still can be decoded with reconstruction quality corresponding to number of bits recovered, thus can be used to satisfy various adaptation needs. Disadvantages of scalable coding: In terms of error resiliency, failure of enhancement layer path makes all enhancement layers above the failed layer become unusable. A failure at the basic layer cause a disruption of the video, all enhancement layers can’t be decoded without the basic layer. SVC consumes more network bandwidth than single layer H.264/AVC to achieve the same video quality. The multilayer structure of SVC splits between video layers, each layer is encapsulated and transported over the network, the video layers separation requires excess bandwidth than consumed by single layer AVC. A user with limited access rate will experience better video quality with AVC than user of same limited access rate with SVC coding. Nowadays scalable coding algorithms have not been really deployed in practical media streaming applications. OptiBand Confidential 11 OptiBand Consortium
  • D3.1 - EXISTING DATA DROPPING ANALYSIS REPORT OPTIBAND 248495 03/05/2010 3. Conclusions This paper presented five different categories relevant to data dropping and rate adaptation. Each category has a wide range of methods and algorithms of implementation, from simple to sophisticated and complicate algorithms. In this paper the categories were generally described and were adapted to the Optiband project requirements to meet the project goals. This report specified the advantages and limitations of each category. Limitations that conflicts with acceptable or with the practical IPTV network implementation that can’t be resolved, are recommended to be removed at this stage of the project and not be further analyzed or researched. The following table summarizes the limitations level of each proposed category: Category Unresolved limitation Limitation to resolve Smoothing MPEG packets 1. Only suitable for VBR streams 2. Smoothing is extremely limited by VBV delay Requantization 1. Brings extra computational burden on network elements which may not be practical 2. Can’t be implemented with encrypted streams Selective frame dropping Freeze and artifacts when more B- frames are dropped Multi-rate streaming 1. Inflation of metro network bandwidth consumption 2. Increased number of H.264/AVC encoders at the head-end Scalable video coding 1. More overhead than H.264/AVC on the expense of video quality 2. Not deployed yet The recommendation at this stage is to focus on both ―Multi-rate streaming‖ and ―Scalable video coding‖ categories, and not dealing with the other categories. OptiBand Confidential 12 OptiBand Consortium
  • D3.1 - EXISTING DATA DROPPING ANALYSIS REPORT OPTIBAND 248495 03/05/2010 4. Bibliography [1] Transport of Scalable Video over ITU-T Rec H.222.0 | ISO/IEC 13818-1 - AMENDMENT 3 [2] Overview of the Scalable Extension of the H.264/MPEG-4 AVC Video Coding Standard - Heiko Schwarz, Detlev Marpe, Member, IEEE, and Thomas Wiegand, Member, IEEE [3] Efficient Selective Frame Discard Algorithms for Stored Video Delivery across Resource Constrained Networks - Zhi-Li Zhang, Srihari Nelakuditi, Rahul Aggarwal, and Rose P. Tsang [4] TRANSRATING EFFICIENCY — BIT-RATE REDUCTION METHODS TO ENABLE MORE SERVICES OVER EXISTING BANDWIDTH - L. Lastein and B. Reul [5] Network Adapted Selective Frame-Dropping Algorithm for Streaming Media - Longshe Huo, Member, IEEE, Qiang Fu, Yuanzhi Zou, and Wen Gao, Member, IEEE [6] RATE-DISTORTION OPTIMIZED VIDEO FRAME DROPPING ON ACTIVE NETWORK NODES - Wei Tu1, Wolfgang Kellerer2 , and Eckehard Steinbach1 OptiBand Confidential 13 OptiBand Consortium