Hybrid Error Control Mechanism for Video Transmission
`in the Wireless IP Networks
`
`Felix Hartanto
`GMD FOKUS
`Kaiserin-Augusta-Allee 31
`D-10589 Berlin, Germany
`E-mail: hartanto@fokus.gmd.de
`
`Harsha R. Sirisena
`Department of Electrical and Electronic Engineering
`University of Canterbury
`Christchurch, New Zealand
`E-mail: sirisehr@elec.canterbury.ac.nz
`
`EXTENDED ABSTRACT
`
`The Internet has recently been experiencing an explosive growth in the use of audio and video applications. With
`the Internet having historically expanded its reach over new communication systems not long after each became
`available, it is not surprising that future wireless IP networks will also need to support video applications, such as
`video-conferencing, video streaming and video broadcast.
`
`Unlike wired networks where packet losses are mainly caused by congestion, in wireless networks packet loss are
`primarily due to bit errors, arising from terrestrial obstructions and reflections of transmission signals. In order to
`achieve a better packet loss rate, comparable to its wired counterpart, some form of error correction mechanism to
`remove the errors and restore the original information must be employed [2,6].
`
`There are two basic error correction mechanisms, namely Automatic Repeat reQuest (ARQ) and Forward Error
`Correction (FEC). ARQ requires the receiver to explicitly (by means of negative acknowledgement) or implicitly
`(using positive acknowledgements and timeouts) request the retransmission of the lost/corrupted packets. On the
`other hand, FEC transmits together with original data some redundant data, called parities, to allow reconstruction
`of lost/corrupted packets at the receiver.
`
`Of the two mechanisms, FEC has been commonly suggested for video applications due to the strict delay
`requirements and semi-reliable nature of video streams [1,5]. However, FEC incurs constant transmission overhead
`even when the channel is loss free. Considering the limited bandwidth of the wireless link, it is important that the
`error control mechanism be spectrally efficient. Towards this end, we investigate the use of hybrid error control,
`where the amount of redundancy is kept to a minimum and ARQ is used to recover lost/corrupted packets, which
`can not be recovered through FEC. The ARQ will send a negative acknowledgement (NAK) to request
`retransmission only if the packets are likely to be received before they are required for the frame playout.
`
`In order to keep the FEC to a minimum, we develop an error control architecture which takes into account the use
`of hierarchical video coding, where the video signal is encoded in a base layer that provides a low quality image
`and additional complementary layers for improved video quality. The amount of redundancy is then selected by
`distinguishing the significance of each layer and by determining the impact of packet lost from each layer onto
`overall video quality. Additionally, the architecture also includes a module which estimates the packet error rate
`and round trip time observed by the receiver and adjusts the level of redundancy to be used based on the estimate.
`
`As a specific implementation example of this adaptive, priority-aware hybrid error control architecture, we
`consider the use of hierarchical video coding in MPEG-2. Among the three MPEG-2 frames (I,P,B) [4], I-frame
`(intra-coded frame) is the most sensitive to loss due to its role as reference frame for inter-frame prediction. The
`loss of this frame will deem other frames (P and B) received until the next I-frame to be useless. On the other hand,
`the loss of a B-frame can normally be fixed using an error concealment method [3]. Thus, I-frames can be
`considered as the base layer while P- and B-frames the enhancement layers. I-frames will be protected with a
`higher amount of redundancy than P-frames, which in turn will be protected with a higher amount of redundancy
`than B-frames.
`
`Hulu
`Exhibit 1010
`Page 0001
`
`

`
`In general, we assume that the architecture retransmits the FEC packets rather than original packets and
`retransmission is carried out after the completion of the transmission of the current segment. By choosing different
`parameters for the architecture, we can derive different error control schemes. In this paper, we investigate the
`performance of five derived error control schemes, namely ARQ, pure FEC, hybrid FEC/ARQ, hybrid FEC/ARQ
`with priority-dependent redundancy, and adaptive hybrid FEC/ARQ with priority-dependent redundancy.
`
`To evaluate the performance of these schemes, we use the dummy MPEG-2 traces of the movie "Terminator 2"
`from [7]. The video has a frame rate of 24 frames per second, encoded with the pattern "IBBPBBPBBPBB". The
`traces contain 40,000 frames with an average bit rate of 2 Mbps. In applying redundancy, each frame is segmented
`into 500 byte packets. The fragmented packets are grouped into blocks of k (=10) packets and redundancy of h
`packets are applied to each block.
`
`We assume a wireless channel with random bit errors. The wireless link has a peak bandwidth of 4 Mbps. We
`assume no error on the wired link and all packet lost are due to bit errors on the wireless link. To cater for the jitter
`in the link, we define playout control time as the duration between the arrival instant and playback point of the first
`frame. The longer the time, the more frames will be buffered before the first frame is played out. This prevents
`playout starvation due to late frame arrivals.
`
`We vary the network conditions, in terms of bit error rate at the wireless link and round trip time, and observe the
`resulting frame loss probability, which takes into account that discarding an I-frame results in the loss of the entire
`group of picture (GOP).
`
`The results show that the pure FEC offers the worst overall performance. For a given playout control time, the
`additional FEC reduces the number of corrupted frames at the cost of increasing the number of late frames due to
`the limited link bandwidth.
`
`The results also show that the adaptive hybrid FEC/ARQ with priority-dependent redundancy provides the best
`overall performance among the five schemes. At low bit error rates, where additional redundancy is not helpful, the
`scheme uses minimum amount of redundancy and thus, approaches the performance of ARQ, which offers the
`best performance under low bit error rate and short round trip time. For other conditions, the priority-aware hybrid
`FEC/ARQ with or without FEC adaptation offers the best performance.
`
`Keywords: VBR video, hierarchical video coding, ARQ, FEC, hybrid FEC, priority FEC, adaptive FEC.
`
`References
`[1] E. Ayanoglu, P. Pancha, A. Reibman and S. Talwar. Forward Error Control for MPEG-2 Video Transport in a
`Wireless LAN. ACM/Baltzer Mobile Networks and Applications Journal, Vol. 1(3), Dec. 1996, pp. 235-244.
`[2] H. Balakhrisnan. Challenges to Reliable Data Transport over Heterogeneous Wireless Links. Ph.D. Thesis,
`University of California, Berkeley, CA, 1998.
`[3] P. Cuenca, A. Garrido, F. Quiles and L. Orozco-Barbosa. Some Proposal to Improve Error Resilience in the
`MPEG-2 Video Transmission over ATM Networks. Proc. of IEEE INFOCOM, San Francisco, CA, Mar. 1998.
`[4] ISO/IEC 13818-2 International Standard. Generic Coding of Moving Pictures and Associated Audio (MPEG-
`2). Nov. 1994.
`[5] H. Ma and M. El Zarki. Broadcast/Multicast MPEG-2 Video over Broadband Fixed Wireless Access
`Networks. IEEE Network Magazine, Vol. 13(6), Nov./Dec. 1998, pp. 80-93.
`[6] G.C. Polyzos and G. Xylomenos. Enhancing Wireless Internet Links for Multimedia Services. Proc. of
`International Workshop on Mobile Multimedia Communications (MoMuc), Berlin, Germany, Oct. 1998.
`[7] D. Saparilla, K.W. Ross and M. Reisslein. Periodic Broadcasting with VBR-Encoded Video. Proc. of IEEE
`INFOCOM, New York, NY, March 1999, pp. 464-471.
`
`Hulu
`Exhibit 1010
`Page 0002
`
`

`
`Hybrid Error Control Mechanism
`for Video Transmission
`in the Wireless IP Networks
`
`Felix Hartanto
`GMD FOKUS, Germany
`E-mail: hartanto@fokus.gmd.de
`Harsha R. Sirisena
`University of Canterbury, New Zealand
`E-mail: sirisehr@elec.canterbury.ac.nz
`
`Background
`
`l Video transmission over Internet is growing in popularity.
`
`l Video can be coded in MPEG, motion JPEG, or H.26x.
`
`l Video characteristics:
`
`n Real-time transmission.
`
`n Semi-reliable.
`
`n Possible use of hierarchical coding.
`l High error rate in wireless link presents the challenge.
` Need error control mechanism.
`
`Error Control Mechanism
`
`MPEG-2 Video Frame Structure
`
`l Basic error control:
`n ARQ - retransmits corrupted/lost packets.
`n FEC - transmits redundancy along with original packets.
`l Real-time requirements of video favours FEC usage.
`l Problem:
`n How to minimize amount of redundancy for a limited
`wireless link bandwidth ?
`l Solution:
`n Adaptive Hybrid FEC/ARQ with Priority-Dependent FEC.
`n Example: MPEG-2.
`
`I B B P B B P B B P B B
`
`l Frame types: I (Intra), P(Predictive) and B(Bidirectional).
`l Significance: I > P > B.
` Amount of priority-dependent redundancy: I > P > B.
`
`Adaptive Hybrid FEC/ARQ with Priority
`
`Video Application
`
`MPEG
`Encoder
`
`Fragmentor
`&
`FEC Encoder
`
`Packet Queue
`&
`Scheduler
`
`Priority-aware
`FEC adapter
`
`Error & delay
`estimator
`
`NACK
`
`Wireless IP Network
`
`MPEG
`Decoder
`
`Defragmentor
`&
`FEC Decoder
`
`Packet
`Receiver
`
`l Fragmentor segments frame into L byte packets and
`groups them into blocks of k (=10) packets
`l FEC encoder applies redundancy of ht (t=I,P,B) packets
`to each block.
`l Priority-aware FEC adapter adjusts ht (t=I,P,B) based on
`feedback from error and delay estimator.
`l Packet queue and scheduler retransmits packets based
`on NACK request before transmitting packets from new
`segments.
`l Packet receiver sends NACK when total received
`packets of a segment is less than expected number of
`packets for the segment.
`
`Hulu
`Exhibit 1010
`Page 0003
`
`

`
`Derived Error Control Schemes
`
`Default Simulation Parameters
`
`l ARQ: ht = 0.
`l Pure FEC: no NACK sent.
`l Hybrid FEC/ARQ: hI : hP : hB = 1 : 1 : 1.
`l Hybrid FEC/ARQ with priority-dependent FEC:
`hI : hP : hB = 1 : 0.25 : 0.05.
`l Adaptive hybrid FEC/ARQ with priority-dependent FEC:
`hI : hP : hB = 1 : 0.25 : 0.05.
`Initially ht (t=I,P,B) are set to zero and updated by FEC
`ratio, estimated every 1000 packets.
`FEC ratio = max(Est. PER*(Est. RTT/Target RTT), 1.0)
`
`l Video trace “Terminator 2” - 40,000 frames.
`l Frame rate = 24 frames/sec.
`l Packet size (L) = 500 bytes, header (H) = 40 bytes.
`l Wireless link bandwidth = 4 Mbps.
`l Wireless link bit error rate (BER) = 10-4.
`l Packet loss is primarily due to packet bit error, where:
`PER = 1-(1-BER)L+H
`l Round trip time (RTT) = 2 msec.
`l Target RTT = 10 msec.
`l Playout control time = 500 msec.
`
`Optimal Amount of Redundancy
`
`Variation of Bit Error Rate
`
`ARQ
`FEC
`Hybrid
`Priority
`Adaptive
`
`10−2
`
`100
`
`10−1
`
`Frame loss rate
`
`FEC
`Hybrid
`Priority
`
`0.2
`
`0.6
`0.4
`Amount of redundancy
`
`0.8
`
`1
`
`10−2
`10−8
`
`10−6
`
`10−4
`Bit error rate
`
`1
`
`0.9
`
`0.8
`
`0.7
`
`0.6
`
`0.5
`
`0.4
`
`0.3
`
`0.2
`
`0.1
`
`0
`0
`
`Frame loss rate
`
`l FEC is limited by link bandwidth.
`l Optimal at around 0.2 for all three schemes.
`l Priority hybrid FEC/ARQ performs better at any level.
`
`l Adaptive scheme offers better overall performance.
`l Adaptive scheme approaches ARQ performance for low
`BER and better than other schemes for high BER.
`
`Variation of Round Trip Time
`
`Conclusion
`
`l A generic error control architecture, which is configurable
`to provide specific error control schemes, has been
`presented.
`l Five schemes are investigated, among them:
`n Adaptive scheme provides best overall performance.
`n Pure FEC demonstrates worst overall performance.
`n ARQ is preferred under low BER (< 10-5) and low RTT.
`n Priority FEC/ARQ with or without adaptation is preferred
`under high BER.
`
`ARQ
`FEC
`Hybrid
`Priority
`Adaptive
`
`0.1
`
`0.3
`0.2
`Round trip time (ms)
`
`0.4
`
`0.5
`
`100
`
`Frame loss rate
`
`10−1
`0
`
`l ARQ degrades the most with increasing RTT.
`l Adaptive scheme offers better overall performance.
`
`Hulu
`Exhibit 1010
`Page 0004