Cross-layer Two-stage FEC Scheme for Scalable Video Transmission over Wireless Networks Yufeng Shan, Su Yi, Shivkumar Kalyanaraman and John W. Woods Rensselaer Polytechnic Institute Google “Shiv RPI”

Outline Problems Our proposed schemes Simulations Conclusions Enhanced wireless MAC/PHY layer Two-stage FEC Simulations Conclusions First, I will talk about the problems why current 802.11 wireless LANs are not efficient on multimedia data transmission.

Problems – 802.11 LANs No cooperation between layers Any bit error can cause a whole packet being dropped No cooperation between layers 802.11 wireless LANs are designed for reliable data transmission. One bit error can cause the whole packet being discarded, even though the packet can be used by error resilience CODEC in the receiver or recovered by application layer error control codes. (2) Each layer has it own protection mechanisms. (3) (a) packets are dropped due to routing disruption and congestion in the intermediate nodes, and (b) packets are discarded in the MAC/PHY layers due to internal bit errors. 802.11 MAC ARQ is not efficient for packet bit error

Our proposed two schemes Enhanced MAC/PHY layer using Header CRC/FEC helps to pass packet with errors to application and to forward more packets to next node Two-stage FEC scheme at application layer to cooperative with enhanced MAC/PHY layer for error recovery, both packet drop and bit error. To solve the above mentioned problems, we propose two scheme. Since packet with errors are passed to application layer, we need to have a scheme to solve both bit level and packet drops Cyclical Redundancy Check/Forward Error Correction

Enhanced Protocol Stack System Diagram Video Encoder Stage1 FEC Encoder Stage2 FEC Encoder Application UDP-lite Enhanced Protocol Stack IP MAC PHY Video Decoder Stage1 FEC Decoder Stage2 FEC Decoder This is the system diagram show how the two scheme works together to improve the efficiency for multimedia over wireless networks We slight modified the protocol stack and packets with errors are passed to application layer. Further, our enhanced MAC/PHY layer using header CRC/FEC increases application layer throughput Enhanced MAC/PHY layer using header CRC/FEC increases application layer throughput Stage 1: Packet level Stage 2: Bit level

Enhanced MAC/PHY layer Only header CRC/FEC Header FEC Headers FEC Packet payload APP UDP IP MAC Enhance MAC/PHY layer for multimedia: Header CRC: only CRC the header part Header FEC: add small bit level FEC to protect header part from bit errors We use two methods to enhance the MAC/PHY layer for multimedia data transmission, header CRC and header FEC.

Enhanced MAC/PHY layer 802.11 physical bandwidth 2 Mbps Packet payload 1000 bytes Packet CRC checks whole packet Header CRC checks only headers Header FEC protect headers with BCH(511,502,1) Here we show the analytical results using matlab of our enhanced MAC/PHY layer compared with conventional 802.11 Single hop application throughput Packet may have errors inside

Two-stage FEC FECs are processed only at application layer Can recover both packet losses and bit errors inside packet Cooperate with enhanced MAC/PHY layer to improve the application layer throughput. To combat with this errors, we proposed two stage FEC scheme Packets with errors are passed to the application layer, to effectively recover the two kind of packets loss, we proposed two stage FEC scheme. Packets first are encoded with packet level FEC across packets, then each packet including FEC packet are further protected by bit level codes within each packet.

Two-stage FEC Error free throughput Protections FEC codes Code Rate 802.11 SW-ARQ Retransmit one time RS only RS(255,239) 239/255 Two-stage FEC + HCRC RS(255,245) + Header CRC 239/245 Two-stage FEC + HFEC BCH(511,502,1) Error free throughput In this simulation Effective application throughput increased Two-stage FEC: payload is protected by BCH(8191,8000,14)

ns-2 simulations ns-2 802.11b wireless module, PHY layer bandwidth is set to 2Mbps; MAC layer retransmission 2; Channel model: Gilbert (burst length: 2) Two-stage FEC: RS code + BCH code Two scenarios: Single hop: from sender to receiver1, CBR traffic is set to 2Mbps Multi-hop: from sender to recever2, receiver 1 is relay node, CBR traffic is set to 1.2Mbps

ns-2 simulations - throughput Single hop Multi-hop Header FEC: BCH(8191,8000,14) for payload, BCH(510,480,3) for header Packet CRC: 802.11b Header CRC: BCH(8191,9000,14) for payload

ns-2 simulations – video Monochrome Foreman, CIF, 30fps, 16 frame/GOP, source coder: MC-EZBC Bitstream is pre-encoded with MD-FEC at 1 Mbps, 10% loss rate Two kinds of simulations: Single hop fixed FEC, CBR 1Mbps Multi-hop w/ adaptive FEC, bitstream and FEC protection adapts based on network conditions

ns-2 simulations - video Video PSNR of the first 7 GOPs PSNR = 0 if no enough bandwidth for base layer Single hop Multi-hop

Conclusions Two-stage FEC with enhanced MAC/PHY layer using header CRC/FEC increases application layer throughput Efficient in wireless multimedia error protection. Cooperation between layers can increase performance for multimedia over wireless networks.

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