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Efficient Framing and ARQ for High-Speed PLC Systems

This paper discusses the evolution of powerline communication (PLC) systems and the goals for achieving high-speed audiovisual (AV) transmission and quality of service (QoS) for PLC. It explores the MAC framing processes, powerline characteristics, channel adaptation, and robust ARQ mechanisms. The paper also presents various MAC framing strategies, including simple concatenation, concatenation with demarcation, and 2-level framing, and evaluates their efficiency. The conclusion highlights the importance of 2-level framing for high efficiency in PLC systems.

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Efficient Framing and ARQ for High-Speed PLC Systems

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  1. Efficient Framing and ARQ for High-Speed PLC Systems Richard Newman Haniph Latchman (Univ. of Florida) nemo@cise.ufl.edu Srinivas Katar Larry Yonge (Intellon) srinivas.katar@intellon.com

  2. PLC Evolution in a Nutshell • > 5 yrs ago: Low speed control applications • 1-5 yrs ago: Medium speed data transfer • Now + future: High speed AV, BPL

  3. QoS Goals for AV PLC • Data speeds - must sustain application rates of • 6 Mbps per SDTV connection • 24 Mbps per HDTV connection • Must be QEF (quasi-error-free) for video • Must meet latency requirements (10 ms for voice, 300 ms for video)

  4. PLC MAC • High attenuation => CS but no CD • Per-channel adaptation => Virtual Carrier Sense • VCS => Broadcast delimiters • Broadcast => high fixed OH per MPDU • High PHY rates => concatenation • Impulse noise….

  5. Framing Processes

  6. Powerline Characteristics • High attenuation • Periodic noise floor variations • Isolated impulse noise • Periodic impulse noise • Continuous impulse noise

  7. Powerline Attenuation Example Typical Frequency Response

  8. Powerline Noise Examples Dimmer switch Hair dryer

  9. Channel Adaptation and MAC Framing • Impulse noise power is high • Adapting channel to eliminate impulse noise effects may be impossible • Even when possible, it may reduce data rate excessively • Hence, need robust ARQ mechanism

  10. MAC Framing Requirements • High efficiency absent errors • Ability to cope with errors from impulse noise • Efficient retransmission • Privacy

  11. MAC Framing Strategies • 1 MSDU per MPDU - low efficiency • 25% efficiency sans errors for 1518 B enet pkt • Require concatenation of MSDUs • Even with concatenation, single acknowledgement per MPDU too inefficient • <8% for 24 FEC blocks at 10% FER • <30% for 24 FEC blocks at 5% FER • Require partial delivery to handle inevitable impulse errors

  12. Viable MAC Framing Strategies • Viable = concatenation and partial delivery • Simple Concatenation • Concatenation with demarcation • 2-level framing

  13. Simple Concatenation

  14. Simple Concatenation • Framing • MSDU sequence number (SN) • MSDU Length (Len) • MSDU Frame Check Sequence (FCS) • Advantages • Low, low overhead • Simplicity • Disadvantages • MPDU padding to fit PPDU • Loss of all data following FEC block error

  15. Concatenation with Demarcation

  16. Concatenation with Demarcation • Framing • add Header Check Sequence (HCS) to resynchronize after FEC block error • ID within MPDU (for bitmap Selective ACK) • Advantages • Can recover data after FEC block error • Selective retransmission of MSDUs • Disadvantages • More complex • still pad • single FEC block error can corrupt two MSDUs

  17. 2-Level Framing 2-Level Framing

  18. 2-Level Framing • Framing per MSDU - Length • Framing per FEC block • FCS per FEC block • FEC block SN • MSDU boundary flag and offset • Advantages • Selective retransmission of FEC blocks • Padding may be avoided • simplifies memory management • Disadvantages • Complexity

  19. Framing Efficiency Metrics • Ratio of total bits of data successfully delivered to total bits sent • Bits sent includes framing overhead bits and retransmissions • Ignore MPDU overheads (same for all and system dependent) • Assume fixed size FEC blocks • Assume FEC block errors independent

  20. Simple Concatenation Efficiency • p = FEC Block error rate • k = location of first error • Lfec = Length of FEC Block • Lmf = Length of MAC Frame • Lmsdu = Length of MSDU • N = number of FEC Blocks

  21. 2-Level Framing Efficiency • p = FEC Block error rate • Lfec = Length of FEC Block • LOH,sb,2L = per FEC Block overhead • Lmsdu = Length of MSDU • N = number of FEC Blocks

  22. MSDU length = 1518 bytes (Ethernet)

  23. Conclusions • Fixed overheads in PLC and wireless communications require concatenation when PHY rates are high • Simple concatenation methods suffer from poor retransmission options • 2-Level framing method solves these problems, is highly efficient; efficiency independent of MPDU length

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