Efficient Framing and ARQ for High-Speed PLC Systems

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

25. MSDU length = 1518 bytes (Ethernet)

26. 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