Efficient Framing and ARQ for High-Speed PLC Systems

Srinivas Katar

Larry Yonge

(Intellon)srinivas.katar@intellon.com

Richard Newman

Haniph Latchman

(Univ. of Florida)nemo@cise.ufl.edu

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

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)

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….

Framing Processes

Powerline Characteristics

• High attenuation• Periodic noise floor variations• Isolated impulse noise• Periodic impulse noise• Continuous impulse noise

Powerline Attenuation Example

Typical Frequency Response

Powerline Noise Examples

Dimmer switch

Hair dryer

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

MAC Framing Requirements

• High efficiency absent errors• Ability to cope with errors from impulse

noise• Efficient retransmission• Privacy

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

Viable MAC Framing Strategies

• Viable = concatenation and partial delivery

• Simple Concatenation• Concatenation with demarcation• 2-level framing

Simple Concatenation

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

Concatenation with Demarcation

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

2-Level Framing2-LevelFraming

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

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

Simple Concatenation Efficiency

fec

msdu

mf

feck

mf

fecN

k

kSC LN

L

L

Lkp

L

Lkpp

111

1

• 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

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

fec

msdu

fec

LsbOHfecL LN

L

L

LLp 2,,

2 1

MSDU length = 1518 bytes (Ethernet)

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