PPR: Partial Packet Recovery

Brad KarpUCL Computer Science

CS 4038 / GZ0623rd January, 2008

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Packet Corruption in Wireless MACs

• 802.11 and 802.15.4 use per-packet CRC– Receiver recomputes CRC; drops whole packet if

doesn’t match– Protects against data corruption by collisions,

interference

• 802.11 and 802.15.4 do link-layer retransmission of whole packets– Recovers faster than transport layer

• CRC failures at receiver are relatively frequent– Collisions: RTS/CTS don’t work so well in practice– Bit errors: can’t predict them, so would need too

much redundancy to guarantee error-free channel

Is it efficient to retransmit entire packet?How much of packet is actually corrupted in practice?

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Anatomy of a Collision

• Recall: receiver only can decode bits correctly when SNR (SINR) above some threshold

• Insight: provided senders in range of receiver, only concurrently received portions of packets should be corrupted!

P1

P2

time (at receiver)

corrupted

corrupted OK

OK

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Goal: Only Retransmit Corrupted Bits

• How can receiver know which bits corrupted, which received correctly?

• Will receiver be able to detect salvageable packet at all during a collision?– Today’s radios must receive a special

preamble bit sequence correctly to detect start of packet

• How can receiver efficiently express which bits should be retransmitted?– ACK itself costs bandwidth; want ACK +

rexmt’ed bits to be smaller than whole packet!

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SoftPHY: Identifying Corrupted Bits

• Idea: PHY can supply MAC not only with decoded data bits, but also degree of confidence in data bits’ correctness

(P2) Preamble

(P1)

Preamble

“Uncertainty”

[Figure: Kyle Jamieson]

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Background: Direct Sequence Spread Spectrum (DSSS)

Transmitter• Sender’s PHY receives packet bits from MAC• PHY maps every b bits of packet into B-bit

(aka B-chip) codeword– Because B > b, space of codewords sparsely

populated• PHY divides each codeword into series of k-

bit symbols (k >= 1)• Radio transmits series of symbols; for packet

size of T bits, TB/bk symbols transmitted• 802.15.4 (common sensor “mote” radio):

– b = 4, B = 32, k = 2– i.e., 4 bits in packet sent as 16 channel symbols of

2 bits each

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Background: DSSS Receiver

• Receiver demodulates signal• Demodulator recovers each successive

symbol independently• Channel decoder maps each successive

codeword-sized group of symbols (i.e., in 802.15.4, 16 symbols, or 32 chips) to closest valid codeword

• Closest: minimal Hamming distance; valid codeword differing in fewest number of bits from demodulated codeword

• Idea: for each codeword, use value of this Hamming distance as measure of confidence in corresponding packet data bits’ correctness

802.11b/g use DSSS and OFDM-DSSS; work similarly

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SoftPHY for DSSS Radios:Hamming Distance Example

• SoftPHY passes hint about confidence level in every 4-bit block in packet up to MAC, along with data

• Uses threshold η: presumes 4-bit block correct when hint < η, incorrect when hint > η

Receive: 11101101000111000011010110100010C1: 11101101100111000011010100100010

Receive: 11001101000111010111011110110111C1: 11101101100111000011010100100010

Uncertainty: 2

Uncertainty: 9

[Figure: Kyle Jamieson]

Will threshold always correctly identify whether block is received without error?

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Accuracy of Threshold-Based Confidence Metric

• At η = 2, few misclassifications of codeword correctness

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Postamble Decoding:Detecting Packets During

Collisions

• Recall: radio hardware only detects packet upon receipt of preamble containing reserved bit sequence

• Receiver can’t hear P2’s preamble!

P1

P2

corrupted

corrupted OK

OK

preamble

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Postamble Decoding (cont’d)

• Postamble: add copy of preamble and packet header at end of packet

• When postamble detected, packet has already passed by!– Must buffer recently received samples at

all times– Upon detecting postamble, retrieve

buffered header just before it, to learn packet length

– Retrieve previous packet in its entirety

P1

P2

corrupted

corrupted OK

OK postamble

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PP-ARQ:Retransmitting Partial Packets

• SoftPHY indicates which bits likely to be correct, which corrupted

• Postamble increases probability of detecting colliding packets

• Need link-layer reliability protocol that allows receiver to send ACKs that request partial retransmissions of packets

• Problem: how to minimize size of partial ACKs?

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PP-ARQ Protocol

2. Codewords are in fact corrector incorrect– Two possibilities for mistakes

• Labeling a correct codeword “bad”• Labeling an incorrect codeword “good”

“Good” bits“Bad” bits

1. Assuming hints correct, which ranges to ask for?– Dynamic programming problem– Forward and feedback channels

[Slide: Kyle Jamieson]

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Implementation

Sender: Telos Tmote Sky sensor node– Radio: CC2420 DSSS/MSK (Zigbee)– Modified to send postambles

Receiver: USRP software radio with 2.4 GHz RFX 2400 daughterboard

– Despreading, postamble synchronization, demodulation

– SoftPHY implementation

[moteiv.com]

[ettus.com]

PP-ARQ: trace-driven simulation using data from above [Slide: Kyle Jamieson]

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• Live wireless testbed experiments– Senders transmit 101-byte

packets, varying traffic rate– Evaluate raw PPR

throughput– Evaluate SoftPHY and

postamble improvements

• Trace-driven experiments– Evaluate end-to-end PP-ARQ performance– Internet packet size distribution– 802.11-size preambles

Experimental design

25 senders6 receivers

[Slide: Kyle Jamieson]

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PP-ARQ performance comparison

• Packet CRC (no postamble)

• Fragmented CRC (no postamble)– Tuned against traces for optimal fragment

size

Preamble

Checksum

Checksum

PreambleChecksum [Slide: Kyle Jamieson]

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Throughput improvement 2.3-2.8x

[Slide: Kyle Jamieson]

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PP-ARQ retransmissions are short

[Slide: Kyle Jamieson]

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25% improvement over fragmented

[Slide: Kyle Jamieson]

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PP-ARQ retransmissions are short

[Slide: Kyle Jamieson]

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PP-ARQ feedback overhead is low

802.11 ACK size[Slide: Kyle Jamieson]

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Conclusion

• Mechanisms for recovering correct bits from parts of packets– SoftPHY interface (PHY-independent)– Postamble decoding

• PP-ARQ improves throughput 2.3–2.8x over ARQ with whole-packet CRCs

• Great example of how software radio enables experimentation with new ideas in wireless systems!

• Software radio can’t yet support online PP-ARQ implementation; latency of processing too great to send ACKs back in timely fashion