20-11-2012, 06:10 PM
Wireless Sensor Networks
1Wireless Sensor.pdf (Size: 206.81 KB / Downloads: 67)
Error control
•Error control has to ensure that data transport is
•Error-free –deliver exactly the sent bits/packets
•In-sequence –deliver them in the original order
•Duplicate-free –and at most once
•Loss-free –and at least once
•Causes: fading, interference, loss of bit synchronization, …
•Results in bit errors, bursty, sometimes heavy-tailed runs (see physical layer chapter)
•In wireless, sometimes quite high average bit error rates –10-2…10-4possible!
•Packet losses (not processes by MAC or LL):
•The receiver fails to acquire bit/frame synchronization
•PHY header checksum fails
•Packets with bits in error
•MAC header ok, but MAC payload unprotected or never discarded
•Basic procedure (a quick recap)
•Put header information around the payload
•Compute a checksum and add it to the packet
•Typically: Cyclic redundancy check (CRC), quick, low overhead, low residual error rate
•Provide feedback from receiver to sender
•Send positive or negative acknowledgement
•Sender uses timer to detect that acknowledgements have not arrived
•Assumes packet has not arrived
•Optimal timer setting?
Standard ARQ protocols
•Alternating bit –at most one packet outstanding, single bit sequence number
•Go-back N –send up to N packets, if a packet has not been acknowledged when timer goes off, retransmit all unacknowledged packets
•Selective Repeat –when timer goes off, only send that particular packet
When to retransmit
•Assuming sender has decided to retransmit a packet –when to do so?
•In a BSC channel, any time is as good as any
•In fading channels, try to avoid bad channel states –postpone transmissions
•Instead (e.g.): send a packet to another node if in queue (exploit multi-user diversity)
•How long to wait?
•Example solution: Probing protocol
•Idea: reflect channel state by two protocol modes, “normal”and “probing”
•When error occurs, go from normal to probing mode
•In probing mode, periodically send short packets (acknowledged by receiver) –when successful, go to normal mode
MultihopFEC
•Three possibilities compared in [943] assuming constraint on received power:
1.Direct FEC-coded transmission from source to destination
2.Intermediate nodes do FEC decoding and re-coding before forwarding
3.Intermediate nodes just forward without FEC processing
•Relative advantages depend on the distance from source to destination:
•1 and 2 preferable over long distances
•2 only pays-off over longest distances: accumulation of too many errors avoided.
Hybrid Alternatives
•Normal packets w/o FEC, retransmitted packets w/ FEC
•Parity packets instead of retransmissions [119]
•Packet combining schemes where receiver uses information in erroneous packets and uses equal-gain combining or bit-by-bit majority voting [334, 394, 860]
Dynamically adapt frame length
•For known bit error rate (BER), optimal frame length is easy to determine
•Problem: how to estimate BER?
•Collect channel state information at the receiver (RSSI, FEC decoder information, …)
•Example: Use number of attempts T required to transmit the last M packets as an estimator of the packet error rate (assuming a BSC)
•Second problem: how long are observations valid/how should they be aged?
•Only recent past is –if anything at all –somewhat credible