3 Wireless Sensor Networks Akyildiz/Vuran Wireless Errors
Causes: Fading, interference, loss of bit synchronization Results
in bit errors, sometimes bursty For low power wireless
communication sometimes quite high average bit error rates (BERs)
(10 -2 10 -4 possible!)
Slide 4
4 Wireless Sensor Networks Akyildiz/Vuran Error Control
Approaches Automatic Repeat reQuest (ARQ) Forward Error Correction
(FEC) Hybrid ARQ (Type I and II)
Slide 5
5 Wireless Sensor Networks Akyildiz/Vuran ARQ (Quick Recap)
Basic procedure Put header information before the payload Compute a
checksum and add it to the end of 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 (ACK/NACK)
Slide 6
6 Wireless Sensor Networks Akyildiz/Vuran ARQ Sender uses timer
to detect that ACKs have not arrived Assumes packet has not arrived
Optimal timer setting? If sender infers that a packet has not been
received correctly, sender can retransmit it What is maximum number
of retransmission attempts?
Slide 7
7 Wireless Sensor Networks Akyildiz/Vuran 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 ACKed when timer goes off, retransmit all packets after
the first unacknowledged packet Selective Repeat when timer goes
off, only send the unacknowledged packet(s)
Slide 8
8 Wireless Sensor Networks Akyildiz/Vuran How to use ACKs ? Be
careful about ACKs from different layers A MAC ACK (e.g., S-MAC)
does not necessarily imply buffer space in the link layer Do not
(necessarily) ACK every packet use cumulative ACKs Tradeoff against
buffer space Tradeoff against number of negative ACKs to send
Slide 9
9 Wireless Sensor Networks Akyildiz/Vuran When to Retransmit ?
Assuming sender has decided to retransmit a packet when to do so?
For a good channel, any time is as good as any For 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) REMARK: If the previous packet was corrupted
then the channel was bad
Slide 10
10 Wireless Sensor Networks Akyildiz/Vuran When to Retransmit ?
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 (ACKed by receiver) when
successful, go to normal mode
Slide 11
11 Wireless Sensor Networks Akyildiz/Vuran Forward Error
Control (FEC) Idea: Endow symbols in a packet with additional
redundancy to withstand a limited amount of random permutations
Additionally: interleaving change order of symbols to withstand
burst errors Header Payload Redundant Bits l DATA n-k k n
Slide 12
12 Wireless Sensor Networks Akyildiz/Vuran Forward Error
Control
14 Wireless Sensor Networks Akyildiz/Vuran Popular Block Codes
Energy Consumption: Encoding: Negligible overhead (linear-feedback
shift register) Decoding: depends on block length (n) and Hamming
distance (t) Eng. Consumption for multiplication Eng. Consumption
for addition
Slide 15
15 Wireless Sensor Networks Akyildiz/Vuran Convolutional Codes
Code rate: ratio of k user bits mapped onto n coded bits Constraint
length k determines coding gain Energy Encoding: cheap Decoding:
Viterbi algorithm, energy & memory depends exponentially (!) on
constraint length
Slide 16
16 Wireless Sensor Networks Akyildiz/Vuran Hybrid ARQ (HARQ)
ARQ Efficient when channel is good - No redundant bits are sent
Inefficient when channel is bad whole packet is sent again FEC
Inefficient when channel is good Redundant bits sent anyway
Efficient when channel is bad No retransmissions Hybrid ARQ Marry
ARQ and FEC Get advantages from both techniques
Slide 17
17 Wireless Sensor Networks Akyildiz/Vuran Hybrid ARQ Send
uncoded or lightly coded packet first If in error, retransmit with
a stronger code Two types based on retransmission technique HARQ
Type I HARQ Type II
Slide 18
18 Wireless Sensor Networks Akyildiz/Vuran Hybrid ARQ Type I
(HARQ-I) Header + Payload Redundant Bits e.g. BCH(128,106,3) Packet
in error Send NACK Send uncoded packet NACK Send coded packet No
errors No ACK sent
Slide 19
19 Wireless Sensor Networks Akyildiz/Vuran Hybrid ARQ Type I
(HARQ-I) Header + Payload e.g. BCH(128,78,7) Packet in error Send
NACK Send lightly coded packet NACK Send more powerful coded packet
No errors No ACK sent Redundant Bits e.g. BCH(128,106,3)
Slide 20
20 Wireless Sensor Networks Akyildiz/Vuran Hybrid ARQ Type II
(HARQ-II) Header + Payload Redundant Bits e.g. BCH(128,106,3)
Packet in error Send NACK Send uncoded packet NACK Send redundant
bits No errors No ACK sent
Slide 21
21 Wireless Sensor Networks Akyildiz/Vuran Hybrid ARQ Type I
Does not require previous packet to be stored Type II Reduces
bandwidth usage
Slide 22
22 Wireless Sensor Networks Akyildiz/Vuran Error Control
through Transmit Power Control Higher transmission power reduces
the packet error rate by improving SNR. Energy consumption and
interference is increased Radio should support different power
levels (may not be applicable in many WSNs (low cost nodes)
Slide 23
23 Wireless Sensor Networks Akyildiz/Vuran Cross-Layer
Comparison: FEC, ARQ, HARQ Investigate the tradeoffs between ARQ,
FEC, and HARQ in terms of Energy consumption Latency Packet error
rate The first work that jointly considers Broadcast wireless
channel Multi-hop communication Realistic channel model 2-D
topology Realistic hardware models (Mica2 and MicaZ) M. C. Vuran
and I. F. Akyildiz, Cross-layer Analysis of Error Control in
Wireless Sensor Networks, IEEE SECON 06, September 2006. M. C.
Vuran and I. F. Akyildiz, Error Control in Wireless Sensor
Networks: A Cross Layer Analysis, IEEE Trans. on Networking, vol.
17, no. 4, pp. 1186-1199, April 2009.
Slide 24
24 Wireless Sensor Networks Akyildiz/Vuran Cross-Layer Effects
Multi-hop communication affects End-to-end packet error rate Energy
consumption Latency Broadcast wireless channel Nodes other than
communicating parties are affected (overhearing) Wireless channel
Channel characteristics drastically influence BER performance
Cross-layer analysis of error control is necessary
Slide 25
25 Wireless Sensor Networks Akyildiz/Vuran Motivation ARQ
Reactive error control Send redundancy in case of errors (lack of
ACK) FEC Proactive error control Incorporate redundancy to correct
errors Improves the error resiliency of a single packet
Slide 26
26 Wireless Sensor Networks Akyildiz/Vuran Error Resiliency
Packet Error Rate (PER) is a function of signal to noise ratio
(SNR), i.e., channel quality ARQ vs. FEC: FEC codes provide the
same PER with lower SNR, i.e., lower channel quality For PER *, SNR
ARQ >SNR FEC As error correction capability, t, increases, error
resiliency improves
Slide 27
27 Wireless Sensor Networks Akyildiz/Vuran Error Resiliency How
can error resiliency be exploited in multi-hop WSN? d ARQ P t,ARQ
-P n SNR ARQ PER * Path Loss ARQ case Transmit Power Noise
Power
Slide 28
28 Wireless Sensor Networks Akyildiz/Vuran Error Resiliency How
can this be exploited in multi-hop WSN? Hop-length Extension
(increase hop length for FEC) P t,FEC = P t,ARQ, d FEC > d ARQ d
FEC P t,ARQ -P n SNR FEC PER * Path Loss FEC case
Slide 29
29 Wireless Sensor Networks Akyildiz/Vuran Error Resiliency How
can this be exploited in multi-hop WSN? Hop-length Extension
(increase hop length for FEC) P t,FEC = P t,ARQ, d FEC > d ARQ
Transmit Power Control (decrease transmit power) P t,FEC < P
t,ARQ, d FEC = d ARQ d ARQ P t,FEC -P n SNR FEC PER * Path Loss FEC
case
Slide 30
30 Wireless Sensor Networks Akyildiz/Vuran Cost FEC codes
improve error resiliency at the cost of Encoding/decoding (energy +
latency) Tx/Rx longer packets (energy + latency) Header Payload
Redundant Bits a l DATA n-k
Slide 31
31 Wireless Sensor Networks Akyildiz/Vuran Network Model
Channel-aware routing Next hop j is selected if received SNR, j
> Th (SNR threshold) Contention-based medium access
RTS-CTS-DATA(-ACK) exchange Duty cycle operation Nodes are active
fraction of the time
Slide 32
32 Wireless Sensor Networks Akyildiz/Vuran Cross-layer Analysis
Channel model Transmit Power Received Power Path loss exponent
Log-normal Shadow fading
Slide 33
33 Wireless Sensor Networks Akyildiz/Vuran Cross-layer Analysis
Channel model then, Received SNR SNR threshold Variance of
log-normal fading (usually ~3.5) Noise power
Slide 34
34 Wireless Sensor Networks Akyildiz/Vuran Total energy
consumed as a flow-based energy consumption analysis Expected
number of hops from a source to sink with distance D E[d next-hop ]
is the expected hop distance. R inf is the approximated
transmission range. Energy Consumption
Slide 35
35 Wireless Sensor Networks Akyildiz/Vuran Effect of Duty Cycle
Consumed energy per hop in one hop for transmitting a packet
Detailed derivations for ARQ, FEC, and HARQ in the paper Eng.
consumption of the transmitter node Eng. consumption of the
receiver node Eng. consumption of the neighbor nodes
Slide 36
36 Wireless Sensor Networks Akyildiz/Vuran Latency Analysis
Expected end-to-end latency Expected number of hops Expected
latency per hop
Slide 37
37 Wireless Sensor Networks Akyildiz/Vuran Decoding Latency and
Energy Encoding for block codes is negligible Decoding latency for
a (n,k,t) block code Both Mica2 and MicaZ use 8-bit
microcontrollers Decoding energy Processor cycle latency (250ns)
Processor current (8mA) Voltage (3V)
Slide 38
38 Wireless Sensor Networks Akyildiz/Vuran Bit Error Rate Mica2
nodes use frequency shift keying (FSK) MicaZ nodes use offset
quadrature phase shift keying (O- QPSK) with direct sequence spread
spectrum (DSSS) Bandwidth Bit rate # of chips per bit (16) =2 for
MicaZ
Slide 39
39 Wireless Sensor Networks Akyildiz/Vuran Numerical
Evaluations Energy consumption, latency and PER are found for Mica2
and MicaZ, and ARQ, FEC, and HARQ schemes
Slide 40
40 Wireless Sensor Networks Akyildiz/Vuran Numerical
Evaluations FEC Codes BCH (n,k,t) - (128,50,13), (128,78,7),
(128,106,3) RS (n,k,t) (7,3,2), (15,9,3), (31,19,6) Hybrid ARQ (I
& II) Notation: HARQ-I (t 1,t 2 ) means: Hybrid ARQ type I t 1
: error correction capability for 1 st transmission (t 1 = 0
uncoded packet) t 2 : 2 nd transmission BCH codes are used for
coded transmission
Slide 41
41 Wireless Sensor Networks Akyildiz/Vuran Hop Length Extension
Energy consumption vs. SNR threshold Lower SNR threshold leads to
longer hop length BCH and RS achieve the same PER for lower SNR
threshold compared to ARQ BCH (t=7) consumes less energy than ARQ
RS (t=3) consumes more energy than ARQ Energy Consumption (MicaZ)
PER ARQ =10 -2 PER BCH =10 -2 PER RS =10 -2
Slide 42
42 Wireless Sensor Networks Akyildiz/Vuran Hop Length Extension
Compared to ARQ, BCH and RS achieve lower latency for the same PER
Lower SNR threshold longer hops less number of hops lower latency
Latency (MicaZ) PER ARQ =10 -2 PER BCH =10 -2 PER RS =10 -2
Slide 43
43 Wireless Sensor Networks Akyildiz/Vuran Hop Length Extension
Taxonomy function Received bit per consumed resources (time*energy)
i.e., resource efficiency
Slide 44
44 Wireless Sensor Networks Akyildiz/Vuran Hop Length Extension
For MicaZ nodes, FEC is more resource efficient than ARQ BCH is 30%
more efficient than RS MicaZ
Slide 45
45 Wireless Sensor Networks Akyildiz/Vuran Hop Length Extension
For Mica2, ARQ is more resource efficient Why? Lower bit rate (16
kbps vs. 250 kbps MicaZ) Packet transmission duration is more
important than hop length extension in Mica2 ARQ outperforms FEC
since shorter packets are sent Taxonomy function, T (Mica2)
Slide 46
46 Wireless Sensor Networks Akyildiz/Vuran Transmit Power
Control Significant reduction in energy consumption with transmit
power control for BCH RS codes are not energy efficient with
transmit power control BCH is more energy efficient than ARQ Energy
Consumption (Mica2)
Slide 47
47 Wireless Sensor Networks Akyildiz/Vuran Transmit Power
Control Trade off latency Decrease in transmit power increase in #
of hops ARQ has lower latency when transmit power is decreased for
FEC codes Latency (Mica2)
Slide 48
48 Wireless Sensor Networks Akyildiz/Vuran Hybrid ARQ HARQ-II
outperforms ARQ and FEC schemes HARQ-I is less energy efficient
whole packet is retransmitted For HARQ-II, energy efficiency
improves if error correction capability (t) increases HARQ-II
implementation cost is also lower than HARQ-I Energy Consumption
(MicaZ)
Slide 49
49 Wireless Sensor Networks Akyildiz/Vuran Hybrid ARQ HARQ-II,
BCH, and RS provide lower latency compared to ARQ HARQ-I latency is
significantly higher Retransmission + Encoding/Decoding overhead
HARQ-II and RS most suitable for delay sensitive traffic
Slide 50
50 Wireless Sensor Networks Akyildiz/Vuran Conclusions A
cross-layer analysis of ARQ, FEC, and HARQ is performed Developed
framework enables a generic analysis with various parameters (e.g.
BCH, RS, Mica2, MicaZ) Careful selection of FEC parameters improve
latency without hampering energy consumption (HARQ and RS suitable
for multimedia traffic) Hardware support for FEC will improve
performance
