Ad hoc and Sensor Networks Chapter 6: Link layer...
Transcript of Ad hoc and Sensor Networks Chapter 6: Link layer...
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Ad hoc and Sensor NetworksChapter 6: Link layer protocols
Holger Karl
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SS 05 Ad hoc & sensor networs - Ch 6: Link layer protocols
Goals of this chapter – Link layer tasks in general
Framing – group bit sequence into packets/framesImportant: format, size
Error control – make sure that the sent bits arrive and no other
One of the most important tasks in data link layerFlow control – ensure that a fast sender does not overrun its slow(er) receiver
Not an issue in sensor networksLink management – discovery and manage links to neighbors
Do not use a neighbor at any cost, only if link is good enough ! Understand the issues involved in turning the radio communication between two neighboring nodes into a somewhat reliable link
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SS 05 Ad hoc & sensor networs - Ch 6: Link layer protocols
Overview
Error controlFramingLink management
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SS 05 Ad hoc & sensor networs - Ch 6: Link layer protocols
Error control
Error control has to ensure that data transport is Error-free – deliver exactly the sent bits/packetsIn-sequence – deliver them in the original orderDuplicate-free – and at most once Loss-free – and at least once
Causes: fading, interference, loss of bit synchronization, …
Results in bit errors, bursty, sometimes (see physical layer chapter)In wireless, sometimes quite high average bit error rates – 10-2 … 10-4 possible!
ApproachesBackward error control – ARQ (Automatic Repeat Request)Forward error control – FEC
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Causes and characteristics of transmission error
Sync: consists of a string of 0s or 1s, alerting the receiver that a potentially receivable signal is present.SFD (start frame delimiter): defines the beginning of a frame.Signal: identifies the type of modulation that the receiver must use to demodulate the signal.Services: reserves for future use.CRC: The Signal, Service, and Length fields are all protected with a CCITT CRC-16 frame check sequence
SS 05 Ad hoc & sensor networs - Ch 6: Link layer protocols
Format of an IEEE 802.11 physical layer frame
Sync(128-bit) SFD(16-bit) Signal(8-bit) Services(8-bit) Length(16-bit) CRC(16-bit) MPDU (variable)
Preamble PHY header
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SS 05 Ad hoc & sensor networs - Ch 6: Link layer protocols
Backward error control – ARQ
Basic procedure (a quick recap)Put header information around the payloadCompute 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 senderSend positive or negative acknowledgement
Sender uses timer to detect that acknowledgements have not arrived
Assumes packet has not arrivedOptimal timer setting?
If sender infers that a packet has not been received correctly, sender can retransmit it
What is maximum number of retransmission attempts? If bounded, at best a semi-reliable protocols results
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SS 05 Ad hoc & sensor networs - Ch 6: Link layer protocols
Standard ARQ protocols
Alternating bit – at most one packet outstanding, single bit sequence numberGo-back N – send up to N packets, if a packet has not been acknowledged when timer goes off, retransmit all unacknowledged packetsSelective Repeat – when timer goes off, only send that particular packet
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SS 05 Ad hoc & sensor networs - Ch 6: Link layer protocols
How to use acknowledgements
Be careful about ACKs from different layersA MAC ACK (e.g., S-MAC) does not necessarily imply buffer space in the link layerOn the other hand, having both MAC and link layer ACKs is a waste
Do not (necessarily) acknowledge every packet – use cumulative ACKs
Tradeoff against buffer space Tradeoff against number of negative ACKs to send
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SS 05 Ad hoc & sensor networs - Ch 6: Link layer protocols
When to retransmit
Assuming sender has decided to retransmit a packet – when to do so?
In a BSC (binary symmetric channel) channel, any time is as good as any In fading channels, try to avoid bad channel states – postpone transmissionsInstead (e.g.): send a packet to another node if in queue (exploit multi-user diversity)
How long to wait? Example solution: Probing protocolIdea: reflect channel state by two protocol modes, “normal” and “probing”When error occurs, go from normal to probing modeIn probing mode, periodically send short packets (acknowledged by receiver) – when successful, go to normal mode
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SS 05 Ad hoc & sensor networs - Ch 6: Link layer protocols
Forward error control
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
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Block Error Correction Codes
TransmitterForward error correction (FEC) encoder maps each k -bit block into an n -bit block codewordCodeword is transmitted; analog for wireless transmission
ReceiverIncoming signal is demodulatedBlock passed through an FEC decoder
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FEC Decoder Possible Outcomes
No errors presentCodeword produced by decoder matches original codeword
Decoder detects and corrects bit errorsDecoder detects but cannot correct bit errors; reports uncorrectable errorDecoder detects no bit errors, though errors are present
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Block Code Principles
Hamming distance- for 2 n -bit binary sequences, the number of different bits
E.g., v 1=011011; v 2=110001; d (v 1, v 2) = 3Example: Data block Codeword00 0000001 0011110 1100111 11110We can correct 1-bit error and detect 2-bit error
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Block Code Principles
Redundancy – ratio of redundant bits to data bits that is (n - k )/kCode rate – ratio of data bits to total bitsCoding gain – the reduction in the required Eb /N0 to achieve a specified BER of an error-correcting coded system
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SS 05 Ad hoc & sensor networs - Ch 6: Link layer protocols
Block-coded FEC
Level of redundancy: blocks of symbolsBlock: k p -ary source symbols (not necessarily just bits)Encoded into n q -ary channel symbols
Injective mapping (code) of pk source symbols into qn channel symbolsCode rate : (k ld p) / (n ld q )
When p=q=2: k/n is code rateFor p=q =2: Hamming bound – code can correct up to t bit errors only if
Codes for (n, k, t) do not always exist
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Hamming Code Process
Hamming codes are designed to correct single bit errorsEncoding: k data bits + (n - k ) check bitsDecoding: compares received (n -k ) bits with calculated (n - k ) bits using XOR
Resulting (n - k ) bits called syndrome wordSyndrome range is between 0 and 2(n -k ) - 1Each bit of syndrome indicates a match (0) or conflict (1) in that bit position and hence2(n -k ) – 1 >= k + (n – k ) = n
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Hamming Code Characteristics
We would like to generate a syndrome with the following characteristics:
If the syndrome contains all 0s, no error has been detectedIf the syndrome contains only one bit set to 1, then an error has occurred in one of the check bitsIf the syndrome contains more than one bit set to 1, then the numerical value of the syndrome indicates the position of the data bit in error
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Hamming Code Generation
Hamming check bits are inserted at the position of power of 2 i.e., positions 1, 2, 4, …, 2(n-k-1 )The remaining bits are data bitsEach data position which has a value 1 is presented by a binary value equal to its position; thus if the 9th bit is 1 the corresponding value is 1001All of the position values are then XORed together to produce the bits of the Hamming code
Example: The 8-bit data block is 00111001
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BCH Codes
For positive pair of integers m and t , a (n , k ) BCH code has parameters:
Block length: n = 2m – 1Number of check bits: n – k = 2t + 1
Correct combinations of t or fewer errorsFlexibility in choice of parameters
Block length, code rate
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Reed-Solomon Codes (Option)
Subclass of nonbinary BCH codesData processed in chunks of m bits, called symbolsAn (n , k ) RS code has parameters:
Symbol length: m bits per symbolBlock length: n = 2m – 1 symbols = m (2m – 1) bitsData length: k symbolsSize of check code: n – k = 2t symbols = m (2t ) bitsMinimum distance: d min = 2t + 1 symbols
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Example
Let t = 1 and m = 2. Denoting the symbols as 00, 01, 10, and 11Then n = 22 – 1 = 3 symbols = 6 bitsCheck code = (n - k ) = 2 symbols = 4 bitsThis code can correct any burst error
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Block Interleaving
Data written to and read from memory in different ordersData bits and corresponding check bits are interspersed with bits from other blocksAt receiver, data are de-interleaved to recover original orderA burst error that may occur is spread out over a number of blocks, making error correction possible
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Block Interleaving
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Convolutional Codes
Generates redundant bits continuously Error checking and correcting carried out continuously
(n , k , K ) codeInput processes k bits at a time Output produces n bits for every k input bitsK = constraint factork and n generally very small
n -bit output of (n , k , K ) code depends on:Current block of k input bitsPrevious K -1 blocks of k input bits
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SS 05 Ad hoc & sensor networs - Ch 6: Link layer protocols
Convolutional codes
Code rate: ratio of k user bits mapped onto n coded bitsConstraint length K determines coding gainEnergy
Encoding: cheapDecoding: Viterbi algorithm, energy & memory depends exponentially on constraint length K
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Convolutional Encoder
Ex: input 1011generates output11100001
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SS 05 Ad hoc & sensor networs - Ch 6: Link layer protocols
Energy consumption of convolutional codes
Tradeoff between coding energy and reduced transmission power (coding gain)Overall: block codes tend to be more energy-efficient
residual bit error prob.!
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SS 05 Ad hoc & sensor networs - Ch 6: Link layer protocols
Comparison: FEC vs. ARQ
FECConstant overhead for each packetNot (easily) possible to adapt to changing channel characteristics
ARQOverhead only when errors occurred (expect for ACK, always needed)
Both schemes have their uses ! hybrid schemes
BCH + unlimited number of retransmissions
Relative energy consumption
t: error correction capacity
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SS 05 Ad hoc & sensor networs - Ch 6: Link layer protocols
Power control on a link level
Further controllable parameter: transmission power Higher power, lower error rates – less FEC/ARQ necessaryLower power, higher error rates – higher FEC necessary
Tradeoff!
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SS 05 Ad hoc & sensor networs - Ch 6: Link layer protocols
Overview
Error controlFramingLink management
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SS 05 Ad hoc & sensor networs - Ch 6: Link layer protocols
Frame, packet size
Small packets: low packet error rate, high packetization overheadLarge packets: high packet error rate, low overheadDepends on bit error rate, energy consumption per transmitted bit
Notation: h(overhead, payload size, BER)
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SS 05 Ad hoc & sensor networs - Ch 6: Link layer protocols
Dynamically adapt frame length
For known bit error rate (BER), optimal frame length is easy to determineProblem: 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)
Details: see the references of text bookSecond problem: how long are observations valid/how should they be aged?
Only recent past is – if anything at all – somewhat credible
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SS 05 Ad hoc & sensor networs - Ch 6: Link layer protocols
Putting it together: ARQ, FEC, frame length optimization
Applying ARQ, FEC (both block and convolutional codes), frame length optimization to a Rayleigh fading channel
Channel modeled as Gilbert-Elliot
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SS 05 Ad hoc & sensor networs - Ch 6: Link layer protocols
Overview
Error controlFramingLink management
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SS 05 Ad hoc & sensor networs - Ch 6: Link layer protocols
Link management
Goal: decide to which neighbors that are more or less reachable a link should be established
Problem: communication quality fluctuates, far away neighbors can be costly to talk to, error-prone, quality can only be estimatedThe quality of a link is not binaryThe quality of a link is time variableThe quality can only to be estimated
Establish a neighborhood table for each nodePartially automatically constructed by MAC protocols
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SS 05 Ad hoc & sensor networs - Ch 6: Link layer protocols
Link quality characteristics
Expected: simple, circular shape of “region of communication” – not realisticSometimes links are asymmetricInstead:
Correlation between distance and loss rate is weak; iso-loss-lines are not circular but irregularAsymmetric links are relatively frequent (up to 15%)Significant short-term PER variations even for stationary nodes
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SS 05 Ad hoc & sensor networs - Ch 6: Link layer protocols
Three regions of communication
Effective region : PER consistently < 10%Transitional region: anything in between, with large variation for nodes at same distancePoor region : PER well beyond 90%
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SS 05 Ad hoc & sensor networs - Ch 6: Link layer protocols
Link quality estimation
How to estimate, on-line, in the field, the actual link quality?Requirements for estimator
Precision – estimator should give the statistically correct resultAgility – estimator should react quickly to changes Stability – estimator should not be influenced by short aberrations Efficiency – avoid too much listening for other nodes’ transmission
There are active or passive estimator
Example: only estimates at fixed intervals
Pn is the prediction of packet loss rate at time t + nT
rn : received packets in interval fn : packet identified as lostPn = αPn-1 + (1- α ) fn /(rn + fn )
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SS 05 Ad hoc & sensor networs - Ch 6: Link layer protocols
Conclusion
Link layer combines traditional mechanismsFraming, Error control, flow control
with relatively specific issuesCareful choice of error control mechanisms – tradeoffs between FEC & ARQ & transmission power & packet size …Link estimation and characterization