ad hoc and sensor networks chapter 6: link layer...

Ad hoc and Sensor NetworksChapter 6: Link layer protocols

Holger Karl

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


Overview

Error controlFramingLink management


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

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


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


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


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


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


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


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


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

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


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

Convolutional Encoder

Ex: input 1011generates output11100001


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.!


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


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!


Overview



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)


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


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


Overview



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


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


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%


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 )


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

ad hoc and sensor networks chapter 6: link layer...

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