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Data Link Control Protocols (DLC)
Wireless Information Networking Group (WING) EEL 6591 Wireless Networks Data Link Control Protocols (DLC)
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Outline Three Fundamental ARQ protocols Hybrid ARQ protocols
Wireless Information Networking Group (WING) Outline Three Fundamental ARQ protocols Stop & Wait (SW) Go-Back-N (GBN) Selective Repeat (SR) Hybrid ARQ protocols Type I Type II
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Wireless Information Networking Group (WING)
Why DLC? Radio signals tend to be distorted, transmission errors do occur! random noise channel fading interference Physical layer does not know what to do with erred signals! DLC is handling such an issue
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Wireless Information Networking Group (WING)
Data Link Layer (DLL) Reliable data delivery from point to point, erred frame will not be passed to the network layer! Point-to-point: only interested in the data flow on the link between the two points Reliability Error detection Error recovery Two error recovery schemes Forward Error Correction (FEC) Automatic Repeat reQuest (ARQ)
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Wireless Information Networking Group (WING)
Error Detection Link condition is not ideal, transmission errors do occur, need to tell right or wrong! ED basic idea: frame is so constructed that a certain relationship is imbedded, if the received frame does not have that relationship, something must be wrong! Redundancy bits added by transmitter for error detection code parity check or CRC related to channel coding: (n,k) coding
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Wireless Information Networking Group (WING)
Error Correction (EC) EC idea: redundancy is added so much to make codes (signals) distinct even under errors Forward Error Correction Control (FEC): some codes have error correction capability, if the wireless channel can be estimated a priori, FEC may be used to improve the channel efficiency (e.g., satellite communications): use FEC to lower the BER (Bit-Error-Rate) ED & EC examples ED: each data bit repeats once--two differing bits indicates error EC: each data bit repeats twice--majority rule
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Automatic Repeat reQuest (ARQ) --- Reactive Control
Wireless Information Networking Group (WING) Automatic Repeat reQuest (ARQ) --- Reactive Control FEC is proactive scheme for error recovery ARQ is a reactive scheme for error recovery detecting errors sending feedback info to sender Positive acknowledgment Negative acknowledgement timeout retransmission
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Three Fundamental ARQ Schemes
Wireless Information Networking Group (WING) Three Fundamental ARQ Schemes Stop and wait (SW) Go back N (GBN) Selective reject (selective repeat) (SR)
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Stop and Wait Simple case: KISS--Keep It Simple and Stupid
Wireless Information Networking Group (WING) Stop and Wait Simple case: KISS--Keep It Simple and Stupid Source transmits frame Destination receives frame and replies with acknowledgement Source waits for ACK before sending next frame Destination can stop flow by not send ACK, transmitter has a timer, when off, the frame will be retransmitted
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Go Back N Use sliding window and sequence number (SN)
Wireless Information Networking Group (WING) Go Back N Use sliding window and sequence number (SN) If no error, ACK as usual with next FrameExpected request, correct frame passed to network layer Use window size (N) to control number of outstanding frames If error, not passed to network layer, reply with rejection Discard that frame and all future frames until erred frame received correctly Transmitter must go back to the erred frame (N frames back) and retransmit that frame and all subsequent frames
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Wireless Information Networking Group (WING)
Go Back N - Diagram Window size selection: W < total number of sequence numbers -1 0,1,2,…,N-1 are all available sequence numbers, then W < N
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Wireless Information Networking Group (WING)
Selective Repeat (SR) Also called selective reject or selective retransmission Only erred frames are retransmitted Correctly received frames are accepted by the receiver and buffered Minimizes retransmissions Receiver must maintain large enough buffer More complex logic in transmitter and receiver Trade memory for BW
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Selective Repeat - Diagram
Wireless Information Networking Group (WING) Selective Repeat - Diagram Window size W <= N/ where N is the total number of SNs The buffered frames and frames in the window all have distinct SNs
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Wireless Information Networking Group (WING)
Throughput Analysis Throughput efficiency (throughput), or channel efficiency: the ratio of the average number of information bits successfully accepted by the receiver per unit time to the total number of digits that could be transmitted per unit of time The proportion of “useful” transmission time EEL5718: frame is counted as “information bits”, not exactly appropriate frame = information bits + redundancy bits (n,k) code: information k and redundancy (n-k) in wirelesss channel, (n-k) will be carefully designed according to channel condition
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Throughput Analysis Some quantities
Wireless Information Networking Group (WING) Throughput Analysis Some quantities Pc = probability that the received frame is error-free Pd = detectable frame error probability Pe = undetectable frame error probability P = Pc + Pe is the probability that the packet in a received frame will be passed to the network layer (n,k) is the code for a frame p = BER i.e., the bit error probability We will neglect undetectable error probability, thus P = Pc
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Throughput Analysis If BER is given, we can find
Wireless Information Networking Group (WING) Throughput Analysis If BER is given, we can find Idea: find how long it will take to send one frame to the receiver SUCCESSFULLY, or how many transmissions needed for one single frame We are dealing with ideal case: buffers at transmitter and receiver are infinite the number of sequence number are plenty (>3N) traffic is highly loaded (plenty of frames available)
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Wireless Information Networking Group (WING)
Selective Repeat Observation: any correctly received frame will NOT be retransmitted, only need to find how many transmissions needed!
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Wireless Information Networking Group (WING)
Go-Back-N Observation: each failed frame transmission will result in N wasted transmissions, need to find how many wasted transmissions per successfully received frame
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Wireless Information Networking Group (WING)
Stop & Wait Observation: each failure will lead to the waste of transmission time plus idle time D: the idle time from the end of frame transmission to the beginning of next frame R: transmission rate
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Performance Comparison
Wireless Information Networking Group (WING) Performance Comparison Infinite buffer Finite Buffer: : N 2: 2N
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ARQ with Mixed Modes of Retransmissions
Wireless Information Networking Group (WING) ARQ with Mixed Modes of Retransmissions SR performs better but performance drops sharply when error rate reaches certain point while GBN’s performance drops slowly SR+GBN: two protocols can be switched according to the performance to keep both advantages! Starts with SR mode: buffer all correctly received If a NACKed frame F has been acked before m retransmissions, the SR will continue Otherwise, the transmitter switches to GBN until positively ACK for F is received, in which case the SR is switched back again
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Wireless Information Networking Group (WING)
Performance of SR+GBN
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Wireless Information Networking Group (WING)
Hybrid ARQ: Basic Idea Information used in ARQ in progressive order: SW-->GBN-->SR Three observations: incorrectly received frames are discarded whole frame will be retransmitted upon request you know wireless channel is bad Information theory tells us: Never throw away useful information discarded frames may contain useful information do not transmit more than you’re asked for: being stingy at transmitter!
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Wireless Information Networking Group (WING)
Hybrid ARQ Depending on how you use the information, different hybrid ARQ can be developed Type I: FEC+ARQ Type II: take action at both ends incremental redundancy packet combining
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Type I Hybrid ARQ Observations:
Wireless Information Networking Group (WING) Type I Hybrid ARQ Observations: ARQ: is efficient when BER is low and degrades significantly when BER is high FEC: too much redundancies will be added if BER is high in order to achieve reasonable QoS Idea: use reasonable length of FEC to reduce the BER, use the ARQ to correct the rest of BER use a FEC subsystem in the ARQ system simple case: use a code (n,k) which has both error correction and error detection capability
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Type I Hybrid ARQ (cont)
Wireless Information Networking Group (WING) Type I Hybrid ARQ (cont) Two coding system k information bits choose a code (k’,k) with error correction capability choose a code (n,k’) with error detection capability (CRC) proof of concept: when a frame is received, if there is no error, k is passed on to network layer; if error, the receiver attempts to recover from (k’,k) code, if can be corrected, pass the corrected k to network layer, otherwise, retransmit One coding system a single FEC coder is used, the FEC decoder will generate retransmission request request retransmission when FEC decoder fails (RS coder) if FEC code can correct t errors, set a threshold t’<t, request retransmission whenever the number of errors corrected > t’
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Type I Hybrid ARQ (cont)
Wireless Information Networking Group (WING) Type I Hybrid ARQ (cont) Most powerful type I HARQ schemes are those based on Reed-Solomon (RS) code A RS code correcting all combo of e errors and s erasures, with minimum distance dmin >2e+s, let de denote the effective diameter of the type I decoder (i.e., threshold) do not retransmit if there are e errors and s erasures satisfying 2e+s <= de retransmit otherwise Adaptive type I HARQ can be designed for time-varying channels such as wireless channels
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Type II Hybrid ARQ Observations Two schemes
Wireless Information Networking Group (WING) Type II Hybrid ARQ Observations incorrectly received frames may contain more information: damaged image effect you may not need to transmit the whole thing, partial information may be enough Two schemes Incremental redundancy: retransmit more redundancy bits packet combining: combine the newly received frame with the previously received frame (similar to image overlaying effect)
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Incremental Redundancy
Wireless Information Networking Group (WING) Incremental Redundancy Simple case need two separate codes C0: (n,k), error detection only C1: (2k,k), invertible, error correction and error detection example transmit (n,k) coded message (f(u),u) with f(u) parity construct invertible (2k,k) code (q(u),u) and put q(u) in the retransmission buffer, q(u) is the parity, the redundancy upon retransmission request, (f(q(u)),q(u)) retransmitted upon reception if no error, use q(u) to find u if error, the received erred version of q(u) and previously saved u will be used to recover u
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Incremental Redundancy
Wireless Information Networking Group (WING) Incremental Redundancy Remarks alternating repetitions may be used if error persists, giving up when a certain threshold is reached incremental redundancy retransmissions can be combined with SW, GBN or SR adaptive version is possible There are many variations or generalizations, most powerful ones is the one combined with RS codes (see Wicker’s book for details)
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Wireless Information Networking Group (WING)
Packet Combining Idea: multiple received versions of a frame can be processed to recover the frame Two distinct approaches code combining diversity combining Code combining individual frame transmission: code rate R (=k/n) when N frames causes retransmissions, concatenated N frames form a single code with code rate R/N for simultaneous decoding more info will be gathered when N large
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Packet Combining Diversity combining
Wireless Information Networking Group (WING) Packet Combining Diversity combining multiple identical copies of a frame to recover the frame two combining methods symbol voting (hard-decision): majority rule symbol averaging (soft-decision) One variation (my wild idea) ith retransmission: (fi(u),u), fi(u) is a parity check bit function, do combining using different redundancies More advanced generalizations are possible Adaptive version (with FEC) can be investigated
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TX and RX Diversity Transmitter diversity Receiver diversity
Wireless Information Networking Group (WING) TX and RX Diversity Transmitter diversity Space-time coding (smartly utilize the multipath) Receiver diversity Turbo coding/decoding (received signals carry channel information as well, utilize it) Turbo decoding could identify “weak” bits Collaborative decoding of multiple receivers Hybrid ARQ: request only the necessary information
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Power Consideration DLC protocols with power conservation
Wireless Information Networking Group (WING) Power Consideration DLC protocols with power conservation reduce the transmissions over the air hybrid ARQ may help: transmit what is only needed stop transmissions when channel is bad and probe further research may be needed Asymmetric DLC mobile need to save power while BS may not, develop protocols to save mobile’s power one example (wild thought) MS to BS: SR BS to MS: GBN
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Wireless Information Networking Group (WING)
Further Reading Error Control Coding, S. Lin and D.J. Costello, Jr., Prentice-Hall, 1983 Error Control Systems for Digital Communications and Storage, S. Wicker, Prentice-Hall, 1995. Two papers under Advanced Reading
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