1. Data Link Control Protocols (DLC)
Wireless Information Networking Group (WING)
EEL 6591 Wireless Networks
Data Link Control Protocols
(DLC)

2. 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

3. 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

4. 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)

5. 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

6. 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

7. 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

8. 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)

9. 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

10. 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

11. 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

12. 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

13. 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

14. 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

15. 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

16. 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)

17. Wireless Information Networking Group (WING)
Selective Repeat
Observation: any correctly received frame will NOT be retransmitted, only need to find how many transmissions needed!

18. 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

19. 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

20. Performance Comparison
Wireless Information Networking Group (WING)
Performance Comparison
Infinite buffer Finite Buffer: : N 2: 2N

21. 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

22. Wireless Information Networking Group (WING)
Performance of SR+GBN

23. 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!

24. 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

25. 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

26. 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’

27. 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

28. 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)

29. 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

30. 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)

31. 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

32. 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

33. 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

34. 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

35. 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