Error Correction and Flow Control
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Transcript of Error Correction and Flow Control
Error Correction and Flow Control
Martin Weiss
Slide 2
Objectives of this Meeting
Describe different flow control techniques
Describe the major error detection techniques
Slide 3
Review of the Last Meeting
Error detection Forward error correction
Slide 4
Error Correction Automatic Response Request
(ARQ) Forward error correction
Slide 5
Types of ARQ
Stop-and-Wait (Idle RQ) Sliding Window (Continuous RQ)
Slide 6
Stop and Wait
1 Data 0
2 ACK0
3 Data 1
4 NACK1
5 Data 1
6 ACK1
7 Data 0
Slide 7
Stop and Wait (Long Channel)
1 Data 0
2 ACK0
3 Data 1
4 ACK1
Increasing bit rate
1 D 0
2 A0
3 D 1
4 A1
Slide 8
Performance of Stop and Wait
t=0
t=tp
t=tt
t=tt+tp
t= x+tt+tp
t= x+ta+tt+tp
t= x+ta+tt+2tp
Slide 9
Performance of Stop and Wait
Error free (assume ta << tt and x=0)
att
tU
tp
t
21
1
2
Slide 10
Error Calculations
How many frames are transmitted for a frame to be properly received?– This is a probability distribution– Assuming independence of errors,
– The mean of this is a geometric dist.
)1(}ions transmissPr{ 1 PPj j
PPjPN
j
j
1
1)1(
1
1
Slide 11
Error Calculations P is the probability of frame error This can be estimated using:
– p is the bit error rate– k is the number of bits per frame
Using burst error rate is more accurate You can assume that the burst error
rate = 1/3 bit error rate
kpP )1(1
Slide 12
Stop and Wait with Errors
The error-free throughput is
Since it takes an average of NT frames to get an error free one, we can compute:
a
RT
21
a
PR
Na
RT
T 21
)1(1
21
a
P
R
TU
21
1
Slide 13
Sliding Window
Transmitter may transmit several messages before awaiting response
The number of unacknowledged messages permitted is the Window Size
Slide 14
Sliding Window (n>3)
0 Data 0
1 Data 1
2 Data 2
3 ACK 20
Slide 15
Sliding Window (Go-Back-N)
4 Data 4
5 Data 5
6 Data 6
7 NACK 41
8 Data 4
9 Data 5
10 Data 6
Slide 16
Performance of Sliding Window Protocols
Error-free (Go-Back-N and Selective Repeat)
12
21
121
aNa
NaN
U
Slide 17
Selective Repeat with Errors
If N is large enough, only the errored frames are retransmitted (occurring at a rate of (1-P)
Thus, the utilization becomes
12
21
1121
aNa
PNaNP
U
Slide 18
Go-Back-N with Errors
The number of retransmitted frames is
– When– Notice that when
»N>(2a+1) then»N<(2a+1) then K=N
P
KPPPPifN
i
ir
1
1)1(
1
1
KiKKiif )1()1(1
12 aK
Slide 19
Go-Back-N with Errors
For N>2a+1 For N<2a+1
Substituting and simplifying, we obtain:
12
121
1
1221
1
aNNPPa
PN
aNaP
P
U
rNU
1
)12(
aN
NU
r
Slide 20
Performance IssuesU
0
0.2
0.4
0.6
0.8
1.0
0.1 1 10 100 1000a
Sliding Window(N=7)Stop and
Wait
P=10-3
Slide 21
Comparing Sliding Window Approaches
Selective Reject Go-Back-N
Slide 22
Types of Protocols
Byte-oriented protocols Bit-oriented protocols
Slide 23
Protocol Environments End-to-End Local
Slide 24
Structure of Byte-Oriented Protocols
SYN SYN SYN HeaderSOH STX Message EOT
Sample Structure
SYN = SynchronizationSOH = Start of HeaderSTX = Start of TextEOT = End of Text
Slide 25
Structure of Bit-Oriented Protocols
Preamble Flag Header Message Trailer Flag
Sample Frame Structure
Preamble: Used for Bit SynchronizationFlag: Identifies Start and End of MessageTrailer: Error Checking
Slide 26
Bit Stuffing
Example
0100101111010101101110000010111110010101111110
Transmission Direction
Locations for Stuff Bits
010010111101010110111000001011111000101011111010
Original Bit Stream
Transmitted Bit Stream
Slide 27
Design of Transmitter and Receiver
Application
NetworkSoftware
Bit Stuffing Flag Insertion
Control
Transmitter Receiver
Application
NetworkSoftware
Bit Destuffing
Control
Flag Detection