Date post: | 18-Jan-2018 |
Category: |
Documents |
Upload: | brianna-elliott |
View: | 219 times |
Download: | 0 times |
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
Physical interfaces Topologies Asynchronous communications
Slide 4
Data Link Layer Issues Type Error detection and control Flow control
Slide 5
Types of DLC Protocols Byte-oriented protocols Bit-oriented protocols
Slide 6
Protocol Environments End-to-End Local
Slide 7
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 8
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 9
Bit Stuffing Example
0100101111010101101110000010111110010101111110
Transmission Direction
Locations for Stuff Bits010010111101010110111000001011111000101011111010
Original Bit Stream
Transmitted Bit Stream
Slide 10
Design of Transmitter and Receiver
Application
NetworkSoftware
Bit Stuffing Flag Insertion
Control
Transmitter Receiver
Application
NetworkSoftware
Bit Destuffing
Control
Flag Detection
Slide 11
Vertical and Longitudinal Redundancy Checks
Bit 1 Bit 2 Bit n Parity
Character 1
Character 2
Character m
Parity
b11
b1m
C1
b2m
C2
bnm Rm
Cn Cn+1
b21 bn1 R1
b12 b22 bn2 R2
LRC
VRC
Slide 12
Cyclical Redundancy Check (CRC)
Based on polynomial division Use standard divisor polynomials
Slide 13
Standard CRC Polynomials P is the bit pattern, or Generator
Polynomial In the previous example, P = X5 + X4
+ X2 + 1 Standard CRC generator polynomials
–CRC-12 = X12 + X11 + X3 + X2 + X + 1–CRC-16 = X16 + X15 + X2 + 1–CRC-CCITT = X16 + X12 + X5 + 1
Slide 14
Error Correction Automatic Response Request
(ARQ) Forward error correction
Slide 15
Types of ARQ Stop-and-Wait (Idle RQ) Sliding Window (Continuous RQ)
Slide 16
Stop and Wait1 Data 0
2 ACK0
3 Data 1
4 NACK1
5 Data 1
6 ACK1
7 Data 0
Slide 17
Stop and Wait (Long Channel)
1 Data 0
2 ACK0
3 Data 1
4 ACK1
Increasing bit rate1 D 0
2 A0
3 D 1
4 A1
Slide 18
Sliding Window Transmitter may transmit several
messages before awaiting response
The number of unacknowledged messages permitted is the Window Size
Slide 19
Sliding Window If an error occurs
– Transmitter resends everything since the error (Go-Back-N)
– Only the errored packet (Selective Retransmission)
Slide 20
Sliding Window (n>3)0 Data 0
1 Data 1
2 Data 2
3 ACK 20
Slide 21
Sliding Window (Go-Back-N)4 Data 4
5 Data 5
6 Data 6
7 NACK 61
8 Data 4
9 Data 5
10 Data 6
Slide 22
Performance Issues Stop and Wait
– Error free
– With errorsatt
tU
frameprop
frame
211
2
aPU21
1
Slide 23
Performance Issues Sliding window (Go-Back-N)
12
1211
1221
1
aNNPPa
PN
aNaPP
U
Slide 24
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
WaitP=10-3
Slide 25
Forward Error Correction Encode data so that errors can be
corrected Requires redundancy in message
Slide 26
Examples of Protocols Byte oriented protocol
–Kermit–Bisync
Bit oriented protocol: HDLC
Slide 27
Kermit Frame Structure
SOH LEN SEQ TYPE data BCC CR