1. ICSA341 Data Communications and Computer Networks 1 Data Link Control •General principles

2. ICSA341 Data Communications and Computer Networks 2 Data Link Control •Coordinating between two end-systems across a Link to send/receive data correctly –Who should send and when – Medium Access •Coordinating sending and receiving – Line Discipline – involves Link establishment etc –What happens if received data is in error – Error control •Error correction •Retransmission –Processing capacity & Buffer and sending and receiving processes – Flow Control

3. ICSA341 Data Communications and Computer Networks 3 Line Discipline •No Device should send unless it has ascertained that the receiver is ready to receive –Link Establishment – logical check •Two Basic Schemes –Enquiry /Ack (peer to peer) –Poll /Select (Master /Slave)

4. ICSA341 Data Communications and Computer Networks 4 ENQ - enquiry ACK - Acknowledgement EOT - end of transmission data ACK ENQ data ACK EOT Establish Transfer Terminate SIMPLE ENQ/ACK protocol (e.g., "peer-to-peer", dedicated line) T R

5. ICSA341 Data Communications and Computer Networks 5 POLL - "have anything?" ACK - Acknowledgement EOT - end of transmission Address the secondary ? data NAK POLL ACK EOT POLL Establish Transfer Terminate POLL protocol (e.g., multipoint) T R Primary Secondary stations ACK POLL

6. ICSA341 Data Communications and Computer Networks 6 SEL - "ready to receive?" ACK - Acknowledgement NAK - Negative Acknowledgement EOT - end of transmission SEL ACK NAK SEL data ACK EOT Establish Transfer Terminate SELECT protocol (e.g., multipoint) T R Primary Secondary stations

7. ICSA341 Data Communications and Computer Networks 7 Flow Control •How fast and How much data to send? •Tx & rx speed physical layer •Memory storage at sender and receiver •Processing capacity at sender and receiver •Two most popular schemes –STOP & WAIT –SLIDING WINDOW

8. ICSA341 Data Communications and Computer Networks 8 ACK - Acknowledgement EOT - end of transmission Frames can be numbered 0,1 data ACK data ACK EOT Establish Transfer Terminate STOP and WAIT T R data WAIT TIME

9. ICSA341 Data Communications and Computer Networks 9 data, n - data, frame sent ACK, n - ACK, expected frame EOT - end of transmission ACK,4 ACK, 7 EOT Establish Transfer Terminate SLIDING WINDOW (without error control) T R WAIT TIME Data,0 Data,1 Data,2 Data,3 Data,6 Data,5 Data,4

10. ICSA341 Data Communications and Computer Networks 10 ACK - Acknowledgement Data 0 ACK 1 Data 0 Data 1 ACK 0... STOP and WAIT ARQ: lost frame T R Data 0 TIME OUT!!

11. ICSA341 Data Communications and Computer Networks 11 ACK - Acknowledgement ACK 1... STOP and WAIT ARQ: lost ACK T R Data 0 TIME OUT!! Data 0 ACK 1

12. ICSA341 Data Communications and Computer Networks 12 ACK - Acknowledgement NAK - Negative Acknowledgement Data 1 ACK 0 NAK ACK 1 Data 0 ACK 1... STOP and WAIT ARQ: damaged frame T R Data 0 WAIT TIME ERROR!

13. ICSA341 Data Communications and Computer Networks 13 Data, n - data, sent frame ACK, n - ACK, expected frame NAK, n - NAK, damaged frame EOT - end of transmission ACK,3 SLIDING WINDOW ARQ: Go-Back-n: Damaged Frame T R Data,0 Data,1 Data,2 Data,3 Data,5 Data,4 ERROR! Discarded NAK,3

14. ICSA341 Data Communications and Computer Networks 14 Data, n - data, sent frame ACK, n - ACK, expected frame NAK, n - NAK, damaged frame EOT - end of transmission ACK, 4 SLIDING WINDOW Go-Back-n: Lost Frame T R Data,0 Data,1 Data,2 Data,3 Data,4 LOST! Data,5 NAK,3 Data,6 Data,3 Discarded

15. ICSA341 Data Communications and Computer Networks 15 Data, n - data, sent frame ACK, n - ACK, expected frame NAK, n - NAK, damaged frame EOT - end of transmission SLIDING WINDOW Select-Reject: Lost Frame T R Data,0 Data,1 Data,2 Data,3 Data,4 LOST! Data,5 NAK,3 Data,6 Data,3 ACK,7