1. Error/Flow Control Modeling (ARQ Modeling)

2. © Tallal Elshabrawy 2 Data Link Layer  Data Link Layer provides a service for Network Layer (transfer of data from the network layer of a sender to the network layer of a receiver)  Data Link Layer uses the Physical Layer to transmit bits of Data Link Frames over the physical medium LLC MAC Network Physical

3. © Tallal Elshabrawy 3 Data Link Layer Functions  Framing (Grouping Bits into Frames)  Error Control  Flow Control  Medium Access Control

4. © Tallal Elshabrawy 4 Bit Errors in Communication Systems  At the physical layer, bit errors are inevitable to occur with small but non zero probability, example:  Bit error probability in the order of 10 -6 for systems using copper wires  Bit error probability in the order of 10 -9 for modern optical fiber systems  High bit error probability in the order of 10 -3 for wireless transmission systems  Some services are tolerant to relatively high bit error rates such as digital speech transmission  Some applications must experience error-free communications such as electronic funds transfer

5. © Tallal Elshabrawy 5 Error Control  Error Control is a system to deal with errors that occur due to disturbances on the physical channel.  Components of an error control system:  Error Correction and Detection  Acknowledgement (ACK) & Non- Acknowledgement Control Messages (NAK)  Timers Sender Receiver 0 0 Data Frame ACK No Errors 1 1 Errors 1 1 ACK No Errors Timer Frame is Good Detection/ Correction Detection/ Correction Detection/ Correction

6. © Tallal Elshabrawy 6 Error Control Mechanisms  Forward Error Correction (FEC)  Detection of erroneous frames or packets  Processing of received frame bits in attempt to correct the errors  Automatic Retransmission reQuest (ARQ)  Detection of erroneous frames or packets  Retransmission of erroneous frames with the hope that no errors would occur in the next attempt

7. © Tallal Elshabrawy 7 Automatic Repeat reQuest (ARQ) Protocols Purpose: to ensure a sequence of information packets is delivered in order and without errors or duplications despite transmission errors & losses (Error Control & Flow Control)

8. Modeling of Stop and Wait Protocol

9. © Tallal Elshabrawy 9 Stop-and-Wait ARQ  Stop after Transmitting a Packet  Wait for an Acknowledgement H: Header CRC: Cyclic Redundancy Check (Error Detection)

10. © Tallal Elshabrawy 10 Stop-and-Wait ARQ Operation Machine A Machine B Physical Channel First Packet-Bit enters Channels Last Packet-Bit enters Channels First Packet-Bit arrives at B Last Packet-Bit arrives at B Last ACK-Bit Arrives at A Processing Time for Error Detection Channel is Idle

11. © Tallal Elshabrawy 11 Stop-and-Wait ARQ Operation Machine A Machine B Physical Channel

12. © Tallal Elshabrawy Stop-and-Wait ARQ Modeling 12 Machine A Machine B Physical Channel

13. © Tallal Elshabrawy Stop-and-Wait ARQ Markov Model 13

14. © Tallal Elshabrawy Stop-and-Wait ARQ Markov Model 14

15. © Tallal Elshabrawy Stop-and-Wait ARQ Performance 15

16. © Tallal Elshabrawy Stop-and-Wait ARQ Efficiency 16 Efficiency Decreases with:  Increase in BER  Increase in Packet Size

17. © Tallal Elshabrawy Stop-and-Wait ARQ Throughput 17 Notes  Throughput does not care how many attempts have been done to successfully transmit a packet  Throughput measures the channel utilization for successful transmission  Efficiency rather measures the delay of a given packet  Both efficiency and throughput represent two faces of the same coin

18. © Tallal Elshabrawy Stop-and-Wait ARQ Simplified Model 18