1. An SSCOP-based Link Layer Protocol for Wireless LANs Haoli Wang and Aravind Velayutham IEEE Global Telecommunications Conference 1-5 December, 2003 San Francisco, USA

2. 2 Outline Introduction Problems Solutions Proposed Protocol Analysis and Performance Conclusions Q&A

3. 3 Introduction The IEEE 802.11 MAC protocol It ’ s the standard for medium access in wireless LANs. It provides fast recovery from frame losses using a rapid link level positive acknowledgement scheme, generally an ACK frame.

4. 4 Positive acknowledgement scheme Retransmission Sender Receiver ACK DATA ACK Timeout

5. 5 Problems At a low error rate, the most of the frame transmissions are correct and the positive ACK scheme is inefficient. Since each data frame requires on ACK, the large traffic overhead of ACK transmission wastes the scarce bandwidth.

6. 6 Solutions To use the negative ACK scheme, in order to reduce the amount of ACK traffic by the receiver only requests the retransmission of the lost frames. The proposed protocol is an adaptation of Service Specific Connection Oriented Protocol (SSCOP) that has already been implemented in ATM networks.

7. 7 Proposed Protocol Basic operation USTAT operation POLL/STAT operation Timeout mechanisms POLL Timeout Data Timeout Determination of  value Buffer management Modified NAV value Frame formats

8. 8 USTAT operation A receiver can detect a frame loss by checking the sequence number in the frame header. When a receiver detect a frame loss, it sends an Unsolicited Status (USTAT) control frame to the transmitter

9. 9 Example of USTAT operation Frame loss Out-of-order frame Retransmission USTAT Sender Receiver

10. 10 POLL/STAT operation After a transmitter sends out a specified number of data frames without acknowledgement, say , a POLL frame is sent out to the receiver. The receiver will report the status of the last  frames to the transmitter using a Status (STAT) frame.

11. 11 Example of POLL/STAT operation STAT Sender Receiver POLL  = 3

12. 12 Buffer management STAT Sender Receiver POLL 3 3 4 4 5 5 Empty buffer 7 6 8 USTAT Retransmission 7 8 Remove 8 from buffer

13. 13 Analysis and Performance Advantages of the proposed protocol To reduce the acknowledgement traffic because of is the removal of ACK frame for each data frame transmissions. To eliminate of ACK timeout from 802.11 Simulation result To compared the proposed protocol with IEEE 802.11 The overhead The throughput

14. 14 To reduce the ACK traffic

15. 15 The occurrence of frame loss

16. 16 The POLL/STAT overhead with increasing a  value

17. 17 Total throughput with increase number of flows at frame loss rate = 0

18. 18 Total throughput with increase number of flows at frame loss rate = 10%

19. 19 Total throughput with increase number of flows at frame loss rate = 20%

20. 20 Conclusions The performance improvement of the proposed protocol increases with increasing bit error rate. The other issues that will be improved buffer management for more link between transmitter and receiver

21. 21 Q & A