1. Improving the scalability of MAC protocols in Wireless Mesh Networks Mthulisi Velempini (Mr.)

2. UCT-COE Seminar 000 Page 2 January 31, 2016 Introduction  WMN are composed of a wired backbone  A Wireless Back haul of static routers  Mobile mesh clients  Broadband wireless communication system  It is scalable and easy to deploy  Offers a cost effective last mile solution

3. UCT-COE Seminar 000 Page 3 January 31, 2016 Mesh Architecture Backbone Back Haul Mobile Mesh Clients

4. UCT-COE Seminar 000 Page 4 January 31, 2016 Research Question and Arguments 1.Is CSMA/CA suitable for WMN? 2.CSMA/CA can not be scaled up to meet new challenges in multi channel multi hop WMN 3.CSMA/CA is not suitable for WMN

5. UCT-COE Seminar 000 Page 5 January 31, 2016 IEEE 802.11 MAC  Consists of two schemes: PCF and DCF  PCF is centralized, real time data scheme  DCF is a CSMA/CA based decentralized scheme designed for non real time data  PCF needs the services of a PC  There is no PC in Ad Hoc networks  PCF is not functional in WMN  Requirements of multimedia data are not guaranteed

6. UCT-COE Seminar 000 Page 6 January 31, 2016 IEEE 802.11 MAC DCF  It is a primary medium access protocol  It is contention based  It uses CSMA/CA for carrier sensing  CSMA/CD can not be implemented due to transmission/reception constraint  CSMA/CA suffers from packet collisions

7. UCT-COE Seminar 000 Page 7 January 31, 2016 CSMA/CA  Consists of three processes:  Virtual Sensing – NAV  Random back off interval ( )  Contention period  Collision result in retransmission of packets  Back off interval is doubled  Delayed ACK is associated with congestion  Degrades significantly as network grows 2 n – 1

8. UCT-COE Seminar 000 Page 8 January 31, 2016 Continuation CCA procedure Contention Window Reset parameters CW = 0? DATA RTS

9. UCT-COE Seminar 000 Page 9 January 31, 2016 Multi hop Communication  IEEE 802.11 performs badly in Multi hop  It is a single hop protocol  Fails to deal with Hidden and exposed nodes  Relay packets degrade throughput  Listen before you talk results in delays  Collisions lead to Hold off and retransmissions  Lack of coordination affects throughput  HOL problem degrades delay bounded data

10. UCT-COE Seminar 000 Page 10 January 31, 2016 Hidden and Exposed Nodes  Y is exposed to WX flow, this problem affects spatial reuse  Z is hidden to XW flow and will interfere with data reception at W.. X W Y Z

11. UCT-COE Seminar 000 Page 11 January 31, 2016 Asymmetric Views  Nodes have different views of the channel  Lead to unfairness and starvation of disadvantaged flows  The asymmetric problems are IA and FIM  In IA, an advantaged flow knows when to contend  In FIM, a middle node continues to sense outer nodes and to hold off  A node can suffer combinational effects of FIM and IA

12. UCT-COE Seminar 000 Page 12 January 31, 2016 Continuation ZY WX W X A Y Z B

13. UCT-COE Seminar 000 Page 13 January 31, 2016 Multi Channel  IEEE 802.11 MAC is single channel  It is not suitable for multi channel  Transmission/reception constraint  Hidden and exposed nodes  Missing receiver problem  Virtual sensing has limitations  RTS/CTS fail to notify neighbours  Lack of coordination is a major problem

14. UCT-COE Seminar 000 Page 14 January 31, 2016 Deafness Problem  Relates to channel assignment  There is a need to trade off deafness and Interference  Common channel is required for connectivity  Channel sharing leads to interference  Minimal connectivity may be a solution X Y Z Channel 1 Channel 2

15. UCT-COE Seminar 000 Page 15 January 31, 2016 Related Work  MAC can be optimized for real time data  IEEE 802.11 MAC is scalable  It has to be modified to ensure compatibility  Use of multi channels should be coordinated  Per flow models to be designed  Back offs, hidden and exposed nodes to be addressed  MAC should guarantee throughput, capacity, latency, reach and QoS requirements

16. UCT-COE Seminar 000 Page 16 January 31, 2016 Summary  MAC promotes unfairness and starvation  Deferred transmissions cause starvation  Radios have transmit/reception constraints  Distributed schemes are preferred  Designs to optimize channel utilization  To improve end to end flow fairness against interference  This leads to the scalability of WMN  Heterogeneity between MR and MC