1. Capacity Scaling with Multiple Radios and Multiple Channels in Wireless Mesh Networks Oguz GOKER

2. Introduction  Emerge of wireless technologies.  Increasing Used of Wifi points everyday

3. Problems  Small sized network  One hop system  NO Interoperability on AP s  Nothing for hidden terminal and other problems

4. Contention  Mai problems  Frequency Re use. 3 orthogonal in 802 11b 3 orthogonal in 802 11b 5 orthogonal in 802 11a 5 orthogonal in 802 11a

5. Advantages   Higher degree of spatial variability of the MAI.   Significant location-dependent node throughput.

6. Leads to   Dynamic interference management   Topology control via techniques such as node clustering and power control.

7. Mesh APs  Can increase channel Re use.  Single Radios vs Multi Radios  802.11b interconnection  802.11a client connection in multi radios

8. SINGLE RADIO MULTI-HOP MESH NETWORKS  It is under study till 1970s.  The end-to-end throughput in such single radio networks reduces with the number of hops.  Operates in Half Duplex Mode

9. Continue  End to end user throughput will decrease if the hop number increase  So throughput is inversely proportional to hop number  One hop is the most effective

10. Single Radio, Single Channel Mesh Networks  Works in 1/n   throughput of a single-channel multi-hop network of 802.11 nodes typically Works in 1/n   is coming from topology and traffic characteristics. Topologies like a single-channel n node linear chain t is based on pessimistic that all network interfere with each other   0.5 for a purely ad-hoc topology and random choice of source-destination pairs.  In larger networks it is better.

11. Single Radio, Multi-channel Mesh Networks  We can re use C different channels. So re use increases.  But works in C/n  is not possible because of half duplex   On the other hand, exploiting the multiple orthogonal channels clearly enhances aggregate 1-hop throughput vis-avis the single channel scenario but at the cost of enhancing the end-to-end delay.

12. Main Problems  Still half duplex so there is no real advantage on a network.   This necessitates channel scanning, selection and switching the radio at each node causes big delay   Few milliseconds to a few hundred microseconds

13. Single vs Multi Channel  Multi Channel give much more frequency re use (spatial re use)  But the final throughput totally depends on topology.  In a linear chain or near to that single is better because of channel change time on hardware

14. Conclusion

15. MULTI-RADIO MESH   Multiple radio nodes are effectively full duplex   We can allocate radios to different channel.

16. Example  R/4 in example a.  R in example b

17. Importance of network design  We have to select true type of nodes. How many channel and how many radios per node is very important.

18. Mistakes

19. Radio/Channel assignment and routing  Classical wired network solutions are not efficient for mesh networks.  Wired Networks use  Wired Networks use both proactive (tabledriven) and reactive (on- demand)   Increased size of overhead makes classical methods not possible on mesh networks.

20. Cont..   Since wireless is essentially a broadcast medium, any transmission between two neighboring nodes impacts (in principle) transmissions anywhere else in the network.   Classical Shortest path algorithms may not be effective.

21. Why no shortest path   Such algorithms simply select the path with the fewest hops without regard to the available link bandwidth, which can vary significantly depending on the interference environment at the local receiver.   It is simple to construct examples where paths with larger number of hops can provide shorter end-to-end delay, depending on the residual and bandwidths available on the links in the respective routes.

22. What is Important  In wireless network for routing important thing is channel diversity.  In another mean re use and empty of channels

23. Cont…   In other words, longer paths (measured in number of hops) that reuse the available channels for better co-channel interference management may provide improved end-to-end throughput/delay than shorter paths which use a fewer number of hops.  So system must be channel aware

24. Cont   Unlike a wired network where each hop is assumed to be isolated from simultaneous transmissions on other links, the interaction between the link and upper layers is a vital and unavoidable element in wireless multi-hop routing.   So this creates a problem.

25. NP Hard Problem  So transmission have to find a way that’s channel are not too busy, low delay, and if possible lowest number of hops. For finding this we must aware of all nodes and channels position. As system is mesh for optimal this will create a NP hard problem

26. Some solution   Raniwala Central system that’s load aware of network updates it self with the move. Synchronization needed.   Kyasanur ynchronization and channel change needed. Nodes can decrease traffic on channels depending on coming traffic. Can change interface. Work done with neighbours so no synchronization and channel change needed.

27. Channel-Aware Path Metrics for Routing   A good channel-aware path metric should incorporate notions of   (i) total link cost (e.g., sum of the trannsmission delays along the links in a path)   (ii) path channel diversity

28. Weighted Cumulative Expected Transmission Time  More common use

29. Problems  WCETT doesnot consider spatial re use, so in a big networks spatial re use can be high and there can be some channels that congestion is low. So WCETT may miss it. It is better on small networks

30. Conclusion  Right now we are using single radios network, with the increase usage of 802 technologies we must start use multi radios systems. But there is still too much need for research on multi radio system considering single channels systems are in research since 1970s.