Multi-Channel Wireless Networks: Capacity and Protocols

1. Multi-Channel Wireless Networks: Capacity and Protocols Pradeep Kyasanur and Nitin H. Vaidya University of Illinois at Urbana-Champaign

2. D Access Point C Wireless channel A B Infrastructure-based Network Multi-hop Network Wireless networks • We consider multi-hop networks • Ad hoc networks, mesh networks, sensor networks

3. Key limitation • Wireless channel is a shared resource • Simultaneous transmissions limited by interference • Throughput reduces with multiple hops • Higher density reduces per-node throughput • Throughput reduces as number of flows increase • New applications require higher throughput • Streaming video, games Improving network capacity is important

4. Multiple channels • Typically, available frequency spectrum is split into multiple channels • Large number of channels may be available • Using all the available channels is beneficial 3 channels 8 channels 4 channels 26 MHz 100 MHz 200 MHz 150 MHz 915 MHz 2.45 GHz 5.25 GHz 5.8 GHz 250 MHz 500 MHz 1000 MHz 24.125 GHz 61.25 GHz 122.5 GHz

5. 1 1 1 B C A B C A 1 2 1 1 D D Multiple channels not used Network is poorly connected Current state of art • Typical multi-hop networks use one channel only • Key challenge: Connectivity vs using multiple channels

6. Multiple interfaces • Nodes may be equipped with multiple interfaces • Common case may be small number of interfaces • Wireless radio interfaces typically support one channel at a time • We assume a half-duplex transreceiver • Interface can switch to any channel Number of interfaces per node expected to be smaller than number of channels

7. Example configuration • IEEE 802.11 has multiple channels • 12 in IEEE 802.11a • Devices can be equipped with multiple interfaces • E.g., one interface per PCMCIA/ mini-PCI slot • Typically, fewer interfaces than channels • 2 interfaces, 12 channels

8. Focus of research • Establish capacity of multi-channel networks • How does capacity vary with channels? • What are the insights from theoretical study? • Design, implement and evaluate protocols • Can we use existing protocols? • Develop suitable protocols optimized for multi-channel networks • How to implement protocols in real systems?

9. Organization • Capacity analysis • Theory to protocols: Overview of challenges • Protocols • Interface Management Protocol • Routing Protocol • Implementation Issues • Summary and Future Work

10. Capacity problem • Per-node capacity decreases as network density increases • Use more channels when network density increases • Challenge: Harder to scale interfaces at the same rate as channels How does the network capacity scale with large number of channels, and fewer interfaces than channels?

11. Related work • [Gupta&Kumar] have studied the capacity of single channel networks • Result applicable for multi-channel networks when number of channels = number of interfaces per node • [Gamal et al.] have studied the throughput-delay tradeoff • Some of our constructions are based on their work • Lot of work on studying capacity in other contexts • Mobility, infrastructure-support, delay constraints, etc.

12. Model • n nodes in the network, all located on a unit torus • c channels are available • m interfaces per node • Interface operates on one channel at a time • Channel model 1: Total bandwidth W, each channel has bandwidth W/c • Channel model 2:Total bandwidth Wc, each channel has bandwidth W

13. Network scenarios [Gupta&Kumar] • Arbitrary network • Nodes can be located anywhere on the torus • Traffic patterns can be arbitrarily chosen • Measure of capacity – aggregate network transport capacity (bit-meters/sec) • Random network • Nodes are randomly placed on the torus • Each node sets up a flow to a random destination • Measure of capacity – minimum of flow throughputs (bits/sec)

14. Results • Established tight bounds • Upper bounds and constructive lower bounds have same order • Capacity depends on ratio of c to m • Derived insights from constructions • Capacity-optimal routing and scheduling strategies

15. Capacity constrained by interference Arbitrary network – Region 1

16. Capacity constrained by interfaces Arbitrary network – Region 2

17. Capacity constrained by connectivity + interference Random network – Region 1

18. Capacity constrained by interference (arbitrary n/w) Random network – Region 2

19. Capacity constrained by bottleneck destination Random network – Region 3

20. Practical implications • When m < c, it is better to use c channels • If only m channels are used, larger capacity loss • Single interface per node often suffices • Up to log(n) channels, 1 interface is sufficient • Switching delay may not affect capacity • Extra hardware has to be provided

21. Insights for protocol development • Multiple interfaces can simplify protocol design • Use one interface for receiving data on a fixed channel • Use second interface for sending data • Routing protocol has to distribute routes • Important for multi-channel networks • Optimal transmission range depends on density of nodes as well as number of channels • Optimum: # of interfering nodes = # of channels

22. Open issues • Impact of switching delay has to be better studied • Is switching required at all? • Capacity under other switching constraints – switch among only a subset of channels • Analyze capacity of deterministic networks • Given a topology, what is the capacity? • What protocols should be used to achieve this capacity?

23. Organization • Capacity analysis • Theory to protocols: Overview of challenges • Protocols • Interface Management Protocol • Routing Protocol • Implementation Issues • Summary and Future Work

24. 3 channels 8 channels 4 channels 26 MHz 100 MHz 200 MHz 150 MHz 915 MHz 2.45 GHz 5.25 GHz 5.8 GHz Assumptions • Homogeneous channels: Channels with similar ranges and rates • Possibly channels in same frequency band • Alternatively, use appropriate power control

25. 1 2 B A C Design choice: Multiple interfaces • Theory indicates single interface may suffice • But, multiple interfaces can hide switching delay • Multiple interfaces simplify protocols • Our proposal, described later, is simple to implement • Multiple interfaces can allow full-duplex transfer • Useful when multiple channels are available

26. Design choice: Protocol separation • Separate protocol design into two components • Interface management • Routing • Interface management – shorter timescales • Map interfaces to channels • Schedule and control interface switching • Routing – longer timescales • Select “channel diverse” routes

27. Routing and Interface assignment User Space Kernel Space IP Stack Interface Switching and Buffering Interface Interface Protocol separation overview

28. 1,2 2 3,4 B A D C Link layer requirements • Utilize all the available channels • Even if number of interfaces < number of channels • E.g.: Interfaces can be switched to different channels • Ensure connectivity is not affected • B should be able to communicate with A and D • Need to be cognizant of switching delay

29. Link layer requirements • Solution should be simple to implement • Avoid the need for complicated co-ordination, tight time synchronization • Allow implementation with existing hardware • Avoid requiring hardware changes • Avoid assuming specific hardware capabilities

30. 1 2 3 B A D C 1 2 B A C 3 4 D E F Routing requirements • Improve single flow throughput by using multiple channels • Both interfaces can be utilized at the relay nodes • Improve network throughput by distributing flows

31. Organization • Capacity analysis • Theory to protocols: Overview of challenges • Protocols • Interface Management Protocol • Routing Protocol • Implementation Issues • Summary and Future Work

32. Key components • Interface assignment strategy • How to map interfaces to channels? • How to ensure neighboring nodes can communicate with each other? • Interface management protocol • Control when interfaces are switched, based on assignment strategy • Buffer packets if interface is busy

33. Interface assignment strategies • Static Interface Assignment • Interface to channel assignment is fixed • Dynamic Interface Assignment • Interface assignment changes with time • Hybrid Interface Assignment • Some interfaces use static assignment, others use dynamic assignment

34. 1,2 1,2 1,2 Not all channels used B A D C Common channel approach (e.g., [Draves2004Mobicom]) 1,2 2 3 B A D C May lead to longer routes 3 3,4 E Not possible Varying channel approach (e.g., [Raniwala2005Infocom]) Static interface assignment • Each interface is fixed to one channel • Does not require frequent co-ordination

35. 1 - 4 1 - 4 1 - 4 B A D C Transmissions can dynamically occur on any channel Co-ordination may be needed for each transmission 1 2 B A D D is unaware of B’s communication Dynamic interface assignment • Interfaces can switch channels as needed • E.g., [So2004Mobihoc, Bahl2004Mobicom]

36. [Nasipuri1999Wcnc] [Jain2001Ic3n] 1 1 1 B A D C 2-4 2-4 2-4 Channel for data transmission negotiated on control channel Hybrid strategies • One common channel used as “control” channel • One interface always fixed to this channel • Remaining channels used as “data” channels • Second interface switches among data channels Common control channel becomes a bottleneck

37. Proposed hybrid assignment • One interface “fixed” on a channel • Different nodes use different fixed channels • Other interfaces “switch” as needed • Dynamic assignment • Fixed interface receives data on well-known channel • Avoids co-ordination issues, deafness problems • Switchable interfaces send on recipient's fixed channel • Retain flexibility of dynamic assignment

38. A B C Fixed (ch 1) Fixed (ch 2) Fixed (ch 3) Switchable Switchable Switchable 2 1 3 2 Switchable interface of B switches to channel 3 when sending to node C, and to channel 1 when sending to node A Hybrid assignment example Any node pairs within transmission range can communicate

39. Identifying fixed channel • Static Approach: Fixed channel as a function of node-identifier • Simple to build, but may not balance assignment • Dynamic approach: Choose fixed channel based on neighborhood information • A node chooses least used channel for fixed channel • Can balance load, and still inexpensive

40. Interface management • Each channel is associated with a queue • Broadcast packets are inserted in to every queue • Fixed interface services fixed channel queue • Switchable interface services other channels • Channels serviced in round-robin fashion • Each channel is serviced for at most MaxSwitchTime

41. UDP throughput – chain topology

42. FTP throughput – chain topology

43. Open issues • Broadcast cost increases linearly with channels • Consider partial broadcasts • Use a separate broadcast channel, with third interface • Fixed channel selection is topology-based • Consider load, channel quality information • Integrate with a routing solution

44. Organization • Capacity analysis • Theory to protocols: Overview of challenges • Protocols • Interface Management Protocol • Routing Protocol • Heterogeneous channels • Summary and Future Work

45. Routing approach • Existing routing protocols can be operated over interface management protocol • May not select channel diverse routes • Does not consider cost of switching interfaces • Our solution • Develop a new channel-aware metric • Incorporate metric in an on-demand source-routed protocol

46. B 1 1 Route A-C-D is better A D 2 C 1 When possible, select routes where different hops are on different channels Selecting channel diverse routes • Most routing protocols use shortest-hop metric • Not sufficient with multi-channel networks • Need to exploit channel diversity

47. B 2 1 Route A-B-D is better D A 1 2 C When possible, select routes that do not require frequent switching 3 E Impact of switching cost • Interface switching cost has to be considered • Switching interfaces incurs a delay • A node may be on different routes, requiring switching

48. B 2 1 D A 1 2 C 3 E Designing a routing metric • Measure switching cost for a channel • Measure total link cost of a hop • Combine individual link costs into path cost

49. D 3 0.1 1 C E 0.9 Measuring switching cost • Switching cost depends on the likelihood a switch is necessary before transmission • Fixed channel has cost 0 • “Active” channel has low switching cost • Switching cost (SC) directly proportional to time spent on other channels

50. Routing protocol • Incorporate metric in on-demand source-routed protocol (similar to DSR) • RREQ messages modified to include link costs • Source initiates RREQ • Intermediate nodes forward RREQ if, • New RREQ • Cost of RREQ smaller than previously seen RREQ • Destination can compute best path • Using link cost information in sent RREQ