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MultiChannel Wireless Networks: Capacity and Protocols

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... in the network, all located on a unit torus. c channels are available ... Nodes can be located anywhere on the torus. Traffic patterns can be arbitrarily chosen ... – PowerPoint PPT presentation

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Title: MultiChannel 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
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
2.45 GHz
915 MHz
5.25 GHz
5.8 GHz
250 MHz
500 MHz
1000 MHz
61.25 GHz
24.125 GHz
122.5 GHz
5
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
  • GuptaKumar 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 GuptaKumar
  • 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
Arbitrary network Region 1
16
Arbitrary network Region 2
17
Random network Region 1
18
Random network Region 2
19
Random network Region 3
20
Practical implications
  • When m lt 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
Assumptions
  • Homogeneous channels Channels with similar
    ranges and rates
  • Possibly channels in same frequency band
  • Alternatively, use appropriate power control

25
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
Protocol separation overview
28
Link layer requirements
  • Utilize all the available channels
  • Even if number of interfaces lt 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
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
Static interface assignment
  • Each interface is fixed to one channel
  • Does not require frequent co-ordination

35
Dynamic interface assignment
  • Interfaces can switch channels as needed
  • E.g., So2004Mobihoc, Bahl2004Mobicom

36
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
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
Selecting channel diverse routes
  • Most routing protocols use shortest-hop metric
  • Not sufficient with multi-channel networks
  • Need to exploit channel diversity

47
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
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
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

51
Throughput in random networks
52
Throughput with varying load
53
Open issues
  • Incorporate load information into MCR metric
  • Support for route caching
  • Metric does not allow route combination
  • Design alternate metrics?
  • Integrated routing and fixed channel selection
  • Can improve performance at cost of increased
    complexity

54
Organization
  • Capacity analysis
  • Theory to protocols Overview of challenges
  • Protocols
  • Interface Management Protocol
  • Routing Protocol
  • Implementation Issues
  • Summary and Future Work

55
Lack of multi-channel support
  • Existing assumptions break with multiple channels
  • Assume of channels of interfaces
  • Routing table has interface information only
  • Not easy to use multiple interfaces
  • Switching channels requires explicit invocation
  • Interfaces and channels not hidden from
    applications
  • Frequent switching not permitted

56
Requirements
  • Hide interface management from data path
  • Allow existing applications to work unmodified
  • Break node-channel mapping
  • Allow channel to be selected based on destination
  • Support multi-channel / single channel broadcast
  • Broadcast primitive required for many applications

57
Proposed architecture
  • Abstraction layer exports single virtual
    interface
  • Channel switching details are hidden
  • Fixed channel selection, and routing protocol is
    implemented as part of channel policy manager

Joint work with Chandrakanth Chereddi
58
Organization
  • Capacity analysis
  • Theory to protocols Overview of challenges
  • Protocols
  • Interface Management Protocol
  • Routing Protocol
  • Heterogeneous channels
  • Implementation Issues
  • Summary and Future Work

59
Summary
  • Goal of the project is to utilize multiple
    channels
  • Research issues considered are
  • Analysis of capacity of multi-channel networks
  • Design of protocols for multi-channel networks
  • Implementing protocol suite in testbed

60
Future work
  • Capacity analysis with switching delay
  • What if there is no switching allowed at all?
  • Flow-aware protocol design
  • Assign channels based on channel quality and load
  • Select routes based on existing routes
  • Implementation and measurement
  • Fully implement all protocols
  • Measure characteristics of multiple channels

61
Questions?
  • More details at
  • http//www.crhc.uiuc.edu/wireless

62
Backup Slides
63
Arbitrary Network Upper bound
  • Interference constraints GuptaKumar Each pair
    of simultaneous receivers must have minimum
    separation
  • Separation depends on transmission radius
  • Bounds the number of simultaneous transmissions
  • Interface constraint Only m interfaces available
  • Each node can send/receive at most m bits/sec

64
Arbitrary Network Lower bound
  • Divide torus in to square cells
  • Each cell has nodes

65
Random Networks Upper bound
  • Arbitrary network constraints Random network is
    a special case of an arbitrary network
  • Connectivity constraint A minimum transmission
    range is needed to ensure network is connected
  • Destination bottleneck constraint The maximum
    number of incoming flows at any node will limit
    per-flow throughput

66
Lower bound Routing
  • Divide torus in to square cells of area a(n)
  • a(n) depends on the number of channels
  • Route through cells on the straight line joining
    source and destination

67
Lower bound Step 1 schedule
  • Divide every second in to hop-color slots
  • Flow scheduling For each hop of a flow, schedule
    its transmission in some hop-color slot
  • Procedure
  • Build a routing graph
  • Vertices are nodes in the network
  • One edge for every hop
  • Edge color the graph
  • Number of colors used number of hop-color slots
  • Map each color to a hop-color slot
  • Every hop is scheduled in slot associated with
    its color

68
Lower bound Step 2 schedule
  • Divide each hop-color slot in to node slots
  • Node scheduling Each node can only transmit in
    its node slot
  • Procedure
  • Build an interference graph
  • Vertices are nodes in the network
  • One edge for every pair of nodes that may
    interfere
  • Vertex color the graph
  • of colors of node slots per hop-color slot
  • Map each color to a slot
  • Each node transmits only in slot associated with
    its color

69
Switching Delay
  • Initial analysis ignores interface switching
    delay
  • Upper bounds do not mandate switching
  • Open question Is interface switching required at
    all
  • Possible that switching delay does not affect
    capacity
  • Lower bound constructions affected by delay
  • Capacity affected only if there are latency
    constraints
  • Even with latency constraints, multiple
    interfaces can hide delay

70
Benefits of Proposed Strategy
  • Frequent co-ordination not required
  • Fixed channel information infrequently exchanged
  • Maintains full-connectivity
  • Any node pairs within transmission range can
    communicate
  • No changes required to MAC protocol
  • Can be built with existing IEEE 802.11 hardware

71
Arbitrary networks
  • Two capacity regions

72
Random networks
  • Three capacity regions

73
One approach
  • Based on ETT measurement Draves2004Mobicom
  • ETT(j) Expected Transmission Time of packet
  • LinkLossRate measurement modified
  • LinkRate measured from probing driver

74
One path metric (MCR)
  • Based on WCETT Draves2004Mobicom
  • Path cost limited by bottleneck channel cost ( Xj
    )
  • Network throughput depends on aggregate cost

75
CBR throughput
76
CBR throughput
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