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

Multi-Channel Wireless Networks: Capacity and Protocols

Pradeep Kyasanur and Nitin H. Vaidya

University of Illinois at Urbana-Champaign

2

Wireless networks

Wireless

channel

Wireless

channel

Access Point

A B

Infrastructure-based Network

C D

Multi-hop Network

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

26 MHz 100 MHz 200 MHz 150 MHz

2.45 GHz 915 MHz 5.25 GHz 5.8 GHz

3 channels 8 channels 4 channels

250 MHz 500 MHz 1000 MHz

61.25 GHz24.125 GHz 122.5 GHz

5

Current state of art

Typical multi-hop networks use one channel only Key challenge: Connectivity vs using multiple channels

Multiple channels not used Network is poorly connected

A1 1

1 11

B C

D

A1

2

B C

D

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

Arbitrary network – Region 1

Capacity constrained by interference

16

Arbitrary network – Region 2

Capacity constrained by interfaces

17

Random network – Region 1

Capacity constrained by connectivity + interference

18


Capacity constrained by interference (arbitrary n/w)

19


Capacity constrained by bottleneck destination

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




24

Assumptions

Homogeneous channels: Channels with similar ranges and rates Possibly channels in same frequency band Alternatively, use appropriate power control

26 MHz 100 MHz 200 MHz 150 MHz

2.45 GHz 915 MHz 5.25 GHz 5.8 GHz

3 channels 8 channels 4 channels

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

A1

B C2

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

Routing andInterface assignment

Interface Switching and Buffering

IP Stack

Interface Interface

User Space

Kernel Space

28

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

A1,2 3,4B CD

2

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

A1 3B CD

2

A1

4

B C

E

2

D F3

31

Organization




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

A1,2

B CD1,2 1,2 Not all channels

usedCommon channel approach (e.g., [Draves2004Mobicom])

Varying channel approach (e.g., [Raniwala2005Infocom])

A1,2

B CD2 3 May lead to longer

routes

E

3

Not possible3,4

35

Dynamic interface assignment

Interfaces can switch channels as needed E.g., [So2004Mobihoc, Bahl2004Mobicom]

A1

B D2

D is unaware of B’s communication

Co-ordination may be needed for each transmission

Transmissions can dynamically occur on any channel

A1 - 4

B CD1 - 4 1 - 4

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

Channel for data transmission negotiated on control channel

A1

B CD1 1

2-4 2-4 2-4

[Nasipuri1999Wcnc]

[Jain2001Ic3n]

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

A

Fixed (ch 1)

Switchable

B

Fixed (ch 2)

Switchable

C

Fixed (ch 3)

Switchable12 3 2

Switchable interface of B switches to channel 3 when sending to node C, and to channel 1 when sending to node A

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

0

5

10

15

20

25

30

35

1 2 3 4 5 6 7 8 9 10

Th

rou

gh

pu

t (M

bp

s)

Chain length (in hops)

DSR - 1MCR - 2MCR - 5

MCR - 12

42

FTP throughput – chain topology

0

5

10

15

20

25

30

1 2 3 4 5 6 7 8 9 10

Th

rou

gh

pu

t (M

bp

s)

Chain length (in hops)


MCR - 12

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



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

A

B

C

D

1 1

2 1

Route A-C-D is better

When possible, select routes where different hops are on different channels

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

A

B

C

D

2 1

2 1

Route A-B-D is better

E

3 When possible, select routes that do not require frequent 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

A

B

C

D

2 1

2 1

E

3

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

C

D

1E

3

0.9

0.1

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

0

2

4

6

8

10

12

14

1 2 3 4 5 6 7 8 9 10

No

rma

lize

d T

hro

ug

hp

ut

Topology Number


MCR - 12

52

Throughput with varying load

0

5

10

15

20

2 4 6 8 10 12 14

No

rma

lize

d T

hro

ug

hp

ut

Number of Flows


MCR - 12

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




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

Channel Policy Manager

Interface and Channel Abstraction Layer

IP Stack

Interface Interface

ARP

User Applications Abstraction layer exports single virtual interface

Channel switching details are hidden

Fixed channel selection, and routing protocol is implemented as part of channel policy managerJoint work with Chandrakanth Chereddi

58

Organization



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 [Gupta&Kumar]: 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

c sender nodes

c receiver 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

Balance route assignment

within each cell

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

capacity is When

capacity is When

72

Random networks

Three capacity regions

capacity is1)

2)

capacity is

3) capacity is

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

Documents

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