Post on 27-Mar-2015
transcript
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Multi-Channel Wireless Networks:Capacity and Protocols
Nitin H. VaidyaUniversity of Illinois at Urbana-Champaign
Joint work with
Pradeep KyasanurChandrakanth Chereddi
Jungmin So
Computer Communications Workshop 2005© 2005
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Multi-hop Wireless Networks
Mesh networks
Ad hoc networks
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Wireless Capacity
Wireless capacity limited
In dense environments, performance suffers
How to improve performance ?
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Improving Wireless Capacity
Exploit physical resources
Exploit diversity/multiplicity of resources
Examples …
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Exploit Infrastructure
Infrastructure provides a tunnel to forward packets
EA
B CD
BS1 BS2
X
Z
infrastructure
Ad hoc connectivity
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Exploit Antennas
Steered/switched directional antennas
A
D
CB A B
D
C
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Improve Spatial ReusePower/Rate/Carrier-Sense Control
A B C D
A B C D
Transmit SpatialPower Rate reuse
High High Low
Low Low High
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Exploiting Diversity
Exploiting diversity
requires suitable protocols
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This Talk
Utilizing multiple channels in wireless networks
Capacity bounds
Insights on protocol design
Implementation issues
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Multiple Channels
Available frequency spectrum often split into channels
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
802.11 in ISM Band
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Multiple Channels
Common practice in multi-hop networks:Tune all interfaces to the same channel
Channel 1
Channel 1
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Better capacity using multiple interfaces One interface per channel
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c c
1
c
Multiple Channels
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Multiple Channels with Interface Constraint
What if interfaces (m) < channels (c) ?
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c = mm = c
1
c
1
m m
Baseline Our system
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This Talk
Utilizing multiple channels in wireless networks
Capacity bounds
Insights on protocol design
Implementation issues
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Capacity = ?[Gupta-Kumar]
Random source-destination pairs among randomly positioned n hosts in unit area, with n ∞
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Capacity = ?
= minimum flow throughput Capacity = n
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Capacity Constraints
Capacity constrained by available spectrum bandwidth
Other factors further constrainwireless network capacity …
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Connectivity Constraint[Gupta-Kumar]
Need routes between source-destination pairs Places a lower bound on transmit range
Not connected Connected
AD A
D
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Interference Constraint [Gupta-Kumar]
Interference among simultaneous transmissions Limits spatial reuse
is aGuardparameter
dAB
(1+)dDC
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Capacity of Wireless Networks[Gupta-Kumar]
Bit rate for each transmission = W
Capacity increases with n as
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Capacity of Wireless Networks[Gupta-Kumar]
Result holds when m = c
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11
cm = c
W/c
W
W/c
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Capacity of Wireless Networks
What if fewer interfaces ?
Additional constraints on capacity become relevant
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Interface Constraint
Throughput is limited by number of interfaces in a neighborhood
•Interfaces, a resource
k nodes in the “neighborhood”
total throughput ≤ k * m * W/c
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Destination Bottleneck Constraint
Per-flow throughput limited bytotal number of flows at a host
Df incomi
ng
flows
If node throughput = T
Per-flow throughput = T / f
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Random network – Region 1
Capacity constrained by connectivity + interference
No dependence on m and
c
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Random network – Region 2
Capacity constrained by interfaces + interference
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Random network – Region 3
Capacity constrained by destination bottleneck
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This Talk
Utilizing multiple channels in wireless networks
Capacity bounds
Insights on protocol design
Implementation issues
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Insights from Capacity Analysis (1)
Static channel allocation does not yield optimal performance in general
Must dynamically switch channels
Need protocol mechanisms for channel switching
A
C
BChannel 1
2 D3
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Insights from Capacity Analysis (2)
Optimal transmission range function of
density of nodes and
number of channels
Goal: # of interfering nodes = # of channels
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Insights from Capacity Analysis (3)
Routes must be distributed within a neighborhood
This is not necessary in single channel networks
A
B
C
D
E
F
A
B
C
D
EF
Multi-Channel (m<c)Optimal strategy
Single Channel (m=c=1)Optimal strategy
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Insights from Capacity Analysis (4)
Channel switching delay potentially detrimental, but may be hidden with
careful scheduling, or
additional interfaces
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Example Configuration
IEEE 802.11a/b devices
2 interfaces per host
Soekris box
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This Talk
Utilizing multiple channels in wireless networks
Capacity bounds
Insights on protocol design
Implementation issues
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Which Layers to beMulti-Channel Aware?
Practical decision:
Above MAC layer
Allows use of unmodified 802.11
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Separation of Responsibility
Interface management: Shorter timescales
Dynamic channel assignmentto interfaces
Interface switching
Routing: Longer timescales
Multi-channel aware route selection metrics
Link
Network
Transport
PhysicalLayer
Upper layers
802.11
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Channel Assignment
2 interfaces much better than 1 Hybrid channel assignment: Static + Dynamic
A
Fixed (ch 1)
Switchable
B
Fixed (ch 2)
Switchable
C
Fixed (ch 3)
Switchable12 3 2
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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
Select routes with greater channel diversity
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Impact of Switching Cost
Interface switching cost has to be considered A node may be on different routes, requiring switching
A
B
C
D
2 1
2 1
Route A-B-D is better
E3
Prefer routes that do not require excessive switching
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Other Issues
Routing table entries need to store interface and channel identifiers
Packet buffering pending channel switch
Multi-channel broadcast
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Testbed Status
Interface and channel abstraction layer implemented
•Can run legacy routing protocol above this
Multi-channel routing implementation in progress
20+ node testbed to be deployed later in Fall 2005
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Conclusion
Capacity results hint at significant
performance benefits using
many channels with few interfaces
Need suitable protocols to exploit the channels
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Thanks!
www.crhc.uiuc.edu / wireless