$ network support for wireless connectivity in the tv bands victor bahl ranveer chandra thomas...
TRANSCRIPT
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Network Support for Network Support for Wireless Connectivity in Wireless Connectivity in
the TV Bandsthe TV Bands
Victor BahlRanveer Chandra
Thomas MoscibrodaSrihari Narlanka
Yunnan WuYuan Yuan
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KNOWS-PlatformKNOWS-Platform
This work is part of our KNOWS project at MSR(Cognitive Networking over White Spaces) [see DySpan 2007]
Prototype has transceiver and scanner Transceiver can dynamically adjust center-frequency
and channel-width with low time overhead (~0.1ms) Transceiver can tune to contiguous spectrum bands
only! Scanner acts as a receiver on control channel when not
scanning
Scanner Antenna
Data Transceiver Antenna
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Design a MAC protocol for cognitive radios in the TV band that leverages device capability -- dynamically adjusting central-freq and channel-width
Goals:◦ Exploit “holes” in spectrum x time x space ◦ Opportunistic and load-aware allocation
Few nodes: Give them wider bands Many nodes: Partition the spectrum into
narrower bands
Problem FormulationProblem Formulation
Frequency
5Mhz20Mhz
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Context and Related WorkContext and Related Work
Context: • Single-channel IEEE 802.11 MAC allocates only
time blocks• Multi-channel Time-spectrum blocks have
pre-defined channel-width
• Cognitive channels with variable channel-width!
tim
e
Multi-Channel MAC-Protocols:[SSCH, Mobicom 2004], [MMAC, Mobihoc
2004], [DCA I-SPAN 2000], [xRDT, SECON
2006], etc… MAC-layer protocols for Cognitive Radio Networks:
[Zhao et al, DySpan 2005], [Ma et al, DySpan 2005], etc…
Regulate communication of nodeson fixed channel widths
Existing work does not
consider channel-width
as a tunable parameter!
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KNOWS ArchitectureKNOWS Architecture
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Allocating Time-Spectrum Allocating Time-Spectrum BlocksBlocksView of a node v:
Time
Frequency
t t+¢t
f
f+¢f
Primary users
Neighboring nodes’time-spectrum blocks
Node v’s time-spectrum block
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OutlineOutline
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CMAC OverviewCMAC Overview
Use a common control channel (CCC)◦ Contend for spectrum access
◦ Reserve a time-spectrum block
◦ Exchange spectrum availability information
(use scanner to listen to CCC while transmitting)
Maintain reserved time-spectrum blocks◦ Overhear neighboring node’s control packets
◦ Generate 2D view of time-spectrum block reservations
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CMAC OverviewCMAC Overview Sender Receiver
DATA
ACK
DATA
ACK
DATA
ACK
RTS
CTS
DTS
Waiting Time
RTS◦ Indicates intention for
transmitting◦ Contains suggestions for
available time-spectrum block (b-SMART)
CTS◦ Spectrum selection (received-
based)◦ (f,¢f, t, ¢t) of selected time-
spectrum blockDTS
◦ Data Transmission reServation◦ Announces reserved time-
spectrum block to neighbors of sender
Tim
e-S
pectru
m B
lock
t
t+¢t
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Network Allocation Matrix (NAM)Network Allocation Matrix (NAM)
Control channel
Frequency
The above depicts an ideal scenario1) Primary users (fragmentation)2) In multi-hop neighbors have different views
Time-spectrum block
Nodes record info for reserved time-spectrum blocks
Time
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Network Allocation Matrix (NAM)Network Allocation Matrix (NAM)
Control channel Time
The above depicts an ideal scenario1) Primary users (fragmentation)2) In multi-hop neighbors have different views
Primary Users
Nodes record info for reserved time-spectrum blocks
Frequency
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B-SMARTB-SMART
Which time-spectrum block should be reserved…?◦ How long…? How wide…?
B-SMART (distributed spectrum allocation over white spaces)
Design Principles
1. Try to assign each flow blocks of bandwidth B/N
2. Choose optimal transmission duration ¢t
B: Total available spectrumN: Number of disjoint flows
Long blocks: Higher delay
Short blocks: More
congestion on control channel
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B-SMARTB-SMART
Upper bound Tmax~10ms on maximum block duration
Nodes always try to send for Tmax
Find placement of ¢bx¢t blockthat minimizes finishing time
and doesnot overlap with any other
block
Tmax
¢b=10MHz
Tmax
¢b=dB/Ne=20MHz
Tmax
¢b=5MHz
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Estimation of NEstimation of N
1 (N=1)
2(N=2)
3 (N=3)
1 2 3 4 5 6
5(N=5)
4 (N=4)
40MHz
80MHz
7 8
6 (N=6)
7(N=7)
8 (N=8)2 (N=8)1 (N=8)3 (N=8)
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We estimate N by #reservations in NAM based on up-to-date information
adaptive!Case study: 8 backlogged single-hop flows
3 Time
Tmax
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Simulation Results - SummarySimulation Results - Summary
Simulations in QualNetVarious traffic patterns, mobility models, topologies
B-SMART in fragmented spectrum:◦ When #flows small total throughput increases with
#flows ◦ When #flows large total throughput degrades very
slowly
B-SMART with various traffic patterns:◦ Adapts very well to high and moderate load traffic patterns◦ With a large number of very low-load flows
performance degrades ( Control channel)
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Conclusions and Future WorkConclusions and Future Work
Summary: ◦ CMAC 3 way handshake for reservation◦ NAM Local view of the spectrum availability◦ B-SMART efficient, distributed protocol for
sharing white spaces
Future Work / Open Problems◦ Control channel vulnerability◦ QoS support◦ Coexistence with other systems