Crowded Spectrum in
Wireless Sensor Networks
Gang Zhou, John A. Stankovic, Sang H. Son
Department of Computer Science
University of Virginia
May, 2006
1
Spectrum Crisis – Single Network
Find objects;
The position is (x,y);
Persons with guns;
What are they talking about? (Audio)
What are they doing? (Video)
Need Higher Throughput!
AA…
University of Virginia 2/16
Spectrum Crisis – Co-existing
Co-existing WSNs & Electric
Appliances
Security WSN
Need Frequency Management
Other Devices
Health Care WSN
University of Virginia 3/16
Outline


The Spectrum Crisis
Initial solutions in three dimensions





Single Network Throughput
Cooperative Networks
Non-cooperative Networks and Electric Appliances
Open Challenges
Summary
University of Virginia 4/16
Single Network Throughput

Limited single-channel bandwidth in WSN


19.2kbps in MICA2, 250kbps in MICAz/Telos
The bandwidth requirement is increasing

Support audio/video streams (assisted living, …)
Multi-channel
design needed
Hardware appearing
Multi-channel support in
MICAz/Telos
 More frequencies available
in the future

Software still lags behind
Collision-based: B-MAC
 Scheduling-based: TRAMA
 Hybrid: Z-MAC

University of Virginia 5/16
State of the Art: Multi-Channel MAC in MANET
① Require more powerful hardware/multiple transceivers

Listen to multiple channels simultaneously

[Nasipuri 1999], [Wu 2000], [Nasipuri 2000], [Caccaco 2002]
② Frequent Use of RTS/CTS Controls


For frequency negotiation
Due to using 802.11
Examples: [Jain 2001], [Tzamaloukas 2001], [Fitzek 2003], [Li 2003],
[Bahl 2004], [So 2004], [Adya 2004], [Raniwala 2005]
University of Virginia 6/16
Basic Problems for WSN

Don’t use multiple transceivers



Packet Size


Cost
Form factor
30 bytes versus 512 bytes (or larger) in MANET
RTS/CTS

Costly overhead
University of Virginia 7/16
RTS/CTS Overhead Analysis [Zhou INFOCOM’06]

RTS/CTS Controls are too heavyweight for WSN:



Mainly due to small packet size: 30~50 bytes in WSN vs.
512+ bytes in MANET
From 802.11: RTS-CTS-DATA-ACK
From frequency negotiation: case study with MMAC
MMAC:


RTS/CTS frequency
negotiation
802.11 for data
communication
University of Virginia 8/16
Design Consideration - Frequency Assignment
F8
F7
F6
F5
F1
Reception Frequency
Complications
• Not enough frequencies
• Broadcast
F4
F2
F3
University of Virginia 9/16
Design Consideration - Media Access
F8
F7
F6
Issues:
• Packet to Broadcast
• Receive Broadcast
• Send Unicast
• Receive Unicast
• No sending/no receiving
F5
F1
F4
F2
F3
See [Zhou,INFOCOM’06] for our solution
University of Virginia 10/16
Co-existing & Cooperative Networks

The Challenges:

QoS Control



Space-Dimension Flexibility


Different priorities for different networks, different
bandwidths
Map to frequency decision
Frequency decision depends on node density & network
density
Time-Dimension Flexibility

Dynamic frequency adjustment
See [Zhou, EmNets’06] for our solution
University of Virginia 11/16
Non-cooperative Networks and Devices


IEEE Standards in 2.4GHz ISM Band
802.11 (1997)
802.11 b
78 channels
(1 MHz Distance)
14 channels
(5 MHz Distance)
802.15.1 (Bluetooth)
802.15.4
79 channels
(1 MHz Distance)
16 channels
(5 MHz Distance)
2.4 GHz Electronic Devices
& Electric Appliances
University of Virginia 12/16
Measurement with Spectrum Analyzer


When MICAz operates on 2.45 GHz, 46%~81% PRR
When MICAz operates on 2.42 GHz, PRR not impacted by presenter
University of Virginia 13/16
Deal With the Crowded Spectrum

New challenges:

Interference from a different radio


Interference from electric appliances


Measurement & metrics
Measurement & metrics
Incorporate these into:



Static frequency assignment
Dynamic frequency adjustment
Media access
University of Virginia 14/16
More Open Challenges






What is/are the best place/places to provide
spectrum management in WSN communication
stack?
More unlicensed frequencies from the FCC?
Tradeoff between #channels and bandwidth:
static/dynamic?
More sophisticated radio hardware?
Take advantage of partially-overlapping channels?
A service between MAC and PHY, supporting
existing single-channel minded MACs?
University of Virginia 15/16
Summary


Present a vision of crowded WSNs & the spectrum
crisis
Initial efforts in three complementary dimensions



Single WSN
Cooperative WSNs
Non-cooperative WSNs
University of Virginia 16/16
Backup Slides
University of Virginia 17/16
Frequency Assignment
When
#frequencies >= #nodes within two hops
Exclusive Frequency
Assignment



When
#frequencies < #nodes within two hops
Implicit-Consensus
Both guarantee that nodes within two
hops get different frequencies
The left scheme needs smaller
#frequencies
The right one has less communication
overhead
Even Selection



Eavesdropping
Balance available frequencies within
two hops
The left scheme has fewer potential
conflicts
The right one has less communication
overhead
University of Virginia 18/16
Media Access Design



Different frequencies for unicast reception
The same frequency for broadcast reception
Time is divided into slots, each of which consists of a
broadcast contention period and a transmission period.
Ttran
… ...
Ttran
University of Virginia
19
Media Access Design
Case 1: When a node has no packet to transmit
Tbc
(a) Snoop (f0)
(b)
Snoop (f0)
(c)
Snoop (f0)
Ttran
Signal(f0)
Receive BC (f0)
Snoop (fself)
Signal(fself)
Receive UNI (fself)
Snoop (fself)
University of Virginia
20
Media Access Design
Case 2: When a node has a broadcast packet to transmit
Tbc
Ttran
(a) Back off (f0)
Signal(f0)
(b) Back off (f0)
Send broadcast packet (f0)
Receive BC (f0)
University of Virginia
21
Media Access Design
Case 3: When a node has a unicast packet to transmit
Tbc
(a) Snoop (f0)
Ttran
Signal(f0)
Receive BC (f0)
(b)
Snoop (f0)
Back off (fself,fdest)
(c)
Snoop (f0)
Back off (fself,fdest)
(d)
Snoop (f0)
Back off (fself,fdest)
(e)
Snoop (f0)
Back off (fself,fdest)
Signal(fself)
Signal(fdest)
Signal(fdest)
Receive UNI (fself)
Snoop(fself)
Signal(fself)
Receive UNI (fself)
Snoop(fself)
Toggle send unicast packet(fdest)
University of Virginia
22
Toggle Snooping

During “back off (fself,fdest) “, toggle snooping is used
TTS
fself
fself
fdest
fself
fdest
fself
fself
fdest
fdest
fdest
University of Virginia
23
Toggle Transmission
 When a node has unicast packet to send
 Transmits a preamble
 f self so that no node sends to me
 f dest so that no node compete for the same channel
TTT
…….
Preamble
PHY Protocol Data Unit
Use fdest
Use
fself
 We let TTS=2TTT
University of Virginia
24
Co-existing & Cooperative Networks

The Challenges:

QoS Control


 i
K
1
percent of available frequency spectrum
Space-Dimension Flexibility


Enforce neti gets  i 
i
Node density & network density
Time-Dimension Flexibility

More dynamics
University of Virginia
25
Co-existing & Cooperative Networks

The Solutions:

Static frequency assignment



Collect (ID, gID,  ) from (two-hop) neighbors
Chained frequency decision: (increasing gID & ID)

The candidate frequency range

 i  N where  i  K i
The range [ Sfrei , Efrei ]
1 i

Randomly choose one of the least chosen frequencies from
the range
Dynamic frequency adjustment


Reassign nodes from crowded frequencies to light ones
Avoid pushing around “hot potatoes”
University of Virginia
26