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State of Multi-Hop Wireless Networking
Nitin VaidyaElectrical and Computer Engineering
University of Illinois at Urbana-Champaign
Sept. 15. 2008
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Deep Thought
So the secret to good self-esteem is
to lower your expectations
to the point where they're already met ?
Calvin (and Hobbes) :Bill Watterson
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Caveat
This talk is based on opinions
not necessarily substantiated by real data
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Multi-Hop Wireless
Many possibilities …
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Multi-Hop Wireless
Mobile ad hoc networks•Interconnect cars, planes,
tanks, soldiers, people
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Multi-Hop Wireless
Mesh networks (roof-top, pole-top)internet
Mesh
Client
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Multi-Hop Wireless
Sensor networks
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Multi-Hop Wireless
OpportunisticDelay/Disruption/Disconnection-tolerant networks
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Why Multi-Hop Wireless ?
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Why Multi-Hop Wireless ?
Lack of infrastructure
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Why Multi-Hop Wireless ?
Some clients difficult to reach directly via infrastructure, due to obstacles
AP
Relay
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Why Multi-Hop Wireless ?
Decreasing dependence on wired infrastructure
Add wireless “infrastructure” internet
Mesh
Client
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Why Multi-Hop Wireless ?
Low-power clients unable to communicate directly with infrastructure
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Why Multi-Hop Wireless ?
For improved capacity
High interference
High transmit power
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Why Multi-Hop Wireless ?
For improved capacity
Low interference
Low transmit power
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Why Multi-Hop Wireless ?
Poor connectivity
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A Selective History
1973-87 DARPA Packet Radio Networks (PRNET/SURAN)
1997 IEEE 802.111997 IETF MANET
1999 TinyOS2000 MeshNetworks founded2000+ CUWiN open-source mesh2000 ACM MobiHoc2001 “Embedded, Everywhere” Sensor networks2001 Interplanetary Internet, IETF draft, Vint Cerf
2003 ACM Sensys2004 Motorola acquires MeshNetworks2004 IEEE 802.11s study group for mesh networking2004 ZigBee
Time
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Research Activityversus
Relevance
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Research Activity
Much activity in
Mobile ad hoc networks
•No infrastructure
•Large diameter
•High mobility
Sensor networks
•Low power
•Large diameter
•Small diameter useful in practice, but not “interesting”
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Unscientific Measure of Interest:Google 9/11/08
Ad hoc networks: 2,290,000
Mesh networks: 764,000
Sensor networks: 1,670,000
Vehicular networks: 1,710,000
Delay tolerant networks: 196,000 Disruption tolerant networks: 206,000 Disconnection tolerant networks: 99,800 Opportunistic networks: 978,000
Magna Carta (1215)2,630,000
United states constitution (1787) 5,790,000
Paris Hilton 68,800,000
Computerarchitecture21,400,000
802.1166,000,000
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Research Activity
Most activity seems to be in
Mobile ad hoc networks•No infrastructure
•Large diameter
•High mobility
Sensor networks•Low power
•Large diameter
Extreme assumptions make the problem exciting But what about relevance ?
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Relevance ?
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Relevance
Not all networks are made equal …
Some are likely to be commonplace
others limited to niche scenarios
Relevance
In increasing order of relevance …
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Delay Tolerant Networks
Limited to niche scenarios
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Interesting Variation
Wireless Graffiti Microblogs “Sticky notes in-the-air”
Users leave information “in the air”at some location
Others can retrieve later from there
May be viewed as opportunistic communication(Not quite the same as DTN)
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Mobile Ad Hoc Networks
Why design networks without infrastructure ? Possible to deploy some infrastructure in
most environments
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Sensor Networks
Wireless sensors are important Important to network the sensors
Sensors + Network ≠ Large diameter
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Infrastructure Extension
Most compelling reason for multi-hop wireless
Only a small number of hops!
AP
Relay
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Infrastructure Extension
Mesh (Wireless “infrastructure”)
internet
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Summary:Most Appealing Scenario
Some wired infrastructure
Capacity scales with the infrastructure
Small diameter wireless extensionfor the infrastructure
Using relays or peer-to-peer
Better reachability Low-power operation Reduced capacity loss
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If only small diameter networks matter,did we waste our time ?
Not quite …
Interference management and MAC-related issuessomewhat independent of network diameter
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State of Multi-Hop Wireless
Very large volume of activity
Beautiful theory
Asymptotic Capacity
Throughput-optimal schedulingNetwork utility optimizationNetwork codingCooperative relaying
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State of Multi-Hop Wireless
Very large volume of activity
Practical protocols & deployments
Many wireless standardsAnd many more MAC & routing protocolsMany experimental deploymentsMesh devicesSensor devicesStart-ups
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State of Multi-Hop Wireless
Very large volume of activity
(Too) Many conferences and workshops
Plenty of research funding Compared to many other areas
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State of Multi-Hop Wireless
Despite the volume of activity
Difficult to enumerate core set of principles for wireless network design
What should we teach in anundergraduate wireless networksclass ?
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State of Multi-Hop Wireless
Despite the volume of activity
Theoretical developments haven’t been translated to practice
Much protocol design ignores physical layer issues
Much talk of cross-layer design,but progress not impressive
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What is Lacking ?
Meaningful contact between
Practice Networking
Theory Comm
Picture from Wikipedia
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Net-X
Theory to
Practice
Multi-channelprotocol
Channel Abstraction Module
IP Stack
InterfaceDevice Driver
User Applications
ARP
InterfaceDevice Driver
OS improvementsSoftware architecture
Capacitybounds
channels
capaci
ty
Net-Xtestbed
CSL
A
B
C
D
EF
Fixed
Switchable
Insights onprotocol design
Linux box
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Things I Wish I Had Learned in Kindergarten
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Those who cannot learn from historyare doomed to repeat it
With apologies to George Santayana
outgrow
1
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Relaying : Multi-hop routes (store-and-forward)
Pre-History of Wireless Communication:Smoke Signals, Fires, Semaphore
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Pre-History of Wireless Communication:Homing Pigeons
Exploiting mobility
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Reusing Ideas Reasonable,but Need to Explore Better Alternatives
No wired-equivalentfor wireless networks No links !
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Wireless Channel Offers Rich Diversity
Current protocolsexploit diversityonly to a limited extent
The vanishing link :Diversity confusesthe notion of a link
Layer 1 : 2+ gap
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Interference is Information
2
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Interference is Information
A
B
D
C
Signal
Interference
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Bits Are Not Automobiles
3
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Bits Are Not Automobiles
We treat information networks same asphysical transportation networks
•Planes, Trains and Automobiles
Bits can be combined (encoded) andseparated, unlike physical objects
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Network Coding
A CBP
P QQ
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Network Coding
A CBP Q
P +QQ+
Q P
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Physics Does Not Know Layers
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Physics Does Not Know Layers
Layering is an abstraction, not a theorem
Backpressure scheduler ( “ throughput-optimal ” ) spans traditional layers 1 through 3:
arg max ∑ W(l) r(l)r Є Rate l Region
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Physics Does Not Know Layers
Layering is useful, but need a principled approach to identifying appropriate cross-layer exchange
Great start towards this: Network utility optimization» Queue as price
Shortcomings:» Not all requirements easy to capture as concave
utility» Framework does not (yet) yield enough insight on
practical “scheduling/routing”
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Opportunism Pays
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Opportunism Pays
Channel variations make it difficult to predict short-term optimal in advance
Late binding can work better
– Opportunistic beamforming– Opportunistic routing– MAC-Layer anycasting– …
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State of Multi-Hop Wireless
Theoretical developments haven’t been translated to practice
Much protocol design ignores physical layer issues
Much talk of cross-layer design,but progress not impressive
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State of Multi-Hop Wireless
Despite the volume of activity
Theoretical developments haven’t been translated to practice
Much protocol design ignores physical layer issues
Much talk of cross-layer design,but progress not impressive
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What Now ?
Four-Point Agenda
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Reduce the unknown unknowns
Increase phy content in CS/CE networking courses
– Awareness of phy necessary to ask better questions
– Phy community should help
Educate phy students about higher layer issues
1. Educate BetterOurselves & Next Generation
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If you have influence at funding agencies …
Resist temptation to create new networking programs
•Partitioning of resources creates false demand
– Remove existing partitionsPossible to encourage research without these
– Past examples: NOSS, FIND?
2. Fewer Research Programs
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3. Fewer “Better” Conferences
Increase venues that encourage diversecommunity interactions(phy-networking , theory-applied)
•More Workshops, fewer “selective” conferences, (fewer papers!)
•Co-located conferences
•Tutorials
Eliminate most (wireless) networking conferences
•Emulate Info Theory model ?
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4. Greater Industry/User Feedback
What are the industry-perceived long-term challenges ?
What do they need from us ?
Not everything needs to be dictated by industry, but practical insights can benefit academic research
– Problem formulations constrained by reality
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Summary:Multi-Hop Wireless Networks
Enormous progress in past 15 years
But potential for much more impact
Need greater attention to cross-layer design
Improved education a prerequisite
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Advertisement
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Illinois Wireless Summer School
August 3-7, 2009
Illinois Center for Wireless Systems (ICWS)at the University of Illinois at Urbana-Champaign
Lectures ranging antennas-to-applications
Opportunities for students to interact
Sponsorships welcome !
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Thanks!
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Thanks!
