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State of Multi-Hop Wireless Networking
Nitin Vaidya
Electrical and Computer Engineering
University of Illinois at Urbana-Champaign
Sept. 15. 2008
1
Deep Thought
Calvin (and Hobbes) :
Bill Watterson
So the secret to good self-esteem is
to lower your expectations
to the point where they're already met ?
2
Caveat
This talk is based on opinions
not necessarily substantiated by real data
3
Multi-Hop Wireless

Many possibilities …
4
Multi-Hop Wireless

Mobile ad hoc networks
• Interconnect cars, planes,
tanks, soldiers, people
5
Multi-Hop Wireless

Mesh networks (roof-top, pole-top)
internet
Mesh
Client
6
Multi-Hop Wireless

Sensor networks
7
Multi-Hop Wireless

Opportunistic
Delay/Disruption/Disconnection-tolerant networks
8
Why Multi-Hop Wireless ?
9
Why Multi-Hop Wireless ?

Lack of infrastructure
10
Why Multi-Hop Wireless ?

Some clients difficult to reach directly via
infrastructure, due to obstacles
AP
Relay
11
Why Multi-Hop Wireless ?


Decreasing dependence on wired infrastructure
Add wireless “infrastructure”
internet
Mesh
Client
12
Why Multi-Hop Wireless ?

Low-power clients unable to communicate directly
with infrastructure
13
Why Multi-Hop Wireless ?

For improved capacity
High transmit power
High interference
14
Why Multi-Hop Wireless ?

For improved capacity
Low transmit power
Low interference
15
Why Multi-Hop Wireless ?

Poor connectivity
16
A Selective History
1973-87 DARPA Packet Radio Networks (PRNET/SURAN)
1997 IEEE 802.11
1997 IETF MANET
Time
1999 TinyOS
2000 MeshNetworks founded
2000+ CUWiN open-source mesh
2000 ACM MobiHoc
2001 “Embedded, Everywhere”  Sensor networks
2001 Interplanetary Internet, IETF draft, Vint Cerf
2003 ACM Sensys
2004 Motorola acquires MeshNetworks
2004 IEEE 802.11s study group for mesh networking
2004 ZigBee
17
Research Activity
versus
Relevance
18
Research Activity
Much activity in

Mobile ad hoc networks

Sensor networks
• No infrastructure
• Large diameter
• High mobility
• Low power
• Large diameter
• Small diameter useful in practice, but not “interesting”
19
Unscientific Measure of Interest:
Google 9/11/08

Ad hoc networks:

Mesh networks:

Sensor networks:
1,670,000

Vehicular networks:
1,710,000




2,290,000
764,000
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
Computer
architecture
21,400,000
802.11
66,000,000
Paris Hilton
68,800,000
20
Research Activity
Most activity seems to be in

Mobile ad hoc networks

Sensor networks
• No infrastructure
• Large diameter
• High mobility
• Low power
• Large diameter
Extreme assumptions make the problem exciting
 But what about relevance ?
21
Relevance ?
22
Relevance

Not all networks are made equal …

Some are likely to be commonplace
Relevance
others limited to niche scenarios
In increasing order of relevance …
23
Delay Tolerant Networks

Limited to niche scenarios
24
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)
25
Mobile Ad Hoc Networks


Why design networks without infrastructure ?
Possible to deploy some infrastructure in
most environments
26
Sensor Networks

Wireless sensors are important
Important to network the sensors

Sensors + Network ≠ Large diameter

27
Infrastructure Extension

Most compelling reason for multi-hop wireless
 Only a small number of hops!
AP
Relay
28
Infrastructure Extension

Mesh (Wireless “infrastructure”)
internet
29
Summary:
Most Appealing Scenario

Some wired infrastructure
 Capacity scales with the infrastructure

Small diameter wireless extension
for the infrastructure
Using relays or peer-to-peer
 Better reachability
 Low-power operation
 Reduced capacity loss
30
If only small diameter networks matter,
did we waste our time ?
Not quite …

Interference management and MAC-related issues
somewhat independent of network diameter
31
State of Multi-Hop Wireless
Very large volume of activity

Beautiful theory
Asymptotic Capacity
Throughput-optimal scheduling
Network utility optimization
Network coding
Cooperative relaying
32
State of Multi-Hop Wireless
Very large volume of activity

Practical protocols & deployments
Many wireless standards
And many more MAC & routing protocols
Many experimental deployments
Mesh devices
Sensor devices
Start-ups
33
State of Multi-Hop Wireless
Very large volume of activity

(Too) Many conferences and workshops

Plenty of research funding
Compared to many other areas
34
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 an
undergraduate wireless networks
class ?
35
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
36
What is Lacking ?
Meaningful contact between


Practice
Networking


Theory
Comm
37
Picture from Wikipedia
capacity
D
Net-X
Theory
to
Practice
Net-X
testbed
E
Fixed
F
B
A
Switchable
C
Capacity
bounds
channels
Insights on
protocol design
OS improvements
Software architecture
User
Applications
Multi-channel
protocol
IP Stack
ARP
Channel Abstraction Module
Linux box
CSL
Interface
Interface
Device Driver
Device Driver
38
Things I Wish I Had Learned in Kindergarten
39
1
outgrow
Those who cannot learn from history
are doomed to repeat it
With apologies to George Santayana
40
Pre-History of Wireless Communication:
Smoke Signals, Fires, Semaphore

Relaying : Multi-hop routes (store-and-forward)
41
Pre-History of Wireless Communication:
Homing Pigeons

Exploiting mobility
42
Reusing Ideas Reasonable,
but Need to Explore Better Alternatives
No wired-equivalent
for wireless networks
No links !
43
Wireless Channel Offers Rich Diversity
Current protocols
exploit diversity
only to a limited extent
Layer 1 : 2+ gap
The vanishing link :
Diversity confuses
the notion of a link
44
2
Interference is Information
45
Interference is Information
B
A
D
C
Signal
Interference
46
3
Bits Are Not Automobiles
47
Bits Are Not Automobiles

We treat information networks same as
physical transportation networks
• Planes, Trains and Automobiles

Bits can be combined (encoded) and
separated, unlike physical objects
48
Network Coding
P
A
P
B
Q
C
Q
49
Network Coding
P ++Q
Q
P
Q
A
B
C
Q
P
50
4
Physics Does Not Know Layers
51
Physics Does Not Know Layers


Layering is an abstraction, not a theorem
Backpressure scheduler ( “ throughput-optimal ” )
spans traditional layers 1 through 3:
arg max
r Є Rate
Region
∑ W(l) r(l)
l
52
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”
53
5
Opportunism Pays
54
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
…
55
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
56
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
57
What Now ?
Four-Point Agenda
58
1. Educate Better
Ourselves & Next Generation

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
59
2.
Fewer Research Programs
If you have influence at funding agencies …

Resist temptation to create new networking programs
• Partitioning of resources creates false demand
– Remove existing partitions
Possible to encourage research without these
– Past examples: NOSS, FIND?
60
3.

Fewer “Better” Conferences
Increase venues that encourage diverse
community 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 ?
61
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
62
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
63
Advertisement
64
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 !
65
Thanks!
66
Thanks!
67