"Open" Problems in Mobile Ad Hoc Networking

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“Open” Problems in Mobile Ad Hoc Networking

Nitin Vaidya

University of Illinois at Urbana-Champaign

nhv@uiuc.edu

www.crhc.uiuc.edu/~nhv

Keynote talk presented at the Workshop on Wireless Local Networks(in conjunction with 26th Conference on Local Computer Networks),

Tampa, Florida, November 14, 2001

© 2001 Nitin Vaidya

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Mobile Ad Hoc Networks

Formed by wireless hosts which may be mobile

Without necessarily using a pre-existing infrastructure

Routes between nodes may potentially contain multiple hops

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Mobile Ad Hoc Networks

May need to traverse multiple links to reach a destination

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Mobile Ad Hoc Networks (MANET)

Mobility causes route changes

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Why Ad Hoc Networks ?

Potential ease of deployment

Decreased dependence on infrastructure

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Many Applications

Personal area networking cell phone, laptop, ear phone, wrist watch

Military environments soldiers, tanks, planes

Civilian environments taxi cab network meeting rooms sports stadiums boats, small aircraft

Emergency operations search-and-rescue policing and fire fighting

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Many Variations

Fully Symmetric Environment all nodes have identical capabilities and responsibilities

Asymmetric Capabilities transmission ranges and radios may differ battery life at different nodes may differ processing capacity may be different at different nodes speed of movement

Asymmetric Responsibilities only some nodes may route packets some nodes may act as leaders of nearby nodes (e.g., cluster

head)

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Many Variations

Traffic characteristics may differ in different ad hoc networks bit rate timeliness constraints reliability requirements unicast / multicast / geocast host-based addressing / content-based addressing /

capability-based addressing

May co-exist (and co-operate) with an infrastructure-based network

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Many Variations

Mobility patterns may be different people sitting at an airport lounge New York taxi cabs kids playing military movements personal area network

Mobility characteristics speed predictability

• direction of movement

• pattern of movement uniformity (or lack thereof) of mobility characteristics among

different nodes

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Challenges

Limited wireless transmission range Broadcast nature of the wireless medium

– Hidden terminal problem Packet losses due to transmission errors Mobility-induced route changes Mobility-induced packet losses Battery constraints Potentially frequent network partitions Ease of snooping on wireless transmissions (security

hazard)

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Research on Mobile Ad Hoc Networks

Variations in capabilities & responsibilities

X

Variations in traffic characteristics, mobility models, etc.

X

Performance criteria (e.g., optimize throughput, reduce energy consumption)

+

Increased research funding

=

Significant research activity

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Hidden Terminals&

RTS/CTS Handshake

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A B C

Hidden Terminal Problem

Node B can communicate with A and C both A and C cannot hear each other

When A transmits to B, C cannot detect the transmission using the carrier sense mechanism

If C transmits, collision will occur at node B

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RTS/CTS Handshake

Sender sends Ready-to-Send (RTS) Receiver responds with Clear-to-Send (CTS) RTS and CTS announce the duration of the transfer Nodes overhearing RTS/CTS keep quiet for that duration RTS/CTS used in IEEE 802.11

D

C

BACTS (10)

RTS (10)

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10

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Problemsin

Ad Hoc Networking

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Problem Space

Practical considerations Consumer demand or lack thereof Standardization Government regulations

Technical issues

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Problem Space

Link

Network

Transport

Physical

Upper layers

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Physical Layer

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Physical Layer

Traditionally, not much interaction between physical layer and upper layers

Many physical layer mechanisms not beneficial without help from upper layers

Example: Adaptive modulation

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Adaptive Modulation

Channel conditions are time-varying

A B

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Choose modulation scheme as a function of channel conditions

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Adaptive Modulation

If physical layer chooses the modulation scheme transparent to MAC MAC cannot know the time duration required for the transfer

Must involve MAC protocol in deciding the modulation scheme Some 802.11-compliant implementations use a sender-

based scheme for this purpose Receiver-based schemes can perform better

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Sender-Based “Autorate Fallback” MAC Protocol

D

C

BA

1Mbps2Mbps

Sender decreases rate after N consecutive ACKS are not received Sender increases rate after Y consecutive ACKS are received

DATA2Mbps

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Performance of Sender-Based“Autorate Fallback”

Expected

ARF

CCK (11Mbps)

CCK (5.5Mbps)

QPSK (2Mbps)

BPSK (1Mbps)

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1Mbps2Mbps

Sender sends RTS containing its best rate estimate Receiver chooses best rate for the conditions and sends it in the CTS Sender transmits DATA packet at new rate Information in data packet header implicitly updates nodes that heard old rate

Receiver-Based Autorate MAC Protocol

D

C

BACTS (1)

RTS (2)

2

1

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Physical Layer

Several other physical layer capabilities call for changes to upper layers of protocol stack

Example: Power control

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Power Control

Transmit power determines “Range” of a transmission Interference caused at other nodes

B C DA

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Power Control

Transmit power determines “Range” of a transmission Interference caused at other nodes

B C DA

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Benefits of Power Control

Transmit a packet with least transmit power necessary to deliver to the receiver

Save energy: Important benefit to battery-powered hosts

Reduce interference Can allow greater spatial reuse

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Power Control

Power control introduces asymmetry

D transmits to C at low power, but B uses high transmit power to transmit to A

B may not about D-to-C transmission, but can interfere with it

B C DA

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Power Control

Proposals for medium access control and routing with power control exist

Do not solve the problem satisfactorily

Ideal solution will Reduce energy consumption, and Maximize spatial reuse

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Directional / Smart Antennas

Various capabilities Sectored antennas (fixed beam positions) Beam steering Tracking a transmitter

MAC and routing protocols for ad hoc networks using such antennas

How to take into account antenna capabilities?

• Network may be heterogeneous

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Physical Layer

Are ad hoc networks benefiting from the progress made at physical layer ?

Other interesting areas Efficient coding schemes Various diversity techniques

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Physical Layer: Simulation Models

Insufficient accuracy in commonly used physical layer models

Physical link state is not binary as often assumed

Reliable packet reception does not depend just on distance Transmit power Interference level Fading

Need to use realistic models

Modulation schemeCoding

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Link Layer

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Interesting Link Layer Issues

Medium access control

Retransmission mechanisms

Transmission scheduling Which pending packet should a node attempt to transmit?

Adaptive parameter selection Frame size Retransmission limit

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QoS in Medium Access Control

Many proposals for achieving fairness

Fair scheduling schemes attempt to provide equitable sharing of channel

Unpredictable nature of transmission errors makes it difficult to make hard guarantees

Need to develop a probabilistic framework

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QoS in MAC

Easier in a centralized protocol (such as 802.11 point coordination function), than in a distributed protocol

Distributed MAC appears more suitable for ad hoc networks, however

Perhaps a hybrid protocol will be best How to design such a protocol ?

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Transmission Scheduling

When multiple packets pending transmission, which packet to transmit next?

Choice should depend on Receiver status (blocked by some other transmission?) Congestion at receivers Noise level at receivers Tolerable delay for pending packets

– Need interaction between upper layers and MAC

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MAC for Multiple Channels

How to split bandwidth into channels?

How to use the multiple channels ?

• Dedicated channel for control ?

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Network Layer

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Reactive versus Proactive Routing

Reactive protocols Maintain routes between nodes that need to communicate

Proactive protocols Maintain routes between all node-pairs

Lot of activity on routing protocol design

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Routing

Reactive and proactive protocols are quite

well-understood

Designing reactive protocols: “Solved” problem Designing proactive protocols: “Solved” problem

At least, when using common assumptions about the network

Interesting problems exist when other issues are considered (such as QoS or physical layer properties)

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Reactive versus Proactive

Choice of protocol depends on Mobility characteristics of the nodes Traffic characteristics

How to design adaptive protocols ?

Existing proposals use a straightforward combination of reactive and proactive Proactive within “radius” K Reactive outside K Choose K somehow

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Reactive versus Proactive

Need a more flexible way to manage protocol behavior

Assign proactive/reactive tag to each route (A,B) ?

How to determine when proactive behavior is better than reactive ?

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Address Assignment

How to assign addresses to nodes in an ad hoc network ?

Static assignment Easier to guarantee unique address

Dynamic assignment How to guarantee unique addresses when partitions merge?

Do we need to guarantee unique addresses ?

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Transport Protocols

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TCP

TCP performance degrades in presence of route failures

TCP cannot distinguish between packet losses due to route change and due to congestion

Reduces congestion window in response

• Unnecessary degradation in throughput

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TCP

Several solutions have been proposed to fix this

These techniques somehow inform TCP sender that the packet losses are due to route failure

TCP does not decrease congestion window in response

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TCP

New route may differ significantly from old route

Proposals for TCP-over-ad-hoc tend to use old timeout and congestion window after a route change

Does not seem like a good idea

How to choose appropriate timeout and congestion window after detecting a route change ?

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Other Issues

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Algorithms

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Distributed Algorithms

Rich body of work on distributed algorithms in traditional distributed environments Shared memory Message ordering Clock synchronization Leader election

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Distributed Algorithms

Existing algorithms can usually be used on ad hoc networks without affecting correctness

Performance on ad hoc networks may not be good

Existing algorithm treat link repairs/failures as random events

With mobility, link failure/repairs are correlated with host movement

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Distributed Algorithms

How to design distributed algorithm exploiting the correlation between mobility and link failure/repair ?

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Distributed Algorithms

Traditionally, complexity is measured as a function of problem “size” Number of nodes Number of failures

How to analyze algorithm complexity as a function of mobility ?

What measure of mobility is amenable to such an analysis ? Need to capture the correlation without making the measure

too complex

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Security Issues

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What’s New ?

Wireless medium easy to snoop on

With ad hoc networking, hard to guarantee connectivity

Easier for intruders to insert themselves into network

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Authentication

How to authenticate a node ?

May not have access to a certification authority

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Resource Depletion Attack

Intruders may send data with the objective of congesting a network or depleting batteries

A

CB

D

T

intruder

U intruder

Bogus traffic

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Routing Attacks

Intruders may mis-route the data not delivering it to the destination at all, or delaying it significantly

How to detect such attacks ?

How to tolerate such attacks ?

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Traffic Analysis

Despite encryption, an eavesdropper can identify traffic patterns

Traffic patterns can divulge information about the operation mode

Traffic analysis can be prevented by presenting “constant” traffic pattern

– Insert dummy traffic

How to make this cheaper ?

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Other Issues

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Incentives for Ad Hoc Routing

Why should I forward packets for some other nodes ?

Need some incentive mechanism

Policies to determine reward for performing each operation

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Applications

New applications for ad hoc networks ?

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Hybrid Environments

Use infrastructure when convenient Use ad hoc connectivity when necessary or superior

EA

BS1 BS2

X

Z

infrastructure

Ad hoc connectivity

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Summary

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Summary

Plenty of interesting research problems

Research community disproportionately obsessed with routing protocols

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Summary

Interesting problems elsewhere at the two ends of the protocol stack

How to design

algorithms and applications ?

How to exploit physical

layer techniques ?• Increase interaction

between physical layer

and upper layers

Link

Network

Transport

Physical

Upper layers

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Summary

Hybrid environments require revisiting protocol design decisions

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Tutorials

Visit http://www.crhc.uiuc.edu/~nhv for my tutorials on

Mobile ad hoc networking

TCP over Wireless

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Thank you !!

Comments/questions tonhv@uiuc.edu

© 2001 Nitin Vaidya