Download - "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
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.
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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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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)
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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
