Asynchronous Ad-hoc Leader Election in Complete

NetworksNolan Irving

Outline• Presentation of problem• Survey of current work• System description• Program description• Data collected• Conclusions

Problem Statement• Ad-hoc network

• No existing backbone to network• Nodes are resource-poor

• Battery life• Processing power

Problem Statement (cont.)• Leader election

• At any given time, there must be at most one leader

• Both links and nodes are unstable• Cannot safely assume reliable channels• Network must adapt to frequent changes

Uses of Ad-hoc Networks• Rapid network deployment

• Combat situations• Search & rescue

Why Elect?• The leader is used to control requests for

access to limited resources• Restoration of tokens• Grant resource requests

Other Research• Multihop networks• Bidirectional links• Movement-based networks

Flawed Assumptions• Algorithms assumed knowledge of number

of participating processors• Nearly all research assumed global ordering• Link representation inappropriate to

wireless networking

Other Problems• Maintenance costs never addressed• Addition/removal of nodes ignored

• Problem increased by initialization requirement

Problem Description• Asynchronous network• Unknown participants• No global ordering• Broadcast communication channel• CD enabled• Complete network

Assumptions• Communications is a shared broadcast channel –

multiple simultaneous transmissions corrupt signal• Nodes can detect a collision – likewise, the sender

can detect a successful transmission• Network is single-hop – all processors can be

reached with a single broadcast• A successful broadcast will reach all participating

nodes

Program Framework• Emulate asynchronous communications

using priority queue• Channel class keeps track of simultaneous

communications and status• Node class handles communications

requests

Simulation Structure

N o d e 1 N o d e 2 N o d e 3 N o d e 4

E le c tio n O b je ct

C h a n n e l C la ss

C o m m un ica tio ns Q u e ue

Program Framework (cont.)

• Leader election protocol• Global ordering• Adding/removing nodes

Results• Times were an average of 1000 runs• Total time is listed in seconds

Timing Results 1

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Timing Results 2

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Timing Results 3

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Comparison of Results

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Timing Results(0.05 second message duration)

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AverageWorst

Summary of Results• Linear relationship between message length

and election time• Polynomial growth of algorithm time and

message complexity with n

Conclusions• Advantages

• System offered a simple asynchronous protocol for leader election

• Protocol allows for only one leader• Maintenance costs minimal• Handles new additions/dropped nodes easily• One of very few designs able to handle an

unknown number of nodes

Conclusions• Disadvantages

• Time not strongly bounded• Delaying technique inefficient• Will not count participating processors• Unsuited to extremely large networks

Sources• Fundamental Control Algorithms in Ad-hoc Networks.

Hatzis, et. Al. 1998.• Leader Election Algorithms for Mobile Ad-hoc Networks.

Malpani, et. Al. 2000.• Randomized Initialization Protocols for Ad-hoc Networks.

Nakano, Koji and Olariu, Stephan. 2000.• Randomized Leader Election Protocols for Ad-hoc

Networks. Nakano, Koji and Olariu, Stephan. 2000.

Questions

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