BitMAC: A Deterministic, Collision-free, and Robust MAC Protocol for - - PowerPoint PPT Presentation

• 31. 1. 2005

BitMAC: A Deterministic, Collision-free, and Robust MAC Protocol for Sensor Networks

Matthias Ringwald, Kay Römer ETH Zurich

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• 31. 1. 2005

Matthias Ringwald, Kay Römer, Institut for Pervasive Computing

Motivation

• Dense wireless sensor networks to collect data
• f physical events
• Event causes communication burst
• Contention-based MAC protocols

=> collisions, long delays, reduced bandwidth GOAL: Collision-Free Multi-Hop MAC Protocol

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• 31. 1. 2005

Matthias Ringwald, Kay Römer, Institut for Pervasive Computing

Collision-Free Concurrent Access ?

Experiment: Two nodes A and B send different On-Off-Keying (OOK) modulated data

1 0 1 0 0 0 1 0 0 0 1 0

A B

1 0 1 0 1 0

A + B

Our communication model: The OR channel

?

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• 31. 1. 2005

Matthias Ringwald, Kay Römer, Institut for Pervasive Computing

111 C

Integer Operations on Physical Layer

• Children synchronously send bits to parent

=> Bitwise OR Side effect: One child is elected, if values are distinct

A D E 110 011 101 1 1 1 1 1 1 1 1 1 1 1 1 1 1

• MAX of in number-of-bits rounds

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• 31. 1. 2005

Matthias Ringwald, Kay Römer, Institut for Pervasive Computing

MAX Operation in Parallel

• What happens with multiple parents?

A C D E 110 011 101 F B 100 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1

Side effect: At most one child elected per two-hop neighborhood, if values are distinct

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• 31. 1. 2005

Matthias Ringwald, Kay Römer, Institut for Pervasive Computing

BitMAC: A MAC Protocol using the OR- Channnel

• BitMAC builds spanning tree of sensor nodes
• without collision
• Application assumptions
• Network contains sink, e.g., gateway
• Data routed mostly from nodes to sink
• Network topology mostly static
• Nodes can use different radio channels
• e.g., TinyDB, directed diffusion, ...

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• 31. 1. 2005

Matthias Ringwald, Kay Römer, Institut for Pervasive Computing

Protocol Overview - Setup Phase

• Ring formation by synchronous

flooding of beacon message with hop count

• OR Channel!

1 2 3

• Establish spanning tree

Reduce number of uplinks to one by assigning separate radio channels

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• 31. 1. 2005

Matthias Ringwald, Kay Römer, Institut for Pervasive Computing

Protocol Overview - Operational

• Every inner node with children

forms star network

• Star networks use TDMA

with single bit send requests

• Children need to have small IDs

1 2 3

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• 31. 1. 2005

Matthias Ringwald, Kay Römer, Institut for Pervasive Computing

Assigning Channels and Local IDs

• Requirements for one ring:
• Channels(ch): ch(B) ≠ ch(C) since B and C share child
• Local IDs (id): id(A) ≠ id(B) since A and B share parent
• Combining both:
• Color := Channel = ID
• Color (col): col(x) ≠ col(y), if x and y share child or parent

Two-hop ring coloring problem

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• 31. 1. 2005

Matthias Ringwald, Kay Römer, Institut for Pervasive Computing

Two-Hop Ring Coloring

• Nodes maintain palette of free colors 1..C
• For C rounds:
• Every node picks free color from palette
• For every color c:

Perform election process within two hop neighborhood

• If elected, a node assigns color and STOPS
• Otherwise, node marks color as used in palette.
• Size of palette for random graphs
• C = 2 x average-node-degree (with high probability)
• see paper for details

Parallel MAX Op

• n MAC Address

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• 31. 1. 2005

Matthias Ringwald, Kay Römer, Institut for Pervasive Computing

QuickTime™ and a TIFF (LZW) decompressor are needed to see this picture.

• Synchronization by flooding a sync bit (see ring formation)
• Receivers lock to the middle
• f this bit
• Transmissions of multiple

senders overlay due to OR channel

• Required precision depends on
• bit length
• network diameter
• See paper for details

Time Synchronization

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• 31. 1. 2005

Matthias Ringwald, Kay Römer, Institut for Pervasive Computing

Status

• Prototype implementation of coloring algorithm
• n BTnodes (only single ring)
• Simulation of coloring for larger networks
• Open issues:
• Bit-synchronous sending in

networks with large diameter

• Bit errors
• Full BitMAC implementation

BTnode Rev3

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• 31. 1. 2005

Matthias Ringwald, Kay Römer, Institut for Pervasive Computing

Conclusion

• “Collisions are not necessarily bad”
• Efficient integer operations (OR, AND, MAX, MIN)

and election on physical layer

• Deterministic and collision-free...
• Algorithm for two-hop ring coloring
• Spanning tree construction
• Multi-hop communication