TDM MAC Protocol Design and Implementation for Wireless Mesh Networks

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TDM MAC Protocol Design and Implementationfor
Wireless Mesh Networks

• D. Koutsonikolas (Purdue University), T.
  Salonidis,
• H. Lundgren, P. LeGuyadec, Y. Charlie Hu, and I.
  Sheriff
• ACM CoNEXT 2008
• Presented by Fajun CHEN

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Background

• MAC protocol for Wireless Mesh Networks
• CSMA MAC 802.11
• Carrier Sense, exponential back-off
• React poorly to heavy traffic load
• TDM (Time Division Multiplex) MAC
• Assume global knowledge of network
• Centralized scheduling better performance
  expectable to heavy traffic load

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Objective of the paper

• Design of TDM MAC protocol
• Identify bottlenecks
• Platform bottlenecks
• Synchronization bottlenecks
• Synchronization algorithm
• Integrates synchronization algorithm in the
  protocol

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Related works TDM MAC

• Driver level or firmware level
• Driver level
• Overlay MAC Layer
• SoftMAC and MadMAC
• Point-to-point synchronization or network-wide
• Point-to-point synchronization
• WiFi based Long Distance networks
• Others
• RT-Link, optimized for energy constrained sensor
  networks, resorts to out-of-band, hardware-based
  synchronization

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Related works - algorithm

• Out-of-band or in-band
• Out-of-band
• GPS clocks need clear sky view
• In-band
• NTP, used for Internet synchronization, yield low
  accuracy (millisecond scale ) to multi-hop
  wireless mesh
• Others, designed for sensor networks and on top
  of CSMA MAC
• Synchronization over a tree or not
• For single-hop or multi-hop

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TDM protocol architecture

• Slotted structure of TDM MAC protocol

GW
MAP
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Bottlenecks

• Platform bottlenecks
• Clock drift rate
• Slot processing
• RxRx, TxRx, RxTx, TxTx turnaround
• Data packet preparation time
• Platform design constraints

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Bottlenecks

• Synchronization bottlenecks
• SCS duration
• Synchronization period
• Synchronization design constraints

Fix to 5000µs in experiment
(4)
(5)
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Synchronization Algorithm

• Slot sequence computation
• Construct maximum reliability tree using
  Dijkstras algorithm
• Link loss rate
• Algorithm failure
  probability
• Choose the best GW

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Synchronization Algorithm
SCS
Beacon
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Each node updates its local clock based on the
newly computed offset at the end of slot SCS.
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Integration

• Step1
• Perform bottlenecks measurements on the platform
• Perform bottlenecks calculation on the
  synchronization algorithm
• Step2
• Solving optimization problem aimed to minimize
  the protocol overhead restricted by platform and
  synchronization constraints.
• Step 3
• Computing all the parameters in the protocol.

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Evaluation
Throughput comparison of TDM MAC and 802.11 for
one-hop and two-hop bi-direction flows
Throughput degrades with increase packet loss
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Comments

• Professional written and easy to follow.
• Based on a real test-bed in Purdue
• D. Koutsonikolas focuses on High Throughput and
  Reliable Multicast in Wireless Meshes
• External interference is mitigated, might
  constraint its practice application in dynamic
  environment

https//engineering.purdue.edu/MESH