Congestion Control and Fairness for ManytoOne Routing in Sensor Networks

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Congestion Control and Fairness for Many-to-One
Routing in Sensor Networks

• Cheng Tien Ee and Ruzena Bajcsy
• University of California, Barkeley

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Outline of the Talk

• Introduction
• Motivation behind this work
• Congestion control
• Fairness
• Results Simulation Implementation
• Conclusion

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Introduction

• Sensor motes send data to the Base
• Intermediate motes relay messages from far away
  motes to the Base
• Routing of messages form a tree rooted at the
  base

base
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Motivation

• Problems
• Distributed system no global knowledge
• Motes near the base has to transmit more packets
  than far away motes bottleneck
• Packets generated by far away motes may be
  dropped (unfairness)
• Loss of packet due to buffer overflow and
  collision (congestion)

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Motivation

• Solutions
• Determine the maximum data generation rate of the
  network and adjust transmission rate accordingly
  thus buffer overflow wont occur
• Implement hop by hop ARQ no packet loss is
  ensured
• TCP
• Global Coordination
• But .

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Motivation

• Limitations
• Dynamic and distributed configuration
• Obtaining maximum data generation global
  information
• Underestimation of generation rate reduces
  effective bandwidth
• Transient congestion
• Local perturbation

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Congestion Control

• The Algorithm
• Simple limited resources
• Distributed no global coordination

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Congestion Control

• Design Network Stack Model

Congestion
Application
Transport
Network
Data-Link
Routing
Physical
MAC
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Congestion Control

• Types of Congestion
• Type A - Simultaneous transmission interference
  packet loss reduction in throughput
• Type B - Data generation rate is faster than the
  transmission rate - Buffer overflow

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Congestion Control

• The idea
• The network should be self-adaptive
• Type A congestion can be reduced by phase
  shifting
• Type B congestion can be reduced by buffer
  monitoring

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Congestion Control

• The idea
• The two different rates we need
• data generation rate rate at which data is
  passed from application
• transmission rate rate at which packets, both
  locally produced and from downstream motes, are
  transmitted upstream
• transmission rate of parent mote should ideally
  be greater than or equal to data generation rate
  of all motes downstream

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Congestion Control

• The idea

Application
Transport
Transmission
Network
Data Generation
Data-Link
Physical
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Congestion Control

• The idea

Transmission rate of parent should be gt
5pkts/sec
Each mote generates data at the rate 1pkt/sec
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Congestion Control

• The Algorithm
• Repeatedly run at each mote (localized algo)
• determines local maximum transmission rate
• divide transmission rate by total number of
  children motes to give data generation rate
• disseminate min(own_rate, parent_rate) downstream

Queue overflow adjustment incorporated
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Congestion Control

• The Algorithm
• Adjustment
• Let the data generation rate for a child
  determined by the current node is denoted by rc
• Let r denote data generation rate for the current
  when the queues are overflowing or about to
  overflow
• disseminate min(parent_rate, r, rc) downstream

Current nodes data gen rate sent by its parent
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Congestion Control

• The Algorithm

Transmission rate 50 pkts/sec
Each childs data generation rate should be lt
10pkts/sec
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Congestion Control

• Lets look into the methods for
• determining effective transmission rate
• determining number of downstream motes
• disseminating data generation rate downstream
• adjusting data generation rate if buffers are
  overflowing

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Congestion Control

• Calculating effective Transmission Rate

ack pkt
data pkt
data pkt
no ack, timeout
data pkt
no ack, timeout
Total time taken
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Congestion Control

• Counting number of Downstream Motes
• each mote sums childrens count, adds 1 (for
  itself), then transmits count to parent (data
  aggregation)

5
2
1
1
1
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Congestion Control

• Disseminating data generation rate
• downstream
• via control packets
• piggy-backed on data packets

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Congestion Control

• Adjusting data generation rate if buffers are
• overflowing

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Congestion Control
Taking mean data gen. rate

• The Solution Explained
• The packet generation rate assignment is fair
• Why?
• Type B congestion is minimized
• How?
• Type A congestion is reduced
• How?

Queue overflow check
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Congestion Control
A
Rate updates takes time to propagate introducing
phase shift
D
B
C
E
E
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Fairness

• Requirement
• The base station should receive the same number
  of packets from each mote
• The idea
• Transmit number of packets from each subtree with
  equal probability
• Within each period of time, transmit number of
  packets from each subtree equal to size of that
  subtree

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Fairness

• Mechanisms
• Per-child packet queues
• Maintenance of per-child tree size
• Obtained as before

• FIFO Queues
• small in size, independent of subtree size

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Fairness

• Algorithm
• Probabilistic Selection

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Fairness

• Algorithm
• Correctness of PS
• Allows packet from all nodes to have equal
  probability of reaching the base station.
• Proof by induction
• Effective only when queues are backlogged
  queues small in size unable to absorb bursty
  traffic

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Fairness

• Algorithm
• Epoch based proportional selection
• We define an epoch to have units of packets and
  is an integer multiple of the total number of
  nodes in this subtree and a positive integer n.
• With each epoch we transmit from each queue
  exactly n times the number of nodes serviced by
  that queue.
• Queues are FIFO
• No work conservation is implemented

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Fairness
In one time period transmit 2 from A, 1 from B, 1
from C and 1 from this parent node

• The idea

C
A
1
B
2
1
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Fairness

• Algorithm

Referring to node A and letting n1, the number
of packets selected for transmission from the
queues corresponding to nodes A, B, C and D are
1, 2, 3, 1 respectively.
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Fairness

• Algorithm
• FIFO Queue

• In node A, packets from B are received according
  to Bs epoch
• No order is imposed on childs transmission
• Within each epoch one packet per child

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Fairness

• Algorithm
• Selecting the next packet to transmit

• In node A, packets from B are received according
  to Bs epoch
• No order is imposed on childs transmission

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Fairness

• Algorithm
• Effect of non-work conservation
• Congestion in any branch of the network will
  cause rates to decrease throughout all other
  parts of the network

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Fairness

• Algorithm
• Correctness of EPS
• Proof by induction
• WLOG, we can assume size of an epoch is equal to
  the subtree size
• Within each epoch one packet from each downstream
  node is transmitted to the parent node

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Fairness

• Algorithm
• Necessity of ARQ
• Implementing EPS without ARQ
• Suppose link between parent A and a child Q is
  very lossy
• A may miss time intervals to transmit as it waits
  for Q to send data
• Entire performance deteriorates due to one single
  link !
• Best solution is to implement hop-by-hop ARQ

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Results

• Simulation
• Bit rate, control packet size, data packet size
• No interference from motes more than one hop away
• Uses MACA as MAC protocol
• Packet level simulation
• Round-robin servicing of queues to show effect
  of loss of packets further away from the base

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Results

• Simulation

• The (Congestion Control EPS) graph
• is a constant at 5000 packets
• The (Congestion Control PS) graph fluctuates
  slightly around 5000 packets

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Results

• Implementation on mica2dot motes
• MACA with transport layer ACKs
• 10 motes deployed indoors, within 15 feet of one
  another
• motes arbitrarily construct routing tree
• compare with round-robin servicing of queues

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Results

• Implementation

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Conclusion

• very scalable
• size of queues can be small, constant
• state required increases linearly with number of
  neighbors
• ARQ achieves fairness with small number of packet
  transmission
• exact, same, simple code runs in each mote
• reduces implementation, debugging time
• network adapts itself automatically

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Conclusion

• implemented in transport layer, thus independent
  of the other network layers
• Slight modification to the MAC layer may be
  required
• Can handle non-constant rate traffic
• Local computation

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Thank you
Questions??