A Cross Layer Approach for Power Heterogeneous Ad hoc Networks

1

• A Cross Layer Approach for Power Heterogeneous Ad
  hoc Networks
• Vasudev Shah and Srikanth Krishnamurthy
• ICDCS 2005

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Motivation

• In the future multifarious devices together will
  form an ad hoc network. e.g laptops, PDAs, low
  power sensor nodes etc.
• Power Control Schemes deploy multiple
  transmission power levels.
• These artifacts will result in link level
  asymmetry.
• Performance of Legacy IEEE 802.11 MAC has been
  shown to degrade in a network having nodes with
  varying transmission power.
• Current MAC layer protocols cannot handle
  inherent link asymmetry.

3
Road Map

• Problem Description
• Contribution
• Assumptions and Definitions
• Our Approach
• Simulations Models/Results
• Conclusions

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Problem Description

• In the presence of power heterogeneity, the IEEE
  802.11 MAC signaling mechanism is inefficient
• ? Number of hidden terminals increases.
• ? Increase in the number of False Link
  Failures reported to the Routing Layer.
• Consequently, low power nodes suffer in terms
  of throughput.
• The inefficiencies at the MAC layer affect the
  performance of higher layers
• ? ROUTING (e.g. AODV, DSR)
• Increase in the number of Route Discoveries.
• Unidirectional routing schemes largely ignore
  MAC dependencies.
• ? TRANSPORT
• TCP re-transmissions.

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Inefficiency of IEEE 802.11
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Quantifying the problem with IEEE 802.11

• MAC layer Metrics 1. Throughput Efficiency ()
• 2. Data
  Success Rate ()
• Simulation Models Simulator ns2, IEEE 802.11
  MAC, 40 nodes (50 nodes at 0.14W 50 at 0.56
  W), Poisson Traffic at 1000 packets/sec, Random
  Waypoint movement with randomly distributed
  nodes, speed varied from 5-10 m/s.
• To avoid Transport and Routing layer artifacts a
  packet generating agent is used just above the
  MAC layer.
• The agent randomly chooses a neighbor to
  communicate.

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Evaluation of IEEE 802.11
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Inability to identify unidirectional links

• H can reach L1 but not vice versa.
• L thinks link exists.
• May send RTS messages to H that fails.
• Repeated attempts -- can lead to wasted route
  discovery attempts.

• While unidirectional routing schemes attempt to
  route around the link they do not examine effects
  at MAC layer.

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What we have done ..

• A framework that spans the MAC and routing layers
  to address the problem of link asymmetry.
• Alleviates the hidden terminal problems at MAC
  layer.
• Enables the identification and effective usage of
  unidirectional links at the routing layer.
• Improvement by as much as 25 in terms of
  throughput over traditional layered approaches.

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Definitions and Assumptions

• In this work we consider an ad hoc network in
  which the different nodes differ in terms of
  their maximum achievable transmission range.
• For simplicity we only consider two types of
  nodes, we refer to these two types as lower power
  nodes and higher power nodes respectively.
• We assume that the transmit power of the high
  power nodes is such that the transmission range
  is doubled.
• We use the terms homogeneous and heterogeneous to
  refer to networks in which all nodes have,
  respectively, identical or non-identical power
  capabilities in terms of the maximum transmission
  range.

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Some simple MAC layer solutions

• We considered standard flooding and GPS based
  schemes ICC 2001 to propagate CTS messages.
• The proposed schemes further degraded the
  performance of IEEE 802.11 MAC in terms of
  throughput of the low power nodes due to the
  overhead incurred due to flooding.
• In our subsequent work we attempted to use (i)
  smart broadcasting and (ii) using a single
  reservation for sending multiple DATA messages
  ICC 2004.
• Performance improved marginally over the
  traditional IEEE 802.11 protocol.

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Related Work Unidirectional Routing

• A plurality of methods for unidirectional routing
  exist.
• They do not consider MAC layer dependencies.
• Some of the previous work correctly identifies
  the need for a MAC layer scheme that can handle
  link asymmetry.

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Key Ideas

• Propagate the CTS message (a variant thereof
  called the BW_RES message is used) to high power
  nodes.
• Enlist the support of the routing layer at the
  MAC layer to route the BW_RES message to
  relevant nodes (as opposed to simply using
  broadcasts).
• Use this structure to identify and effectively
  tunnel packets to span unidirectional links.

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Topology Aware CTS Propagation (TACP)

• Nodes exchange HELLO messages
• Each message includes a list of the transmitters
  inbound n-hop neighbors (as this information
  becomes available).
• Using these HELLO messages, a node constructs a
  localized graph
• The graph represents the nodes local
  neighborhood.
• The node then constructs a minimum cost tree to
  reach all of the high power nodes on this graph.
• It is essential to restrict n to small values
  to keep the size of these HELLO messages small.
• High power nodes are used to the extent feasible
  for data distribution (tree construction) so as
  to minimize the number of data broadcasts.

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Benefits of TACP

• TACP helps in reducing the overhead of
  disseminating propagated BW_RES messages.
• We can use (and do use) the multi-reservation
  technique for making a single reservation for
  multiple data packets, in conjunction with TACP.
• The use of high power nodes to the extent
  possible minimizes the latency incurred in BW_RES
  messages.
• And ... TACP inherently facilitates the use of
  unidirectional links.

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Identifying reverse tunnels

• Using the localized graph, L1 can construct a
  reverse path to H1.
• A tunnel is established using this reverse path.
• The tunnel is used for disseminating both
  control and data messages.
• Unidirectional links are therefore transparent
  to the routing protocol.

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MAC layer Simulations -- Set up and Metrics

• Throughput Efficiency Fraction of total time
  that is used for sending successful data.
• Data Success Rate Percentage of successful Data
  transmissions after a successful RTS/CTS exchange.

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Comparisons

• We compare the results from four cases
• Case A The legacy IEEE 802.11 Protocol.
• Case B With TACP
• Case C With Multi-reservations
• Case D With TACP Multi-reservations.

• All nodes use TACP irrespective of whether they
  are high power or low power nodes -- CTS
  propagation was seen to help high power nodes as
  well.

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Improvement in MAC layer Data Success Rate.
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Reduction in BW_RES overhead
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Improved MAC layer Throughput Efficiency
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Simulation Settings Higher Layers Included
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Reduction in Route Discovery Attempts
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Increase in Packet Delivery Ratio at Higher Layers
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Penalty Marginal Increase in Delay
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Open Issues

• We have examined a network wherein nodes use
  static but fixed transmission power levels.
• What if nodes could vary the transmission powers
  ?
• Goal in a power controlled setting is much more
  lofty -- achieving an even higher capacity (in
  terms of throughput) than, in a homogeneous
  setting.
• Current approaches consider only one layer --
  energy efficient routing/ MAC layer solutions.
• How can one integrate the two to achieve
  application layer end to end performance ?