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					  Topology Control of Multihop
Wireless Networks Using Transmit
        Power Adjustment
Paper By:   Ram Ramanathan, Regina Resales-Hain
Slides adapted from R. Jayampathi Sampath
Presenter: Shahram Mohrehkesh, CS852, ODU, Spring 2011
Outline
 lNTRODUCTION

 PROBLEM STATEMENT

 STATIC NETWORKS: OPTIMUM CENTRALIZED ALGORITHMS
   CONNECT
   Separation Edges and Vertices
   Biconnected Graph
   BICONN-AUGMENT

 MOBILE NETWORKS : DISTRIBUTED HEURISTICS
   LINT Description
   LILT Description

 EXPERIMENTAL RESULTS
lNTRODUCTION
 “Topology” in ad hoc netwoek
 set of communication links between node pairs used
 explicitly or implicitly by a routing mechanism.

    uncontrollable factors: mobility, weather, noise
    controllable factors: transmit power, antenna direction

 This paper addresses the problem of controlling the
 topology of the network by changing the transmit powers
 of the nodes.

 Controlling the set of neighbors to which a node talks to is
 the basic approach.
lNTRODUCTION(Contd.)
 Why do we need to control the topology?
   Draw back of a wrong topology
     Reduce the capacity
     Increase the end-to-end packet delay
     Decrease the robustness to node failures
   Example 1 – Too sparse network
     A danger of network partitioning
     High end to end delays
   Example 2 – Dense network
     Many nodes interfere with each other
     Recompute routes even if small node movements
PROBLEM STATEMENT
 Definition 1: A multihop wireless network is represented
 as M = (N, L), where N is a set of nodes and L
 is a set of coordinates on the plane denoting the locations
 of the nodes.

 Definition 4: The least-power function gives the
 minimum power needed to communicate a distance of d.

 Definition 6: Problem Connected MinMax Power (CMP).
 Given an M = (N, L), and a least-power function   find a
 per-node minimal assignment of transmit powers
 such that the induced graph of (M, p) is connected, and
                 is a minimum.
PROBLEM STATEMENT (Contd.)
 Definition 7: Problem Biconnectivity Augmentation with
 MinMax Power (BAMP). Given a multihop wireless net M =
 (N, L), a least-power function     and an initial assignment
 of transmit powers                   such that the induced
 graph of (M,     p) is connected, find a pernode minimal set
 of power increases         such that the induced graph of
                     is biconnected, and
                        is a minimum.
STATIC NETWORKS: OPTIMUM
CENTRALIZED ALGORITHMS




                             s-p

               step number          power assigned
                             d(s)

                distance            step number
Algorithm CONNECT (Contd.)

                                                 side-effect edge




 A side effect edge may form a loop with other edges and
 may allow the lowering of some power levels and the
 elimination of some edges added previously.
Per Node Minimalize
 reduction of power of A and B to 1 again.




                                      A      B
Separation Edges and Vertices
 Definitions
    Let G be a connected graph
    A separation edge of G is an edge whose removal disconnects G
    A separation vertex of G is a vertex whose removal disconnects G
 Applications
    Separation edges and vertices represent single points of failure in a
    network and are critical to the operation of the network
 Example
    3, 5 and 6 are separation vertices
    (3,5) is a separation edge

                                         4
                  1                                       7

                                                6

      2                3                 5                8
Biconnected Graph
 Equivalent definitions of a bi-connected graph G
    Graph G has no separation edges and no separation vertices
    For any two vertices u and v of G, there are two disjoint
    simple paths between u and v (i.e., two simple paths between
    u and v that share no other vertices or edges)
    For any two vertices u and v of G, there is a simple cycle
    containing u and v
 Example


                                   4
                1                                7

                                         6

     2              3             5              8
Algorithm BICONN-AUGMENT
                Identify the bi-connected
                components in the graph
                induced by the power
                assignment from
                algorithm CONNET
                This is done using method
                based on depth-first
                search
                Node pairs are selected in
                non-decreasing order of
                their mutual distance and
                joined only if they are in
                different bi- connected
                components
                This is continued until the
                network is biconnectd.
Implementation
 40 nodes spread out with a density of 2 nodes/sq mile
MOBILE NETWORKS :
DISTRIBUTED HEURISTICS
 The topology is continually changing
   Solution: continually readjust the transmit powers of
   the nodes to maintain the desired topology.
      The solution must use only local or already available
      information. Eg. Positions

 Centralized solutions not available in a mobile
 context.

 Present two distributed heuristics
   Local Information No Topology (LINT)
   Local Information Link-State Topology (LILT)

 Zero overhead protocols; they do not use any
 special control messages for their operation
LINT
 Uses locally available information collected by a routing
 protocol
 Attempt to keep degree of each node bounded.
 if d(Ni)>dh
      reduce transmit power
 if d(Ni)<dl
      increase transmit power
 dh High threshold on the node degree
 dl Low threshold on the node degree
 dd desired degree, dc current degree
 Gamma is factor of propagation loss function based on
 environment, between 2 and 5 , set 4 in emulations
 New power
LILT
 significant shortcomings of LINT
    Unaware of network connectivity
    Danger of a network partitioning

 LILT uses global information available in locally to recognize and
 repair network partitions

 Two main parts
    Neighbor reduction protocol (NRP)
        LINT mechanism
    Neighbor addition protocol (NAP)
        Triggered whenever an event driven or periodic link-state updates arrives

 The purpose triggering is to override the high threshold bounds
 and increase the power if the topology change indicated by the
 routing update results in undesirable connectivity.
Experiment
 Emulation by a prototype of Rooftop communication nodes
 40 nodes
 3 min of simulation
 In mobile scenarios, random movement with speed of 72
 miles/hour
 256 bytes packet
 12 stream
 Arrival rate of 4Kbps per stream
EXPERIMENTAL RESULTS- static




                  AVG




  BICONN better   BICONN uses more power
   EXPERIMENTAL RESULTS -mobile




-LINT is better. Not an ideal scenario
- Density 1 is where the network is connected   No significant changes
and increase in density cause drop in           Delay is calculated only for
throughput
                                                successful transferred
-Bkz of hidden terminal and inaccurate link     messages
state information LILT start to downgrade
Thanks



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posted:9/17/2011
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