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Location-Aided Routing _LAR_

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					Location-Aided Routing (LAR)

       Dr. R. B. Patel


                               1
                Organization
• Location-Aided Routing (LAR) protocol
  –   Route Discovery using Flooding
  –   Location information
  –   Expected Zone and Request Zone
  –   LAR Scheme 1
  –   LAR Scheme 2
  –   Error in location estimate
• Variations and Optimizations
                                          2
                        Basics
Mobile Ad hoc Network(MANET)
• Node mobility which is the cause of frequent and
  unpredictable topology changes leads to difficult task of
  route maintenance in MANET
• Many protocols have been proposed for MANET to
  achieve efficient routing
• An approach to decrease overhead of route discovery by
  utilizing location information (GPS or other method to
  obtain location information)
• Two LAR protocols for route discovery presented in this
  lecture uses location information(may be out of date) to
  limit search space which results in fewer route discovery
  messages                                              3
   Route Discovery Using Flooding
• The basic flooding algorithm and location-aided route
  discovery based on limited “flooding”
Basic Flooding Algorithm
   – A source node S needs to find a route to destination node D,
     node S broadcasts a route request to all its neighbors
   – Intermediate node X receives a route request and compares the
     destination with its own identity
   – If it does not match, then node X broadcast the request to its
     neighbors(sequence numbers used to detect duplicate and
     eliminate/avoid redundant transmissions)
   – Node D responds by route reply messages to sender which
     traverse the path in reverse of the path received by D (route
     request packet contains path of all nodes traversed starting S)
                                                                  4
   Route Discovery Using Flooding
Basic Flooding Algorithm
  – Timeout scheme is also used to re-initiate route request with
    new sequence number due to transmission error or node D is
    unreachable from S




                                                               5
   Route Discovery Using Flooding
• It is assumed that node S can know that route is
  broken only if it attempts to use the route by
  sending data packet and receiving route error
  messages – it initiates route discovery for D
• Note that route request may reach every node in
  the network that is reachable from S (potentially
  all nodes in the MANET)
• Using location information reduces the number of
  nodes to whom route request is propagated (limit
  the scope of route request propagation)
                                               6
             Location Information
• Location information can be obtained by the use of
  Global Positioning System (GPS)
• With use of GPS, mobile host can know its physical
  location – note that GPS includes some degree of error
  compared to the real coordinates and GPS-calculated
   – NAVSTAR GPS has positional accuracy of 50-100 meters
   – Differential GPS has positional accuracy of few meters
• Each node knows its current location precisely –
  possibility of error in location are discussed separately in
  the performance evaluation
• Also assume that the mobile nodes are moving in a two-
  dimensional plane
                                                              7
                   Expected Zone
• The Expected Zone is the region where source node S
  thinks that the destination node D may contained at some
  time t – only an estimate made by S
   – Assume that node S knows that the node D was at location L at
     time t0 and current time is t1
   – From the viewpoint of S, expected zone of node D is the region
     that node S expects to contain node D at time t1 based on the
     knowledge that node D was at location L at time t0
• If S knows that D travels with average speed v, then S
  assumes that the expected zone is the circular region of
  radius v(t1- t0) centered at location L
• Note that if actual speed is faster than the average, then
  the destination may be outside the expected zone at t1
                                                              8
                 Expected Zone
• Without the previous knowledge of the location of D, S
  will assume that the entire region is the expected zone
  and implementation uses the basic flooding algorithm
• The size of expected zone can be reduced if node has
  more information about the mobility of a destination D




                                                       9
                          Request Zone
• Node S defines (implicitly or explicitly) a request zone for the route
  request
•   Node forwards a route request only if it belongs to the request zone (it
    does not forward a route request to its neighbor if outside of the request
    zone)
•   Two LAR scheme differ in determining the membership of request zone
•    The request zone includes expected zone in addition to (possibly) other
    surrounding zone around the request zone
•    If a route is not discovered within the timeout period, S initiates a new
    route discovery with expanded request zone – all paths from S to D
    include nodes that are outside the request zone
•    Note that the probability of finding path can increase as size of request
    zone increases (route discovery overhead also increases with the size of
    the request zone
                                                                          10
Request Zone




               11
                     LAR Scheme 1
• The request zone is rectangular in shape
• Assume S knows that the node D was at location (Xd,Yd) at time
  t0
• Assume S knows the average speed v with which D can move
• From above two, S defines the expected zone at time t1 with
  radius R = v(t1- t0) centered at location (Xd,Yd)
• The request zone is the smallest rectangle that includes current
  location S and the expected zone such that the sides of the
  rectangle are parallel to the X and Y axes
• Node D sends route reply message with its current location and
  time (may include average speed but simulation assumes all
  nodes knows each other’s average speed)                   12
LAR Scheme 1




               13
LAR Scheme 1




               14
                     LAR Scheme 1
• Size of the request Zone is proportional to
  (i)average speed of movement v and
  (ii)elapsed time since recorded last location of the destination
• Recall that R = v(t1- t0) is used to determine the size of request
  zone
• In general, a smaller request zone may be formed at speed that
  are neither too small nor too large
• For instance, at low speed, factor (i) is small but route discovery
  occur after long intervals making (ii) larger (t1- t0 is large)

                                                               15
                       LAR Scheme 2
• Node S includes two pieces of information with its route request
   – Assume that S knows the location (Xd,Yd) of D at some time t0 which
     route discovery is initiated by S at t1 where t1  to
   – S calculates its distance from location (Xd,Yd) denoted DISTs and
     included with the route request
   – The coordinate (Xd,Yd) are also included with the route request
• When node I receives the route request from S, node I calculates
  its distance from (Xd,Yd) denoted DISTi and:
   – For some parameter , if DISTs +   DISTi, then I forwards request to
     its neighbors – this request includes (Xd,Yd) and DISTi replacing original
     DISTs and (Xd,Yd) from S
   – Else DISTs +   DISTi, node I discards the route request
• Each intermediate nodes repeat the process above                       16
Comparison of the two LAR Schemes




                               17
Comparison of the two LAR Schemes




                               18
      Error in Location Estimate
• Both LAR schemes assume that each node knows its own
  location accurately. However, just like GPS, there may be
  some error in the estimated location
• Let e (location error) denote maximum error in the coordinates
  estimated by a node
• If a node N believes that it is at location (Xn,Yn), then the actual
  location of node N may be anywhere in the circle of radius e
  centered at (Xn,Yn)
• If LAR Scheme 1 is modified to take e into account, then the
  expected zone is a circle of radius e + v (Xn,Yn) which makes
  request zone larger since it includes larger expected zone
• No modification is made to the LAR Scheme 2
                                                                19
         Variations and Optimizations
• Accuracy of a request zone can be improved by adapting the request
zone determined by the source node S initially
• Idea is to use location information at some intermediate node which
may know more recent location for destination node than the source node
(assuming source information is out of date compare to intermediate
node)
• Thus, using this up-to-date information at the intermediate node with its
expected zone and adopting the request zone of source node
• In Scheme 2, intermediate node may calculate distance from the more
recent location of destination D, and use this distance in making decision
whether or not to discard a route request

                                                                   20
Continued…




             21
      Location-Aided Routing
• Advantages
  – reduces the scope of route request flood
  – reduces overhead of route discovery


• Disadvantages
  – Nodes need to know their physical locations
  – Does not take into account possible existence of
    obstructions for radio transmissions
                                                  22

				
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posted:8/9/2012
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Description: mobile computing