Geocasting in Mobile Ad Hoc Networks: LocationBased Multicast Algorithms
Young-Bae Ko and Nitin H. Vaidya
LOGO
2016/7/15
授課老師：陳定宏 老師
指導老師：許子衡 老師
姓名：羅英辰
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學號：M97G0216
Introduction
When an application must send the same
information to more than one destination,
multicasting is often used, because it is much more
advantageous than multiple unicasts in terms of the
communication costs.
We consider a different approach, namely,
geocasting.
A geocast is delivered to the set of nodes within a
specified geographical area.
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Unlike the traditional multicast schemes, the
multicast group is implicitly defined as the set of
nodes within a specified area.
We will refer to the specified area as the “multicast
region”, and the set of nodes in the multicast region
as the location-based multicast group.
In wireless ad hoc environments, two approaches can
be used for multicasting:
 multicast flooding
 multicast treebased approach.
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Location-Based Multicast Protocols
 Two approaches may be used to implement locationbased multicast:
1. Maintain a multicast tree, such that all nodes
within the multicast region at any time belong to
the multicast tree. The tree would need to be
updated whenever nodes enter or leave the
multicast region.
2. Do not maintain a multicast tree. The multicast
may be performed using some sort of “flooding”
scheme.
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Multicast Flooding
 The flooding algorithm can be used to deliver
packets to nodes within a location-based multicast
group.
 The multicast flooding algorithm can be
implemented as Figure 1:
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Preliminaries
 The proposed approach is termed location-based
multicast, as it makes use of location-based
multicast groups and utilizes location information
to reduce multicast delivery overhead.
 Multicast Region and Forwarding Zone
 Multicast Region:
• Consider a node S that needs to multicast a message to
all nodes that are currently located within a certain
geographical region. We call this specific area as
“Multicast Region”.
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• The multicast region would be represented by some
closed polygon such as a circle or a rectangle (see
Figure 2).
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• Accuracy of multicast delivery can be defined as ratio
of the number of group members that actually receive
the multicast packet, and the number of group members
which were in the multicast region at the time when the
multicast is initiated.
 Forwarding Zone:
• The proposed location-based multicast algorithms use
multicast flooding with one modification.
• Node S defines (implicitly or explicitly) a “Forwarding
Zone” for the multicast data packet. A node forwards
the multicast packet only if it belongs to the forwarding
zone
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• To increase the probability that a data packet will reach
all members in the multicast group, the forwarding zone
should include the multicast region.
• Additional region must be included in the forwarding
zone, so that node S and other nodes in the multicast
region both belong to the forwarding zone.
• Accuracy of the protocol can be increased by increasing
the size of the forwarding zone
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Determining Membership of the Forwarding Zone
 Implementing location-based multicast schemes
requires that a node be able to determine if it is in
the forwarding zone for a particular multicast
packet two algorithms presented here differ in the
manner in which this determination is made.
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 Location-Based Multicast Scheme 1
• Our first scheme uses a forwarding zone that is
rectangular in shape (refer to Figure 4).
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
Size of the forwarding zone:
•
Note that the size of a rectangular forwarding zone
above is dependent on
1. Size of the multicast region
2. Location of the sender
•
When δ is positive, the rectangular forwarding zone is
extended in positive and negative X and Y directions
by δ.
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 Location-Based Multicast Scheme 2
• Without including the forwarding zone explicitly, node
S includes three pieces of information with its multicast
packet:
– The multicast region specification.
– The location of the geometrical center, (Xc,Yc), of the multicast
region. Distance of any node Z from (Xc,Yc) will be denoted as
DISTs in the rest of this discussion.
– The coordinates of sender S, (Xs,Ys).
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 Node I calculates its distance from location
(Xc,Yc) , denoted as DISTs, and:
• If DISTs + δ≧ DISTi, then node I forwards the packet to
its neighbors. Before forwarding the multicast packet,
node I replaces the (Xs,Ys) coordinates received in the
multicast packet by its own coordinates (Xi,Yi).
• Else DISTs + δ< DISTi. If S is in the multicast region,
then node I forwards the packet to its neighbors.
Otherwise, I discards the packet.
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Performance Evaluation
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 Simulation Model
• Number of nodes in the network was chosen to be 30.
• The nodes in the mobile ad hoc network are confined to
a 1000 unit x 1000 unit square region.
• Each node moves with an average speed ν .
• The actual speed is uniformly distributed in the range να and ν+α units/second, where, we use α = 2.5.
• Each node makes several “moves” during the simulation.
• During a given move, a node travels distance d, where is
exponentially distributed with mean 20.
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• If during a move, a node “hits” a wall ofthe 1000x1000
region, the node bounces and continues to move after
reflection, for the remaining portion of distance.
 Simulation Results
• We measure two parameters:
– Accuracy of multicast delivery
– Total number of multicast packets received by nodes per
multicast:
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Optimizations of Location-Based
Multicast
Forwarding Zone Adaptation
 By adapting the forwarding zone at any
intermediate node I, the performance of the
scheme 1 can be improved.
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Use of Directed Antennas
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Conclusion
This paper focuses on the problem of geocasting
broadcasting to every node in a specified
geographical area in mobile ad hoc environments.
The specified geographical area is called the multicast
region, and the set of nodes that reside within the
specified multicast region is called a location-based
multicast group
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