lectue 10 : an overview of position based routing in ad hoc networks

Mobile and Wireless Computing

Institute for Computer Science, University of Freiburg

Western Australian Interactive Virtual Environments Centre (IVEC)

Lectue 10 : An Overview of Position Based Routing in Ad Hoc Networks

Lecture 10 : We will review several position based routing strategies in this lecture.




Position Unaware Routing

All the routing protocols that we have discussed so far can be called position unaware routing.

Each node has an address and each node keeps track of the neighbours´ addresses.

However, when a message has to be sent from a source to a destination, the source node is not aware of the current position of the destination.




Position Aware Routing

In contrast, in position aware routing, a source node knows the position or location of the destination node.

The position information can be collected in different ways : – From the direction and strength of the received

wireless signals– Through interfacing with a low-power Global

Positioning System (GPS) and a satellite updating the positions of the nodes by sending signals to this GPS device.




Position Aware Routing

There is a large body of literature dealing with position aware routing in ad hoc mobile networks.

The purpose of this lecture is to give an overview of different position aware routing algorithms.

We will discuss only the basic strategies used in these algorithms rather than the details of the routing schemes.




Position Based Routing Schemes

There are four main categories of position based routing schemes : – Basic distance, progress and direction based

methods,– Partial flooding and multi-path based path strategies,– Depth first search based routing with guaranteed

delivery– Nearly stateless routing with guaranteed delivery




Basic Distance, Progress and Direction Based Methods

The notion of progress is the key concept of several GPS based methods.

Given a transmitting node S, the progress of a node is determined depending on its projection on the line connecting S to the destination.

A

S

dest

B

A has the highest positive progress and B has negative progress




Random Progress Method

In the random progress method, packets towards the destination are routed with equal probability towards one intermediate neighbouring node that has positive progress.

The idea behind this method is that, if all nodes are sending packets frequently, the probability of collision grows with the distance between nodes.

Thus there is a trade-off between progress and transmission success.




Progress Methods

In the most forward within radius (MFR) routing algorithm, a packet is sent to the neighbour with the greatest progress.

MFR can be proven to be loop-free. In other variations of the MFR algorithm, the

packet is sent to the nearest node with positive progress.

In the greedy scheme, the paket is sent to the closest known node to the destination.




Greedy Scheme

When none of the known nodes is closer to the destination compared to the current node, the algorithm floods the network until a node closer to the destination is found.

The algorithm does not guarantee delivery, nor optimize flooding rate.

In a variant of the greedy strategy called GEDIR, the message is dropped by a node when the best option is to send back the packet to the node from where it came.




Other Greedy Methods

In the alternate greedy method, the i-th received copy of the message is forwarded to the i-th neighbour.

In the disjoint greedy method, each node will forward the message to its best neighbour who never received this message.

These methods increase message delivery rate compared to the original greedy method.

However, most of these methods are not loop-free.




Partial Flooding and Multi-Path Based Strategies

In directional flooding based routing methods, a node n transmits a message m to several neighbours whose direction is closest to the direction of destination D.

In order to control flooding, flooding based methods require nodes to remember past traffic, to avoid forwarding same message more than once.

There are several strategies in this class.





Flooding can be partial because it is directed towards nodes in a limited sector of the network.

Also, flooding can be stopped after a certain number of hops.

Moreover, partial flooding can be used only for path discovery or for packet forwarding.

In the DREAM protocol, a message is forwarded by node n to all neighbours within a selected range.





The range is determined by the tangents from node n to a circle centered at the destination and with radius equal to a maximal possible movement of the destiation since the last location update.

In the location aided routing strategy, the request zone is the area containing the circle and two tangents from the original source of the message. Nodes outside the request zone do not forward route requests.




Geometry Based Partial Flooding

Methods like V-GEDIR and CH-MFR provide loop-free routing and reduces flooding using geometric techniques.

A message m is forwarded to exactly those neighbours that are the best choices according to progress or distance criteria.

In V-GEDIR method, these neighbours are determined by intersecting the Voronoi diagram of neighbours with a circle representing the possible positions of the destination.




Partial Flooding Methods

In CH-MFR, the convex hull of neighbouring nodes is used as a region for message forwarding.

In the multi-path method, the source node S forwards the message to c best neighbours according to distance from the destination.

Each of the c copies afterwards follows the greedy, alternate or disjoint methods.

Experiments indicate significant gain in delivery rate for c=2, but less gains for higher values of c.




Terminode Routing

Terminode routing takes a different approach. This routing scheme is a combination of two

protocols called Terminode Local Routing (TLR) and Terminode Remote Routing (TRR).

TLR is a mechanism that is used for reaching destinations in the vicinity of a terminode and does not use location information for packet forwarding.




Terminode Routing

TRR is used to send packets to remote destinations and uses geographic location information.

The main novelty is the Anchored geodesic packet forwarding (AGPF) approach taken in TRR.

Instead of using traditional source paths (i.e., a list of nodes), it uses anchored paths.

An anchored path is a list of fixed geographical points called anchors.




Terminode Routing

AP1 AP2

S D

A B

From S, the message is sent in the direction of the closest anchor point.

When terminode A detects that it is close to anchor point AP1, It forwards the packet towards the next anchor point AP2. Finally B sends the message to D through TLR routing.




Depth First Search Based Routing with Guaranteed Delivery

Several DFS based strategies have been proposed for routing in ad hoc mobile networks and they are similar to a greedy strategy with disjoint routing.

Each node along the path from S to D forwards the message to a node that minimizes the distance to D.

Also, the message is forwarded by a node only once unless the message is stuck.




Nearly Stateless Routing

In nearly stateless routing, nodes maintain only some local information to perform routing.

All routing decisions are based on : – Location of neighbouring nodes– Location of the destination– The position of the neighbouring node that forwarded

the message in the previous step

A Gabriel graph is a spanning subgraph of the original network. It is constructed as follows.




Gabriel Graph

Given any two adjacent nodes U and V in the network, the edge (U,V) belongs to the Gabriel Graph (GG) if and only if no other node W of the network is located in the disk with (U,V) as its diameter.

The construction of the GG is completely localized and does not require any other information except for the positions of the neighbours.




Gabriel Graph

U V P Q

W

GG is planar and hence no two edges intersect.

The intersection of GG and the network is connected since both of them contain a spanning tree as a subgraph.




Routing Using Gabriel Graph

The routing strategy is a combination of greedy strategy and recovery mode traversal.

The greedy strategy is followed as long as there is progress in terms of reaching intermediate nodes closer to the destination.

The recovery mode is used when it is impossible to make progress with the greedy strategy.

The recovery mode traversal usually consists of traversing the faces of the Gabriel Graph. Whenever there is a node with progress, the algorithm switches over to the greedy mode.




An Example

SDEST

A

B

CK

M




Other Issues in Ad Hoc Mobile Networks

There are many other important issues related to routing in ad hoc mobile networks.

Power-aware routing, QoS support, multicasting, support for multimedia in ad hoc networks are some of these issues that we have not covered in this course.

Also, there are many specific routing problems for static wireless networks like packet radio networks and sensor networks.

lectue 10 : an overview of position based routing in ad hoc networks

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