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82256202سيد عليرضا كارداني 82206906مجتبي اميرخاني

Path Set Selection in Mobile Ad Hoc Networks

1382زمستان

Contents

• Introduction

• Routing with Quality of Service

• Single path versus Multi path

• DPSP protocols

Introduction

• The nature of ad hoc network involves many different constraints occurring in different layer of the network (MAC layer, network layer, etc.)

• Different problems can be met :– Unpredictable link properties– Node mobility– Limited battery life– Hidden and Exposed terminal problem– Route maintenance– Security

Routing with Quality of Service

• Quality of Service (QoS) : « QoS is usually defined as a set of service requirements

that need to be met by the network while transporting a packet stream from a source to its destination »

Routing with Quality of Service

• Different metrics to consider QoS :– Delay– Bandwidth– Probability of packet lost– Delay variance– Etc.

• All applications don’t need the same QoS!

Single path versus Multi path

• Why should we use Multi-Path protocols?– Several paths can collectively ensure the QoS

– To use at best the network resources

– To improve network reliability

• Which set of paths must be selected?– Shortest paths (simple solution)

– DPSP (Disjoint Pathset Selection Protocol)

A simple solution

• shortest paths are found one after the other, and are removed from graph.

• This solution does not work well in practice

• we wish to find as many disjoint paths as possible that are as reliable as possible

An example

Simple algorithm find only one path We can find two disjoint path which are more reliable

DPSP : presentation

• The goal of this protocol : to propose a set of highly reliable disjoint paths (D)

• To perform this goal, it uses a path reliability metric which guides the route selection process

• Each node continuously monitors the reliability of each of its incident links

• This approach is independent of the routing protocol

DPSP - Path Selection

• The network is modeled as probabilistic graph Gp = (V, E) where each vertex, edge or arc has a weight p, its operating probability (and therefore, its failure probability (q = 1 – p) )

• For a source node s and a destination t, t s, Rel s t (GP ) denotes the probability that there exists at least one path connecting s and t over GP

DPSP - Path Selection

• Four steps :1. A search for the more reliable path between s and t

2. A decision to know if this path improves the path set reliability or no

3. A means of adding path to the path set

4. An update of the basic graph

• These steps are followed until no new path can be discovered

Path selection - first step

• Forward arc : edge that belongs to one path of D and ‘follows’ the direction from s to t

• Backward arc : edge that belongs to one path of D and ‘follows’ the direction from t to s

• A shortest path algorithm (SP) is use to find the different paths

• The backward arcs can temporarily be used to define a path. Hence, interlacing can occur.

Path selection - definition

• a weight is assigned to each arc.

• for an s t path Pi, with (vi, vj) the arcs that belong to Pi,

• The path set operate with probability define as:

Path selection - definition (cont)

• We also define the constant

• We used this constant to change the weight of arcs.

Path selection - second step

• Two metrics are provided to determinate when it is suitable to include a new path or not

• They compute the reliability of the path set before and after the new path is added

• The reliability of a path can be seen as the product of its different links reliabilities

Path selection - third step

• If the new path is not interlacing with another path, the protocol simply adds it to the set (without metrics computation)

• If the overall reliability of the path set is not improved, it drops the path and the weights of the links are changed in such way that they can not be used by SP after this

Path selection - fourth step

• When a path is added, all forward arcs weights are changed to a high positive value (Cp) and backward arcs weights to a negative value (Cn) (so SP can use them to find new interlacing path)

• If the interlacing is removed, the weights of the backward arcs that belong to the interlacing are changed back to their original value

An Example of DPSP Operation

• we assume only three different link reliability values:

– pe = 0.9 for links shown as thick dashed lines,– pe = 0.95 (thick solid lines) and– pe = 0.7 for the normal lines.

Operations for example

• First path find : P1 = {1,2,4,5,8,9,11}• The second path : P2 = {1,4,8,11} . No interlaces between P1 , P2

• The third path : P3 = {1,3,7,8,5,4,6,10,11} . Interlaces with P1 {(8,5),(5,4)} and would result npath1 ={1,2,4,6,10,11} and npath2 ={1,3,7,8,9,11} . metric1 > metric2 (actual values: 0.7632 > 0.5233) and reject.

• The fourth path : P4 = {1,3,7,8,4,6,10,11} . Interlaces with P2 {(8,4)} and would result npath1 ={1, 4,6,10,11} and npath2 ={1,3,7,8,11} . metric1 > metric2 (actual values: 0.7577 > 0.5221) and reject.

• Two disjoint path find {1,2,4,5,8,9,11} , {1,4,8,11}

Operations for example

• If P(4,8) = 0.5 then three disjoint path find {1,2,4,5,8,9,11} , {1,4,6,10,11} and {1,3,7,8,11}.

DPSP OPTIMIZATION

• worst-case computational complexity, is still polynomial

• Optimization :– If the improvement in the reliability of the path set

drops below a given threshold, the method can conclude.

– The threshold value can be a design parameter regulating a trade-off between path set reliability and computational overhead.

EVALUATION

• generate random multihop topologies for 100 nodes distributed in an area of 1000 m. x 1000 m.

• assign random reliability values to links from a normal distribution with mean pe ={0.25, 0.5, 0.85, 0.97} and standard deviation 0.0707

• For each setting, 1000 Monte Carlo iterations are performed.

References

• P. Papadimitratos, Z. J. Haas, E.G. Sirier,

«Path Set Selection in mobile ad hoc network» in Proc. MOBIHOC’02 Copyright 2002 ACM

• P. Mohapatra, J. Li and C. Gui,

«QoS in mobile ad hoc network»

IEEE Wireless Communicatons 10 (2003)