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A Position-based Deployment and Routing Approach for

Directional Wireless Mesh Networks

Speaker: Weisheng Si

Supervisors: Dr. S. Selvakennedy

Prof. A. Zomaya

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Outline

• Background

• Overview of our work

• Literature survey and Uniqueness of our work

• Theoretical Background

• The PDT generation algorithm

• The BE-GF protocol

• Conclusions and future work

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Background

• What is wireless mesh networks (WMNs)?

• Two developing trends in WMNs

• Architecture of Directional WMNs

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WMNs

• Provide the backhaul connection for WLANs• Perform routing for the backhaul.

A WMN is a set of mesh routers that communicate with each other via wireless links and form a mesh topology. The basic functionalities of these mesh routers are:

WLAN WLAN WLANWLAN

MR

INTERNET

MR MR

Mesh Router

Mesh Router

AP & Mesh Router

wired connection

wireless connectionMR MR

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WMNs (cont’d)

• gateways: also interface with the Internet.• APs: also serve as Access Points (APs) for WLANs.• pure mesh routers: only have the above basic

functionalities.

Based on their additional functionalities, the mesh routers can be classified into three categories:

My research mainly focuses on the basic functionalities of the mesh routers. For brevity, the mesh routers are also referred to as “nodes” hereafter.

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Two developing trends in WMNs

• Multiple radios and multiple channels: – Each node is equipped with multiple radios, each of

which uses a distinct channel– By enabling multiple channels to carry network traffic

simultaneously, this trend multiplies the available bandwidth for network nodes.

• Directional antennas: – Used in the backhaul connections, enabling the point-

to-point communication.– The interference among links sharing identical

channels is eliminated.

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Architecture of Directional WMNs

• We call such WMNs with multiple radios/channels and point-to-point backhaul links the Directional WMNs (DWMNs). • Our research is targeted at the DWMNs.

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Overview of our work

• A strategy of position-based joint deployment and routing for DWMNs proposed

• A concrete approach under this strategy given

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The position-based deployment and routing strategy

• Main philosophy:– Perform whatever can be planned to simplify the

operation of WMNs, so as to achieve efficiency and scalability.

– A lesson from the Internet is the Simplicity Principle: Complexity is the primary aspect that impedes the scalability and increases the expenditures of the networks.

• Main idea:– First, deploy the DWMNs with certain kind of geometric

graph as the network topology– Then, design a position-based routing protocol by

exploiting this graph’s routing properties.

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Justifications for the proposed strategy

• Possible:– The network topologies of many WMNs can be planned by the

deployers, making this joint deployment and routing strategy possible.

• Localized:– Position-based routing protocols are localized protocols in that the

routing decision is solely based on a constant amount of information stored in the packets and the positions of the current forwarding node, its neighbors and the destination. Thus, position-based routing protocols are highly efficient and scalable.

• Dynamic:– WMNs have significant network dynamics such as link failure and

congestion, making the static routing protocols unsuitable. However, position-based routing protocols can easily handle such network dynamics.

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Justifications for the strategy (cont’d)

• Making maintenance of WMNs easy:– This localized property also significantly eases the maintenance of

WMNs, since the reconfiguration to one part of the network does not need to be notified to the other parts.

• Low overhead:– The periodical exchange of positions among them is obviated. In

contrast, such overhead is considerable when the position-based routing protocols is used in mobile wireless networks.

• Practical:– With the interference among backhaul links eliminated, the metrics

related to positions become practical for making routing decisions. Otherwise, the interference-aware routing metrics (e.g., WCETT and MGF) seem more promising.

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The concrete approach

• The PDT Generation Algorithm: we use Delaunay triangulations (DT) as the basis to generate the network topologies. Specifically, this algorithm produces first refined and then pruned DTs (called PDTs by us). The PDTs have the advantages of (1) being practical for the deployment of DWMNs and (2) retaining good routing properties of the complete DTs.

• The Backward-Enabled Greedy Forwarding protocol (BE-GF): this protocol extends the well-known greedy forwarding algorithm by enabling the delivery of a packet farther from the destination at certain hops without causing loops. BE-GF has the advantages of (1) being localized and (2) considering the network dynamics of link failure and congestion.

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Literature survey and uniqueness of our work

• To our best knowledge, ours is the first joint deployment and routing approach based on the positions for DWMNs.

• Next, the related work and our uniqueness in two areas are discussed: – Node Deployment– Position-Based Routing

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Node Deployment

• WMN Deployment

• Wireless Sensor Network (WSN) Deployment

• Cellular Base Station (BS) Planning

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WMN Deployment

• With WMN as a new research area, we have not seen a concrete approach proposed for the WMN deployment in community up to now.

• We only find an InfoCom07 paper that conducts a performance study on the influence of the following factors on the WMN deployment. This study is done by Monte Carlo simulations.– Network Capacity– Connectivity– Node Density– Installation Cost

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WSN Deployment

• Main factors to consider– Coverage– Event Detection– Connectivity– Cost

• Compared with coverage and connectivity, routing is not an important factor for deployment

• Mostly use a Poisson or grid deployment strategy

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Cellular BS Planning

• Main factors to consider– Coverage– Traffic Distribution– Signal Quality– Installation Cost

• No routing

• Mostly use a hexagonal deployment strategy

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Our Uniqueness in Deployment

• The main factors considered:– Routing: the most important functionality

provided by the WMNs

– Installation cost: a primary concern of deployers.

• A novel kind of geometric graph, PDT, is proposed as the network topology.

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Position-Based Routing

• There is a vast literature in exploiting various geometric graphs to facilitate routing in wireless networks. These graphs mainly include:– Relative neighborhood graph (RNG)– Gabriel graph (GG)– Yao graph (YG)– Delaunay triangulation

• The wireless network environment is modeled by the unit disk graph (UDG)– All network nodes use omni-directional antennas with an identical

transmission range– Two nodes have a link between them if their distance is no more

than the transmission range. – There exists significant interference among links that are near

each other, making the routing protocols solely based on position information not appealing.

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Position-Based Routing (cont’d)

• The network topology needs to be maintained by exchanging control messages among the nodes, which adds significant overhead to the network.

• In forwarding a packet to the destination, if the forwarding node does not have a neighbor closer to the destination than itself, face routing is used to guarantee the delivery.

• Face routing can guarantee the delivery in static networks, it cannot do this in dynamic networks where link failures make the forwarding loops possible.

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Our Uniqueness in Position-Based Routing

• The UDG model is replaced with the point-to-point link model due to the use of directional antennas, thus the interference from nearby links is avoided.

• With the network topology planned in advance, the overhead of maintaining network topology is obviated.

• When the forwarding node does not have a neighbor closer to the destination, a simple technique of routing backward under certain conditions is used to increase the delivery ratio. This technique has the advantage that no forwarding loops can occur under network dynamics.

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Theoretical Background

• The greedy forwarding (GF) algorithm

• The Delaunay triangulation (DT) graph

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The GF algorithm

• A position-based routing algorithm• Finds a path from a source node s to a destination

node t in the following method:– At each node (say u) along the path to t, u chooses the

neighbor v that has the smallest d(v, t) as the next hop, where d(x, y) denotes the direct Euclidean distance between node x and y

– Ties are broken arbitrarily.

• Characterized by simplicity– Localized– Makes the routing decision simply by one search of the

neighbor list.

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The DT graph

Definition: No node lies inside the circumcircle of any triangle. Also, the dual of the Voronoi diagram.

Definition: No node lies inside the circumcircle of any triangle. Also, the dual of the Voronoi diagram.

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The appealing routing properties of DT

• Let n denotes the number of nodes, e the number of edges, k the number of convex hull edges, we have e = 3n – 3 – k. This implies that in a DT, the average node degree is slightly less than 6 and hence bounded.

• For any node u to any destination t, there always exists a neighbor v of u satisfying that d(v, t) < d(u, t). Hereafter, we call this property backward-free. Since a DT is backward-free, GF can always find a path between any two nodes. Besides, we also say that u routes a packet backward, if u routes this packet to its neighbor v with d(v, t) d(u, t).

• The length of the shortest path between any two nodes u and v is guaranteed to be less than c·d(u, v), where c is proved a constant.

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The PDT Generation Algorithm

• Problem formulation

• Algorithm description

• Algorithm evaluation

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Problem formulation

• The positions of the AP nodes are essentially given. • A deployer can decide:

– The positions of gateways and pure mesh routers– The existence of links between any pair of nodes

• The angle between any two links incident on a node should be no less than a threshold value θ_min, so as to fully avoid the inter-channel interference. Hereafter, an angle less than θ_min is referred to as a bad angle.

• The length of a point-to-point link cannot exceed a threshold value l_max. Hereafter, a link longer than l_max is referred to as a bad link.

We make the following assumptions on the DWMNs:

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Problem formulation (cont’d)

The DTAP of given 39 AP nodes

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Problem formulation (cont’d)

• T has no bad angles or bad links. • T is backward-free.

• The number of pure mesh routers added is as small as

possible.

Given a set of AP nodes and the thresholds θ_min and l_max, find a graph T as the network topology by adding pure mesh routers into the DTAP and recalculating the triangulation, such that:

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Algorithm description

• DT construction: construct the DTAP of the given AP nodes.

• DT refinement: add pure mesh routers to remove bad angles and links in the interior of the DTAP, producing a refined DT (denoted DTR hereafter) that has bad angles and links only near the boundary.

• DT pruning: remove the remaining bad angles and links in DTR by eliminating certain related edges, while guaranteeing that the resulting graph (called PDT) is still backward-free.

Our PDT generation algorithm has the following three stages:

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Algorithm description — stage 1

The DT construction DTAP

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Algorithm description — stage 2

The DT refinement DTR

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Algorithm description — stage 3

The DT pruning PDT

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Algorithm description — the DT pruning algorithm1. // Initialization

graph T = DTR; v = an arbitrary node on the boundary of T;2. // Traversing each node on the boundary of T clockwisedo { // examining triangles at node v for (each triangle with v as a vertex) { // to keep the triangulation structure during the // examination, only mark for deletion here if (it is a bad triangle) mark the edge opposite to its right or obtuse angle, the longest edge in this triangle, for deletion; } T = T – those marked edges; // advance v in clockwise order v = next node on the boundary of T;}until (no bad triangles are found on a boundary node)3. Output T as the final PDT;

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Algorithm description — properties

To justify the backward-free property of PDTs, we prove the following theorem.

Theorem 1: The PDTs obtained by our topology generation algorithm are backward-free.

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Algorithm evaluation

• Experiment setup

• The ratio of added pure mesh routers

• The ratio of pruned edges

• The path efficiency of hops

• The path efficiency of Euclidean distance

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Experiment setup

• The given AP nodes are assumed to be randomly distributed in a square area with a constant density– Note that since a DT exists for any set of points on the

plane, our PDT generation algorithm works for any kind of AP distributions.

• A minimum distance of 50m between any two AP nodes is assumed.

• Experiments are conducted on network sizes of 50, 100, 200, 400, 700, and 1000 given AP nodes respectively.

• For each network size, 200 random topologies are generated and the average result of them is calculated.

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The ratio of added pure mesh routers

Defined as the number of added pure mesh routers divided by the total number of nodes after the topology generation.

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The ratio of pruned edges

Defined as the number of pruned edges divided by the total number of edges after the topology generation.

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The path efficiency of hops

Given a routing algorithm, the path efficiency of hops for a pair of source and destination (s, t) is defined as the ratio of the minimum number of hops between s and t versus the number of hops in the path found by this algorithm from s to t.

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The path efficiency of Euclidean Distance

Given a routing algorithm, the path efficiency of Euclidean Distance for a pair of source and destination (s, t) is defined as the ratio of the distance of shortest path between s and t versus the distance of the path found by this algorithm from s to t.

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The BE-GF Protocol

• Protocol Overview

• Protocol description

• Protocol evaluation

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Protocol Overview• Based on the GF algorithm. When no network dynamics exist,

it reduces to GF and fully exploits the backward-free property of PDTs.

• Extends GF by considering two kinds of network dynamics: – link failure– link congestion

• Both kinds of network dynamics can be monitored locally by nodes, such that no exchanges of network-layer control packets are involved.

• Supports routing a packet backward to increase its delivery ratio. To prevent loops, BE-GF stipulates that a node u can route a packet with destination t backward, only when d(u, t) < d(b, t), where b is the node that routes this packet backward last time (if exists); otherwise, this packet is dropped.

• A packet only needs to remember one node (i.e., b) in its header while traversing the network.

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BE-GF versus Face routing

• Advantages:– In case of network dynamics, no forwarding loops can occur,

while face routing can incur loops.– The amount of information stored in the packet header is

significantly less than that of face routing; for instance, in GPSR, a well-known face routing protocol, five new fields are added into the packet header.

• Disadvantages: – Since packets are dropped under the aforementioned condition

d(u, t) ≥ d(b, t), they may not be delivered to the destination when there exists a path.

– Face routing performs better in this aspect, because it guarantees to find a path if the network is connected and static.

– Note that, with the PDTs being backward-free and having an average node degree of approximately six, the packets drops in BE-GF is not significant, which is verified in our simulations.

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Protocol description — packet format

• Dst Coords: the (x, y) coordinates of the destination node.

• Backward Flag (BF): this flag is used to indicate whether a packet has ever been routed backward. That is, whenever a packet is routed backward by a node, its BF is set to 1 by this node and will not change later.

• Backward Coords: This field is present only when the BF is 1. When a node sets the BF of a packet to 1, it also copies its (x, y) coordinates into this field. If a packet is routed backward by one node and later by another node, the later node’s coordinates will overwrite the previous node’s coordinates.

Required by BE-GF, three new fields are added to a packet’s network-layer header:

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Protocol description — pseudo-code1. // search the neighbor list u looks for a neighbor v with the shortest d(v, t) that satisfies: (1) link uv is up; (2) packet buffer of link uv is not full; // avoid sending P back to b (3) v is not b (if Backward Coords exists in P);2. // all neighbors are not available if (v is not found) { u drops P; return; } 3. // v is closer to t than u if (d(v, t) < d(u, t)) { u forwards P to v; return; } 4. // otherwise, check whether to route this packet backward if ( BF in P == 0 || d(v,t) < d(b,t) ) { u puts 1 into BF of P and puts its coordinates into Backward Coords of P; u forwards P to v; } else { u drops P; // BF in P == 1 and d(v,t) >= d(b,t) }

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Protocol description — properties

We prove the following two theorems regarding the loop-freeness and complexity of BE-GF.

Theorem 2: For any network topology, BE-GF is loop-free in that it either delivers a packet (denoted by P) to the destination (denoted by t) or drops P.

Theorem 3: For any network topology with constant maximum node degree, BE-GF runs with O(1) complexity in both time and space at a node.

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Protocol evaluation

• Experiment setup• Network throughput

• Packet drop ratio

• The ratio of packet drops due to no available neighbors

• Backward ratio

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Experiment setup• The BE-GF protocol is implemented in ns-2 (version ns-

allinone-2.31) • The link failures are generated using the Exponential Model

included in ns-2.– The up/down time for a link is exponentially distributed with the

mean up-interval and down-interval respectively, – Both intervals are configurable.

• Experiments are conducted for each network size with the following four scenarios:– Static: no link failures– Dyna1: for each link, up-interval = 10s, down-interval = 2s– Dyna2: for each link, up-interval = 10s, down-interval = 4s– Dyna3: for each link, up-interval = 10s, down-interval = 6s

• In each experiment, every node is found a peer randomly, and then for each pair of peering nodes, two ftp flows in opposite directions are generated.

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Network throughput

Defined as the total throughputs of all communicating node pairs in the network.

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Packet drop ratio

Defined as the total number of data packets dropped by all nodes during routing divided by total number of data packets generated by all nodes.

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The ratio of packet drops due to no available

neighbors BE-GF drops a packet in two cases: (1) no neighbors are available for

forwarding and (2) routing backward is prohibited. Defined as the number of packet drops in case (1) divided by the total

number of packet drops.

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Backward ratio

Defined as the total number of backward routing decisions made by all nodes divided by total number of routing decisions made by all nodes.

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Conclusions

• Proposed a position-based deployment and routing strategy

• Gave a concrete approach under this strategy, which consists of the following two parts– The PDT generation algorithm– The BE-GF protocol

• Both parts possess appealing properties– The PDTs are feasible and cost-effective for the deployment of

DWMNs, and also support GF by being backward-free.– BE-GF is a localized routing protocol, capable of dealing with

network dynamics, and loop-free.

• Formal proofs for these properties were provided when applicable.

• Experimental results on both parts justified our approach

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Future work

• Our proposed strategy of position-based deployment and routing for WMNs opens a new area of research.

• Under this strategy, there are many novel problems to explore.

• We plan to address the problems in both theoretical and application aspects.

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Future work — theoretical aspect

• Investigating the sufficient and necessary conditions for a geometric graph to be backward-free. – Whether a network topology satisfies the backward-free

property significantly influences the performance of position-based routing protocols.

• Investigating the sufficient and necessary conditions for a geometric graph to be GF-perfect.– A geometric graph is GF-perfect if its path efficiency of

hops is one with GF as the routing algorithm.– This property can also be defined in terms of the path

efficiency of Euclidean distance.

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Future work — application aspect

• Exploring other kinds of backward-free graphs that can be used as the network topologies according to the various requirements of network deployment and routing protocols.

• Comparing BE-GF against existing face routing protocols with various traffic scenarios.

• Designing a routing protocol with less packet drop ratio for DWMNs, especially when the network suffers from frequent link failures.

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Thank you!

Questions?

Slides and technical report available at my homepage