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Wireless Sensor Network Cluster Formation at the Presence of a

Wireless Mesh Network

Pantelis Angelidis, Member, IEEE

Abstract— Countries under development have recently benefited from the emergence of networked communities operating in an infrastructure poor environment. We describe a WSN protocol for monitoring quality of life in such networked communities. The protocol bridges a IEEE 802.15.4 to a IEEE 802.11 network using a presence advertising algorithm.

I. INTRODUCTION

Health status (together with education) represent the two major challenges for those parts of the developing world that have found solutions on drinkable water and nutrition. An interconnected community (even with limited or low-quality access to a backbone network) has the means to support activities aiming at facilitating disease management and health status control within a larger (to the community) population. Such activities may include the implementation of scenarios in which a Wireless Sensor Network (WSN) coupled with an electronic community infrastructure supports monitoring, processing and transmitting of personal, ambient and environmental parameters.

Our main contribution is the proposed novel simple and energy-efficient cluster tree reorganisation algorithm of the WSN as a result of topology changes caused by mobility of the Wireless Mesh Network (WMN) nodes. Our aim is the performance analysis of the protocol in terms of connection time, traffic and power management..

II. NETWORK FORMATION

In our problem definition we identify two networks. A WSN that collects QoL parameters, like the environment (water, soil, air, volcano), vital signs, health related human receptors, behavioral patterns. This is referred as cloud A. Usually in the literature the sinks are considered part of the WSN. Our case scenario examines a different setting in which the sinks form a different network, Cloud B, which acts as a store & forward facility for the acquired data. In our framework Cloud B is implemented by networked communities that pre-exist for some other reason or are formed for this particular case. Examples of such network communities may be found in a OLPC equipped village [1], a mobile phones sharing cooperation [2] or a hospital on wheels [3].

Manuscript received July 21,2010. This work was supported in part by the EC via grant agreement FP7-234995 -CLAP.

P.A. Angelidis is a visiting researcher of MIT Media Lab, USA. He is with Un. Of W. Macedonia, Creece (phone: 617-510-2167; e-mail: pantelis@media.mit.edu).

For the purposes of our work here we assume that Cloud B is an ad-hoc WMN IEEE 802.11s network and Cloud A an IEEE 802.15.4 WSN. The generality of these assumptions is enough to ensure wide applicability of the proposed solution. The problem of bridging IEEE 802.15.4 and 802.11b networks has recently been studied for healthcare applications [5]. However, in that case the two networks are assumed static unrelated networks. In our case, however, the 802.11s network nodes move arbitrarily and when arriving in the WSN proximity they act as 802.15.4 sinks. This formation represents the realistic scenario of building a monitoring sensor network in the vicinity of a networked electronic community. Cluster heads and network coordination is assumed by Cloud B nodes. This results in higher energy efficiency and longer lifetime for Cloud A. A beacon mode with a superframe is used.

Each node advertises itself as a sink to Cloud A. This is achieved by having each node broadcast a Presence Entry. All motes that receive the Entry and do not have a one-hop relation to another node set the advertising node as their sink. Motes that already have a one-hop relation with a node ignore the invitation In this case the network topology does not change in the child tree branches of these motes.

The motes that decide to accept the node as a cluster head, become hop 0 motes for this cluster. The first node that arrives in the proximity of Cloud A assumes the role of the coordinator of Cloud A (Fig. 1-top). All subsequent nodes will form independent clusters. The coordinator could act as a gateway to the outer world as well; other nodes may also act as gateways. The coordinator role may be transferred between nodes.

Nodes broadcast Presence Entries as they move. When a mote establishes a direct connection with a node, it informs its neighbors; for this purpose it transmits a Presence Entry itself. Motes with a 2 or higher hop distance transverse their traffic to the mote in question. It may be that the parent of this mote will now become its child (Fig. 1-bottom). In general, whenever a node sends a Presence Entry the following changes in the routing path may occur (in all cases motes disassociate from their past parent node and associate with the new one):

a. Cloud A links break for the motes that connect

directly to the node.

b. Cloud A links reverse for the parent motes that

decide to use a (new) route to case a. motes.

c. New cloud A links are formed; for each link formed

one link disappears.