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ZigBee is a robust wireless communication standard managed by the ZigBee Alliance (ZigBee, 2006) and based on the IEEE 802.15.4 physical and MAC layer standard (IEEE, 2006). It defines a network layer, application framework as well as security services. ZigBee aims at handling low data rate, low cost devices and long-life batteries making it very suitable to wireless sensor networks (WSNs) (Akyildiz, Su, & Cayirci, 2002). It can be embedded in a wide range of products and applications. Nowadays, the availability of low-cost CMOS cameras and microphones, Wireless Multimedia Sensor Networks (WMSNs) (Akyildiz, Melodia, & Chowdhury, 2007) gained more interest and research effort. In a WMSN, the scalar WSN is strengthened by introducing the ability of retrieving richer information content through image and video/audio sensors (Rahimi et al., 2005). This can significantly enhance a wide range of applications like object detection, surveillance, recognition, localization, and tracking.
ZigBee supports three types of network topologies namely: star, peer-to-peer and its special case cluster tree topologies. The routing algorithm specified by the network layer of ZigBee depends on the topology used in the ZigBee sensor network. In the simple star topology, there is no routing, data is assumed to be directly transmitted in one-hop to the destination. In the tree topology, a Tree Routing (TR) protocol (Kim, Kim, Yoo, & Lopez, 2007; Nefzi & Song, 2007) is used. Data is routed along the parent-child links established as a result of join operations. Peer-to-peer topology uses a table driven routing basically similar to the Ad hoc On demand Distance Vector (AODV) routing protocol (Baronti et al., 2007; Perkins & Royer, 2003; Karthikeyan, 2010; Ran, Sun, & Zou, 2006). For these two latter topologies, devices can communicate with each other in a multi-hop manner.
ZigBee technology suffers from its limited bandwidth (250 kbps at 2.4 GHz) and extending it to meet WMSN requirements is a real challenge. In the literature, few works already considered multimedia applications over IEEE 802.15.4. In order to guarantee transmitting such dataflow, the authors in (Garcia-Sanchez, Losilla, & Garcia-Haro, 2008) made use of the Guaranteed Time Slot (GTS) (IEEE, 2006) (part of the ZigBee standard). In Deshpande (2006) the GTS mechanism is not used, streaming metrics such as packet loss and latency are analyzed in 802.15.4 networks using the NS2 simulator (Information Sciences Institute, n.d.). A cross-layer solution is proposed in Garcia-Sanchez, Garcia-Sanchez, and Losilla (2010) where the feasibility of transmitting streaming video flows is evaluated. It uses application-level QoS parameters to tune the MAC and physical layers.
In this paper, we explore the use of multipath routing to handle high data rate applications. We chose the cluster-tree topology (IEEE, 2006) of ZigBee and made extensions of the Tree Routing (TR) protocol to allow the formation of multiple disjoint paths. This choice is motivated by the fact that the tree-based topology is efficient, easy to establish and to maintain.