## 3 Recent Advances on IEEE 802.15.4-Based WMSN Approaches
### WMSN Solutions Developed on Top of the IEEE 802.15.4 PHY and MAC (Non-beacon Mode) Layers  
#### 3.2.1 Zigbee(r) Pro  
Regarding the non-beacon mode category at MAC layer, Zigbee(r) Alliance is one of the greatest players in the WSN market due to its Zigbee standard, released in 2004 (later revised in 2006), and to Zigbee Pro (2007), both mainly addressed to the industrial market. These specifications [11] coincide in that both implement the upper layers over the protocol stack of the IEEE 802.15.4-2003 recommendation. However, the major difference between both standards is in the topology supported: while Zigbee implements cluster-tree scenarios [29], Zigbee Pro sets the rules to generate and operate mesh networks. Then, in line with our work, we focus on discussing the Zigbee Pro [28, 30, 31], and, in particular, how it operates in order to deploy WMSNs. Under ZigBee Pro, devices are identified by two different types of addresses: a MAC address (based on 64-bit IEEE), and a logical address (based on 16-bit IEEE). Each network device exclusively has a MAC address which is employed for the formation of the mesh topology. In this same phase, each node is assigned a specific logical address in order to later perform the communication tasks. Note that by making use of a short address scheme (16 bits), devices demand less memory and computing processing resources to create and manage the routing tables. However, one of the main disadvantages of Zigbee Pro lies in its address assignment mechanism. It follows a random procedure in which each device individually obtains a logical address from an available set. In particular, this set is the same for all network nodes, what may provoke that two or more devices get the same logical address. This fact generates conflicts in the operation of the network, concern known as _duplicate addresses_. In order to overcome it, Zigbee Pro devices execute an address resolution mechanism based on the employment of MAC addresses instead of the logical ones in order to later accomplish communications. However, this solution negatively impacts the nodes because of the significant increase in memory utilization. In any case, the major concern of Zigbee Pro (and also Zigbee) standard is related with its Ad-hoc On-demand Distance Vector (AODV) [32] routing protocol. Due to its reactive nature, _AODV does not guarantee network scalability_ and it is unable to distribute the traffic load efficiently. This may generate bottleneckswhich, among other problems, lead to a greater waste of energy in those devices involved in the bottleneck than in the remaining network nodes. This circumstance is even worse because _Zigbee Pro lacks energy-saving mechanisms_, deteriorating the lifetime of the Zigbee Pro nodes and, in general, of the entire mesh network. As regards the public accessibility, Zigbee and Zigbee Pro are open-access standards [11], although contributions addressed to developing new functionalities are only available by fee payment. Finally, despite the fact that all these drawbacks have had an important influence on the reduction of the number of Zigbee's users in the last years, this recommendation has currently a dominant position in the WSN and WMSN markets.