## 4 The IEEE 802.15.5 WPAN Mesh Standard
### Synchronous Communications: SES  
SES mode [50] is an energy saving mechanism that, thanks to its synchronous character, facilitates the support of delay sensitive applications. In this type of applications, the goal is to guarantee a fast response of the mesh network whenever an event of interest is detected, as well as to ensure a strict delay in the delivery of information at the destination. To this end, in SES mode, all devices are network-wide synchronized, that is, the periods of activity (denoted by the standard as _Active Duration_) and inactivity of each network node (both periods comprise a duty-cycle interval, called _Wakeup Interval_) trigger at the same time. It guarantees that all intermediate devices placed along the source-destination path/-s be in active mode (ON) for the reception and/or forwarding of data. Nevertheless, to obtain strict synchronism during the entire network operation, we should consider two important concerns: (i) the error-prone nature of the wireless channel and (ii) the intrinsic clock drift of devices, that is, a slight random deviation of the clock's oscillator from their nominal frequency, due to impure crystals, temperature changes or variations of power supply and air pressure (e.g., the clock drift value for TelosB devices is, on average, above 30 \(\upmu\)s per second [57]). As a result, even if two nodes have the same type of oscillator and are initiated at same time, the difference between both nodes' clocks can become significant as time goes by. Thus, both concerns provoke the desynchronization of the devices' _wakeup intervals_ and, as a consequence, failures in the transmission of the information (e.g., loss of data). To deal with synchronization, SES is founded on two main aspects: on the one hand, SES divides the mesh network into \(n\) regions sized in \(h\) hops (parameter defined by the user) and, on the other hand, SES employs a straightforward synchronization algorithm initiated by the MC. Figure 4(a) exemplifiesa network divided into two regions of size \(h\) equal to 2 hops. The synchronization algorithm operates as follows: the MC begins the synchronization process with every device placed in the first region. This synchronization process is accomplished within a specific period, denoted as _synchronization duration_, where nodes remaining in the active mode dispatch only synchronization messages. In this period, every node synchronizes with its parent device. To this end, parents transmit two consecutive synchronization messages to their children, which, upon receipt, synchronize with the time of its parent device. Children confirm the success of the synchronization process by means of a reply message. Meanwhile, the rest of network nodes out of the first region carry on with their usual operations (sensing, transmission and reception of information or sleep mode) in their corresponding _wakeup intervals_. Then, when the synchronization of the first region finalizes, the same algorithm is executed by the last nodes previously synchronized by the MC. Now, these nodes become synchronizers (namely, _Region Synchronizers_) of the second region, thus finalizing the synchronization process illustrated in Figure 4(a). Note that, regardless of the number of regions contained by the mesh network, each one is synchronized consecutively from the MC's region until the leaf nodes (last region) are reached. An advantage of this synchronization process lies in its reduced network-wide synchronization error which entails that the number of regions can be increased. This represents a clear improvement in the scalability of the network [19]. The synchronization algorithm is run periodically (usually, once each time period denoted as _synchronization interval_, which comprises several _wakeup intervals_) by the MC guaranteeing the synchronization among all network devices and avoiding possible failures in the data transmission due to clock drifts.