## 4 The IEEE 802.15.5 WPAN Mesh Standard
### Synchronous Communications: SES
In addition, this continuous synchronization process helps to solve other issues such as detecting new devices joining the network or nodes leaving the network because of the depletion of their batteries. Figure 4: Example of SES (**a**) scheduling and topology; (**b**) transmission methods. SES defines a recovery procedure to cope with the loss of synchronization messages. The recovery is based on the premise of retransmitting the synchronization messages when, for instance, parent nodes do not receive the reply messages from any of their children and the synchronization duration has not expired. Another significant aspect of this procedure lies in its capability of counting the number of synchronization attempts failed at every synchronization interval. In this case, if the number of failed attempts exceeds the threshold value of a parameter defined by the IEEE 802.15.5 standard as _meshMaxLostSynchronization_ (default value equal to 3), the recovery method stops the process of synchronization of a node with its parent. Under these circumstances, the node must find another neighbor different from its parent to accomplish the synchronization. To this aim, the node under consideration listens to the physical medium to receive synchronization messages from any neighbor within the synchronization duration. Thereby, once the node is synchronized successfully with one of its neighbors, it will always employ the same neighbor for future synchronization processes. Regarding data transmission, a key feature of SES mode is its two methods to dispatch information. They differ on how the MAC layer manages the channel access mechanism: while method (1), denoted as _contention-based_ method, achieves the transmission of the information by means of a contention-based channel access algorithm, method (2), called _reservation-based_ method, operates allocating time slots for exclusive use of nodes. In particular, in method (1), all sender nodes access the physical medium (always within their _active durations_) by using the unslotted CSMA-CA algorithm [20]. Figure 4(b) shows the operation of this algorithm for SES mode. If we study it carefully, two senders in coverage range nodes A and B must transmit a data message to the same destination, which is also in the same coverage area. Firstly, sender A listens to the physical medium. If the medium is free, the message is automatically transmitted. Then, sender B senses the same physical medium which is busy while sender A is dispatching its message. In this case, sender B aborts its transmission attempt and waits for a random time, in order to avoid a collision between messages. Once this time is over and if the _active duration_ has not expired yet, sender B triggers the CSMA-CA algorithm again. Otherwise, if the _active duration_ has expired, sender B switches to sleep mode and will attempt to transmit the message again during the next _active duration_. The second mechanism to transmit data, the _reservation-based method,_ allows the sources that dispatch critical information (e.g., a node equipped with a physical sensor that detects toxic gases) to establish a dedicated path through several intermediate nodes towards the destination. This dedicated path is accomplished by the thorough coordination of all the nodes that belong to the path with the aim of guaranteeing the transmission of information with strict delay requirements. The contribution of the _reservation-based method_ is that it divides the _inactive duration_ into slots, each of them dedicated exclusively to transmissions between two nodes in coverage range. Thus, SES takes full advantage of the total bandwidth of each slot, because only one sender and one receiver belonging to the source-destination path dispatch information within their reserved slot without the interference of other transmissions in the vicinity. This is extended along the dedicated path, thus giving support to delay-sensitive applications. Figure 4(b) shows an example of this operation.