## 2 Selected Wireless Sensor Networks Standards: State-of-the-art
### Architecture  
The WirelessHART standard is presented by the HART Communication Foundation, as a reliable, secure, and robust standard. We consider successively the data link, network as well as transport and application layers that compose WirelessHART's protocol stack. At the data link layer, the WirelessHART standard coordinates and manages each device's transmission time by using TDMA with timeslots of 10 ms. Each time slot may be allocated to one source or may be shared between several sources using the _Carrier Sense Multiple Access with Collision Avoidance_ (CSMA/CA) mechanism. In the former case, minimal latency can be reached, whereas the latter case supports efficient bandwidth utilization. Possible collisions due to multiple sources sending simultaneously are reduced by setting random back-off intervals for each source. During the timeslot assignment, the data to transmit are first prioritized by the network manager. The data originating from the network manager have the highest priority, followed by sensed data and event information. The timeslot allocation is then communicated by the network manager in the superframe to each device. At least one superframe (Fig. 5) is continuously repeated at fixed rate and further superframes can be added to support additional traffic [20]. The length of the superframes can be adapted to the needs of the application. However, the length is fixed once the superframe becomes active. In addition to the timeslot allocation, the network manager indicates the transmission channel in the superframe. This frequency hopping helps in reducing multi-path fading and interferences. Moreover, the faulty channels are eliminated by blacklisting. The WirelessHART standard uses two possible routing protocols [22] at the network layer: graph and source routing. In _graph routing_, the network manager determines the different paths forming the graph. The paths are stored by each device and are then used to identify the next node to forward the packet. In case of _source routing_, the packet header contains the list of the devices from the source to the destination. In addition to the paths stored for the graph routing, each device maintains tables about its communication statistics and neighbor activities. Furthermore, the WirelessHART network layer offers broadcast, multicast and unicast transmissions. At the transport layer, the WirelessHART standard supports connection-oriented as well as connectionless communication. The _connection-oriented_ communications are set up for applications requiring a reliable transfer of data between the host application and the field device for example. The connection set up starts by opening a dedicated port on the targeted field device with a specific HART command and the transmission data rate is then negotiated with the network manager, before the data transmission between the both entities can begin. Once all packets have been transmitted, the connection established between both entities is removed. The reliability of the data transfer is ensured by the acknowledgement and the order of the packets is maintained from the source to the destination. Nevertheless, these guarantees introduce additional overhead and a trade-off between reliability and overhead has to be found. For applications supporting out-of-order packet delivery, _connectionless_ communication can be sufficient. Each datagram contains the full destination address and is routed independently from the others through the network [23]. Depending on the required level of reliability and the tolerance of packet loss, end-to-end acknowledgement can be introduced, but would also cause additional overhead. In connection-oriented and connectionless communication, the transport layer is also responsible for the segmentation of the data blocks at the source and their reassembling at the destination in order to allow transparent transfers for the upper layers.