## 2 Selected Wireless Sensor Networks Standards: State-of-the-art
### Architecture  
The ISA100.11a standard addresses all OSI layers: physical, data link, network, transport and application layers. The _physical layer_ is based on the IEEE 802.15.4 standard. However, additional requirements were defined. First of all, the ISA100.11a-2009 supports only the frequency band at 2.4 GHz; the lower sub-bands are not supported. In addition to the IEEE 802.15.4 standard, the ISA100.11a standard supports frequency hopping and also blacklisting that eliminates faulty frequency bands in order to improve the robustness against interferers. Additionally, the carrier sensing scheme can be disabled to reduce possible delay transmission.  
The ISA100.11a _data link layer_ is responsible for the management of the employed TDMA schemes by configuring the timeslot durations and managing the superframes. The configuration of timeslots can be done according to two different patterns: slotted channel hopping and slow channel hopping. The former scheme optimizes the bandwidth utilization and is adapted to energy-constrained routers, while the latter smooths the time synchronization requirements between neighbors by powering their receivers continuously during well-defined periods. The last scheme is therefore only adapted to routers offering unlimited energy budget. Both patterns can however be combined in a hybrid fashion by mixing superframes of both types. In addition to manage TDMA, the network manager assigns paths and links between the devices composing the WSNs. Each link is associated to one or multiple timeslots of a superframe and its type can be _transmit_ and/or _receive_ (Fig. 7). Information about the neighbors, the  
Figure 6: ISA100.11a network elements [25].  
channel offset from the superframe hopping scheme as well as possible alternatives for the transmission and reception are also included. The ISA100.11a data link layer supports the previously described graph routing as well as source routing.  
The _network layer_ provides schemes for routing and QoS. It allows energy and bandwidth savings by mapping and translating the 128-bit addresses of the application endpoints to 16-bit short addresses used within subnets and vice versa. These savings can be increased by adapting the packet formats in function of the desired addressing, routing or QoS. Moreover, the network layer headers are compatible with the headers specifications conceived by the IETF 6 LoWPAN working group and described in [26] in order to support future compatibility. Packet fragmentation and reassembly are also ensured at this layer. The packets can be routed at the backbone and the mesh levels, as defined in the standard. The first routing level is ensured by the data link layer, while the second is performed at the network layer by the end devices with routing capabilities. QoS is addressed in detail in the next section.  
Depending on the level of reliability required by the application, the ISA100.11a _transport layer_ can support end-to-end acknowledgements as well as unacknowledged communication. Additionally, flow control, segmentation and reassembly as well as security (see Section 4) are supported at transport layer, whereas the application layer ensures standard interoperability by using tunneling and native protocols at the gateways. The former carry protocols used in existing standards such as HART or FOUNDATION Fieldbus, while the latter provide efficient bandwidth utilization and therefore increase the battery lifetime.