## 3 Wireless Sensor Networks
### ISA 100.11a  
The International Society of Automation (ISA) has developed a wireless mesh networking standard known as ISA 100.11a [37] that guarantees a deterministic communication latency, while increasing the communication reliability. It focuses on process control and monitoring applications, with latency requirements around 100 ms. ISA 100.11a can coexists with other wireless technologies such as cell phones, IEEE 802.11\(\times\), IEEE 802.15\(\times\), IEEE 802.16\(\times\), and can provide tunneling for legacy protocols (HART, Foundation Fieldbus, Profibus, Modbus).  
A typical ISA 100.11a network is presented in Figure 2. It may be composed of seven types of devices: gateway, system manager, security manager, router, backbone router, input/output (IO) devices and portable devices. Each device has a specific role definition that control its functions. The IO device is responsible for monitoring the environment. If minimization of the energy consumption is configured, the IO device only transmits messages. Otherwise, the IO device can also route messages. In addition to the routing functionality, a router device shares the function of provisioning devices to join the network. A router device can use slow slotted hopping to send advertising messages about the network for joining devices. On the other hand, backbone router devices are used to encapsulate external networks in order to carry native protocols over ISA 100.11a. The gateway device provides a connection between the wireless sensor network and the plant automation network. There is support for multiple gateways and backbone routers [38]. The most important tasks are performed by the security manager and the system manager. The system security management function is controlled by the security manager whereas the system manager governs all the network, devices and communications.  
Similarly to WirelessHART, ISA 100.11a has a physical layer based on IEEE 802.15.4. On the other hand, the data link layer is slightly different from the one used on WirelessHART. The slot time has duration of 10 ms or 12 ms. The schedule mechanism was designed in a more flexible way than the WirelessHART schedule. There is support for slotted channel hopping (TDMA), slow channel hopping (CSMA) and a hybrid combinations of both. The TDMA approach is similar to the WirelessHART schedule. In the CSMA (Carrier Sense Multiple Access) approach, contiguous slot times are grouped into a single radio channel with a period ranging from 100 ms to 400 ms. During this period the radio of devices are always activated. This approach is indicated for neighborhood discovery procedures and frequency hopping in the case of overlapping with 802.11\(\times\) networks, for example. On the other hand, the hybrid approach is more suitable for a flexible retry procedure. Other improvements when compared with the WirelessHART standard are related with the frequency hopping pattern. ISA 100.11a has defined five default hopping patterns to mitigate the influence of external communication interference. For example, the pattern 1 is configured to eliminate the overlap with the same channels of IEEE 802.11\(\times\).  
The network and transport layers support mesh networks, similarly to WirelessHART. However, addressing in ISA 100.11a is compatible with the 6 LoWPAN [39] (IPv6 over IEEE 802.15.4). ISA 100.11a also introduces a new mechanism to detect failures in the network based on the transmission of alert messages.