## 2 WirelessHART
### The IEEE 802.15.4 Physical Layer
#### 2.2.1 WirelessHART Scheduling  
For the scheduling process, the WirelessHART specification [6] only suggests an algorithm that meets the requirements defined by the specification. The requirements are presented in Table 2, and those requirements must be implemented by the algorithms that intend to work with WirelessHART. \begin{table}
\begin{tabular}{l|l} \hline \hline  & – The maximum number of concurrent active channels is determined by the number of enabled channels and limited by black-listing. \\  & – No device can be scheduled to listen twice in a slot. \\  & – More than one device can transmit to the same device. A broadcast link and dedicated links to each of the listening devices can coexist. \\  & – On a multi-hop path, early hops are scheduled first. \\
**Features** & – The supported update rates are defined as \(2^{n}\), where \(n\) is a positive or a negative integer. \\  & For example, update rate selection can be 250 ms, 500 ms, 1 s, 2 s, 4 s, 8 s, 16 s, 32 s, 60 s, \\  & or more. \\  & – Base network management and publish data communications should not exceed 30\% of the available communication bandwidth (100 slots/s max). \\  & – The network manager takes into account the service requirements. \\  & – The final schedule (not counting the gateway) should have 50\% free slots (_i.e._, allocated for retri  
\begin{table}
\begin{tabular}{l|l} \hline \hline  & – The maximum number of concurrent active channels is determined by the number of enabled channels and limited by black-listing. \\  & – No device can be scheduled to listen twice in a slot. \\  & – More than one device can transmit to the same device. A broadcast link and dedicated links to each of the listening devices can coexist. \\  & – On a multi-hop path, early hops are scheduled first. \\
**Features** & – The supported update rates are defined as \(2^{n}\), where \(n\) is a positive or a negative integer. \\
**Features** & For example, update rate selection can be 250 ms, 500 ms, 1 s, 2 s, 4 s, 8 s, 16 s, 32 s, 60 s, \\  & or more. \\  & – Base network management and publish data communications should not exceed 30\% of the available communication bandwidth (100 slots/s max). \\  & – The network manager takes into account the service requirements. \\  & – The final schedule (not counting the gateway) should have 50\% free slots (_i.e._, allocated for retri  
Figure 6: Channel overlap for 802.15.4 and 802.11b. This algorithm suggestion states that an alternate route should be used, if possible, beyond the main route from the data source to the data destination. For each packet to be transmitted by a station, two time slots must be reserved for scheduling purposes: the first for the transmission and a later one for a possible retry (both through the main route). If an alternate route is defined for that link, a third copy packet must be sent through that alternate route. For the sake of understanding the scheduling approach, a simple example is presented in Figure 7.  
In the first time slot of the example described in Figure 7, the data are produced at the source station \"S\" and sent to the destination station \"D\" using the main route S-1-D and the alternate route S-2-D. According to the aforementioned scheduling algorithm, the two first time slots (Ts1 and Ts2) are reserved for the packet's transmission and retransmission through the main route on Channel 0. On the other hand, Ts3 reserves Channel 1 for the transmission through the alternate route. Again, Channel 0 in Ts3 and Ts4 is reserved for the main route's transmission and retransmission. Finally, in Ts5, Channel 0 is reserved for the transmission of the alternate route's sole try. It is worth noting that after the definition of the channel offset, WirelessHART uses time slotted channel hopping (TSCH) [47] to determine the actual channel in which the data will be transferred in that slot.