802.11 Co-existence: WirelessHART
WirelessHART is based on the 802.15.4 standard and forms a flat mesh network where each client acts as a source and a repeater. Alternative routes are set up during the initialization to add a layer of redundancy. WirelessHART is coordinated using time division multiple access (TDMA). TDMA is a method used to provide access to a channel based on allocated time slots, in this case, the time slot is 10ms. The schedule of time slots is provided to the clients through the use of a superframe.
Each client is allocated a 10ms slot and can only transmit during this window, unless the time slots are shared. If shared time slots are used, collision avoidance will be used. Shared timeslots are generally implemented when traffic is irregular and throughput requirements are low.
WirelessHART also utilises frequency hopping spread spectrum. Each of the 16 channels defined within 802.15.4 are used in parallel. Channel blacklisting is implemented to avoid channels that contain interference, or a high duty cycle.
There are 5 datalink frames defined within WirelessHART. If an unknown frame is received by a client, the client must discard the frame without acknowledgement.
The 5 defined frames are: Data, Ack, Advertise, Keep-Alive, Disconnect.
Advertise frames are used to build and maintain the neighbour lists, whilst keep alive frames are transmitted to each neighbour until another frame is received
WirelessHART must have a coordinator to form the mesh network. The coordinator will receive reports of path failures, provide the time slot schedule through use of super frames, and advertise the network so that devices can join.
Per best practice recommendations, a WirelessHART network will generally have at a minimum, 3 neighbors. For large networks of WirelessHART devices, 25% of the devices should have a direct connection to the coordinator.