The WirelessHART protocol provides redundancy to secure against path failure. A device forwards a message according to the two-byte graph ID embedded in the message. The graphs are created by the network manager and stored in each device. Graph routing is similar to a quarterback option play in American football.
A touchdown to win
Seven seconds left to play in the championship game. The home team is behind by a field goal and needs a touchdown to win. The ball is on the 23 yard line, so a pass seems the obvious call.
The ball is hiked. The quarterback has time to exam the field, because the defense has opted for increased pass coverage rather than a blitz. On the left side of the field, his best receiver is in the end zone with triple coverage. On the right side of the field, the tight end is just over the goal line and double-teamed.
The halfback, not needed for defense, is in mid-field just five yards shy of the goal line. The quarterback lets fly. The halfback and the defender both reach for the ball.
Graph routing to handle path failure
When the network manger creates a graph to route traffic between tow points, redundancy is built in. For instance, a route connecting A and D needs to be created. Devices B and C are neighbors to A and D and each other. A message with A as the source and D as the sink might take the following paths:
- A -> B -> D
- A -> C -> D
- A -> B -> C -> D
Note the graph does not allow the message to return to A, nor does it allow C to forward the message to B. This prevents loops from occurring involving A, B and C passing the message endlessly between themselves.
In this example, C would represent the neighbor with the strongest, most reliable connection with D. At least it would be stronger and more reliable than the connection B has with D.
The mesh network inherent in WirelessHART ensures reliable connectivity between field devices and the gateway as well as field device to field device. Like the quarterback option, the graph routing enables the network to find the best path to achieve the goal of delivering a transmission.
The network manager routinely updates the graphs connected devices to reflect the changing dynamics of the network. As devices enter or leave the network or transmission conditions change, the network manager adapts to minimize path failure in the network.
As time runs out
The halfback catches the ball, but finds himself being tackled as a second defender dives and grabs his legs. As he starts to go down, he tosses the ball back to the full back who is running up from behind. The fullback crosses the fullback is hit from the side, turn, stumbles and falls backwards across the goal line just as the clock runs out. All in slow motion.