08-17-2017, 12:15 AM
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Wireless Building Control Communications Using WLAN Mesh
Route Discovery
On Demand route discovery
Triggered whenever a source node wants to communicate with other node for which there is no entry in the former s routing table.
Source node broadcasts a RREQ (Route REQuest) across the network. Nodes receiving RREQ will update their information by setting up backward pointers to the source node in their route tables (Reverse Route set up)
A RREP (Route REsPonse) packet is sent back to the source node from the destination. RREP is a unicast message unlike RREQ (which is a broadcast message)
RREQ are generated only if
The destination is previously unknown to the node.
Previously valid route to the destination expires or is marked invalid.
Every RREQ message contains a unique sequence number called RREQ ID.A node increments its own sequence number Immediately before originating a RREQ.When a node receives RREQ, it updates a route to a previous hop and then
Checks to determine whether it has received a RREQ with the same source MAC Address and RREQ ID
If yes, the node updates its route discovery records if the new request received has a better route path cost than the previous message
If not, it creates a new route discovery record and updates the path cost information in the RREQ message and re-broadcasts it
A node generates RREP if it is the destination. When generating RREP message, a node copies the Destination address and the originator sequence number from the RREQ message into the corresponding fields in the RREP message. Once created, the RREP is unicast to the next hop towards the originator of the RREQ. Upon receiving a RREP, the node verifies if the route cost is lesser for the received RREP than its current record and updates its route record accordingly. All intermediate nodes receiving RREP would update their current entries if the new route cost is lesser than the current cost and updates it accordingly.
If the route table entry to the destination is created or updated
The route is marked as active
The next hop in the route entry is assigned to be the node from which the RREP is received
The route cost is updated to the best of the available routes