Practical Internet of Things Security - Second Edition

Device redundancy 

WSNs are a great example of the power of device redundancy. Wireless nodes can often operate as either a node or a router, passing traffic from other nodes in the network to gateway devices. When energy levels are reduced on these router nodes, other nodes step up and take on the routing function.

Architect your WSNs to ensure that sufficient coverage across a geographic region is provided, to allow nodes that are either offline or no longer capable of more expensive routing functions to hand off their responsibilities to other nodes.