Ad hoc On-Demand distance vector routing (AODV)
Like its sibling Wireless HART, AODV (including the variant used in the Digi Mesh) is designed to meet the very low power sensor networks where battery powered routers are required.

Key Characteristics. Our AODV variant is available in multiple frequency bands, 2.4 GHz DSSS and 900 MHz FHSS and does not rely on a full 802.15.4 implementation as it has some of these functions internal.

For both message routing and discovery, it uses a variant of AODV, meaning that routing tables are built only for needed destinations, leaving it to be referred to as a peer-to-peer mesh instead of a cluster-tree.

All nodes are viewed as equal participants meaning that they are all routers and they can all sleep. Channel access is a sort of time synchronized CSMA method, enabling bursty traffic, but the benefits of few collisions. It has a full security suite.

Network Architecture. Figure 8 below illustrates a typical ad hoc network topology. Unlike the Cluster-Tree method described in ZigBee, routes are only determined on an as needed basis. This means that routes that are never used never get routing table entries and routes that are used frequently are continuously updated, optimizing their efficiency.

Figure 8. The Digi Mesh AODV wireless network topology.

One of the other keys to note about the topology is that there is no coordinator or gateway function. Time synchronization is accomplished through a nomination and election process, enabling the network to operate autonomously.

Routing Methodology. Figure 9 below shows the process of how routing failures are handled. The first shows the initial network configuration where a route has been established from one point to another.

The second illustrates a failure where one of the nodes has been removed for an unknown reason, removing relationships in the center of the route. Finally, the last figure shows how this route is reconstituted using a path that didn't previously exist. The relationships were there, but they had never been used, but were newly discovered using AODV after the failure.

Figure 9. Here's how routing failures are handled with an AODV wireless mesh network configuration.

Strengths. Every node is a router at very low power consumption. Further, because every message is acknowledged and routes are determined on an as needed basis, the network is not overwhelmed with unnecessary discovery traffic " very important if the routers are battery powered and sleeping.

Efficient route discovery and routing means that the network only learns routes that actually get used (AODV). Frequency agility is supported and security meets the requirements of both encryption and authentication. Reliability is projected at 99.99%. Finally, the system supports larger Payloads with support for message fragmentation.

Limitations. Unfortunately, efficient power management means latency is long and non-deterministic. Even though throughput is not limited by time slots, it is still limited depending on loading and discoveries. The network can scale to a moderate size of around 500+ nodes and can be very large if traffic is light and message flow doesn't change much.