A second key attribute of mesh networks includes re-routing, or self-healing. Many of you have experienced an unexpected event while driving a car — perhaps a highway exit is closed for repairs, or an unfamiliar street takes you in the wrong direction. In these situations, we usually turn to our mobile phone’s mapping app, which typically offers an alternative route. That is the idea behind 802.15.4 mesh network re-routing.
In wireless networks, there are many issues that arise, such as dead batteries, temporary interference caused by human movement, permanent interference caused by changes in the environment, new nodes being introduced into the network and more. When these disturbances occur, mesh networks based on the 802.15.4 standard can self-heal. In other words, the connection from the node to the coordinator can be re-routed through a different FFD that offers a more optimal path. This feature dramatically improves the strength of the network and therefore the reliability of the communication.
A third benefit of nodes in 802.15.4 mesh networks is persistence. Unlike network technologies such as Ethernet or Wi-Fi® which “age-out” uncommunicative nodes within the network, 802.15.4 networks feature permanent membership, allowing nodes to stop communicating for extended periods of time. A node may sleep for a week, then wake, immediately join the network and transmit data — in as little as 30 milliseconds. This is a tremendous advantage for power consumption. Transmitting and listening functions consume most of the power in IoT devices, therefore this feature greatly reduces the ratio of radio-to-sleep activity.
Frequency matters for reliability
There is an inverse relationship between radio carrier frequency and its ability to penetrate solid objects in the immediate environment. The most highly used frequency today is 2.4 GHz. This is the frequency used throughout our homes for Wi-Fi, Bluetooth® and microwave ovens. This frequency band is known for its high data rate transmission, but due to the relatively poor penetration that 2.4 GHz offers over lower frequency bands, it’s also likely to run into coverage issues throughout the home. However, the unlicensed 800-/900‑MHz bands offer superior penetration ability, at lower data rates, when used in environments with solid objects such as walls, trees, furniture and doors. Therefore, sub-GHz frequencies offer superior performance when looking to build a network that can perform well in harsh or confined environments.