Wireless connectivity protocols for embedded systems

Wireless is everywhere! It's a mantra that's not new, but perhaps is taking on invigorated meaning with the explosion of wireless devices in seemingly every application space where embedded systems are deployed.

Driven by our insatiable need for information and the desire for finer control over every resource of value, it seems everything is becoming connected, and much of it is being done wirelessly. Fortunately today, adding wireless connectivity has never been easier.

In this article I'll introduce wireless terminology and basic design considerations, explain popular connectivity options such as Wi-Fi, Bluetooth, ZigBee and others, and examine hardware and software trade-offs, including design strategies, for easily and quickly incorporating wireless connectivity into an embedded design.

Wireless Considerations
The first, and sometimes most difficult, decision to make when incorporating wireless is selecting which protocol to use. The protocol selection can have implications of interoperability (or not) with other products, which could even be supplied by a competitor.

On the other hand, a proprietary protocol could restrict access to a product-set under your control if you can forego the wider market for interoperable products. Additionally, some proprietary protocols can provide a solution tuned to specific performance parameters like simplicity, network resiliency or security.

Which frequency bands to use is often determined by market requirements, especially with the larger industry alliances like Wi-Fi. But the choice of frequency bands can be a trade-off with propagation range; wherin doubling the frequency halves the range. It can also be a trade-off with power consumption and over-the-air data rate.

In the U.S., the Federal Communications Commission (FCC) has set aside portions of the RF spectrum as designated Industrial-Scientific-Medical (ISM) bands, which are unlicensed and unrestricted, meaning they are open for anyone to use without requiring a license. However, these frequencies are not unregulated. All products which intentionally radiate in these frequency bands must pass certification testing to demonstrate that emissions are within the limits prescribed by CFR Part 15.

Unlicensed bands vary from country to country, so products designed for worldwide distribution will also have to address the certification requirements imposed by the governments of their ultimate destination. The unlicensed 2.4-GHz band is particularly unique in that it's an unlicensed band nearly worldwide – the primary reason for its popular use as the frequency band used by Wi-Fi and Bluetooth.

Another important consideration is the topology needed by the network. Star topologies are an aggregation of point-to-point links, with a central master node that manages a fixed number of slave nodes and serves as the conduit for all upstream communication. Master nodes can also link with other master nodes to extend a star network into various configurations, sometimes called tree or cluster-tree networks. Wi-Fi and Bluetooth use the star topology.

One of the drawbacks of a star topology is that if a master node fails, the entire network (or sub-network) fails. Mesh networks provide the most resiliency and flexibility. In a mesh topology, nodes have routing capability so that every node has multiple pathways to every other node, and the network can “self-heal” around failed nodes. The IEEE Std 802.15.4 radio, used by ZigBee and many other sensor networking protocols, uses a mesh topology.