The IEEE (Institute of Electrical and Electronic Engineers) defines the standards for radio just as it does for Ethernet and many other things. In Ethernet the standard is known as 802.3. The 802 is the set of standards that talks about local area networks and something called Metropolitan Area Networks. In general the 802 standard is for variable packet size networks and subset 3 (802.3) is Ethernet.
In 802.3 the IEEE defines the physical network and the media access for Ethernet. The physical network is the electrical properties of the bits on the wire. It defines the voltage levels, the duration of a bit, the speeds that are used; anything and everything that defines the electrical characteristics of the signals on the wire. The media access part defines how and when messages can be inserted into the network. It defines the basic message structure, how collisions are handled, how messages are validated and, most importantly, when messages can be inserted onto the network.
One of the most important parts of the media access is collision processing. In Ethernet, collisions are really bad news. If two nodes try to transmit at the same time, there’s a collision, the message is lost and, more importantly, that message slot goes unused. Both nodes try again at some random later time. As more and more nodes try to use the network, the number of collisions increases, there’s more unused message slots and the throughput drops.
The IEEE has specified similar kinds of characteristics for wireless networks. These characteristics define the physical and media access for each of the different kinds of wireless networking. There are a bunch of these standards. Here are a few that are important to industrial and building automation:
802.11 – Wireless LAN, the standards for Local Area Networks, the kinds of networks we use in Homes, Airports and Offices .
802.15 – Wireless PAN, the standards for Personal Area Networks. The idea behind PAN is that they are smaller networks but the technology has changed such that Mesh neworks can now have hundreds and thousands of nodes . There are two important PAN substandards:
802.15.1 – Bluetooth (True PAN since it is in your immediate area)
802.15.4 – Mesh Networking
802.16 – Wireless Broadband MAN, standards for Metropolitan Area Networks.
802.20 – Wireless Mobile Networking
802.11 is the standard that is used for most of the PC wireless connections we work with every day. All our home wireless, office wireless and hotspots we use are all 802.11. 802.11 devices are great but don’t really fit a lot of wireless automation needs. 802.11 has an incredibly high data rate but that’s sure not needed in most automation applications. For example, most devices that report temperatures don’t need to report them very often and when they do, there is only a tiny bit of data to report.
802.11 devices also require quite a bit of power and that’s something that we don’t have in a remote device powered by a small battery. 802.11 devices are designed to support applications where the devices are constantly connected instead of devices that wake up, transmit some data and go dormant again.
For all these reasons and more, automation applications use 802.15.4 as the basis for most industrial and building wireless. We’ll look at 15.4 in more detail in the next installment of this series.