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. If you want to get specific for a second, the 802 standard covers variable packet size networks and subset 3 (802.3) within the standard defines the Ethernet we all know and love.
If we talk about Ethernet for a moment, the 802.3 standard defines the physical network and how devices access that physical network. 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 about the electrical characteristics of the signals on the wire. The media access part defines the how and when of messaging on the network. It defines the lowest level 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. This protocol for detecting and managing collisions is known as CSMA (Carrier Sense Multiple Access).
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. This is our everyday, long range, high speed, high throughput, power intensive wireless.
802.15 – Wireless PAN, the standards for Personal Area Networks. The original standard covered small numbers of nearby devices typically within 10 meters of each other. These radios are now advanced to the point where the practical range for some of them is up to a mile and advances in routing technology are now making networks of tens of thousands of devices possible. Though the range and count have expanded, low power is still a key factor for devices on PAN networks.
There are two important PAN substandards:
802.15.1 – Bluetooth (True PAN since it is in your immediate area), the radios in our cell phones linking our cell phones to our ears.
802.15.4 – Mesh Networking. This standard covers radios designated for ISM (Instrument, Scientific and Medical) uses. Applications for 15.4 radios are growing at an exponential rate in all these areas.
802.16 – Wireless Broadband MAN, standards for Metropolitan Area Networks.
802.20 – Wireless Mobile Networking
IEEE 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 all follow the 802.11 standard. 802.11 devices are great but don’t really fit a lot of wireless automation needs. 802.11 offers an incredibly high data rate but that’s not necessary for a lot of automation applications. For example, most temperature sensors don’t need to report 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 many remote devices. In fact we want these remote devices battery powered so we don’t have to run power lines. 802.11 devices are really designed to support applications where the devices are constantly connected not devices that sleep 99% of the time and wake occasionally to transmit a tiny bit of data.
For all these reasons and more, automation applications use 802.15.4 as the basis for most industrial and building wireless. In our next article we’ll take a close look at 802.15.4.