Industrial Wi-Fi

A recent article about the difficulties of Machine-to-Machine wireless reminded me of when I first learned about industrial wireless nine years ago.  I was starting a job in the wireless department of a company focused on industrial automation connectivity.  The product I was working on used Wi-Fi chipset used in commercial grade Wi-Fi “routers”.  When my supervisor explained they are used in the links that control oil pumps and municipal water pumps at distances of miles across a city, I thought he was joking.  The average link between a commercial Wi-Fi access point (AP) and a laptop client has trouble covering a large coffee shop.  How could Wi-Fi possibly link stretch for miles across a city that probably has a hundred Wi-Fi networks between them?

(The picture to the right is an actual installation of a 50mW industrial Wi-Fi radio.)

A Wi-Fi connection in the coffee shop scenario is limited by objects obstructing line of sight and low-gain antennas.  If you use higher gain antennas and plug the numbers into free-space path loss (FSPL) equation, you’ll find you can get many miles out of 2.4GHz radios with 90dB of link budget.  The radios do see countless APs in the direction the antenna is pointing, but you can set a somewhat higher carrier sense threshold allowing the radios to transmit over the other networks if necessary.  It doesn’t cause much interference to those networks if the nodes those APs are communicate with have strong signals.  Usually installers do not even change the carrier sense threshold.  They allow the radios to wait “politely” for their turn to use the channel.

Wi-Fi’s channel access scheme, the distributed coordination function (DCF), is very good at dealing with crowded channels.  I have done tests, with real antennas and with cables, splitters, and attenuators, to see how it deals with interferers.  Even frequency hoppers, which hop around the band and spend a good deal of their time in any Wi-Fi channel, share the band nicely.  The hoppers usually do not do any clear channel assessment (CCA), but Wi-Fi’s CCA prevents collisions.  Like clockwork, if you’re using 50% of the hopper’s throughput capacity, you’ll get 50% of the maximum Wi-Fi throughput. 

The hoppers typically have a smaller bandwidth, taking up 1 MHz of spectrum during each hop.  A Wi-Fi signal is 20MHz wide, 40Mhz or 80MHz optionally using 802.11(n) or 802.11(ac).  Reducing receiver bandwidth from 20MHz to 1MHz, results in a 20-fold (13dB) increase in sensitivity. 

Why would industrial users use 20MHz Wi-Fi if signalling with 1MHz bandwidth would be equivalent to increasing transmit power by 20 times?  One answer is even if the channel is crowded (and 2.4GHz is crowded) and Wi-Fi can only access it 20% of the time, the user will get a several Mbps of throughput.  A typical 1MHz signalling scheme provides about 1Mbps of throughput. 

The amazing fact is there are countless outdoor Wi-Fi installations used to control equipment and transfer industrial data over long distances, and generally the users never have to think about how Wi-Fi’s channel access scheme squeezes the data through short periods when 20MHz crowded ISM spectrum is free.  Intuitively I would expect Wi-Fi could only be used for optional links, such as accessing the Internet in a coffee shop. Experience shows, however, that in many locations it is a reliable way to send important industrial data over long line-of-sight paths.

Next week's post will be on Multipath in Industrial Wi-Fi Links.