Received Signal Strength Indication (RSSI)

Many SCADA radios and even some cellular modems have a Received Signal Strength Indication (RSSI) as one of their diagnostic parameters. It's a good thing, too, considering how important RSSI is. RSSI is basically a measurement of how well the radio is receiving or 'hearing' data. It's typically measured in -dBm, which is the power ratio in decibel (dB) of the measured power referenced to one milliwatt (mW). The general rule is that the closer you get to zero, the better. In other words, -70 dBm is better than -80 dBm and so on. However, the closer you get to zero, the more the front end of the receiver can get overloaded. Imagine someone standing next to you and screaming in your ear. You could certainly hear them, but you wouldn't be able to understand a word they were saying. Same thing with radio. When people (and you know who you are) start putting on power amplifiers and huge gain antennas, you are going to cause problems with your system!

Personally, I like to shoot for right in between -60 and -70 dBm. It's a good, solid signal without being overwhelming to the receiver.

Here's a good reference for RSSI and how it relates to reliability:

§-100 dBm

–50% reliability; fading may cause frequent data loss

§-90 dBm

–90% reliability; fading may cause occasional data loss

§-80 dBm

–99% reliability; reasonable tolerance to most fading

§-70 dBm

–99.9% reliability; high tolerance to fading

Cellular Modems and SCADA?

I've always been a private network radio guy. Well, at least as private as spread-spectrum is, but at least you still own the network. However, more and more, I'm hearing about the use of the public RF networks, in this case the cellular networks, being used for SCADA applications that have traditionally only used private radio. I think this is the case for three primary reasons:

1) People are becoming much more accepting of the cellular networks.
2) Cellular coverage (both CDMA and GSM) is much better than it used to be, and gets better every day.
3) There are fewer and fewer true RF 'experts' around now.

The last one bugs me, but it's the way of the world. Back in the day, a systems integrator or an engineering consultant always had at least one crusty, old RF guy that blessed all system designs, manufacturers, etc. Now, the younger generation views SCADA radios as simply 'wireless,' "plug them in, and they should work." This is a generation that grew up with cell phones and Wi-Fi. Why should they expect their SCADA wireless to be any different?

I've noticed that these youngsters turn their noses up at licensed radio, but are much more accepting of spread-spectrum, and even more enthusiastic about cellular modems. I recently took part in a seminar with a consulting engineering firm, and cellular modems garnered a significant amount of interest. Why? "Because you plug them in, and they work. There are no path-surveys, there is no network design. It's easier."

Anyway, I'm starting to come around to the idea. I have some customers and prospects evaluating these cellular modems right now, and the feedback has been very good. I think that these modems go beyond traditional SCADA and into the realm of M2M, but if it works in the M2M world, there's no technical reason why it can't work in the SCADA world as well.

Licensed or License-Free Radio For SCADA?

I get asked this question all the time, and opinions vary. Some folks are absolutely loyal to licensed radio for SCADA. Others won't touch it, and will only put in license-free, spread-spectrum systems. The bottom line is that both technologies have their benefits and drawbacks. So, what are they?

Licensed radio (VHF and UHF) simply 'talk' farther than spread spectrum radios. This fact (and yes, it is a fact) is due to two reasons: 1) Licensed radios can transmit at up to five watts of power and 2) VHF and UHF are less prone to attenuation by rain, fog, tree leaves, etc. Add those two basic facts of physics together, and you've got a long radio link. This is the case with all radio manufacturers; like I said, it's just physics and we've all got the same rules there.

Another benefit of licensed RF for SCADA is that you own the frequency. No one else can use it. If they do, the FCC tends to get rather annoyed.

So, licensed radio is better, right? Well, that depends. You DO have to get a license. It is a common misconception that you can't get licenses for SCADA applications. That is not true in many, if not most, cases. I have coordinated hundreds of licenses over the last five years, and I've only ever had three refused by the FCC. It really is a painless process, much less so than it was ten years ago or so. Just a few pages of information, and send it in. You get temporary approval within about 4 to 6 weeks. Why are they available now? Two reasons: 1) licenses are becoming available because users such as fire, EMT, police, etc. are switching over to (you guessed it) cellular technology and letting their licenses lapse. 2) The FCC is refarming the spectrum from 25 kHz channels to 12.5 and then 6.25 kHz. In other words, they are basically quadrupling the number of license available. So, when someone tells you, "You can't get an FCC license for SCADA," they are usually wrong.

Now, here's the rub: it IS still hard to get licenses in the metro areas of the United States. Not impossible, but the three I had refused were all within highly-populated areas.

Another drawback of licensed radio can be seen by those channels I mentioned above. Pretty narrow channels, eh? Well, you can't push a lot of data through narrow channels. Therefore, if your application REQUIRES high bandwidth, licensed radio might not be for you. Just think about how much data you NEED, and see if licensed RF would work. You get the benefits mentioned above.

Now, good old spread spectrum. Everyone loves spread spectrum (I know I do). But, just like licensed RF, it has its benefits and its drawbacks. The first is benefit is obvious: you don't need a license. So, in those areas where you can't get a license, it's a great fit. However, everyone and anyone can come in right next to your system and put in a system. Let's look at the 900 MHz spread spectrum (ISM) band. What other devices use that spectrum? Well, other SCADA radios for one. Baby monitors. Garage door openers. RC toys. You name it. Lots of noise. Also, the FCC limits transmit power to one watt at the antenna port and 36 dBm at the antenna (EIRP). Just make sure you get your gains right, and it's okay.

Now, line-of-sight is much more of an issue at 900 MHz. As you get higher in the spectrum, the wavelength gets shorter and becomes much more susceptible to attenuation by rain, fog and leaves. So, 900 MHz doesn't travel as far as licensed. But, the channels are much wider (sometimes 500 kHz or so), so you can pump more data through them. In other words, it's better for high-throughput requirements.

Okay, so what's the answer? Yes, you guessed it: It depends. Where is the system going in? What speeds do you require? If they tell me their system is going into the middle of Nebraska and they need 9600 baud, I recommend licensed radios. If they tell me they need 512 kbps and it's going to be deployed around Denver, I recommend spread-spectrum. BOTH technologies work and both are available. BOTH have their benefits AND their drawbacks. You just need to decide which works for your specific application.

Whew... now my fingers hurt.

New From Dataradio

Dataradio has just released a remote-only version of their HiPR-900 spread spectrum IP radio. Dataradio figured (correctly, I might add) that not all users need the horsepower of the original HiPR-900, now known as the HiPR-900 Enhanced ( or the 'e' version), so they basically removed some of the higher-end features and created the Standard, or 's' version to act as a remote-only with the HiPR-900 (e) intended as the master radio or as an individual repeater when needed. Just like the HiPR-900 'e,' the 's' incorporates Power-over-Ethernet (and/or 10-30 VDC) for power input, is capable of full-blown IP routing, two active serial ports and one Ethernet port. And, yep, you guessed it, the 's' version is considerably lower priced than the 'e' version.

All in all, a very good decision from Dataradio.

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