819-GPRS Serial Cellular Data Modem

Okay, okay. I know what you're thinking: "A serial cellular data modem? Where's the Ethernet port?!" While Ethernet is quickly becoming the interface of choice, the fact remains that thousands and thousands of devices out there only have a serial port. I'm pretty sure I don't even have a serial cable in my briefcase, but the fact is I should. One of these days I'm going to come across a serial-only device and I'm going to have to find the local Radio Shack. In all likelihood, it will be an 819 from CalAmp. This little cellular data modem has been around for years and there are thousands of them out there, in both CDMA and GSM versions. Here's the blurb about the 819-GPRS from CalAmp:

The LandCell 819-GPRS cellular data modem gets you connected with a quad-band integrated platform through a serial connection. With plug-and-play installation in a diverse range of applications, the 819 from CalAmp helps you un-wire any communication-ready machines, systems and next generation devices. The 819 features packet data transmission speeds up to 86 kbps and offers a JAVA application development platform (J2ME) for quick system integration.

Click here to visit the website.

CalAmp's LandCell Cellular Modems

CalAmp has really gotten a hold of the LandCell line of modems. I think that the most exciting model is the 882, since it is capable of EVDO Rev. A speeds (or HSDPA on the GSM side) and the fact that it is a full-blown IP router (NAT, RIPv2, SNMP, etc.). From a SCADA perspective, that kind of speed isn't always necessary, but with more and more requirements for video over the air coming along, it sure is nice to have the option. I would imagine that the 822, which has the same functionality as the 882 but at 1xRTT (or GPRS/EDGE) speeds, will be more applicable to classic SCADA/M2M.

Looks like the manuals, data sheets and firmware are here.

WEFTEC

WEFTEC 2007 is coming up in a few weeks. In case you haven't heard of it, WEFTEC is a water/wastewater trade show with a stunning number of exhibitors. Dataradio, Freewave and a few other SCADA radio manufacturers will be there.

Don’t miss the largest water quality event in North America! Join thousands of water and wastewater professionals at WEFTEC.07, where the newest water quality research, technology, and services will be on display. WEFTEC offers unparalleled education and training for anyone committed to water, sanitation, public health, and the environment.

WEFTEC.07 Exhibition Hours:

Monday, October 15 9:30 am – 4:30 pm
Tuesday, October 16 9:30 am – 4:30 pm
Wednesday, October 17 9:30 am – 4:30 pm

Ethernet IP Industrial-Grade Wireless Radio Modem

Looks like Dataradio is ready to release their new licensed, UHF/VHF, IP radio. 32 kbps and the long-range benefit of licensed RF. Nice.

ViPR

CalAmp DataCom proudly announces the Dataradio ViPR Ethernet IP industrial-grade wireless radio modem.

ViPR is the industry's first long-range licensed IP router. delivering up to 32 kbps in point-to-point and point to multi-point SCADA applications, ViPR features neighbor discovery to automatically detect the presence of other sites in the coverage area to identify primary and secondary back-up routes. System expansion is made easy with the remote site repeater feature. Simply over-the-air re-program a remote site into a repeater and let it send the message while continuing to operate as a remote site. Three active ports give the operator the advantage of operating two serial and one Ethernet device at the same time! Plus, each ViPR comes standard with Dataradio’s real time non-intrusive diagnostics to continuously monitor and report the health of your radio system. With programmable 10 watt capability, you’ll have the power to keep the information moving throughout your network.

VSWR

Troubleshooting RF systems can be difficult and having good diagnostics, both locally and over-the-air to remote sites, is critical in making an accurate determination of your system. One of the most important is VSWR or 'voltage standing wave ratio.' It is basically the measurement of the impedance match (or mismatch) between a radio and its antenna/feedline. Most radios won't actually give you the ratio, but the better manufacturers do provide a 'reverse power' reading. Reverse power is the amount of RF energy that is reflected back into the radio. The higher the reverse power, the more likely that something is wrong.

For example, say you've got a radio that is transmitting at 5 Watts and it has a reverse power of .2 Watts. That's actually pretty normal, since achieving a 1:1 ratio is quite hard to do. In that scenario, everything's okay. But, if you see that reverse power start to climb, to say .6 or higher, something is not right. You've got a bad radio, a loose connector, a squirrel's been gnawing on your feedline, something. Verify that it isn't the radio itself by putting on a different antenna, and see if it goes back down. After that, inspect the feedline, connectors and antenna for damage or water.

"How Far Do Your Radios Talk?"

One of my favorite questions! I get this one at every trade show I attend, without fail. And, it makes sense. My response is always, "Well, how far do you NEED them to talk?"

Radios can talk a very long way, given extremes. One of my pet peeves is when radio manufacturers put on their spread spectrum datasheets: "Range = up to 60 miles point-to-point *" Note the asterisk. Sure, they CAN talk 60 miles, with 1000 feet high gain antenna on either end! In fact, the longest spread spectrum shot I ever managed was 38.7 miles, but that was mountain peak to mountain peak.

In a real world SCADA application, however, ranges shouldn't have asterisks. Here are general range limits by frequency, with realistic gain antenna and realistic antenna heights (say 50-75 feet at the master and 20 feet at the remotes):

VHF, 5 watts = 20 miles
UHF, 5 watts = 15 miles
900 MHz, 1 watt = 8 miles

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