The moral of this wireless LAN story is: don’t assume your hardware is to blame if you experience poor results. It could be your installation.

Thankfully, for one Queensland company experiencing major wireless communication problems, Siemens was able to come to the rescue and rectify the situation without the need to purchase or replace any hardware.

Sunstate Cement Ltd, a leading Australian-owned supplier of high quality cement product to Queensland and northern New South Wales, sought to improve its productivity through wireless communications.

The company had completed work recently on an $85 million expansion program that increased capacity to 1.5 million tonnes per year of its bulk and bagged cement products. Part of that expansion included the installation of wireless LAN network between three massive ship unloaders and four corresponding access points.

And that’s where the problems began. From day one of the installation, the WLAN didn’t function properly, meaning the loaders couldn’t communicate interrupt free back to the main controller, posing huge safety and productivity issues.

“These three shiploaders needed to work as a team but without a reliable communication link, this was impossible,” said Siemens Australia PLC and Networks Product Manager Falk Hohmann.

“At first, Sunstate Cement thought the problem was with the Siemens equipment that had been installed. The company had paid to have installed a high quality wireless LAN network that didn’t function as expected. They had very low signal strength, even from only a couple of meters away from the WLAN access points and there were several communication drop outs between the main Controller and the three ‘RTU’ PLC which were installed on those unloaders. (Fig . 1 shows these three unloaders in the parked position).

When contacted about the problems, Falk flew to the company’s site to investigate first hand. What he found was a comedy of errors. But understandably, no-one at Sunstate Cement was laughing.

“The first thing I found was that the installer of the system hadn’t conducted any WLAN channel planning. WLAN is a shared media, not like switched Ethernet, and there were so many collisions/interferences because the traffic was not coordinated between the 4 WLAN access points,” he said (see Fig. 2).

He explained that access collision could be avoided by conducting space division multiple access (SDMA) or frequency division multiple access (FDMA).

WLAN installations like this usually show strong signal strength but a very poor signal quality. This is like a class room where everyone starts to talk at the same time. The result is a very noisy class room but the so called ”signal to noise ratio” or also called signal quality is so small that nobody could actually follow a conversation.

In order to avoid overlaps and interferences, WLAN simulation software like Siemens Sinema E can be used in order to improve the communication performance even before the actual installation starts. (Figs. 3 and 4 show the connection between strong signal strength and low quality.)

“We also discovered that the engineer had installed a 5 GHz antennae but the network was configured for a 2.4 GHz frequency range. Also, the access points antennae were not only installed below structures, they were too low and adjacent to solid concrete walls,” he said.

So, after selecting the 5 GHz range for that network, we reallocated channels to each of the access points and the corresponding loaders.”

Falk said channels available for the network ranged between 149 and 165 (5745 MHz-5825 MHz) for this outdoor application. But because there is a 2 MHz overlap between channels, Falk arranged the channels in an adhoc order to prevent any interference (see Figs. 5 and 6).

Using a Cisco spectrum analyser, the second round of tests showed a slightly weaker signal strength (-70 dBm or 48 percent) but much greater signal quality. “Unfortunately, we still experienced communication drop outs. So, what else was wrong and why was it only 48 percent?”

Falk looked to the client antennae, which were installed on each of the hoppers.

“The installer had used omni directional antennae, which can have high gain but a very small vertical lobe. When the loaders were in the parked position, it was impossible for the antennae to have line-of-sight communication due to a big difference in the height of the mounting positions,” he said.

“Therefore the antenna pattern should always be considered in order to determine the antennae opening angel. In order to do so the -3dB curve is the most interesting section. (See Fig. 7).

“And we noticed that the antennae were mounted too close together. In order to use antenna diversity (which we would always recommend in industrial application) properly in a 5 GHz radio cell we recommended to use 20 times the wavelength as the clearance.”

The wavelength of a 5 GHz electro-magnetic wave is ~5.5 cm. Therefore, the resulting distance between the two access point antennae should be around 1.1 m. Falk said the antennae were also mounted too close to the wall (around 4 cm), causing strong interference due to reflecting waves, resulting in signal cancellations. A minimum distance would be twice the wavelength (2 x 5.5 cm = 11 cm).

“And being located underneath a belt conveyor certainly didn’t help matters, either!” (See Fig. 8).

Raising the antennae from its current height of 2m to around 5 m proved very worthwhile. Another  ‘hidden’ issue Falk uncovered within the cable installation was the amount of redundant cable.

“Here we found 5 m of excess cable stuffed into a 1 m channel. As we all know, buckled or wound up cables cause a very strong cable attenuation problem. Luckily, we were able to utilise the additional cable when the aerials were relocated away from the overhead structures and raised higher,” said Falk.

After making the changes, and using Siemens iPCF (rapid roaming functionality) the system is now operating as it was originally designed to. It has been running without interruption for more than 12 months without any replacement or additional hardware.

For more information, contact: falk.hohmann@siemens.com , call Siemens on 137 222 or visit: www.siemens.com.au