With the proliferation of Ethernet in the Industrial, Utility and ITS industries, wireless Ethernet is also being used more often, especially for extremely remote sites and sites where only a few end devices need communications to the network. Wireless, however, does not provide as stable a “physical” connection as a wired network does. For this reason extra planning must be undertaken when designing a wireless network, or a wireless extension to an existing network.
The first step that should be taken in setting up a wireless network is a full, in depth site survey. This involved many steps and points that need to be considered. The first is obviously the terrain at the site. The terrain needs to be checked between each point of a wireless link, to make sure that line of sight (LOS) is not interrupted. An important point that also needs to be taken into account here is that when talking about wireless connections, there are two different “types” of LOS, the direct LOS, which is a straight line from point A to point B, and then the Radio line of sight, which is an oval shaped zone surrounding the Direct LOS. For a truly reliable wireless link at least 60% of the Fresnel zone must remain clear of obstructions. This is due to the fact that obstructions will cause reflection of the radio waves, too much of which can cause interference that may bring down the link or cause the link to perform erratically.
As part of the full site survey, a wireless spectrum scan is required. For this step, industrial Ethernet specialists will use special hardware to scan the areas that wireless links will be installed. By walking the area over which the wireless links will pass, the specialist will be able to analyse the interference in the area, put forth by other Wi-Fi devices, and other wireless devices that operate in the same frequency band (2.4 GHz). Also one will be able to determine the current status of the wireless channels in the area. A wireless channel is a subdivision of the 2.4 GHz frequency, for instance channel 1 begins at 2.412 and ends at 2.416. Most wireless devices by default will, in the process of creating a wireless link, determine the least busy channel. However occasionally it can be the case that a wireless link will be established on a channel that is reserved for some other devices that could have been off at the time the wireless link was established. Two or more wireless links on the same channel in the same area will interfere with each other, meaning both links could experience instability or could fail completely. For this reason a proper spectrum scan (Whilst the site is in a full running state) will help to determine the best channel to be used for the link, and rather than allowing the wireless devices to choose their own channel for the link, this can be forced to a certain channel if the hardware allows it. Another important reason for performing a full site survey using a spectrum analyser is to find out what other interference is in the area. Many electronic devices can cause interference in the wireless spectrum, which could cause links to become unstable or drop completely. These site surveys must always be done when the site is active (i.e. machinery operating etc.) as one needs to be able to determine the true interference that will be affecting the wireless during a running state. Also any devices or machinery in the area that are used intermittently should be turned on during the survey to determine their interference, such as microwave ovens, passive perimeter beams, smelters etc.
This leads us to correct selection and placement of antennas. Antennas need to be selected based on their intended role in the wireless setup. For instance, for a wireless node at the edge of the wireless network that only needs to connect to a single access point (AP) it would be better to use a directional antenna pointing directly at the AP. However, for an AP that needs to allow multiple client nodes to connect to it would require an omnidirectional antenna to cover the area all around it. The gain of the antennas used is also important. For a relatively short wireless link a lower gain antenna would be recommended so as not to overpower the receivers, but for a long wireless link one would look at using a higher gain antenna to provide the required power to bridge the gap. As wireless is so easily affected by external conditions, when installing it is recommended that a variety of antennas be taken along, as this will allow different antennas to be tested to find the combination that gives the strongest, most reliable link.
Next in the process comes the initial install of the system. When installing the hardware, one thing that needs to be taken into account is the distance from the external antenna to the wireless unit. Due to the fact that cable will always impose some dB loss on the wireless signals, the antenna cable should not be too long. The actual length of antenna cable one can use will depend on a lot of factors, such as the power of the wireless signals as they are pushed out of the device, the actual dB loss of the cable and the dB gain of the antenna being used. These need to all be taken into account to make sure that the signal will be strong enough on the receiving end. This is why it is recommended to ensure there are multiple different antennas with varying gains, as this means they can be tested to find the most suitable for the installation. The reason one cannot just the highest gain antenna is that in some cases too strong a signal can cause large amounts of signal reflection, which can cause a link to become unstable the same as with too low a power.
Once the equipment is installed and the basic link is up, the link should be fully tested for its intended application, and any relevant tweaking should take place. This can include trying different antennas or changing the alignment of antennas currently in place, as well as changing the configuration settings of the wireless unit. Many top-end wireless units have many configuration settings that can be tweaked to get the strongest, most stable wireless link available, such as raising or lowering the output power of the wireless signals to the antenna, configuring the physical distance between antennas and so forth.
The wireless link is in place and running smoothly, but this does not mean that the installer can wash their hands of the project. Wireless links should be checked on a regular basis to make sure that the link integrity does not deteriorate. This can generally be done remotely, with the end user collecting the logs of the units and sending them, along with any relevant screenshots of links strength etc., on to the support team for the product to check for any potential problems. Also if any construction or new machinery is planned in close proximity to the Fresnel zone or to the wireless units themselves, another spectrum scan should be undertaken once the building/machinery is in place to analyse the possible impact on the link.
Wireless links can be invaluable in the industrial, utility and military environments, due to the time and money saved by not having to plan for laying of cables. However the planning, installation, configuration and maintenance need to be properly implemented in order to make sure that the link will be as reliable and stable as required for the application. For more information on wireless units and projects please contact H3iSquared.
Tel: +27 (0)11 454 6025
Email: info@h3isquared.com
Website: www.h3isquared.com