In the industrial, utility and ITS industries, there has recently been a migration towards using Ethernet as the network of choice. As a world standard, Ethernet is the logical choice as more and more manufacturers start to develop hardware with Ethernet interfaces. With the mechanisms inherent in Ethernet, a network can be installed and configured for almost any application. Ethernet provides redundancy, prioritization, VLANs, and many other options that can enable a single network to be used for everything from critical applications such as control and automation, to less important applications such as email or simple file transfers.
However, at times Ethernet connectivity will be required in remote locations or areas where it is difficult or impossible to lay physical cabling for connectivity. This could be due to existing structures, terrain and a variety of other factors. Also at times one only requires connectivity to a single device to send very small amounts of non-critical data, and in these cases it is sometimes not feasible to lay cable.
In cases like these we need to look at an alternative communication medium (rather than copper or fibre optic cabling), such as wireless. Wireless communications come in various different forms. Wireless can be used for relatively short distances (Up to approximately 3km or more depending on the hardware used), in which case 802.11, more commonly know as WiFi, will be recommended.
To set up wireless over slightly longer distances than WiFi (Up to around 12km, again depending on hardware) one would look at using WiMAX, also known as 802.16/e. If one needs to reach distances greater than this, cellular or satellite connections can be investigated, which allows us to bring connectivity to almost any location on the globe, depending on the application.
One of the most obvious pros when using WiFi, or any wireless communications technology, is of course the lack of cables. When an overflow meter 3 km away from the main network needs to be connected to the network in order to send an update once every hour, it is not feasible to lay thousands of rands of cabling in order to accomplish this. These are the situations where wireless communications can save time and money. However there are cons to using wireless over actual physical cabling, and these need to all be taken into consideration.
One of the worst cons when using wireless communications is the stability of the link. Wireless links are adversely affected by external conditions such as weather or surrounding buildings and this must all be taken into account on installation. 802.11 links require line of site between the antennas, and so must be kept away from surrounding buildings. A common misconception when using wireless it that the signals travel in a completely straight line without dispersion, and for this reason people can often be tricked into thinking that wireless antennas that can “see” each other through very tight gaps should still be able to connect strongly. This is not the case, and this is due to a phenomenon with wireless communications called the Fresnel zone. As we can see in the below diagram, if we draw a direct line of sight (DLoS) line between the antennas, the Fresnel zone will occupy a large area around the DLoS, sometime also called the Radio Line of Sight. As a rule of thumb 60% (or more) of the Fresnel zone must always remain clear of obstructions for us to achieve a strong, stable link.
Figure 1 – The Fresnel Zone
Besides fixed obstructions such as buildings or trees, one must also take into account temporary obstructions that could occur, such as low lying cloud cover or heavy rainfalls. In these cases of adverse weather the stability of the link could be greatly affected, or the link could even drop completely. High winds can also pose a problem if antennas have not been properly installed, as shifting antennas can cause the link to fail completely, especially in the case of long distance links where an antenna shift of a couple of centimetres can mean the antennas become completely out of alignment thus breaking the link.
These are the common pros and cons that need to be considered for any wireless scenario, whether using basic 802.11, WiMAX, Cellular or Satellite wireless. Now we will delve a bit deeper into the different technologies and their specific pros and cons.
802.11 wireless communications do not need to be licensed, which in itself provides a pro and a con. The pro is that there are no hidden costs, and that 802.11 hardware can simply be purchased and installed at the user’s discretion. The con to this, however, is that we can get interference being caused by other surrounding wireless devices, as it is legal for anyone to setup their own wireless. For this reason a site survey including a wireless frequency scan is essential when planning for a wireless installation. A frequency scan will show what frequencies are being used in the area, and using this scan, the installer can determine which frequency to use so as to avoid interference. In more populated areas it is worth performing frequency scans every few months to make sure that no new wireless systems have been put in place that could cause interference.
WiFi (802.11) is not considered the strongest or most stable of links, and so should not be used for critical applications where a link going down can cause major problems, site shutdowns or even death. However in cases where non-critical remote connections are required, 802.11 wireless can be a great time and money saver.
Figure 2 – 802.11 (WiFi) Access Point
Next we can look at WiMAX, or IEEE 802.16. WiMAX provides wireless mesh network with much greater range than 802.11 wireless (Up to approximately 12 km radius or more, depending on the hardware). WiMAX also does not require Line-of-Sight (Although bandwidth and performance may be affected in a non-LoS scenario, depending on the environment). The 802.16e (Enhanced) standard also calls for an allowance for WiMAX subscribers to be able to travel at up to vehicular speeds (120 km/h) and to be able to cross over to different WiMAX base stations without renegotiating the link (i.e. no drop in communications as a new link is negotiated). This is different to 802.11 wireless which has to renegotiate the link to each new access point as it moves around. This means that for applications such as VoIP (Voice over IP) 802.11 is not suitable if users need to hold calls whilst moving around the site, however 802.16e will allow this movement between different WiMAX base stations without dropping the call.
The con to 802.16 (which in its own way is also a pro) however, is the requirement that frequencies need to be licensed for specific areas before they can be used. This licensing is handled by ICASA, the regulator for the South African communications sector, or the equivalent body in countries other than South Africa. This licensing can prove to be a time consuming task, and will also raise the cost of the solution, however having a licensed frequency means that interference from an outside source can not happen legally. This means that interference on a WiMAX link is not so much of a concern, unlike with 802.11 WiFi.
Finally WiMAX will require regular renewals of subscription fees for the licensing of the frequencies, unlike WiFi which is a once off payment for the hardware (Not counting spares or possible repairs on the hardware).
Figure 3 – 802.16e (WiMAX) Base Station
Finally in some cases WiFi or even WiMAX cannot achieve the distances needed. For instance look at a single camera or PLC in a remote area that is 100km away from the network of interest. If we wanted to use WiMAX we would end up needing multiple different WiMAX links, all back to back. In the end the amount of expenditure required to set this up is completely unfeasible, not to mention the maintenance that would be required and the fact that if any one of the links has a problem the end device would be unreachable. So using WiMAX or WiFi is not even an option in these cases.
When dealing with situations like this we can look at using either cellular or satellite links, both of which have their own pros and cons to consider. Without going too far into the technical working of these two mechanisms, they are both very similar. The greatest pro to using either of these options is the fact that they can be used almost anywhere to gain network connectivity. For cellular communications you will still require a cellular tower within range to get a connection, however in today’s day and age most areas will have cellular towers in range. For satellite all you require is an unobstructed view of the sky.
Figure 4 – Industrial Cellular Router
However both of these technologies come with their own bunch of cons. First off is the fact that they will require monthly costs, unlike WiFi. Cellular connections require the user to pay for the amount of data transferred, whilst satellite connections require a monthly rental cost. This also brings up the fact that both of these technologies are relying on a 3rd party. If there is a problem with a cellular tower or a satellite these links could be disrupted until such time as the third party manages to sort out the problem. In these situations the end user will have very little control over how quickly the problem is sorted, unlike WiFi and WiMAX where the end user can address the problem directly.
So we can see that although wireless might not always be the first choice, especially in mission critical applications, it can be one of the best methods when needing to transfer non-critical data from remote sites, or areas where physical cabling is unfeasible. However, using wireless links should only be considered after proper planning and consideration of all the factors involved. Site surveys are essential in order to properly assess the installation requirements prior to purchasing of equipment, and professional installers must be used that will ensure the antennas are correctly mounted so as to provide a strong and stable link.
For more information on using wireless in your network feel free to contact H3iSquared.
Tim Craven, H3iSquared
+27 (0)11 454 6025