Wireless Ethernet communications
December 2011, Fieldbus & Industrial Networking
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, prioritisation, 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 or a variety of other factors. Also at times connectivity is only required for a single device to send very small amounts of non-critical data, and then it is sometimes not feasible to lay cable.
In cases like these it is necessary to look at an alternative communication medium to copper or fibre-optic cabling, such as wireless. Wireless communications come in various forms. Wireless can be used for relatively short distances up to approximately 3 km, in which case 802.11, more commonly known as WiFi, will be recommended. To set up wireless over slightly longer distances than WiFi, up to around 12 km, WiMAX, also known as 802.16/e, is the common choice. To reach distances greater than this, cellular or satellite connections can be investigated. These allow connectivity to reach almost any location on the globe, depending on the application.
One of the most obvious advantages when using WiFi, or any wireless communications technology, is the lack of cables. When an overflow meter 3 km away from the main network needs to be connected in order to send an update once every hour, it is not feasible to lay thousands of rands of cabling. In these situations wireless communications can save time and money. However, there are disadvantages to using wireless rather than physical cabling, and these need to be taken into consideration.
Stability of the link
One of the biggest disadvantages 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 be taken into account on installation. 802.11 links require line-of-sight between the antennas and so must be kept away from surrounding buildings. Common misconceptions when using wireless are that the signals travel in a completely straight line without dispersion, and 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 is due to a phenomenon with wireless communications called the Fresnel zone. If a direct line-of-sight (DLoS) line is drawn between the antennas, the Fresnel zone will occupy a large area around the DLoS, sometimes 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 to achieve a strong, stable link.
Besides fixed obstructions such as buildings or trees, temporary obstructions such as low lying cloud cover or heavy rainfall must be considered. In the case of adverse weather, the stability of the link could be greatly affected or it could drop completely. High winds can also pose a problem if antennas have not been properly installed. Shifting antennas can cause the link to fail completely, especially in the case of long distance links. An antenna shift of a couple of centimetres can put it completely out of alignment.
Advantages and disadvantages
These are the common advantages and disadvantages that need to be considered for any wireless scenario, whether basic 802.11, WiMAX, cellular or satellite wireless.
802.11 wireless communications do not need to be licensed, which can be both an advantage and a disadvantage. Other advantages are that there are no hidden costs, and 802.11 hardware can simply be purchased and installed at the user’s discretion. The disadvantage, however, is that interference can be 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 the installer can then 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 is not considered the strongest or most stable of links, and so should not be used for critical applications where a link failure could cause major problems, site shutdowns or even death. However, in cases where non-critical remote connections are required, 802.11 wireless can be an excellent time and money saver.
WiMAX, or IEEE 802.16, provides a wireless mesh network with a much greater range than 802.11 wireless – up to 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 of 120 km/h and to be able to cross over to different WiMAX base stations without renegotiating the link. 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 it 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 disadvantage to 802.16 (which again could also be an advantage) is the requirement that frequencies 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 also raises the cost of the solution. However, having a licensed frequency means that interference from an outside source cannot happen legally. This means that interference on a WiMAX link is less of a concern than 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).
Cellular or satellite
In some cases WiFi or even WiMAX cannot achieve the distances needed. A single camera or PLC in a remote area 100 km away from a network would need many different WiMAX links, all back to back, in order to use WiMAX. The expenditure required to set this up and the ongoing maintenance required would not be feasible. In addition if any one of the links has a problem, the end device would be unreachable. The use of WiMAX or WiFi is not even an option in this case.
In these situations either cellular or satellite links would be an option, both having their own advantages and disadvantages. They can be used almost anywhere to gain network connectivity. Cellular communications will still require a cellular tower within range to get a connection, but nowadays most areas will have cellular towers in range. For satellite, all that is needed is an unobstructed view of the sky.
However, both of these technologies come with disadvantages. Unlike WiFi they incur monthly costs. Cellular connections require the user to pay for the amount of data transferred, whilst satellite connections require a monthly rental cost. Also, both of these technologies rely on a third party and the end user does not have direct control in the case of problems.
Although wireless might not always be the first choice, especially in mission-critical applications, it can be one of the best methods when transferring non-critical data from remote sites, or areas where physical cabling is not feasible. However, the use of 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 to ensure the antennas are correctly mounted so as to provide a strong and stable link.
For more information contact Tim Craven, H3iSquared, +27 (0)11 454 6025, firstname.lastname@example.org, www.h3isquared.com