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High-Speed Ethernet radios. How well do they work in traffic industry applications?by Oystein Konsmo, Principal of Arizona Technology Trading & Consulting October, 2004
However, not everything is as rosy as it may seem on the surface. In order to understand what the benefits and disadvantages of these “broadband” Ethernet radios really are, it is necessary to look at the entire range of radio specifications. By carefully studying the specifications, one can determine that many of these radios were designed for the indoors (as most Ethernet products originally were) and that the advertised speed is a theoretical speed rather than the true output that can be realized in an industrial setting. Let’s take a look at some of the important parameters that will determine how well a wireless device will work in the harsh, outdoors of Arizona. But First: Know Your Application Requirements Unless you understand the demands your application will place on your equipment, it will be very difficult to make the right decision. Before you determine which type of radio technology will be right for your application, you need to carefully consider the real requirements of your system. Some of your considerations should be:
To help you make a sound decision, let’s examine the following important specification parameters: Making the correct choice of wireless technology will be your most important decision. If you decide to use a radio that operates in one of the license free areas of the wireless spectrum, oftentimes referred to the ISM bands, you need to select from two core spread spectrum technologies: Direct Sequence (DSSS) or Frequency Hopping (FHSS). To make a sound decision, one must first understand the basic technology on which spread spectrum is based. Spread spectrum is a wireless communication technology where the signal is spread across the available frequency band. Spreading the data across the frequency spectrum greatly increases the bandwidth (the amount of data that can be transmitted at one time), and makes the signal more resistant to noise, interference, and eavesdropping. Simply put, the transmitter takes the input data and spreads it in a pre-defined method. As well, each receiver must understand this pre-defined method and de-spread the signal before the data can be interpreted. There are two methods to perform the spreading: FHSS and DSSS. FHSS spreads the signal by “hopping” a narrow band signal as a function of time. DSSS spreads the signal by expanding it over a broad portion of the radio band. In choosing between these two technologies, one must consider which would be more reliable in the “non-licensed” environment of the ISM bands. The basic operation of DSSS radio technology presents some inherent reliability limitations and concerns. A DSSS system uses a wide frequency channel in which to transmit and receive information. One can see that operating with a fixed wide frequency channel makes DSSS vulnerable to interference: An interfering signal with a frequency near the DSSS radio's frequency can block the receiver, rendering the radio inoperable. In contrast, an FHSS radio does just what the name implies: It “hops” from frequency to frequency over a wide band. Therefore, FHSS presents the real advantage of being highly immune to interference, as it is always changing RF channels. This technology also offers further interference immunity because it allows for a large number of user-selectable RF channels as well as a large number of different “hop patterns”. Many of the “broadband” Ethernet radios are using DSSS technology, because high-speed capacity requires a wide frequency channel which FHSS cannot provide. The choice of a frequency band will affect your system’s data throughput, how immune your system is to interference, total transmission distance, and finally how robust or critical line-of-sight will be. There are three available frequency ISM bands (license free) in North America: 900 MHz, 2.4 GHz and 5.8 GHz. The higher frequency bands allow for a wider frequency channel, which provides the potential for higher data throughput. However, the lower the frequency band, the more robust and better penetration the RF signals will have. Also, the transmission distance will be reduced when you go up to a higher frequency band and the requirement for a true line-of-sight will increase. Hence, you have to make some tough choices. What is more important: Data throughput or system robustness and transmission distance? Many wireless devices have been manufactured to operate in the 2.4 GHz band. The 2.4 GHz band is now shared by many wireless devices such as garage door openers, indoor wireless LANs, hospital monitoring equipment, retail bar code readers and even cordless telephones, to name a few. The competition for space is fierce! As thousands of these devices crowd the 2.4 GHz band, the 900 MHz band has become - and will continue to become - less congested. So not only is the 900 MHz band a more robust frequency to use, it is a better choice both for today and for the future. Most high-speed Ethernet radios will operate either in the 2.4 GHz or 5.8 GHz band, and as mentioned before, spread their signal over as wide portion of the available spectrum as possible. This means these radios will be very prone to interference not only from other systems operating in the same frequency band, but also from radios within the same system. Even though you can get most of these Ethernet systems to work - at least for a while – it may take a significant investment in infrastructure and labor. How well can your receiver “hear” your transmitter? This is an important parameter since a radio with poor “listening” capability can dramatically increase your infrastructure cost. Receiver sensitivity is measured in negative dBm and it represents the least amount of signal strength that the radio requires to process error free data. In essence, it tells you what the receiver radio’s listening capability is. The rule is: The lower the dBm value, the better the listening capability of the radio. A good industrial strength wireless radio should have a sensitivity of -108 dBm or lower while many wireless radios developed for the indoors will have a dBm of -90 or higher. A dBm of -108 represents a 64 times better listening capability than a dBm of -90. There is always a trade-off between throughput and receiver sensitivity. Generally, if you increase the transmission speed of a particular radio, the sensitivity will be sacrificed. That’s why many specifications will read: -90 dBm at 1 MBPS, -85 dBm at 5 MBPS, etc. Many Ethernet radios can have low listening capability at speeds of 1 MBPS, but the sensitivity will actually decrease as the speed (data throughput) increases. Hence, to achieve the maximum speed that some of these radios are advertised at, significant investments in antennas and other auxiliary equipment will be needed. If you use a radio with a low output power, the result may be that you have to employ expensive and difficult to install dish antennas to compensate for the lack of power. The maximum allowable Radio Output Power in the ISM bands is 1 Watt. A good industrial radio will provide several output power options. Many of the Ethernet radios have a much lower maximum RF Output Power (150 milliwatts), which means large directional antennas may be required to compensate for this low output power level. These large dishes may require expensive installation and reinforced mountings. The number of available channels used for transmission of data will affect the speed of the radio, how secure the radio is and how immune it is to interference. The number of available channels is dependent on the wireless technology you chose (DSSS or FHSS) and the frequency band of operation. The width of the 900 MHz ISM frequency band is 26 MHz while the 2.4 GHz band is 84 MHz wide. A DSSS radio spreads it signal over 20MHz channels while an FHSS radio uses a much narrower channel spread (200 KHz). Hence, a DSSS radio operating in the 2.4 GHz band will typically have up to 5 channels, while the maximum available channels with FHSS could be approximately 200 in the same frequency band. The more channels you can chose from, the better chance of selecting a channel that will give you minimum interference. The wide frequency channel limitation of DSSS is something to be concerned about even more when using DSSS technology in high-speed Ethernet radios. This is because high-speed capacity requires a wide frequency channel, but DSSS has fewer alternate frequency channels to use when interference is encountered. How important is it to protect your data from intruders? Many DSSS Ethernet radios employ one of the open Wi-Fi standards (802.11X) which originally were designed for indoor use. These radios typically send and receive data on one channel only, and many hacking programs are available. So, unless the radios are equipped with heavy encryption algorithms, these radios are not very secure. Radios built on the FHSS technology, however, can choose among a large numbers of channels, and in addition, can employ many unique hopping patterns. Most of the FHSS technologies are in addition proprietary which makes these radios very secure, indeed. How important is high-speed data throughput? Sometimes, purchasing high-speed-but-designed-for-indoors Ethernet radios is like discovering the Ferrari you purchased has bicycle tires. You will be able to achieve the potential speed of the engine, but only after meeting the additional costs of tires and suspension, because a high-speed engine alone is not enough; every part of the car must also be capable of supporting that speed. Without the rest of the infrastructure system, it’s a Ferrari on bicycle tires; sure, you’ve got high-speed specs, but you can only go 5 miles per hour (and only a few yards). Do not be seduced by the high-speed specs of an Ethernet radio that brings with it hidden infrastructure costs. There is more to total performance than just speed specifications. Wireless products designed expressly for traffic applications, although their speed specs may appear slow, work very efficiently in real-world conditions and actually out-perform and are much more cost-effective than the Ferrari-on-bicycle-tires. Enduring high temperatures - the big difference between radios designed for indoor and outdoor. Wireless radios designed to operate in harsh, outdoor environments should be able to endure temperatures of up to +80 degrees Celsius. Many of the Ethernet radios that were originally designed for indoor use and now are being marketed and sold as “industrial’ radios have temperature specs of maximum +55 degree Celsius. It is just a matter of time before these radios will start to malfunction if installed in a controller cabinet in Phoenix unless you install fans or other cooling devices that can cool the temperature during the hot summer months. Ethernet radios in traffic applications Where do Ethernet radios fit into the traffic industry? Examples of Ethernet wireless in traffic include video detection and video surveillance to a control room monitoring traffic flow. As pointed out earlier, you must look for more than the maximum theoretical data speed on a radio specification. All Ethernet radios must be suited for rugged outdoor use, and not bring with them hidden infrastructure costs. Wireless Ethernet is ideal for large network applications because it offers a standard protocol. As opposed to serial-based systems, which, because they are timing-driven, are limited to exist as pockets of twenty or thirty devices, wireless Ethernet allows for mass control. Total cost of ownership – A final word of advice. Finally, one word of advice: When you compare prices on radios, don’t just look at the initial investment in the radio devices. You really need to consider the total cost of ownership. This will include: 1) Antennas and other auxiliary hardware such as mounting devices, cables and connectors. Some radios will require expensive and hard to install dish antennas while others only need simple to install, directional Yagi antennas. 2) Installation costs. Radios operating in the higher frequencies have more stringent requirements to line-of-sight and may require dish antennas to work. This will increase your labor costs of installing the system. 3) If you deploy a large system of DSSS radios, you may experience interference between your own radios as well as from other wireless devices operating in the same frequency band. The “broadband” Ethernet radios typically use only one channel, and serial DSSS radios perhaps five channels, giving you significantly fewer options than an FHSS radio before you have to move antennas around to get the signals through. 4) The environment is dynamic and not static which means the level of interference will most certainly change during the life of a system. If you do not have a system where most of these changes can be addressed via simple software settings, the maintenance costs of your system will dramatically increase.
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