As the use of wireless technologies is increasing in the process and manufacturing industry, so is the installed base of IEEE 802.11b/g/n products that operate in the worldwide free 2.4 GHz ISM band. Besides Wireless LAN IEEE 802.11b/g/n, other wireless technologies like Bluetooth technology, IEEE 802.15.4/ ZigBee/Wireless HART and several proprietary technologies operate in the 2.4 GHz band.
With so many technologies crowding the same frequency band, interference problems can occur. To make sure that industrial wireless solutions are robust, there are basically two solutions: either to carry through extensive frequency planning and use special antenna solutions (like leakage cables) in the industrial 2.4 GHz setting, or to use the 2.4 GHz band for office and IT communication and then use the 5 GHz band for the manufacturing and M2M communication.
Differences in channels and usage for the 2.4 and 5 GHz bands
The Wireless LAN IEEE 802.11b/g radios utilize the 2.4 GHz frequency band (2.412 – 2.472 GHz) and the IEEE 802.11a radio utilizes the 5 GHz frequency band (5.180 – 5.825 GHz). IEEE 802.11n radios can operate in either frequency band. There are the following worldwide implementation attributes:
- The 2.4 GHz ISM band provides 13 overlapping channels spread equally over the frequencies plus a fourteenth channel used in Japan with the center frequency of 2.484 GHz. This leaves available only three non-overlapping channels in the 2.4 GHz band. In order to avoid interferences between the Wireless LAN connected devices, these channels have to be used very efficiently. Installation requires careful frequency planning or expensive installation of solutions such as leakage cables. In other words, the installation costs can easily become higher than the actual wireless equipment installed.
- The 5 GHz ISM band is divided up into sub-bands called U-NII bands (Unlicensed National Information Infrastructure) and are usually named U-NII-1, U-NII-2, U-NII-2e, and U-NII-3 where U-NII-3 is not freely available worldwide. In total, this gives 23 non-overlapping channels where four of these have limitations based on location*. Today, most available Wireless LAN solutions in the 5 GHz band use the U-NII-1 band (5.18-5.24 GHz) with frequency channels 36-48. However, there are also some suppliers that have extended the range to include the U-NII-2/2e band (5.26-5.70 GHz) with frequency channels 52-140.
|Channel||U-NII Band||Frequency (MHz)||United States 40/20 MHz||Europe 40/20 MHz|
Table 1: Table of U-NII bands in the 5 GHz frequency band (ref. wikipedia.org).
Pros and cons of the 2.4 GHz and 5 GHz standards
Wireless LAN IEEE 802.11b/g/n is already well-established with its huge installed base and a wide range of products made available. Besides its wide use, the 2.4 GHz band offers the advantage of operating in an ISM band available worldwide. Further, the achieved range using the same output power is better on 2.4 GHz compared to radios using the higher frequency 5 GHz band.
As shown in the table above, the entire 5 GHz ISM band is not available for worldwide use. Further, availability of components and products are still somewhat limited compared to the 2.4 GHz band.
The greatest strength of the 5 GHz band is the availability of 23* non-overlapping channels; 20* more channels than what is available in the 2.4 GHz band. Since there is no other wireless technology that “fights” for the radio space, the 23* available non-overlapping channels can provide a possibility for easier planning of an interference-free and stable wireless communication. Another advantage of the 5 GHz band is that the greater number of available channels provides for increased density, which means that more wireless devices can be connected in the same radio environment.
5 GHz band radar detection – Dynamic Frequency Selection (DFS)
The use of Wireless LAN in the U-NII-2/2e bands (channel 52 -140, frequency range 5.260––5.725 GHz) requires radar detection. Within the operation context of the Dynamic Frequency Selection (DFS) function, a device shall operate as either a master or a slave. The requirements for a slave device, which is typically a client in an infrastructure, are as follows:
- A slave device shall not transmit data before receiving an appropriate enabling signal from a master device.
- A slave device shall stop all its data transmissions whenever instructed by a master device.
- Devices operating as slaves shall only operate in a network controlled by a device operating as a master.
- A master device shall detect radar signals.
- A master device shall only start operations on available channels.
- During normal operation, a master device shall monitor the operating channel (in-service monitoring).
- If a master device has detected a radar signal during in-service monitoring, the master device shall instruct all its associated slave devices to stop transmitting on this channel.
Range and performance
The radio wavelength in the 5 GHz band is half of the wavelength in the 2.4 GHz band. As a consequence, a radio module using the 5 GHz band will have a narrower range than a radio operating on the 2.4 GHz band using the same output power. How much less the range will be is hard to predict as it depends on the radio conditions at the location in consideration. Further, diverse materials absorb frequencies differently which, in turn, dramatically affects the range. In order to learn the exact range, the solution has to be tested live.
Tests in factories in the 5 GHz band have shown that the range can be between 50 meters to 100 meters in free line-of-sight. Obstacles, interference, materials, and use of large data packages can decrease the range substantially.
By using the 5 GHz band for Wireless LAN communication, a number of advantages and cost decreases can be achieved. By adding 23* possible Wireless LAN channels, frequency planning, density (the number of active wireless devices within the radio coverage space) and the installation complexity can be dramatically improved. An extra bonus would be the freeing up of the 2.4 GHz band for other radio technologies.
These advantages, together with increasing availability of 5 GHz industrial products, will increase the use of the 5 GHz band greatly in the near future. Up till now, the use of the 5 GHz band in industrial applications has been more or less limited to products such as smaller access points and small compact clients (based on the same platforms as the access points). Already available on the market are OEM wireless modules for integration in various industrial products as well as serial Wireless LAN clients for integration of smaller devices and existing serial communication based products.
* For FCC channels 120 – 132, use is restricted near airports due to the interference risk of the Terminal Doppler Weather Radar (TDWR). (ref. FCC KDB 443999). Canada requires a restriction on the channels 120 – 128.
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