Industrial Communication. WiFi in the 5 GHz Band

Transcription

1 Industrial Communication Whitepaper WiFi in the 5 GHz Band

2 Contents Introduction... 3 IEEE Standards... 3 ETSI Standards... 4 European Harmonization... 4 Dynamic Frequency Selection... 4 Transmission Power Control TPC... 5 Differences between USA and Asia... 5 Radio Channels GHz Band GHz Band... 6 The Advantages of the 5 GHz Range

3 Introduction The historical development of wireless LAN technology is closely connected to the standards that have been introduced by organizations throughout the years. The two most important of these organizations are the Institute of Electrical and Electronics Engineers (IEEE) and the European Telecommunications Standards Institute (ETSI). IEEE Standards In 1997, the IEEE adopted the initial version of the standard for wireless networks. The original version used the license-free ISM band (Industrial, Scientific, Medical 2.4 to GHz) for transmission, making data transfer rates of max. 2 Mbps (gross) possible. In the following years, the standard expanded with a variety of enhancements. One of the enhancements introduced in 1999 was the 5 GHz band in which provided for gross data rate of 54 Mbps. However, the standard, which was termed a, was of little interest to some countries as this band was not authorized for private use due to interference with military radio networks and radar applications. The wider use of 5 GHz WLANs was also restricted by its exclusive use in closed spaces and the relatively low transmission power. The b standard was also adopted in 1999 as an extension to the 2.4 GHz band. With gross data transfer rates of 11 Mbps, this marked the first time in history that speeds could be reached in wireless networks that were truly useful in productive implementations - as compared to the widespread use of 10 Mbps Ethernet networks. Since this frequency range could be used privately in many countries without a license, the b technology rapidly spread. Devices developed under the b standard could be certified by the WiFi Alliance which drove interoperability between products developed by different manufacturers. Due to the proliferation of WiFi products, the WiFi seal was often erroneously regarded as a standard in and of itself. Shortly thereafter, in 2003, the IEEE introduced the new g standard. While it continued to use the ISM frequency band, it was able to achieve a gross data rate of 54 Mbps with the same modulation methods as a (OFDM). Sharing the same 2.4 GHz frequency band made the g standard backwards compatible with devices manufactured in accordance with the b version, which had gained a great deal of popularity by then. This compatibility and competitive performance helped the g standard achieve such great success. With the h enhancement in September 2003, the private use of the 5 GHz band was finally possible even outside of closed spaces. To protect military applications in the 5 GHz band, the DFS and TPC procedures were prescribed. However, the use of DFS and TPC allowed significantly higher transmission power (maximum 1000 mw) than in any of the other standards that were valid until then. The next level of the hierarchy, the n variation, has been released in September This version promises gross data rates of up to 600 Mbps over extremely long distances. In order to protect current technology investments, n devices are compatible with the a/b/g components currently in use. 3

4 ETSI Standards ETSI adopted the first standard for controlling remote data transfers as early as 1996 under the name of Hiperlan (High Performance Radio Local Area Networks). The first version (Hiperlan Type 1) was intended for use in the frequency range of 5.15 to 5.30 GHz with a transmission rate of 20 Mbps. As no manufacturers took up this standard, Hiperlan initially had no practical significance. With the new version in 2000, Hiperlan Type 2, ETSI introduced a WLAN solution that operates in the 5 GHz band similar to IEEE a, and also provides a gross data rate of 54 Mbps. However, as the frequencies and the OFMD modulation method that was also used for a overlapped, it was necessary to adapt the standards between IEEE and ETSI to avoid disruptions to the systems. European Harmonization To standardize the use of the 5 GHz band in Europe, the European Commission issued the ETSI standard on July 11, The member states of the EU were obliged to implement this by October 31, Instead of the three sub-bands described in the a/h standards ( MHz, MHz and MHz for the UK), the ETSI standard regulates three frequency bands with different specifications: 5150 MHz MHz 5250 MHz MHz 5470 MHz MHz The guidelines focus on preventive measures for avoiding disruptions to other systems that use the same frequency band. This includes radar equipment that counts as "primary applications". The "secondary applications" such as WLAN have to change the frequency as soon as a conflict is detected. Dynamic Frequency Selection Dynamic Frequency Selection (DFS) was stipulated to prioritize primary applications. DFS initially assumes that no channel is available in the corresponding frequency band. The WLAN device selects an arbitrary channel at the start and performs what is known as a Channel Availability Check (CAC). Before sending to a channel for 60 seconds (Channel Observation Time, COT), a check is run to see if a different device is already working on this channel and the channel is therefore occupied. If this is the case, then a different channel is checked by the CAC. If not, then the WLAN device can perform the transmission operation. Even during operation, a check is run to see if a primary application such as a radar device is using this channel. This exploits the fact that radars frequently work according to the rotation method, whereby a tightly bundled directional transmission signal is transmitted by a rotating antenna. A remote receiver perceives the radar signal as a short pulse (radar peak). If a device receives such a radar peak, it pauses the transmission operation and monitors the channel for further pulses. If additional radar peaks occur during the COT, then a new channel is selected automatically. A check of this type is required to be carried out every 24 hours. This is why interrupting the data transmission for 60 seconds is unavoidable. DFS is stipulated for the frequency ranges of MHz and from MHz. It is optional for the frequency range of MHz. 4

5 Transmission Power Control TPC Dynamic adjustment of the transmission power is intended to reduce radio interference. Dynamically adjusting the transmission power facilitates the shared use of the MHz and MHz frequency bands with satellite services. TPC should cause an average reduction in the transmission power by at least 3 db compared with the maximum permitted transmission power. TPC determines the minimum transmission power necessary to maintain the connection with the partner (such as an access point). If TPC is not used within these frequency bands, then the highest permissible average EIRP and the corresponding maximum EIRP density are reduced by 3 db. This restriction does not apply to the frequency range of MHz. Without DFS and TPC, a maximum of only 30 mw EIRP is permitted. When DFS and TPC are used, a maximum 1000 mw EIRP is permitted as the transmission power (compared with 100 mw with b/g, 2.4 GHz, DFS and TPC are not possible here). The higher maximum transmission power not only compensates for the higher attenuation of 5 GHz radio waves in air, it also makes noticeably longer ranges possible than in the 2.4 GHz range. Differences between USA and Asia The USA and Asia use different frequency bands and maximum signal strengths that are different than the European standard. In the USA, three sub-bands, each 100 MHz wide, are used for wireless networks in the 5 GHz band. The "lower band" ranges from MHz, the "middle band" ranges from MHz and the "upper band" ranges from MHz. In the lower band, a maximum average EIRP of 50 mw is permitted; in the middle band this is 250 mw and 1 W in the upper band. In Japan, the use of the 5 GHz band is possible to a limited extent: only the lower band of MHz is released for private use. 5

6 Radio Channels 2.4 GHz Band In the EU, up to 13 DSSS channels are available within the usable frequency range of 2400 to 2483 MHz. In the rest of the world, only the first 11 of these are available. Due to the fact that the frequency ranges used on the channels partially overlap, no more than three channels can be operated in parallel without interference between the frequencies. 5 GHz Band In the usable frequency space of 5.13 to GHz, up to 19 channels are available in Europe, divided into frequency ranges to which different conditions of use can apply: MHz (channels 36, 40, 44 and 48) MHz (channels 52, 56, 60 and 64) MHz (channels 100, 104, 108, 112, 116, 120, 124, 128, 132, 136 and 140) Band 3: MHz (channels 147, 151, 155 and 167) Note: The frequency ranges and radio channels in band 3 may only be used in Great Britain. The following table shows which channels may be used in the different regions. Table 1: Channels to be used in different regions 6

7 Frequency Ranges for Indoor and Outdoor Use The use of the methods described in ETSI for reducing mutual interference in the 5 GHz band (TPC and DFS) is not stipulated for all fields of application. The following table provides information about the permitted use and corresponding transmission powers within the EU: Table 2: Permitted Transmission Powers within the EU Note: Other regulations may apply to use in other countries. Please refer to the current wireless network regulations for the country in which you want to operate a wireless LAN device, and make sure you set the country of operation in the WLAN settings. The Advantages of the 5 GHz Range When evaluating the various standards in the group, the obvious question arises as to why the a standard should be further enhanced by h despite gross data rates that are identical to the g standard and a lack of compatibility with older equipment. The question regarding compatibility lost momentum when dual-band components entered the market. These devices can selectively transmit according to the a/b/g standards rendering them capable of adapting ideally to the possibilities provided in the respective environment. Since the compatibility mode between the standards in the 2.4 GHz band is paid for by significantly reduced transfer rates, b/g compatibility does not play a major role in professional applications. The a/h standard's main advantages are clearly derived from the frequency band used: Free channels: In the 2.4 GHz band, only 13 channels are available. Moreover, these channels overlap excessively with one another. Due to this overlapping, the maximum possible number of parallel independent connections is limited to 3 channels. Low level of interference: Since the ISM band has been released for use for a number of years, a great number of radio applications have been established for the 2.4 GHz band. As such, b/g standard WLANs are often forced to compete with interference from these applications. In contrast, the relatively recent release of the 5 GHz band for private use gives WLAN usage a very prominent position in this frequency range. Other than military applications, which are isolated from a/h by special mechanisms, there are virtually no sources of interference. Higher performance: The 5 GHz band allows for significantly higher performance than the 2.4 GHz band. This makes 5 GHz band WLANs much better suited for outdoor applications and directional radio connections (point-to-point) than wireless connections in the 2.4 GHz band. 7

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