Published:2011/8/18 2:26:00 Author:Amy From:SeekIC
Stefan Tauschek
Wireless local area networks
Now that you understand the techniques described above, we can enlighten you a bit regarding the various types of wireless networks currently in use. As can be seen in Figure S, frequency hopping is the technique used by Bluetooth and the original primitive WIAN standard, which bears the IEEE designation ’802.1 1’- The more recent 802.1 1 a, 802. II b and 802.1 1 y standards are based on DSSS, since this spread-spectrum technique is easier to implement. Naturally, other typos of radio links using conventional AM or FM modulation can also be used in the ISM bands; they ore typically used for applications such as remote garage-door controls and other types remote-action system (using narrow-band FM).
Fourteen years after work began on developing the 802.1 1 family of standards, rapidly increasing market penetration of WIAN products can be seen in the form of public ’hot spots’ in airports and railway stations as well as wireless networks inside companies. The ranks of technology addicts and enthusiastic readers of Elektor Electronics hove even anticipated (his technology, with some rather remarkable results. What would you say to the idea of a WIAN antenna for less than €5? It’s actually quite easy. All you need lo make a working directional antenna is an empty tin, a few screws and a coaxial RF connector. This little marvel is called a ’cantenna’, which is a combination of con and antenna (Figure 6). Using inexpensive materials from o DIY shop, you con build an 802.1 1 b antenna with o range of more than 50 km.
Currently, the most widely used standard is 802.1 lb, which employs the license-free ISM band from 2.4000 GHz lo 2.4835 GHz using the DSSS spread-spectrum technique The range is 30-100 m with a transmitted power of 100 mW, which is adequate for most applications, although the raw data rote of 11 Mbit/s leaves something to be desired, since the value obtained in practice is rarely more than 4-6 Mbit/s. An additional drawback arises from the fact that in most WLAN networks a large number of clients want to communicate with each other, so the number of available channels can be quickly used up. The 802.1 I b standard defines 14 channels with a bandwidth of 22 MHz pet channel, beginning with channel 1 at 2.412 GHz. Channel 2 starts at 2.417 GHz, offset from channel I by 5 MHz. The channels overlap, so they cannot all be used at the same time in a single network (Figure 7). If several access points are needed in a WLAN, it is possible to use (for example) channels 1, 7 and 14 concurrently.
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