The Institute of Electrical and Electronics Engineers (IEEE), an international professional organisation, has published a family of 802.11 standards for WLANs; their output characteristics are shown in Table 1.
Max. transmission power (mW)
Mean transmission power of beacon (mW)
Mean transmission power (max.) (mW)
2400 – 2483.5
2400 – 2483.5
up to 200
Max. raw data rate (MBits/s)
Table 1: Properties of the various IEEE WLAN standards
The most widely used standard nowadays is 802.11g. The frequency ranges of the a and h standards are also used for other services in Switzerland (and Europe). This is why a-standard devices may only be used at reduced power and inside buildings. The h standard has been adapted for Europe in such a way that it can free up the frequency immediately if it is needed by another service.
The more recent standards 802.11 a, g and h incorporate high data rates. If several devices try to use an access point at the same time (e.g. several computers in a classroom), the transmission capacity of the connection is split, with the data rate for each device dropping accordingly.
Regulating transmission power
In the h standard the transmission power is regulated automatically depending on the reception quality. In addition, the transmission power of g-standard and h-standard access points can be regulated by the software to suit the area to be served.
The level of energy emitted is determined primarily by the volume of data being transmitted. Even when no data are being transmitted, the access point still sends a signal (the beacon) lasting 0.5 ms every 100 ms to enable the other devices to synchronise with it. If a 100 mW access point is only transmitting the beacon, the mean energy emitted over time is 0.5 mW. However, if a large volume of data is being transmitted, the mean energy emitted can be up to 70 mW.
The radiation pattern is very irregular because a device can transmit as soon as no other data transfer is taking place. The beacon transmitted by the access point produces relatively evenly pulsed energy with a repetition frequency of 10 Hz, for example.
The effect of distance
The energy emitted by an antenna decreases greatly with increasing distance. It can also be weakened or reflected by obstructions such as walls. The data rate can therefore drop if there is a considerable distance between the access point and the networked devices, or if data have to contend with obstacles.
Devices that use the h standard have a longer range in the open because their transmission power is greater. At the same time, however, the energy emitted by these devices is attenuated to a greater degree by walls because of the higher frequency at which they operate, resulting in a lower range inside buildings.
WLANs are very sensitive; in other words, they can network even if the energy level is very low.