Transmitter power and health protection
Since WLANs operate in microwave frequencies, the range of the radio link of the equipment must be restricted by using power limiters. An isotrop antenna serves as reference guide to determine this. (Isotrop antenna means a theoretical, perfect antenna which radiates uniformly in all spatial directions.)
The range of the wireless connection in WLANs is restricted by limiting the power of the devices for health protection reasons:
- max. 100 mW Equivalent Isotropic Radiation Power (EIRP) at 2,4 – 2,4835 GHz indoors and outdoors
- max. 200 mW EIRP at 5,18 – 5,32 GHz indoors
- max. 1.000 mW EIRP at 5,47 – 5,725 GHz indoors and outdoors
This gives an environment-dependent range in buildings of 30-50 m and outdoors over open ground of 100–300 m in practice (manufacturer specifications).
Connections for WLAN
To prevent a legally prohibited increase in the transmitter power, special coaxial connectors are used at the WLAN access points and their antennas.
This approach originates from the USA, where only one designated antenna for the whole system may be used, and which must be fitted with a unique interface.
In the majority of cases, this unique connection is based on the well-known SMA and TNC connector series and differ only in a socket contact installed as an inner conductor in the plug for example. These connectors are therefore referred to as reverse polarised from which the series designation suffix R-SMA (or synonymously RP-SMA) and R-TNC (or synonymously RP-TNC) is derived. In contrast to the SMA and TNC series, the reverse polarised connectors R-SMA und R-TNC are not internationally standardised.
In Europe a different approach is taken, which makes the operator legally responsible for preventing the permitted transmission power level to be exceeded.
For improving the wireless range, the access point and antenna are often separated, or point-to-point antennas are used. Since with point-to-point antennas, (as opposed to omni-antennas) the signal is radiated in one direction only, the transmission power is increased accordingly in this particular direction. It may be necessary, therefore, to reduce the transmission power of the Access Points by using an attenuator.
Attenuators with 3 dB, 6 dB, 10 dB und 20dB are available for individually reducing transmission power.
WLAN networks which most of us are familiar with privately from the WLAN-capable router for wireless Internet access at home represent a usually free alternative to the commercial wireless networks such as UMTS (Universal Mobile Telecommunications System) or recently LTE (Long-Term-Evolution). Freeof charge because the two frequency bands that they use are allocated worldwide without licence and there are therefore no fees. They are also usually provided by non-commercial providers such as associations, public institutions or private persons.
The name of the wireless standard IEEE 802.11 is often used synonymously for WLAN. This is not quite correct because this is a standard which enables the installation of wireless networks. With IEEE 802.11 (compiled by the Institute of Electrical and Electronical Engineers) there has been a binding air interface for wireless networks since 1997. The letter after the standard IEEE 802.11 indicates the data transmission speed.
- IEEE 802.11g with 54 Mbps and a frequency range of 2.4 – 2.4835 GHz is the most widely distributed in Germany
- IEEE 802.11h also achieves 54 Mbps in a frequency range of 5.15 – 5.725 GHz
(divided into the sub-bands 1: 5.18 – 5.32 GHz and 2: 5.47 – 5.725 GHz)
It should be noted here that the specified data transmission values are only gross values which are never reached in practice even under optimal conditions.
Another term is Wi-Fi (Wireless Fidelity). Wi-Fi-certified devices have passed an interoperability test with other 802.11 devices.
Ad-hoc and infrastructure networks
A distinction is made between two different types of WLANs:
In the ad-hoc network two or more WLAN-using devices (clients) are wirelessly connected directly without a co-ordination point. These also include the so-called mesh networks in which activated computers are networked spontaneously with each other when they are within mutual wireless range.
The computers involved receive and send data via the built-up wireless network so that, with the appropriate number of clients, they can considerably increase their own wireless range by passing on the data via other devices to the actual recipient. The range is reduced accordingly when one of the participating devices is switched off.
Unlike the ad-hoc networks, infrastructure networks are similar in structure to a mobile radio network whereby a WLAN router or access point as a so-called hot spot takes over the co-ordination of the clients and their access to the Internet in place of a mobile radio base station.