The wireless networks specified exclusively for data transports are termed as wireless data networks. These are classified in accordance with their coverage range. The most extensive network covering a wide area is the wireless wide area network (WWAN) that may span an entire country. The network that connects the residents and visitors in a metropolitan area wirelessly is termed the wireless metropolitan area network (WMAN).
A wireless community area network (wireless CAN) includes the systems intended for a small coverage area such as a university or a hospital campus. The wireless CAN, in a restricted sense when deployed within a localized area such as a building, office etc., becomes a wireless local area network (WLAN). It can be wired to the legacy LAN in the premises such as the Ethernet.
Lastly, a wireless personal area network (WPAN) refers to the implementation of a wireless network in the smallest area possible, for example, a sub-region of a building or home, where the cluster of furniture or other contents in the surroundings could have an influence on the nature of the wireless propagation
For wireless LAN operation, the regulatory bodies have permitted to share the socalled ISM bands with existing systems. WLAN is classified as an “intentional radiator” in its scope of implementation in the ISM bands. The Part 15 Rules of the FCC allows unrestricted (unlicensed) radio communications at these ISM bands, however, with constricted maximum power in the transmissions.
In applications up to 1 W use of spread-spectrum (SS) technique is mandatory. In such cases, the radio implementations should ensure that the spreading ratio (ratio of signal bandwidth after spreading the spectrum to raw or “unspread” signal bandwidth) exceed a factor of ten in direct sequence spread-spectrum (DSSS) and requires a minimum of 50 and 75 hopping frequencies at 910 MHz and 2.4 GHz in the frequency hopped spread-spectrum (FHSS).
The typical WLAN implementations in the United States refer to AT & T’s 2-Mbps WaveLANTM. Motorola’s proprietary WLAN, namely AltairTM, operates at 18 GHz and provides a 10 Mbps Ethernet interface. In Europe, the ETSIdefined WLAN standard, known as HIPERLAN, operates in the 5.1-5.3 GHz range and supports 23.5 Mbps.
Its standard specifies 50 m coverage indoor, with options for relaying via HIPERLAN nodes and/or wired infrastructure. In the United States, the FCC has also allocated 300 MHz of bandwidth in the same 5 GHz regime as per the petition from the National Information Infrastructure (NII) and Supernet.
Other formal workgroups (such as MM wave Working Group) have obtained a spectrum for millimetre wave medium access for WLAN deployment. The WLAN standards are specified by the IEEE 802.11 working group, which define a number of services that need to be provided by the wireless LAN with the functionality equivalent to that of legacy wired LANs.
The legacy LANs, interconnect the computers through copper and/or fibre lines with clients and servers placed at fixed locales. With roaming/mobile end-entities, the wireless connectivity is facilitated via WLAN systems. The physical media prescribed for WLAN transmissions are the infrared (IR), the radio frequency (RF/UHF) and the microwave (including millimeter wave) ranges of the electromagnetic spectrum.
Excluding the IR-band, the RF/microwave ranges conform to ISM bands indicated earlier. Relevant United States prescriptions are: 902-928 MHz, 2.4000-2.4853 GHz, and 5.725-5.850 GHz. The operation is based on the spread-spectrum technique and the emitted spectrum is confined to the prescribed band. Further, these are low-power systems with a maximum permissible power level of 500 mW.
The IR system uses the same wavelengths used on fibre links. (Typically, the wavelengths are in the range 1300 nm and 1500 nm.) These systems are not bandwidth constrained and operate on line-of-sight (LoS) principle. The modulation scheme prescribed for WLAN, as stated before, is the spread-spectrum technology of either DSSS or FHSS type.
A major consideration as regard to the electromagnetics of WLAN transmissions deployed in the indoor environment is the prevalence of intense multipath transmissions and the resultant fading effects. The statistics of such fading is largely refers to the Rayleigh fading in which the signal intensity fluctuations follow the Rayleigh distribution. The antenna designs, therefore, are made to cope with such RF signal fading environments.
