The 802.11 standard for WLANs presents itself completely differently in the various versions in terms of transmission rate, frequency ranges, modulation methods, number of channels, etc., which is due not least to the development time of the standard and technological advances. For the user, the 802.11 standards are confusing because they are largely incompatible with each other.
The basic 802.11 standard was developed in the 1990s and adopted in 1997. It has a transmission rate of 2 Mbit/s, which was increased to up to 54 Mbit/s in the 802.11a standard in 1999. In the meantime, various 802.11 standards have been developed with transmission rates of several hundred Mbit/s and, with the 802.11a and 802.11ad standards, even those with several Gbit/s.
Transmission techniques and processes in WLANs
WLANs according to 802.11 transmit the signals by means of infrared( IR) or via microwaves in the frequency bands at 2.4 GHz, 5 GHz and in the 60 GHz band. In the 2.4 GHz band, the ISM band, in which Bluetooth and HomeRF also transmit, the basic 802.11 standard uses spread-band technology( DSSS) and frequency hopping( FHSS). This frequency band is also used by the 802.11b and 802.11g standards, whereas the 5 GHz band is used by 802.11a and Hiperlan. The different standards also use different methods for modulation and coding.
For example, 802.11b uses Barker code and complementary code keying( CCK), 802.11g uses orthogonal frequency division multiplexing( OFDM) in addition to the methods mentioned, and finally 802.11a uses only OFDM multiplexing. These different methods result in completely different transmission rates, which range between 1 Mbit/s and 54 Mbit/s. Further differences can be seen in the number of channels, which varies between three 20 MHz channels (802.11b, 802.11g) and 19 channels in 802.11a. Higher data rates in the gigabit range are achieved with the 802.11ac and 802.11ad standards.
The IEEE 802.11 working group defines the standards for the physical layer, such as modulation and media access protocols Wireless Media Access Control( WMAC). The specification supports three different transmission techniques: Frequency hopping (FHSS), spread spectrum (DSSS) and infrared (IR) transmission. The latter technology has not yet been implemented in technical products.
Both radio methods operate in the frequency range between 2.412 GHz and 2.484 GHz. The maximum transmitting power( EIRP) for the European systems is 100 mW, while up to 1,000 mW is permitted in the USA. The standard provides for transmit power control and automatic channel selection. These functions are called Transmit Power Control( TPC) and Dynamic Frequency Selection( DFS).
The transmission rates are 1 Mbit/s to 2 Mbit/s for DSSS technology and 1 Mbit/s for FHSS technology. For the most commonly used FHSS technique, the standard specified that transmit power be limited to 1 W, 75 frequency bands with a maximum bandwidth of 1 MHz be available for transmission that do not overlap each other, and the minimum channel hop rate be 2.5 hops/ second. To eliminate mutual interference and the resulting diffuse spectra as much as possible, the FHSS procedure was revised. In a revision of the standard, the number of FHSS frequency bands was reduced to 15; with each channel having a bandwidth of 5 MHz.
For infrared LANs, the spectral range from 850 nm to 950 nm is used. The transmission speeds for this technology are also between 1 Mbit/s and 2 Mbit/s, with two- phase shift keying(BPSK) being used for the 1 Mbit/s transmission speed and quadrature phase shift ke ying( QPSK) for the 2 Mbit/s transmission. 802.11 uses CSMA/ CA(Collision Avoidance) as the MAC protocol.
The various 802.11 standards define different data rates, service levels, encryption and mesh networks.
According to the standard, an 802.11 LAN is formed by at least two radio stations, called a Basic Service Set( BSS). The BSS stations are connected to the Distribution System( DS) via the access point.
802.11 takes into account voice services such as VoIP and VoWLAN in 802.11e and 802.11r, and has mechanisms for roaming and fast switching between different access points(APs) so that this occurs without interruption or delay. The problem here lies in authentication, which can take some time. To shorten this time when the subscriber leaves one access point, logs on to another and returns to the previous one, the PMK mechanism was developed, which stands for Pairwise Master Key. In this case, re-authentication is reduced to a few data packets by confirming the PMKID.
In addition to the basic 802.11 standard, there are other standards that have already been adopted and proposed:
802.11a: 802.11a describes the High Speed Physical Layer in the 5 GHz band. The standard is based on OFDM and the spread spectrum technique (DSSS).
802.11b: 802.11b deals with the Higher Speed Physical Layer. Extension in the 2.4 GHz band. This technology provides for transmission rates of 5.5 Mbit/s through 11 Mbit/s to 20 Mbit/s.
802.11c: Supplement to Bridge Standard. 802.11c specifies MAC layer bridging to 802.11d. Standard for the wireless coupling of two networks via a WLAN. When bridging, the access points ( AP) of the WLANs establish a dedicated radio link between each other via the MAC address.
802.11d: Regulatory Domain Updates. Standard for adapting WLAN devices in countries with different frequency usage regulations. This includes, among other things, the selection of radio channels.
802.11e: 802.11e defines MAC enhancement. Definition of procedures with which Quality of Service( QoS) is made available to the user. This is particularly important for VoWLANs and for real-time behavior in fieldbuses.
802.11g: 802.11g supports transmission rates of up to 54 Mbit/s, transmission in the 2.4 GHz band. The modulation method is Orthogonal Frequency Division Multiplex (OFDM).
802.11h: Frequency spectrum management of 802.11a. This standard, which has not yet been adopted, prescribes dynamic frequency and power selection for use in Europe, for example.
802.11i: The 802.11i security standard is to replace the WEP protocol. Parts of 802.11i are published in the WiFi Protected Architecture(WPA). Extension of the initialization vector from 24 bits to 128 bits to increase WLAN security.
802.11j: The Japanese variant of 802.11a in the frequency range between 4.9 GHz and 5 GHz.
802.11m: This standard defines the prioritization of VoWLAN over data traffic.
802.11n: The 802.11nworking group is working on high-speed transmissions of up to 600 Mbps. This technology uses multiple input multiple output( MIMO) and OFDM. The specifications for this standard were developed by the Enhanced Wireless Consortium( EWC).
802.11q: Support for virtual WLANs.
802.11r: 802.11r is about fast roaming between access points. Switching between access points (AP) causes audible interruptions in wireless telephony, which are further prolonged with port-based authentication.
802.11s: The IEEE 802.11s working group is about Wireless Mesh Network( WMN). The topic of mesh networks deals with new network structures in which the access points (AP) are connected to each other by radio technology. The typical hub structure will dissolve in the future and all nodes will form a large meshed wireless network with each other.
802.11t: Wireless Performance Prediction (WPP) specifies, among other things, performance metrics, measurement methods and test procedures for measuring the performance of WLAN components and making it easier to predict the performance of the application.
802.11v: Wireless Network Management defines network management functions forwireless networks.
802.11w: Protection of Management Frames deals with the protection of management connections.
802.11y: Frequency range 3.650 GHz to 3.700 GHz, which can be used in the USA.
802.11z: Direct connections between WLAN clients without the detour via the base station.
802.11aa: : Robust Streaming ofAudioVideo Transport Streams.
802.11ad: 802.11ad stands for a gigabit WLAN in the 60 GHz band with 2 GHz wide radio channels. In such a 2 GHz band, data rates of up to 6.7 Gbit/s are to be realized with quadrature amplitude modulation (QAM64).
802.11ae: QoS management.
802.11af: The 802.11ay working group deals with the white spaces of the TV bands.
802.11ah: The 802.11ah working group deals with low power WAN in the frequency range below 1 GHz.
802.11ai: Fast Initial LinkSetup
802.11aj: China, microwaves in the 45 GHz range.
802.11ak: General Links
802.11aq: Pre-association Discovery of Services
802.11ax: The 802.11ax working group deals with High Efficiency WLAN
802.11ay: The 802.11ay working group deals with Enhancements for Ultra High Throughput in the 60 GHz band
802.11az: Next Generation Positioning.
802.11ba: Wake-Up Radio With(WUR) Legacy IEEE 802.11 Transmitters
802.11bg: 802.11bg operates in the 2.4 GHz range with Single Input Single Output( SISO) and a bandwidth of 20 MHz.