Consumer demand for WLAN applications has been predicted in all industrialized countries with widespread use of personal computers (PCs) since the early 1970s.
The objective need to ensure the compatibility of WLAN equipment from different manufacturers has led to the need to develop appropriate standards, which were simultaneously produced by the standardization bodies of three regions: wlan meaning
1) in the USA - by the Institute of Electrical and Electronics Engineers (IEEE);
2) in Europe - by the European Telecommunications Standards Institute (ETSI);
3) in Japan - by the Association of Radio Industries and Business (ARIB).
The most famous are the standards developed under the auspices of the IEEE and ETSI.
In the IEEE, the WLAN standards were created by the 802.11 Work Group WG of the 802 LAN / MAN Standards Committee. Within the IEEE 802 committee, the WG 802.11 working group develops WLAN standards, and the related standards are known as IEEE 802.11 standards. The original (basic) IEEE 802.11 standard was adopted in 1987. In the future, he was continuously undergoing improvements, which correspond to versions with different letter designations - from a to x.
The WLAN standards developed by ETSI are known as HIPERLAN (High Performance Radio LAN). The development of these standards was carried out almost simultaneously with the IEEE 802.11 standard (with a year ahead). Initially, it was supposed to develop 4 versions of these standards, but in reality it was limited to two versions: HIPERLAN 1 and HIPERLAN2. According to the plan, it was assumed that WLAN of the HIPERLAN standards, with the use of the frequency resource equivalent to the IEEE 802.11 standard, should have higher data transfer rates. Standards development was supported by well-known hardware manufacturers, in particular Ericsson.
However, the process of practical implementation of WLAN (taking into account the current state of the market) has led to the need for both developers and standardization bodies to choose only one of the directions for the development of wireless network technologies. This direction turned out to be the networks of the IEEE 802.11 family of standards.
WLAN equipment manufacturers associations actively participated in the development of standards. The IEEE 802.11 standard complies with the formation of the Wireless Ethernet Compatibility Alliance (WECA), known for its popular Wi-Fi Alliance brand. The role of this Association was manifested in the development of a certification system for IEEE 802.11 products, due to which they are known as Wi-Fi products. Similar associations of manufacturers of HIPERLAN products: the HIPERLAN Alliance and the HIPERLAN2 Global Forum are currently less active. However, it is likely that it would be impractical to completely negate some of the inherent advantages of HIPERLAN and the possibility of their future use is not excluded.
Factors Determining WLAN Architecture
One of the most significant distinguishing features of local digital networks in comparison with global ones is the presence of autonomous telecommunication lines between their nodes. The difference between the architecture of wireless and wired LAN is due to the properties of the propagation medium of the signals used:
- conductive guiding medium in wired LAN;
- natural environment in wireless LAN (Wireless LAN -WLAN).
A related use of both environments is that they are Multiple Access (MA) access environments . Signals from different subscribers in the conditions of their independent operation can be transmitted simultaneously, which leads to a superposition of signals in the environment. This superposition results in the difference between the total signal and each of the transmitted ones and significantly complicates the possibility of their correct reception. Signal collisions can occur in a shared environment, regardless of its physical properties . Elimination of collisions presupposes the coordinated use of the environment, a mandatory component of which is monitoring its employment. Occupancy-based media access called carrier sense multiple access (Carrier Sense Multiple Access - CSMA). LANs of different flavors (wired and wireless) use several derivatives of this access method.
The access of nodes to the environment of both types is carried out using network adapters (Network Interface Card - NIC, Wireless NIC - WNIC), which perform the functions of the two lower layers of the basic reference model for the interaction of open systems ISO / OSI, namely:
- physical layer (Physical Layer - PHY);
- sublayer of media access control (MAC) of the data link layer (DLL).
Network adapters (wired and wireless) provide monitoring of the environment, coordinated access of various nodes to it, generation, transmission and reception of signals.
In a wired environment, which is a two-wire long line, such as a cable, the transmission of signals between nodes is accompanied by relatively low attenuation during their propagation. A collision of signals from two (or more) nodes leads to a significant change in the characteristics of the total signal (first of all, the energy level), as compared to single signals. Accordingly, each node using the NIC can detect the fact of a collision of signals during their transmission and take actions to ensure the order of access to the environment of different nodes. Ordered Multiple Access for Collision Reduction in Wired Environments has come to be known as Carrier Sense Multiple Access with Collision Detection (CSMA / Collision Detection - CSMA / CD). Collision detection is an inherent property of wired environments.
In a wireless environment, which is the natural and artificial filling of the space surrounding the nodes, signals propagate, attenuating significantly with distance from the source. The total signal of several sources does not have energy features corresponding to the fact of collision. The latter can be detected by checking for errors in the received and processed WNIC digital signal after the completion of its transmission. Accordingly, the reaction to the fact of a collision can be carried out with a delay, and the prevention of collisions becomes an essential necessity. An associated wireless multiple access method was developed in preparation for the IEEE 802.11 standard. It is named Carrier Sense Multiple Access and Collision Avoidance. (CSMA / Collision Avoidance - CSMA / CA).
Local digital networks of large organizations are, as a rule, a combination of wired and wireless segments. Accordingly, the LAN architecture should provide for a distribution system that performs the following functions:
- ensuring interconnection between different LAN segments (including wired and wireless segments);
- ensuring access of all LAN segments to the backbone Internet environment.
Access to the backbone network is provided through Web servers (service servers) with appropriate interface properties. The network and transport layer (Network Layer, Transport Layer) issues of open systems interoperability go beyond the IEEE 802.11 standard, which is limited to the MAC and PHY issues of the WLAN implementation. Accordingly, consideration of the WLAN architecture is limited in the standard to issues of building wireless segments and LAN distribution systems.
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