WiFi Networks: Introduction

Under the denomination “WiFi Networks”, where WiFi comes from Wireless Fidelity, we group a variety of wireless local area networks which are based on the IEEE 802.11 standards.

IEEE 802.11 Standard

The first version of the IEEE 802.11 standard was ratified in 1997 and then in 1999, when the first revisions where made. IEEE standards are organized in such a way that extensions to any given standard are published as modifitions and are named by adding a letter to the original standard. In the case of 802.11, we have extensions 802.11a, 802.11b, etc. See the references for further information on these revisions. Actually, 802.11 is only a part of larger set of standards: IEEE 802. Figure 1 (copied from [1]) shows the structure of the IEEE 802 set of standards, concerned with the lowest layers of the network architecture.

Figure 1

The lowest layer is the Physical Layer in Figure 1. This is the logical layer in charge of defining the physical details of the networks, say, transmitted power, modulation scheme, etc. The physical layer is just below the Media Access Control (MAC Layer). This layer enables the coordination of the use of the shared transmission medium among all stations which want to communicate. In the seven-layer OSI model, it is hte lowest sublayer of the Data Link Layer. Uno de los componentes más conocidos es el estándar 802.3, correspondiente a la especificación de redes de área local (LAN) Ethernet. El éxito de 802.3 hizo que éste se convierta en una de las principales fuentes de las cuales toma el diseño del estándar IEEE 802.11.

Industrial Standards

Besides the IEEE 802.11 standard, there are several standards published by asociations of companies. Although industrial standards tend to follow IEEE 802.11, it often happens that market pressure pushes the industry to incorporate performance improvements before they can be translated into a revision of the corresponding IEEE standard. One of the best known associations of companies is the Wi-Fi Alliance (see http://www.wi-fi.org). Some of its members are Cisco, IBM, Intel Nokia, 3Com, Hewlett Packard, AMD, NEC. Avaya, Apple, Motorota and Microsoft. These names give an idea of the relevance of the association.

Mobile stations and basic networks

In this section we introduce some of the basic nomenclature of WiFi networks which will be used in the remaining of these course notes. We shall call mobile station or simple station to any equipment connected to a wireless network. The basic structure of a wireless network is denominated BSS, Basic Service Set. A BSS can be seen as the minimal structure in which a group of mobile stations which are interconnected can be organized. Also, as a first (not completly accurate) approximation to the problem, we can view a BSS as the coverage area network of mobile stations, that is, the area which can be reached by radiofrequency waves transmitted by any station. Figure 2 (modified from an illustration in [1]) shows two BSSs with two mobile stations each.

Figure 2

Infrastructure

There are two different types of networks: 1. Infrastructure networks: In this case, each BSS is organized around an specialized mobile station which can grant access to a another network, say, a wired LAN. The specialized station is called an access point and we shall refer to it as an AP. 2. Independent (ad hoc) networks: They are networks made up of a single BSS, called IBSS (Independent BSS), which is not structure around any specialized station. On the contrary, coordination tasks are equally distributed among all mobile stations in the network. Several BSSs in an infrastructure network can be group to be part of a larger network, known as an Extended Service Set (ESS). An ESS is simply a network made up of a set of BSSs where the access points act as bridges between the the BSSs. The network which connects the access points (which could be a wired LAN such as Ethernet) is called the Distribution System (DS). On the other hand, the access medium in any given BSS is called a Wireless Distribution System (WDS). Figure 3 (taken with some modifications, from [1]), represents theses concepts schematically.

Figure 3

Overlapping networks

Although we have referred to BSSs as if they were well-defined areas, this is an oversimplification of reality. On one hand, BSSs are not trully “areas”, but complex volumes whose actual shape depends on the peculiarities of the transmission of radiofrequency waves. On the other hand, coverage regions of the BSSs are not well-defined either and a mobile station may be inside the coverage regions of, say, two access points, as it is shown in Figure 4 (copied from [1]).

Figure 4

Pablo I. Fierens PhD
Centro Avanzado de Comunicaciones
(Center for Advanced Studies in Communications (CAT))
Buenos Aires Institute of Technology (ITBA)

References

[1] ANSI/IEEE 802.11. Information technology-Telecommunications and information exchange between systems-Local and metropolitan area networks-Specific requirements-Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications. 1999. Available online at http://standards.ieee.org/getieee802/.

[2] IEEE 802.11a. Supplement to IEEE Standard for Information technology. Telecommunications and information exchange between systems-Local and metropolitan area networks-Specific requirements-Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) specifications. High-speed Physical Layer in the 5 GHz Band Adopted by the ISO/IEC and redesignated as ISO/IEC 8802-11:1999/Amd 1:2000(E). 2000. Available online at http://standards.ieee.org/getieee802/.

[3] IEEE 802.11b. Supplement to IEEE Standard for Information technology-Telecommunications and information exchange between systems-Local and metropolitan area networks-Specific requirements-Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) specifications: Higher-Speed Physical Layer Extension in the 2.4 GHz Band. 1999. Available online at http://standards.ieee.org/getieee802/.

[4] IEEE 802.11b. IEEE Standard for Information technology-Telecommunications and information exchange between systems-Local and metropolitan area networks-Specific requirements-Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) specifications. Amendment 2: Higher-speed Physical Layer (PHY) extension in the 2.4 GHz band-Corrigendum 1. 2001. Available online at http://standards.ieee.org/getieee802/.

[5] IEEE 802.11g. IEEE Standard for Information technology-Telecommunications and information exchange between systems-Local and metropolitan area networks-Specific requirements-Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) specifications. Amendment 4: Further Higher Data Rate Extension in the 2.4 GHz Band. 2003. Available online at http://standards.ieee.org/getieee802/.

[6] Matthew S. Gast, 802.11 Wireless Networks. The Definitive Guide, O’Reilly, 2002.