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Electronic Bulletin / Number 14 - August, 2005

Introduction to WiFi Networks

Introduction

Under the heading “WiFi Networks,” where WiFi stands for Wireless Fidelity, we bring together a set of local area networks where access is by wireless. At present, WiFi networks are based on a series of standards referred to as IEEE 802.11 (IEEE: Institute of Electrical and Electronics Engineers).

IEEE 802.11 Standard

The first component of the IEEE 802.11 standard was ratified in 1997 and then again in 1999, when the first extensions were also made. The structure of the IEEE standards is such that the extensions are drawn up as modifications of the original standards and are identified by adding a letter to the name of the standard. In the case of 802.11, we have extensions 802.11a, 802.11b, etc. Some of these extensions are provided in the references.

Actually, the 802.11 standard is only one part of a broader set of IEEE standards: the 802. Figure 1 (copied from [1]) provides a diagram of the structure of the set of 802 standards, dedicated to the lower layers of network architecture.

The lowest layer is the Physical Layer, as indicated in Figure 1. This is the logical layer in charge of defining the physical details of the network, such as electric power transmitted, modulation scheme, etc.

Over the physical layer, there is the Media Access Control Layer (MAC Layer). This is the layer that permits coordinating the use of the common transmission medium between all the stations that wish to communicate with each other. It pertains to a lower under-layer of the Data Link Layer of the seven-layered OSI model.

One of the best-known components is the 802.3 standard, which corresponds to the specification of the Ethernet local area networks (LAN). The success of 802.3 made it one of the principal sources from which the IEEE 802.11 standard took its design.

Industrial standards

In addition to the IEEE 802.11 standard, there are standards established by manufacturers’ associations. Although industry standards basically observe IEEE 802.11, oftentimes industry goes ahead (for marketing reasons) and incorporates improvements that take more time to be incorporated into an IEEE standard.

One of the best-known manufacturers’ associations is the Wi-Fi Alliance (see http://www.wi-fi.org). Some of the members of this association are Cisco, IBM, Intel, Nokia, 3Com, Hewlett Packard, AMD, NEC, Avaya, Apple, Motorola, and Microsoft. The importance of these companies gives an idea of the relevance of the association.

Mobile stations and basic networks

Under this section, we present some of the basic classifications in WiFi networks that will be used throughout the course.

We will refer to equipment connected to a wireless network as mobile stations or simply stations. The basic structure of a wireless network is called BSS, that is, Basic Service Set. The BSS can be viewed as the minimum structure in which a group of mobile stations communicating with each other can be organized. Another way of understanding a BSS, in a first approach (although not precisely, as we will soon see), is like the area of coverage of the radiofrequency transmission. Figure 2 (copied and modified from [1]) shows two BSSs each with two mobile stations.

From the modified 802.11:

Infrastructure

There are two different types of networks:

1. Infrastructure networks:

In this case, every BSS is organized around one station that can permit access to a larger network, for example, to a wired LAN. Because of this, the special station is called the access point, and therefore it is oftentimes simply referred to as the AP.

2. Independent or ad hoc networks:

They are networks comprised of one single BSS, called IBSS (Independent BSS), which are not structured around any station with specific functions, but rather distribute coordination tasks amongst each other.

The BSSs of the infrastructure networks can be grouped together to constitute a larger unit known as the Extended Service Set (ESS). An ESS is simply a network comprised of a group of BSSs where the connectivity between the BSSs is provided owing to the bridging functions of the access points. The medium through which the access points are connected (which can be a wired LAN, as mentioned earlier) is called a Distribution System (DS). The medium that is common to a BSS, however, is called a Wireless Distribution System (WDS). Figure 3 (copied with some changes from [1]), provides a diagram of these concepts.

Network overlapping

Although we have been talking of BSSs as well-defined areas, this is not actually the case. On the one hand, they are not truly areas, because the coverage of the radio waves that are transmitted is not uniform throughout the volume, rather it can be directional. In other words, a mobile station on the fourth floor does not necessarily belong to the same BSS as the other mobile station exactly below, on the third floor.

Furthermore, the coverage “areas” of the BSSs are not well-defined because a station can be inside the coverage area of, for example, two access points, as shown in Figure 4 (taken from [1]).

References

[1] ANSI/IEEE Standard 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.