WLAN: Elements and Characteristics

The basic elements in a WLAN domain are as follows:

The wireless client, also known as the STA (station) or supplicant, is usually a card or embedded chip in a device. These devices include PCs, PDAs, tablets, or smart phones. They communicate over a radio link.
The access point (AP) is the receiving (or sending, depending on the context) end of the radio link, and it has the capability to reach back to the LAN. The AP, obviously, has a range called the coverage area (actually, it is three-dimensional and should be called coverage volume). The coverage area could overlap or be disjointed between the APs. The AP's signal strength and antenna characteristics usually determine the coverage area. The antenna determines the shape and volume of the coverage area, and the power of the AP determines the signal strength. Factors like interference (from other devices) and objects (like walls) affect the overall coverage area.
The coverage area is also essentially a Basic Service Set (BSS), the lowest membership, group, or organizational granularity in a WLAN.
Note

Unless there are mechanisms to coordinate between the APs, a mobile device jumping from one coverage area/BSS to another loses the connectivity. That is, the BSSs are independent; one device can be associated with only one BSS, and the STA-BSS association is dynamic. Later you will learn how the Extended Service Set (ESS) solves this problem of disjointed BSS and provides continuity for a client to move from one AP to another seamlessly.
The distribution service (DS) is the wired network that provides connectivity from the WLAN to the rest of the world. This could be a corporate LAN, access provider, wireless service provider (WSP), or even the Internet.

Figure 4-1 shows these elements.

Figure 4-1. WLAN Basic Elements

There are two modes of WLAN operation: independent BSS (IBSS) and ESS. Figure 4-2 shows the topology of the two modes.

Figure 4-2. WLAN Operation Modes

In the IBSS mode, the devices form ad-hoc connections with each other. IBSS mode is not used very often, but it has the potential to add a lot of value to the Internet?military, peer-to-peer, ad-hoc networks, sensor networks, mesh networks, and other applications are emerging in this space. The Mobile Ad-Hoc Network (MANet) protocols such as Ad-Hoc On-Demand Distance Vector (AODDV) and Adaptive Demand Driven Multicast Routing (ADMR) are being worked on at the standards level (IETF) and at research labs.

The ESS consists of STAs connected to APs that, in turn, are connected to the DS. The APs and the DS enable the STAs to reach back and connect to a network infrastructure with more "permanency." ESS mode is the most common WLAN mode, at least for now. ESS provides the capability for sustained mobility?that is, a client/STA can move from AP to AP or from BSS to BSS connected to the same ESS and will not lose the connectivity context. However, the connectivity context is lost when an STA moves between ESSs. In plain words, the concept of an ESS is to give mobility across APs in an organization, but moving across organizations is not supported. Also remember that the applications might not be able to tolerate the connection transition time from one BSS to another in the same ESS.

Note

IEEE 802.11 distinguishes between the distribution system medium (DSM), which is usually the wired environment, and the wireless medium (WM), which is usually radio signals over the air. But the specification does not assume or mandate any specific DS infrastructure; rather, it specifies a set of services that the DS would provide to support a WLAN. This level of decoupling has proved to be a good architecture to evolve the 802.11 in terms of security, speed, and functionality. As you saw in Chapter 3, the various subsequent standards were able to add QoS, 802.1x security primitives, support for video, and audio to the basic 802.11 standard incrementally.