A WLAN is a type of LAN technology. In fact, the IEEE 802.11 standard that governs the basic operations and functions of WLANs says that a WLAN is required to appear to higher protocol layers as a regular wired IEEE 802 LAN.
WLAN Components and Concepts
WLANs comprise several components, some purely logical and others physical. The following sections introduce some of the components that make up a WLAN and define some of the lingo used in describing the entities in a WLAN.
Wireless Medium
The wireless medium (WM) is used for the actual transfer of information between the entities of a WLAN—that is, air and space.
Various types of data can be encoded into a type of electromagnetic disturbance called radio waves. These radio waves are then transmitted over the air (our medium) to their destination, where they are decoded back into useful data.
Wireless Station
Any device that implements the IEEE 802.11 standard is called a wireless station (STA).
A STA is therefore a single physical entity that can perform the 802.11 song and dance. Wireless STAs are not very useful by themselves; they need other wireless or wired devices to send information to and receive information from in order to make them useful.
Figure 6-1 shows some sample wireless STAs.
A STA can operate in one or more of the following modes:
■■ Infrastructure mode Also referred to as client mode STA, this mode requires the use of an infrastructure device, such as an access point (AP), for communicating with other STAs, as well as with the wired LAN. Devices operating in this mode can be considered wireless clients, which implies that the wireless STA acts as a client in a WLAN. Other literature may refer to client mode STAs as mobile units (MUs). Despite the name, MUs can, of course, be either mobile or in a fixed location. Infrastructure mode is probably the most typical and common mode of operation for the vast majority of wireless devices.
■■ Ad hoc mode Ad hoc STAs form autonomous networks that do not require an AP to communicate with other STAs. They can be either mobile or in a fixed location.
■■ Access control mode As implied by the name, an access control STA is used for controlling access between STAs or controlling STA access to the wired LAN. A wireless access point (WAP) is a good example of a STA that operates in this mode.
Distribution System
The distribution system (DS) is an important part of any network and serves as a “glue” for interconnecting similar or dissimilar networks together.
The fabric of the DS can be based on any of the common wired technologies such as Ethernet. It can also be entirely wireless using regular wireless technologies such as those based on IEEE 802.11.
In wireless networks, the DS can be used for linking the wireless STAs to the wired resources. In fact, the most common use of the DS is for bridging a wireless network to the wired network.
Access Point
The access point (AP) is a type of STA. Specifically, it helps link wireless stations to the wired stations or resources, or it may simply be used for connecting wireless STAs to one another.
Basic Service Set
The basic service set (BSS) is a logical entity in a WLAN. The BSS can also be viewed as a type of WLAN topology. BSSs come in two “official” types: independent BSS and infrastructure BSS.
A third, but “unofficial,” topology type exists, called a wireless distribution system (WDS). It’s unofficial because it is not officially sanctioned or described in the 802.11 specification, and neither is it sanctioned by the Wi-Fi Alliance special interest group. But it is widely used in WLAN setups.
Infrastructure BSS
Infrastructure BSS, frequently referred to simply as a BSS, makes use of an AP. Most WLANs operate in this mode. The STAs that participate in the infrastructure BSS do not communicate directly with each other; all communications go through the AP.
The IEEE 802.11 standard defines the BSS as “a set of stations (STAs) that have successfully synchronized amongst themselves using the appropriate functions.” These functions are called primitives in the 802.11 technical jargon.
Infrastructure mode STAs operate in infrastructure BSS. The following illustration shows a sample infrastructure BSS:
Independent BSS
The Independent BSS (IBSS) does not make use of an AP; the STAs communicate directly with one another in a peer-to-peer fashion. A minimum of two STAs are required to form an IBSS.
Ad hoc mode STAs operate in IBSS mode. The following illustration shows an IBSS:
Wireless Distribution System
Wireless distribution systems (WDS) are used to describe WLAN topologies in which APs are connected together. Strictly speaking, it means that the infrastructure devices (or APs) are linked together using a wireless medium (air or space) as the distribution system. A WDS is used for creating a wireless backbone link between the APs in a WLAN. This is in contrast to the traditional method of linking the APs in a wireless
network via a wired distribution system.
The WDS often requires that all participants employ and share various characteristics, such as a common radio frequency (RF) channel and a common security mechanism.
The next illustration shows a sample WDS.
Service Set ID
The service set ID (SSID) is used to identify an extended service set (ESS) or IBSS.
Specifically, it is a human-friendly means of identifying the ESS or IBSS. The SSID is the name that users often see when they are presented with a list of available wireless networks detected by their wireless device.
The SSID can be 0 to 32 bytes long.
Basic Service Set ID
It only makes sense that if we have a human-friendly way of identifying wireless networks, we should also have a machine-friendly way of doing the same thing. This is the job of the BSSID, which is used for identifying each BSS. It is 48 bits long and is very similar to the MAC address used on Ethernet-based networks.
The exact value of the BSSID depends on the service set in use (Infrastructure or Independent).
In the Infrastructure BSS, the BSSID is easily determined, because it is the MAC address currently in use by the wireless STA that is acting as the AP. Recall that the Infrastructure BSS always has an AP present. This MAC address is a universally administrated type, and it is always bound to be unique. A sample BSSID for an infrastructure BSS is 00:ab:34:56:78:9a.
In the ad hoc, or independent mode, BSS, the BSSID is a locally administered (nonuniversally unique) type of MAC address. A locally administered MAC address is one in which the value of universal or local (U/L) field bit is set to one (1). Specifically this means that the second least significant bit of the most significant byte of the MAC address has a value of one (1). The first byte is then followed by a 46-bit randomly generated number. A sample BSSID for an ad hoc network would be 02:12:34:56:78:9a.
Basic Service Area
The basic service area (BSA) defines the physical area or boundary spanned by any BSS.
It is used to describe the physical area containing the members of a BSS. This area may also span other BSSs, which means that it may contain members of other BSSs.
Extended Service Set
The IEEE 802.11 standard defines an extended service set (ESS) as “a set of one or more interconnected basic service sets (BSSs) and integrated local area networks (LANs) that appears as a single BSS to the logical link control (LLC) layer at any station (STA) associated with one of those BSSs.” The LLC is a data communication protocol sublayer in the OSI reference model. Specifically, it is a sublayer of the Data Link layer. (More about the OSI in the following section.)
An ESS is normally a union of BSSs, but the foundation for the ESS is laid as soon as a BSS is born. This means that the mere existence of a BSS implies the existence of an ESS.
The following illustration shows a sample ESS; it also shows the relative positions of some of the components of WLANs that have been discussed thus far.
WLAN Standard (IEEE 802.11)
As you know, the IEEE 802.11 is a family of standards that governs the operations and functions of WLANs. But the standard does not define or manage absolutely every aspect of WLAN operations—it specifically concerns itself only with the functions of WLANs at the Physical (PHY) layer and Media Access Control sublayer of the OSI reference model. We examine these functions next.
Figure 6-2 shows the entire OSI model.
PHY
The Physical layer is the first layer (Layer 1) in the OSI reference model. It defines the relationship between a device and the physical communication medium.
For our purposes, a good example of the device is any IEEE 802.11 compliant hardware, and the physical communication medium is the airspace.
The PHY layer for IEEE 802.11 specifies the wireless signaling techniques used for transmitting and receiving information over the airwaves. Sample signaling techniques are listed in Table 6-1.
The PHY layer provides service to the IEEE 802.11 MAC sublayer, which is discussed next.
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