The Basic Service Set (BSS) is the basic building block of the IEEE 802.11 architecture. A BSS is defined as a group of stations that coordinate their access to the medium under a given instance of the medium access control. The geographical area covered by the BSS is known as the basic service area (BSA), which is analogous to a cell in a cellular co communication network. A BSA may extend over an area with a diameter of tens of meters. Conceptually, all stations in a BSS can communicate directly with all other stations in a BSS. Note that two unrelated BSS's may be colocated. IEEE 802.11 provides a means for these BSS's to coexist.
A single BSS can be used to form an ad hac network. An ad hoc network consists of a group of stations within the range of each other. Ad hoc networks are typically temporary in nature. They can be formed spontaneously anywhere and be disbanded
after a limited period of time.
The smallest building block of wireless LAN is a basic service set (BSS), which consists of some stations executing the same MAC protocol and competing for access to the same shared wireless medium. A BSS may be isolated or it may connect to a backbone distribution system (DC) through an access point (AP). The access point functions as a bridge. The MAC protocol may be fully distributed or controlled by a central coordination function housed in the access point. The BSS generally corresponds to what is referred to as a cell in the literature. The DS can be a switch, a wired network or a wireless network.

The simplest configuration is shown in Figure 9.2. Here, each station belongs to a single basic service set (BSS); that is, each station is within wireless range only of another station within the same BSS. It is also possible for two BSS to overlap geographically, so that a single station could participate in more than one BSS. Further, the association between a station and a BSS is dynamic; station may turn off, come within range, and go out of range.
An extended service set (ESS) consists of two or more basic services set interconnected by a distribution system. Typically, the distribution system is a wired backbone LAN, though it can be any communication network. The extended services set appears as a single logical LAN to the logical link control (LLC) level.Figure indicates that an access point (AP) is implemented as part of a station; the AP is the logic within a station that provides access to the DS by providing DS services in addition to acting as station. To integrate the IEEE 802.11 architecture with
= traditional wired LAN, a portal is used. The portal logic is implemented in device, as a bridge or a router, that is part of the wired LAN and is attached to the DS.
  PROTOCOL LAYER
S02- 11 standard defines a layered protocol architecture to implement the services as given below:
Association: Establishes an initial association between a station and an access point.
Reassociation:   Enables an established association to be transferred from one access point to another, allowing a mobile station to move.
Diassociation:    A notification from either a station or an access point that an existing association is terminated.
Authentication:     Used to establish the identity of stations to each other. Privacy:     Used to prevent the contents of messages from being read by other than the intended recipient. The standard provides for the optional use of encryption to assure privacy.
At the lowest layer, or the physical layer, the frequency band, data rate, and other details of the actual radio

transmission are defined. Above that is the medium access control (MAC) layer, which regulates access to the shared radio frequency band, so that station transmissions do not interfere with one another. The lower sublayer of the MAC layer is the distributed coordination function (DCF). DCF uses an Ethernet-style connection algorithm to provide access to all traffic. Ordinary asynchronous traffic directly uses DFC. The point coordination function (PCF) is a centralized MAC algorithm used to provide concentration free service; which is done by polling stations in turn. Higher-priority traffic, or traffic with greater timing requirements, makes use of the PCF. Finally, the logical link control (LLC) layer provides an interface to the higher layer and performs basic link-layer functions such as error control.
  PHYSICAL LAYER
The physical layer for IEEE 802.11 was issued in three stages; the first part was issued in 1997 and the remaining two in 1999. The first part, simply called IEEE 802.11, includes the MAC layer and three physical layer specifications, two in the 2.4 Ghz band and one in the infrared, all operating at 1 and 2 Mbps. IEEE 802.11 operates in the 5 Ghz band at data rates up to 54 Mbps. IEEE 802.11 operates in the 2.4 Ghz band at 5.5 and 11 Mpbs.
Three physical layers are defined in the original 802.11 standard.
(1) Direct sequence spread spectrum (DSSS) operating in 2.4 Ghz ISM band, at data rates of 1 Mbps and 2 Mbps.
(2) Frequency hopping spread spectrum (FHSS) operating in 2.4 Ghz ISM band, at data rates of 1 Mbps and 2 Mbps.
(3) Infrared at 1 Mbps and 2 Mbps operating at a wavelength between 850 and 950 nm.
The infrared option never gained market support. The other two schemes use spread spectrum approaches. In essence, spread spectrum involves the use of much wider bandwidth than actually necessary to support a given data rate. The result of using a wider bandwidth is to minimize interference and drastically reduce the error rate. In the case of FHSS, spread spectrum is achieved by frequently jumping from one carrier frequency to another; thus, if there is interference or performance degradation at a given frequency, it only affects a small fraction of the transmission. DSS effectively increases the data rate of a signal mapping each data bit into a string of bits, with one string used for binary 1 and another used for binary 0. The higher data rate uses a greater bandwidth. The effect is to spread each bit out over time, which minimizes the effects of interference and degradation. FHSS, which is similar, was employed in most of the early 802.11 networks. Products using DHSS, which is more effective in the 802.11 scheme, followed. However, all of the original 802.11 products were of limited utility because of the low data rates.
IEEE 802.11b is an extension of the IEEE 802.11 DS-SS scheme, providing data rates of 5.5 and 11 Mbps. A higher data rate is achieved by using a more complex modulation technique. The 802.11b specification quickly led to product offerings, including chipsets, PC cards, access points, and systems. Apple computer was the first company to offer 802.11b products, with its i-block portable computer using the Airport Wireless network option. Other companies, including Cisco, 3Com, and Dell have followed. Although these new products are all based on the same standard, there is always a concern whether products from different vendors will successfully interoperate. To meet this concern, the Wireless Ethernet Compatibility Alliance (WECA) created a test suite to certify interoperability for 802.11b products. Interoperability tests have 'been going on since last year and a number of products have achieved certification.
One other concern for both the original 802.11 and the 802.11b products is interference with other systems that operate in the 2.4 Ghz band, such as Bluetooth, Home RF, and many other devices that use the same portion of the spectrums. A
coenexistence study group (IEEE 802.15) is examining this issue and so far the prospects are encouraging.
Although 802.11b is achieving a certain level of success, its limited data rate results in limited appeal. To meet the needs for a truly high-speed LAN, IEEE 802.11a has been developed. The IEEE 802.11a specifications make use of the 5 Ghz band. Unlike the 2.4 Ghz specifications, IEEE 802.11 does not use a spread spectrum scheme, but an orthogonal frequency division multiplexing (OFDM). OFDM, also called multi­carrier modulation, uses multiple carrier signals (up to 52) at different frequencies, sending some of the bits on each channel. The possible data rates for IEEE 802.11a are 6 9. 12, 18, 24, 36, 48 and 54 Mbps. First generation 802.11b products should appear with WECA interoperability and compliance testing.

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