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`Overview of the IEEE 802.11 Standard
`
`Dec 6, 2001
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`This chapter is from the book
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`Wireless LANs, 2nd Edion
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`IEEE 802.11 Topology
`The IEEE 802.11 topology consists of components interacng to provide a wireless LAN that enables
`staon mobility transparent to higher protocol layers, such as the LLC. A staon is any device that
`contains funconality of the 802.11 protocol (in other words, the MAC layer, the PHY layer, and an
`interface to a wireless medium). The funcons of the 802.11 standard reside physically in a radio NIC,
`the soware interface that drives the NIC, and the access point. The 802.11 standard supports the
`following two topologies:
`
`Independent Basic Service Set (IBSS) networks
`
`Extended Service Set (ESS) networks
`
`These networks use a basic building block the 802.11 standard refers to as a BSS, providing a coverage
`area whereby staons of the BSS remain fully connected. A staon is free to move within the BSS, but it
`can no longer communicate directly with other staons if it leaves the BSS.
`
`NOTE
`
`Harris Semiconductor (now Intersil) was the first company to offer a complete radio chipset (called
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`PRISM) for direct sequence spread spectrum that is fully compliant with IEEE 802.11. The PRISM chip
`set includes six integrated microcircuits that handle all signal processing requirements of 802.11.
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`Independent BSS (IBSS) Networks
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`An IBSS is a standalone BSS that has no backbone infrastructure and consists of at least two wireless
`connue”, you agree to the storing of cookies on your device in accordance with our
`staons (see Figure
` 3.6). This type of network is oen referred to as an ad hoc network because it can be constructed
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`quickly without much planning. The ad hoc wireless network will sasfy most needs of users occupying
`a smaller area, such as a single room, sales floor, or hospital wing.
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`Extended Service Set (ESS) Networks
`For requirements exceeding the range limitaons of an independent BSS, 802.11 defines an Extended
`Service Set (ESS) LAN, as illustrated in Figure
` 3.7. This type of configuraon sasfies the needs of large coverage networks of arbitrary size and
`complexity.
`
`Figure 3.6 An independent BSS (IBSS) is the most basic type of 802.11 wireless LAN.
`
`Figure 3.7 An Extended Service Set (ESS) 802.11 wireless LAN consists of mulple cells
`interconnected by access points and a distribuon system, such as ethernet.
`
`The 802.11 standard recognizes the following mobility types:
`
`No-transion This type of mobility refers to staons that do not move and those that are
`moving within a local BSS.
`
`BSS-transion This type of mobility refers to staons that move from one BSS in one ESS to
`another BSS within the same ESS.
`
`ESS-transion This type of mobility refers to staons that move from a BSS in one ESS to a
`BSS in a different ESS.
`
`The 802.11 standard clearly supports the no-transion and BSS-transion mobility types. The standard,
`though, does not guarantee that a connecon will connue when making an ESS-transion.
`
`The 802.11 standard defines the distribuon system as an element that interconnects BSSs within the
`ESS via access points. The distribuon system supports the 802.11 mobility types by providing logical
`services necessary to handle address-to-desnaon mapping and seamless integraon of mulple
`BSSs. An access point is an addressable staon providing an interface to the distribuon system for
`staons located within various BSSs. The independent BSS and ESS networks are transparent to the LLC
`layer.
`
`Within the ESS, the 802.11 standard accommodates the following physical configuraon of BSSs:
`
`BSSs parally overlap This type of configuraon provides conguous coverage within a
`defined area, which is best if the applicaon cannot tolerate a disrupon of network
`service.
`
`BSSs are physically disjointed For this case, the configuraon does not provide conguous
`coverage. The 802.11 standard does not specify a limit to the distance between BSSs.
`
`BSSs are physically collocated This may be necessary to provide a redundant or higher-
`performing network.
`
`The 802.11 standard does not constrain the composion of the distribuon system; therefore, it may
`be 802 compliant or some non-standard network. If data frames need transmission to and from a non-
`IEEE 802.11 LAN, then these frames, as defined by the 802.11 standard, enter and exit through a logical
`point called a portal. The portal provides logical integraon between exisng wired LANs and 802.11
`LANs. When the distribuon system is constructed with 802-type components, such as 802.3
`(ethernet) or 802.5 (token ring), then the portal and the access point become one and the same.
`
`NOTE
`
`Before deeming their devices as 802.11 compliant, manufacturers should follow the protocol
`implementaon compliance procedures that the 802.11 standard specifies in its appendix. The
`procedures state that the vendor shall complete a Protocol Implementaon Conformance Statement
`(PICS) pro forma. The structure of the PICS pro forma mainly includes a list of quesons that the
`vendor responds to with yes or no answers, indicang adherence to mandatory and oponal
`porons of the standard.
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`For Wi-Fi cerficaon, refer to the test matrix document located at hp://www.wi-
`fi.com/downloads/test_matrix.PDF.
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