Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications
Section2 Components of the IEEE 802.11 architecture
更新于2008-06-25 21:42:45

5.2 Components of the IEEE 802.11 architecture

  • The IEEE 802.11 architecture consists of several components that interact to provide a WLAN that supports STA mobility transparently to upper layers.
  •  The basic service set (BSS) is the basic building block of an IEEE 802.11 LAN. Figure 5-1 shows twoBSSs, each of which has two STAs that are members of the BSS.
  • It is useful to think of the ovals used to depict a BSS as the coverage area within which the member STAs of the BSS may remain in communication. (The concept of area, while not precise, is often goodenough.) This area is called the Basic Service Area (BSA). If a STA moves out of its BSA, it can nolonger directly ommunicate with other STAs present in the BSA.

5.2.1 The independent BSS (IBSS) as an ad hoc network

  • The IBSS is the most basic type of IEEE 802.11 LAN. A minimum IEEE 802.11 LAN may consist of onlytwo STAs. Since the BSSs shown in Figure 5-1 are simple and lack other components (contrastthis with Figure 5-2), the two can be taken to be represenative of two IBSSs.
  • This mode of operation is possible when IEEE 802.11 STAs are able to communicate directly. Because this type of IEEE 802.11 LAN is often formed without pre-planning, for only as long as the LAN is needed, this type of operation is often referred to as an ad hoc network.

5.2.2 STA membership in a BSS is dynamic

  • A STA’s membership in a BSS is dynamic (STAs turn on, turn off, come within range, and go out of range). To become a member of a BSS, a STA joins theBSS using the synchronization procedure described in To access all the services of aninfrastructure BSS, a STA shall become “associated.” These associations are dynamic andinvolve the use of the distribution system service (DSS), which is described in 5.3.2.

5.2.3 Distribution system (DS) concepts

  • PHY limitations determine the direct station-to-station distance that may be supported. For somenetworks this distance is sufficient; for other networks, increased coverage is required.
  • Instead of existing independently, a BSS may also form a component of an extended form of networkthat is built with multiple BSSs. The architectural component used to interconnect BSSs is theDS.
  • iEEE Std 802.11 logically separates the WM from the distribution system medium (DSM). Each logical medium is used for different purposes, by a different component of the architecture. The IEEE 802.11 definitions neither preclude, nor demand, that the multiple media be either the same or different.
  • Recognizing that the multiple media are logically different is key to understanding the flexibility of the architecture. The IEEE 802.11 LAN architecture is specified independently of the physical characteristics of any specific implementation.
  • An access point (AP) is any entity that has STA functionality and enables access to the DS, via the WM for associated STAs.
  • Figure 5-2 adds the DS, DSM and AP components to the IEEE 802.11 architecture picture.TheDS enables mobile device support by providing the logical services necessary tohandle address to destination mapping and seamless integration of multiple BSSs.

  • Data move between a BSS and the DS via an AP. Note that all APs are also STAs; thus they are addressable entities. The addresses used by an AP for communication on the WM and on the DSM are not necessarily the same.
  • Data sent to the AP’s STA address by one of the STAs associated with it are always received at theuncontrolled port for processing by the IEEE 802.1X port access entity. In addition, if the controlledport is authorized, these frames conceptually transit the DS. Extended service set (ESS): The large coverage network

  • The DS and BSSs allow IEEE Std 802.11 to create a wireless network of arbitrary size and complexity.IEEE Std 802.11 refers to this type of network as the ESS network. An ESS is the union of the BSSs connected by a DS. The ESS does not include the DS.
  • The key concept is that the ESS network appears the same to an LLC layer as an IBSS network. STAswithin an ESS may communicate and mobile STAs may move from one BSS to another (withinthe same ESS) transparently to LLC.
  • Nothing is assumed by IEEE Std 802.11 about the relative physical locations of the BSSs in Figure 5-3.

All of the following are possible:

a) The BSSs may partially overlap. This is commonly used to arrange contiguous coverage within a physical volume. 

b) The BSSs could be physically disjoint. Logically there is no limit to the distance between BSSs.

c) The BSSs may be physically collocated. This may be done to provide redundancy.

d) One (or more) IBSS or ESS networks may be physically present in the same space as one (or more) ESS networks. This may arise for a number of reasons. Some examples are when an ad hoc network is operating in a location that also has an ESS network, when physically overlapping IEEE 802.11 networks have been set up by different organizations, and when two or more different access and security policies are needed in the same location. RSNA

  • An RSNA defines a number of security features in addition to wired equivalent privacy (WEP) and IEEE 802.11 authentication. These features include the following:
  • — Enhanced authentication mechanisms for STAs
    — Key management algorithms
    — Cryptographic key establishment
    — An enhanced data cryptographic encapsulation mechanism, called Counter mode with Cipher-block chaining Message authentication code Protocol (CCMP), and, optionally, Temporal Key Integrity Protocol (TKIP).
  • An RSNA relies on several components external to the IEEE 802.11 architecture.
  • The first component is an IEEE 802.1X port access entity (PAE). PAEs are present on all STAs in an RSNA and control the forwarding of data to and from the medium access control (MAC). An AP always implements the Authenticator PAE and Extensible Authentication Protocol (EAP) Authenticator roles, and a non-AP STA always implements the Supplicant PAE and EAP peer roles. In an IBSS, each STA implements both the Authenticator PAE and Supplicant PAE roles and both EAP Authenticator and EAP peer roles.
  • A second component is the Authentication Server (AS). The AS may authenticate the elements of the RSNA itself, i.e., the non-AP STAs; and APs may provide material that the RSNA elements can use to authenticate each other. The AS communicates throughtheIEEE802.1XAuthenticator with the IEEE 802.1X Supplicant on each STA, enabling the STA to beauthenticated to the ASand vice versa. An RSNA depends upon the use of an EAP method that supports mutual authentication of the AS and the STA, such as those that meet the requirements in IETF RFC 4017. In certain applications, the AS may be integrated into thesame physical device as the AP, or into a STA in an IBSS.

5.2.4 Area concepts

  • For wireless PHYs, well-defined coverage areas simply do not exist. Propagation characteristics aredynamic and unpredictable. Small changes in position or direction may result in dramatic differences in signal strength. Similar effects occur whether a STA is stationary or mobile (as movingobjects may impact station-to-station propagation).
  • Figure 5-4 shows a signal strength map for a simple square room with a standard metal desk and an open doorway. Figure 5-4 is a staticsnapshot;thepropagationpatternchangedynamically as STAs and objects in the environment move. In Figure 5-4 the dark (solid) blocks in the lower left are a metal desk and there is a doorway at the top right of the figure. The figure indicates relative differences in field strength with different intensities and indicatethevariability of field strength even in a static environment. The difference between the greatest signal strength and the least signal strength in Figure 5-4 is 50 dB.

  • While the architecture diagrams show sharp boundaries for BSSs, this is an artifact of the pictorial representation, not a physical reality. Because dynamic three-dimensional field strength pictures aredifficult to draw, well-defined shapes are used by IEEE 802.11 architectural diagrams to represent the coverage of a BSS.
  • Further description difficulties arise when attempting to describe collocated coverage areas. Consider Figure 5-5, in which STA 6 could belong to BSS 2 or BSS 3.
  • While the concept of sets of STAs is correct, it is often convenient to talk about areas. For many topics the concept of area is sufficient. Volume is a more precise term than area, though still not technically correct. For historical reasons and convenience, this standard uses the common term area.

5.2.5 Integration with wired LANs

  • To integrate the IEEE 802.11 architecture with a traditional wired LAN, a final logical architectural component is introduced—a portal.
  • A portal is the logical point at which MSDUs from an integrated non-IEEE-802.11 LAN enter the
    IEEE 802.11 DS. For example, a portal is shown in Figure 5-6 connecting to a wired IEEE 802 LAN.

  • All data from non-IEEE-802.11 LANs enter the IEEE 802.11 architecture via a portal. The portal is thelogical point at which the integration service is provided. The integration service is responsible forany addressing or frame format changes that might be required when frames pass between theDSand the integrated LAN. It is possible for one device to offer both the functions of an AP and aportal. 

 5.2.6 QoS BSS: The QoS network  

  • The IEEE 802.11 QoS facility provides MAC enhancements to support LAN applications with QoS equirements. The QoS enhancements are available to QoS STAs associated with a QoS access point in a QoS BSS. A subset of the QoS enhancements is available for use between STAs that are members of the same QoS IBSS. Because a QoS STA implements a superset of STA functionality, as defined in this standard, the STA may associate with a non-QoS access point in a non-QoS BSS, to provide non-QoS MAC data service when there is no QoS BSS with which to associate.
  • The enhancements that distinguish QoS STAs from non-QoS STAs and QoS APs from non-QoS APs are collectively termed the QoS facility. The quantity of certain, QoS-specific, mechanisms may vary among QoS implementations, as well as between QoS STAs and QoS APs, over ranges specified in subsequent clauses. All service primitives, frame formats, coordination function and frame exchange rules, and management interface functions except for the Block Acknowledgment (Block Ack) function, direct-link setup (DLS), and automatic power-save delivery (APSD) are part of the core QoS facilities. A QoS STA or QoS AP must implement those core QoS facilities necessary for its QoS functions to interoperate with otherSTAs in the BSS. Functions such as the Block Ack, DLS, and APSD are separate from thecore QoS facilities; and the presence of these functions is indicated by STAs separately from the core QoS facilities. A comprehensive statement on mandatory and optional functionalities is available in Annex A.
  • This standard provides two mechanisms for the support of applications with QoS requirements.
  • The first mechanism, designated the enhanced distributed channel access (EDCA), delivers traffic based on differentiating user priorities (UPs). This differentiation is achieved by varying the following for different UP values:
    — Amount of time a STA senses the channel to be idle before backoff or transmission, or
    — The length of the contention window to be used for the backoff, or
    — The duration a STA may transmit after it acquires the channel.
  • These transmissions may also be subject to certain channel access restrictions in the form of admission control. Details of this mechanism are provided in 9.9.1. The second mechanism, designated the hybrid coordination function (HCF) controlled channel access (HCCA), allows for the reservation of transmission opportunities (TXOPs) with the hybrid coordinator (HC). A non-access point (non-AP) STA based on its requirements requests the HC for TXOPs, both for its own transmissions as well as for transmissions from the AP to itself.13 The request is initiated by the station management entity (SME) of the non-AP STA. The HC, which is collocated at the AP, either accepts or rejects the request based on an admission control policy. If the request is accepted, the HC schedules TXOPs for both the AP and the non-AP STA. For transmissions from the non-AP STA, the HC polls thenon-AP STA based on the parameters supplied by the non-AP STA at the time of its request. For transmissions to the non-AP STA, the AP directly obtains TXOPs from the collocated HC and delivers the queued frames to the non-AP STA, again based on the parameters supplied by the non-AP STA. Details of themechanism are provided in 9.9.2 and 11.4. This mechanism may be used for applications such as voice and video, which may need periodic service from the HC. If the application constraints dictate the use of this mechanism, the application initiates this mechanism by using the management service primitives.
  • Non-QoS STAs may associate in a QoS BSS, if allowed to associate by the AP. All directed frames that are sent to non-QoS STAs by an AP shall not use the frame formats associated with the QoS facility.
  • A QoS STA associated in a non-QoS BSS shall act as a non-QoS STA.