1998 IEEE 802.11-98 WLAN标准：MAC与PHY层详解

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802.11-98 WLAN 是一个重要的无线局域网络 (WLAN) 标准，由 IEEE LAN/MAN 标准委员会制定，于 1998 年 11 月 16 日发布。该标准文档主要关注 MAC 层（Media Access Control）和 PHY 层（Physical Layer），这两个层在实现无线通信中起着至关重要的作用。在 ISO/IEC DIS 8802-11 和 IEEEP 802.11/D9.0 的框架下，802.11-98 定义了一个统一的接入方法，旨在为桌面用户提供各种公共和私有骨干网络（如 ANSIFDDI、IEEE 802.x 和 ISDN）之间的集成服务（Integrated Services，IS）。它支持多种类型的网络环境，包括本地区域网络 (LAN) 和城市区域网络 (MAN)，旨在提供高效、兼容且可管理的服务。在 MAC 子层中，关键概念包括逻辑链路控制 (LLC) 和介质访问控制 (MAC) 子层。LLC 负责数据链路层的协议处理，而 MAC 子层则负责管理多个用户间的无线资源分配，通过定义诸如数据包（Protocol Data Unit，PDU）和管理对象（Managed Object）来确保有序传输。此外，它还定义了接入单元 (AU) 和混合多路复用器 (Hybrid Multiplexer，HMUX) 的概念，这些技术用于优化网络带宽和信号效率。 PHY 层则关注物理媒介依赖 (Physical Medium Dependent, PMD) 和物理信令 (Physical Signalling)，它们负责无线信号的传输和接收，确保信号在无线介质上的可靠传输。标准还定义了物理层接口和物理层管理实体 (PHY Layer Management Entity)，以及与物理介质相关的参数设置，如信道选择和功率控制。此外，802.11-98 WLAN 标准还涉及管理信息库 (Management Information Base, MIB)，这是一个存储网络配置和性能数据的结构，便于网络管理员进行监控和维护。集成服务终端设备 (Integrated Service Terminal Equipment, ISTE) 和私有交换网络 (Private Switching Network) 的概念也在该标准中有所体现，以满足不同应用场景的需求。总结来说，802.11-98 WLAN 标准是构建现代无线网络基础设施的核心组成部分，它不仅规定了无线接入技术的基本规则，还强调了服务质量 (QoS)、标准化接口和管理的统一性，对推动无线通信技术的发展起到了关键作用。

ISO/IEC DIS 8802-11

IEEE P802.11/D9.0, 16 Nov. 1998 LOCAL AND METROPOLITAN AREA NETWORKS: WIRELESS LAN

This is an unapproved IEEE Standards Draft, subject to change.

PC point coordinator

PCF point coordination function

PDU protocol data unit

PHY physical (layer)

PHY-SAP physical layer service access point

PIFS point (coordination function) interframe space

PLCP physical layer convergence protocol

PLME physical layer management entity

PMD physical medium dependent

PMD-SAP physical medium dependent service access point

PN pseudonoise (PN code sequence)

PPDU PLCP protocol data unit

ppm parts per million

PPM pulse position modulation

PRNG pseudorandom number generator

PS power save (mode)

PSDU PLCP SDU

RA receiver address

RF radio frequency

RSSI received signal strength indication

RTS request to send

RX receive or receiver

SA source address

SAP service access point

SDU service data unit

SFD start frame delimiter

SIFS short interframe space

SLRC station long retry count

SME station management entity

SMT station management

SQ signal quality (PN code correlation strength)

SRC short retry count

SS station service

SSAP source service access point

SSID service set identiﬁer

SSRC station short retry count

STA station

TA transmitter address

TBTT target beacon transmission time

TIM trafﬁc indication map

TSF timing synchronization function

TU time unit

TX transmit or transmitter

TXE transmit enable

WAN wide area network

WDM wireless distribution media

WDS wireless distribution system

WEP wired equivalent privacy

WM wireless medium

ISO/IEC DIS 8802-11

MEDIUM ACCESS CONTROL (MAC) AND PHYSICAL (PHY) SPECIFICATIONS IEEE P802.11/D9.0, 16 Nov. 1998

This is an unapproved IEEE Standards Draft, subject to change.

5. General description

5.1 General description of the architecture

This subclause presents the concepts and terminology used within IEEE Std 802.11-1997 (referred to

throughout the text as IEEE 802.11). Speciﬁc terms are deﬁned in Clause 3. Illustrations convey key IEEE

802.11 concepts and the interrelationships of the architectural components. IEEE 802.11 uses an architec-

ture to describe functional components of an IEEE 802.11 LAN. The architectural descriptions are not

intended to represent any speciﬁc physical implementation of IEEE 802.11.

5.1.1 How wireless LAN systems are different

Wireless networks have fundamental characteristics that make them signiﬁcantly different from traditional

wired LANs. Some countries impose speciﬁc requirements for radio equipment in addition to those speciﬁed

in this standard (e.g., Japan in MPT notice No. 759,1992 for the content for identiﬁcation signals as meant in

Article 25 of the ITU Radio Regulations).

5.1.1.1 Destination address does not equal destination location

In wired LANs, an address is equivalent to a physical location. This is implicitly assumed in the design of

wired LANs. In IEEE 802.11, the addressable unit is a station (STA). The STA is a message destination, but

not (in general) a ﬁxed location.

5.1.1.2 The media impact the design

The physical layers used in IEEE 802.11 are fundamentally different from wired media. Thus IEEE 802.11

PHYs

a) Use a medium that has neither absolute nor readily observable boundaries outside of which stations

with conformant PHY transceivers are known to be unable to receive network frames.

b) Are unprotected from outside signals.

c) Communicate over a medium signiﬁcantly less reliable than wired PHYs.

d) Have dynamic topologies.

e) Lack full connectivity, and therefore the assumption normally made that every STA can hear every

other STA is invalid (that is, STAs may be “hidden” from each other).

f) Have time-varying and asymmetric propagation properties.

Because of limitations on wireless PHY ranges, wireless LANs intended to cover reasonable geographic dis-

tances may be built from basic coverage building blocks.

5.1.1.3 The impact of handling mobile stations

One of the requirements of IEEE 802.11 is to handle

mobile

as well as

portable

stations. A

portable

station

is one that is moved from location to location, but is only used while at a ﬁxed location.

Mobile

stations actu-

ally access the LAN while in motion.

For technical reasons, it is not sufﬁcient to handle only portable stations. Propagation effects blur the distinc-

tion between portable and mobile stations; stationary stations often appear to be mobile due to propagation

effects.

Another aspect of mobile stations is that they may often be battery powered. Hence power management is an

important consideration. For example, it cannot be presumed that a station’s receiver will always be powered on.

ISO/IEC DIS 8802-11

IEEE P802.11/D9.0, 16 Nov. 1998 LOCAL AND METROPOLITAN AREA NETWORKS: WIRELESS LAN

This is an unapproved IEEE Standards Draft, subject to change.

5.1.1.4 Interaction with other IEEE 802 layers

IEEE 802.11 is required to appear to higher layers [logical link control (LLC)] as a current style 802 LAN.

This requires that the IEEE 802.11 network handle station mobility within the MAC sublayer. To meet reli-

ability assumptions (that LLC makes about lower layers), it is necessary for IEEE 802.11 to incorporate

functionality that is untraditional for MAC sublayers.

5.2 Components of the IEEE 802.11 architecture

The IEEE 802.11 architecture consists of several components that interact to provide a wireless LAN that

supports station mobility transparently to upper layers.

The

basic service set

(BSS) is the basic building block of an IEEE 802.11 LAN. Figure 1 shows two BSSs,

each of which has two stations 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 stations

of the BSS may remain in communication. (The concept of area, while not precise, is often good enough.) If

a station moves out of its BSS, it can no longer directly communicate with other members of the BSS.

5.2.1 The independent BSS as an ad hoc network

The independent BSS (IBSS) is the most basic type of IEEE 802.11 LAN. A minimum IEEE 802.11 LAN

may consist of only two stations.

Figure 1 shows two IBSSs. This mode of operation is possible when IEEE 802.11 stations are able to com-

municate 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

5.2.1.1 STA to BSS association is dynamic

The association between a STA and a BSS is dynamic (STAs turn on, turn off, come within range, and go out

of range). To become a member of an infrastructure BSS, a station shall become “associated.” These associ-

ations are dynamic and involve the use of the distribution system service (DSS), which is described later.

Figure 1—Basic service sets

ISO/IEC DIS 8802-11

MEDIUM ACCESS CONTROL (MAC) AND PHYSICAL (PHY) SPECIFICATIONS IEEE P802.11/D9.0, 16 Nov. 1998

This is an unapproved IEEE Standards Draft, subject to change.

5.2.2 Distribution system concepts

PHY limitations determine the direct station-to-station distance that may be supported. For some networks

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 network that is

built with multiple BSSs. The architectural component used to interconnect BSSs is the

distribution system

(DS).

IEEE 802.11 logically separates the wireless medium (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.

The DS enables mobile device support by providing the logical services necessary to handle address to des-

tination mapping and seamless integration of multiple BSSs.

access point

(AP) is a STA that provides access to the DS by providing DS services in addition to acting

as a STA.

Figure 2 adds the DS and AP components to the IEEE 802.11 architecture picture.

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.

5.2.2.1 Extended service set (ESS): The large coverage network

The DS and BSSs allow IEEE 802.11 to create a wireless network of arbitrary size and complexity. IEEE

802.11 refers to this type of network as the

extended service set

(ESS) network.

Figure 2—Distribution systems and access points

ISO/IEC DIS 8802-11

IEEE P802.11/D9.0, 16 Nov. 1998 LOCAL AND METROPOLITAN AREA NETWORKS: WIRELESS LAN

This is an unapproved IEEE Standards Draft, subject to change.

The key concept is that the ESS network appears the same to an LLC layer as an IBSS network. Stations

within an ESS may communicate and mobile stations may move from one BSS to another (within the same

ESS) transparently to LLC.

Nothing is assumed by IEEE 802.11 about the relative physical locations of the BSSs in Figure 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 disjointed. 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. Two of the most common are when an ad hoc

network is operating in a location that also has an ESS network, and when physically overlapping

IEEE 802.11 networks have been set up by different organizations.

5.2.3 Area concepts

For wireless PHYs, well-deﬁned coverage areas simply do not exist. Propagation characteristics are dynamic

and unpredictable. Small changes in position or direction may result in dramatic differences in signal

strength. Similar effects occur whether a station is stationary or mobile (as moving objects may impact sta-

tion-to-station propagation).

Figure 4 shows a signal strength map for a simple square room with a standard metal desk and an open door-

way. Figure 4 is a static snapshot; the propagation patterns change dynamically as stations and objects in the

environment move. In Figure 4 the dark (solid) blocks in the lower left are a metal desk and there is a door-

way at the top right of the ﬁgure. The ﬁgure indicates relative differences in ﬁeld strength with different

intensities and indicates the variability of ﬁeld strength even in a static environment.

While the architecture diagrams show sharp boundaries for BSSs, this is an artifact of the pictorial represen-

tation, not a physical reality. Since dynamic three-dimensional ﬁeld strength pictures are difﬁcult to draw,

well-deﬁned shapes are used by IEEE 802.11 architectural diagrams to represent the coverage of a BSS.

Figure 3—Extended service set

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