IEEE 802.11 Tutorial

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Mustafa Ergen June 2002 ergen@eecs.berkeley.edu Department of Electrical Engineering and Computer Science University of California Berkeley 2 Abstract This document describes IEEE 802.11 Wireless Local Area Network (WLAN) Standard. It describes IEEE 802.11 MAC Layer in detail and it briefly mentions IEEE 802.11a, IEEE 802.11b physical layer standard and IEEE 802.11e MAC layer standard. Acknowledgement I quoted some of the materials from the “IEEE 802.11 Handbook- A Designer‘s Companion” book. I want to thank Haiyun Tang for his contribution in finite state machine representations. Contents 1 Overview 1.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.1.1 Goals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.1.2 Architecture . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 5 5 6 2 Medium Access Control 2.1 MAC Functionality . . . . . . . . . . . . . . . . . . . 2.2 MAC Frame Exchange Protocol . . . . . . . . . . . . 2.2.1 Dealing with Media . . . . . . . . . . . . . . 2.2.2 The Hidden Node Problem . . . . . . . . . . 2.2.3 Retry Counters . . . . . . . . . . . . . . . . . 2.2.4 Basic Access Mechanism . . . . . . . . . . . . 2.2.5 Timing Intervals . . . . . . . . . . . . . . . . 2.2.6 DCF Operation . . . . . . . . . . . . . . . . . 2.2.7 Centrally Controlled Access Mechanism . . . 2.2.8 Frame Types . . . . . . . . . . . . . . . . . . 2.2.9 Control Frame Subtypes . . . . . . . . . . . . 2.2.10 Data Frame Subtypes . . . . . . . . . . . . . 2.2.11 Management Frame Subtypes . . . . . . . . . 2.2.12 Components of the Management Frame Body 2.2.13 Other MAC Operations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 11 11 12 12 13 14 16 17 17 20 22 24 25 27 30 3 MAC Management 3.1 Tools Available to Meet the Challenges 3.1.1 Authentication . . . . . . . . . . 3.1.2 Association . . . . . . . . . . . . 3.1.3 Address Filtering . . . . . . . . . 3.1.4 Privacy MAC Function . . . . . 3.1.5 Power Management . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36 36 36 37 38 38 38 . . . . . . . . . . . . . . . . . . 2 . . . . . . . . . . . . . . . . . . . . . . . . 3.2 3.1.6 Synchronization . . . . . . . . . . . . . . . . . Combining Management Tools . . . . . . . . . . . . . 3.2.1 Combine Power Saving Periods with Scanning . 3.2.2 Preauthentication . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40 42 42 43 4 MAC Management Information Base 44 4.1 Station Management Attributes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44 4.2 MAC Attributes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46 5 The Physical Layer 5.1 Physical Layer (PHY) Functionality . . . . . . . . . . . . . . . . . . 5.2 Direct Sequence Spread Spectrum (DSSS) PHY . . . . . . . . . . . . 5.2.1 DSSS PLCP Sublayer . . . . . . . . . . . . . . . . . . . . . . 5.2.2 Data Scrambling . . . . . . . . . . . . . . . . . . . . . . . . . 5.2.3 DSSS Modulation . . . . . . . . . . . . . . . . . . . . . . . . 5.2.4 Barker Spreading Method . . . . . . . . . . . . . . . . . . . . 5.2.5 DSSS Operating Channels and Transmit Power Requirements 5.3 The Frequency Hopping Spread Spectrum (FHSS) PHY . . . . . . . 5.3.1 FHSS PLCP Sublayer . . . . . . . . . . . . . . . . . . . . . . 5.3.2 PSDU Data Whitening . . . . . . . . . . . . . . . . . . . . . 5.3.3 FHSS Modulation . . . . . . . . . . . . . . . . . . . . . . . . 5.3.4 FHSS Channel Hopping . . . . . . . . . . . . . . . . . . . . . 5.4 Infrared (IR) PHY . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.4.1 IR PLCP Sublayer . . . . . . . . . . . . . . . . . . . . . . . . 5.4.2 IR PHY Modulation Method . . . . . . . . . . . . . . . . . . 5.5 Geographic Regulatory Bodies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 Physical Layer Extensions to IEEE 802.11 6.1 IEEE 802.11a - The OFDM Physical Layer . . . . . . . . . . . . . . . 6.1.1 OFDM PLCP Sublayer . . . . . . . . . . . . . . . . . . . . . . 6.1.2 Data Scrambler . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.1.3 Convolutional Encoding . . . . . . . . . . . . . . . . . . . . . . 6.1.4 OFDM Modulation . . . . . . . . . . . . . . . . . . . . . . . . . 6.1.5 OFDM Operating Channels and Transmit Power Requirements 6.1.6 Geographic Regulatory Bodies . . . . . . . . . . . . . . . . . . 6.2 IEEE 802.11b-2.4 High Rate DSSS PHY . . . . . . . . . . . . . . . . . 6.2.1 HR/DSSS PHY PLCP Sublayer . . . . . . . . . . . . . . . . . 6.2.2 High Rate Data Scrambling . . . . . . . . . . . . . . . . . . . . 6.2.3 IEEE 802.11 High Rate Operating Channels . . . . . . . . . . . 6.2.4 IEEE 802.11 DSSS High Rate Modulation and Data Rates . . 3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49 49 49 49 51 51 52 52 53 53 54 55 55 56 57 59 59 . . . . . . . . . . . . 60 60 60 61 62 62 63 63 64 64 65 66 66 6.2.5 6.2.6 6.2.7 Complementary Code Keying (CCK) Modulation . . . . . . . . . . . . . . . . . . . . . . 66 DSSS Packet Binary Convolutional Coding . . . . . . . . . . . . . . . . . . . . . . . . . 66 Frequency Hopped Spread Spectrum (FHSS)Inter operability . . . . . . . . . . . . . . . 66 7 System Design Considerations for IEEE 802.11 WLANs 7.1 The Medium . . . . . . . . . . . . . . . . . . . . . . . . . . 7.2 Multipath . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.3 Multipath Channel Model . . . . . . . . . . . . . . . . . . . 7.4 Path Loss in a WLAN System . . . . . . . . . . . . . . . . . 7.5 Multipath Fading . . . . . . . . . . . . . . . . . . . . . . . . 7.6 Es/No vs BER Performance . . . . . . . . . . . . . . . . . . 7.7 Data Rage vs Aggregate Throughput . . . . . . . . . . . . . 7.8 WLAN Installation and Site Survey . . . . . . . . . . . . . 7.9 Interference in the 2.4 GHz Frequency Band . . . . . . . . . 7.10 Antenna Diversity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67 67 67 68 68 69 69 69 70 70 70 8 IEEE 802.11 PROTOCOLS 8.1 Overview of IEEE 802.11 Standards . . . . . . . . . . 8.2 IEEE 802.11E MAC PROTOCOL . . . . . . . . . . . 8.2.1 Enhanced Distribution Coordination Function . 8.2.2 Hybrid Coordination Function . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72 72 74 74 76 . . . . . . . . . . 78 78 78 78 79 79 80 80 83 83 85 A 802.11 Frame Format A.1 MAC Frame Formats . . . . . . . . . . A.1.1 General Frame Format . . . . . A.1.2 Frame Fields . . . . . . . . . . A.2 Format of individual frame types . . . A.2.1 Control frames . . . . . . . . . A.2.2 Data Frames . . . . . . . . . . A.2.3 Management frames . . . . . . A.3 Management frame body components A.3.1 Fixed Fields . . . . . . . . . . . A.3.2 Information Elements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . B IEEE 802.11a Physical Layer Parameters 87 B.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87 B.2 IEEE 802.11a OFDM PHY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87 4 Chapter 1 Overview 1.1 Introduction • In 1997, the IEEE adopted the first standard for WLANs and revised in 1999. • IEEE defines a MAC sublayer, MAC management protocols and services, and three physical (PHY) layers. • PHY Layers: 1. IR at baseband with 1-2 Mbps, 2. FHSS at 2.4GHz with 1-2 Mbps, 3. DSSS at DSSS with 1-2 Mbps. • IEEE 802.11a ; PHY Layer - OFDM at UNII bands with 54 Mbps • IEEE 802.11b ; PHY Layer - DSSS at 2.4 GHz with 11Mbps 1.1.1 Goals • to deliver services previously found only in wired networks. • high throughput • highly reliable data delivery • continuous network connection. 5 1.1.2 Architecture Architecture is designed to support a network where mobile station is responsible for the decision making. Advantages are • very tolerant of faults in all of the WLAN equipment. • eliminates any possible bottlenecks a centralized architecture would introduce. Architecture has power-saving modes of operation built into the protocol to prolong the battery life of mobile equipment without losing network connectivity. Components Station the component that connects to the wireless medium. Supported services are authentication, deauthentication, privacy, and delivery of the data. Basic Service Set A BSS is a set of stations that communicate with one another. A BSS does not generally refer to a particular area, due to the uncertainties of electromagnetic propagation. When all of the stations int the BSS are mobile stations and there is no connection to a wired network, the BSS is called independent BSS (IBSS). IBSS is typically short-lived network, with a small number of stations, that is created for a particular purpose. When a BSS includes an access point (AP), the BSS is called infrastructure BSS. When there is a AP, If one mobile station in the BSS must communicate with another mobile station, the communication is sent first to the AP and then from the AP to the other mobile station. This consume twice the bandwidth that the same communication. While this appears to be a significant cost, the benefits provided by the AP far outweigh this cost. One of them is, AP buffers the traffic of mobile while that station is operating in a very low power state. Extended Service Set (ESS) A ESS is a set of infrastructure BSSs, where the APs communicate among themselves to forward traffic from one BSS to another and to facilitate the movement of mobile stations from one BSS to another. The APs perform this communication via an abstract medium called the distribution system (DS). To network equipment outside of the ESS, the ESS and all of its mobile stations appears to be a single MAC-layer network where all stations are physically stationary. Thus, the ESS hides the mobility of the mobile stations from everything outside the ESS. Distribution System the distribution system (DS) is the mechanism by which one AP communicates with another to exchange frames for stations in their BSSs, forward frames to follow mobile stations from one BSS to another, and exchange frames with wired network. Services • Station Services: Authentication, De-authentication, privacy, delivery of data • Distribution Services: Association, Disassociation, Reassociation, Distribution, Integration 6 Station Services Similar functions to those that are expected of a wired network. The wired network function of physically connecting to the network cable is similar to the authentication and de-authentication services. Privacy is for data security. Data delivery is the reliable delivery of data frames from the MAC in one station to the MAC in one or more other station, with minimal duplication and minimal ordering. Distribution Services provide services necessary to allow mobile stations to roam freely within an ESS and allow an IEEE 802.11 WLAN to connect with the wired LAN infrastructure. A thin layer between MAC and LLC sublayer that are invoked to determine how to forward frames within the IEEE 802.11 WLAN and also how to deliver frames from the IEEE 802.11 WLAN to network destinations outside of the WLAN. • The association service makes a logical connection between a mobile station and an AP. It is necessary for DS to know where and how to deliver data to the mobile station. the logical connection is also necessary for the AP to accept data frames from the mobile station and to allocate resources to support the mobile station. The association service is invoked once, when the mobile station enters the WLAN for the first time, after the application of power or when rediscovering the WLAN after being out of touch for a time. • The reassociation service includes information about the AP with which a mobile station has been previously associated. Mobile station uses repeatedly as it moves in ESS and by using reassociation service, a mobile station provides information to the AP with which the mobile station was previously associated, to obtain frames. • The disassociation service is used to force a mobile station to associate or to inform mobile station AP is no longer available. A mobile may also use the disassociation service when it no longer require the services of the AP. • An AP to determine how to deliver the frames it receives uses the distribution service. AP invoke the distribution service to determine if the frame should be sent back into its own BSS, for delivery to a mobile station that is associated with the AP, or if the frame should be sent into the DS for delivery to another mobile station associated with a different AP or to a network destination. • The integration service connects the IEEE 802.11 WLAN to other LANs, The integration service translates IEEE 802.11 frames to frames that may traverse another network, and vice versa. Interaction between Some Services The IEEE 802.11 standard states that each station must maintain two variables that are dependent on the authentication, de-authentication services and the association, reassociation, disassociation services. The variables are authentication state and association state and used in a simple state machine that determines the order in which certain services must be invoked and when a station may begin using the data delivery service. A station may be authenticated with many different stations simultaneously. However, a station may be associated with only one other station at a time. 7
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