17-WirelessLANs - Data and Computer Communications...

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Unformatted text preview: Data and Computer Communications Communications Chapter 17 – Wireless LANs Chapter LAN Eighth Edition by William Stallings Lecture slides by Lawrie Brown High Speed LANs LAN Investigators have published numerous reports of birds taking turns vocalizing; the bird spoken to gave its full attention to the speaker and never vocalized at the same time, as if the two were holding a conversation Researchers and scholars who have studied the data on avian communication carefully write the (a) the communication code of birds such has crows has not been broken by any means; (b) probably all birds have wider vocabularies than anyone realizes; and (c) greater complexity and depth are recognized in avian communication as research progresses. —The Human Nature of Birds, Theodore Barber Overview of Wireless LANs Overview use wireless transmission medium issues of high prices, low data rates, occupational safety concerns, & licensing requirements now addressed requirements key application areas: LAN extension cross-building interconnect nomadic access ad hoc networking Single Cell LAN Extension LAN Multi Cell LAN Extension LAN Cross-Building Interconnect Cross-Building connect LANs in nearby buildings point-to-point wireless link Not a LAN per se connect bridges or routers Nomadic Access Nomadic link LAN hub & mobile data terminal laptop or notepad computer enable employee to transfer data from enable portable computer to server portable also useful in extended environment such also as campus or cluster of buildings as users move around with portable computers may wish access to servers on wired LAN Infrastructure Wireless LAN Infrastructure Ad Hoc Networking Ad temporary peer-to-peer network temporary peer Wireless LAN Requirements Wireless throughput - efficient use wireless medium no of nodes - hundreds of nodes across multiple cells no of connection to backbone LAN - using control modules service area - 100 to 300 m llow power consumption - for long battery life on mobiles ow on transmission robustness and security collocated network operation license-free operation handoff/roaming dynamic configuration - addition, deletion, and relocation deletion, of end systems without disruption to users of Technology Technology infrared (IR) LANs iindividual cell of IR LAN limited to single room ndividual cell IR light does not penetrate opaque walls IR light mostly operate in ISM (industrial, scientific, and medical) bands medical) no Federal Communications Commission (FCC) licensing is required in USA USA microwave frequencies but not use spread spectrum some require FCC licensing spread spectrum LANs narrowband microwave Infrared LANs Infrared constructed using infrared portion of spectrum strengths spectrum virtually unlimited hence high rates possible unregulated spectrum iinfrared shares some properties of visible light nfrared shares • reflection covers room, walls isolate networks inexpensive and simple background radiation, e.g. sunlight, indoor lighting background radiation, power limited by concerns for eye safety and power consumption consumption weaknesses Infrared LANs Infrared Transmission Techniques directed-beam IR point-to-point links range depends on power and focusing for indoor use can set up token ring LAN IR transceivers positioned so data circulates in ring single base station with line of sight to other stations acts as a multiport repeater other stations use directional beam to it use stations focused / aimed at diffusely reflecting ceiling at omnidirectional diffused configuration Spread Spectrum LAN Spread Configuration usually use multiple-cell arrangement usually use adjacent cells use different center frequencies configurations: hub hub • • • connected to wired LAN connected to wired connect to stations on wired LAN and in other cells may do automatic handoff peer-to-peer • no hub • MAC algorithm such as CSMA used to control access • for ad hoc LANs Spread Spectrum LANs Spread Transmission Issues Transmission llicensing regulations differ between countries icensing regulations USA FCC allows in ISM band: spread spectrum (1W), very low power (0.5W) spread spectrum 1W), very • 902 - 928 MHz (915-MHz band) • 2.4 - 2.4835 GHz (2.4-GHz band) • 5.725 - 5.825 GHz (5.8-GHz band) 2.4 GHz also in Europe and Japan many devices around 900 MHz: cordless telephones, wireless microphones, and amateur radio wireless fewer devices at 2.4 GHz; microwave oven little competition at 5.8 GHz interference IEEE 802 Standards IEEE Standard Scope Medium access control (MAC): One common MAC for WLAN applications IEEE 802.11 Physical layer: Infrared at 1 and 2 Mbps Physical layer: 2.4-GHz FHSS at 1 and 2 Mbps Physical layer: 2.4-GHz DSSS at 1 and 2 Mbps IEEE 802.11a IEEE 802.11b IEEE 802.11c IEEE 802.11d IEEE 802.11e IEEE 802.11f IEEE 802.11g IEEE 802.11h IEEE 802.11i IEEE 802.11j Physical layer: 5-GHz OFDM at rate s from 6 to 54 Mbps Physical layer: 2.4-GHz DSSS at 5. 5 and 11 Mbps Bridge operation at 802.11 MAC lay er Physical layer: Extend operation of 802.11 WLANs to new regulatory domains (countries) MAC: Enhance to improve quality of service and enhance security mechanisms Recommended practices for multiven dor access point interoperability Physical layer: Extend 802.11b to data rates >20 Mbps Physical/MAC: Enhance IEEE 802.11a to add indoor and outdoor channel selection and to improve spectrum and transmit power management MAC: Enhance security and authenti cation mechanisms Physical: Enhance IEEE 802.11a to conform to Japanese requirements Radio resource measurement enhance ments to provide interface to higher layers for rad io and network measurements Maintenance of IEEE 802.11-1999 st andard with technical and editorial corrections Physical/MAC: Enhancements to enab le higher throughput Physical/MAC: Wireless access in vehicular environments Physical/MAC: Fast roaming (fast BSS transition) Physical/MAC: ESS mesh networking Recommended practice for the Evalu ation of 802.11 wireless performance Physical/MAC: Interworking with ex ternal networks IEEE 802.11k IEEE 802.11m IEEE 802.11n IEEE 802.11p IEEE 802.11r IEEE 802.11s IEEE 802.11,2 IEEE 802.11u IEEE 802 Terminology IEEE Access point (AP) Any entity that has station functionality and provides access to the distribution system via the wireless medium for associated stations A set of stations controlled by a single coordination function The logical function that determines when a station operating within a BSS is permitted to transmit and may be able to receive PDUs A system used to interconnect a set of BSSs and integrated LANs to create an ESS A set of one or more interconnected BSSs and integrated LANs that appear as a single BSS to the LLC layer at any station associated with one of these BSSs The unit of data exchanged between two peer MAC entites using the services of the physical layer Information that is delivered as a unit between MAC users Any device that contains an IEEE 802.11 conformant MAC and physical layer Basic service set (BSS) Coordination function Distribution system (DS) Extended service set (ESS) MAC protocol data unit (MPDU) MAC service data unit (MSDU) Station IEEE 802.11 Architecture IEEE IEEE 802.11 - BSS IEEE basic service set (BSS) building block may be isolated may connect to backbone distribution may system (DS) through access point (AP) system BSS generally corresponds to cell BSS DS can be switch, wired network, or DS wireless network wireless have independent BSS (IBSS) with no AP Extended Service Set (ESS) Extended possible configurations: simplest is each station belongs to single BSS simplest each can have two BSSs overlap can BSSs a station can participate in more than one BSS association between station and BSS dynamic ESS is two or more BSS interconnected by DS BSS DS appears as single logical LAN to LLC IEEE 802 Services IEEE Service Association Authentication Deauthentication Dissassociation Distribution Integration MSDU delivery Privacy Reassocation Provider Distribution system Station Station Distribution system Distribution system Distribution system Station Station Distribution system Used to support MSDU delivery LAN ac cess and securi ty LAN ac cess and securi ty MSDU delivery MSDU delivery MSDU delivery MSDU delivery LAN ac cess and securi ty MSDU delivery Services - Message Distribution Distribution distribution service primary service used by stations to exchange primary MAC frames when frame must traverse DS MAC iif stations in same BSS, distribution service f logically goes through single AP of that BSS logically enables transfer of data between 802.11 LAN enables station and one on an integrated 802.x LAN one integration service Association Related Services Association DS requires info about stations within ESS provided by association-related services station must associate before communicating communicating 3 mobility transition types: no transition - stationary or in single BSS BSS transition - between BSS in same ESS BSS ESS transition: between BSS in different ESS ESS between Association Related Services Association DS needs identity of destination statio stations must maintain association with AP within current BSS within Association - establishes initial association Association between station and AP between Reassociation - to transfer an association to ransfer association another AP AP Disassociation - by station or AP station 3 services relate to this requirement: Medium Access Control Medium MAC layer covers three functional areas reliable data delivery access control access control security Reliable Data Delivery Reliable 802.11 physical / MAC layers unreliable noise, interference, and other propagation effects noise, result in loss of frames result even with error-correction codes, frames may not even successfully be received successfully can be dealt with at a higher layer, e.g. TCP more efficient to deal with errors at MAC level 802.11 includes frame exchange protocol station receiving frame returns acknowledgment (ACK) frame (ACK) exchange treated as atomic unit iif no ACK within short period of time, retransmit f ACK retransmit Four Frame Exchange Four can use four-frame exchange for better reliability can source issues a Request to Send (RTS) frame to dest destination responds with Clear to Send (CTS) after receiving CTS, source transmits data after destination responds with ACK RTS alerts all stations within range of source that RTS exchange is under way exchange CTS alerts all stations within range of destination CTS other stations don’t transmit to avoid collision RTS/CTS exchange is required function of MAC RTS/CTS but may be disabled but Media Access Control Media Distributed Coordination Function Function DCF sublayer uses CSMA DCF uses if station has frame to send it listens to medium if medium idle, station may transmit else waits until current transmission complete else no collision detection since on wireless network network DCF includes delays that act as a priority DCF scheme scheme IEEE 802.11 Medium Access Control Logic Logic Priority IFS Values Priority SIFS (short IFS) SIFS for all immediate response actions (see later) used by the centralized controller in PCF scheme when issuing polls scheme used as minimum delay for asynchronous used frames contending for access frames PIFS (point coordination function IFS) DIFS (distributed coordination function IFS) SIFS Use SIFS SIFS gives highest priority highest over stations waiting PIFS or DIFS time Acknowledgment (ACK) • station responds with ACK after waiting SIFS gap station responds • for efficient collision detect & multi-frame transmission SIFS used in following circumstances: Clear to Send (CTS) • station ensures data frame gets through by issuing RTS station ensures • and waits for CTS response from destination Poll response • see Point coordination Function (PCF) discussion next PIFS and DIFS Use PIFS PIFS used by centralized controller for issuing polls for polls has precedence over normal contention traffic but not SIFS DIFS used for all ordinary asynchronous DIFS traffic traffic IEEE 802.11 MAC Timing IEEE Basic Access Method Point Coordination Function (PCF) (PCF) alternative access method implemented on top of DCF polling by centralized polling master (point coordinator) uses PIFS when issuing polls uses PIFS point coordinator polls in round-robin to stations configured for polling configured when poll issued, polled station may respond using SIFS iif point coordinator receives response, it issues another f poll using PIFS poll iif no response during expected turnaround time, f coordinator issues poll coordinator coordinator could lock out async traffic by issuing polls have a superframe interval defined PCF Superframe Timing PCF IEEE 802.11 MAC Frame Format Format Control Frames Control Power Save-Poll (PS-Poll) Power request AP transmit buffered frame when in power-saving mode first frame in four-way frame exchange second frame in four-way exchange second frame Request to Send (RTS) Clear to Send (CTS) Acknowledgment (ACK) Contention-Free (CF)-end announces end of contention-free period part of PCF announces PCF acknowledges CF-end to end contention-free period and release acknowledges stations from associated restrictions associated CF-End + CF-Ack: CF-End Data Frames – Data Carrying Data eight data frame subtypes, in two groups eight in first four carry upper-level data first Data simplest data frame, contention or contention-free use or carries data and acknowledges previously received data during contention-free period data used by point coordinator to deliver data & req send combines Data + CF-Ack and Data + CF-Poll Data + CF-Ack Data Data + CF-Poll Data + CF-Ack + CF-Poll Data Frames – Not Data Carrying Not other four data frames do not carry user data Null Function carries no data, polls, or acknowledgments carries power mgmt bit in frame control field to AP indicates station is changing to low-power state other three frames (CF-Ack, CF-Poll, CF-Ack + CF-Poll) same as corresponding frame in preceding list but without data preceding Management Frames Management used to manage communications between used stations and Aps stations such as management of associations requests, response, reassociation, response, dissociation, and authentication dissociation, 802.11 Physical Layer 802.11 802.11 Available bandwidth Unlicensed frequency of operation Number of nonoverlapping channels Data rate per channel Compatibility 83.5 MHz 802.11a 300 MHz 5.15 - 5.35 GHz OFDM 5.725 - 5.825 GHz OFDM 4 indoor 4 (indoor/outdoor) 4 outdoor 6, 9, 12, 18, 24, 36, 48, 54 Mbps Wi-Fi5 802.11b 83.5 MHz 802.11g 83.5 MHz 2.4 - 2.4835 GHz DSSS, FHSS 2.4 - 2.4835 GHz DSSS 2.4 - 2.4835 GHz DSSS, OFDM 3 (indoor/outdoor) 3 (indoor/outdoor) 3 (indoor/outdoor) 1, 2, 5.5, 6, 9, 11, 12, 18, 24, 36, 48, 54 Mbps Wi-Fi at 11 Mbps and below 1, 2 Mbps 1, 2, 5.5, 11 Mbps Wi-Fi 802.11 Original 802.11 Physical Layer - DSSS DSSS Direct-sequence spread spectrum (DSSS) 2.4 GHz ISM band at 1 Mbps and 2 Mbps up to seven channels, each 1 Mbps or 2 Mbps, up can be used can depends on bandwidth allocated by various national regulations regulations 13 in most European countries one in Japan each channel bandwidth 5 MHz encoding scheme DBPSK for 1-Mbps and DQPSK for 2-Mbps using an 11-chip Barker seq DQPSK Original 802.11 Physical Layer - FHSS FHSS Frequency-hopping spread spectrum Frequency-hopping 2.4 GHz ISM band at 1 Mbps and 2 Mbps at 23 channels in Japan 70 channels in USA 70 channels signal hopping between multiple channels based on a pseudonoise sequence pseudonoise 1-MHz channels are used hopping scheme adjustable two-level Gaussian FSK modulation for 1 Mbps four-level GFSK modulation used for 2 Mbps Original 802.11 Physical Layer – Infrared Infrared omnidirectional range up to 20 m 1 Mbps uses 16-PPM (pulse position modulation) Mbps 16-PPM 4 data bit group mapped to one of 16-PPM symbols each symbol a string of 16 bits each 16-bit string has fifteen 0s and one binary 1 2-Mbps has each group of 2 data bits is mapped 2-Mbps into one of four 4-bit sequences into each sequence consists of three 0s and one binary 1 intensity modulation is used for transmission 802.11a 802.11a uses 5-GHz band (different to other variants) supports higher data rates, is less cluttered multiple carrier signals at different frequencies some bits on each channel orthogonal frequency division multiplexing (OFDM) up to 48 subcarriers modulated using BPSK, QPSK, 16-QAM, or 64-QAM QPSK, subcarrier frequency spacing 0.3125 MHz convolutional code at rate of 1/2, 2/3, or 3/4 provides forward error correction forward combination of modulation technique and coding rate combination determines data rate determines 802.11a Physical Frame 802.11a 802.11b 802.11b extension of 802.11 DS-SS scheme extension of with data rates of 5.5 and 11 Mbps same as original DS-SS scheme Complementary Code Keying (CCK) Complementary modulation gives higher data rate with same modulation bandwidth & chipping rate bandwidth also Packet Binary Convolutional Coding also (PBCC) for future higher rate use (PBCC) chipping rate 11 MHz chipping rate 11-Mbps CCK Modulation Scheme Scheme 802.11b Physical Frame 802.11b 802.11g 802.11g higher-speed extension to 802.11b operates in 2.4GHz band compatible with 802.11b devices combines physical layer encoding techniques used in 802.11 and 802.11b to provide service at a variety of data rates provide ERP-OFDM for 6, 9, 12, 18, 24, 36, 48, ERP-OFDM 54Mbps rates 54Mbps ERP-PBCC for 22 & 33Mbps rates Data Rate vs Distance (m) Data Data Rate (Mbps) 1 2 5.5(b)/6(a/g) 9 11(b)/12(a/g) 18 24 36 48 54 802.11b 90+ 75 60 Ñ 50 Ñ Ñ Ñ Ñ Ñ 802.11a Ñ Ñ 60+ 50 45 40 30 25 15 10 802.11g 90+ 75 65 55 50 50 45 35 25 20 Access and Privacy Services Access - Authentication authentication used to establish station identity station wired LANs assume physical connection gives authority to use LAN authority LAN not a valid assumption for wireless LANs 802.11 supports several authentication schemes does not mandate any particular scheme does not from relatively insecure handshaking to public-key from encryption encryption 802.11 requires mutually acceptable, successful 802.11 authentication before association authentication Access and Privacy Services Access Deauthentication & Privacy Deauthentication iinvoked whenever an existing authentication is to be nvoked terminated terminated used to prevent messages being read by others 802.11 allows optional use of encryption 802.11 allows Privacy original WEP security features were weak subsequently 802.11i and WPA alternatives subsequently evolved giving better security evolved Summary Summary wireless LAN alternatives IEEE 802.11 architecture and services 802.11 Media Access Control 802.11 Physical Layers 802.11, 802.11a, 802.11b, 802.11g Security considerations ...
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