Wireless LANs. Outline. Outline II. Benefits Applications Technologies Issues Configurations Overview of Standard

Transcription

1 Wireless LANs Outline Benefits Applications Technologies Issues Configurations Overview of Standard Dr. Michael S. Boykin Spring 02-2 Local Area Networks 2 Outline II MAC layer protocols PHY layer protocols Implementation concerns Dr. Michael S. Boykin Spring 02-2 Local Area Networks 3 1

2 Introduction Standard vs.proprietary protocols Interoperable hardware Interoperable software Reduced prices New applications anduses Dr. Michael S. Boykin Spring 02-2 Local Area Networks 4 Benefits Wireless LAN Benefits Mobility Simple/fast installations Temporary connectivity Difficult to wire situations Reliability Immunity from cable plant problems Reduced TCO for MAC s Moves, adds and changes Dr. Michael S. Boykin Spring 02-2 Local Area Networks 6 2

3 Applications Wireless LAN Applications Retail Warehousing Healthcare Hotel & Restaurant Conference Hosting SOHO s Certain Enterprise LANs Dr. Michael S. Boykin Spring 02-2 Local Area Networks 8 Technologies 3

4 Wireless LAN Technologies IEEE Mbps and2 Mbps 2.4GHz spectrum 2-PHYs FHSS, DSSS IEEE b Most common Extended data rates thru 11 Mbps 2.4 Ghz IEEE a Up to 54 Mbps 5 GHz spectrum OFDM PHY modulation Dr. Michael S. Boykin Spring 02-2 Local Area Networks 10 Wireless LAN Technologies HiperLAN/1 Up to 24 Mbps QoS support 5 GHz specturm HiperLAN/2 in development Up to 54 Mbps QoS support Ethernet, ATM and IP transport standards Dr. Michael S. Boykin Spring 02-2 Local Area Networks 11 Wireless LAN Technologies HomeRF SWAP SharedWireless Access Protocol Support communications between PCs andconsumer devices 1 to 2 Mbps support FHSS modulated PHY 10 Mbps in developent Dr. Michael S. Boykin Spring 02-2 Local Area Networks 12 4

5 Wireless LAN Technologies Bluetooth Radio-based wireless PAN PAN: personal area network FHSS modulated PHY 2.4Ghz spectrum Dr. Michael S. Boykin Spring 02-2 Local Area Networks 13 Issues Wireless LAN Issues Multipath propagation Office Furniture Tx Office Wall Rx Dr. Michael S. Boykin Spring 02-2 Local Area Networks 15 5

6 Wireless LAN Issues Hidden terminal problem Access Point Barrier Terminal A Terminal B Dr. Michael S. Boykin Spring 02-2 Local Area Networks 16 Wireless LAN Issues Path loss Signal interference Inwardinterference Outwardinterference W Inward Battery life W Outward Dr. Michael S. Boykin Spring 02-2 Local Area Networks 17 Wireless LAN Issues Interoperability Network Security Property A Bldg Property B Bldg Public Road Dr. Michael S. Boykin Spring 02-2 Local Area Networks 18 6

7 Wireless LAN Issues Application Connectivity Problems Addressing Connection reliability Installation Issues Omnidirectional antenna radiation Dr. Michael S. Boykin Spring 02-2 Local Area Networks 19 Configurations Ad-Hoc Networks (Peer to Peer) Single Cell Radio LAN PC Client PC Client PC Client Dr. Michael S. Boykin Spring 02-2 Local Area Networks 21 7

8 Overlapping Cells Cell A Cell B Cell C Access Point Access Point Access Point Wired Network Dr. Michael S. Boykin Spring 02-2 Local Area Networks 22 General LAN Configuration Access Point Wired Network Dr. Michael S. Boykin Spring 02-2 Local Area Networks 23 Wireless Bridges Local Bridge Network B Network A Remote Bridges Network C Dr. Michael S. Boykin Spring 02-2 Local Area Networks 24 8

9 Infrared LAN Configuration Ceiling Infrared Light Dr. Michael S. Boykin Spring 02-2 Local Area Networks Overview Goals of Asynchrounous, time-bounded delivery Continuity of servic via distribution network Transmission speeds of 1 Mbps and 2 Mbps Wide application support Multicast services Network management services Registration and authentication Dr. Michael S. Boykin Spring 02-2 Local Area Networks 27 9

10 Overview Target Environments Inside buildings Select outdoor areas Target concerns Power management Bandwidth efficiency Security Addressing Dr. Michael S. Boykin Spring 02-2 Local Area Networks 28 Benefits of Appliance interoperability Fast product development Stable future migration Price competition Silo avoidance Dr. Michael S. Boykin Spring 02-2 Local Area Networks 29 IEEE Topology 2 standard topologies composed of stations, access points, distribution systems portals. IBSS- Independent Basic Service Set Standalone BSS with no backbone infrastructure ESS- Extended Service Set Multiple BSS s interconnectedby access points anda distribution system Dr. Michael S. Boykin Spring 02-2 Local Area Networks 30 10

11 IEEE Topology Station any device that contains functionality of the protocol The functions named in the standard physically reside in the NIC Access point an addressable station providing an interface to the distribution system for stations located in various BSS s Distribution system network element used to connect BSSs within the ESS via access points Portals logical point of entry and exit for frames The access point for 802-type distribution systems Dr. Michael S. Boykin Spring 02-2 Local Area Networks 31 IBSS Topology Basic Service Set (BSS) Single Cell Propagation Boundary Station A Station B Dr. Michael S. Boykin Spring 02-2 Local Area Networks 32 ESS Topology BSS 1 Access Point Distribution System Access Point BSS 2 Dr. Michael S. Boykin Spring 02-2 Local Area Networks 33 11

12 IEEE Topology BSS configurations within an ESS Partially overlap Contiguous coverage, no disruption Physically disjointed Non-contiguous coverage, possible disruption Physically co-located Used for redundancy and high performance Dr. Michael S. Boykin Spring 02-2 Local Area Networks 34 IEEE Topology Mobility Support No-transition Fixed or local stations only BSS-transition Stations move between BSSs of the same ESS ESS transition Stations move between BSSs of different ESSs Dr. Michael S. Boykin Spring 02-2 Local Area Networks 35 IEEE 802 Family Local and Metropolitan Area Network Standards Committee IEEE Carrier Sense IEEE Logical Link Control (LLC) IEEE Token Bus IEEE Token Ring IEEE Wireless MAC PHY OSI Layer 2 (Data Link) OSI Layer 1 (Physical) Dr. Michael S. Boykin Spring 02-2 Local Area Networks 36 12

13 Services Functions required by the LLC to send MAC SDUs 2 types Station services Distribution services Dr. Michael S. Boykin Spring 02-2 Local Area Networks Station Services Authentication Open systems Sharedkey De-authentication Irrefutable Privacy optional All data frames Some management frames Dr. Michael S. Boykin Spring 02-2 Local Area Networks 38 Optional Privacy Service Plain Text Wireless Medium Encryption Cipher Text Decryption Plain Text Dr. Michael S. Boykin Spring 02-2 Local Area Networks 39 13

14 Network Join Passive scanning Listen for access point beacons with correct SSID Active scanning- Probe request packet with SSID of desired network Probe response Probe broadcast packet to have all reachable network respond Dr. Michael S. Boykin Spring 02-2 Local Area Networks Distribution Services Association Station associates with single access point Access point can associate with multiple stations First step to support station mobility Indicates whether a station is pollable Disassociation When leaving network When going offline Dr. Michael S. Boykin Spring 02-2 Local Area Networks System Services Distribution Transmission of MAC frame across distribution network Integration Media and address translation for non LANs Reassociation Initiatedby mobile station to support BSStransitions Dr. Michael S. Boykin Spring 02-2 Local Area Networks 42 14

15 Station State Transition State 1 (Unauthenticated, Unassociated) Class 1 Frames Permitted Class 1, 2 & 3 Frames Permitted Class 1 & 2 Frames Permitted State 3 (Authenticated, Associated) State 2 (Authenticated, Unassociated) Dr. Michael S. Boykin Spring 02-2 Local Area Networks 43 Station State Transition State 3 (Authenticated, Associated) State 1 (Unauthenticated, Unassociated) Class 1 Frames Permitted Successful Authentication Class 1, 2 & 3 Deauthentication Frames Permitted Notification Class 1 & 2 Frames Permitted State 2 (Authenticated, Unassociated) Dr. Michael S. Boykin Spring 02-2 Local Area Networks 44 Station State Transition State 3 (Authenticated, Associated) State 1 (Unauthenticated, Unassociated) Class 1 Frames Permitted Class 1, 2 & 3 Deauthentication Frames Permitted Notification Class 1 & 2 Frames Permitted Disassociation Notification Successful Association, Reassociation Successful Authentication State 2 (Authenticated, Unassociated) Dr. Michael S. Boykin Spring 02-2 Local Area Networks 45 15

16 Station State Transition State 3 (Authenticated, Associated) State 1 (Unauthenticated, Unassociated) Class 1 Frames Permitted Successful Authentication Deauthentication Class 1, 2 & 3 Notification Deauthentication Frames Permitted Notification Class 1 & 2 Frames Permitted Disassociation Notification Successful Association, Reassociation State 2 (Authenticated, Unassociated) Dr. Michael S. Boykin Spring 02-2 Local Area Networks 46 Frame Types Class1Frames Control Frames RTS/CTS, ACK, Contention-free Management Frames Probe request/response, beacon, Authentication, deauthentication Announcement traffic indication message Data Frame Dr. Michael S. Boykin Spring 02-2 Local Area Networks 47 Frame Types Class 2 Frames Management Frames Association request/response Reassociation request/response Disassociation Class 3 Frames Data & Management Frames Deauthentication Control Frames PowerSavePoll Dr. Michael S. Boykin Spring 02-2 Local Area Networks 48 16

17 Physical Layer Physical Layer Management Convergence Procedure (PLCP) Communicates with MAC layer to handle Tx andrx of wireless frames Media Dependent Sublayer (PMD) Provides actual Tx and Rx functions De/Modulates on wireless medium Dr. Michael S. Boykin Spring 02-2 Local Area Networks 50 Physical Layer Management Operations Carrier Sense Transmit Receive Dr. Michael S. Boykin Spring 02-2 Local Area Networks 51 17

18 Physical Layer Management Carrier Sense function at multiple levels MAC level performs clear channel assessment CCAcausesPLCPtodirectPMDtocheck whether the medium is busy or idle If station is not transmitting a frame, PLCP searches for preamble Dr. Michael S. Boykin Spring 02-2 Local Area Networks 52 Physical Layer Management Transmit Function MAC layer sends data and rate directive to PHY PHY sends preamble to antenna within 20 msec Preamble and header 1 Mbps Payload sent at specified rate Dr. Michael S. Boykin Spring 02-2 Local Area Networks 53 Physical Layer Management Receive Function PLCP senses busy medium PLCP receives error-free header PLCP interprets data rate and length field PLCP transfers header to MAC PLCP receives payload PLCP delivers payload to MAC Dr. Michael S. Boykin Spring 02-2 Local Area Networks 54 18

19 Physical Layer Management PHYs 2.4 GHz Initially 1 or 2 Mbps FHSS, DSSS or IR b DSSS extended to HR-DSSS Employs CCK for 5.5 and11 Mbps operation a Uses OFDM for data rates up to 54 5 GHz Employs multiple frequencies each supporting multiple phase andamplitude modulation Dr. Michael S. Boykin Spring 02-2 Local Area Networks 55 PLCP Frame format FHSS PHY BITS Variable SYNC Start Frame Delimiter PLW PSF Header Error Check Whitened PSDU PLCP Preamble PLCP Header Dr. Michael S. Boykin Spring 02-2 Local Area Networks 56 FHSS Modulation Hop from channel to channel to channel in a pseudo-random sequence. Tx/Rx at each frequency for a specific amount of time dwell time Binary 0 is represented by a fixed negative deviation from the center frequency Binary 1 is represented by a positive deviation of the same magnitude Dr. Michael S. Boykin Spring 02-2 Local Area Networks 57 19

20 PLCP Frame format SYNC Start Frame Delimiter DSSS PHY #BITS Variable Signal Service Length FCS MPDU PLCP Preamble PLCP Header PPDU Dr. Michael S. Boykin Spring 02-2 Local Area Networks 58 DSSS Modulation Upto14channelseach22MHzwide Differential Binary PSK for 1 Mbps Differential Quadrature PSK for 2 Mbps Dr. Michael S. Boykin Spring 02-2 Local Area Networks 59 Infrared (IR) PHY PLCP Frame format Slots SYNC Start Frame Delimiter Data Rate DC Level Adjustment Length FCS PSDU PLCP Preamble PLCP Header Dr. Michael S. Boykin Spring 02-2 Local Area Networks 60 20

21 IR Modulation 16 PPM for 1 Mbps transmission Creates a 16 position encoding for 4 bits 16 entry table in gray-code order Column 1 has the 4 data bit values Column 2 has the 16 position encoding Encoding places a 1 in the position equal to the table row number. E.g. Row 1: Row 2: Row 3: Etc. Dr. Michael S. Boykin Spring 02-2 Local Area Networks 61 IR Modulation 4 PPM for 2 Mbps transmission Creates a 4 position encoding for 2 bits 4 entry table in gray-code order Column 1 has the 2 data bit values Column 2 has the 4 position encoding Encoding places a 1 in the position equal to the table row number. Row 1: Row 2: Row 3: Row 4: Dr. Michael S. Boykin Spring 02-2 Local Area Networks b PHY (HR-DSSS) Same as DSSS for 1 and 2 Mbps Similar frame format Shorter sync field56 bits Expandedsignal fieldfor 5.5 and11 Mbps Expanded service field values Uses CCK to spread the frequencies Uses bit-position dependent phases to signal multiple bits Dr. Michael S. Boykin Spring 02-2 Local Area Networks 63 21

22 802.11a PHY (OFDM) PLCP frame format BITS V 6 V PLCP Preamble RATE RESERVED LEN PARITY TAIL SERVICE PSDU TAIL PAD Dr. Michael S. Boykin Spring 02-2 Local Area Networks a PHY (OFDM) Divide high-speed serial data stream across multiple lower speed sub-signals Transmit the multiple sub-signals simultaneously at different frequencies Dr. Michael S. Boykin Spring 02-2 Local Area Networks 65 MAC Layer 22

23 MAC Layer Transimission Services Asynchronous data service (mandatory) Time-bounded service Access Methods CSMA/CA Station reservation (hidden terminal) Contention-free polling Dr. Michael S. Boykin Spring 02-2 Local Area Networks MAC Layer Access method Implementation DistributedCoordination Function (DCF) Point Coordination Function (PCF) Frame Spacing DIFS DCF Interframe Spacing Lowest priority for asynchronous data PIFS PCF Interframe Spacing Medium priority for time-bounded data SIFS Short Interframe Spacing Highest priority for control, polling and acks Dr. Michael S. Boykin Spring 02-2 Local Area Networks 68 DCF with CSMA/CA Random Backoff NO NO YES NAV=0 sense the channel IDLE? Tx frame Collision? Success! Dr. Michael S. Boykin Spring 02-2 Local Area Networks 69 23

24 MAC Layer DIFS DIFS PIFS medium busy SIFS contention next frame Dr. Michael S. Boykin Spring 02-2 Local Area Networks MAC Layer Contention Interval Issues Backoff timer management Randomized backoff timer Residual backoff timer Congestion window management Load dependent Exponential increase Light load short delay Heavy load reduce probability of additional collisions Dr. Michael S. Boykin Spring 02-2 Local Area Networks 71 DCF with CSMA/CA Station 1 DIFS DIFS BOe BOr DIFS DIFS BOe B Station 2 BOe B Station 3 B Station 4 BOe B BOe BOr Station 5 BOe BOr BOe B BOe BOr Dr. Michael S. Boykin Spring 02-2 Local Area Networks 72 24

25 DCF with CSMA/CA UnicastACKs Sender Receiver Other Stations DIFS B data SIFS ACK wait time contention DIFS C B Dr. Michael S. Boykin Spring 02-2 Local Area Networks 73 DCF RTS/CTS Hidden terminal solution Two stations unable to sense each other s transmission that collides at the receiver Goal have senders schedule their transmission via the NAV Collisions possible only at the initial phase Threshold based due to overhead Dr. Michael S. Boykin Spring 02-2 Local Area Networks 74 DCF RTS/CTS DIFS Sender Receiver RTS SIFS SIFS CTS data SIFS ACK Other Stations NAV (RTS) NAV (CTS) DIFS defer access Dr. Michael S. Boykin Spring 02-2 Local Area Networks 75 25

26 DCF RTS/CTS Fragment Solution May be useful for error-prone environs Transparent to LLC anduser Sendnew NAV with each fragment Fragment and ACK transmission cause NAV update Dr. Michael S. Boykin Spring 02-2 Local Area Networks 76 PCF Polling SuperFrame concept Contention-free interval reservedvia NAV Contention interval Point Coordinator Frame Types Data Frame CF Poll Frame Data+CF Poll Frame CF EndFrame Dr. Michael S. Boykin Spring 02-2 Local Area Networks 77 Frame Format BYTES [0-2313] 4 FRAME Duration ADDRESS ADDRESS ADDRESS SEQ ADDRESS CONTROL ID CONTROL 4 DATA CRC Dr. Michael S. Boykin Spring 02-2 Local Area Networks 79 26

27 Frame Control Field BITS PROTOCOL VERSION TYPE SUBTYPE TO FROM MORE PWR RETRY DS DS FRAG MGMT MORE DATA WEP ORDER Bit 0 Bit 15 Dr. Michael S. Boykin Spring 02-2 Local Area Networks 80 Transmission Types Ad hoc network Infrastructure network, from AP Infrastructure network, to AP Infrastructure network, within DS Dr. Michael S. Boykin Spring 02-2 Local Area Networks 81 Address Interpretation Tx Type To DS From Ds Addr 1 Addr 2 Addr 3 Addr 4 Ad hoc 0 0 DA SA BSSID - From AP 0 1 DA BSSID SA - To AP 1 0 BSSID SA DA - W/I DS 1 1 RA TA DA SA Dr. Michael S. Boykin Spring 02-2 Local Area Networks 82 27

28 Station Synchronization Distributed clock management Infrastructure-based networks Access point transmits (quasi) periodic beacon w/ timestamp Ad hoc networks Each station schedules beacon transmission Winning station causes others to reschedule Dr. Michael S. Boykin Spring 02-2 Local Area Networks 83 Power Management Goal Allow stations to enter sleep mode to conserve power Have senders buffer data for receivers in sleep mode All stations wake up for beacon Delivery outstanding frames Allowstationstogobackto sleep Dr. Michael S. Boykin Spring 02-2 Local Area Networks 86 Power Management Infrastructure networks TIM traffic indication map Sent with beacon message to identify receivers with outstanding unicast frames DTIM delivery traffic indication map Special TIM interval for sending broadcast & multicast traffic Multiple of the the TIM interval Dr. Michael S. Boykin Spring 02-2 Local Area Networks 87 28

29 Power Management Ad hoc networks ATIMs- adhoc TIMs Sent by all stations during beacon interval Lists all stations with buffered frames Dr. Michael S. Boykin Spring 02-2 Local Area Networks 88 Power Management Issues TIM, DTIM sizing Too small little power savings Too large many buffered frames ATIM scalability Large number of sleeping stations imply large ATIM window.see above. Large number of stations with large ATIMs imply higher collision rate Dr. Michael S. Boykin Spring 02-2 Local Area Networks 91 Mobility Support Typical networks require multiple access points to cover all areas Access point coverage 10-20m radius Moving between access points Roaming Dr. Michael S. Boykin Spring 02-2 Local Area Networks 92 29

30 Mobility Support Roaming steps Scan for access points Select best access point for association Transmit association request The access point transmits an association response Access points updates Its BSS station database the DS to allow it to update its station database May inform old access point of update information Dr. Michael S. Boykin Spring 02-2 Local Area Networks 93 30