Page 1. Key Points from Last Lecture Frame format. EEC173B/ECS152C, Winter Wireless LANs

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1 EEC173/ECS152C, Winer 2006 Key Poins from Las Lecure Wireless LANs Frame forma MAC managemen Synchronizaion, Handoffs, Power MAC mehods: DCF & PCF CSMA/CA wih posiive ACK Exponenial backoff Prioriized access via differen IFS values Hidden/exposed erminal problems RTS/CTS clearing Virual sensing using received NAV Acknowledgmen: Seleced slides from Prof. Schiller Chuah Winer Frame forma Frame forma Frame Types Daa: unicas (ACKed); broadcas/mulicas (no ACKed) Conrol: RTS/CTS, ACKs Managemen (beacon, probe reques/response, auhenicaion, associaion, ec) Sequence numbers Imporan agains duplicaed frames due o los ACKs Addresses Receiver, ransmier (physical), SS idenifier, sender (logical) Miscellaneous Sending ime, checksum, frame conrol, daa byes Frame Duraion/ Address Address Address Sequence Address Daa CRC Conrol ID Conrol 4 bis Proocol version Type Subype To DS From DS More Frag Power More Rery Mgm Daa WEP Order 3 4 Page 1

2 MAC Address Forma Special Frames: ACK, RTS, CTS scenario o DS from address 1 address 2 address 3 address 4 DS ad-hoc nework 0 0 DA SA SSID - infrasrucure 0 1 DA SSID SA - nework, from AP infrasrucure 1 0 SSID SA DA - nework, o AP infrasrucure nework, wihin DS 1 1 RA TA DA SA Acknowledgemen Reques To Send ACK RTS byes Frame Duraion Receiver CRC Conrol Address byes Frame Conrol Duraion Receiver Transmier Address Address CRC DS: Disribuion Sysem AP: Access Poin DA: Desinaion Address SA: Source Address SSID: asic Service Se Idenifier RA: Receiver Address TA: Transmier Address Clear To Send CTS byes Frame Duraion Receiver CRC Conrol Address MAC Managemen Sublayer Regisraion/Synchronizaion Try o find a LAN, ry o say wihin a LAN Timer, ec. Handoff: Associaion/Reassociaion Inegraion ino a LAN Roaming, i.e. change neworks by changing access poins Scanning, i.e. acive search for a nework Power managemen Sleep mode wihou missing a message Periodic sleep, frame buffering, raffic measuremens Securiy MI Managemen Informaion ase Managing, read, wrie Regisraion A managemen frame called beacon is ransmied periodically by he AP o esablish he iming synchronizaion funcion (TSF) TSF conains: SS id, imesamp, raffic indicaion map (TIM), power managemen, and roaming informaion RSS measuremens are done on he beacon message Associaion: process by which an MS regisers wih an AP 7 8 Page 2

3 Synchronizaion using a eacon (infrasrucure) Synchronizaion using a eacon (ad hoc) beacon inerval beacon inerval access poin medium busy busy busy busy valueof heimesamp beacon frame saion saion medium busy busy busy busy valueof heimesamp beacon frame random delay 9 10 Handoff Roaming Mobiliy Types: No ransiion MS is saic or moving wihin a SA SS ransiion MS moves from one SS o anoher wihin he same ESS (exended service se) ESS ransiion MS moves from one SS o anoher SS which belong o a differen ESS (no suppored) SSs in an ESS communicae via Disribuion Sysem Reassociaion service is used when an MS moves from one SS o anoher wihin he same ESS No or bad connecion? Then perform: Scanning Scan he environmen, i.e., lisen ino he medium for beacon signals or send probes ino he medium and wai for an answer Reassociaion Reques Saion sends a reques o one or several AP(s) Reassociaion Response Success: AP has answered, saion can now paricipae Failure: coninue scanning AP acceps Reassociaion Reques Signal he new saion o he disribuion sysem The disribuion sysem updaes is daa base (i.e., locaion informaion) Typically, he disribuion sysem now informs he old AP so i can release resources Page 3

4 Managemen Operaions: Scanning Power Managemen (1) Passive scanning Lisen o S beacons Acive scanning MS sends probe reques S responds o probe How o power off during idle periods? Idea: swich he ransceiver off if no needed Saes of a saion: sleep and awake IEEE buffers daa a he AP, and sends he daa when he MS is awakened Timing Synchronizaion Funcion (TSF) Using TSF, all MSs are synchronized hey wake up a he same ime o lisen o beacon Wih every beacon a Traffic Indicaion Map (TIM) is sen ha has a lis of saions having buffered daa An MS learns ha i has buffered daa by checking beacon and TIM Power managemen (2) Power saving wih wake up paerns (infrasrucure) Infrasrucure Traffic Indicaion Map (TIM) Lis of unicas receivers ransmied by AP Delivery Traffic Indicaion Map (DTIM) Lis of broadcas/mulicas receivers ransmied by AP Ad hoc Ad hoc Traffic Indicaion Map (ATIM) Announcemen of receivers by saions buffering frames More complicaed no cenral AP Collision of ATIMs possible (scalabiliy?) access poin medium saion TIM inerval D busy T TIM D DTIM DTIM inerval T T d D busy busy busy broadcas/mulicas p PS poll p awake d d daa ransmission o/from he saion Page 4

5 Power saving wih wake up paerns (ad hoc) saion 1 A D 1 1 saion beacon frame awake ATIM window beacon inerval random delay a acknowledge ATIM a d A ransmi ATIM d acknowledge daa D ransmi daa Wifi IEEE b (1) Daa rae 1, 2, 5.5, 11 Mbi/s, depending on SNR User daa rae max. approx. 6 Mbi/s Transmission range 300m oudoor, 30m indoor Max. daa rae ~10m indoor Frequency Free 2.4 GHz ISM band Securiy Limied, WEP insecure, SSID Cos $20 $100 base saion, dropping Availabiliy Many producs, many vendors Wifi IEEE b (2) Channel Selecion (Non overlapping) Connecion se up ime Connecionless/always on Typ. es effor, no guaranees (unless polling is used, limied suppor in producs) Qualiy of Service Manageabiliy Limied (no auomaed key disribuion, sym. Encrypion) Special Advanages Many insalled sysems, lo of experience, available worldwide, free ISM band, many vendors, inegraed in lapops, simple sysem Disadvanages Heavy inerference on ISM band, no service guaranees, slow relaive speed only Europe (ETSI) channel 1 channel 7 channel MHz [MHz] US (FCC)/Canada (IC) 2400 channel 1 channel 6 channel MHz [MHz] Page 5

6 IEEE a (1) IEEE a (2) Daa rae 6, 9, 12, 18, 24, 36, 48, 54 Mbi/s, depending on SNR User hroughpu (1500 bye packes): 5.3 (6), 18 (24), 24 (36), 32 (54) 6, 12, 24 Mbi/s mandaory Transmission range 100m oudoor, 10m indoor E.g., 54 Mbi/s up o 5 m, 48 up o 12 m, 36 up o 25 m, 24 up o 30m, 18 up o 40 m, 12 up o 60 m Frequency Free , , GHz ISM band Securiy Limied, WEP insecure, SSID Cos $100 Availabiliy Some producs, some vendors Connecion se up ime Connecionless/always on Qualiy of Service Typ. bes effor, no guaranees (same as all producs) Manageabiliy Limied (no auomaed key disribuion, sym. Encrypion) Special Advanages/Disadvanages Advanage: fis ino 802.x sandards, free ISM band, available, simple sysem, uses less crowded 5 GHz band Disadvanage: sronger shading due o higher frequency, no QoS IEEE a PHY frame forma Operaing channels for a / US U NII variable 6 variable bis channel rae reserved lengh pariy ail service payload ail pad PLCP header [MHz] 16.6 MHz channel cener frequency = *channel number [MHz] PLCP preamble signal daa 12 1 variable 6 Mbi/s 6, 9, 12, 18, 24, 36, 48, 54 Mbi/s symbols [MHz] 16.6 MHz Page 6

7 OFDM in IEEE a Orhogonal FDM wih 52 used subcarriers (64 in oal) 48 daa + 4 pilo (plus 12 virual subcarriers) khz spacing pilo khz Concluding Remarks IEEE WLAN is becoming real popular hese days There is sill a big room o improve he curren sysems Imporan o consider how any improved sysem co exiss wih legacy sysems channel cener frequency subcarrier number Characerisics of Wireless LANs Characerisics of Wireless LANs Advanages Very flexible wihin he recepion area Ad hoc neworks wihou previous planning possible (Almos) no wiring difficulies (e.g. hisoric buildings, firewalls) More robus agains disasers like, e.g., earhquakes, fire or users pulling a plug... Disadvanages Typically very low bandwidh compared o wired neworks (1 10 Mbi/s) Many proprieary soluions, especially for higher biraes, sandards ake heir ime (e.g. IEEE ) Producs have o follow many naional resricions if working wireless, i akes a vary long ime o esablish global soluions like, e.g., IMT Page 7