Wireless Ethernet. Wireless LANs: Design Goals. Structure of a WLAN. Infrastructure Network

Transcription

1 L apo p L apo p Lapop L apo p Lapop Lapop Lapop Lapop Wireless Eherne Wireless equivalen o Eherne: Wireless LAN (WLAN) Exclusively daa-oriened, wide-band Inerne access soluion Sandardized by he IEEE as IEEE 802. IEEE 802. (daa rae of 2 MBi/s), sandardised in 997 IEEE 802.a wih 54 MBi/s, use of a 5 GHz frequency band IEEE 802.b wih MBi/s in a 2.4 GHz frequency range IEEE 802.g: enhancemen of 802.b wih up o 54 MBi/s IEEE 802.n: daa raes up o several hundreds of MBi/s (no finished) 802.a 802. or 2 MBi/s 2.4 GHz FHSS, DSSS 54 MBi/s 5 GHz OFDM 802.b MBi/s 2.4 GHz DSSS 802.g 54 MBi/s 2.4 GHz OFDM, DSSS Page Wireless LANs: Design Goals Global, seamless operaion Low power for baery use No special permissions or licenses needed o use he LAN Robus ransmission echnology Simplified sponaneous cooperaion a meeings Easy o use for everyone, simple managemen Proecion of invesmen in wired neworks Securiy (no one should be able o read my daa), privacy (no one should be able o collec user profiles), safey (low radiaion) Transparency concerning applicaions and higher layer proocols, bu also locaion awareness if necessary Page 2 Srucure of a WLAN Infrasrucure Nework. Infrasrucure nework Access Poins (APs) are aached o an exising fixed nework (Eherne, Saellies, ) Each AP manages all communicaion in is recepion range APs using he same frequency range mus have enough disance o avoid disurbances Conrol funcionaliy (medium access, mobiliy managemen, auhenicaion, ) are realized wihin he infrasrucure, wireless devices only need a minimum of funcionaliy 2. Ad-hoc Nework If no AP is available, saions also can build up an own LAN The ransmission now akes place direcly beween he saions Higher complexiy needed wihin he saions (conrol funcionaliy) AP AP Fixed nework AP Page 3 ESS STA 802. LAN BSS Access Poin BSS 2 Poral Disribuion Sysem Access Poin 802.x LAN STA LAN STA 3 Saion (STA) Compuer wih access mechanism o he wireless medium and by his radio connecion o he AP Access Poin (AP) Saion which is inegraed boh in he radio and he wired nework (disribuion sysem) Basic Service Se (BSS) Group of saions incl. he AP wihin an AP ransmission range Poral Gaeway o anoher fixed nework Disribuion sysem Connecion of differen AP areas o one logical nework (EES: Exended service se). Simples principle: swich Page 4

2 Ad-hoc Nework 802. Proocols 802. LAN STA STA 3 IBSS STA 2 IBSS 2 STA 5 STA LAN Direc communicaion wihin limied range Saion (STA) Compuer wih access mechanism o he wireless medium Independen Basic Service Se (IBSS) Group of saions which use he same carrier frequency wihin a ransmission range Differen IBSS are possible by spaial separaion or by using differen carrier frequencies No designaed saions for he forwarding of daa, rouing, Page 5 Applicaions should no be aware of he exisence of he wireless nework (excep capaciy, longer access imes) Medium Access Conrol Access mechanism, fragmening, encrypion MAC managemen: synchronizaion, roaming beween APs, power managemen Physical layer Channel selecion, modulaion, coding Page 6 IEEE 802. Varians IEEE 802. Varians 802.a 802.b 802.c 802.d 802.e 802.f 802.g 802.h 802.i 802.j 802.k 54 MBi/s WLAN in he 5 GHz band MBi/s WLAN in he 2,4 GHz band Wireless Bridging beween Access Poins "World Mode", Adapaion o regional regulaions (e.g. used frequency ranges) QoS und sreaming enhancemen for 802.a/g/h Iner Access Poin Proocol (IAPP), allows communicaion beween Access Poins of differen vendors, e.g. for exchanging roaming informaion 54 MBi/s WLAN in he 2,4 GHz band 54 MBi/s WLAN in he 5 GHz band wih dynamic adapaion of channel and frequency choice as well as auomaic adapaion of ransmission power (enhancemen of IEEE 802.a for Europe) Auhenicaion/encrypion for 802.a/b/g, e.g. WPA Japanese varian of 802.a for he frequency range of 4,9 GHz - 5 GHz Improved measuremen/evaluaion/managemen of radio parameers (e.g. signal srengh), e.g. for enabling locaion based services 802.m 802.n 802.p 802.q 802.r 802.s u 802.v 802.w 802.y Summary of earlier enhancemens, correcion of errors in former specificaions (mainenance) Enhancemen for a fuure, faser WLAN wih daa rae of MBi/s WAVE - Wireless Access for he Vehicular Environmen (such as ambulances and passenger cars) Suppor of Virual WLANs Fas roaming beween APs o avoid gaps in Voice over WLAN audio ESS Mesh Neworking Wireless Performance Predicion (WPP) - es mehods and merics Inerworking wih non-802 neworks (for example, cellular) Wireless nework managemen Proecion of Managemen Frames MHz Operaion in he U.S. Page 7 Page 8

3 802. Physical Layer IEEE 802.b Varians for ransmission: 2 using radio (in he 2.4 GHz band), using infrared FHSS (Frequency Hopping Spread Specrum) 79 differen channels wih MHz bandwidh each Hopping beween 2 channels for MBi/s, beween 4 channels for 2 MBi/s Min. 2.5 hops/sec GFSK modulaion Max. ransmission power: W (USA)/00 mw (EU), min. mw DSSS (Direc Sequence Spread Specrum) DBPSK modulaion for MBi/s (Differenial Binary Phase Shif Keying), DQPSK for 2 MBi/s (Differenial Quadraure PSK) Chipping sequence: (+, -, +, +, -, +, +, +, -, -, -), a Barker-Code Max. ransmission power: W (USA)/00 mw (EU), min. mw Infrared nm, diffuse ligh, ypically 0 m range Daa rae, 2, 5.5, MBi/s, depending on SNR User hroughpu max. approx. 6 MBi/s Transmission range 00m oudoor, 30m indoor (direced links: several km) Max. daa rae ~ 0m (indoor) Frequency range Unlicensed 2.4 GHz ISM band Securiy SSID, WPA2 Connecion seup ime Connecionless, always on QoS Bes effor, no guaranees (some defined in bad way, laer on much beer sandardized in 802.e) Manageabiliy Limied (no auomaic key disribuion, symmerical encrypion) Special advanages/disadvanages Advanages: free ISM band, many vendors, simple sysem Disadvanage: heavy inerferences on he ISM band, no QoS, relaively low daa raes Usage Preferred version in Europe Page 9 Page 0 Channels in IEEE 802.b Channels in IEEE 802.b Two APs using he same frequency would have inerferences in he overlapping area hus: divide he whole frequency range in channels Each channel in IEEE 802.b has a bandwidh of 22 MHz 3 channels in Germany (242, 247, 2422,, 2472 MHz), in USA/Canada Channels overlap! Non-overlapping choice of channels: Available in he ISM band (mos of Europe): ,5 MHz Channel USA/Canada: channel - Channel Channel Channel Carrier frequency Channel Channel 6 Channel Channel Channel Channel MHz Ideal case: only use channels, 6 und : 6 6 [MHz] Page Channel Channel Channel Channel Channel Channel Channel Channel Japan ( 4) MHz Page 2

4 Dynamic Rae Shifing Adjusmen of he daa rae o he ransmission qualiy: Daa Rae Mbi/s 2 Mbi/s 5,5 Mbi/s Mbi/s Code lengh (barker code) 8 (CCK) Modulaion DSSS/PSK DSSS/QPSK Modified DSSS/QPSK Used Symbol Rae MS/s,375 MS/s Bis/Symbol CCK: Complemenary Code Keying Use of an 8-chip spreading sequence where each chip is modulaed wih QPSK QPSK has 4 saes, chipping sequence has lengh resuling saes Selec 64 (for Mbi/s) resp. 4 (for 5,5 Mbi/s) of he saes which have as good cross correlaion characerisics as possible (i.e. are as differen as possible) Tha means: make use of 4 resp. 6 code words which can be ransferred insead of only as wih he barker code (i.e. skip some robusness) Page 3 Channels The whole 2.4GHz ISM band is divided ino resp. 3 overlapping channels. On each channel, DSSS is used for signal spreading: One sub-band has a bandwidh of 22 MHz. The sen daa are spread o hose bandwidh o avoid environmenal disurbances The chips of he barker code resp. CCK are sen in sequence his increases he number of symbols per second compared wih pure sending of he daa, hus a larger bandwidh is needed Purpose: even if he frequency range Channel n is disurbed parly, enough of he signal power reaches he receiver on he res of he channel; if a non-spread ransmission would ake place, he whole daa would be los in case of narrowband inerference 22 MHz If CCK is used, we use several codes insead of he same chipping sequence everyime - he ransmission becomes more suscepible for disurbances han wih use of he barker code, if we have a disorion (maybe caused by an overlapping channel)! Page 4 Range of IEEE 802.b Range of 802.b Due o abused spreading in case of CCK, he higher daa ransmission raes are more suscepible for disurbances. Thus, a smaller range resuls: Daa rae 0 Mbi/s b m Disance Page 5 Page 6

5 IEEE 802.a Daa raes 6, 9, 2, 8, 24, 36, 48, 54 MBi/s, depending on SNR User Throughpu: max. 32 MBi/s 6, 2, 24 MBi/s mandaory Transmission range 00m oudoor, 0m indoor (e.g. 54 Mbi/s up o 5 m, 48 up o 2 m, 36 up o 25 m, 24 up o 30 m, 8 up o 40 m, 2 up o 60 m) Frequency range Free GHz ISM band Securiy SSID, WPA2 Connecion seup ime Connecionless, always on QoS Bes effor, no guaranees (same as for 802.b) Manageabiliy Limied (same as for 802.b) Special advanages/disadvanages Advanages: uses less crowded free ISM band, available worldwide, simple sysem, many vendors Disadvanages: srong shading due o high frequencies, no QoS Usage Preferred version in USA Channels in IEEE 802.a Channels are also overlapping, as in 802.b: [MHz] 6,6 MHz [MHz] 6,6 MHz channel-no. channel-no. cener frequency = channel-no. [MHz] Page 7 Page 8 Modulaion in 802.a: OFDM Medium Access Conrol OFDM wih 52 subcarriers (64 in oal, 6 as guard space on each side) Subcarriers overlap wih 32,5 khz spacing, bu orhogonaliy of chosen frequencies allows for clear separaion 48 daa subchannels + 4 subchannels for phase reference (pilo) Pilos are used by he receiver o deal wih mulipah propagaion: phase references for he whole band are sen here, he receiver can inerpolae phase shifs for he daa carriers 32,5 khz phase reference (pilo) channel cener frequency And: IEEE 802.g simply is inroducing OFDM on he exising 802.b sysem, i.e. replacing of DSSS by OFDM for higher daa raes (while keeping he abiliy o swich o DSSS for inerworking wih 802.b) subcarrier number Page 9 We can assign one channel wih an AP bu hen we have o coordinae all mobile saions in heir communicaion wih he AP. Chosen for IEEE 802.a/b/g/ : Wireless Eherne MAC proocol is oriened a CSMA/CD Hidden Saion Problem Exposed Saion Problem Soluion of he problems, especially Hidden Saion CSMA/CA CSMA wih Collision Avoidance Types of raffic Asynchronous daa service (sandard) Exchange of daa by bes effor Suppor of broadcas and mulicas Time-bound services (opional) Implemenaion of some degree of QoS Only for infrasrucure neworks Page 20

6 802. MAC Layer: DFWMAC 802. MAC Layer Access sraegies DFWMAC-DCF CSMA/CA (sandard) DFWMAC: Disribued Foundaion Wireless MAC DCF: Disribued Coordinaion Funcion collision avoidance by random access wih backoff mechanism Minimum ime beween wo frames s for acknowledging correc receip (no for broadcas) Prioriies for medium access defined hrough differen iming inervals no guaraneed prioriies (Shor Iner Frame Spacing) 0µs highes prioriy, used for, CTS, polling response PIFS (PCF IFS) 30µs medium prioriy, for ime-bounded services using PCF DFWMAC-DCF wih RTS/CTS (opional) Avoidance of Hidden Saions MACA varian (Muliple Access wih Collision Avoidance) (DCF IFS) 50µs lowes prioriy, für asynchronous daa service DFWMAC-PCF (opional) PCF: Poin Coordinaion Funcion Collision-free, cenralized Polling sraegy where he AP has a lis of all conneced saions Page 2 Medium PIFS direc access, if ime he medium is free conenion nex frame Page CSMA/CA Mehod Example - Backoff Medium PIFS conenion window (randomized backoff mechanism) nex frame B = 25 wai B = 5 daa waiing ime ime slo (20 µs) Mandaory for all implemenaions Before sending, a saion performs carrier sense If he medium is free for a leas he duraion of a, he saion may send If he medium is occupied, when becoming free he saion wais for one and hen randomly chooses a backoff ime (collision avoidance, in muliples of a slo ime). The saion coninues o lisen o he medium If he medium is occupied by anoher saion during he backoff ime, he backoff imer sops. In he nex ry, no new backoff ime is chosen randomly, bu he old imer is gone on wih. Also usable for broadcas B2 = 20 daa B2 = 5 wai B and B2 are backoff inervals a nodes and 2 B2 = 0 Page 23 Page 24

7 Compeing Saions CSMA/CA Mehod Saion Saion 2 Saion 3 bo e bo bo e r bo e bo bo e bo e r bo e bo e bo e bo r Unicas ransmission: he receip is addiionally confirmed, since collisions possibly are no deeced by he ransmier Daa can be sen afer waiing for Receivers answer immediaely (afer, wihou addiional backoff ime), if he frame arrived correcly (CRC) In case of an error he frame is repeaed auomaically. No special reamen of a ransmission repeiion, same access mechanism as before Saion 4 Saion 5 Sending reques bo bo e bo e r Medium (Frame,, ec.) bo e bo e bo r bo e bo r elapsed backoff ime remaining backoff ime The size of he compeiion window (Conenion Window, CW) affecs he efficiency. Therefore (similar o Eherne) i sars wih CW = 7 and is doubled wih each collision up o CW max = 255 sender receiver oher saions Daa waiing ime conenion Daa Page 25 Page 26 Compeing Saions (wih ) Saion Saion 2 Saion 3 Saion 4 Saion 5 Sending reques bo bo 2 bo 5 The size of he compeiion window (Conenion Window, CW) affecs he efficiency. Therefore (similar o Eherne) i sars wih CW = 8 and is doubled wih each collision up o CW max = 256 Page 27 Medium occupied (Frame,, ec.) bo ij bo bo 4 bo 5 bo bo 42 bo 52 j h backoff ime of saion i Acknowledgemen 802. DFWMAC wih RTS/CTS Opional exension for he avoidance of he hidden saion problem: RTS wih holding ime as parameer can be sen afer waiing for (plus backoff ime) Confirmaion of he receiver by CTS afer (also conaining holding ime) Immediae sending of he daa is possible, confirmaion by Oher saions sore he holding ime, which were sen in he RTS and CTS, in heir NAV (Nework Allocaion Vecor) Collisions are only possible wih RTS/CTS messages, bu subsanial overhead hrough RTS/CTS messages sender receiver oher saions RTS CTS daa NAV (RTS) NAV (CTS) waiing ime conenion daa Page 28

8 802. DFWMAC wih RTS/CTS Fragmening daa can decrease he damage caused by ransfer errors Special mechanism: adap size of he fragmens o curren error rae of he medium Firs: normal reservaion wih RTS/CTS Fragmens and s (excep he las for each case) conain reservaion duraions sender receiver oher saions RTS CTS frag NAV (RTS) NAV (CTS) frag 2 2 NAV (frag ) NAV ( ) conenion daa Page 29 DFWMAC-PCF PCF for guaranees concerning bandwidh and access delay AP conrols medium access and cyclic queries all saions (Polling) Super-frames wih compeiion-free period and compeiion period (like before) If he medium ges free ( ) afer he begin of he super-frame ( 0 ), he coordinaor cyclic asks all saions x (D x ) for sending needs. If necessary, hey answer wih U x (he daa o be sen) If he phase is ended earlier han planned ( 2 insead of 3 ), more ime remains for he compeiion phase (end is announced by a conrol frame CF end ) 0 PIFS D U D2 super-frame U 2 PIFS D 3 D4 NAV conenion-free period U 4 CFend conenion Page 30 Wha is implemened? Any vendor has o implemen he sandard CSMA/CA varian, he oher wo are opional RTS/CTS very ofen is implemened by AP manufacurers, bu: disabled! Usual mehod: A frame size hreshold is defined, and only frames longer han he hreshold are sen wih RTS/CTS (o avoid overhead for small frames) The hreshold value in basic configuraion is sen o maximum allowed frame lengh Changing he hreshold value allows you o enable he RTS/CTS Only possibiliy o really avoid collisions PCF mechanism usually is no implemened No needed in many cases, and no possible in ad-hoc neworks Would allow for real-ime daa ransmission, bu is no good in i, hus i doesn became prominen insead, a QoS enhancemen for real-ime ransmission was defined (IEEE 802.e) Frame Forma Types Conrol frames, adminisraive frames, daa frames Sequence numbers For deecing duplicaed frames due o los s Addresses Receiver, ransmier (physical), sender (logical), BSS idenifier Misc Duraion of ransmission, daa byes Duraion/ ID Address Address 2 Address 3 Sequence Conrol Frame Conrol Address Daa 4 bis Proocol version To Type Subype DS From DS More Frag Power Rery Mgm More Daa WEP Order CRC Page 3 Page 32

9 Frame Forma MAC Address Forma Frame Conrol Proocol version, frame ype (adminisraion, conrol, daa), fragmening, encrypion informaion, meaning of he following address fields Duraion ID Sen along wih RTC, CTS for seing he NAV Sequence Conrol Recogniion of duplicaed frames by sequence numbers CRC Checksum for deecing ransmission errors Addresses Each field conains a 48-Bi MAC address. MAC frames can be ransferred beween wo saions, beween saion and AP or beween wo APs wihin he disribuion sysem. In he field Frame Conrol, wo bis are deermining he curren meaning of he addresses. Addresses can be: Final desinaion, source address, BSS Idenifier, inermediae sender address, inermediae receiver address Scenario o DS from DS address address 2 address 3 address 4 ad-hoc nework 0 0 DA SA BSSID - infrasrucure 0 DA BSSID SA - nework, from AP infrasrucure 0 BSSID SA DA - nework, o AP infrasrucure nework, wihin DS RA TA DA SA DS: Disribuion Sysem AP: Access Poin DA: Desinaion Address SA: Source Address BSSID: Basic Service Se Idenifier RA: Receiver Address TA: Transmier Address Page 33 Page 34 Special Frames FHSS Frame Forma (PHY) Acknowledgemen, Reques o Send, RTS Clear o Send, CTS byes Frame Duraion Receiver CRC Conrol Address byes Frame Duraion Receiver Transmier CRC Conrol Address Address byes Frame Duraion Receiver CRC Conrol Address Synchronizaion Synchronizaion of receivers by he paern SFD (Sar Frame Delimier) o announce sar of frame PLW (PLCP_PDU Lengh Word) Lengh of payload including he 32 Bi CRC (a he end of he payload). Allowed values are beween 0 and 4095 PSF (PLCP Signaling Field) Daa rae of payload ( or 2 Mbi/s) HEC (Header Error Check) CRC wih x 6 +x 2 +x variable Bis Synchronizaion SFD PLW PSF HEC Payload Page 35 Preamble ransmission wih Mbi/s Header ransmission wih or 2 Mbi/s Page 36

10 DSSS Frame Forma (PHY) Synchronizaion Synchronizaion, gain seing, energy deecion, frequency offse compensaion SFD (Sar Frame Delimier) as sar paern Signal Daa rae of payload (0A: Mbi/s DBPSK; 4: 2 Mbi/s DQPSK) Service Reserved for fuure use, sandard: 00 for 802. frames Lengh (lengh of payload) and HEC (CRC) as for FHSS variable Bis Synchronizaion SFD Signal Service Lengh HEC Payload IEEE 802.b Frame Forma (PHY) Long frame forma: variable Bis synchronizaion SFD signal service lengh HEC payload Preamble Shor frame forma, opional: Header 92 µs a Mbi/s DBPSK, 2, 5.5 or Mbi/s variable Bis shor synch. SFD signal service lengh HEC Payload Preamble ransmission wih Mbi/s Header ransmission wih or 2 Mbi/s Preamble ( Mbi/s, DBPSK) Header (2 Mbi/s, DQPSK) 96 µs 2, 5.5 or Mbi/s Page 37 Page 38 IEEE 802.a Frame Forma (PHY) MAC Managemen variable 6 variable rae reserved lengh pariy ail service payload ail pad Bis Synchronizaion Find a LAN, ry o remain in he LAN Synchronizaion of inernal clocks (e.g. FHSS, PCF, power saving mechanisms) Timer ec. PLCP-Header Power managemen Sleep mode wihou missing a message Periodic sleeping, frame buffering, raffic monioring Preamble, SFD Signal Daa 2 variable Symbols Associaion/Re-associaion Inegraion ino a LAN Roaming, i.e. moving beween neworks from one Access Poin o anoher Scanning, i.e. acive search for a nework 6 Mbi/s 6, 9, 2, 8, 24, 36, 48, 54 Mbi/s MIB - Managemen Informaion Base Managing, read and wrie of managemen aribued and sae variables inside APs, he disribuion sysem, ec Page 39 Page 40

11 Synchronizaion using a Beacon Beacon frame conains ime samps and adminisraive informaion for power saving mechanisms and roaming Varying imes beween beacon frames, since he medium can be occupied In infrasrucure neworks: AP akes over he sending of he beacons Synchronizaion using a Beacon (Ad-hoc) All saions ry o send a Beacon frame in fixed inervals Sandard access procedure wih backoff One saion wins and sends a beacon frame a firs. All oher saions synchronize o his frame. Inerval of he periodic radio signal (beacon): 20ms - s Saion beacon inerval B B AP Medium B B B B value of he imesamp B beacon frame Saion 2 B 2 B 2 Medium value of he imesamp B beacon frame random backoff Page 4 Page 42 Power Managemen Power Managemen wih Wake-up Paerns (Infrasrucure) Idea: Swich off he sending/receiving device when no needed Timing Synchronizaion Funcion Regular acivaion of all saions. Transmissions for sleeping saions are buffered; when waking up, he saions receive he ransmission Infrasrucure: AP can sore all pending frameworks for sleeping saions Wih each beacon frame, a Traffic Indicaion Map (TIM) is sen along which indicaes, for which saions frames are buffered. Addiionally: Lis for broadcas/mulicas receivers (Delivery Traffic Indicaion Map, DTIM) Ad-hoc Similar o he infrasrucure mod, an aa-hoc Traffic Indicaion Map (ATIM) is defined Saions, which have daa o send, announce he receivers of sored packages More complex, no cenral AP: all saions have o emporarily sore frames Collisions of ATIMs possible (scalabiliy?) AP Medium Saion D TIM inerval T TIM D DTIM B B Broadcas/Mulicas DTIM inerval T T d D B p PS Poll p awake d d Daa ransmission from/o he saion Page 43 Page 44

12 Power Managemen wih Wake-up Paerns (Ad-hoc) B B Saion A D B Saion 2 B 2 B 2 beacon frame awake ATIM window random backoff a for ATIM beacon inerval a d A ATIM ransmission D daa ransmission d for daa Roaming Bad or even no connecion? Scanning Scanning of environmen (lisen for beacons of APs or send a probe and wai for a response) Reassociaion Reques Saion requess joining he nework o AP(s) Reassociaion Response If an AP responds, he saion akes par in he nework Oherwise, go on scanning AP acceps Reassociaion Reques Announce new saion o he Disribuion Sysem Disribuion Sysem updaes is daabases (locaion informaion) The old AP is informed by he Disribuion Sysem Page 45 Page 46 Qualiy of Service IEEE 802.e Exended Disribued Channel Access The PCF varian of CSMA/CA should allow some qualiy in daa ransmission: By polling a cerain imes, allow for deerminisic delay of informaion Also, guaranee a cerain daa rae o each paricipan Bu frames in polling can be beween 0 and 2304 byes and he daa rae on physical layer can change due o channel condiions no way o calculae ransmission ime of a frame in advance, hus he above qualiy canno be given Soluion: define addiional CSMA/CA varians which can give prioriy o real-ime daa (defined in IEEE 802.e) Only an add-on he IEEE 802.a/b/g, no a sand-alone WLAN sandard Definiion of Exended Disribued Channel Access (EDCA) as beer version of DCF using several classes of access prioriy by refining he iner-frame gaps and inroducing so-called Transmission Opporuniies (TXOP) Hybrid Coordinaion Funcion Conrolled Channel Access (HCCA) as beer version of PCF also using TXOP Page 47 The scheme from before (all saions use he ime inerval) is refined: Assign differen prioriies o differen daa sreams (raffic classes, TC) As before, prioriy is given by waiing imes: he Arbiraion Iner-Frame space (AIFS) AIFS[TC 0 ] AIFS[TC 6 ] = AIFS[TC 7 ] PIFS conenion window RTS Classify all daa sreams in raffic classes regarding heir QoS 8 prioriy classes, TC 7 has highes prioriy Give longer waiing imes o lower prioriy hus higher prioriy sreams can sar sending earlier Fairness is given even high prioriy senders can draw a large backoff number TC Access Caegory (AC) Purpose Bes Effor Background Background Video Probe Video Video Voice Voice Page 48

13 EDCF Implemenaion Wih EDCF, each saion has o handle up o 8 queues performing he same access procedure as plain DCF wih backoff couner (BC) and conenion window (CW): HCCA As in PCF, HCCA is a combinaion of a conenion-free period and a conenion period In he conenion-free period he AP polls he saions Difference o PCF: saions can place reservaions for he polling phase The AP polls saions by graning a TXOP oriened a reservaion wishes and curren raffic load In he conenion period, EDCF is used Quesion: why giving QoS? Why no overprovisioning, i.e. only increase he daa rae? One more enhancemen: each class also a TXOP is assigned, which is a maximum sending duraion afer geing medium access, for ime of TXOP several frames can be sen (Conenion Free Burs) Page 49 Page 50 Faser! MIMO No an end wih 802.a/g go on wih 802.n up o 600 MBi/s! over m! How o achieve such a daa rae while keeping compaibiliy o 802.a/b/g? Applied o 2.4 as well as 5 GHz ISM band o only have a single varian for he fuure Modify OFDM wih increasing symbol rae and slighly enlarge he bandwidh: increase daa rae from 54 MBi/s o 65 MBi/s Opional: Greenfield mode, i.e. skip suppor for 802.a/b/g (an increasing number of legacy devices reduces he average hroughpu in he whole nework) Opional: increase a channel s bandwidh o 40 MHz (dynamic adapaion o oher WLANs in he environmen necessary!) Use MIMO muliple inpu muliple oupu MIMO means: use several anennas in parallel o send daa o one receiver Apply Space Division Muliplexing (SDM) i.e. spli he daa sream ino muliple pars (called spaial sream) and ransmi each par wih a separae anenna (for up o 4 anennas) Necessary: power conrol only use MIMO if necessary, oherwise los of power is consumed Apply beam-forming o focus he sender s anennas o he receiver s anennas By anenna diversiy, a receiver can find ou he angle of incidence of cerain spaial sreams and hus disinguish beween several sreams Opional: apply diversiy on improving signal srengh, i.e. improve signal by receiving he same sream wih several anennas and combine he oupus (for up o 4 anennas, bu only if he number of receiver anennas is larger han he number of spaial sreams) Page 5 Page 52

14 802.n MAC Layer Many improvemens on PHY layer, only a few on he MAC layer: Inroduce Reduced Iner-Frame Space (RIFS) o shoren he waiing ime afer deecing he medium o be idle Use frame aggregaion, i.e. pack ogeher several frames of one saion and remove redundan header informaion 802.s WLAN Mesh Neworking Oher WLAN varian: mesh neworks Classical WLAN: wired infrasrucure beween APs Someimes called Wireless Paradox Availabiliy of 802.n? Draf version 2 finished his year Lo of producs of several vendors (compliance o a non-finished sandard?) Poenial problems wih a paen? Planned release dae varies beween Sepember 2008 and March 2009 Le APs inerconnec in wireless manner, also using WLAN (lower coss, simple insallaion, resilien, ) Figures from: IEEE 802.s uorial Page 53 Page 54 Mesh Topology Secure or no Secure Mesh Poral As mesh poin, bu addiionally connecs o some oher nework Figures from: IEEE 802.s uorial Changes in he 802. sandard regarding: Addresses MAC scheme (oriened a 802.e) Synchronizaion / power modes Securiy And: rouing (layer 3!) Mesh Poin Special componen, esablishes peer links wih neighbors Mesh AP As mesh poin, bu addiionally implemens AP funcionalliy Page 55 Wihin a WLAN daa are flying free hrough he air. Wihin WLAN everybody in ransmission range can share your Access Poin. Thus: securiy! Regisraion of allowed MAC addresses Bu: MAC addresses can be faked, large effor for large neworks Hiding of SSID Broadcas of SSID in beacons can be swiched of, hus only someone knowing he SSID can join he nework (bu: inuiive names? Defaul names?) WEP: Wired Equivalen Privacy Auhenicaion a he Access Poin, encrypion of daa before ransmission Connecion is only possible if knowing he WEP key Bu: no key managemen, shor keys Thus: WPA/WPA2 (Wi-Fi Proeced Access) oday give much beer securiy... bu many users are overaxed wih configuring an Access Poin even if oday a good user guide o insall securiy funcions is implemened on APs, here is a lo of open neworks... Page 56

15 Wardriving New kind of spors: search for open WLANs. Jus ake: A noebook wih WLAN card and a connecor for a GPS device A sofware for decing Access Poins, e.g. Nework Sumbler Warchalking Wha can be found a walls afer a wardiver has passed... A GPS receiver Time for driving around Page 57 Page vs /Blueooh f [MHz] b 000 bye 3 channles (separaed by insallaion) 500 bye 500 bye 500 bye channels bye bye bye bye bye (separaed by 2402 hopping paern) Blueooh may ac like a rogue member of a 802. nework does no know anyhing abou gaps, IFS ec. IEEE discusses hese problems Proposal: Adapive Frequency Hopping (only co-exisence, no collaboraion) Real effecs? Many differen opinions, ess, formulae, Resuls from complee breakdown o almos no effec Blueooh (FHSS) seems o be more robus han 802.b (DSSS) Maybe Blueooh adapive frequency hopping has beer effec Page 59