Wireless LANs: MAC. Wireless LAN: MAC. IEEE protocol stack. Module W.lan MAC

Transcription

1 Wireless LANs: W.lan.3-2 Wireless LAN: MAC Shanghai Jiaoong Universiy Shanghai, China Module W.lan.3 Universiy of New Mexico Albuquerque, NM, USA managemen PANs & Blueooh: module W.lan.4 1 IEEE proocol sack W.lan : MAC W.lan.3-4 App Transpor Nework Link PHY LLC MAC Layer PLCP (Physical Layer Convergence Procedure) PMD (Physical Medium Dependen) IEEE MAC managemen PHY managemen Saion managemen MAC Access mechanisms Fragmenaion Encrypion Link MAC Layer MAC managemen Synchronizaion Roaming MIB Power managemen MAC managemen

2 W.lan.3-5 W.lan.3-6 MAC services : DCF & PCF Wireless ransmissions are inherenly broadcas Conenion/collision only can be reduced, bu ineviable Basic service: daa ransmission Mandaory: asynchronous daa service, uni or muli-cas DCF (Disribued Coordinaion Funcion) exchange of daa packes based on bes-effor Opional: real-ime service, infrasrucure-based only PCF (Poin Coordinaion Funcion) Primary algorihm defined DFWMAC (Disribued Foundaion Wireless Medium Access Conrol) Access mechanism DCF CSMA/CA (mandaory) collision avoidance via randomized back-off mechanism minimum disance beween consecuive packes ACK packe for acknowledgemens (for unicas) DCF w/ RTS/CTS (opional) Disribued Foundaion Wireless MAC avoids hidden erminal problem PCF (opional) Access poin polls erminals according o a lis : Prioriy W.lan.3-7 : parameers W.lan.3-8 Prioriies are implemened by IFS (Iner-Frame Spacing) IFS = ime inerval beween ransmission of wo successive frames. Slo ime, DSSS 20 μs, FHSS 50 μs, funcion of various delay (Shor Iner Frame Spacing, DSSS 10 μs, FHSS 28 μs) highes prioriy, for ACK, CTS, polling response PIFS (PCF IFS, PIFS = + 1 slo ime medium prioriy, for ime-bounded service using PCF (DCF IFS, = + 2 slo ime) lowes prioriy, for asynchronous daa service medium PIFS conenion nex frame Parameer CWmin CWmax T slo PIFS (FHSS) μs 28 μs (DSSS) μs 10 μs (IR) 63 1 K 8 μs 10 μs + T slo + T slo +T slo b μs 10 μs a 15 9 μs 16 μs direc access if medium is free

3 Carrier sensing & collision deecion W.lan.3-9 Wireless LANs: W.lan , cable as a medium The receiver reads back he peak volage on he cable Compare i agains a hreshold Sensing idle or deec collision , radio as a medium Rely on PHY layer s CCA (Clear Channel Assessmen) CCA is based on sensing of air inerface To sense he deeced bis in he air Slow, relaively reliable To check he received signal srengh (RSS) agains a hreshold Possible false alarm due o level of inerference Collision deecion is no always possible managemen PANs & Blueooh: module W.lan DCF: CSMA/CA W.lan DCF: CSMA/CA W.lan.3-12 Sender/receiver proocol, overall sender has o wai for before sending daa receivers afer receiving daa if he packe was received correcly (CRC) acknowledge a once afer waiing for Else, waiing sender s waiing imer off auomaic reransmission of daa packes if no ACK sender receiver oher saions daa waiing ime ACK conenion daa medium conenion window (randomized back-off mechanism) nex frame direc access if medium is free slo ime Ini: saion ready o send sars sensing he medium w/ CCA Ligh loaded: if he medium is free for he duraion of an IFS, sar sending, shor access delay If he medium is, he saion has o wai for, hen ener a conenion phase Back-off: wai T w, collision avoidance T w is randomly chosen from a conenion window CW (>CW min, <CW max ) T w is muliple of slo-ime

4 DCF: CSMA/CA W.lan DCF CSMA/CA example W.lan.3-14 medium direc access if medium is free slo ime conenion window (randomized back-off mechanism) nex frame During back-off duraion, coninue o check CCA If deec a channel, (ransmission is going) he back-off imer sops: freeze resume remainder of he imer afer channel becomes idle for Upon back-off imer reaches zero sar sending Mus execue he conenion phase a leas once beween every successive ransmission boe bo r bo e bo r saion 1 boe saion 2 bo e saion 3 saion 4 saion 5 bo e bo r bo e bo e bo e bo e bo r bo r medium no idle (frame, ack ec.) bo e elapsed backoff ime packe arrival a MAC bo r residual backoff ime W.lan.3-15 W.lan.3-16 Wireless LANs: Hidden erminal problem managemen PANs & Blueooh: module W.lan.4 One can receive from wo ohers, which canno hear each oher Saion A can exchange frames wih B, bu no C Saion C can exchange frames wih B, bu no A Boh A and C wan o communicae wih B Two frames collide wih each oher a B A and C are said o be hidden nodes wih respec o each oher Soluion: RTS/CTS exension: small reservaion packes RTS Reques To Send CTS Clear To Send

5 DCF: RTS/CTS W.lan DCF: RTS/CTS W.lan.3-18 Sender/receiver proocol, overall Sender can send RTS wih NAV afer waiing for NAV (Nework Allocaion Vecor): reservaion deermines amoun of ime needs he medium Receiver acknowledges via CTS afer if ready o receive Sender can now send daa a once, acknowledgemen via ACK Oher saions (hidden nodes) sore NAV disribued via RTS & CTS sender receiver oher saions RTS CTS daa NAV (RTS) NAV (CTS) defer access ACK conenion daa Overhead involved in RTS/CTS Pro: avoid collisions Con: overhead When o use RTS? Define RTS hreshold ϕ If frame size > ϕ Acivae RTS 4-way handshaking RTS/CTS/Daa/ACK If frame size < ϕ Disable RTS 2-way handshaking Daa/ACK DCF: RTS/CTS example W.lan.3-19 Wireless LANs: W.lan.3-20 managemen PANs & Blueooh: module W.lan.4

6 PCF: polling W.lan PCF: polling W.lan.3-22 PCF (Poin Coordinaion Funcion) Applicable only in real-ime service, infrasrucure-based Access is cenrally conrolled by AP, poin coordinaor AP perform poll & response proocol Build a cenralized conenion resoluion scheme AP polls he nodes in BSS Time pariioned ino super frame periods CFP (Conenion Free Period), run PCF CP (Conenion Period), run DCF Boh DCF & PCF operae simulaneously, PCF has a higher prioriy due o PIFS < Once AP gains access, send beacon o sar CFP wih NAV Possible o define super frame for CP is zero, all CFP Once in CFP PC wais for PIFS, send daa D1 o STA1, PC ransmis packes o STA and polls STA ha requesed conenion-free service All spacing uses PIFS or raher han o remain in CFP D: downsream poll or daa from PC U: daa from STA o PC poin coordinaor wireless saions saions NAV 0 medium 1 PIFS D 1 U 1 SuperFrame NAV D 2 U 2 Wireless LANs: W.lan.3-23 : frame forma W.lan.3-24 managemen PANs & Blueooh: module W.lan.4 Types conrol frames, managemen frames, daa frames Sequence numbers imporan agains duplicaed frames due o los ACKs Addresses, four 48-bi MAC addresses receiver, ransmier (physical), BSS idenifier, sender (logical) Miscellaneous sending ime, checksum, frame conrol, daa byes Duraion/ Address Address Address Sequence ID Conrol Frame Conrol Proocol version Type Subype To DS Address Daa 4 bis From DS More Order Frag ReryPower More Mgm Daa WEP CRC

7 : frame forma W.lan.3-25 : address forma W.lan.3-26 Frame conrol 2-bi proocol version 2-bi ype 00: managemen; 01: conrol; 10: daa 4-bi subype 10/0000: daa, 00/0000: associaion reques 00/1000: beacon; 01/1011: RTS; 01/1100: CTS; ec 1-bi more fragmen Having anoher fragmen o follow 1-bi rery As a reransmission 1-bi power managemen 1: STA goes ino power-save mode 0: STA says acive 1-bi more daa 1-bi WEP 1-bi order: 1: frames mus be processed in sric order Frame conrol 1-bi o DS & 1-bi from DS DS: Disribuion Sysem AP: Access Poin DA: Desinaion Address SA: Source Address BSSID: Basic Service Se Idenifier RA: Receiver Address TA: Transmier Address scenario o DS from DS addr1 addr2 addr3 adddr4 ad-hoc nework 0 0 DA SA BSSID - infrasrucure 0 1 DA BSSID SA - nework, from AP infrasrucure 1 0 BSSID SA DA - nework, o AP infrasrucure nework, wihin DS 1 1 RA TA DA SA : address forma Frame conrol IBSS (ad-hoc nework) 00: DA, SA, BSSID BSS (infrasrucure nework) 01: from AP 10: o AP 11: STA1 o STA2 via AP scenario o DS from DS addr1 addr2 addr3 adddr4 ad-hoc nework 0 0 DA SA BSSID - infrasrucure 0 1 DA BSSID SA - nework, from AP infrasrucure 1 0 BSSID SA DA - nework, o AP infrasrucure nework, wihin DS 1 1 RA TA DA SA W.lan.3-27 : special frames ACK RTS CTS byes Frame Duraion Receiver CRC Conrol Address byes Frame Duraion Receiver Transmier CRC Conrol Address Address byes Frame Duraion Receiver CRC Conrol Address W.lan.3-28

8 Wireless LANs: W.lan.3-29 managemen W.lan.3-30 managemen PANs & Blueooh: module W.lan.4 Synchronizaion ry o find a LAN, say wihin a LAN imer ec. Power managemen sleep-mode wihou missing a message periodic sleep, frame buffering, raffic measuremens Associaion/Reassociaion inegraion ino a LAN roaming, i.e. change neworks by changing access poins scanning, i.e. acive search for a nework MIB - Managemen Informaion Base managing, read, wrie Synchronizaion W.lan.3-31 Sync using a beacon (infrasrucure) W.lan.3-32 Hardware & sofware suppor in each node Inernal clock TSF (Timing Synchronizaion Funcion) Use of synchronizaion Power managemen Coordinaion of PCF Frequency hopping Timer of IFS (, PIFS,, ec) Synchronizaion mechanism Beacon = imesamp + oher managemen info BBS (infrasrucure-based), Beacon sen by AP IBBS (ad-hoc-based), Beacon sen by a random STA access poin medium beacon inerval B B B B value of he imesamp B beacon frame AP ransmi he quasi-periodic beacon signal All STAs adjus heir local imer by beacon s imesamp IF medium is, beacon will be delayed wihin is inerval, bu no accumulaive Beacon s imesamp always reflecs he real ransmi ime, no he scheduled ime Beacon inerval =??? around 100ms

9 Sync using a beacon (ad-hoc) saion 1 medium beacon inerval B 1 saion 2 B 2 B 2 value of he imesamp B beacon frame Every node mainains is own synchronizaion imer B 1 random delay A he beacon inerval, sars ransmission of beacon frame Since he sandard random back-off algorihm will be applied, only one STA wins All oher STAs receive he beacon and suppress heir beacons for his inerval W.lan : Power managemen Idea: swich he ransceiver off if no needed Saes of a saion: sleep and awake Saions wake up a he same ime by TSF Announce going o sleep by mark he bi in frame conrol Sender or AP need o buffer frames if receiver is no awake BBS/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 IBBS/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?) W.lan.3-34 Power saving wih wake-up paerns W.lan.3-35 Power saving wih wake-up paerns W.lan.3-36 Infrasrucure example TIM inerval DTIM inerval Ad-hoc ATIM window beacon inerval access poin medium saion D B T TIM D DTIM B T T d D B awake broadcas/mulicas p PS poll Assumpion: beacon inerval = TIM inerval DTIM inernal = muliple of TIM inerval p d d daa ransmission o/from he saion B saion 1 A D B 1 1 saion 2 B 2 B 2 B beacon frame awake random delay a acknowledge ATIM A ransmi ATIM a d d acknowledge daa Announce an ATIM window (< beacon inerval) o say awake Send ATIM o is receiver if buffered any Lisen for any incoming ATIM, if no addressed, sleep again D ransmi daa

10 802.11: Roaming W.lan : Message ypes W.lan.3-38 Roaming moving beween APs wihin a single ESS Poor connecion from AP 1? Scanning for anoher AP Passive: lisen ino he medium for beacon signals Acive: send probes ino he medium and wai for an answer Pick he bes AP from scanning Sends a Reassociaion reques Wai for Reassociaion response success: AP has answered, saion can now paricipae failure: coninue scanning AP 2 acceps Reassociaion Reques Signal he new saion o he DS, which updaes is daa base (i.e., locaion informaion) Joining AP 2, leaving AP 1, via IAPP (Iner-AP Proocol), non sandard Managemen Type (2-bi) 00 Subype (4-bi) Descripion 0000/0001 Associaion reques/response 0010/0011 Rassociaion reques/response 0100/0101 Probe reques/response 1000 Beacon 1001 Announcemen of TIM 1010 Dissociaion 1011/1100 Auhenicaion/deauhenicaion W.lan.3-39 W.lan : Message ypes : Message ypes Conrol Type (2-bi) 01 Daa Type (2-bi) 10 Subype (4-bi) Descripion Subype (4-bi) Descripion 1010 Power-save poll, reques he sender o ransmi he buffered frame while his STA was in power-save mode /0011 Daa Daa + CF-Ack 1011/1100 RTS /CTS 0100/0101 Daa + CF-Poll ACK CF-End (Conenion Free), he las message in PCF No daa CF_ACK CF_Poll 1111 CF-End & CF-ACK 1011/1100 CF_Ack, C+CF-Poll

11 LLC (Link layer conrol, no ) W.lan.3-41 Wireless LANs: W.lan.3-42 Framing, link access: encapsulae daagram ino frame, adding header, railer physical addresses used in frame header o idenify source, des example: MAC address Flow conrol: pacing beween adjacen sending and receiving nodes Error deecion and correcion Reliable delivery beween adjacen nodes seldom used on low bi error link (fiber, some wised pair) On he op of MAC managemen PANs & Blueooh: module W.lan.4