Media Access Control Chapter 6

Transcription

1 Home Auomaion Media Access Conrol Chaper 6 Cooperaive Lighs, Blinds, Fans Energy Monioring Securiy Inrusion Deecion Fire Alarm Digialsrom EU: 10 billion elecrical devices 95% are no neworked cheap neworking (over power) rue sandby remoe conrol universal ID Ad Hoc and Sensor Neworks Roger Waenhofer 6/1 Ad Hoc and Sensor Neworks Roger Waenhofer 6/2 Raing Overview Area mauriy Firs seps Tex book Pracical imporance No apps Mission criical Moivaion Classificaion MAC layer echniques Case sudy: Theory appeal Boooooooring Exciing Ad Hoc and Sensor Neworks Roger Waenhofer 6/3 Ad Hoc and Sensor Neworks Roger Waenhofer 6/4

2 Moivaion Moivaion Hidden Terminal Problem Can we apply media access mehods from fixed neworks? Example CSMA/CD Carrier Sense Muliple Access wih Collision Deecion send as soon as he medium is free, lisen ino he medium if a collision occurs (original mehod in IEEE 802.3) A sends o B, C canno receive A C wans o send o B, C senses a free medium (CS fails) collision a B, A canno receive he collision (CD fails) A is hidden for C Problems in wireless neworks signal srengh decreases quickly wih disance senders apply CS and CD, bu he collisions happen a receivers Energy efficiency: having he radio urned on coss almos as much energy as ransmiing, so o seriously save energy one needs o urn he radio off! A B C Ad Hoc and Sensor Neworks Roger Waenhofer 6/5 Ad Hoc and Sensor Neworks Roger Waenhofer 6/6 Moivaion Exposed Terminal Problem Moivaion - Near and Far Terminals B sends o A, C wans o send o D C has o wai, CS signals a medium in use since A is ouside he radio range of C waiing is no necessary C is exposed o B Terminals A and B send, C receives he signal of erminal B hides A s signal C canno receive A A B C A B C D This is also a severe problem for CDMA neworks precise power conrol required Ad Hoc and Sensor Neworks Roger Waenhofer 6/7 Ad Hoc and Sensor Neworks Roger Waenhofer 6/8

3 Access Mehods Muliplexing: Space Muliplexing channels k i SDMA (Space Division Muliple Access) segmen space ino secors, use direced anennas Use cells o reuse frequencies FDMA (Frequency Division Muliple Access) assign a cerain frequency o a ransmission channel permanen (radio broadcas), slow hopping (GSM), fas hopping (FHSS, Frequency Hopping Spread Specrum) TDMA (Time Division Muliple Access) assign a fixed sending frequency for a cerain amoun of ime Muliplex channels (k) in four dimensions space (s) ime () frequency (f) code (c) Goal: muliple use of a shared medium Imporan: guard spaces needed! k 1 s 1 c k 2 k 3 k 4 k 5 k 6 f c s 2 c f CDMA (Code Division Muliple Access) Example: radio broadcas s 3 f Combinaions! Ad Hoc and Sensor Neworks Roger Waenhofer 6/9 Ad Hoc and Sensor Neworks Roger Waenhofer 6/10 Example: Cellular nework Signal-o-Noise Simplified hexagonal model Signal propagaion ranges: Frequency reuse only wih a cerain disance beween he base saions Can you reuse frequencies in disance 2 or 3 (or more)? Example: fixed frequency assignmen for reuse wih disance 2 Digial echniques can wihsand a signal o noise raio (S/N) of approximaely 9 db, depending on he echniques Assume he pah loss exponen = 3. Then, R D Graph coloring problem Inerference from neighbor cells (oher color) can be conrolled wih ransmi and receive filers which gives D/R = 3. Reuse disance of 2 migh jus work Remark: Signal-o-noise is also known as, e.g. carrier-oinerference raio C/I. Ad Hoc and Sensor Neworks Roger Waenhofer 6/11 Ad Hoc and Sensor Neworks Roger Waenhofer 6/12

4 Frequency Division Muliplex (FDM) FDD/FDMA - general scheme, example 900Mhz Separaion of he whole specrum ino smaller frequency bands A channel ges a cerain band of he specrum for he whole ime + no dynamic coordinaion necessary 960 MHz f works also for analog signals wase of bandwidh if raffic is disribued unevenly inflexible c k 1 k 2 k 3 k 4 k 5 k 6 f MHz 915 MHz MHz 200 khz Example: broadcas radio MHz 1 Ad Hoc and Sensor Neworks Roger Waenhofer 6/13 Ad Hoc and Sensor Neworks Roger Waenhofer 6/14 Time Division Muliplex (TDM) TDD/TDMA - general scheme, example DECT A channel ges he whole specrum for a cerain amoun of ime + only one carrier in he medium a any ime + hroughpu high even for many users precise synchronizaion necessary k 1 k 2 k 3 k 4 k 5 k μs Example: Eherne c f downlink uplink Ad Hoc and Sensor Neworks Roger Waenhofer 6/15 Ad Hoc and Sensor Neworks Roger Waenhofer 6/16

5 Time and Frequency Division Muliplex Code Division Muliplex (CDM) Combinaion of boh mehods A channel ges a cerain frequency band for some ime + proecion agains frequency selecive inerference + proecion agains apping + adapive precise coordinaion required Example: GSM c k 1 k 2 k 3 k 4 k 5 k 6 f Each channel has a unique code All channels use he same specrum a he same ime + bandwidh efficien + no coordinaion or synchronizaion + hard o ap + almos impossible o jam lower user daa raes more complex signal regeneraion Example: UMTS Spread specrum U. S. Paen , Hedy K. Markey (a.k.a. Lamarr or Kiesler) and George Anheil (1942) k 1 k 2 k 3 k 4 k 5 k 6 c f Ad Hoc and Sensor Neworks Roger Waenhofer 6/17 Ad Hoc and Sensor Neworks Roger Waenhofer 6/18 Code Division Muliple Access (CDMA) CDMA is a novel Physical/MAC concep. Example: Direc Sequence Spread Specrum (DSSS) Each saion is assigned an m-bi code (or chip sequence) Typically m = 64, 128,... (in our examples m = 4, 8, ) To send 1 bi, saion sends chip sequence To send 0 bi, saion sends complemen of chip sequence Insead of spliing a 1 MHz band shared beween 100 channels ino 100 x 10kHz bands, every saion can use he whole band, wih 100 chips. CDMA does no increase he oal bandwidh, bu i may simplify he MAC layer a he expense of complicaing he physical layer. CDMA Basics Each saion s has unique m-bi chipping code S or complemen S Bipolar noaion: binary 0 is represened by 1 (or shor: ) Two chips ST, are orhogonal iff S T 0 1 S T is he inner (scalar) produc: S T m Noe: S S 1, S S 1 Noe: S T 0 S T 0 m i 1 ST i i Ad Hoc and Sensor Neworks Roger Waenhofer 6/19 Ad Hoc and Sensor Neworks Roger Waenhofer 6/20

6 CDMA Example CDMA Example (2) Assume ha all saions are perfecly synchronous Assume ha all codes are pair wise orhogonal Assume ha if wo or more saions ransmi simulaneously, he bipolar signals add up linearly Example S = ( ) T = ( ) U = ( ) Check ha codes are pair wise orhogonal E.g., if S,T,U ransmi simulaneously, a receiver receives R = S+T+U = (+3, 1, 1, 1, 1, 1, +3, 1) To decode a received signal R for sender s, one needs o calculae he normalized inner produc R S. R S = (+3, 1, 1, 1, 1, 1, +3, 1) ( )/8 = ( )/8 = 8/8 = 1 by acciden? R S = (S+T+U) S = S S +T S +U S = = 1 Wih orhogonal codes we can safely decode he original signals Ad Hoc and Sensor Neworks Roger Waenhofer 6/21 Ad Hoc and Sensor Neworks Roger Waenhofer 6/22 CDMA: Consrucion of orhogonal codes wih m chips CDMA: How much noise can we olerae? Noe ha we canno have more han m orhogonal codes wih m chips because each code can be represened by a vecor in he m- dimensional space, and here are no more han m orhogonal vecors in he m-dimensional space. Walsh-Hadamard codes can be consruced recursively (for m = 2 k ): The se of codes of lengh 1 is C0 {( )}. For each code ( c) Ck we have wo codes ( cc) and ( cc) in Ck 1 Code ree: C0 {( )} C1 {( ),( )} C {( ),( ),( ),( )} 2 We now add random noise: R = R + N, where N is an m-digi noise vecor. Assume ha chipping codes are balanced (as many + as ) If N = (α, α,, α) for any (posiive or negaive) α, hen he noise N will no maer when we decode he received signal. R S = (R+N) S = S S +(orhogonal codes) S +N S = = 1 How much random (whie) noise can we olerae? Noe: Random codes are also quie balanced and prey orhogonal. Ad Hoc and Sensor Neworks Roger Waenhofer 6/23 Ad Hoc and Sensor Neworks Roger Waenhofer 6/24

7 CDMA: Problems CDMA: Summary Some of our assumpions are problemaic: A) I is no possible o synchronize chips perfecly. Wha can be done is ha he sender firs ransmis a long enough known chip sequence on which he receiver can lock ono. B) No all saions are received wih he same power level. CDMA is ypically used for sysems wih fixed base saions. Then mobile saions can send wih he reciprocal power hey receive from he base saion. (Alernaively: Firs decode he bes saion, and hen subrac is signal o decode he second bes saion ) + all erminals can use he same frequency, no planning needed + reduces frequency selecive fading and inerference + base saions can use he same frequency range + several base saions can deec and recover he signal + sof handover beween base saions + forward error correcion and encrypion can be easily inegraed precise power conrol necessary higher complexiy of receiver and sender Example: UMTS C) We didn discuss how o ransmi bis wih elecromagneic waves. Ad Hoc and Sensor Neworks Roger Waenhofer 6/25 Ad Hoc and Sensor Neworks Roger Waenhofer 6/26 Cockail pary as analogy for muliplexing Comparison SDMA/TDMA/FDMA/CDMA Space muliplex: Communicae in differen rooms Frequency muliplex: Use soprano, alo, enor, or bass voices o define he communicaion channels Time muliplex: Le oher speaker finish Code muliplex: Use differen languages and hone in on your language. The farher apar he languages he beer you can filer he noise : German/Japanese beer han German/Duch. Can we have orhogonal languages? Approach SDMA TDMA FDMA CDMA Idea segmen space ino spread he specrum cells/secors using orhogonal codes Terminals Signal separaion Advanages Disadvanages Commen only one erminal can be acive in one cell/one secor cell srucure, direced anennas very simple, increases capaciy per km² inflexible, anennas ypically fixed only in combinaion wih TDMA, FDMA or CDMA useful segmen sending ime ino disjoin ime-slos, demand driven or fixed paerns all erminals are acive for shor periods of ime on he same frequency synchronizaion in he ime domain esablished, fully digial, flexible guard space needed (mulipah propagaion), synchronizaion difficul sandard in fixed neworks, ogeher wih FDMA/SDMA used in many mobile neworks segmen he frequency band ino disjoin sub-bands every erminal has is own frequency, uninerruped filering in he frequency domain simple, esablished, robus inflexible, frequencies are a scarce resource ypically combined wih TDMA (frequency hopping paerns) and SDMA (frequency reuse) all erminals can be acive a he same place a he same momen, uninerruped code plus special receivers flexible, less frequency planning needed, sof handover complex receivers, needs more complicaed power conrol for senders sill faces some problems, higher complexiy, lowered expecaions; will be inegraed wih TDMA/FDMA [J.Schiller] Ad Hoc and Sensor Neworks Roger Waenhofer 6/27

8 MAC Alphabe Soup Tradiional MAC Proocol Classificaion μ-mac Aloha AI-LMAC B-MAC BiMAC BMA CMAC Crankshaf CSMA-MPS CSMA/ARC DMAC E2-MAC EMACs f-mac FLAMA Funneling-MAC G-MAC HMAC LMAC LPL MMAC nanomac O-MAC PACT PCM PEDAMACS PicoRadio PMAC PMAC Preamble sampling Q-MAC Q-MAC QMAC RATE EST RL-MAC RMAC RMAC S-MAC S-MAC/AL SMACS SCP-MAC SEESAW Sif SS-TDMA STEM T-MAC TA-MAC TRAMA U-MAC WiseMAC X-MAC Z-MAC [TU Delf] Cenralized/Single-Hop Proocols A base saion coordinaes all raffic Conenion Proocols (CSMA) Transmi when you feel like ransmiing Rery if collision, ry o minimize collisions, addiional reservaion modes Problem: Receiver mus be awake as well Scheduling Proocols (TDMA) Use a pre-compued schedule o ransmi messages Disribued, adapive soluions are difficul Hybrid proocols E.g. conenion wih reservaion scheduling Specific ( cross-layer ) soluions, e.g. Dozer for daa gahering Ad Hoc and Sensor Neworks Roger Waenhofer 6/29 Ad Hoc and Sensor Neworks Roger Waenhofer 6/30 Polling mechanisms Example: Inhibi Sense Muliple Access (ISMA) If one erminal can be heard by all ohers, his cenral erminal (a.k.a. base saion) can poll all oher erminals according o a cerain scheme Use a scheme known from fixed neworks The base saion chooses one address for polling from he lis of all saions The base saion acknowledges correc packes and coninues polling he nex erminal The cycle sars again afer polling all erminals of he lis Curren sae of he medium is signaled via a busy one he base saion signals on he downlink (base saion o erminals) wheher he medium is free erminals mus no send if he medium is busy erminals can access he medium as soon as he busy one sops he base saion signals collisions and successful ransmissions via he busy one and acknowledgemens, respecively (media access is no coordinaed wihin his approach) Example: for CDPD (USA, inegraed ino AMPS) Ad Hoc and Sensor Neworks Roger Waenhofer 6/32

9 TDMA Moivaion TDMA Sloed Aloha Sysem wih n saions (0,1,2,,n 1) and one shared channel The channel is a perfec broadcas channel Single ransmissions are received by every oher saion. No hidden or exposed erminal problem. Inerference if more han one saion ransmis. Round robin algorihm: saion k sends afer saion k 1 (mod n) If a saion does no need o ransmi daa, hen i sends ε There is a maximum message size m ha can be ransmied How efficien is round robin? Wha if a saion breaks or leaves? We assume ha he saions are perfecly synchronous In each ime slo each saion ransmis wih probabiliy p. n 1 P1 Pr[Saion 1 succeeds] p(1 p) P Pr[any Saion succeeds] np1! dp n 2 maximize P : n(1 p) (1 pn) 0 dp pn 1 1 n 1 1 hen, P (1 ) n e In Sloed Aloha, a saion can ransmi successfully wih probabiliy a leas 1/e, or abou 36% of he ime. All deerminisic TDMA proocols have hese (or worse) problems Ad Hoc and Sensor Neworks Roger Waenhofer 6/33 Backoff Proocols Demand Assigned Muliple Access (DAMA) Backoff proocols rely on acknowledgemens only. Binary exponenial backoff If a packe has collided k imes, we se p = 2 -k Or alernaively: wai from random number of slos in [1..2 k ] I has been shown ha binary exponenial backoff is no sable for any arrival rae λ > 0 (if here are infiniely many poenial saions) [Proof skech: wih very small bu posiive probabiliy you go o a bad siuaion wih many waiing saions, and from here you ge even worse wih a poenial funcion argumen sadly he proof is oo inricae o be shown in his course ] Ineresingly when here are only finie saions, binary exponenial backoff becomes unsable wih λ > 0.568; Polynomial backoff however, remains sable for any λ < 1. Ad Hoc and Sensor Neworks Roger Waenhofer 6/35 Channel efficiency is only 36% for Sloed Aloha, and even worse for backoff proocols. Pracical sysems herefore use reservaion whenever possible. Bu: Every scalable sysem needs an Aloha syle componen. Reservaion: a sender reserves a fuure ime-slo ypical scheme for saellie sysems sending wihin his reserved ime-slo is possible wihou collision reservaion also causes higher delays Examples for reservaion algorihms on he following slides Ad Hoc and Sensor Neworks Roger Waenhofer 6/36

10 DAMA: Explici Reservaion DAMA: Implici Reservaion Aloha mode for reservaion: compeiion for small reservaion slos, collisions possible. Reserved mode for daa ransmission wihin successful reserved slos (no collisions possible). I is imporan for all saions o keep he reservaion lis consisen a any poin in ime and, herefore, all saions have o synchronize from ime o ime. collisions Aloha Aloha Aloha Aloha reserved reserved reserved reserved A cerain number of slos form a frame, frames are repeaed. Saions compee for empy slos according o he sloed aloha principle. Once a saion reserves a slo successfully, his slo is auomaically assigned o his saion in all following frames. Compeiion for his slos sars again as soon as he slo was empy in he las frame. reservaion ACDABA-F frame 1 ACDABA-F frame 2 AC-ABAFframe 3 A---BAFD frame 4 ACEEBAFD frame ime-slo A C D A B A F A C A B A A B A F A B A F D A C E E B A F D collision a reservaion aemps Ad Hoc and Sensor Neworks Roger Waenhofer 6/37 Ad Hoc and Sensor Neworks Roger Waenhofer 6/38 DAMA: Reservaion TDMA Muliple Access wih Collision Avoidance (MACA) Every frame consiss of n mini-slos and x daa-slos Every saion has is own mini-slo and can reserve up o k daaslos using his mini-slo (i.e. x = nk). Oher saions can send daa in unused daa-slos according o a round-robin sending scheme (bes-effor raffic) Use shor signaling packes for collision avoidance Reques (or ready) o send RTS: a sender requess he righ o send from a receiver wih a shor RTS packe before i sends a daa packe Clear o send CTS: he receiver grans he righ o send as soon as i is ready o receive N mini-slos Nk daa-slos n=6, k=2 Signaling packes conain sender address receiver address packe size reservaions for daa-slos oher saions can use free daa-slos based on a round-robin scheme Example: Wireless LAN (802.11) as DFWMAC Ad Hoc and Sensor Neworks Roger Waenhofer 6/39 Ad Hoc and Sensor Neworks Roger Waenhofer 6/40

11 MACA examples Energy Efficien MAC Proocols MACA avoids he problem of hidden erminals A and C wan o send o B A sends RTS firs RTS C wais afer receiving CTS from B CTS CTS A B C MACA avoids he problem of exposed erminals B wans o send o A, and C o D now C does no have RTS o wai as C canno receive CTS from A RTS CTS A B C D In sensor neworks energy is ofen more criical han hroughpu. The radio componen should be urned off as much as possible. Energy managemen consideraions have a big impac on MAC proocols. Idle lisening coss abou as much energy as ransmiing In he following we presen a few ideas, solen from some known proocols ha ry o balance hroughpu and energy consumpion. S-MAC, T-MAC, B-MAC, or WiseMAC Many of he hundreds of MAC proocols ha were proposed have similar ideas Ad Hoc and Sensor Neworks Roger Waenhofer 6/41 Ad Hoc and Sensor Neworks Roger Waenhofer 6/42 Sensor MAC (S-MAC) Coarse-grained TDMA-like sleep/awake cycles. frame lisen sleep lisen sleep increased laency ime Sensor MAC (S-MAC) Problem: Nodes may have o follow muliple schedules o avoid nework pariion. Schedule 1+2 Schedule 1 Schedule 2 All nodes choose and announce awake schedules. synchronize o awake schedules of neighboring nodes. Uses RTS/CTS o resolve conenion during lisen inervals. And allows inerfering nodes o go o sleep during daa exchange. A fixed sleep/awake raio is no always opimal. Variable load in he nework. Idea: Adap lisen inerval dependen on he curren nework load. T-MAC Ad Hoc and Sensor Neworks Roger Waenhofer 6/43 Ad Hoc and Sensor Neworks Roger Waenhofer 6/44

12 Low Power Lisening (B-MAC) Low Power Lisening (B-MAC) overhearing problem Nodes wake up for a shor period and check for channel aciviy. Reurn o sleep if no aciviy deeced. If a sender wans o ransmi a message, i sends a long preamble o make sure ha he receiver is lisening for he packe. preamble has he size of a sleep inerval Problem: All nodes in he viciniy of a sender wake-up and wai for he packe. Soluion 1: Send wake-up packes insead of preamble, wake-up packes ell when daa is saring so ha receiver can go back o sleep as soon as i received one wake-up packe. Soluion 2: Jus send daa several imes such ha receiver can une in a any ime and ge ail of daa firs, hen head. channel sniff preamble Very robus No synchronizaion required Insan recovery afer channel disrupion daa lisen Ad Hoc and Sensor Neworks Roger Waenhofer 6/45 Communicaion coss are mosly paid by he sender. The preamble lengh can be much longer han he acual daa lengh. Idea: Learn wake-up schedules from neighboring nodes. Sar sending preamble jus before inended receiver wakes up. WiseMAC encode wake-up paern in ACK message Ad Hoc and Sensor Neworks Roger Waenhofer 6/46 Hybrid Proocols Sandards Proocols may use informaion from upper layers o furher improve heir performance. Informaion abou neighborhood Rouing policies Minimize cosly overhearing of neighboring nodes Inform hem o change heir channel sniff paerns opimizaion for WiseMAC IEEE physical & MAC layer sar, clique (peer-o-peer), and cluser ree opology Full funcion (wih coordinaor) and reduced funcion nodes Unsloed mode (nonbeacon) CSMA/CA: Send when medium is free Sloed mode (beacon) Similar o beacons in Dozer: Coordinaor sends beacon o indicae period when nodes can send schedule collision Use randomizaion o resolve schedule collisions like in Dozer Proocols on higher layers using ZigBee Goals: low cos, low power (no really), plug-in and shor range TSMP (Time Synchronized Mesh Proocol) Goals: reliabiliy and low power Ad Hoc and Sensor Neworks Roger Waenhofer 6/47 Ad Hoc and Sensor Neworks Roger Waenhofer 6/48

13 Case Sudy: Design Goals Characerisics Global, seamless operaion Low power consumpion for baery use No special permissions or licenses required Robus ransmission echnology Simplified sponaneous cooperaion a meeings Easy o use for everyone, simple managemen Ineroperable wih 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 + Very flexible (economical o scale) + Ad-hoc neworks wihou planning possible + (Almos) no wiring difficulies (e.g. hisoric buildings, firewalls) + More robus agains disasers or users pulling a plug Low bandwidh compared o wired neworks (20 vs Mbi/s) Many proprieary soluions, especially for higher bi-raes, sandards ake heir ime Producs have o follow many naional resricions if working wireless, i akes a long ime o esablish global soluions (IMT-2000) Securiy Economy Ad Hoc and Sensor Neworks Roger Waenhofer 6/49 Ad Hoc and Sensor Neworks Roger Waenhofer 6/ Infrasrucure vs. ad hoc mode Proocol archiecure Infrasrucure nework mobile erminal server fixed erminal AP AP: Access Poin AP wired nework AP infrasrucure nework Ad-hoc nework applicaion TCP IP access poin applicaion TCP IP LLC LLC LLC MAC MAC MAC MAC PHY PHY PHY PHY Ad Hoc and Sensor Neworks Roger Waenhofer 6/52

14 The lower layers in deail MAC layer: DFWMAC PMD (Physical Medium Dependen) modulaion, coding PLCP (Physical Layer Convergence Proocol) clear channel assessmen signal (carrier sense) PHY Managemen channel selecion, PHY-MIB Saion Managemen coordinaion of all managemen funcions PHY DLC LLC MAC PLCP PMD MAC Managemen PHY Managemen MAC access mechanisms fragmenaion encrypion MAC Managemen Synchronizaion roaming power managemen MIB (managemen informaion base) Saion Managemen Traffic services Asynchronous Daa Service (mandaory) exchange of daa packes based on bes-effor suppor of broadcas and mulicas Time-Bounded Service (opional) implemened using PCF (Poin Coordinaion Funcion) Access mehods DFWMAC-DCF CSMA/CA (mandaory) collision avoidance via binary exponenial back-off mechanism minimum disance beween consecuive packes ACK packe for acknowledgemens (no used for broadcass) DFWMAC-DCF w/ RTS/CTS (opional) avoids hidden erminal problem DFWMAC-PCF (opional) access poin polls erminals according o a lis Ad Hoc and Sensor Neworks Roger Waenhofer 6/53 Ad Hoc and Sensor Neworks Roger Waenhofer 6/54 MAC layer defined hrough differen iner frame spaces no guaraneed, hard prioriies (Shor Iner Frame Spacing) highes prioriy, for ACK, CTS, polling response PIFS (PCF IFS) medium prioriy, for ime-bounded service using PCF (DCF, Disribued Coordinaion Funcion IFS) lowes prioriy, for asynchronous daa service medium busy PIFS direc access if medium is free conenion nex frame CSMA/CA medium busy direc access if medium is free slo ime conenion window (randomized back-off mechanism) nex frame saion ready o send sars sensing he medium (Carrier Sense based on CCA, Clear Channel Assessmen) if he medium is free for he duraion of an Iner-Frame Space (IFS), he saion can sar sending (IFS depends on service ype) if he medium is busy, he saion has o wai for a free IFS, hen he saion mus addiionally wai a random back-off ime (collision avoidance, muliple of slo-ime) if anoher saion occupies he medium during he back-off ime of he saion, he back-off imer sops (fairness) Ad Hoc and Sensor Neworks Roger Waenhofer 6/55 Ad Hoc and Sensor Neworks Roger Waenhofer 6/56

15 Compeing saions - simple example CSMA/CA 2 saion 1 saion 2 busy bo e bo e bo r busy bo e bo r bo e busy Sending unicas packes saion has o wai for before sending daa receivers acknowledge a once (afer waiing for ) if he packe was received correcly (CRC) auomaic reransmission of daa packes in case of ransmission errors saion 3 saion 4 saion 5 busy bo e busy bo e bo r bo e bo r bo e busy bo e bo r medium no idle (frame, ack ec.) bo e elapsed backoff ime packe arrival a MAC bo r residual backoff ime backoff sender receiver oher saions daa ACK waiing ime conenion daa Ad Hoc and Sensor Neworks Roger Waenhofer 6/57 Ad Hoc and Sensor Neworks Roger Waenhofer 6/58 DFWMAC saion can send RTS wih reservaion parameer afer waiing for (reservaion deermines amoun of ime he daa packe needs he medium) acknowledgemen via CTS afer by receiver (if ready o receive) sender can now send daa a once, acknowledgemen via ACK oher saions sore medium reservaions disribued via RTS and CTS sender receiver oher saions RTS CTS daa NAV (RTS) NAV (CTS) defer access ACK conenion daa Fragmenaion If packe ges oo long ransmission error probabiliy grows A simple back of he envelope calculaion deermines he opimal fragmen size sender receiver oher saions RTS CTS frag 1 NAV (RTS) NAV (CTS) ACK 1 frag 2 ACK2 NAV (frag 1 ) NAV (ACK 1 ) conenion daa Ad Hoc and Sensor Neworks Roger Waenhofer 6/59 Ad Hoc and Sensor Neworks Roger Waenhofer 6/60

16 Fragmenaion: Wha fragmen size is opimal? Fragmenaion: Wha fragmen size is opimal? Toal daa size: D bis Overhead per packe (header): h bis Overhead beween wo packes (acknowledgemen): a bis We wan f fragmens, hen each fragmen has k = D/f + h daa + header bis Channel has bi error probabiliy q = 1-p Probabiliy o ransmi a packe of k bis correcly: P := p k Expeced number of ransmissions unil packe is success: 1/P Expeced oal cos for all D bis: f (k/p+a) Goal: Find a k > h ha minimizes he expeced cos For he sake of a simplified analysis we assume a = O(h) If we furher assume ha a header can be ransmied wih consan probabiliy c, ha is, p h = c. We choose k = 2h; Then clearly D = f h, and herefore expeced cos If already a header canno be ransmied wih high enough probabiliy, hen you migh keep he message very small, for example k = h + 1/q Ad Hoc and Sensor Neworks Roger Waenhofer 6/61 Ad Hoc and Sensor Neworks Roger Waenhofer 6/62 DFWMAC-PCF DFWMAC-PCF 2 An access poin can poll saions poin coordinaor wireless saions saions NAV 0 medium busy 1 PIFS D 1 U 1 SuperFrame NAV D 2 U 2 poin coordinaor wireless saions saions NAV D 3 PIFS D 4 NAV conenion free period U 4 CFend conenion period Ad Hoc and Sensor Neworks Roger Waenhofer 6/63 Ad Hoc and Sensor Neworks Roger Waenhofer 6/64

17 Frame forma MAC address forma Duraion Address Address Address Sequence ID Conrol Frame Conrol 2 4 Address Daa 4 Bye 1: version, ype, subype Bye 2: wo DS-bis, fragm., rery, power man., more daa, WEP, order Type conrol frame, managemen frame, daa frame Sequence conrol imporan agains duplicaed frames due o los ACKs Addresses receiver, ransmier (physical), BSS idenifier, sender (logical) Miscellaneous sending ime, checksum, frame conrol, daa byes CRC scenario o DS from address 1 address 2 address 3 address 4 DS 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 DS: Disribuion Sysem AP: Access Poin DA: Desinaion Address SA: Source Address BSSID: Basic Service Se Idenifier RA: Receiver Address TA: Transmier Address Ad Hoc and Sensor Neworks Roger Waenhofer 6/65 Ad Hoc and Sensor Neworks Roger Waenhofer 6/66 Special Frames: ACK, RTS, CTS MAC managemen Acknowledgemen Reques To Send Clear To Send 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 Synchronizaion ry o find a LAN, ry o 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 Ad Hoc and Sensor Neworks Roger Waenhofer 6/67 Ad Hoc and Sensor Neworks Roger Waenhofer 6/68

18 Synchronizaion Synchronizaion In an infrasrucure nework, he access poin can send a beacon In an ad-hoc nework, he beacon has o be sen by any saion beacon inerval beacon inerval access poin medium B B B B busy busy busy busy value of imesamp B beacon frame saion 1 saion 2 medium B 1 B 1 B 2 B 2 busy busy busy busy value of he imesamp B beacon frame backoff delay Ad Hoc and Sensor Neworks Roger Waenhofer 6/69 Ad Hoc and Sensor Neworks Roger Waenhofer 6/70 Power managemen Power saving wih wake-up paerns (infrasrucure) Idea: if no needed urn off he ransceiver Saes of a saion: sleep and awake Timing Synchronizaion Funcion (TSF) saions wake up a he same ime 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 D TIM inerval busy T TIM D DTIM B B DTIM inerval T T d D busy busy busy broadcas/mulicas p PS poll p awake d d B daa ransmission o/from he saion Ad Hoc and Sensor Neworks Roger Waenhofer 6/71 Ad Hoc and Sensor Neworks Roger Waenhofer 6/72

19 Power saving wih wake-up paerns (ad-hoc) WLAN: IEEE b saion 1 saion 2 B awake ATIM window B 1 A D B 1 beacon frame beacon inerval random delay B 2 B 2 a acknowledge ATIM a d A ransmi ATIM d acknowledge daa D ransmi daa 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 Low Availabiliy Declining Ad Hoc and Sensor Neworks Roger Waenhofer 6/73 Ad Hoc and Sensor Neworks Roger Waenhofer 6/74 WLAN: IEEE b IEEE b PHY frame formas Connecion se-up ime Connecionless/always on Qualiy of Service Typically bes effor, no guaranees unless polling is used, limied suppor in producs Manageabiliy Limied (no auomaed key disribuion, sym. encrypion) + 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 Long PLCP PPDU forma variable bis synchronizaion SFD signal service lengh HEC payload PLCP preamble Shor PLCP PPDU forma (opional) PLCP header 192 μs a 1 Mbi/s DBPSK 1, 2, 5.5 or 11 Mbi/s variable bis shor synch. SFD signal service lengh HEC payload PLCP preamble (1 Mbi/s, DBPSK) PLCP header (2 Mbi/s, DQPSK) 96 μs 2, 5.5 or 11 Mbi/s Ad Hoc and Sensor Neworks Roger Waenhofer 6/75

20 Channel selecion (non-overlapping) WLAN: IEEE a Europe (ETSI) channel 1 channel 7 channel MHz [MHz] US (FCC)/Canada (IC) 2400 channel 1 channel 6 channel MHz [MHz] 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 $50 adaper, $100 base saion, dropping Availabiliy Some producs, some vendors No really deployed in Europe (regulaions!) Ad Hoc and Sensor Neworks Roger Waenhofer 6/77 Ad Hoc and Sensor Neworks Roger Waenhofer 6/78 WLAN: IEEE a Quiz: Which sandard? Connecion se-up ime Connecionless/always on Qualiy of Service Typically bes effor, no guaranees (same as all producs) Manageabiliy Limied (no auomaed key disribuion, sym. Encrypion) + Advanages: fis ino 802.x sandards, free ISM-band, available, simple sysem, uses less crowded 5 GHz band Disadvanages: sronger shading due o higher frequency, no QoS Ad Hoc and Sensor Neworks Roger Waenhofer 6/79

21 Open Problem Alhough he MAC alphabe soup is consanly growing, he radeoffs delay, hroughpu, energy-efficiency, localiy, dynamics, fairness, are sill no undersood. In paricular here is no Swiss Army Knife of MAC proocols, wih good guaranees in delay and hroughpu, even in dynamic siuaions. Ad Hoc and Sensor Neworks Roger Waenhofer 6/81