Chapter 3 MEDIA ACCESS CONTROL

Transcription

1 Chaper 3 MEDIA ACCESS CONTROL Overview Moivaion SDMA, FDMA, TDMA Aloha Adapive Aloha Backoff proocols Reservaion schemes Polling Disribued Compuing Group Mobile Compuing Summer 2003 Disribued Compuing Group MOBILE COMPUTING R. Waenhofer 3/2 Moivaion Moivaion Hidden erminal 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 a leas proporional o he square of he disance senders apply CS and CD, bu he collisions happen a receivers A B C Disribued Compuing Group MOBILE COMPUTING R. Waenhofer 3/3 Disribued Compuing Group MOBILE COMPUTING R. Waenhofer 3/4

2 Moivaion Exposed erminal problem Moivaion - near and far erminals 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 Disribued Compuing Group MOBILE COMPUTING R. Waenhofer 3/5 Disribued Compuing Group MOBILE COMPUTING R. Waenhofer 3/6 Access mehods SDMA/FDMA/TDMA FDD/FDMA - general scheme, example GSM SDMA (Space Division Muliple Access) segmen space ino secors, use direced anennas Use cells o reuse frequencies f 960 MHz 24 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) MHz 95 MHz MHz 200 khz TDMA (Time Division Muliple Access) assign a fixed sending frequency for a cerain amoun of ime MHz Disribued Compuing Group MOBILE COMPUTING R. Waenhofer 3/7 Disribued Compuing Group MOBILE COMPUTING R. Waenhofer 3/8

3 TDD/TDMA - general scheme, example DECT TDMA Moivaion 47 µs downlink uplink We have a sysem wih n saions (0,,2,,n ) and one shared channel The channel is a perfec broadcas channel, ha is, if any single saion ransmis alone, he ransmission can be received by every oher saion. There is no hidden or exposed erminal problem. If wo or more ransmi a he same ime, he ransmission is garbled. Round robin algorihm: saion k sends afer saion k (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? All deerminisic TDMA proocols have hese (or worse) problems Disribued Compuing Group MOBILE COMPUTING R. Waenhofer 3/9 Disribued Compuing Group MOBILE COMPUTING R. Waenhofer 3/0 TDMA Sloed Aloha Queuing Theory he basic basics in a nushell We assume ha he saions are perfecly synchronous In each ime slo each saion ransmis wih probabiliy p. n P = Pr[Saion succeeds] = p( p) P = Pr[any Saion succeeds] = np! dp n 2 maximize P : = n( p) ( pn) = 0 dp pn = n hen, P = ( ) n e In sloed aloha, a saion can ransmi successfully wih probabiliy a leas /e. How quickly can an applicaion send packes o he radio ransmission uni? This quesion is sudied in queuing heory. Simples M/M/ queuing model (M=Markov): Poisson arrival rae λ, exponenial service ime wih mean /µ λ In our ime slo model, his means ha he probabiliy ha a new packe is received by he buffer is λ; he probabiliy ha sending succeeds is µ, for any ime slo. To keep he queue bounded we need ρ = λ/µ <. In he equilibrium, he expeced number of packes in he sysem is N = ρ/( ρ), he average ime in he sysem is T = N/λ. μ Disribued Compuing Group MOBILE COMPUTING R. Waenhofer 3/ Disribued Compuing Group MOBILE COMPUTING R. Waenhofer 3/2

4 Sloed Aloha vs. Round Robin Adapive sloed aloha Sloed aloha uses no every slo of he channel; he round robin proocol is beer. + Wha happens in round robin when a new saion joins? Wha abou more han one new saion? Sloed aloha is more flexible. Example: If he acual number of saions is wice as high as expeced, here is sill a successful ransmission wih probabiliy 30%. If i is only half, 27% of he slos are used successfully. Idea: Change he access probabiliy wih he number of saions How can we esimae he curren number of saions in he sysem? Assume ha saions can disinguish wheher 0,, or more han saions send in a ime slo. Idea: If you see ha nobody sends, increase p. If you see ha more han one sends, decrease p. Model: Number of saions ha wan o ransmi: n. Esimae of n: ˆn Transmission probabiliy: p = / ˆn Arrival rae (new saions ha wan o ransmi): λ; noe ha λ < /e. Disribued Compuing Group MOBILE COMPUTING R. Waenhofer 3/3 Disribued Compuing Group MOBILE COMPUTING R. Waenhofer 3/4 Adapive sloed aloha 2 Adapive sloed aloha Q&A ˆn n We have o show ha he sysem sabilizes. Skech: Q: Wha if we do no know λ, or λ is changing? A: Use λ = /e, and he algorihm sill works P ( λ ) P 2 P + P 0 ( P + P )λ 0 2 n Q: How do newly arriving saions know ˆn? A: We send ˆn wih each ransmission; new saions do no send before successfully receiving he firs ransmission. Q: Wha if saions are no synchronized? A: Aloha (non-sloed) is wice as bad nˆ nˆ + λ, if success or idle nˆ nˆ + λ +, if collision e 2 Q: Can saions really lisen o all ime slos (save energy by urning off)? Q: Can saions really disinguish beween 0,, and more han sender? A: No. One can use sysems ha only rely on acknowledgemens Disribued Compuing Group MOBILE COMPUTING R. Waenhofer 3/5 Disribued Compuing Group MOBILE COMPUTING R. Waenhofer 3/6

5 Backoff Proocols Backoff proocols rely on acknowledgemens only. Binary exponenial backoff, for example, works as follows: If a packe has collided k imes, we se p = 2 -k Or alernaively: wai from random number of slos in [..2 k ] I has been shown ha binary exponenial backoff is no sable for any 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 <. Demand Assigned Muliple Access (DAMA) Channel efficiency only 36% for Sloed Aloha, and even worse for Aloha or 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 sending wihin his reserved ime-slo is possible wihou collision reservaion also causes higher delays ypical scheme for saellie sysems Examples for reservaion algorihms: Explici Reservaion (Reservaion-ALOHA) Implici Reservaion (PRMA) Reservaion-TDMA Muliple Access wih Collision Avoidance (MACA) Disribued Compuing Group MOBILE COMPUTING R. Waenhofer 3/7 Disribued Compuing Group MOBILE COMPUTING R. Waenhofer 3/8 DAMA: Explici 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 DAMA: Packe Reservaion MA (PRMA) 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 as long as he saion has daa o send compeiion for his slos sars again as soon as he slo was empy in he las frame collisions Aloha Aloha Aloha Aloha reserved reserved reserved reserved reservaion ACDABA-F frame 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 Disribued Compuing Group MOBILE COMPUTING R. Waenhofer 3/9 Disribued Compuing Group MOBILE COMPUTING R. Waenhofer 3/20

6 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 : a sender requess he righ o send from a receiver wih a shor 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.) as DFWMAC Disribued Compuing Group MOBILE COMPUTING R. Waenhofer 3/2 Disribued Compuing Group MOBILE COMPUTING R. Waenhofer 3/22 MACA examples MACA varian: DFWMAC in IEEE802. MACA avoids he problem of hidden erminals A and C wan o send o B A sends firs C wais afer receiving CTS CTS CTS from B 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 o wai for i canno receive CTS from A CTS A B C D ACK sender idle RxBusy ime-ou or NAK fi wai for ACK wai for he righ o send CTS fi daa ime-ou fi ACK: posiive acknowledgemen NAK: negaive acknowledgemen RxBusy: receiver busy receiver idle ime-ou daa or corrup fi daa fi ACK NAK wai for daa fi CTS fi RxBusy Disribued Compuing Group MOBILE COMPUTING R. Waenhofer 3/23 Disribued Compuing Group MOBILE COMPUTING R. Waenhofer 3/24

7 Polling mechanisms 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 Example: Randomly Addressed Polling base saion signals readiness o all mobile erminals erminals ready o send can now ransmi a random number wihou collision wih he help of CDMA or FDMA (he random number can be seen as dynamic address) he base saion now chooses one address for polling from he lis of all random numbers (collision if wo erminals choose he same address) he base saion acknowledges correc packes and coninues polling he nex erminal his 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) Disribued Compuing Group MOBILE COMPUTING R. Waenhofer 3/25 Disribued Compuing Group MOBILE COMPUTING R. Waenhofer 3/26 Comparison SDMA/TDMA/FDMA/CDMA 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] Disribued Compuing Group MOBILE COMPUTING R. Waenhofer 3/27