1. Introduction 1.2 Medium Access Control. Prof. JP Hubaux

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1 1. Introduction 1.2 Medium Access Control Prof. JP Hubaux 1 Modulation and demodulation (reminder) analog baseband digital signal data digital analog modulation modulation radio transmitter radio carrier analog demodulation analog baseband signal synchronization decision digital data radio receiver radio carrier 2

2 Motivation Can we apply media access methods from fixed networks? Example of CSMA/CD Carrier Sense Multiple Access with Collision Detection send as soon as the medium is free, listen into the medium if a collision occurs (original method in IEEE 802.3) Problems in wireless networks signal strength decreases proportional to the square of the distance or even more the sender would apply CS and CD, but the collisions happen at the receiver it might be the case that a sender cannot hear the collision, i.e., CD does not work furthermore, CS might not work if, e.g., a terminal is hidden 3 Hidden and exposed terminals Hidden terminals A sends to B, C cannot receive A C wants to send to B, C senses a free medium (CS fails) collision at B, A cannot receive the collision (CD fails) A is hidden for C Exposed terminals B sends to A, C wants to send to another terminal (not A or B) C has to wait, CS signals a medium in use but A is outside the radio range of C, therefore waiting is not necessary C is exposed to B A B C 4

3 Motivation - near and far terminals Terminals A and B send, C receives signal strength decreases proportional to the square of the distance the signal of terminal B therefore drowns out A s signal C cannot receive A A B C If C for example was an arbiter for sending rights, terminal B would drown out terminal A already on the physical layer Also severe problem for CDMA-networks - precise power control needed! 5 Access methods SDMA/FDMA/TDMA/CDMA SDMA (Space Division Multiple Access) segment space into sectors, use directed antennas cell structure FDMA (Frequency Division Multiple Access) assign a certain frequency to a transmission channel between a sender and a receiver permanent (e.g., radio broadcast), slow hopping (e.g., GSM), fast hopping (FHSS, Frequency Hopping Spread Spectrum) TDMA (Time Division Multiple Access) assign the fixed sending frequency to a transmission channel between a sender and a receiver for a certain amount of time CDMA (Code Division Multiple Access) assign an appropriate code to each transmission channel 6

4 Frequency multiplex Separation of the whole spectrum into smaller frequency bands A channel gets a certain band of the spectrum for the whole time Advantages: no dynamic coordination necessary k 1 works also for analog signals c k 2 k 3 k 4 k 5 k 6 Disadvantages: waste of bandwidth if the traffic is distributed unevenly inflexible guard spaces t f 7 Time multiplex A channel gets the whole spectrum for a certain amount of time Advantages: only one carrier in the medium at any time throughput high even for many users k 1 k 2 k 3 k 4 k 5 k 6 Disadvantages: precise synchronization necessary c f t 8

5 Time and frequency multiplex Combination of both methods A channel gets a certain frequency band for a certain amount of time Example: GSM Advantages: better protection against tapping k 1 protection against frequency selective interference c But: precise coordination required k 2 k 3 k 4 k 5 k 6 f t 9 Code multiplex Each channel has a unique code k 1 k 2 k 3 k 4 k 5 k 6 All channels use the same spectrum at the same time Advantages: bandwidth efficient no coordination and synchronization necessary good protection against interference and tapping Disadvantages: lower user data rates more complex signal regeneration Implemented using spread spectrum technology t c f 10

6 Some medium access control mechanisms for wireless SDMA FDMA TDMA CDMA FDD Fixed Used in Aloha CSMA Reservations DAMA GSM Pure Slotted Non-persistent p-persistent CSMA/CA Used in (mandatory) Multiple Access with Collision Avoidance Copes with hidden and exposed terminal RTS/CTS Used in (optional) Polling Used in (optional) CSMA: CSMA: Carrier Carrier Sense Sense Multiple Multiple Access Access CA: CA: Collision Collision Avoidance Avoidance DAMA: DAMA: Demand-Assigned Demand-Assigned Multiple Multiple Access Access MACA-BI: MACA-BI: MACA MACA by by invitation invitation FAMA: FAMA: Floor Floor Acquisition Acquisition Multiple Multiple Access Access CARMA: CARMA: Collision Collision Avoidance Avoidance and and Resolution Resolution Multiple Multiple Access Access FDD: FDD: Frequency Frequency Division Division Duplex Duplex MACAW MACA-BI FAMA CARMA 11 FDMA/FDD example: GSM 960 MHz f downlink MHz khz 915 MHz MHz MHz 1 uplink t 12

7 TDMA/TDD example: DECT 417 µs downlink uplink t DECT: Digital Enhanced Cordless Telecommunications TDD: Time Division Duplex 13 Mechanism random, distributed (no central arbiter), time-multiplex Slotted Aloha additionally uses time-slots, sending must always start at slot boundaries Aloha collision sender A sender B Aloha/slotted aloha sender C Slotted Aloha collision t sender A sender B sender C t 14

8 Performance of Aloha (1/3) First transmission Retransmission (if necessary) t 0 -X t 0 t 0 +X t 0 +X+2t prop t 0 +X+2t prop +B Vulnerable period Time-out Backoff period t prop : maximum one-way propagation time betwwen 2 stations Information about the outcome of the transmission is obtained after the reaction time 2 t prop B: backoff time 15 Performance of Aloha (2/3) S: new packets S: throughput of the system G{ G : total load S : arrival rate of new packets Assumption: Poisson distribution of the aggregate arrival process, with an average number of arrivals of 2G arrivals/2x seconds [ transmissions in 2 Xseconds] ( 2 ) k G e 2G k Pk =, = 0,1,2,... k! Throughput S: total arrival rate G times the prob. of a successful transmission: S = GP = GP X [ no collision] [ 0 transmissions in 2 seconds] 0 ( 2G) 2G = G e 0! 2G = Ge Peakvalue at G = 0.5 : S = e 16

9 Performance of Aloha (3/3) Computation of the average packet transmission time Average number of transmission attempts/packet: G 2G = e attempts per packet S Average number of unsuccessful attempts per packet: 2G ε = G 1= e 1 S The first transmission requires X + t seconds, and each subsequent retransmission requires 2t + X + B [ ] [ ] prop Thus the average packet transmission time is approx: 2G aloha = + prop + ( 1)( + 2 prop + ) ET X t e X t B expressed relatively to X: 2G ETaloha / X= 1 + a+ ( e 1)(1+ 2 a+ B ) X t prop where a = is the normalized one-way propagation delay X prop 17 Performance of Slotted Aloha First transmission Retransmission (if necessary) t 0 =kx (k+1)x t 0 +X+2t prop t 0 +X+2t prop +B Vulnerable period Time-out Backoff period S = Ge -G Peakvalue at G = 1 : S = e Average packet delay: [ ] G ETslotaloha / X= 1 + a+ ( e 1)(1+ 2 a+ B ) X 18

10 Carrier Sense Multiple Access (CSMA) Goal: reduce the wastage of bandwidth due to packet collisions Principle: sensing the channel before transmitting (never transmit when the channel is busy) Many variants: Collision detection (CSMA/CD) or collision avoidance(csma/ca) Persistency (in sensing and transmitting) Station A begins transmission at t=0 A Station A captures the channel at t=t prop A 19 1-Persistent CSMA Stations having a packet to send sense the channel continuously, waiting until the channel becomes idle. As soon as the channel is sensed idle, they transmit their packet. If more than one station is waiting, a collision occurs. Stations involved in a collision perform a the backoff algorithm to schedule a future time for resensing the channel Optional backoff algorithm may be used in addition for fairness Consequence : The channel is highly used (greedy algorithm). 20

11 Non-Persistent CSMA Attempts to reduce the incidence of collisions Stations with a packet to transmit sense the channel If the channel is busy, the station immediately runs the back-off algorithm and reschedules a future sensing time If the channel is idle, then the station transmits Consequence : channel may be free even though some users have packets to transmit. 21 p-persistent CSMA Combines elements of the above two schemes Stations with a packet to transmit sense the channel If it is busy, they persist with sensing until the channel becomes idle If it is idle: With probability p, the station transmits its packet With probability 1-p, the station waits for a random time and senses again 22

12 Throughput expression Protocol Pure ALOHA Slotted ALOHA Unslotted 1-persistent CSMA Slotted 1-persistent CSMA Unslotted nonpersistent CSMA Slotted nonpersistent CSMA G S = G G [ G ag( G ag )] e ( 1+ 2a / 2 ) ag G ( ) ( ) ( ) ( 1+ a 1+ 2a 1 e + 1+ ag e ) ag G G[ a e ] e ( 1+ a 1+ ) S = ag G ( a)( e ) ae ( 1+ a ) S = G Throughput S = Ge S = Ge Ge ( 1+ 2a) age S = 1 e 2G G ag ag + e ag ag + a 23 Throughput plot Normalized propagation delay is a =

13 CSMA/CD (reminder) Repeater Terminator Station CS: Carrier Sense (Is someone already talking?) MA: Multiple Access (I hear what you hear!) CD: Collision Detection (We are both talking!!) Three states for the channel : contention, transmission, idle Operating principle Check whether the channel is idle before transmitting Listen while transmitting, stop transmission when collision If collision, one of the 3 schemes above (1-persistent, nonpersistent or p-persistent) 25 Why CSMA/CD is unfit for WLANs Collision Detection requires simultaneous transmission and reception operations (which a radio transceiver is usually unable to do) detecting a collision is difficult Carrier Sensing may be suitable to reduce interference at sender, but Collision Avoidance is needed at receiver CSMA/CD does not address the hidden terminal problem 26

14 CSMA/CA Is described in the section devoted to IEEE DAMA - Demand Assigned Multiple Access Channel efficiency only 18% for Aloha, 36% for Slotted Aloha Reservation can increase efficiency to 80% a sender reserves a future time-slot sending within this reserved time-slot is possible without collision reservation also causes higher delays typical scheme for satellite links Examples for reservation algorithms: Explicit Reservation (Reservation-ALOHA) Implicit Reservation (PRMA) Reservation-TDMA 28

15 DAMA / Explicit Reservation Explicit Reservation (Reservation Aloha): two modes: ALOHA mode for reservation: competition for small reservation slots, collisions possible reserved mode for data transmission within successful reserved slots (no collisions possible) it is important for all stations to keep the reservation list consistent at any point in time and, therefore, all stations have to synchronize from time to time collision Aloha reserved Aloha reserved Aloha reserved Aloha t 29 DAMA / Packet reservation (PRMA) Implicit reservation based on slotted Aloha a certain number of slots form a frame, frames are repeated stations compete for empty slots according to the slotted aloha principle once a station reserves a slot successfully, this slot is automatically assigned to this station in all following frames as long as the station has data to send competition for a slot starts again as soon as the slot was empty in the last frame reservation ACDABA-F frame 1 ACDABA-F frame 2 AC-ABAFframe 3 A---BAFD frame 4 ACEEBAFD frame time-slot 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 at reservation attempts t 30

16 DAMA / Reservation-TDMA Reservation Time Division Multiple Access every frame consists of N mini-slots and x data-slots every station has its own mini-slot and can reserve up to k data-slots using this mini-slot (i.e. x = N * k). other stations can send data in unused data-slots according to a round-robin sending scheme (best-effort traffic) N mini-slots N * k data-slots e.g. N=6, k=2 reservations for data-slots other stations can use free data-slots based on a round-robin scheme 31 MACA - collision avoidance MACA (Multiple Access with Collision Avoidance) uses short signaling packets for collision avoidance Designed especially for packet radio networks (Phil Karn, 1990) Principle: RTS (request to send): a sender request the right to send from a receiver with a short RTS packet before it sends a data packet CTS (clear to send): the receiver grants the right to send as soon as it is ready to receive Signaling packets contain sender address receiver address packet size Variants of this method can be found in IEEE as DFWMAC (Distributed Foundation Wireless MAC) 32

17 MACA principle MACA avoids the problem of hidden terminals A and C want to send to B A sends RTS first C waits after receiving CTS from B RTS CTS CTS A B C MACA avoids the problem of exposed terminals B wants to send to A, C to another terminal now C does not have to wait for it cannot receive CTS from A RTS RTS CTS A B C 33 MACA example A RTS B D A CTS B D C E C 1 2 E A DATA B D : blocked from Tx C 3 E 34

18 MACA variant: application in IEEE sender receiver ACK idle RxBusy time-out NAK; RTS wait for ACK packet ready to send; RTS wait for the right to send CTS; data time-out; RTS data; ACK time-out Data with errors; NAK idle wait for data RTS; CTS RTS; RxBusy ACK: positive acknowledgement NAK: negative acknowledgement RxBusy: receiver busy 35 Polling mechanisms If one terminal can be heard by all others, this central terminal (e.g., base station) can poll all other terminals according to a certain scheme all schemes known from fixed networks can be used (typical mainframe - terminal scenario) Example: Randomly Addressed Polling base station signals readiness to all mobile terminals terminals ready to send can now transmit a random number without collision with the help of CDMA or FDMA (the random number can be seen as a dynamic address) the base station now chooses one address for polling from the list of all random numbers (collision if two terminals choose the same address) the base station acknowledges correct packets and continues polling the next terminal this cycle starts again after polling all terminals of the list 36

19 ISMA (Inhibit Sense Multiple Access) Current state of the medium is signaled via a busy tone the base station signals on the downlink (base station to terminals) if the medium is free or not terminals must not send if the medium is busy terminals can access the medium as soon as the busy tone stops the base station signals collisions and successful transmissions via the busy tone and acknowledgements, respectively (media access is not coordinated within this approach) mechanism used, e.g., for CDPD (Cellular Digital Packet Data) Similar approach was proposed for Packet Radio Networks (Kleinrock + Tobagi, 1975) 37 CDMA (Code Division Multiple Access) Principles all terminals send on the same frequency and can use the whole bandwidth of the transmission channel each sender has a unique code The sender XORs the signal with this code the receiver can tune into this signal if it knows the code of the sender tuning is done via a correlation function Disadvantages: higher complexity of the receiver (receiver cannot just listen into the medium and start receiving if there is a signal) all signals should have approximately the same strength at the receiver Advantages: all terminals can use the same frequency, no planning needed huge code space (e.g., 2 32 ) compared to frequency space interferences (e.g. white noise) is not coded more robust to eavesdropping and jamming (military applications ) forward error correction and encryption can be easily integrated 38

20 DSSS (Direct Sequence Spread Spectrum) (1/2) XOR of the signal with pseudo-random number (chipping sequence) many chips per bit (e.g., 128) result in higher bandwidth of the signal Advantages reduces frequency selective fading in cellular networks base stations can use the same frequency range several base stations can detect and recover the signal soft handover Disadvantages precise power control necessary complexity of the receiver t b 0 1 t c t b : bit period t c : chip period user data XOR chipping sequence = resulting signal 39 DSSS (Direct Sequence Spread Spectrum) (2/2) user data X spread spectrum signal modulator transmit signal chipping sequence radio carrier transmitter correlator received signal demodulator lowpass filtered signal products X integrator sampled sums decision data radio carrier chipping sequence receiver 40

21 CDMA: principle (very simplified) Spreading Despreading A k A k A d X A s X C+D A d A s + B s B k B k B d X B s X C+D B d C+D: C+D: Correlation and and Decision 41 CDMA: example Sender A sends A d = 1, key A k = (assign: 0 = -1, 1 = +1) sending signal A s = A d * A k = (-1, +1, -1, -1, +1, +1) Sender B sends B d = 0, key B k = (assign: 0 = -1, 1 = +1) sending signal B s = B d * B k = (-1, -1, +1, -1, +1, -1) Both signals superimpose in space interference neglected (noise etc.) A s + B s = (-2, 0, 0, -2, +2, 0) Receiver wants to receive signal from sender A apply key A k bitwise (inner product) A e = (-2, 0, 0, -2, +2, 0) A k = = 6 result greater than 0, therefore, original bit was 1 receiving B B e = (-2, 0, 0, -2, +2, 0) B k = = -6, i.e. 0 42

22 Spreading of signal A data A d key sequence A k A d +A k signal A s Real systems use much longer keys resulting in a larger distance between single code words in code space. 43 Spreading of signal B signal A s data B d key sequence B k B d +B k signal B s A s + B s 44

23 Despreading of signal A data A d A s + B s A k (A s + B s ) * A k correlator output decision output Note: the the received signal is is inverted 45 Despreading of signal B data B d A s + B s B k (A s + B s ) * B k correlator output decision output Note: the the received signal is is inverted 46

24 Despreading with a wrong key A s + B s wrong key K (A s + B s ) * K correlator output decision output (1) (1)? 47 SAMA - Spread Aloha Multiple Access Aloha has only a very low efficiency, CDMA needs complex receivers to be able to receive different senders with individual codes at the same time Idea: use spread spectrum with only one single code (chipping sequence) for spreading for all senders accessing according to aloha collision sender A narrow sender B band send for a shorter period with higher power spread the signal e.g. using the chipping sequence ( CDMA without CD ) Problem: find a chipping sequence with good characteristics t 48

25 Comparison SDMA/TDMA/FDMA/CDMA Approach SDMA TDMA FDMA CDMA Idea segment space into spread the spectrum cells/sectors using orthogonal codes Terminals Signal separation Advantages Disadvantages Comment only one terminal can be active in one cell/one sector cell structure, directed antennas very simple, increases capacity per km² inflexible, antennas typically fixed only in combination with TDMA, FDMA or CDMA useful segment sending time into disjoint time-slots, demand driven or fixed patterns all terminals are active for short periods of time on the same frequency synchronization in the time domain established, fully digital, flexible guard space needed (multipath propagation), synchronization difficult standard in fixed networks, together with FDMA/SDMA used in many mobile networks segment the frequency band into disjoint sub-bands every terminal has its own frequency, uninterrupted filtering in the frequency domain simple, established, robust inflexible, frequencies are a scarce resource typically combined with TDMA (frequency hopping patterns) and SDMA (frequency reuse) all terminals can be active at the same place at the same moment, uninterrupted code plus special receivers flexible, less frequency planning needed, soft handover complex receivers, needs more complicated power control for senders still faces some problems, higher complexity, lowered expectations; will be integrated with TDMA/FDMA In practice, several access methods are used in combination Example :SDMA/TDMA/FDMA for GSM and IS References J. Schiller: Mobile Communications, Addison-Wesley, 2000 Leon-Garcia & Widjaja: Communication Networks, McGrawHill,

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