COM-405 Mobile Networks Module A (Part A2) Introduction Prof. JP Hubaux http://mobnet.epfl.ch Note: some of the slides of this and other modules and derived from Schiller’s book 1
Modulation and demodulation (reminder) analog baseband digital signal data digital analog radio transmitter modulation modulation 101101001 radio carrier analog baseband digital signal data analog synchronization radio receiver demodulation decision 101101001 radio carrier 2
About CSMA/CD Can we borrow media access methods from fixed networks? Example of CSMA/CD q C arrier S ense M ultiple A ccess with C ollision D etection q 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 q a radio can usually not transmit and receive at the same time q signal strength decreases proportionally to the square of the distance or even more q the sender would apply CS and CD, but the collisions happen at the receiver q it might be the case that a sender cannot “hear” the collision, i.e., CD does not work q furthermore, CS might not work if, e.g., a terminal is “hidden” 3
Hidden and exposed terminals Hidden terminals q A sends to B, C cannot receive A q C wants to send to B, C senses a “free” medium (CS fails) q collision at B, A cannot receive the collision (CD fails) q A is “hidden” for C A B C Exposed terminals q B sends to A, C wants to send to another terminal (not A or B) q C has to wait, CS signals a medium in use q but A is outside the radio range of C, therefore waiting is not necessary q C is “exposed” to B 4
Motivation - near and far terminals Terminals A and B send, C receives q signal strength decreases (at least) proportionally to the square of the distance q the signal of terminal B therefore drowns out A’s signal q è 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/TDMA/FDMA/CDMA SDMA (Space Division Multiple Access) q segment space into sectors, use directed antennas q cell structure TDMA (Time Division Multiple Access) q assign the fixed sending frequency to a transmission channel between a sender and a receiver for a certain amount of time FDMA (Frequency Division Multiple Access) q assign a certain frequency to a transmission channel between a sender and a receiver q permanent (e.g., radio broadcast), slow hopping (e.g., GSM), fast hopping (FHSS, Frequency Hopping Spread Spectrum) CDMA (Code Division Multiple Access) q assign an appropriate code to each transmission channel (DSSS, Direct Sequency Spread Spectrum) q frequency hopping over separate channels (FHSS, Frequency Hopping Spread Spectrum) 6
Some medium access control mechanisms for wireless SDMA CDMA TDMA FDMA Fixed FHSS DSSS • Used in GSM • Used in Bluetooth • Used in UMTS Fixed Aloha CSMA Reservations Multiple Polling DAMA Access with • Used in 802.11 • Used in Collision (optional) GSM Pure Slotted Avoidance • Copes with hidden and exposed terminal Non-persistent p-persistent CSMA/CA • RTS/CTS • Used in 802.11 • Used in 802.11 (mandatory) (optional) FHSS: Frequency-Hopping Spread Spectrum DSSS: Direct Sequence Spread Spectrum MACA-BI FAMA MACAW CSMA: Carrier Sense Multiple Access CA: Collision Avoidance DAMA: Demand-Assigned Multiple Access CARMA MACA-BI: MACA by invitation FAMA: Floor Acquisition Multiple Access 7 CARMA: Collision Avoidance and Resolution Multiple Access
Some medium access control mechanisms for wireless SDMA CDMA TDMA FDMA Fixed FHSS DSSS • Used in GSM • Used in Bluetooth Fixed Aloha CSMA Reservations Multiple Polling DAMA Access with • Used in 802.11 • Used in Collision (optional) GSM Pure Slotted Avoidance • Copes with hidden and exposed terminal Non-persistent p-persistent CSMA/CA • RTS/CTS • Used in 802.11 • Used in 802.11 (mandatory) (optional) FHSS: Frequency-Hopping Spread Spectrum DSSS: Direct Sequence Spread Spectrum MACA-BI FAMA MACAW CSMA: Carrier Sense Multiple Access CA: Collision Avoidance DAMA: Demand-Assigned Multiple Access CARMA MACA-BI: MACA by invitation FAMA: Floor Acquisition Multiple Access 8 CARMA: Collision Avoidance and Resolution Multiple Access
Time multiplex A channel gets the whole spectrum for a certain amount of time. Advantages: q only one carrier in the medium at any time k 1 k 2 k 3 k 4 k 5 k 6 Disadvantages: q precise c synchronization f required t 9
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: q looser coordination q works also for analog signals k 1 k 2 k 3 k 4 k 5 k 6 c Disadvantages: f q wastage of bandwidth if the traffic is distributed unevenly q inflexible q guard spaces t 10
Time and frequency multiplex Combination of both methods. A channel gets a certain frequency band for a certain amount of time. Example: GSM Advantages: q more flexibility But: precise coordination k 1 k 2 k 3 k 4 k 5 k 6 required c f t 11
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 c Advantages: q bandwidth efficient q good protection against interference and eavesdropping Disadvantage: f q more complex signal regeneration Implemented using spread spectrum technology t 12
TDMA/TDD – example: DECT 417 µ s 1 2 3 11 12 1 2 3 11 12 t downlink uplink DECT: Digital Enhanced Cordless Telecommunications TDD: Time Division Duplex 13
FDMA/FDD – example: GSM downlink f 960 MHz 124 200 kHz 1 935.2 MHz 20 MHz 915 MHz 124 1 890.2 MHz t uplink FDD: Frequency Division Duplex 14
Some medium access control mechanisms for wireless SDMA CDMA TDMA FDMA Fixed FHSS DSSS • Used in GSM • Used in Bluetooth Fixed Aloha CSMA Reservations Multiple Polling DAMA Access with • Used in 802.11 • Used in Collision (optional) GSM Pure Slotted Avoidance • Copes with hidden and exposed terminal Non-persistent p-persistent CSMA/CA • RTS/CTS • Used in 802.11 • Used in 802.11 (mandatory) (optional) FHSS: Frequency-Hopping Spread Spectrum DSSS: Direct Sequence Spread Spectrum MACA-BI FAMA MACAW CSMA: Carrier Sense Multiple Access CA: Collision Avoidance DAMA: Demand-Assigned Multiple Access CARMA MACA-BI: MACA by invitation FAMA: Floor Acquisition Multiple Access 15 CARMA: Collision Avoidance and Resolution Multiple Access
Aloha/slotted aloha Mechanism q random, distributed (no central arbiter), time-multiplex q Slotted Aloha additionally uses time-slots, sending must always start at slot boundaries Aloha collision sender A sender B sender C t Slotted Aloha collision sender A sender B sender C t 16
Performance of Aloha (1/4) Retransmission First transmission (if necessary) t 0 t 0 +X t 0 +X+2t prop t 0 +X+2t prop +B t 0 -X Vulnerable period Backoff period B Time-out • t prop : maximum one-way propagation time between 2 stations • Information about the outcome of the transmission is obtained after the reaction time 2 t prop • B: backoff time 17
Performance of Aloha (2/4) 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 ( ) k 2 G [ ] − = = 2 G Pr k transmissions in 2 X second s e , k 0,1,2,... k ! Throughput S: total arrival rate G times the prob. of a successful transmission: [ ] [ ] = = S G .Pr no collision G .Pr 0 transmissions in 2 X seconds ( ) 0 2 G − 2 G = G e 0! − 2 G = Ge 18 1 = = ≈ Peakvalue at G 0 .5 : S 0.184 2 e
Performance of Aloha (3/4) Detail of computation of throughput of previous slide: Define: T : Transmission by a given station A : Absence of transmission by any other station Throughput: N ∑ 1.Pr( T , A ) = N . Pr( T , A ) i = 1 = N .Pr( A | T ).Pr( T ) = N .Pr( T ). Poisson (0,2 G ) = G . Poisson (0,2 G ) = G . e − 2 G 19
Performance of Aloha (4/4) Computation of the average packet transmission time Average number of transmission attempts/packet: G = 2 G e attempts per packet S Average number of unsuccessful attempts per packet: G ε − = − 2 G = 1 e 1 S + The first transmission requires X t seconds, prop + + and each subs equent retransmission requires 2 t X B prop Thus the average packet transmission time is approx: [ ] = + + − + + 2 G E T X t ( e 1)( X 2 t B ) aloha prop prop expressed relatively to X: [ ] B = + + − + + 2 G E T / X 1 a ( e 1)(1 2 a ) X aloha t = where a prop i s the normalized one-way propagation delay X 20
Performance of Slotted Aloha Retransmission First transmission (if necessary) t 0 =kX (k+1)X t 0 +X+2t prop t 0 +X+2t prop +B Vulnerable Backoff period period Time-out = - G S Ge 1 = = ≈ Peakvalue at G 1 : S 0.368 e Average packet delay: [ ] B = + + − + + G E T / X 1 a ( e 1)(1 2 a ) X slotaloha 21
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