1 Mobile Networks Module A (Part A2) Introduction Prof. JP Hubaux http://mobnet.epfl.ch.

  • Published on
    29-Dec-2015

  • View
    213

  • Download
    0

Transcript

  • *Mobile Networks

    Module A (Part A2)Introduction

    Prof. JP Hubaux

    http://mobnet.epfl.ch

  • *Modulation and demodulation (reminder)synchronizationdecisiondigitaldataanalogdemodulationradiocarrieranalogbasebandsignal101101001radio receiverdigitalmodulationdigitaldataanalogmodulationradiocarrieranalogbasebandsignal101101001radio transmitter

  • *About CSMA/CDCan we apply media access methods from fixed networks?

    Example of CSMA/CDCarrier Sense Multiple Access with Collision Detectionsend as soon as the medium is free, listen into the medium if a collision occurs (original method in IEEE 802.3)Problems in wireless networksa radio can usually not transmit and receive at the same timesignal strength decreases proportionally to the square of the distance or even morethe sender would apply CS and CD, but the collisions happen at the receiverit might be the case that a sender cannot hear the collision, i.e., CD does not workfurthermore, CS might not work if, e.g., a terminal is hidden

  • *Hidden terminalsA 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 terminalsB sends to A, C wants to send to another terminal (not A or B)C has to wait, CS signals a medium in usebut A is outside the radio range of C, therefore waiting is not necessaryC is exposed to BHidden and exposed terminalsBAC

  • *Terminals A and B send, C receivessignal strength decreases proportional to the square of the distancethe signal of terminal B therefore drowns out As signalC cannot receive A

    If C for example was an arbiter for sending rights, terminal B would drown out terminal A already on the physical layerAlso severe problem for CDMA-networks - precise power control needed!Motivation - near and far terminalsABC

  • *Access methods SDMA/TDMA/FDMA/CDMASDMA (Space Division Multiple Access)segment space into sectors, use directed antennas cell structureTDMA (Time Division Multiple Access)assign the fixed sending frequency to a transmission channel between a sender and a receiver for a certain amount of timeFDMA (Frequency Division Multiple Access)assign a certain frequency to a transmission channel between a sender and a receiverpermanent (e.g., radio broadcast), slow hopping (e.g., GSM), fast hopping (FHSS, Frequency Hopping Spread Spectrum)CDMA (Code Division Multiple Access)assign an appropriate code to each transmission channel (DSSS, Direct Sequency Spread Spectrum)frequency hopping over separate channels (FHSS, Frequency Hopping Spread Spectrum)

  • *Some medium access control mechanisms for wirelessTDMACDMAFDMASDMAFixedAlohaReservationsDAMAMultiple Access withCollisionAvoidancePollingPureCSMA Used in GSMSlottedNon-persistentp-persistentCSMA/CA Copes with hidden and exposed terminal RTS/CTS Used in 802.11 (optional)MACAWMACA-BIFAMACARMA Used in 802.11 (mandatory) Used in 802.11 (optional)FHSS: Frequency-Hopping Spread SpectrumDSSS: Direct Sequence Spread SpectrumCSMA: Carrier Sense Multiple Access CA: Collision AvoidanceDAMA: Demand-Assigned Multiple AccessMACA-BI: MACA by invitationFAMA: Floor Acquisition Multiple AccessCARMA: Collision Avoidance and Resolution Multiple AccessFHSSDSSS Used in GSMFixed Used in Bluetooth Used in UMTS

  • *Some medium access control mechanisms for wirelessTDMACDMAFDMASDMAFixedAlohaReservationsDAMAMultiple Access withCollisionAvoidancePollingPureCSMA Used in GSMSlottedNon-persistentp-persistentCSMA/CA Copes with hidden and exposed terminal RTS/CTS Used in 802.11 (optional)MACAWMACA-BIFAMACARMA Used in 802.11 (mandatory) Used in 802.11 (optional)FHSS: Frequency-Hopping Spread SpectrumDSSS: Direct Sequence Spread SpectrumCSMA: Carrier Sense Multiple Access CA: Collision AvoidanceDAMA: Demand-Assigned Multiple AccessMACA-BI: MACA by invitationFAMA: Floor Acquisition Multiple AccessCARMA: Collision Avoidance and Resolution Multiple AccessFHSSDSSS Used in GSMFixed Used in Bluetooth

  • *ftck2k3k4k5k6k1Time multiplexA channel gets the whole spectrum for a certain amount of time.

    Advantages:only one carrier in the medium at any timethroughput high for many users

    Disadvantages:precise synchronization necessary

  • *Frequency multiplexSeparation 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 necessaryworks also for analog signals

    Disadvantages:waste of bandwidth if the traffic is distributed unevenlyinflexibleguard spaces

    k2k3k4k5k6k1ftc

  • *fTime and frequency multiplexCombination of both methods.A channel gets a certain frequency band for a certain amount of time.Example: GSM Advantages:better protection against tappingprotection against frequency selective interferenceBut: precise coordination requiredtck2k3k4k5k6k1

  • *Code multiplexEach channel has a unique code

    All channels use the same spectrum at the same timeAdvantages:bandwidth efficientno coordination and synchronization necessarygood protection against interference and tappingDisadvantages:lower user data ratesmore complex signal regenerationImplemented using spread spectrum technologyk2k3k4k5k6k1ftc

  • *TDMA/TDD example: DECT12311121231112tdownlinkuplink417 sDECT: Digital Enhanced Cordless TelecommunicationsTDD: Time Division Duplex

  • *FDMA/FDD example: GSMft1241124120 MHz200 kHz890.2 MHz935.2 MHz915 MHz960 MHzdownlinkuplinkFDD: Frequency Division Duplex

  • *Some medium access control mechanisms for wirelessTDMACDMAFDMASDMAFixedAlohaReservationsDAMAMultiple Access withCollisionAvoidancePollingPureCSMA Used in GSMSlottedNon-persistentp-persistentCSMA/CA Copes with hidden and exposed terminal RTS/CTS Used in 802.11 (optional)MACAWMACA-BIFAMACARMA Used in 802.11 (mandatory) Used in 802.11 (optional)FHSS: Frequency-Hopping Spread SpectrumDSSS: Direct Sequence Spread SpectrumCSMA: Carrier Sense Multiple Access CA: Collision AvoidanceDAMA: Demand-Assigned Multiple AccessMACA-BI: MACA by invitationFAMA: Floor Acquisition Multiple AccessCARMA: Collision Avoidance and Resolution Multiple AccessFHSSDSSS Used in GSMFixed Used in Bluetooth

  • *Mechanismrandom, distributed (no central arbiter), time-multiplexSlotted Aloha additionally uses time-slots, sending must always start at slot boundaries Aloha

    Slotted AlohaAloha/slotted alohasender Asender Bsender Ccollisionsender Asender Bsender Ccollisiontt

  • *Performance of Aloha (1/3) First transmissionRetransmission(if necessary)t0t0+Xt0-XVulnerable periodt0+X+2tpropTime-outt0+X+2tprop+BBackoff period tprop : maximum one-way propagation time betwwen 2 stations Information about the outcome of the transmission is obtained after the reaction time 2 tprop B: backoff time

  • *Performance of Aloha (2/3)S: new packetsS: throughput of the system{G

  • *Performance of Aloha (3/3)Computation of the average packet transmission time

  • *Performance of Slotted Aloha First transmissionRetransmission(if necessary)t0=kX(k+1)XVulnerableperiodt0+X+2tpropTime-outt0+X+2tprop+BBackoff period

  • *Some medium access control mechanisms for wirelessTDMACDMAFDMASDMAFixedAlohaReservationsDAMAMultiple Access withCollisionAvoidancePollingPureCSMA Used in GSMSlottedNon-persistentp-persistentCSMA/CA Copes with hidden and exposed terminal RTS/CTS Used in 802.11 (optional)MACAWMACA-BIFAMACARMA Used in 802.11 (mandatory) Used in 802.11 (optional)FHSS: Frequency-Hopping Spread SpectrumDSSS: Direct Sequence Spread SpectrumCSMA: Carrier Sense Multiple Access CA: Collision AvoidanceDAMA: Demand-Assigned Multiple AccessMACA-BI: MACA by invitationFAMA: Floor Acquisition Multiple AccessCARMA: Collision Avoidance and Resolution Multiple AccessFHSSDSSS Used in GSMFixed Used in Bluetooth

  • *Carrier Sense Multiple Access (CSMA)Goal: reduce the wastage of bandwidth due to packet collisionsPrinciple: 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=0AStation A captures the channel at t=tpropA

  • *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).

  • *Non-Persistent CSMA

    Attempts to reduce the incidence of collisionsStations with a packet to transmit sense the channelIf the channel is busy, the station immediately runs the back-off algorithm and reschedules a future sensing timeIf the channel is idle, then the station transmits Consequence : channel may be free even though some users have packets to transmit.

  • *p-Persistent CSMA

    Combines elements of the above two schemesStations with a packet to transmit sense the channelIf it is busy, they persist with sensing until the channel becomes idleIf it is idle:With probability p, the station transmits its packetWith probability 1-p, the station waits for a random time and senses again

  • *Throughput expressionPure ALOHASlotted ALOHAUnslotted 1-persistent CSMASlotted 1-persistent CSMAUnslotted nonpersistent CSMASlotted nonpersistent CSMAProtocol Throughput

  • *Throughput plotNormalized propagation delay is a =0 .01

  • *CSMA/CD (reminder)Operating principleCheck whether the channel is idle before transmittingListen while transmitting, stop transmission when collisionIf collision, one of the 3 schemes above (1-persistent, non-persistent or p-persistent)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, idleStationRepeaterTerminator

  • *Why CSMA/CD is unfit for WLANsCollision Detection requires simultaneous transmission and reception operations (which a radio transceiver is usually unable to do) detecting a collision is difficultCarrier Sensing may be suitable to reduce interference at sender, but Collision Avoidance is needed at receiverCSMA/CD does not address the hidden terminal problem

  • *CSMA/CAIs described in the module devoted to IEEE 802-11

  • *Some medium access control mechanisms for wirelessTDMACDMAFDMASDMAFixedAlohaReservationsDAMAMultiple Access withCollisionAvoidancePollingPureCSMA Used in GSMSlottedNon-persistentp-persistentCSMA/CA Copes with hidden and exposed terminal RTS/CTS Used in 802.11 (optional)MACAWMACA-BIFAMACARMA Used in 802.11 (mandatory) Used in 802.11 (optional)FHSS: Frequency-Hopping Spread SpectrumDSSS: Direct Sequence Spread SpectrumCSMA: Carrier Sense Multiple Access CA: Collision AvoidanceDAMA: Demand-Assigned Multiple AccessMACA-BI: MACA by invitationFAMA: Floor Acquisition Multiple AccessCARMA: Collision Avoidance and Resolution Multiple AccessFHSSDSSS Used in GSMFixed Used in Bluetooth

  • *DAMA - Demand Assigned Multiple AccessChannel efficiency only 18% for Aloha, 36% for Slotted AlohaReservation can increase efficiency to 80%a sender reserves a future time-slotsending within this reserved time-slot is possible without collisionreservation also causes higher delaystypical scheme for satellite linksExamples for reservation algorithms:Explicit Reservation (Reservation-ALOHA)Implicit Reservation (PRMA)Reservation-TDMA

  • *DAMA / Explicit ReservationExplicit 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 timeAlohareservedAlohareservedAlohareservedAlohacollisiont

  • *DAMA / Packet reservation (PRMA)Implicit reservationbased on slotted Alohaa certain number of slots form a frame, frames are repeatedstations compete for empty slots according to the slotted aloha principleonce 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 sendcompetition for a slot starts again as soon as the slot was empty in the last frame frame1frame2frame3frame4frame512345678time-slotcollision at reservation attemptsACDABA FAC ABA A BAF A BAF DACEEBAFDtACDABA-FACDABA-FAC-ABAF-A---BAFDACEEBAFDreservation

  • *DAMA / Reservation-TDMAReservation Time Division Multiple Access every frame consists of N mini-slots and x data-slotsevery 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-slotsN * k data-slotsreservations for data-slotsother stations can use free data-slotsbased on a round-robin schemee.g. N=6, k=2

  • *Some medium access control mechanisms for wirelessTDMACDMAFDMASDMAFixedAlohaReservationsDAMAMultiple Access withCollisionAvoidancePollingPureCSMA Used in GSMSlottedNon-persistentp-persistentCSMA/CA Copes with hidden and exposed terminal RTS/CTS Used in 802.11 (optional)MACAWMACA-BIFAMACARMA Used in 802.11 (mandatory) Used in 802.11 (optional)FHSS: Frequency-Hopping Spread SpectrumDSSS: Direct Sequence Spread SpectrumCSMA: Carrier Sense Multiple Access CA: Collision AvoidanceDAMA: Demand-Assigned Multiple AccessMACA-BI: MACA by invitationFAMA: Floor Acquisition Multiple AccessCARMA: Collision Avoidance and Resolution Multiple AccessFHSSDSSS Used in GSMFixed Used in Bluetooth

  • *MACA - collision avoidanceMACA (Multiple Access with Collision Avoidance) uses short signaling packets for collision avoidanceDesigned 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 packetCTS (clear to send): the receiver grants the right to send as soon as it is ready to receiveSignaling packets containsender addressreceiver addresspacket size

    Variants of this method can be found in IEEE802.11 as DFWMAC (Distributed Foundation Wireless MAC)

  • *MACA avoids the problem of hidden terminalsA and C want to send to BA sends RTS firstC waits after receiving CTS from B

    MACA avoids the problem of exposed terminalsB wants to send to A, C to another terminalnow C does not have to wait for it cannot receive CTS from AMACA principleBRTSCTSCTSBRTSCTSRTS

  • *MACA exampleABCDEABCDEABCDERTSCTSDATA: blocked from Tx123

  • *MACA variant: application in IEEE802.11idlewait for the right to sendwait for ACKsenderreceiverpacket ready to send; RTStime-out; RTSCTS; dataACKRxBusyidlewait fordataRTS...

Recommended

View more >