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Istituto Superiore Mario Boella

Istituto Superiore Mario Boella. Introduction to MS- Aloha. R. Scopigno, Networking Lab – scopigno@ismb.it www.ms-aloha.eu. It simulatively overtakes CSMA/CA. It works under mobility. Introduction : C oncepts and Figures First Proprietary Mechanisms : RR- Aloha +

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Istituto Superiore Mario Boella

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  1. Istituto Superiore Mario Boella Introduction to MS-Aloha R. Scopigno, Networking Lab– scopigno@ismb.it www.ms-aloha.eu

  2. Itsimulativelyovertakes CSMA/CA Itworks under mobility • Introduction: Concepts and Figures • First ProprietaryMechanisms: RR-Aloha+ • Proposed Extensions • Simulative Settings • The Final Version: MS-Aloha • Proposed Extensions for Scalability • RR-Aloha+ & MS-AlohaSimulations • Preemption and Conclusions

  3. Introduction: Concepts and Figures • First ProprietaryMechanisms: RR-Aloha+ • Proposed Extensions • Simulative Settings • The Final Version: MS-Aloha • Proposed Extensions for Scalability • RR-Aloha+ & MS-AlohaSimulations • Preemption and Conclusions

  4. Requirements for slotted Vanets • Basedon reservation • Aimedatachievingdeterminism • Completelydistributed • Infrastructurewould be a requesttoo strong • Dynamicclustering and master electionwouldnot scale • Itrequirestoomuch time and reactsslowly: notcompatible with MAC needs • Preventinghidden terminal issue • Frequent in urban area • Supportingpriority (preemption) for emergencymessages • Blocking must be prevented for suchmessages • Efficientlysupport target lengthmessages and typicalfrequency • In thisstudyfixedat 200B, 10 Hz

  5. MS-Aloha Base Mechanisms (i) • Each node who has obtained a slot appends to the slot its view of all the slots (FI) • Against hidden station and to enable collision detection • Potentially dangerous overhead • Contention Phase (slot reservation) • A node starts competing for slot assignment listening to • Slot (free busy) • N FIs coming from its neighbors • The node transmits a data packet into a slot considered idle, together with its FIs

  6. MS-Aloha Base Mechanisms (ii) • The reservation of a slot is performed through two distinct phases • The slot reservation through the FI • True slot occupation • In the period between slot(K) and slot(K+N) the channel is monitored to detect any reservation • Check on slot and by FI analysis • When slotK begins, the node transmits its packet if it still has the reservation. • Continuousmonitoring to face mobility

  7. MS-Aloha Format (i) • Slot: channel time space dedicated to a single host for data transmission. • N: number of slots within a single frame. • FI: (Frame Information): Structure containing information about the status of each slot. • Required to prevent hidden station • In this presentation: • Same Physical Layer of 802.11p (12Mbps, 10Mhz ch @5.9GHz, QAM16-1) • Frame: 100ms (10Hz application Rate) • Payload: 200 Bytes • If FI=12 bits per slot and Tg: 1 us, then 224 slots (of 446 us) • Other setting (e.g. relaxed guard time) in other studies available in www.ms-aloha.eu

  8. MS-Aloha Format (ii) STI (8bit) Address1(48 bit) Address2(48 bit) SequenceNumber (12bit) FragmentNumber (4bit) FIbit(1bit) • STI: source identification • Address1: source address • Address2:destinationaddress • SequenceNumber: field indicating the sequence number of each packet • FragmentNumber: used in case of frame fragmentation • FIbit: bit indicating the presence of the FI before the payload (sent in slot0 only) • Payload: • CRC: used to highlight any errors during transmission

  9. FI field • FI: (Frame Information): Structure containing information about the status of each slot • Each slot information is composed of: • STI: the short identifier of the node • PSF (Priority Status Field): field indicating the priority of data transmitted in the slot. The values ranging from 1 to 3 (growing priority). • STATE: 2-bit flag indicating channel state PSF (2) State (2) STI (8 bits)

  10. Time EfficiencyThe Issue of Overhead (i) • The mainconcernisabout the overheadimplied by MS-Aloha • The overhead of MS-Alohaisfixed • CSMA/CA introduces a protocoloverheadtoo, butitisvariable and hard to be measured • Comparison by simulations in case of unicast • Both broadcast and Unicast: • In Broadcast CSMA/CA doesnot involve backoff (no ACKs)  no real OH • The side effect of collisionsshould be takeninto account • 100-200 fixednodes on twolanes • Point-to-point full duplex trafficatvariableapplication rate • Peers in distinctLanes • Inter-Node-Dist 4m; Inter-Peer-Dist 60m • 37dbm TX, -85dbm RX (benefits for CSMA)

  11. The Issue of Overhead (ii)Unicast (100) • Inter-packet time inside a flow (Average on the 100 flows) • Time betweentwo consecutive packetscorrectlyreceived • CSMA/CA saturationstartsat 15Hz • variable, fixed on average, higherthan MS-Aloha

  12. The Issue of Overhead (iii)Unicast (200) • Inter-packet time inside a flow (Average on the 100 flows) • Time betweentwo consecutive packetscorrectlyreceived • CSMA/CA saturationstartsat 10Hz • variable, fixed on average, higherthan MS-Aloha

  13. The Issue of Overhead (iv)Broadcast (100) • Inter-packet time inside a flow (Average on the 100 flows) • Time betweentwo consecutive packetscorrectlyreceived • CSMA/CA saturation starts at 15Hz • variable, fixed on average, higher than MS-Aloha

  14. The Issue of Overhead (v)Broadcast (200) • Inter-packet time inside a flow (Average on the 100 flows) • Time betweentwo consecutive packetscorrectlyreceived • CSMA/CA saturationstartsat <10Hz • variable, fixed on average, higherthan MS-Aloha

  15. The Issue of Overhead (vi) • MS-Aloha (224 slots, 200B Appl.Layer, 12Mbps) • 446s per slot (includingguard-time) • Payload_Time = 200*8/12Mbps = 133s • Overhead_Time= 313s (3.756 bit_time @ 12Mbps) • Overhead/Payload = 2,35 • η = 1/(1+2.35) ≈ 0,3 (including Ethernet-likeOverhead) • CSMA/CA (200B Appl.Layer, 12Mbps) 8-50 Hz Appl. Rate • From interpacket time inside a flow to interpacket time in the air • 1.000-3.500 s IPT unicast; 500-5.000 s IPT broadcast • Payload_Time = 200*8/12Mbps = 133s • Overhead_Time= 867-3.367s unicast; 367-4.867s broadcast • Overhead/Payload = 6.5-25 unicast; 2.7-36 broadcast • η = 1/(1+{OH}) ≈ 0,13  0.04 unicast; 0,27  0.03 broadcast

  16. Itsimulativelyovertakes CSMA/CA Itworks under mobility • Introduction: Concepts and Figures • First ProprietaryMechanisms: RR-Aloha+ • Proposed Extensions • Simulative Settings • The Final Version: MS-Aloha • Proposed Extensions for Scalability • RR-Aloha+ & MS-AlohaSimulations • Preemption and Conclusions

  17. Itworks under mobility • Introduction: Concepts and Figures • First ProprietaryMechanisms: RR-Aloha+ • Proposed Extensions • Simulative Settings • The Final Version: MS-Aloha • Proposed Extensions for Scalability • RR-Aloha+ & MS-AlohaSimulations • Preemption and Conclusions

  18. Typical UnresolvedIssues of Other Slotted Solutions • TDMA algorithms are usually for fixed or slowlyvaryingtopologies • Fixednetworks (RR-Aloha) or free-space (line-of-sight), lowdensity and slowlyvaryingmutual positions (STDMA) • Evenif standard the mayNOT be suitable (!) • They do notfit the requirements of dynamicenvironmentssuchthat of Vanet • A node can appearsuddenly due to obstructions • Hiddenterminals are much more frequentthan in free space • The density of nodesisso high to makehiddencollisions more frequent • Thesehave a direct impact on the efficiency and the quality of the services • MS-Alohasolvestheseissues with a first set of proprietarymechanisms • Mechanisms first published under the name of «RR-Aloha+ functions» • Three tricks: memoryrefresh, signalingsemantics, scalability of labelspace • The properness of the solutionshasbeenvalidatedthroughsimulations.

  19. 1. Memory Refresh • Simulationshighlightthe first simple, yetunresolvedissueconcern the refreshing rate for the information on channel state • In case the information isnotrefreshed, once a slot jisassigned to nodeM, the slot state would be frozen • The slot would be continuouslyannouncedbusyalsoifthe nodegetsswitched-off • Additionallythe information wouldjumptoomanyhops • In a vehicularenvironment, the samewouldhappenif the nodeMgot far from the radio range of itspreviousneighbourhood • MoreoverMwouldannouncefallacious information - basedon a radio rangewhichisnotactual • On eachnode the memoryneeds to be refreshedperiodically • Simulationsinvolvingnodemobilityhighlightthisas the primary cause of inefficient slot allocation • Itisshown to worksifinformationisrefreshed once per MS-AlohaPeriod • Additionallyinformation on slot jisrefreshedwhenthe elapsedtime hasreached the position j

  20. 2. SignalingSemantic(i) • In DTDMA and MS-Aloha the problem of hidden terminal iscounteracted by messagebroadcasting with FI • In case of fixednodes, eachnodeexpectsconfirmation of slot assignementsby all the nodesin itsneighbourhood • The assignementisresult a logicANDamongreceivedFis • If the ad-hoc network iscontinuouslychangingitis hard to knowwhatone'sneighbourhoodislike • Notall the nodes can be required to be alwaysconnectedto confirm • Ifa new nodeswitcheson, it ‘0’ states in the FI will reset all the connections • The information carried by FI ismanaged by a logicalOR • The semanticischanged: conflictingFIs - ratherthanacks – drive changes

  21. 2. SignalingSemantic(ii) • Ifchannel state are managed by AND, 1 bit isenough to describechannel state • Onlyifall FI agree on the assignment, the busy state isconfirmed • If a collisionisdetected, itisannounced just by “free” message (thanks to AND logic) • In steady state itmay work; with mobility and OR itgetsambiguous examplefollows • In order to solve thisissue, the STATE subfieldisextended to two bits • 2 bits allows to ditinguishthe followingslots: free, busy and collision • An additionalvariation in the semantic: collisionsrequire an explicitindication • Simulations show that the overhead and latencesintroduced by the additional bit are negligiblewhilemake the VANET stable

  22. Example: Why an Additional Bit isRequired Trasmission Order Slot 0: Slot 1: Slot 2: During slot 3, nodewillsendacknowledgementaboutinto slot 2 of its FI FI nodes receiving from nodes receiving from Slot 3:

  23. Example: Why an Additional Bit isRequired In the nextFIs, the nodeswhichhavedetected a collisionwillsend slot2 status as free of its FI This way the collisionnotificationgetsmissed! The remainingnodeswillsend an ackabout slot2 assignment, withoutdetectingproperly the collision, also due to the OR operation. Trasmission Order Slot 0: Slot 1: So the nodessense a collision status affecting slot 2, then set itas free (Busy=0), while the nodes do notchange the slot 2 status Slot 2: The node notices the collision and send slot 2 as free on its FI Receive from FI Receive from Slot 3: Busy = 0 Slot 4:

  24. 3. Scalability of STI Label Space (i) • 8-bit labels STI usedto identifyeachnode inside the communicationarea: 256 possiblevalues • STI are used to identifywhatnodeisusingeachreservednode • STI are usedinsteadof MAC addresses (typically 48-bit wide numbers) to avoidexcessiveoverheads in the FIs • In urbanareas the labelspacemay be a very strong limit • However the samelabelcan be re-used in differentslots • The purposeof STI iscollisiondetection - differentnodesusing the sameslot • Label+Slot NodeIdentification • Stillstatisticallynot-negligibleevent of twohiddenterminalschosingthe same slot and the sameSTI • Scalabilityfinallysolvedassigning STI a “temporarymeaning” • STI changedby the nodesdirectlyreceiving from nodeAintoSTI’ • TheyknowalsoA’s MAC and compute a new STI’ based on STI and MAC • The nodeswhich do notreceive from A just know STI’. The otherknowthat STI and STI’ represent the samenode A • At nextperiod the STI’ ischanged by A intoSTI’’ and so on. Collision are, soon or later, detected

  25. 3. Scalability of STI Label Space (ii)

  26. Itsimulativelyovertakes CSMA/CA Itworks under mobility • Introduction: Concepts and Figures • First ProprietaryMechanisms: RR-Aloha+ • Proposed Extensions • Simulative Settings • The Final Version: MS-Aloha • Proposed Extensions for Scalability • RR-Aloha+ & MS-AlohaSimulations • Preemption and Conclusions

  27. Itsimulativelyovertakes CSMA/CA • Introduction: Concepts and Figures • First ProprietaryMechanisms: RR-Aloha+ • Proposed Extensions • Simulative Settings • The Final Version: MS-Aloha • Proposed Extensions for Scalability • RR-Aloha+ & MS-AlohaSimulations • Preemption and Conclusions

  28. MS-Aloha • Twomainissues can stillhinder the exploitation of MS-Alohain a VANET scenario: • The scalability of the protocol (number of available free slots) • Itscapacity to stronglyreact to changingconditions due to mobility • Simulations show that the unconstrainedmultihopforwardingof channel state isharmful • Slot reservationisextendedbeyondthe bounds of wireless coverage • Causingresourcewaste and slot depletion • Mobilityintroduces a notnegligibleprobability of gettingcloser to nodeswhichhavebeenassigned the sameslot • Thisbecomes more relevantwhennodesmove in opposite directions • The number of collisionsgrows high • The effectisdisruptive if slot re-use ishindered • Among the causes: slot state forwarding with no limitations on the number of hops

  29. Limitation of FIForwarding • So far the channel-state isdescribed by twobits (State) • Only 3 states are used (free ‘00’, busy‘10’, collision‘01’) • One free configuration (say ‘11’) • The free configuration can be exploited to keep trace of number of hopsthe information isforwarded over • When some information on slot reservationisnotdirectlydetected, itisannouncedas2-hop(’11’) • Nodeswhichreceiveittheyknowthattheyshouldnot use the slot butshouldnotforwardthis information either • Thissolutionshavebeendemonstrated, by simualtions, to: • Decrease the logicalradius of propagation of a slot reservation • Improve of resource re-use. Busy Busy 2-Hop Free

  30. ImprovingSlot Re-Use (i) • Slot re-use can be furtherimprovedsetting a higherthreshold on minimum reception power • If the receivedpowerislowerthana giventhresholdTHR the message IS consideredfor MS-Alohabutdoesnotcontribute to the FI messages • Itconceptuallycorresponds to lowering the radius of cluster of nodeswhichperceive a slot xassigned to a nodeA • Instead, actingon the transmittedpowerwouldaffect the SNR • Furtherimprovement by introducing a mechanismwhichregulates the THR dynamically • THR defined on eachnodeseparatelybasedon itsperception • Blockingcompletelyprevented • Simulations show thatitworks • Effects on slot reuse (increased) • Effectson Packet Delivery Rate (PDR) • Loweredathigherdistancesbutkept high close to the transmitter

  31. Improving Slot Re-Use (ii) • The average does not change much • Slot re-use is also a statistical event: the point is to make it possible • However potentially still scalable • Less blocked nodes and for less time • More unused slot Slot Reuse: -96 dbm: 1.968 -86 dbm: 2.040 -80 dbm: 2.174 Sent Packets: CSMA/CA: 100% (*) MS-Aloha -96: 92,50% MS-Aloha -86: 94,75% MS-Aloha -80: 99,50% (*) far from saturation

  32. Simulation: Settings • The MS-Alohahasbeenimplemented on NS-2 • Mostsimulations use MS-Alohaset asfollows • each slot lasts 0.447 ms • 224 slot per frame (the overall frame takesabout 0.1sec) • Packet generation rate of 10Hz • Alsoothersettingsadopted • 200 slots and over 78.5 µs guard time – relaxedsynchronization • The simulationadopts the followingscenarios: • Simulationlasts 2000 sec. • Nakagamimodel wasused to model propagation and urbangrid with corner obstruction (extra attenuation) • Transmittedpower 7dbm or 20 dbm • Wireless reception sensitivity -96dbm • 400-900 nodes (speed in the range 50-120Km) • Circulartopology (radius R=1Km) with fourlanes or • Urban topology with grid150m blocks and 750m-wide map • In all the simulationsMS-Alohaperformsbetterthan(or aswellas) CSMA/CA in terms of PDR and determinism

  33. Simulation: Metrics • In order to quantifyresultsthe followingmetricsadopted • PDR (Packet Delivery Rate): functionthat shows howmuch a nodeislikely to receive a packetvarying the distance from the transmittingnode; • Suitable for both MS-Aloha and CSMA/CA • In CSMA/CA itisaffected with high congestion • MeanCollisions: the averagenumber of collisionsdetected on the same slot, over the wholesimulation and all the nodes; • Suitable for only MS-Aloha • Slot Re-Use: number of times a slot is re-used by differentnodes (at a given time). • Suitable for alohaMS-Aloha • Seepreviousslides • Determinismishardlymeasuredbutitis • Close to 100% for MS-Aloha (fixeddelays and high PDR, onlyaffected by slot collisions) • Lower in CSMA/CA, due to unpredictable delay (non-deterministictransmission time due to collisionavoidance) andlower PDR (non- deterministic reception)

  34. Simulations: PDR Whatever the threshold MS-Alohaachives a higher PDR than CSMA/CA and a negligibleworseningwhere reception isalreadylow Higherthresholds in MS-Aloha force slot re-use which cause interferences and worsen PDR butquite far fram the transmitter: MS-Alohapreserves time/space-coordination

  35. Simulations: CollisionsMulti-hop FI forwarding vs 2-hop Number of Collisions 2-Hop Number of Collisions MultiHop Only collisions due to mobility!

  36. Itsimulativelyovertakes CSMA/CA Itworks under mobility • Introduction: Concepts and Figures • First ProprietaryMechanisms: RR-Aloha+ • Proposed Extensions • Simulative Settings • The Final Version: MS-Aloha • Proposed Extensions for Scalability • RR-Aloha+ & MS-AlohaSimulations • Preemption and Conclusions

  37. Introduction: Concepts and Figures • First ProprietaryMechanisms: RR-Aloha+ • Proposed Extensions • Simulative Settings • The Final Version: MS-Aloha • Proposed Extensions for Scalability • RR-Aloha+ & MS-AlohaSimulations • Preemption and Conclusions

  38. Preemption(i) • Preemptionas an additionalsolutionagainstchannelblocking • Acting on service differentiation and aimedatQoSguarantee • Each station accesses the channel with a priority, variable in [1-4] (2 bits) • The priorityisannounced in a subfield of the FI field • Whenevera node with higherpriorityneeds to transmist, itcan override a node with lowerpriority • E.g.Node 1 can transmit in slot 5 evenifitisalreadyoccupied by node 2, ifnode 2 has a lowerpriority • In a possiblepractical scenario nodeshave the highestpriorityonly for emergencymessages • Normalaccessuses 3 lowerclasses • E.g.: 1-emergency; 2-channel-access; 3-assistance, 4-entertainment

  39. Preemption(ii) • Questions to be answered: • Can preemption help saturate the channel? • Doespreemption work also under saturation? Can itreally gain channelaccess • Severalsimulations. Followingresultsachieved with: • 858 nodes, averagespeed80km, TX power: 2 dbm • 5x5 grid (150m distance); 2-lane roads • Application rate at 30Hz • With and withoutpreemption • With preemptioneachnodestries to have a High-Priorityslot and a Low-Priority slot • Results (2.000 sec of simulated time) • Transmittedpackets: with preemption-34.360; w/o preemption-18.028; • With preemption: HP packets: 17.980; LP packets 16.378

  40. Preemption(iii) Collisionswithoutpreemption Collisionsat slot0 with preemption

  41. ThankYoufor Your KindAttention R. Scopigno, Networking Lab– scopigno@ismb.it www.ms-aloha.eu

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