Advanced Topics in Next-Generation Wireless Networks

  • Published on
    21-Jan-2016

  • View
    30

  • Download
    0

Embed Size (px)

DESCRIPTION

Advanced Topics in Next-Generation Wireless Networks. Qian Zhang Department of Computer Science HKUST. Basic, PHY and MAC of Ad Hoc Network. What is an Ad Hoc Network?. Definitions: - PowerPoint PPT Presentation

Transcript

  • Qian ZhangDepartment of Computer Science HKUSTAdvanced Topics in Next-Generation Wireless Networks Basic, PHY and MAC of Ad Hoc Network

  • Definitions:An ad-hoc network is one that comes together as needed, not necessarily with any assistance from the existing Internet infrastructure Instant infrastructureA MANET is a collection of mobile platforms or nodes where each node is free to move about arbitrarilyA MANET: distributed, possibly mobile, wireless, multihop network that operates without the benefit of any existing infrastructure (infrastructure-less), except the nodes themselves

    What is an Ad Hoc Network?

  • Mobile Ad Hoc NetworksMay need to traverse multiple links to reach a destination

  • Mobile Ad Hoc Networks (MANET)Mobility causes route changes

  • Why Ad Hoc Networks ?

    Ease of deployment

    Speed of deployment

    Decreased dependence on infrastructure

  • HistoryIn 1972, DoD-sponsored Packet Radio Network (PRNet)Initial network used centralized control stationEvolved into a distributed architectureA network of broadcast radiosMinimal central controlUse multi-hop store-and-forward routingUse ALOHA/CSMA, spread-spectrumDemonstrated the technologies needed to create a MANET

  • History (Cont.)In 1983, Survivable Radio Network (SURAN) program Motivations:Move towards smaller size, low-cost, low power radioDevelop and demonstrate scalable algorithms, up to 10ks of nodes.Develop and demonstrate robustness and survivability against sophisticated attacksTechnology: spread spectrum improvements, hierarchical network topology, dynamic clustering, and even cross-layer design

  • History (Cont.)Army (Army Research Office-ARO)The army did not embrace the new MANET technology until it was demonstrated experimentally in mid-1980sUsed it for primarily land-based applicationsUtilized mainly as overlays to existing networksNavy (Office of Naval Research-ONR)Primarily for use by ships at seaNetwork is not as dense as a ground networkRequired integration with satellite linksAir ForceExplored utilizing aircrafts to provide communications between ground stations

  • Commercialization(Arguably) two papers, both addressing the routing problem, helped spur the ad-hoc networks research[Perkins-Bhagwat94, Johnson94]The community started to adopt the term Ad-Hoc NetworksCommercial applications started to appearA number of standards activities evolved in the mid90sNotably, the MANET working group (within the IETF) to standardize routing protocols for ad hoc networksReactive and proactive routingThe 802.11 subcommittee standardized an ad-hoc mode MAC layerMade it possible to build ad-hoc networks using laptops

  • Fundamental ChallengesIt is better to know some of the questions than all of the answers. James Thurber (1835-1910)

  • 1. Energy EfficiencyNo infrastructure means must rely on batteries (or, in general, limited energy resources)

    Possible solutionsSelectively sending nodes into a sleep modeUsing transmitters with variable power (the Power Control problem)Using energy-efficient pathsUsing co-operative techniques (still relatively new)

  • 2. MobilityMobility-induced route changesMobility-induced packet losses

    Mobility patterns may be differentControlled e.g. robotsOffers opportunities for improving the network functions e.g. connectivityUncontrolled e.g. nomadic usersOffers challenges to network designBut also offers opportunities for improvement, e.g.Users carry delay-tolerant data closer to destinationDelay Tolerant Network (Challenge Networks)

  • 3. QoSProviding QoS even in wired networks (e.g. the Internet) is a challenging problemWireless RF channels further complicate the problemUnpredictabilityMedium access: broadcast medium with hidden terminal problemPossible solutions:New MAC design Cross-layer integration: allow different layers to adapt depending on available information at other layers

  • 4. ScalabilityLimited wireless transmission rangeWhether the network is able to maintain an acceptable level of service even as the number of nodes is increasedHow fast the network protocol control overhead increases as N increases

    Possible solutions:Introducing hierarchyUtilizing location informationLimiting reactions to changesFixing things (e.g. paths) locally

  • 5. Utilizing New TechnologiesWhat are the gains that could be achieved by using newly available technologies such asSmart directional (beamforming) antennasIncreases the spatial reuse in cellular, but how about ad-hoc?Can several nodes together act as an antenna array? Practical issues?Software RadioThe ability to quickly switch the operating frequency may provide opportunities, but also challengingGPSLocation information may help

  • 6. SecurityEase of snooping on wireless transmissionsFrom crypto point of view, lack of a trusted authority is one of the main challengesHow to generate/share keys reliably Harder to track or even detect attackers in a wireless environment, given that:Network relies on in-situ connections to other nodes which may be maliciousMalicious nodes may be especially harmful by injecting bogus control packetsDoS attacks that deplete a nodes battery

  • 7. Lack of ReferenceLack of sufficient experimental data to confirm modelsWhat does a multi-hop path really mean? What is a link?Simplistic models that do not capture the complexities, or complex models that do not lead to insights?Are the protocols good enough, have they reached closed to the best possible?Good balance between mathematical and experimental work

  • Multiple-Layer ProblemPHYAdapt to rapid changes in link characteristicsMACMinimize collision, allow fair access, and semi-reliably transport under rapid change and hidden/exposed terminalsNetworkDetermine efficient transmission paths when links change often and bandwidth is at a premiumTransportHandle delay and packet loss statistics that are very different than wired networksApplicationhandle frequent disconnection and reconnection as well as varying delay and packet loss characteristics

  • Topics in PHY and MAC

    FairnessEnergy efficiencyPower savePower controlAdaptive modulation (multi-rate)

  • Fairness IssueAssume that initially, A and B both choose a backoff interval in range [0,31] but their RTSs collideNodes A and B then choose from range [0,63]Node A chooses 4 slots and B choose 60 slotsAfter A transmits a packet, it next chooses from range [0,31]It is possible that A may transmit several packets before B transmits its first packet

    ABCDTwo flowsUnfairness occurs when one node has backed off much more than some other nodes

  • Fairness in Multi-Hop Networks

    Several definitions of fairness [Ozugur98,Vaidya99MSR,Luo00Mobicom, Nandagopal00Mobicom]

    Hidden terminals make it difficult to achieve a desired notion of fairness

  • Topics in PHY and MAC

    FairnessEnergy conservationPower savePower controlAdaptive modulation (multi-rate)

  • Energy ConservationSince many mobile hosts are operated by batteries, MAC protocols which conserve energy are of interest

    Two approaches to reduce energy consumptionPower save: turn off wireless interface when desirablePower control: reduce transmit powerPower Characteristics for a Mica2 Mote Sensor

  • Power Save in IEEE 802.11 Ad Hoc ModeTime is divided into beacon intervals

    Each beacon interval begins with an ATIM windowATIM = Beacon intervalATIMwindow

  • Power Save in IEEE 802.11 Ad Hoc Mode (Cont.)ATIMReqATIMAckAckDataSleepNode ANode CNode BIf host B has a packet to transmit to A, B must send an ATIM Request to A during an ATIM Window

    On receipt of ATIM Request from A, B will reply by sending an ATIM Ack, and stay up during the rest of the beacon intervalIf a host does not receive an ATIM Request during an ATIM window, and has no pending packets to transmit, it may sleep during rest of the beacon interval

  • Power Save in IEEE 802.11 Ad Hoc Mode (Cont.)Size of ATIM window and beacon interval affects performanceIf ATIM window is too large, reduction in energy consumption reducedEnergy consumed during ATIM windowIf ATIM window is too small, not enough time to send ATIM request

    How to choose ATIM window dynamically?Based on observed load [Jung02infocom]

  • Power Save in IEEE 802.11 Ad Hoc Mode (Cont.)

    How to synchronize hosts?If two hosts ATIM windows do not overlap in time, they cannot exchange ATIM requestsCoordination requires that each host stay awake long enough (at least periodically) to discover out-of-sync neighbors [Tseng02infocom]ATIMATIM

  • Impact on Upper LayersIf each node uses the 802.11 power-save mechanism, each hop will require one beacon intervalThis delay could be intolerable for some applications

    Allow upper layers to dictate whether a node should enter the power save mode or not [Chen01mobicom]

  • Power ControlRecall:Power control has two potential benefit

    Reduced interference & increased spatial reuse

    Energy saving

  • Power Control with 802.11Transmit RTS/CTS/DATA/ACK at least power level needed to communicate with the receiver

    A/B do not receive RTS/CTS from C/D. Also do not sense Ds data transmission

    Bs transmission to A at high power interferes with reception of ACK at CBCDA

  • Data sensedA Plausible SolutionRTS/CTS at highest power, and DATA/ACK at smallest necessary power level

    A cannot sense Cs data transmission, and may transmit DATA to some other hostThis DATA will interfere at CThis situation unlikely if DATA transmitted at highest power levelInterference range ~ sensing rangeBCDARTSDataInterference rangeAck

  • Other ConsTransmitting RTS at the highest power level also reduces spatial reuse

    Nodes receiving RTS/CTS have to defer transmissions

  • Modification to Avoid InterferenceTransmit RTS/CTS at highest power level, DATA/ACK at least required power levelIncrease DATA power periodically so distant hosts can sense transmission [Jung02tech]

    Need to be able to change power level rapidlyPowerlevel

  • Topics in PHY and MAC

    FairnessEnergy efficiencyPower savePower controlAdaptive modulation (multi-rate)

  • Adaptive ModulationChannel conditions are time-varyingReceived signal-to-noise ratio changes with time Multi-rate radios are capable of transmitting at several rates, using different modulation schemesChoose modulation scheme as a function of channel conditionsDistanceThroughputModulation schemes providea trade-off betweenthroughput and range

  • Adaptive Modulation (Cont.)If physical layer chooses the modulation scheme transparent to MACMAC cannot know the time duration required for the transfer

    Must involve MAC protocol in deciding the modulation schemeSome implementations use a sender-based scheme for this purposeReceiver-based schemes can perform better

  • Sender-Based Autorate Fallback [Kamerman97]

    Probing mechanisms

    Sender decreases bit rate after X consecutive transmission attempts fail

    Sender increases bit rate after Y consecutive transmission attempt succeed

  • Autorate FallbackAdvantageCan be implemented at the sender, without making any changes to the 802.11 standard specification

    DisadvantageProbing mechanism does not accurately detect channel stateChannel state detected more accurately at the receiverPerformance can sufferThe sender will periodically try to send at a rate higher than optimalWhen channel conditions improve, the rate is not increased immediately

  • Receiver-Based Autorate MAC [Holland01mobicom]Sender sends RTS containing its best rate estimateReceiver chooses best rate for the conditions and sends it in the CTSSender transmits DATA packet at new rateInformation in data packet header implicitly updates nodes that heard old rateDCBACTS (1 Mbps)RTS (2 Mbps)Data (1 Mbps)NAV updated using rate specified in the data packet

    *

    Note: check this NIST webpagehttp://www.antd.nist.gov/wctg/manet/adhoclinks.html#ASSOCIATIONS

    *premium*Our work: How to integrate adaptive modulation with 802.11 medium access control ?

    **