MAC schemesConsider wireless linksShared links in wired domainSlide 4Classification of MAC protocolsFDMA (Frequency Division Multiple Access)TDMA (Time Division Multiple Access)Frequency vs. timeControl aspectUse of fixed assignment schemes on one shared linkOn wireless linksConcept of upstream vs. downstreamExample of FDMA scheme: Advanced Mobile Phone System (AMPS)Duplex techniquesImpact of FDDHexagonal cell frequency planReuse factorSecond example of fixed-assignment scheme: IS136 NA-TDMAThe TDMA aspect: frames and time slotsData rate of a carrier (frequency)What about CDMA?Random access MAC protocolsVarious random-access MAC schemesALOHASlotted ALOHACSMADifferent techniquesCSMA/CDExample of CSMA/CD: EthernetCollisions in EthernetCollisions in EthernetExponential Backoff AlgorithmEthernet performanceEthernet frame formatWireless 802.11 LANMechanisms for CA802.11 MACDCF Distributed Coordination FunctionDCF mode transmission without RTS/CTSDCF MACWhat frequencies are used in 802.11?Frequency Hopping (FH)802.11 FH PHYDemand assignment schemesShared link as a LAN: relation between MAC protocols and LANsShared links as “access” linksShared link in the presence of basestations/APs?Compare TDMA on an access link with TDM on an inter-switch linkSummary1M. VeeraraghavanMAC schemesProf. Malathi Veeraraghavan2M. VeeraraghavanConsider wireless links•Naturally broadcast medium–One transmitter sends data; multiple receivers can receive the signal and obtain the data–Need a MAC (Medium Access Control) protocol to share the “naturally broadcast” wireless mediumEndpointEndpointEndpoint3M. VeeraraghavanShared links in wired domain•Distance limitation between farthest hosts – Shannon’s capacity; SNR; attenuationoutboundinboundMultipoint drops: potential interference on inbound line – polling; e.g. multidrop telephone linesHub or opticalpassive star couplerHost HostHostHubs/Optical passive star couplers: any data received on one line is broadcast to all other lines4M. VeeraraghavanShared links in wired domain•Ring networks:–ring adapters pass through the signal in cut-through fashion–hosts pick up data destined for themselves•Multitapped shared buses–in coaxial cable transmisison systems users can inject a signal that propagates in both directions of the mediumHostringadapterHostHostHostHostpoint-to-pointtransmissionlinks5M. VeeraraghavanClassification of MAC protocolsMAC protocolsFixed-assignment schemesRandom-access schemesDemand assignment schemesCircuit-switched(e.g., FDMA, TDMA)Connectionlesspacket-switched(e.g., Ethernet,802.11)Connection-orientedpacket-switched(e.g., CDMA, polling)Channelization6M. VeeraraghavanFDMA (Frequency Division Multiple Access)•Similar to broadcast radio and TV, assign a different carrier frequency per call•Modulation technique determines the required carrier spacing•Each communicating wireless user gets his/her own carrier frequency on which to send data•Need to set aside some frequencies that are operated in random-access mode to enable a wireless user to request and receive a carrier for data transmission7M. VeeraraghavanTDMA(Time Division Multiple Access)•Each user transmits data on a time slot on multiple frequencies•A time slot is a channel•A user sends data at an accelerated rate (by using many frequencies) when its time slot begins•Data is stored at receiver and played back at original slow rate8M. VeeraraghavanFrequency vs. timeFrequencyTimeCarrierFDMATimeFrequencyTDMATimeFrequencyHybrid FDMA/TDMABasic principle of communication: Two regions in the time-frequency plane with equal areas can carry the same amount of information9M. VeeraraghavanControl aspect•How is the fixed assignment of a frequency and/or timeslot made?–Who makes it?–How long does an endpoint get to hold the frequency/timeslot?•Need a control channel on which stations can compete using a random-access protocol to obtain a channel assignment–Set aside control frequencies and/or timeslots10M. VeeraraghavanUse of fixed assignment schemes on one shared link •All endpoints/hosts receive/transmit on common frequencies–many if FDMA is used•Use control channels to allocate frequency/timeslot for user communication between one pair–Release when done•In Ethernet hub, baseband transmission – 1 frequency – random access not fixed-assignmentEndpointEndpointEndpointHub or opticalpassive star couplerHost HostHost11M. VeeraraghavanOn wireless links•Previous slide shows what’s referred to as an “adhoc” architecture•More common is to use an “infrastructure” based architecture–basestation or access point present12M. VeeraraghavanConcept of upstream vs. downstreamPSTNBase stations/cell sitesReversechannelsForward channelsDownstreamUpstreamInternetAccess points•Send data between endpoints via a basestation or AP –Can be placed at a height allowing all endpoints to see the basestation/AP–Endpoints that cannot “hear” each other communicate via the basestation/AP–MAC problem only in upstream direction13M. VeeraraghavanExample of FDMA scheme: Advanced Mobile Phone System (AMPS)•FDMA/FDD–Spectrum allocation by FCC: A and B allocations to different providersAB825 8451 667AB870 8901 667channelsKhzMhz6663020OriginalReverse ForwardA B825 8451023667A824 849991 1799BA A B870 8901023667A869 894991 1799BAchannelsKhzMhz8323025Extended14M. VeeraraghavanDuplex techniques•Separates signals transmitted by base stations from signals transmitted by terminals–Frequency Division Duplex (FDD): use separate sets of frequencies for forward and reverse channels (upstream and downstream)–Time Division Duplex (TDD): same frequencies used in the two directions, but different time slots15M. VeeraraghavanImpact of FDD•In AMPS, endpoints cannot “hear” each other–receivers at endpoints (phones) tuned only to downstream frequencies–adhoc mode essentially disabled!16M. VeeraraghavanHexagonal cell frequency planRD•D: Distance between a base station and the nearest base station that uses the same channels•R: Radius of a cell•Reuse distance = D/R•Channel plan: method ofassigning channels to cells to guarantee a minimumreuse distance between cells thatuse the same channel reqISISreqRDRDwhich is the minimum reuse distance for which17M. VeeraraghavanReuse factor•Divide available channels into N groups•N: reuse