ADHOC – MAC : a new, flexible and reliable MAC architecture for ad-hoc networksAd-Hoc NetworksInter-vehicles ad-hoc NetworksMAC problem: Hidden terminalMAC problems: exposed terminalMAC problems: broadcast serviceADHOC MACSlide 8Reservation ALOHASlide 10Reliable Reservation ALOHASlide 12RR-ALOHA : Frame InformationRR-ALOHA : slot statusRR-ALOHA : accessSlide 16Slide 17RR ALOHA : common frameRR ALOHA : slot reuseADHOC MAC : Reserving additional bandwidthSlide 21ADHOC MAC : Point-to-point channelsADHOC MAC : Point-to-point channelsADHOC MAC : Multi-hop Broadcast serviceMulti-hop Broadcast mechanismRR ALOHA PERFORMANCE Implementation overheadSlide 27RR ALOHA PERFORMANCE Time responsivenessConclusionsADHOC – MAC : a new, ADHOC – MAC : a new, flexible and reliable MAC flexible and reliable MAC architecturearchitecture for ad-hoc for ad-hoc networks networks F. Borgonovo, A. Capone, M. Cesana, L. FrattaDipartimento Elettronica e InformazionePolitecnico di Milano2•No fixed infrastructure •Limited propagation range•Need for terminal relaying/routingAd-Hoc NetworksAd-Hoc Networks3•Traffic control•Entertainment•Internet accessInter-vehicles ad-hoc Inter-vehicles ad-hoc NetworksNetworksSpeed poses stringent requirements No centralized operation4not completely solved by IEEE 802.11(CSCA)MAC problem: Hidden MAC problem: Hidden terminalterminal Impact on : –radio access–local broadcast5unsolved by IEEE 802.11 (RQS/CLS)MAC problems: exposed MAC problems: exposed terminal terminal Impact on efficiency since paralleltransmissions can be prevented6how to chose bridgesMAC problems: broadcast MAC problems: broadcast serviceserviceTree-based protocols not applicable due to dynamic topologyFlooding highly inefficient with high degree of connectivity(n transmissions instead of 1)7ADHOC MACADHOC MAC•Features:•Layer two connectivity information•Access to a reliable single-hop broadcast•QoS support for different applications•Efficient point-to-point communication (parallel transmissions)•Efficient multi-hop broadcast8•Time slotted channel (eg, using GPS time synch)•Basic Channel (BCH) •Each active terminal owns a slot (Basic Channel) •It periodically transmits channel status information in it•Slots are grouped into virtual frames (VF) of length N•Transmissions are received by all terminals within one hop rangeADHOC MACADHOC MACterminal jterminal iterminal k.......BCH is established using the Reliable Reservation ALOHA protocol9a distributed way to establish TDMA channels Reservation ALOHAReservation ALOHAk k+N k+2Na slot successfully captured is periodically reserved (every N slots) until released10needs a centralized radio environment with central station feedback, so that all terminals “see” the same slot status: busy, free, collidedReservation ALOHAReservation ALOHA11•operates in a distributed radio environment•each terminal propagates slot status information (Frame Information) using BCHReliable Reservation ALOHAReliable Reservation ALOHAFIFIFIFIFI12•all active terminals transmit the Frame Information every N slots (within the virtual frame)• FI specifies the status of the previous N slots (in the Sliding Virtual Frame) as observed by the terminal•BUSY correct transmission•FREE no transmission or collisionReliable Reservation ALOHAReliable Reservation ALOHAB BBBsliding frame NFFFFFFFFFTransmittingterminal131243567FI-35 2 4 63 1 2 7 4 63 5FI-5 2 45 1FI-1 4 63 5 1 2FI-27 63 5 1 2 4FI-4 65 7 4FI-73 5 2 7 4 6FI-63RR-ALOHA : RR-ALOHA : Frame Frame InformationInformationTransmissions5 1 24 675 1 24 6714 1 2 7 4 63 5FI-5FI-1FI-27 63 5 1 2 4FI-4 65 7 4FI-73 5 2 7 4 6FI-6RR R RR 7RR-ALOHA : slot statusRR-ALOHA : slot statusRESERVED if at least one FI says “BUSY”AVAILABLE otherwiseAAAAFrame status processed by terminal 7RFI-315RR-ALOHA : accessRR-ALOHA : accessRR R RRAAAA AFrameAvailable slot•AVAILABLE slots can be used:•by a new active terminal (as in R-ALOHA )•by an already active terminal to increase its transmission bandwidth•No Hidden-Terminal problem16RR-ALOHA : accessRR-ALOHA : accessThe transmission is successful if•the slot is coded as BUSY with the same station ID in all the received FIThe ID of the slot “owner” must be included in the FI7 683 1 2FI-4 65 97FI-7Collisions17RR-ALOHA : accessRR-ALOHA : accessOne terminalattempting access:Multiple terminals attempting access:• All terminals in the same cluster recognize the transmission. • All FIs will mark the slot as BUSY.• All other terminals will receive FI with the slot marked as BUSY. • The slot is declared RESERVED.• Each terminal upon detecting collision leaves the slot as FREE. • The slot remains AVAILABLE.18RR ALOHA : common frameRR ALOHA : common frame1243567•a unique frame is established among non disjoint radio broadcast domains based on FIs transmitted by nodes in common19RR ALOHA : slot reuseRR ALOHA : slot reuseA B C DAB BC CDB AB B BC ABAABB A BC BC A AFrame 1C C B AB CD B BC C ABABB BC BCFrame 2C C CD BC CCDD DBC DBC D CDFrame 3Frame 1Frame 2Frame 323 transmissions in 13 slots20ADHOC MAC : Reserving ADHOC MAC : Reserving additional bandwidthadditional bandwidth13574•Each active station sets up and manages a BCH•Payload can be transmitted in the BCH slots•Additional available slots can be reserved for increasing transmission bandwidth (additional channels )773521ADHOC MAC : Reserving ADHOC MAC : Reserving additional bandwidthadditional bandwidth• Usin g RR- ALOHA procedure on the AVAILABLE Slots• Usin g estabilished BCH. •New cha nnel requests are signaled •Possibility of priority management •FI guarantees reservation collision detection22ADHOC MAC : Point-to-point ADHOC MAC : Point-to-point channels channels •To exploit slot reuse in the same or adjacent clusters (parallel transmissions)•PTP flag is needed in the FI for each slot•PTP flag is set by a terminal if:–The packet received is broadcast or–The packet is destined to the terminal itself•A reserved slot can be accessed if:–The PTP flag is off in all received FI and–The FI received from the intended destination marks the slot FREE•Due to concurrent access attempts: the transmission is successful if the slot is coded as BUSY in the FI of the destination terminal.23ADHOC MAC : Point-to-point ADHOC MAC : Point-to-point
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