ADHOC – MAC : a new, flexible and reliable MAC architecture for ad-hoc 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 Networks3• Traffic control• Entertainment• Internet accessInter-vehicles ad-hoc NetworksSpeed poses stringent requirements No centralized operation4not completely solved by IEEE 802.11(CSCA)MAC problem: Hidden terminalImpact on : – radio access– local broadcast5unsolved by IEEE 802.11 (RQS/CLS)MAC problems: exposed terminal Impact on efficiency since paralleltransmissions can be prevented6how to chose bridgesMAC problems: broadcast serviceTree-based protocols not applicable due to dynamic topologyFlooding highly inefficient with high degree of connectivity(n transmissions instead of 1)7ADHOC 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 MACterminal jterminal iterminal k.......BCH is established using the Reliable Reservation ALOHA protocol9a distributed way to establish TDMA channels Reservation 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 ALOHA11• operates in a distributed radio environment• each terminal propagates slot status information (Frame Information) using BCHReliable 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 SlidingVirtual Frame) as observed by the terminal• BUSY correct transmission• FREE no transmission or collisionReliable Reservation ALOHAB BBBsliding frame NFFFFFFFFFTransmittingterminal131243567FI-35 2 4 631 2 74 63 5FI-52 45 1FI-14 63 5 1 2FI-27 63 5 1 2 4FI-465 74FI-73 5 2 74 6 FI-63RR-ALOHA : Frame InformationTransmissions5 1 24 675 1 24 67141 2 74 63 5FI-5FI-1FI-27 63 5 1 2 4FI-465 74FI-73 5 2 74 6 FI-6RR R RR 7RR-ALOHA : slot statusRESERVED if at least one FI says “BUSY”AVAILABLE otherwiseAAAAFrame status processed by terminal 7RFI-315RR-ALOHA : accessR R 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 : 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-465 97FI-7Collisions17RR-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 frame1243567• a unique frame is established among non disjoint radio broadcast domains based on FIs transmitted by nodes in common19RR 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 additional 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 additional bandwidth• Using RR- ALOHA procedure on the AVAILABLE Slots• Using estabilished BCH. • New channel requests are signaled• Possibility of priority management • FI guarantees reservation collision detection22ADHOC MAC : Point-to-point 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 channels24ADHOC MAC : Multi-hop Broadcast service{ }{} AND OR AND ijijijjiIDIDCCCCCS>=>⊆i of neighbors ofset theiCk slot inpacket broadcast the receivednot have that neighbors ofsubset theiiCS⊆Terminal i relays the broadcast packet received in slot k if and the following condition is not satisfied for all j0>iSfrom FIs25Multi-hop Broadcast mechanismA B C DAB BC CDOne terminal for each set AB, BC and CD is elected as relay terminal1243567ABC IDLowest in if innot if satisfied satisfied infor=>⊆ABCiCCABCiCCCSABCjijijji26RR ALOHA PERFORMANCEImplementation overhead• N slots >= M terminals (in the cluster)• For inter-vehicles applications M=100 N=200FI must contain:• BUSY status (1 bit)• Terminal temporary ID (8 bits)• Priority field (2 bits)• PTP service flag (1 bit)• Overhead due to FI 2400 bits /slot• Overhead due to other information 100 bits/slot• Packet length 5000 bits • Payload 2500 bits/slot in BCH• At 10 Mbit/s frame duration 100 ms:25kb/s in BCH• 5Mb/s for reservation27RR ALOHA PERFORMANCEImplementation overhead• Overhead reduction:– Insert ID and priority information in the FI once every kframes– Used by the MAC in the access phase only and needed to be repeated for new active terminals– Ex: Add information once every 10 frames• FI reduces to 400 bits 90% of
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