2-1Medium Access Control (MAC) Protocols for Ad hoc Wireless Networks -IV Drs. Baruch Awerbuch & Amitabh MishraDepartment of Computer ScienceJohns Hopkins University©CS: 647Advanced Topics in Wireless Networks2-2Outline❒ Wireless MAC Issues❍ Hidden terminal problem❍ Exposed terminal problem❍ Capture❒MAC Performance Metrics❒ Wireless MAC Classification❒ Distributed Wireless MAC Protocols❍ CSMA/CA❍ 802.11 MAC DCF Backoff❍ Fairness❍ Energy/Power2-3Fairness Issue❒ Many definitions of fairness plausible❒ Simplest definition: All nodes should receive equal bandwidthA BC DTwo flows2-4Fairness Issue❒ Assume that initially, A and B both choose a backoff interval in range [0,31] but their RTSs collide❒ Nodes A and B then choose from range [0,63]❍ Node A chooses 4 slots and B choose 60 slots❍ After 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 packetA BC DTwo flows2-5Fairness Issue❒ Unfairness occurs when one node has backed off much more than some other nodeTwo flowsA BC D2-6MACAW Solution for Fairness❒ When a node transmits a packet, it appends the cwvalue to the packet, all nodes hearing that cwvalue use it for their future transmission attempts❒ MACAW proposes maintaining cwindependently for each receiver❒ Using per-receiver cwis particularly useful in multi-hop environments, since congestion level at different receivers can be very different2-7Another MACAW Proposal❒ For the scenario below, when node A sends an RTS to B, while node C is receiving from D, node B cannot reply with a CTS, since B knows that D is sending to C❒ When the transfer from C to D is complete, node B can send a Request-to-send-RTS to node A[Bharghavan94Sigcomm]❍Node A may then immediately send RTS to node BABCD2-8❒ This approach, however, does not work in the scenario below❍ Node B may not receive the RTS from A at all, due to interference with transmission from CABCD2-9Outline❒ Wireless MAC Issues❍ Hidden terminal problem❍ Exposed terminal problem❍ Capture❒MAC Performance Metrics❒ Wireless MAC Classification❒ Distributed Wireless MAC Protocols❍ CSMA/CA❍ 802.11 MAC DCF Backoff❍ Fairness❍ Priority Scheduling❍ Power2-10Priority Scheduling❒ Given packets belonging to different priority classes, packets with higher priority should be transmitted first❒ Since the packets may be at different nodes sharing the wireless channel, how to coordinate access ?A BHigh Low2-11Priorities in 802.11❒ CTS and ACK have priority over RTSAfter channel becomes idle❒ If a node wants to send CTS/ACK, it transmits SIFS duration after channel goes idle❒ If a node wants to send RTS, it waits for DIFS > SIFS2-12SIFS and DIFSDATA1ACK1SIFSbackoffDIFSRTSDIFS2-13Variation in Backoff Interval [Aad01]❒ For high priority packets❍ Backoff interval in [0,CWh]❒ For low priority packet❍ Backoff interval in [CWh+1, CWl]❒ Higher priority packets use small backoffintervals❍ Higher probability of transmitting a high priority packet before a pending low probability packet2-14❒ With this scheme, if two high priority packets collide, they will have to choose a new backoffinterval, and may be transmitted after a low priority packets❒ Example:❍ Packet H1 : backoff interval 9 slots❍ Packet H2 : backoff interval 9 slots❍ Packet L : backoff interval 13 slots❒ When H1 and H2 collidePacket L : backoff interval is now 4 slotsAssume that H1 and H2 pick backoff intervals 6 and 7 slots respectively, after collisionPacket L will be transmitted first2-15Second Mechanism [Aad01]❒ High priority packets always choose backoff in [0,CWh]❒ Low priority packets wait for LIFS idle period before counting down whereLIFS = DIFS + CWh❒ Ensures that high priority packets will always get a chance to transmit before a low priority packet can2-16ExamplebusyHDIFSH backoffHLbusyLIFSLIFSL backoffH2-17❒ Disadvantage: When no high priority packets, low priority packet unnecessarily wait for long periods of time❒ How to avoid priority reversal, and also minimize wait for low priority packets ? [Yang02Mobihoc]2-18Priority Using Black Bursts [Hiperlan/1,Sobrinho96,99]❒ All nodes begin the priority contention phase together❒ Higher priority node transmit a longer burst than low priority node❒ After transmitting its burst, a node listens to the channel❒ If channel still busy, the node has lost contention to a higher priority node2-19Outline❒ Wireless MAC Issues❍ Hidden terminal problem❍ Exposed terminal problem❍ Capture❒MAC Performance Metrics❒ Wireless MAC Classification❒ Distributed Wireless MAC Protocols❍ CSMA/CA❍ 802.11 MAC DCF Backoff❍ Fairness❍ Priority Scheduling❍ Energy Conservation2-20Energy Conservation❒ Since many mobile hosts are operated by batteries, MAC protocols which conserve energy are of interest❒ Two approaches to reduce energy consumption❍ Power save: Turn off wireless interface when desirable❍ Power control: Reduce transmit power2-21Power Aware Multi-Access Protocol (PAMAS) [Mobicom’98Singh]❒ A node powers off its radio while a neighbor is transmitting to someone elseNode A sending to BNode C stays powered offCBA2-22Power Aware Multi-Access Protocol (PAMAS)❒ What should node C do when it wakes up and finds that D is transmitting to someone else❍ C does not know how long the transfer will lastNode A sending to BC stays powered offCBADENode D sending to EC wakes up andfinds medium busy2-23PAMAS❒ PAMAS uses a control channel separate from the data channel❒ Node C on waking up performs a binary probe to determine the length of the longest remaining transfer❍ C sends a probe packet with parameter L❍ All nodes which will finish transfer in interval [L/2,L] respond❍ Depending on whether node C see silence, collision, or a unique response it takes varying actions2-24Disadvantages of PAMAS❒ Use of a separate control channel❒ Nodes have to be able to receive on the control channel while they are transmitting on the data channel❍ And also transmit on data and control channels simultaneously❒ A node (such as C) should be able to determine when probe responses from multiple senders collide2-25Power Save in IEEE 802.11 Ad Hoc Mode❒ Time is divided into beacon intervals❒ Each beacon interval begins with an ATIM window❍ ATIM = ??Beacon intervalATIMwindow2-26Power Save in IEEE