IEEE 802.11, Token RingsMedium Access ControlEthernet MAC AlgorithmCSMA/CD in WLANs?Hidden Terminal ProblemMACA Solution for Hidden Terminal ProblemExposed Terminal ProblemMACA Solution for Exposed Terminal ProblemCollisionsReliabilityA Simple Solution to Improve Reliability - MACAWRevisiting the Exposed Terminal ProblemRevisiting the Exposed Terminal Problem - MACAWDeafnessInterframe SpacingIEEE 802.11 - CSMA/CATypes of IFSSlide 18Backoff IntervalDCF ExampleBackoff IntervalSlide 22Binary Exponential Backoff in DCFToken RingSlide 25Slide 26Token Ring: Topology and ComponentsToken Ring: Dual RingFDDIMultistation Access UnitToken Ring: Basic ConceptsSlide 32Token ReleaseToken Ring: Media Access Control Parameters802.5 Reliability802.5 MonitorToken Maintenance: 802.5Timing Algorithm: 802.5Traffic Classes: FDDITiming Algorithm: FDDISlide 41FDDI exampleSlide 43Slide 44FDDI PerformanceSlide 46Token Maintenance: FDDI01/16/19 CS/ECE 438 - UIUC, Fall 2006 1IEEE 802.11, Token Rings01/16/19 CS/ECE 438 - UIUC, Fall 2006 2Medium Access ControlWireless channel is a shared mediumNeed access control mechanism to avoid interferenceWhy not CSMA/CD?01/16/19 CS/ECE 438 - UIUC, Fall 2006 3Listen for carrier sense before transmittingCollision: What you hear is not what you sent!Ethernet MAC AlgorithmNode A Node B01/16/19 CS/ECE 438 - UIUC, Fall 2006 4CSMA/CD in WLANs?Most (if not all) radios are half-duplexListening while transmitting is not possibleCollision might not occur at senderCollision at receiver might not be detected by sender!01/16/19 CS/ECE 438 - UIUC, Fall 2006 5Hidden Terminal ProblemNode B can communicate with both A and CA and C cannot hear each otherWhen A transmits to B, C cannot detect the transmission using the carrier sense mechanismIf C transmits, collision will occur at node BA B CDATA DATAABCA’s signalstrengthspaceC’s signalstrength01/16/19 CS/ECE 438 - UIUC, Fall 2006 6MACA Solution for Hidden Terminal ProblemWhen node A wants to send a packet to node BNode A first sends a Request-to-Send (RTS) to AOn receiving RTSNode A responds by sending Clear-to-Send (CTS)provided node A is able to receive the packetWhen a node C overhears a CTS, it keeps quiet for the duration of the transferRTSCTSCTSA B C01/16/19 CS/ECE 438 - UIUC, Fall 2006 7Exposed Terminal ProblemB talks to AC wants to talk to DC senses channel and finds it to be busyC stays quiet (when it could have ideally transmitted)CTSRTSRTSA B C D01/16/19 CS/ECE 438 - UIUC, Fall 2006 8MACA Solution for Exposed Terminal ProblemSender transmits Request to Send (RTS)Receiver replies with Clear to Send (CTS)NeighborsSee CTS - Stay quietSee RTS, but no CTS - OK to transmitCTSRTSRTSRTSA B C D01/16/19 CS/ECE 438 - UIUC, Fall 2006 9CollisionsStill possibleRTS packets can collide!Binary exponential backoff Backoff counter doubles after every collision and reset to minimum value after successful transmissionPerformed by stations that experience RTS collisionsRTS collisions not as bad as data collisions in CSMA Since RTS packets are typically much smaller than DATA packets01/16/19 CS/ECE 438 - UIUC, Fall 2006 10ReliabilityWireless links are prone to errorsHigh packet loss rate detrimental to transport-layer performanceMechanisms needed to reduce packet loss rate experienced by upper layers01/16/19 CS/ECE 438 - UIUC, Fall 2006 11A Simple Solution to Improve Reliability - MACAWWhen node B receives a data packet from node A, node B sends an Acknowledgement (ACK)If node A fails to receive an ACKRetransmit the packetRTSCTSCTSA B CDATAACKACK01/16/19 CS/ECE 438 - UIUC, Fall 2006 12Revisiting the Exposed Terminal ProblemProblemExposed terminal solution doesn't consider CTS at node CWith RTS-CTS, C doesn’t wait since it doesn’t hear A’s CTSWith B transmitting DATA, C can’t hear intended receiver’s CTSC trying RTS while B is transmitting is uselessCTSRTSRTSA B C DRTSCTS01/16/19 CS/ECE 438 - UIUC, Fall 2006 13Revisiting the Exposed Terminal Problem - MACAWOne solutionHave C use carrier sense before RTSAlternativeB sends DS (data sending) packet before DATA:Short packet lets C know that B received A’s CTSIncludes length of B’s DATA so C knows how long to wait01/16/19 CS/ECE 438 - UIUC, Fall 2006 14DeafnessFor the scenario belowNode A sends an RTS to BWhile node C is receiving from D, Node B cannot reply with a CTSB knows that D is sending to CA keeps retransmitting RTS and increasing its own BO timeoutRTSRTSA B C DCTSCTS01/16/19 CS/ECE 438 - UIUC, Fall 2006 15Interframe SpacingInterframe spacing Plays a large role in coordinating access to the transmission mediumVarying interframe spacings Creates different priority levels for different types of traffic!802.11 uses 4 different interframe spacingstmedium busySIFSPIFSDIFSDIFSnext framecontentiondirect access if medium is free DIFS01/16/19 CS/ECE 438 - UIUC, Fall 2006 16IEEE 802.11 - CSMA/CASensing the medium If free for an Inter-Frame Space (IFS)Station can start sending (IFS depends on service type)If busyStation waits for a free IFS, then waits a random back-off time (collision avoidance, multiple of slot-time) If another station transmits during back-off time The back-off timer stops (fairness)tmedium busyDIFSDIFSnext framecontention window(randomized back-offmechanism)slot timedirect access if medium is free DIFS01/16/19 CS/ECE 438 - UIUC, Fall 2006 17Types of IFS SIFSShort interframe spaceUsed for highest priority transmissionsRTS/CTS frames and ACKs DIFSDCF interframe spaceMinimum idle time for contention-based services (> SIFS)01/16/19 CS/ECE 438 - UIUC, Fall 2006 18Types of IFS PIFSPCF interframe spaceMinimum idle time for contention-free service (>SIFS, <DIFS) EIFSExtended interframe spaceUsed when there is an error in transmission01/16/19 CS/ECE 438 - UIUC, Fall 2006 19Backoff Interval When transmitting a packet, choose a backoff interval in the range [0,cw]cw is contention windowCount down the backoff interval when medium is idleCount-down is suspended if medium becomes busyWhen backoff interval reaches 0, transmit RTS01/16/19 CS/ECE 438 - UIUC, Fall 2006 20DCF ExampledatawaitB1 = 5B2 = 15B1 = 25B2 = 20datawaitB1 and B2 are backoff intervalsat nodes 1 and 2cw = 31B2 = 1001/16/19 CS/ECE 438 - UIUC,
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