19/26/07 CS/ECE 438 - UIUC, Fall 2007 1IEEE 802.11, Token Rings9/26/07 CS/ECE 438 - UIUC, Fall 2007 2Medium Access Control Wireless channel is a shared medium Need access control mechanism toavoid interference Why not CSMA/CD?9/26/07 CS/ECE 438 - UIUC, Fall 2007 3 Listen for carrier sense before transmitting Collision: What you hear is not what you sent!Ethernet MAC AlgorithmNode A Node B⊗9/26/07 CS/ECE 438 - UIUC, Fall 2007 4CSMA/CD in WLANs? Most (if not all) radios are half-duplex Listening while transmitting is notpossible Collision might not occur at sender Collision at receiver might not bedetected by sender!9/26/07 CS/ECE 438 - UIUC, Fall 2007 5Hidden Terminal Problem Node B can communicate with both A and C A and C cannot hear each other When A transmits to B, C cannot detect thetransmission using the carrier sense mechanism If C transmits, collision will occur at node BA B CDATA DATAABCA’s signalstrengthspaceC’s signalstrength9/26/07 CS/ECE 438 - UIUC, Fall 2007 6MACA Solution for HiddenTerminal Problem When node A wants to send a packet to node B Node A first sends a Request-to-Send (RTS) to A On receiving RTS Node A responds by sending Clear-to-Send (CTS) provided node A is able to receive the packet When a node C overhears a CTS, it keeps quiet for theduration of the transferRTSCTSCTSA B C29/26/07 CS/ECE 438 - UIUC, Fall 2007 7Exposed Terminal Problem B talks to A C wants to talk to D C senses channel and finds it to be busy C stays quiet (when it could have ideallytransmitted)CTSRTSRTSA B C D9/26/07 CS/ECE 438 - UIUC, Fall 2007 8MACA Solution for ExposedTerminal Problem Sender transmits Request to Send (RTS) Receiver replies with Clear to Send (CTS) Neighbors See CTS - Stay quiet See RTS, but no CTS - OK to transmitCTSRTSRTSRTSA B C D9/26/07 CS/ECE 438 - UIUC, Fall 2007 9Collisions Still possible RTS packets can collide! Binary exponential backoff Backoff counter doubles after every collision and reset tominimum value after successful transmission Performed by stations that experience RTS collisions RTS collisions not as bad as data collisions inCSMA Since RTS packets are typically much smaller than DATApackets9/26/07 CS/ECE 438 - UIUC, Fall 2007 10Reliability Wireless links are prone to errors High packet loss rate detrimental totransport-layer performance Mechanisms needed to reduce packetloss rate experienced by upper layers9/26/07 CS/ECE 438 - UIUC, Fall 2007 11A Simple Solution to ImproveReliability - MACAW When node B receives a data packet fromnode A, node B sends an Acknowledgement(ACK) If node A fails to receive an ACK Retransmit the packetRTSCTSCTSA B CDATAACKACK9/26/07 CS/ECE 438 - UIUC, Fall 2007 12Interframe Spacing Interframe spacing Plays a large role in coordinating access to thetransmission medium Varying 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 ≥ DIFS39/26/07 CS/ECE 438 - UIUC, Fall 2007 13IEEE 802.11 - CSMA/CA Sensing the medium If free for an Inter-Frame Space (IFS) Station can start sending (IFS depends on service type) If busy Station 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 ≥ DIFS9/26/07 CS/ECE 438 - UIUC, Fall 2007 14Types of IFS SIFS Short interframe space Used for highest priority transmissions RTS/CTS frames and ACKs DIFS DCF interframe space Minimum idle time for contention-basedservices (> SIFS)9/26/07 CS/ECE 438 - UIUC, Fall 2007 15Types of IFS PIFS PCF interframe space Minimum idle time for contention-freeservice (>SIFS, <DIFS) EIFS Extended interframe space Used when there is an error intransmission9/26/07 CS/ECE 438 - UIUC, Fall 2007 16Backoff Interval When transmitting a packet, choose abackoff interval in the range [0,cw] cw is contention window Count down the backoff interval whenmedium is idle Count-down is suspended if medium becomesbusy When backoff interval reaches 0, transmitRTS9/26/07 CS/ECE 438 - UIUC, Fall 2007 17DCF ExampledatawaitB1 = 5B2 = 15B1 = 25B2 = 20datawaitB1 and B2 are backoff intervalsat nodes 1 and 2cw = 31B2 = 109/26/07 CS/ECE 438 - UIUC, Fall 2007 18Backoff Interval The time spent counting down backoffintervals is a part of MAC overhead Large cw Large backoff intervals Can result in larger overhead Small cw larger number of collisions (when twonodes count down to 0 simultaneously)49/26/07 CS/ECE 438 - UIUC, Fall 2007 19Backoff Interval The number of nodes attempting totransmit simultaneously may changewith time Some mechanism to manage contentionis needed IEEE 802.11 DCF Contention window cw is chosendynamically depending on collisionoccurrence9/26/07 CS/ECE 438 - UIUC, Fall 2007 20Binary Exponential Backoff inDCF When a node fails to receive CTS inresponse to its RTS, it increases thecontention window cw is doubled (up to an upper bound) When a node successfully completes adata transfer, it restores cw to Cwmin cw follows a sawtooth curve9/26/07 CS/ECE 438 - UIUC, Fall 2007 21Token Ring Example Token Ring Networks IBM: 4Mbps token ring IEEE 802.5: 16Mbps9/26/07 CS/ECE 438 - UIUC, Fall 2007 22Token Ring Focus on Fiber Distributed Data Interface (FDDI) 100 Mbps Was (not is) a candidate to replace Ethernet Used in some MAN backbones (LAN interconnects) Outline Rationale Topologies and components MAC algorithm Priority Feedback Token management9/26/07 CS/ECE 438 - UIUC, Fall 2007 23Token Ring Why emulate a shared medium with point-to-point links? Why a shared medium? Convenient broadcast capabilities Switches costly Why emulation? Simpler MAC algorithm Fairer access arbitration Fully digital (802.3 collision detection requiresanalog)9/26/07 CS/ECE 438 - UIUC, Fall 2007 24Token Ring: Topology andComponents Relay Single Relay Multistation access unitsHostHostHostHostFrom PreviousMSAUTo Next
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