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U of I CS 438 - Token Rings

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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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U of I CS 438 - Token Rings

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