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10/11/06 CS/ECE 438 - UIUC, Fall 2006 1IEEE 802.11, Token Rings10/11/06 CS/ECE 438 - UIUC, Fall 2006 2Medium Access Control Wireless channel is a shared medium Need access control mechanism toavoid interference Why not CSMA/CD?10/11/06 CS/ECE 438 - UIUC, Fall 2006 3 Listen for carrier sense before transmitting Collision: What you hear is not what you sent!Ethernet MAC AlgorithmNode A Node B10/11/06 CS/ECE 438 - UIUC, Fall 2006 3 Listen for carrier sense before transmitting Collision: What you hear is not what you sent!Ethernet MAC AlgorithmNode A Node B⊗10/11/06 CS/ECE 438 - UIUC, Fall 2006 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!10/11/06 CS/ECE 438 - UIUC, Fall 2006 5Hidden Terminal ProblemA B CABCA’s signalstrengthspaceC’s signalstrength10/11/06 CS/ECE 438 - UIUC, Fall 2006 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 CABCA’s signalstrengthspaceC’s signalstrength10/11/06 CS/ECE 438 - UIUC, Fall 2006 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 CDATAABCA’s signalstrengthspaceC’s signalstrength10/11/06 CS/ECE 438 - UIUC, Fall 2006 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 signalstrength10/11/06 CS/ECE 438 - UIUC, Fall 2006 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 signalstrength10/11/06 CS/ECE 438 - UIUC, Fall 2006 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 transferA B C10/11/06 CS/ECE 438 - UIUC, Fall 2006 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 transferRTSA B C10/11/06 CS/ECE 438 - UIUC, Fall 2006 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 C10/11/06 CS/ECE 438 - UIUC, Fall 2006 7Exposed Terminal ProblemA B C D10/11/06 CS/ECE 438 - UIUC, Fall 2006 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)A B C D10/11/06 CS/ECE 438 - UIUC, Fall 2006 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)RTSRTSA B C D10/11/06 CS/ECE 438 - UIUC, Fall 2006 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 D10/11/06 CS/ECE 438 - UIUC, Fall 2006 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 transmitA B C D10/11/06 CS/ECE 438 - UIUC, Fall 2006 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 transmitRTSRTSA B C D10/11/06 CS/ECE 438 - UIUC, Fall 2006 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 transmitCTSRTSRTSA B C D10/11/06 CS/ECE 438 - UIUC, Fall 2006 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 D10/11/06 CS/ECE 438 - UIUC, Fall 2006 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 DATApackets10/11/06 CS/ECE 438 - UIUC, Fall 2006 10Reliability Wireless links are prone to errors High packet loss rate detrimental totransport-layer performance Mechanisms needed to reduce packetloss rate experienced by upper layers10/11/06 CS/ECE 438 - UIUC, Fall 2006 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 packetA B C10/11/06 CS/ECE 438 - UIUC, Fall 2006 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 packetRTSA B C10/11/06 CS/ECE 438 - UIUC, Fall 2006 11A Simple Solution to ImproveReliability - MACAW When node B receives a data packet fromnode A, node B sends an
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