DOC PREVIEW
U of I CS 438 - Token Rings

This preview shows page 1-2-3-22-23-24-45-46-47 out of 47 pages.

Save
View full document
View full document
Premium Document
Do you want full access? Go Premium and unlock all 47 pages.
Access to all documents
Download any document
Ad free experience
View full document
Premium Document
Do you want full access? Go Premium and unlock all 47 pages.
Access to all documents
Download any document
Ad free experience
View full document
Premium Document
Do you want full access? Go Premium and unlock all 47 pages.
Access to all documents
Download any document
Ad free experience
View full document
Premium Document
Do you want full access? Go Premium and unlock all 47 pages.
Access to all documents
Download any document
Ad free experience
View full document
Premium Document
Do you want full access? Go Premium and unlock all 47 pages.
Access to all documents
Download any document
Ad free experience
View full document
Premium Document
Do you want full access? Go Premium and unlock all 47 pages.
Access to all documents
Download any document
Ad free experience
View full document
Premium Document
Do you want full access? Go Premium and unlock all 47 pages.
Access to all documents
Download any document
Ad free experience
View full document
Premium Document
Do you want full access? Go Premium and unlock all 47 pages.
Access to all documents
Download any document
Ad free experience
View full document
Premium Document
Do you want full access? Go Premium and unlock all 47 pages.
Access to all documents
Download any document
Ad free experience
Premium Document
Do you want full access? Go Premium and unlock all 47 pages.
Access to all documents
Download any document
Ad free experience

Unformatted text preview:

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 ControlWireless channel is a shared mediumNeed access control mechanism to avoid interferenceWhy not CSMA/CD?01/16/19 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01/16/19 CS/ECE 438 - UIUC, Fall 2006 4CSMA/CD in WLANs?Most (if not all) radios are half-duplexListening while transmitting is not possibleCollision might not occur at senderCollision at receiver might not be detected by sender!01/16/19 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 the transmission using the carrier sense mechanismIf 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 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 the duration of the transferRTSCTSCTSA B C01/16/19 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 ideally transmitted)CTSRTSRTSA B C D01/16/19 CS/ECE 438 - UIUC, Fall 2006 8MACA Solution for Exposed Terminal 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 D01/16/19 CS/ECE 438 - UIUC, Fall 2006 9CollisionsStill possibleRTS packets can collide!Binary exponential backoff Backoff counter doubles after every collision and reset to minimum value after successful transmissionPerformed by stations that experience RTS collisionsRTS 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 10ReliabilityWireless links are prone to errorsHigh packet loss rate detrimental to transport-layer performanceMechanisms needed to reduce packet loss rate experienced by upper layers01/16/19 CS/ECE 438 - UIUC, Fall 2006 11A Simple Solution to Improve Reliability - MACAWWhen node B receives a data packet from node A, node B sends an Acknowledgement (ACK)If node A fails to receive an ACKRetransmit the packetRTSCTSCTSA B CDATAACKACK01/16/19 CS/ECE 438 - UIUC, Fall 2006 12Revisiting the Exposed Terminal ProblemProblemExposed terminal solution doesn't consider CTS at node CWith RTS-CTS, C doesn’t wait since it doesn’t hear A’s CTSWith B transmitting DATA, C can’t hear intended receiver’s CTSC trying RTS while B is transmitting is uselessCTSRTSRTSA B C DRTSCTS01/16/19 CS/ECE 438 - UIUC, Fall 2006 13Revisiting the Exposed Terminal Problem - MACAWOne solutionHave C use carrier sense before RTSAlternativeB sends DS (data sending) packet before DATA:Short packet lets C know that B received A’s CTSIncludes length of B’s DATA so C knows how long to wait01/16/19 CS/ECE 438 - UIUC, Fall 2006 14DeafnessFor the scenario belowNode A sends an RTS to BWhile node C is receiving from D, Node B cannot reply with a CTSB knows that D is sending to CA keeps retransmitting RTS and increasing its own BO timeoutRTSRTSA B C DCTSCTS01/16/19 CS/ECE 438 - UIUC, Fall 2006 15Interframe SpacingInterframe spacing Plays a large role in coordinating access to the transmission 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  DIFS01/16/19 CS/ECE 438 - UIUC, Fall 2006 16IEEE 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  DIFS01/16/19 CS/ECE 438 - UIUC, Fall 2006 17Types 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-based services (> SIFS)01/16/19 CS/ECE 438 - UIUC, Fall 2006 18Types of IFS PIFSPCF interframe spaceMinimum idle time for contention-free service (>SIFS, <DIFS) EIFSExtended interframe spaceUsed 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 windowCount down the backoff interval when medium is idleCount-down is suspended if medium becomes busyWhen 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,


View Full Document

U of I CS 438 - Token Rings

Documents in this Course
Routing

Routing

5 pages

TCP

TCP

26 pages

TROLL

TROLL

3 pages

Load more
Download Token Rings
Our administrator received your request to download this document. We will send you the file to your email shortly.
Loading Unlocking...
Login

Join to view Token Rings and access 3M+ class-specific study document.

or
We will never post anything without your permission.
Don't have an account?
Sign Up

Join to view Token Rings 2 2 and access 3M+ class-specific study document.

or

By creating an account you agree to our Privacy Policy and Terms Of Use

Already a member?