Lecture-07 - Lecture #07 Todays Lecture Objectives Review...

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Lecture #07
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Today’s Lecture Objectives • Review • Medium Access Control
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Data Link Layer Objectives (Reminder) • Functions – framing – error control – flow control – medium access control • Point-to-Point protocol examples: PPP • MAC functionality – techniques – protocols (mainly Ethernet) • MAC addressing
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Medium Access Control Part of Data Link Layer sometimes called a “sub-layer” Function assumes a “shared medium” deals with who gets to transmit and when
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Medium Access Control “Metrics” What makes a good MAC protocol? Factors Fairness: stations should get equal opportunity to transmit Of course, assuming it’s an explicit goal Simplicity: both operationally and from a management erspective perspective Efficiency: use the channel as much of the time as possible Challenges Tradeoff between minimizing wait time and maximizing throughput Too much load leads to collapse
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Types of MAC Protocols Scheduled Random-Access Fixed Assignment On- Demand No Sense Sensing TDMA FDMA WDMA Polling Token Aloha Slotted Aloha CSMA CSMA/CD
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Misc Medium Access Control Polling: master/slave architecture with master seeing asking if stations want to transmit centralized Token: token allows station to transmit, must be passed decentralized Aloha just transmit simple, but very inefficient key: collision period Slotted Aloha just transmit… but only on slot boundaries a bit less simple, still mostly inefficient, but better than Aloha key: collision period is halved
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ALOHA and Slotted-ALOHA 0.3 0.4 0.5 (ALOHA) Slotted ALOHA: Re -R Ideal (no collisions): R 0 0.5 1 1.5 2 2.5 3 0 0.1 0.2 R Throughput ( Pure ALOHA: Re -2R
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Carrier Sense Multiple Access Listen for transmission, i.e. see if “the line is
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This note was uploaded on 02/19/2012 for the course ENGR 361 taught by Professor Drexel during the Spring '12 term at Bloomsburg.

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Lecture-07 - Lecture #07 Todays Lecture Objectives Review...

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