2a_ethernet - Ethernet CSMA/CD Review CSE 6590 Winter 2012...

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1 Ethernet – CSMA/CD Review CSE 6590 Winter 2012 February 14, 2012
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Characteristics of Some High-Speed LANs 2
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3 Ethernet (CSMA/CD) Medium access control: Carrier Sense Multiple  Access with Collision Detection Xerox - Ethernet IEEE 802.3
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CSMA/CD Precursors 4
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5 ALOHA Developed for packet radio networks. When station has frame, it sends. Station listens (for max round trip time) plus small  increment. If ACK, fine.  If not, retransmit. If no ACK after repeated transmissions, give up. Frame check sequence (as in HDLC). If frame OK and address matches receiver, send ACK. Frame may be damaged by noise or by another station  transmitting at the same time (collision). Any overlap of frames causes collision. Max utilization 18%.
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6 Slotted ALOHA Time in uniform slots equal to frame  transmission time. Need central clock (or other sync mechanism). Transmission begins at slot boundary. Frames either miss or overlap totally. Max utilization 37%.
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CSMA Propagation time is much less than transmission time. All stations know that a transmission has started almost  immediately. First listen for clear medium (“carrier sense”). If medium idle, transmit. If two stations start at the same instant, collision. Wait reasonable time (round trip  plus ACK contention).
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2a_ethernet - Ethernet CSMA/CD Review CSE 6590 Winter 2012...

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