Last 2008 - Ethernet(CSMA/CD Carriers Sense Multiple Access...

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Unformatted text preview: Ethernet (CSMA/CD) Carriers Sense Multiple Access with Collision Detection IEEE 802.3 IEEE802.3 Medium Access Control Random Access Stations access medium randomly Contention Stations contend for time on medium What were the precursors for CSMA/CD? ALOHA Packet Radio 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% 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% 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) No ACK then retransmit Max utilization depends on propagation time (medium length) and frame length Longer frame and shorter propagation gives better utilization Nonpersistent CSMA 1. If medium is idle, transmit; otherwise, go to 2 2. If medium is busy, wait amount of time drawn from probability distribution (retransmission delay) and repeat 1 Random delays reduces probability of collisions Consider two stations become ready to transmit at same time While another transmission is in progress If both stations delay same time before retrying, both will attempt to transmit at same time Capacity is wasted because medium will remain idle following end of transmission Even if one or more stations waiting Nonpersistent stations deferential 1-persistent CSMA To avoid idle channel time, 1-persistent protocol used Station wishing to transmit listens and obeys following: 1. If medium idle, transmit; otherwise, go to step 2 2. If medium busy, listen until idle; then transmit immediately 1-persistent stations selfish If two or more stations waiting, collision guaranteed Gets sorted out after collision P-persistent CSMA Compromise that attempts to reduce collisions Like nonpersistent And reduce idle time Like1-persistent Rules: 1. If medium idle, transmit with probability p, and delay...
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Last 2008 - Ethernet(CSMA/CD Carriers Sense Multiple Access...

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