Ethernet.pdf - CS4344 – Computer Networks and Distributed Systems Ethernet Dr Padmaraj Nair Traditional Ethernet Earliest was ALOHA • • • •

Ethernet.pdf - CS4344 – Computer Networks and Distributed...

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CS4344 – Computer Networks and Distributed Systems Ethernet Dr. Padmaraj Nair
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Traditional Ethernet Earliest was ALOHA Developed for packet radio networks Station may transmit a frame at any time If frame is determined invalid, it is ignored Maximum utilization of channel about 18% Next came slotted ALOHA Organized slots equal to transmission time Increased utilization to about 37%
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CSMA Carrier Sense Multiple Access (CSMA) Station listens to determine if there is another transmission in progress If idle, station transmits Waits for acknowledgment If no acknowledgment, collision is assumed and station retransmits Utilization far exceeds ALOHA
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Channel Busy Ready 1-Persistent: 7UDQVPLW DV VRRQ DV channel goes idle ,I FROOLVLRQ± EDFN RII Nonpersistent: 7UDQVPLW LI LGOH ,I EXV\± ZDLW UDQGRP WLPH and repeat process ,I FROOLVLRQ± EDFN RII &RQVWDQW RU YDULDEOH GHOD\ P-Persistent: 7UDQVPLW DV VRRQ DV FKDQQHO JRHV LGOH ZLWK SUREDELOLW\ 3 2WKHUZLVH± GHOD\ RQH WLPH VORW and repeat process ,I FROOLVLRQ± EDFN RII WLPH CSMA persistence and back off
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Nonpersistent CSMA If the medium is idle, transmit; otherwise, go to step 2 If the medium is busy, wait an amount of time drawn from a probability distribution and repeat step 1 Disadvantage: Capacity is wasted because the medium will generally remain idle following the end of a transmission even if there are one or more stations waiting to transmit
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1-Persistent CSMA Avoids idle channel time Rules: 1. If medium is idle, transmit 2. If medium is busy, listen until idle; then transmit immediately Stations are selfish If two or more stations are waiting, a collision is guaranteed
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P-Persistent CSMA A compromise to try and reduce collisions and idle time P-persistent CSMA rules: 1. If medium is idle, transmit with probability p , and delay one time unit with probability (1– p ) 2. If medium is busy, listen until idle and repeat step 1 3. If transmission is delayed one time unit, repeat step 1 Issue of choosing effective value of p to avoid instability under heavy load
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Value of p ? Have n stations waiting to send At end of transmission, expected number of stations is np If np >1 on average there will be a collision Repeated transmission attempts mean collisions are likely Eventually all stations will be trying to send, causing continuous collisions, with throughput dropping to zero To avoid catastrophe
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