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L19 - CAPTURE EFFECT If two packets are transmitted and one...

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CAPTURE EFFECT. If two packets are transmitted and one is considerably stronger, the stronger one might be successful (capture effect). J.J. Metzner, "On improving utilization in ALOHA Networks", IEEE Transactions On Communications, April 1976. Basic idea of paper - have two classes of sendings: high power and low power. A high power transmission is successful iff no other high power transmission occurs at the same time. A low power transmission is successful iff no other transmission occurs at the same time. Maximum efficiency rises from .368 To about .53 in slotted ALOHA case.
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Carrier Sense Multiple Access (CSMA) and with collision detection CSMA/CD. CSMA/CD forms basis for ETHERNET. By listening before transmission, the chance of collision is greatly reduced, especially if propagation time << frame duration. But there still can be collisions: Given A starts, there will be a collision if some B 1 starts somewhere in the time interval between t 2 and t .
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Efficiency can be improved if a station stops as soon as collision is detected (CSMA/CD) - less time wasted with collision.
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Ethernet uses CSMA/CD. What to do on collision? ETHERNET uses "exponential backoff". Retransmission scheduled at a randomly chosen waiting time between 0 and some T. Each time another collision, the value of T is doubled . (Up to some maximum) Exponential backoff helps stability: if there lots of collisions the colliders, wait gets longer, which reduces attempts/slot.
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What to do when sense channel busy?
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