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6610-l2- - Medium Access Control Puzzle How can a toss be...

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Medium Access Control
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Puzzle How can a toss be called over the phone (without requiring trust)?
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Overview Medium Access Control ALOHA, slotted-ALOHA, CSMA, CSMA/CD Scheduling FCFS, Priority scheduling Fair queuing, Weighted fair queuing Round robin, Weighted round robin Deficit round robin Class based queuing
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Medium Access Control When multiple stations share a common channel, the protocol that determines which station gets access to the shared channel Key characteristics based on which MAC protocols are evaluated: utilization and fairness
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ALOHA ALOHA When a station wants to transmit, it transmits Collisions detected at a higher layer and retransmissions done as required Simple logic Highly inefficient at large loads. Maximum utilization of 18% at a mean load of 0.5 transmissions/slot
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Slotted ALOHA Stations can transmit only at the beginning of pre-determined slots Reduces the “vulnerable period” for collisions More efficient Maximum utilization of 36% at a mean load of 1 transmission/slot
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Carrier Sense Multiple Access Station wishing to transmit senses channel. If channel idle, transmits. Else, backs-off and tries later Carrier sensing hardware required More efficient than both versions of ALOHA 3 flavors of CSMA
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CSMA (contd.) 1-persistent CSMA On finding channel busy, station continues listening and transmits when channel becomes idle p-persistent CSMA On finding channel idle, station transmits with a probability of p, backs-off and tries again when channel is busy non-persistent CSMA On finding channel busy, station backs-off for a random amount of time and tries later
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CSMA/CD In CSMA, when there is a collision of two transmissions, it is detected only after the entire frames have been transmitted … irrespective of when the collision occurs CSMA/CD includes a collision detection mechanism that can detect collisions even as stations are transmitting Transmitting stations terminate transmissions upon collision detection and try later
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10 CSMA/CD (Contd.) When will the performance of CSMA/CD be the same as that of CSMA? IEEE 802.3 Ethernet LAN Standard 802.3u – fast ethernet 802.3ab – gigabit ethernet 802.3ae – 10G ethernet Full-duplex Ethernet
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Other MAC Schemes Collision free protocols Bit map protocol Binary countdown protocol Limited contention protocols Adaptive tree walk protocol
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Recap Class goals and overview Grading and other administrative stuff Communication networks OSI Protocol Stack
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13 Scheduling
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14 Scheduling When a station gets to transmit, the protocol that determines which packet gets to be transmitted Fairness the primary consideration Weighted fairness … an extension Default scheduling in today’s Internet?
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15 Scheduling Policies … FCFS Packets queued on a first-come-first-served basis No fairness Priority
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