4605-l2- - Medium Access Control Puzzle How can a toss be...

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Unformatted text preview: Medium Access Control Puzzle How can a toss be called over the phone (without requiring trust)? 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 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 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 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 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 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 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 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 Other MAC Schemes Collision free protocols Bit map protocol Binary countdown protocol Limited contention protocols Adaptive tree walk protocol Recap Class goals and overview Grading and other administrative stuff Communication networks OSI Protocol Stack 13 Scheduling 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 todays Internet? 15 Scheduling Policies FCFS Packets queued on a first-come-first-served basis No fairness...
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4605-l2- - Medium Access Control Puzzle How can a toss be...

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