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Part_8_macprocedures

Part_8_macprocedures - Medium Access Control EE450...

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EE450, USC, Zahid 1 Medium Access Control EE450: Introduction to Computer Networks Professor A. Zahid
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EE450, USC, Zahid 2 Medium Access Control z Single shared broadcast channel z Two or more simultaneous transmissions by nodes: interference z collision if node receives two or more signals at the same time Multiple Access Protocol z Distributed algorithm that determines how nodes share channel, i.e., determine when node can transmit z Communication about channel sharing must use channel itself!
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EE450, USC, Zahid 3 Classifications of MAC Protocols Three broad classes: z Channel Partitioning z Divide channel into smaller “pieces” (time slots, frequency, code) z Allocate piece to node for exclusive use z Random Access z Channel not divided, allow collisions z “Recover” from collisions z “Taking turns” z Nodes take turns, but nodes with more to send can take longer turns
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EE450, USC, Zahid 4 IEEE802 Standards for LANs
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EE450, USC, Zahid 5 IEEE802.3 ( Based on Ethernet) “Carrier Sense Multiple Access”
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EE450, USC, Zahid 6 Carrier Sense Multiple Access z CSMA/CD: Carrier sense, multiple access with collision detection z collisions detected within short time z colliding transmissions aborted, reducing waste z Persistent, non-persistent and P-persistent retransmission z Collision Detection: z On baseband bus, collision produces much higher signal voltage than transmitted signal z For twisted pair (Hub-topology) activity on more than one port is collision
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EE450, USC, Zahid 7 Behavior of Three Persistent Scenarios
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EE450, USC, Zahid 8 Collisions in CSMA/CD Collisions can still occur: propagation delay means two nodes may not hear each other’s transmission When collision occur, entire frame is wasted Collision is detected by comparing transmitted and received signal strengths (Hard to do in WLANs, TBD)
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EE450, USC, Zahid 9 Vulnerable Time in CSMA
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EE450, USC, Zahid 10 Collision Detection
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EE450, USC, Zahid 11 Flow Chart of CSMA/CD
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