lecture7 - Alan Mislove amislove at ccs.neu.edu...

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Unformatted text preview: Alan Mislove amislove at ccs.neu.edu Northeastern University 1 CS4700/CS5700 Fundamentals of Computer Networks Lecture 7: Ethernet / Wi-Fi media access control Slides used with permissions from Edward W. Knightly, T. S. Eugene Ng, Ion Stoica, Hui Zhang Alan Mislove amislove at ccs.neu.edu Northeastern University 2 Overview Ethernet and Wi-Fi are both multi-access technologies Broadcast medium, shared by many hosts Simultaneous transmissions will result in collisions Media Access Control (MAC) protocol required Rules on how to share medium Alan Mislove amislove at ccs.neu.edu Northeastern University 3 Media Access Control Protocols Channel partitioning Divide channel into smaller pieces (e.g., time slots, frequency) Allocate a piece to node for exclusive use E.g. Time-Division-Multi-Access (TDMA) cellular network Taking-turns Tightly coordinate shared access to avoid collisions E.g. Token ring network Contention Allow collisions recover from collisions E.g. Ethernet, Wi-Fi Alan Mislove amislove at ccs.neu.edu Northeastern University 4 Contention Media Access Control Goals Share medium If two users send at the same time, collision results in no packet being received (interference) If no users send, channel goes idle Thus, want to have only one user send at a time Want high network utilization TDMA doesnt give high utilization Want simple distributed algorithm no fancy token-passing schemes that avoid collisions Alan Mislove amislove at ccs.neu.edu Northeastern University 5 Evolution of Contention Protocols Alan Mislove amislove at ccs.neu.edu Northeastern University 5 Evolution of Contention Protocols Aloha Alan Mislove amislove at ccs.neu.edu Northeastern University 5 Evolution of Contention Protocols Developed in the 1970s for a packet radio network Aloha Alan Mislove amislove at ccs.neu.edu Northeastern University 5 Evolution of Contention Protocols Developed in the 1970s for a packet radio network Aloha Slotted Aloha Alan Mislove amislove at ccs.neu.edu Northeastern University 5 Evolution of Contention Protocols Developed in the 1970s for a packet radio network Aloha Slotted Aloha Improvement: Start transmission only at fixed times (slots) Alan Mislove amislove at ccs.neu.edu Northeastern University 5 Evolution of Contention Protocols Developed in the 1970s for a packet radio network Aloha Slotted Aloha Improvement: Start transmission only at fixed times (slots) CSMA Alan Mislove amislove at ccs.neu.edu Northeastern University 5 Evolution of Contention Protocols Developed in the 1970s for a packet radio network Aloha Slotted Aloha Improvement: Start transmission only at fixed times (slots) CSMA CSMA = Carrier Sense Multiple Access Improvement: Start transmission only if no transmission is ongoing Alan Mislove amislove at ccs.neu.edu Northeastern University 5 Evolution of Contention Protocols Developed in the 1970s for a packet radio network...
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lecture7 - Alan Mislove amislove at ccs.neu.edu...

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