20-Ethernet - Multiple Access and Spanning Tree EE122 Fall...

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1 Multiple Access and Spanning Tree EE122 Fall 2011 Scott Shenker http://inst.eecs.berkeley.edu/~ee122/ Materials with thanks to Jennifer Rexford, Ion Stoica, Vern Paxson and other colleagues at Princeton and UC Berkeley
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Upcoming lectures • Next week: Security by Vern Paxson – I will be absent • Following Monday: Yahel the wireless rock star… – I will be in the mosh pit • That Wednesday: you are on your way home – Enjoy! – and then finals…. 2
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Announcements • Office hours: – This week: as scheduled – Next week: stay tuned… – Available after class • HW4: only one week to finish it (later in semester) – With no slip days (so we can post answers) – But will be quite easy – With a very hard worksheet….that is never due. 3
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Today • Study two algorithms that are dying out – But both important conceptually! • Multiple Access in wired media (extinct) • Spanning Tree (endangered algorithms list) 4
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5 Point-to-Point vs. Broadcast Media • Point-to-point: dedicated pairwise communication – Long-distance fiber link – Point-to-point link between Ethernet switch and host • Broadcast: shared wire or medium – Traditional Ethernet – 802.11 wireless LAN
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6 Multiple Access Algorithm • Single shared broadcast channel – Must avoid having multiple nodes speaking at once – Otherwise, collisions lead to garbled data – Need distributed algorithm for sharing the channel – Algorithm determines which node can transmit • Classes of techniques Channel partitioning : divide channel into pieces Taking turns : scheme for trading off who gets to transmit Random access : allow collisions, and then recover
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7 Channel Partitioning: TDMA TDMA: Time Division Multiple Access • Access to channel in "rounds" – Each station gets fixed length slot in each round • Time-slot length is packet transmission time Unused slots go idle • Example: 6-station LAN with slots 0, 3, and 4 Rounds 0 1 2 3 4 5 0 1 2 3 4 5 Slots =
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8 Channel Partitioning: FDMA FDMA: Frequency Division Multiple Access • Channel spectrum divided into frequency bands • Each station assigned fixed frequency band • Unused transmission time in frequency bands go idle • Example: 6-station LAN, 1,3,4 have pkt, frequency bands 2,5,6 idle frequency bands FDM cable
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9 Taking Turns MAC protocols Polling • Master node invites slave nodes to transmit in turn • Concerns: – Polling overhead – Latency – Single point of failure (master) Token passing • Control token passed from one node to next sequentially • Node must have token to send • Concerns: – Token overhead – Latency – At mercy of any node master slaves poll data data
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None of these are the “Internet way”… • Why not? • What’s wrong with
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This note was uploaded on 01/09/2012 for the course ELECTRICAL 122 taught by Professor Shenker during the Spring '11 term at Berkeley.

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20-Ethernet - Multiple Access and Spanning Tree EE122 Fall...

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