lecture_21_bridges - CS536DataCommunica0ons...

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CS 536 Data Communica0ons and Computer Networks Lecture 21: Bridges and Switches 11/25/2008 Some slides courtesy George Varghese and Nick McKeown 1 CS536 : Computer Networks
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Link Layer 5.1 Introduction and services 5.2 Error detection and correction 5.3 Multiple access protocols 5.4 Link-layer Addressing 5.5 Ethernet 5.6 Link-layer switches 5.7 PPP 5.8 Link Virtualization: ATM, MPLS 2 CS536 : Computer Networks
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Hubs … physical-layer (“dumb”) repeaters: bits coming in one link go out all other links at same rate all nodes connected to hub can collide with one another no frame buffering no CSMA/CD at hub: host NICs detect collisions twisted pair hub 3 CS536 : Computer Networks
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Bridges (precursor to switches) Circa 1980 at DEC: Ethernets under attack Ethernet had limited distance (2.5km) and limited number of stations (8000) Perception that Ethernet collapsed under high loads. Question: How to extend 2 Ethernets to make a larger ethernet that has twice the distance, bandwidth and # of stations. 4 CS536 : Computer Networks
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Bridges Solution 1: Use repeaters Does not work since repeaters/hubs repeat each and every bit, so every node still part of the collision domain. Solution 2: Use routers Routers are expensive Many routers are required if want to interconnect several types of LANs (XEROX, DEC, etc.) Main problem : How to extend LANs transparently. 5 CS536 : Computer Networks
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Bridge Basic idea: Learn the locations of individual machines and selectively repeat frames A B C D Bridge 6 CS536 : Computer Networks
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Code 7 CS536 : Computer Networks
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Bridge Generaliza0ons Any LAN or Data Link that puts both source and destination addresses in frames (includes all types of 802 LANs) Any topology that has no cycles More than two bridge ports. 8 CS536 : Computer Networks
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Realiza0on Need much higher performance than router which handles only frames address to it Need to make decision with 51.2usec on each port. First DEC implementation by Mark Kempf using 1984 technology. Achieved forwarding in min frame time within 1000$s. 9 CS536 : Computer Networks
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Basic architecture 3 bus design to avoid congestion on Proc Bus 4 port memory design Hardware Binary Search takes log (8K)=13 memory accesses of 100nsec ~ 1.3 usec Processor stays in loop after a packet interrupt servicing as many packets to reduce context switching overheads 10 CS536 : Computer Networks
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Switch (new versions of bridges)
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This note was uploaded on 10/24/2009 for the course CS 536 taught by Professor Sonia,f during the Spring '08 term at Purdue University-West Lafayette.

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lecture_21_bridges - CS536DataCommunica0ons...

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