The Way Networks Work

Computer Networks

Kwangwoon University

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Shared Ethernet

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(Ethernet) Switches

• A switch can– interconnects two or more Ethernets, or

– replace the hub of an Ethernet. ( Switched Ethernet)

• A switch can transmit multiple packets at the same time

Larger throughput than a shared Ethernet

• Redundant switches for reliability– If one switch fails, there is another path for the packets.

– But it creates loops in which packets are copied forever!

• Spanning tree algorithm– To prevent a loop, the switches run the algorithm to select a tree.

– Only the switches that are on the tree copy packets.

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Switched Ethernet

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Loop in Switched Ethernet

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Spanning Tree

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Router

• Ethernets can be interconnected also by routers.

• The differences between routers and switches:

– Router decides where to send a packet next. (Routing)

– The decision is made for the best path to the destination.

– Router uses network address to forward a packet.

– Router modifies the Ethernet address of a packet to relay it among multiple Ethernets.

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Ethernets interconnected by Routers

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The Internet

• The Internet is a global network of networks, in which– The WWW hiperlinks documents around the world.

– The documents, written by HTML, include text, audio, video, and other links.

– Users navigate the WWW through Web browsers.

• Shortcomings of the Internet– Best-effort services : No guarantee on the delays or transmission losses

– Unreliable network and security

• The Internet has self-organizing mechanisms– Packets find their ways to their destination

– If a link or a router break down, packets automatically transit along other paths.

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A Small Internet

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Virtual Circuits and Routing Tables