1. The Way Networks Work Computer Networks Kwangwoon University

2. Fall 2006Computer Networks, Kwangwoon University2 Shared Ethernet

3. Fall 2006Computer Networks, Kwangwoon University3 (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.

4. Fall 2006Computer Networks, Kwangwoon University4 Switched Ethernet

5. Fall 2006Computer Networks, Kwangwoon University5 Loop in Switched Ethernet

6. Fall 2006Computer Networks, Kwangwoon University6 Spanning Tree

7. Fall 2006Computer Networks, Kwangwoon University7 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.

8. Fall 2006Computer Networks, Kwangwoon University8 Ethernets interconnected by Routers

9. Fall 2006Computer Networks, Kwangwoon University9 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.

10. Fall 2006Computer Networks, Kwangwoon University10 A Small Internet

11. Fall 2006Computer Networks, Kwangwoon University11 Virtual Circuits and Routing Tables