UCI University of California, Irvine EECS 161 - Introduction to Computer Networking Scott Jordan Local Area Networks / Ethernet Ethernets connected by Routers: (Walrand fig. 2.5) A Packet Router is like a Packet Switch, but it can also modify the packet header (in particular the Ethernet addresses). UCI University of California, Irvine EECS 161 - Introduction to Computer Networking Scott Jordan Local Area Networks / Ethernet Ethernets connected by Routers: Example: At source, Ethernet header contains (s,r1). IP header contains (S,D). Router R s routing table contains an entry (D, E2). It copies the packet to Ethernet E2 and modifies the Ethernet header from (s,r1) to (r2,d). UCI University of California, Irvine EECS 161 - Introduction to Computer Networking Scott Jordan Local Area Networks / Ethernet Ethernets connected by Routers: Note the router needs both a routing table and an ARP table. Also note that the router completely segments the Ethernets, e.g. E1 doesn t need to know Ethernet addresses on E2. UCI University of California, Irvine EECS 161 - Introduction to Computer Networking Scott Jordan Local Area Networks / Ethernet Store and Forward architecture (Walrand fig. 1.4) Each computer/switch/router waits until it has received the entire packet before beginning transmission of the packet onto the next link. Note transmission delay versus propagation delay versus queueing delay !! UCI University of California, Irvine EECS 161 - Introduction to Computer Networking Scott Jordan Local Area Networks / Ethernet Packet Switching UCI University of California, Irvine EECS 161 - Introduction to Computer Networking Scott Jordan Local Area Networks / Ethernet Circuit-Switching versus Packet-Switching: Circuit-Switching less per packet processing lower loss & delay Packet-Switching less call set-up more efficient use of capacity UCI University of California, Irvine EECS 161 - Introduction to Computer Networking Scott Jordan Local Area Networks / Ethernet Terminology gateway - also does protocol conversion (layer 4) router - modifies packet headers (layer 3) switch - simultaneous routing of multiple packets (layer 2) bridge - connects two LANs and simply copies all packets from one side to the other