CS 101 – Nov. 23 Communication, continued LANs –Bus (ethernet) communication –Token ring communication How the Internet works: TCP/IP

Bus topology All machines share the same channel All continuously listening “Ethernet” protocol (dinner table) –don’t talk when someone else is talking –collisions Amplify signal with repeaters

Token ring Fast messaging over larger distances Logically arranged in loop Messaging: Token passed around the ring. –Am I busy? –From –To –message

Token action When you receive token: Is the message for me? –If so, read and change to ack. –If not, just pass token. Do I need token? –Wait until it comes back as not “busy”

Example 4 machines: A, B, C, D. A has message for C. –When A gets token, writes message for C. –B passes token. –C receives msg, sends ack message to A. –D passes token. –A receives ack, clears token. …

Example #2 We have 4 machines A, B, C, D. What happens when… –Token starts at D. –A has a message for C. –C has a message for A. –B has a message for D. …………………………………………

TCP Transfer Control Protocol Break up message into packets Sequence number on each packet Receiver detects lost packets How big should a packet be?

IP Internet Protocol Every machine has IP address – Packets are individually addressed Routers Not all packets go the same way! –Just like 2 people driving to Seattle might not take the exact path.

Behind the scenes “ping” program –Send empty packets, check for ack –75 ms to San Diego… 840 ms to Pretoria “traceroute” program –Lists all routers between here and another machine

Snooping The “dark side” of TCP/IP Web site can track you by IP address –Localized marketing –Privacy concerns Anonymous IP servers –Ex. “Anonymouse”