CS 101 – Nov. 20 Communication, continued (Ch. 15) Cryptography –examples Error detection Local Area Networks –Different ways to hook up machines

Encryption examples Caesar cipher √ Cryptogram One-time Pad & Japanese Naval Codes –Dictionary table –Additive table –Destroy each page as you use it

Breaking codes Like solving a cryptogram Distribution of letters (‘e’ versus ‘q’) Digraphs, trigraphs Common words Eric Nave’s “Betrayal at Pearl Harbor”

Errors in transmission Random flipping of bits Prepare by using parity bit –Add 9 th bit to each byte –Goal is that each byte has even # of 1’s –Receiver checks each byte –Doesn’t solve every error!

Local Area Networks Sharing resources in a lab –File server –Printer Communication based on topology –point-to-point  –star  –bus (like speaking at dinner table) –ring (need token to send message)

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. …………………………………………