CS 101

1
CS 101 Nov. 23

• Communication, continued
• LANs
• Bus (ethernet) communication
• Token ring communication
• How the Internet works TCP/IP

2
Bus topology

• All machines share the same channel
• All continuously listening
• Ethernet protocol (dinner table)
• dont talk when someone else is talking
• collisions
• Amplify signal with repeaters

3
Token ring

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

4
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

5
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.

6
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.

7
TCP

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

8
IP

• Internet Protocol
• Every machine has IP address
• 156.143.128.20
• 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.

9
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

10
Snooping

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