1. CIS 235: Networks Fall, 2007 Western State College Welcome to CIS 235 Computer Networks Fall, 2007 Prof Peterson

2. CIS 235: Networks Fall, 2007 Western State College Homework This is due on Friday in class! Please post only the last question in the wiki – the rest is to submitted by email. We’ll be addressing math and timelines today. Questions?

3. CIS 235: Networks Fall, 2007 Western State College The Internet How does the Internet work? What do we need to understand about the Internet?

4. CIS 235: Networks Fall, 2007 Western State College The Internet All computers have an “IP address”, like a street address, by which they can be referred to. This can change with context Messages are broken into small chunks (packets) Routers move packets to the right place Communication links are multiplexed Protocols handle errors nicely Symbolic names (like western.edu) allow you to avoid seeing IP addresses directly

5. CIS 235: Networks Fall, 2007 Western State College The Internet Your computer can respond to unsolicited outside requests via “services ” Encryption prevents eavesdroppers from listening in. Resources are shared “nicely” when capacity is limited Public protocols are shared by all users Network applications (like web browsers) use application specific protocols Caching improves performance

6. CIS 235: Networks Fall, 2007 Western State College Who Owns the Internet?

7. CIS 235: Networks Fall, 2007 Western State College Who Owns the Internet? Nobody! It’s just a bunch of computers that happen to talk to each other. Contracts may provide a legally enforceable obligations on operators but for the most part you can’t force other people to do things for you. Basic protocols are managed by a worldwide consortium but nobody forces you to do what they say.

8. CIS 235: Networks Fall, 2007 Western State College Math Basics We need to understand the numbers inside the network. These are typically: Sizes – number of bits (bytes) Speed – almost always the speed of light for a transportation device Rate – how fast you can push information into a link. Usually in bits / second

9. CIS 235: Networks Fall, 2007 Western State College Conversions Bits to bytes: multiply / divide by 8 Inches to feet to miles to kilometers: constants you need to look up (.6 mi = 1km, 1.6 km = 1mi) Powers of 10: know these: Kilo = 1,000 (mille) Mega = 1,000,000 (micro) Giga = 1,000,000,000 (nano) Tera = 1,000,000,000,000 (pico) Speed of light: ~200,000 miles / second Equator to pole distance: 10,000 km

10. CIS 235: Networks Fall, 2007 Western State College Math Problems Assume: Distance to Taylor Hall is 500’ Packet size = 1000 Bytes Link speed = 1 Gbit / sec Draw a timeline of this packet going to Taylor Hall. If the computer in Taylor responds with a packet of the same size, how long will the round trip take?

11. CIS 235: Networks Fall, 2007 Western State College Math Problems Assume an average packet is 1500 Bytes How many packets per second would arrive over a 100mB internet connection at full speed? Over a 1gB connection? How many packets would a 1gB router have to dispatch per second?

12. CIS 235: Networks Fall, 2007 Western State College Math Problems Suppose the distance to Australia is 12,000 miles. How long is the delay when a packet is sent assuming it is moving at the speed of light?

13. CIS 235: Networks Fall, 2007 Western State College Math Problems How long does it take a packet moving at the speed of light to cross the US if the US is 3000 miles across? (Assume that the speed of light is 200,000 miles per second) If a link is transmitting information at 19 MBit/second (average 802.11g speed), how long would it take to move a 4 megabyte file through this link?