1. Unit 1, Lesson 3 Evolution of Networks AOIT Computer Networking Copyright © 2008–2013 National Academy Foundation. All rights reserved.

2. Why do we need networks? Say you’re stranded on a desert island. What’s the best way to get rescued? Make smoke signals: SOS? Send a message in a bottle? Strap a message to the leg of a carrier pigeon? Use a mirror to flash a code to a passing ship? Call your friends via cell phone? What are the elements you need to create a successful network?

3. The first networks used visual signals such as torches, smoke signals, and flags Ancient Greece and medieval Europe used these. Towers signaled from town to town. Cities signaled each other when under attack. Limitations Geography and weather limit visual range. Long distances require relay stations.

4. The telegraph is an electromechanical device How does it work? Electronic pulses are sent across a wire. The pulses move a magnet. The magnet moves a marker to write codes on a slip of paper. The operator translates the code to English.

5. The telegraph was the “Victorian Internet” In the 1840s, the telegraph was revolutionary for its time: Complex, quick messages via Morse code dits and dahs No visual contact needed Instantaneous communication: “lightning wires” No barriers across space: “the annihilation of space”

6. Wireless radio was developed in the early 1900s Soon after the telegraph became popular, wireless radio was developed. Guglielmo Marconi broadcast the first transoceanic message in 1901. The radiotelegraph broadcast Morse code wirelessly via radio waves. It was used for ship-to- ship and ship-to-shore communication. When the Titanic sank, the radio distress call saved 700 lives.

7. Radio broadcasting uses channels The radio station’s call numbers—like 90.5 in the image—represent the broadcast frequency. Frequencies are assigned and “licensed” (sold) by the FCC. Different kinds of technologies use different frequencies. How many different spectrum technologies can you think of? (Radio, TV, microwave…)

8. Electromagnetic waves are on a spectrum Visible Light FM Radio/TV Shortwave & AM Radio Low Frequency Generators

9. Broadcast radio changed people’s lives Do you know the difference between AM and FM radio?

10. Television has advanced from 1930s to today How is TV transmitted? Sound uses one channel Video (light) uses another Color is represented through a phase shift, like so: Modern TV signals are transmitted digitally instead of analog

11. Networks have evolved a lot in two millennia 146 BC — Greeks use torch signals 1830s — Electric telegraph is demonstrated 1870s — First telephones developed 1901 — Wireless telegraphy is demonstrated 1920s — AM radio begins broadcasting 1930s — Television begins broadcasting 1960s — FM radio is developed 1970s — The first computer networks develop