1. Sending Bits on the Internet
Lesson 1-3
AP Computer Science Principles

2. Objectives Students will be able to:
Explain how synchronization and coordination enable the transmission of binary messages.
Develop a protocol for exchanging binary messages in two directions.
Calculate the bit rate for a binary message exchange.
Provide a definition of "bit" and relate it to the binary messages they have seen so far.

3. Previously on AP CSP You all made your own binary message devices.
We learned that we could compose any number of messages by sending a sequence of states.
In order to interpret the message we needed to know
which signal meant A and which meant B and
some kind of mapping between sequences of signals and a possible message.

4. What’s Next?
What we were really doing was beginning to develop a communication protocol
Today you’re going to develop a protocol to solve a problem.

5. Case Study:
A binary message has been encoded as a single long colored line. State A was encoded by making the colored line red, and state B was encoded by making the colored line blue.

6. Case Study:
Use the protocol to decode this message into a sequence of A’s and B’s:
Red = A, Blue = B

7. Case Study:
Your friend realizes they actually made a mistake in encoding the message from before and sends this updated version of the message to you. Decode this new version of the message.
Red = A, Blue = B

8. Case Study:
Does this new information change your solution to the first question?
Original message:
New message:
Red = A, Blue = B

9. Vocabulary
We need to get some terminology down so that we can speak about our problems and solutions more efficiently.
Protocol - For our purposes today a “protocol” is simply a set of rules about sending, receiving and interpreting binary messages.
Bit - We will call each element of a binary message a bit. “Bit” is short for binary digit. So for example if you have a binary message A B B A, we would say that is a 4-bit message.

10. Internet Simulator
Today we will be using the Internet Simulator to explore some of the challenges of sending bits on the Internet.

11. Internet Simulator
This tool simulates a single wire connecting two people who cannot otherwise see or speak to each other.
The Internet Simulator helps to enforce “the binary rule” for sending messages

12. Explore the Tool
Your job is to explore this tool with a partner - click all the buttons, type in the text areas what you can.
You cannot break it so don’t worry.
There is a bit of a mystery in what the tool does…and doesn’t do. Can you figure it out?
You and your partner have 5 minutes to poke around and see what you can find.

13. Internet Simulator
Video

14. Activity Guide… Pair up. Download the Activity Guide from the website.
Follow the activity guide. Complete it and turn it in at the end of the lesson.

15. Wrap up…
Go to code studio. Complete all steps through the end of Stage 3