Teaching Tips

Teaching Tips: The Codebreaker

Synopsis:
In this module, students use a simulation to code and decode binary messages to communicate with their friends. The lesson investigates communication through the ages, and the fact that humans have long used patterns to communicate. Issues related to technological tradeoffs (size vs. accuracy) are examined. Students can create coded messages to give to their lab partners who in turn use the simulator to decode them.

Big Ideas:
Before electronic communication, humans used patterns to communicate
The use of technology always involves trade-offs
Digital communication involves binary data
Images can be reduced to a series of zeros and ones

Misconceptions Addressed:
Technology is inherently efficient
Communication between humans always implies the use of words
Base 10 is the only number system

NSES Grade 5-8 Standards
E. Science and Technology
i. Abilities of technical design
ii. Understandings about science and technology
F. Science in Personal and Social Perspectives
i. Science and technology in society
G. History and Nature of Science
i. Science as a human endeavor

Communication Patterns

The Codebreaker PSI Sim™ is designed to motivate students to think about communication patterns in general, and computer communication in particular. Often textbook discussions on binary coding are taught as a mathematics lesson, with no context to the larger issue of human communication. Nested within The Codebreaker Experiment is the idea of cleverness, that human beings have been clever in their communications choices. Communication patterns are sometimes efficient, sometimes descriptive, sometimes intentionally vague, and sometimes fleeting. An interesting classroom discussion might focus on how human communication changed when the written word became common. The printing press made it possible to archive complex legal documents and maintain a reference library. Even the way we speak was affected by the permanence of the written word.

Students using The Codebreaker should realize that in order to decode the messages they create, the decoder has to use the same system (in this case The Codebreaker decoder). In other words, if the decoder didn't know that each letter had 25 digits and was built in a square grid, it would be harder to decode the message. Similarly, there had to be and understanding between parties who used smoke signals. Experts believe that different Native American tribes used different signals, that is, that there was no standard code. People in China also used smoke signals, and high towers on the great wall served as signal stations so that the signals could be seen at a great distance. Smoke signals are formed by covering a fire (often created in a stone bowl) with a blanket and then quickly removing it. With practice, one can control the size and timing of the smoke puffs. Some examples of smoke signal patterns known to exist are: one short puff meant an enemy was approaching, while many short puffs meant the enemy was formidable (many well-armed individuals). Boy scouts teach that three signals of any kind ALWAYS indicates danger or a call for help, and they caution scouts about using that signal for frivolous purposes. In fact, they teach the scouts to use three shouts, or three whistles, or three fires in a row, or three shots if they are in serious trouble.

In a completely different manner, smoke signals are used as part of an age old tradition by Cardinals electing a new Pope. In the morning and evening, they place handwritten ballots in a chalice on top of an alter. To insure secrecy, the ballots are counted and then burned. If a two-thirds majority of votes for a candidate is not obtained, the ballots are mixed with a chemical before they are burned. The chemical makes the smoke appear black, and observers outside the voting hall know that the vote was unsuccessful.

Binary Considerations

The Codebreaker can be used to discuss compression algorithms, but care must be taken with number systems. The codes in The Codebreaker are binary, using only two digits. If you ask students to think of a way to compress a single letter, they might say something like 5 ones, two zeros, 1 one, 4 zeros, etc. While this might seem like a good compression method, in a way it is cheating! the numbers 2, 5, and 4 do not exist in base two, so they are using base 10 to compress a message in base 2. If we had only two digits (for example, zero and one, or on and off, or light or dark) we couldn't express "five" directly (that is, using only one digit. For example the number 5 in base two is 101, three digits long, not one.

Pixels

The Codebreaker offers a great opportunity to discuss the idea of tradeoffs. For example, the idea of the length of a message is directly related to tradeoffs. In simulation, each letter is built with 25 squares (5 X 5), or picture elements (pixels). This makes it difficult to make some letters look correct. K for example, is a bit difficult. It would be easier to make letters if we had more resolution, that is pixels in our letter grid. If we had and 8 X 8 grid, we could get more detail. That would be better, but there would be a price! It would mean that each letter would take 64 digits instead of 25. More resolution, more storage overhead. Technological considerations often involve tradeoffs. Another example has to do with color. To express a black and white pixel, we need two digits (zero and one), to express color, we need more digits.

Bar Codes and RFID Tags

Students might be asked to discuss the impact of bar codes on shopping. Although bar codes have been around as long as they remember, they might be asked for the advantages and disadvantages of using bar codes. Contrast these with RFID tags that are often found on more expensive clothing and goods. A basic RF tag, is a one-bit tag that signals an alarm if it is on. When goods are purchased, the tag is swiped by the clerk and becomes a zero, no longer triggering the alarm. Newer RFID tags will carry much more information (and be more expensive). These tags will allow a product to be tracked, or allow appliances to "know" what products they are dealing with (a refrigerator, for example, could regulate the temperature based on what products were in it). One manufacturer claims that their scanner can read 1000 items at a time within a 4 meter range. This means that check-outs could be much faster, and people could bag their groceries as they shop. It also means that stores could see what products were in your bag when you walk in, and salespeople (or sales stands) could be alerted to tailor their sales pitch to you. For this reason, privacy groups are concerned with the widespread use of RFID tags.