ASCII messages:

ASCII MESSAGES

Download the Word file.

Approximate Time:

1:30 hours

Materials needed:

Pencils (one per student)

ASCII charts (at least one per group)

lots of slips of paper with squares for writing the code that look like this:

At least two helpers to be �servers�

Process:

Split class into groups of two or three.� Each group is a �computer�.� Name these computers with a letter of the alphabet: A, B, C, D, etc. Assign each �computer� another �computer� to send a message to (for example: A sends to B; B sends to C; C sends to D; D sends to A)

Explain to the class that computers send messages over networks like the internet by first encoding the messages, then sending.� The receiving computer must then take the encoded message and translate them back into data interpretable to the viewer.� Explain that ASCII is a type of coding used by computers, changing letters into numbers because a computer can only send numerical values [ASCII= American Standard Code for Information Interchange].� Hand out the ASCII charts and have each group create a short message (no more than 20 characters, including spaces) to be sent to another �computer� and put it into ASCII.� For example:

R - 082

o - 111

p - 112

e - 101

� - 032

t - 116

r - 114

i - 105

c - 099

k - 107

s - 115

� - 032

a - 097

r - 114

e - 101

� - 032

f - 102

u - 117

n - 110

! - 033

Next, explain that a computer, when decoding, must have some way to ensure that the packets have not been compromised, and that the way to do this is to have a way to check that none of the numbers have been accidentally changed.� So we add a number to the end, a �parity bit,� that will be used to check the validity of that letter.� We will do this by adding a number to the end that will make the sum of the digits for that letter divisible by ten:

R: 0 + 8 + 2 = 10 + 0 = 10; 0820

o: 1 + 1 + 1= 3 + 7 = 10; 1117

Or if the digits add up to more than 10, have them add to 20:

c: 0 + 9 + 9 = 18 + 2 = 20; 0992

You can have the students figure out the pattern before you tell them how.� Explain how patterns are important for mathematicians/computer science/computational science and have them find the algorithm.

1) All groups should now have a message and its code: four numbers per letter.� Have each computer fill out ONE AND ONLY ONE packet to begin to send their message.�

Make sure the students recognize that the packets do not cleanly fit into �letters�.� Here we have 2 and 3/4 letters encoded.� The next packet would start with the last digit of the third letter and then start on the fourth.

Have the �servers� collect the packets and bring them to the front of the room.� The servers will look at the packets, be confused, and crumple the packets up and throw them away.�

[REASON: Packets have no direction to servers on where to send them]

2) Next, have the groups fill out another packet with the letter of who they�re sending it to:

The �servers� collect them again, and take a packet to its destination.� This time, the servers repetitively ask the computer for confirmation: �OK, computer B? OK, computer B? OK, computer B? OK, computer B?��

[REASON: Server needs to send verification back to sending computer]

Eventually, the group will get annoyed and say something like �you can go now.�� Explain that this is exactly what the server needs to be told, and have the computers write �VERIFY� on the back of the packet after WRITING DOWN THE NUMBERS!!!� The servers will bring the verified messages to the front of the room, look at them, and then throw them away.

[REASON: Packets have no return address]

3) Have the groups fill out yet another packet, this time with both to and from addresses.� Let the class decide on the protocol. Explain what �protocol� means and how you use it in transferring information.� Explain that all of what we having been doing so far has been defining protocol: 4 numbers for a letter/to and from spaces in a packet/etc. One example of protocol is �to� at the beginning and �from� at the end:

Have the students now fill out THREE packets in this manner and send them.� The servers collect them, give them OUT OF ORDER to the computers, and return the verified packets to the senders.� Ask the students how they can tell which packets come in what order (they can�t), and decide to add yet another thing to our protocol:

Packet number

From

****THIS IS THE FINAL PROTOCOL FOR PACKETS*****

Finally, explain that the parity bit can be used to ensure that the Servers don�t mess things up in delivery, so the Computers should check that the numbers add up before writing �VERIFY�.� If they don�t (sometimes they will have to wait for another packet to get all of a letter�s numbers.� Remember that the messages don�t start at the beginning of a letter, just where the letter code left off), write �REJECT� on the back.� The rejected message will go back to the sending Computer and that Computer will know to resend the packet.� If a packet doesn�t come at all, a Computer can request a resend by the following protocol:

Computer to send info to

Requested Packet number

Computer that has the information

and �RESEND� written on the back

Example:

Computer B didn�t get packet 03 from Computer A.� The request for resend would look like this:

with �RESEND� written on the back.

When A receives this message, it simply fills in the packet numbers and crosses out the �RESEND� on the back, and sends out the packet again.

****At this point, the Computers and Servers can just go about their business of transferring messages.� Emphasize teamwork: messages will be going out and coming in simultaneously.

Summary of Server Protocol:

If you have a packet that you can�t give to a computer, crumple it up and throw away (first with no �to�, then with no �from� on verified message)

If you have a packet that has something on the back, give it to the computer that is in the �from� position.� Otherwise, give it to the computer in the �to� position.

During the final, unsupervised sending of messages, you can �accidentally� mess up some numbers, but DO NOT MESS UP THE PACKET NUMBERS OR THE TO OR FROM ADDRESSES.� Also, DO NOT CHANGE MORE THAN ONE NUMBER IN A PACKET.� The Computers should recognize the problem when checking with the parity bit, and write reject. Don�t change most of the packets, just a couple.� You may also wish to fail to deliver a packet to a group, requiring them to request a resend.� This should be done with the ability of the students and their apparent ease with this protocol in mind.