Labs from Chicago, Fall 1993 :
Packet Radio.

Dr. Jim Sweitzer
Labs written for the CARA Space Explorers, Fall 1993/Yerkes Winter Institute (YWI) 1993.

This is meant to be handed out to the students.

Introduction

Today we're going to learn about how computers "talk" with one another and see how they can do that with the help of amateur radios. The process is exactly what happens when they communicate over phone lines. It is also how a new breed of satellites communicate too. If the timing is right, we may even be able to catch one of these satellites going over and decode what it's saying.

First, however, we need to understand the code computers use. This is like learning how they write letters. Then, we will see how they transmit this code in an error-free manner to each other. This will introduce us to the concept of "packets." Finally, we'll try it out and communicate with another computer using radios.

Computer Codes

Computers can't automatically read letters and other characters. As a result, we need to create a code. Maybe you have made a code before by assigning a number to a letter? 1 would be A, 2 would be B and so on.

This is on the right track, except that computers can't understand the normal Arabic numerals that we use. Computers can only read two numbers! I won't go into the reasons for this, but it's a major fact of computers. What allows them to get away with this and get anything done is that they can do it very fast.

For simplicity let's use two numbers we know: 0 (zero) and 1 (one). This is called a binary number system. "Bi-" means two. How would we count with such a system? Well it's actually not that hard. You just have to think of it as similar to the usual system we always use, which is called the decimal system. "Deci-" means ten. Here's how you would count to five in binary.

It's just like doing the counting you're used to, except that you don't have the other digits. When you add 1 to 1 you get 10, because you have to go to another place. In the spaces below, write the numbers up to 26 in the second column of the following table.

Decimal Number	Binary Number	
      1		
      2		
      3		
      4		
      5	      	
      6		
      7		
      8		
      9		
     10		
     11		
     12		
     13		
     14		
     15		
     16		
     17		
     18		
     19		
     20		
     21		
     22		
     23		
     24		
     25		
     26

Now, create a code for the 26 letters we know as follows. Put zeros in front of the binary numbers above, as necessary, to make all the number have the same number of digits. This doesn't really change the value of the number, it just makes them consistent. It's like saying the number 09=9. Finally, then write the alphabet starting at the top of the third column and working your way down.

Make sure you get the letters in the right order! It would be a big problem if we didn't all have the same match between letters and numbers. We'd have different codes and wouldn't be able to communicate!

My name is JIM. Write it below in this binary code:

___________________________________________________

Now, write your own name in binary:

___________________________________________________

Transmitting the Infomration

Computers exchange information by converting text into these codes and then sending the long sequence of 0's and 1's. Here's what it would look like if you could see the binary code of a block of text.

011100010110100101101110010101010101111110010111001010
110010101010101010100010100101110110000001001001010110
000111010101010101001010010000100010111010101010000011

Radios transmit the information by giving a separate tone for each number. They send a 1200 Hz tone for a 0 and a 2400 Hz tone for a 1. (Hz is the symbol for frequency units. It stands for Hertz. Hertz means one cycle per second. The frequencies we can hear are typically 20 to 20,000 Hz. The higher the frequency the higher the tone.)

Let's see what it's like to transmit binary information. Let the word BEE stand for a 1 and the word BOP stand for a 0. That would mean the binary number 10011101010 would be:

BEE-BOP-BOP-BEE-BEE-BEE-BOP-BEE-BOP-BEE-BOP.

Now, write your name in the binary code, only in terms of BEEs and BOPs.

__________________________________________________

Try and say it.

In the class we'll see who can say their name the fastest. How many letters were in the winner's name?

____________________________________

How many binary digits make up each letter? _______________

So, how many total binary digits in the name? _______________

How long did it take to say the name (someone will have to help with a stopwatch here)?

____________________

Now, divide the number of digits by the time it took. Write your answer here:

__________________________

This last number is called the baud rate of transmission. Baud means "bits" per second. In our case a digit, whether it is a 1 or a 0 or a BEE or a BOP is a "bit" of data.

A really fast computer link can send its BEEBOPS at a baud rate of 56,000 bits per second. We'll now look at some radios and computers that send at a slower rate of 1,200 baud. How much faster is 1,200 baud than the fastest person in the class?

___________________

Packets

Computers don't just send very long strings of binary numbers. If they did and made a mistake, then they'd have to re-send the entire message. This would be a terribly inefficient way to move information. They handle this much more effectively by transmitting only a small group of characters at a time. They then check to see that they have done it correctly. If they have, then they send the next packet. If the first one was sent incorrectly, then they send it again until they get it right. Here's what it might look like if you could see the process.

Let's break up the following sentence into two packets.

The quick brown fox jumped over the lazy dog's back.

Packet #1 = P1= The quick brown fox jump
Packet #2 = P2= ed over the lazy dog's back.

  Computer A                   Computer B

This might seem to be very slow, but it happens very fast in reality and leads to an accurate way of transmitting information from one computer to another.

Let's do a little figuring. Suppose we had a book that had 200 pages and that each page had 60 lines. Further, suppose that each packet was 3 lines long. How many packets would it take to transmit the entire book?

_______________________________________

Further, suppose that each line contained 40 characters and that each character requires 6 bits of information. How many bits in the entire book?

_______________________________

Suppose the effective baud rate of the packet transmissions is 2,400 baud. How long will it take to transmit the entire book? (Convert your answer to minutes.)

_________________________________

Packet Radio

Finally, today we're going to actually see some computers exchange messages. We're going to do it using packet radio. Here is a short glossary of some of the terms you'll hear:

Connect: This happens when the two computers recognize each other and agree on how they will send the packets back and forth. You have to do this first. When they are finished communicating with each other they Disconnect.
Modem: This is a small electronic device that turns tones produced by the radio into voltage changes that become 1s and 0s in the computer. It also turns the string of 1s and 0s from the computer into tones and sends them to the radio to be transmitted.
Transceiver: This is a radio that transmits and receives radio waves. The ones we will use work at a radio frequency of about 145 million Hz.

Here's a block diagram of one of the two communicating stations.

Important Disclaimers and Caveats