Broadcast communication: One to All

Chapter 2 image

Introduction

Wireless (radio) communication, for example, WiFi and mobile phones, is a popular way to connect to the Internet. In Communication over Wires, you connected two micro:bits via wires. In this chapter, you will connect your micro:bits using the embedded radios in your micro:bits.

Doing this, you will not only learn how to use your micro:bit’s radio but also, broadcast communication. Wireless communication is typically broadcast: one micro:bit can send messages to all micro:bits in its communication range.

In summary, this chapter covers:

  • wireless communication and how to configure the micro:bit radio

  • the concept of broadcast and broadcast address

  • receiving and sending different message types (for example, a number or a string) using broadcast

  • when broadcast communication is useful, and when it isn’t

What you’ll need

2 micro:bits
2 battery holders, and 4 AAA batteries
1 teammate

Background

Wireless communication uses electromagnetic radiation - radio waves and microwaves - to send information. Radio waves are electromagnetic waves radiating from an antenna (like the antennas of a WiFi router). So, wireless communication is always broadcast. In other words, the signals from the WiFi routers can be heard by other WiFi devices tuned in to the same radio frequency. Read more about frequency in the Further Reading section at the end.

Definition 1: Broadcast

In networking, broadcast communication means the message of a single sender is transmitted to all receivers in a network.

But, does this mean that broadcast is only possible with wireless communications? No, but it is more cumbersome. For instance, for a wired communication medium, a sender would broadcast by repeating the same message over all the wires.

Finally, receivers may refuse to receive broadcast messages if they are not labelled with a broadcast address.

Definition 2: Broadcast address

A broadcast address is a special address which says all devices in the network should receive this message.

In a micro:bit, the broadcast address can be configured by setting the group ID of micro:bit’s radio. All the micro:bits need to have the same group ID for the broadcast to work.

Further reading

Let’s look at wireless communication in a bit more detail. You already learned that radio waves are electromagnetic waves. Scientists have found that electromagnetic waves can be arranged together on a scale called the electromagnetic spectrum. The figure below shows the electromagnetic spectrum and the different electromagnetic waves.

Electromagnetic spectrum

Figure 1: Electromagnetic spectrum

One thing to notice in the figure that radio waves are within the frequencies 30 kHz and 300 GHz in the electromagnetic spectrum. Radio waves include microwaves, which have frequencies between 300 MHz and 300 GHz. Radio waves travel fast - they move at the speed of light, which is around 300,000 km per second!

Let’s define frequency more formally. The frequency of a wave is the number of waves passing a point in one second. The unit of frequency is hertz (Hz). Like the examples above, you will typically see that frequencies are given as megahertz (MHz) or gigahertz (GHz). 1 MHz is equal to 1 million (10^6) Hz. 1 GHz is equal to 1 billion (10^9) Hz. Your micro:bit’s radio operates in the frequency range of 2402 MHz to 2480 MHz. What other wireless technologies work in the same range as the micro:bit’s radio? Hint: You will find the resources section at the end of this chapter useful to answer this question.

In addition to frequency, another critical parameter of electromagnetic waves is wavelength. The wavelength is the distance between a point on the wave and the same point on the next wave. The unit of wavelength is a meter. The figure below shows an example of a wavelength1.

Wavelength

Figure 2: Wavelength

Frequency and wavelength are related. The relationship between frequency and wavelength is given by a formula:

wavelength (meter) = Speed of light (meter/second)/Frequency (hertz)

From this equation, we see that the higher the frequency, the shorter the wavelength. You can see this also in the spectrum figure. How long do you think your micro:bit’s radio waves are?

Programming: Receiving and sending broadcast messages

In this activity, you will learn how you can receive a message from a broadcasting micro:bit. Also, your micro:bit will broadcast messages.

You may be running this activity with your teacher in the classroom. Then, your teacher’s micro:bit will be the broadcast sender, and you will try to receive from this micro:bit.

If you are running this activity alone or with a friend, you can download the code to run on the broadcasting micro:bit from here. You can use this example to test your receiver code by downloading it to a second micro:bit.

You will complete three tasks to experiment with broadcasting:

Task 1: Configure your radio

Description: For broadcast communication, you need all your micro:bits to have their radios turned on and set to the same radio group ID. This group ID will be the broadcast address. This is like tuning in to the correct channel to receive a TV broadcast.

Instruction: Program your receiver micro:bit to turn on its radio. To do this, you'll first need to import the radio module:

import radio

To turn on your radio, you need the following instruction:

radio.on()

The radio needs to be explicitly turned on since the radio draws power and takes up memory that you may otherwise need in your micro:bit programs.

By default, the group ID is set to 0. This is the group ID used in the example broadcast sender programs.

To change the group ID, for instance, to 5, you need the following instruction:

radio.config(group=5)

Look at the documentation on the radio module to figure out what other parameters you can configure for your radio with the radio.config(**kwargs) function: https://microbit-micropython.readthedocs.io/en/latest/radio.html

Task 2: Receive a broadcast message

Description: In this task, you will program your micro:bits to receive a message from a broadcasting micro:bit. You will use the example broadcast sender program to test your receiver program.

When writing your receiver programs, there are two questions you need to think about.

  1. What function in Python do you need to use to receive a radio message?

  2. Will this function allow you to receive any type of message, for example, a number or a string?

Instruction: First, you will program your micro:bit to receive a message. Download SendNumberOrString.hex into your sender micro:bit. This sender program uses the radio group 0 to broadcast. It sends a number between 0 and 9, whenever button A is pressed and, it sends a string, whenever button B is pressed. Program your micro:bit to receive and display this string. Test your program using the sender micro:bit.

Task 3: Send a broadcast message

Description: Now, it is your turn sending broadcast messages. If you run this exercise in a large group, with several micro:bits, you should notice that you are receiving a lot of messages! Can you guess who is sending which message?

Instruction: Program your micro:bit so that it can send a number when you press the button A and a string if you press button B.

Extended activity

Exercise 1

Extend your programs so that you can either send or receive. Send a "Hello" message when you shake your micro:bit. Display a “Sad” face on your micro:bit’s display until you receive a “Hello” message. Then display a “Happy” face for 2 seconds before switching back to "Sad" face.

Exercise 2

Discuss possible issues with broadcast communication. Is it always useful or necessary to send messages to everybody? What about privacy? Is this a problem that everybody receives all messages?

Problems

  1. Which frequency range does your micro:bit’s radio work in?

  2. What is the speed of light?

  3. Using the wavelength equation, calculate the wavelength of your micro:bit’s radio.

  4. Is it easier to broadcast using wired or wireless communication? Why?

Solutions

Solutions for this chapter can be found in the Github folder.

Resources


  1. Image by Dicklyon (Richard F. Lyon) - Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=7184592