groupcommunication.md

Group communication: One to Many

Introduction

In the previous chapter, you experimented with broadcast: sending messages to everybody. In this chapter, you will learn about sending a message so that it just goes to a smaller group of people. This is an activity that is best carried out with a large group of friends or class mates so that you can experiment with different groups and group sizes.

Group communication (also known as multicast) is an interesting concept, and enables several of today’s Internet technologies. For example, it enables to send videos as fast as possible over the Internet. In this chapter, you will learn:

• The concept of group communication and group or multicast address

• When group communication is useful and when it isn’t

What you’ll need

2 micro:bits
1 whiteboard/board
boardmarkers/post-it notes
1 teammate

Background

In the previous chapter, all micro:bits received messages from all the other micro:bits. This might have got confusing (or amusing!). Now, let’s try limiting who you can send messages to and receive messages from. This is called group communication. Group communication is used in the Internet to send to many people at the same time. For example, Internet television and videoconferencing use group communication.

!!! hint "Definition 1: Communications medium" In group communication or multicast, a message is sent only to the computers in the group.

For this, the messages need to be labeled with a group or multicast address.

!!! hint "Definition 2: Group address" A group or multicast address is a special address which says all devices in the group should receive this message.

To set a group address (or group ID), you will again use the “radio set group” block under the Radio menu like in Broadcast communication:One to all. The main challenge of this chapter is creating the groups for communication. How do computers learn about and join these groups? What happens when they leave a group? In this chapter, you will have a chance to think about these questions when you experiment with creating groups.

Further reading

When configuring group IDs for micro:bits, you will notice that the group IDs range from 0 to 255. This is the decimal (base 10) representation of group IDs. But we can also write these group IDs in binary (base 2). For the binary case, we will need 8 bits to get to a maximum group ID of 255.

Let’s think about the binary representation of group IDs. The figure below shows an example for the group ID 172 in 8 bits: 10101100. Notice that, we start reading bits from right to left. Each bit is numbered 1 to 8, corresponding to a power of two. The leftmost bit, bit 1, means $2^0 = 1$. Bit 2 means $2^1 = 2$ and we continue like this until we reach bit 8, which means $2^7 = 128$. Each bit location may contain either 0 or 1. To find the decimal value of 10101100, we need to do some maths. For a bit location $x$, we multiply its bit value with $2^{x-1}$. For bit location 8, the bit value is 1, and we need to multiply $2^(8-1) =2^7=128$ with 1. After doing this for all bit locations, we add all the values we found. The result of this addition is 172. Now, check for the case 11111111. Is it really equal to 255? For further reading, see the BBC Bitesize, Binary revision page in the Resources section.

!!! note "" Figure 1: Binary representation of group IDs

Programming: Creating groups and messaging within groups

In this chapter, you need to work together in pairs or small groups, with at least 2 micro:bits in each group. You will complete two tasks to program your micro:bits to send messages to and receive messages within your group.

Task 1: Create groups

Description: In this task, you will choose a unique group ID for your group, and configure your radios with this group ID. You will use the radio block radio set group in your program in the JavaScript Blocks editor. When choosing group IDs, you have to think about the best way to choose this number. Hint: What would happen if two groups choose the same number, and how would you make sure that doesn’t happen?

Instruction: Use the board and post-it notes to choose a group ID. Make sure your group ID is not the same as any other group ID

Task 2: Send and receive messages

Description: You will use the programs from the previous chapter to send and receive messages to your group. You will change these programs to count the number of messages you receive. This way, you will test whether you receive messages that only come from your group.

Instruction: Write a sender program that sends a random number between 0 and 9, when you press the button A. Write a receiver program that increments a counter each time it receives a number. When you press the button A at the receiver, it displays the value of the counter. With your group, test that you are receiving the correct number of messages. Test together with other groups that you are not receiving their messages.

Extended activity

!!! attention "Exercise 1" How easy or difficult would it be if micro:bits could create groups automatically themselves? How would they pick a group ID? How would they make sure nobody else had that number? Would broadcast be useful? Discuss with your teammates.

!!! attention "Exercise 2" Can a micro:bit be part of two groups or more? How would you program your micro:bit to do that?

Problems

1. Fill in the blank in this sentence: “A one-to-may communication between one sender and a group of receivers is --- communication.”

   1. unicast

   2. multicast

   3. broadcast

   4. none of the above

2. Let’s assume the group ID is 3 bits. For example, 010 is a group ID. What is the maximum number of groups can you have in a network?

3. If the group ID were 6 bits, what is the largest group ID you could choose for your micro:bit?

4. “Compared to broadcast, the receivers in group communication receive more messages.” True or False?

Resources

BBC Bitesize, Binary revision - http://www.bbc.co.uk/education/guides/z26rcdm/revision