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Handling Errors: Retransmissions

Chapter 8 image

Introduction

In the previous chapters, you probably noticed wireless communication isn’t always reliable. In other words, not every message you send may be received by the other side. In this chapter, you will learn about how to increase the chance your messages are received. What would you do if a message gets lost? This chapter will cover one simple but effective method: retransmissions.

In summary, you will learn about:

  • Wireless communication errors

  • Retransmissions as a method of reliability.

What you’ll need

2 micro:bits
1 teammate

Background

In wireless communications, an error can occur for several reasons. For example, there may be physical obstructions, like walls, doors, and even people. The wireless signals lose power as they go through these obstructions and sometimes, bounce off them! The more obstructions there are between a sender and a receiver, the more chance there is of an error. Also, if the sender and receiver are too far away from each other, they may not always be able to communicate. Imagine there are many obstacles between two people, they may not always hear what the other is saying!

Another reason for a wireless error may be radio interference. This is because wireless communication is broadcast (remember Wireless and Broadcast Communication). This means that there may be many broadcasters, and their transmissions may collide at the receivers. These broadcasters interfere with each other.

!!! hint "Definition 1: Interference" In wireless communications, interference is any other signal that disrupts a signal as it travels to its destination.

Imagine, in a classroom, when everybody is talking at the same time. You will miss half of the things your friend says. Other people’s signals interfere with your friend’s signal on its way to you. In networking, this is a packet loss.

!!! hint "Definition 2: Packet loss" Packet loss happens when one or more data packets traveling in a computer network do not reach their destination. Packet loss is measured as the ratio of packets lost and the packets sent (see Equation below).

Packet loss = (Packets lost)/(Packets sent)

Also, if there is too much interference, you may receive messages incorrectly! For instance, you may hear “Bat!” when your friend is shouting “Cat!”. In networking, this is a packet error. Packet errors are measured as packet error rates.

!!! hint "Definition 3: Packet error rate" Packet error rate is the ratio of packets that have been received with one or more errors and the packets sent.

Packet error rate = (Packets with errors)(Packets sent)

In this chapter, we will cover one simple method to handle these errors, retransmissions, where the sender automatically retransmits messages multiple times to increase the chance of reception.

!!! hint "Definition 4: Retransmissions" Retransmissions mean sending messages many times.

In the Figure below, let’s assume the sender knows that the communications medium loses half of its packets. In other words, the packet loss is 0.5 (or 50%). The sender micro:bit decides to send each packet twice to increase the chance to get its messages through. The first packet is the transmission, and the second packet is the retransmission. So, the number of retransmissions is 1.

Retransmissions may increase message success. In the example, the sender sends each message twice by default. So, even if the first “Hello” failed, the second “Hello” was received by the receiver!

!!! note "" Figure 1: Retransmissions may increase message success. In the example, the sender sends each message twice by default. So, even if the first “Hello” failed, the second “Hello” was received by the receiver!

It is common to use retransmissions combined with another method. For instance, senders may choose retransmit only when they are sure there has been an error. We will explore this option in the next chapter, Handling errors: Acknowledgements.

Programming: Retransmissions

This activity is best done with a teammate. You will start with creating packet errors in Task 1, and then test different packet error rates in Task 2. In Task 3, you will program the retransmissions solution to handle these errors. In this task, you will also run a series of experiments to measure how well retransmissions works.

Task1: Create packet errors

Description: In wireless communication, packets may fail randomly. This may make the testing your code for this chapter difficult. To test errors, you will use a custom block in the JavaScript Blocks editor to send messages with errors.

The ErrorRadio functions are like the Radio functions, but have an extra error parameter. This parameter is set to 20 by default, which means the packet error rate is 0.2 or 20%.

Error radio custom blocks

!!! note "" Figure 2: Error radio custom blocks

Instruction To use the custom blocks in the JavaScript Blocks editor, import the ErrorRadio.hex file in this folder. With your teammate, decide who will have the sender micro:bit, and who will have the receiver micro:bit. Follow the approach in Unicast Communication: One to One to put sender and receiver address in your packets. You may copy and change one of the programs you have written for Unicast Communication: One to One to use the ErrorRadio blocks.

Write a small sender program that sends a number with an error. Download it to the sender micro:bit. Write a small receiver program that receives a number and displays it on screen. Download this program to the receiver micro:bit.

Change the packet error rate using these error values in your program: 0, 50 and 100. Test packet errors by observing the receiver display.

Task 2: Send a sequence of messages

Description: In this section, you will send a sequence of messages to the receiver micro:bit.

Instruction: Extend your program in Task 1, to send this sequence:

Start 1 2 3 4 5 6 7 8 9 10 End

You can send the Start and End using the normal Radio blocks to send them without an error. But remember that your micro:bit’s radio may drop messages. So, there may be errors even in sending Start and End. No radio is perfect!

Extend the receiver program to count the number of messages it receives in this sequence. Run experiments setting the error parameter to 25, 50, and 75. Calculate the packet loss using the Packet loss equation. Repeat each experiment three times. Fill the table below with the result of your experiments. For example, when error is set to 25 in experiment number 1, if you received:

Start 1 5 6 7 8 9 10 End

This means, you received 7 packets, and lost 3. Your packet loss is 0.3%. The first row of the table is filled based on this example. Add in the values from your own experiment Based on your experiment results, discuss with your teammate how the experiment results change as you change the value of error.

Error value Experiment no. Packets received Packet loss
25 (Example) 7 0.3
25 1
25 2
25 3
50 1
50 2
50 3
75 1
75 2
75 3

Task 3: Retransmit by default

Description: In this task, you will program the automatic retransmissions at the sender side.

Instructions: Change your sender code from Task 2 to send each number in your sequence more than once. To try out your code, set error to 75. For example, by setting number of retransmissions to 1, you will send the following sequence:

Start 1 1 2 2 3 3 4 4 5 5 6 6 7 7 8 8 9 9 10 10End

This means the sender sent 20 packets in total, including 10 retransmissions.

Change your receiver code to count the unique numbers received. Count also the duplicates. Calculate the packet loss. For example, let’s assume your receiver received, for the case of 1 retransmission:

Start 1 1 2 3 5 5 6 8 9 9 10 End

This means, the receiver received 8 unique numbers (1, 2, 3, 5, 6, 8, 9 and 10) and 3 duplicates (1, 5 and 9). Note that the packet loss is 9 packets out of 20 (45%). But with retransmissions, the receiver only lost 2 numbers out of 9 (it did not receive 4 and 7). Let’s call this improved packet loss information loss. So, the information loss with retransmissions is 0.2. The first row of the table below is filled based on this example.

Run each experiment three times each, for different retransmission values and fill in the rest of the table.

Retransmissions Experiment Unique Packets Received Duplicates Packet loss Information loss
1 (example) 8 3 0.45 0.2
1 1
1 2
1 3
3 1
3 2
3 3
5 1
5 2
5 3

Extended activity

!!! attention "Exercise 1" Based on your experiments, discuss with your teammate how the increase in retransmissions helps. In your discussion, answer the following questions:

- How does the infromation loss improve as you increase the number of retransmissions?
- Does the method guarantee all messages are received at least once?
- How would you improve your method?

!!! attention "Exercise 2" Imagine you are going to survey packet loss in different locations inside a room using two micro:bits. Write a receiver and a sender program to measure packet loss. What do you observe? How does the packet loss change in different locations?

Problems

  1. What is interference? Why does it happen?

  2. If the sender sent 20 messages, and 11 messages were lost on the way to the destination, what is the packet loss?

  3. If the packet error rate is 20% and the sender sent 40 packets, how many packets had errors?

  4. Assume you do not know how many numbers that will be in the message sequence. But, you know the numbers will start from 1, and will increment by 1. For example, the sent message sequence may be: Start 1 2 3 4 5 6 7 8 9 10 11 12 End. What happens if you lose Start or End messages? Which one is worse: the loss of Start or End message? If the only message you receive is a 4, what can you say about the number of messages you lost?

  5. Assume you do not know how many numbers that will be in the message sequence. And they do not follow any order. For example, the sent message sequence may be: Start 3 5 10 2 End. What happens if you lose Start or End messages in the sequence Which one is worse: the loss of Start or End message? If the only message you receive is a 5, what can you say about the number of messages you lost?

Resources

Video: The Internet: Packet, Routing and Reliability - https://youtu.be/AYdF7b3nMto