Two-way unicast

Chapter 6 image

Introduction

In this chapter, you will learn about bidirectional communication: sending a message to another micro:bit and getting a response to your message. You will also learn about the Ping program, which is a commonly used tool to check if computers are still connected to the internet.

This chapter will build on the learnings from the previous chapter, Unicast Communication: One to One. The new ideas are:

  • The idea of 2-way communication (bidirectional communication)

  • The Ping program

  • The concept of round-trip-time

What you’ll need

2 micro:bits
1 whiteboard/board
Boardmarkers/postit notes
1 teammate

Background

Bidirectional communication enables two-way communication between two computers.

Definition 1: Bidirectional communication

This is a communications mode in which data is transmitted in both directions but not at the same time.

In the previous chapter, your micro:bits had clear roles: there were a sender and a receiver. In bidirectional communication, either of the micro:bits can send and receive messages. This way, it becomes possible to create two-way protocols. In these protocols, when a computer sends a message, it waits to receive a response to its message.

Definition 2: Ping

Ping is an example of a two-way protocol. It is widely used in the internet to test whether a networked computer is connected OK.

Ping program sends a Ping message and expects this message to be echoed back, for example, with a Pong message. This is like playing ping pong but with computers and over networks. If the sender does not receive a Pong, this shows there is a problem with the receiver. It is also a problem if it takes a long time before the sender receives the Pong response. To spot these problems, the Ping program measures the round-trip-time between the two computers.

Definition 3: Round-trip-time (RTT)

Round-trip-time is the time it takes for a message to go from a sender to a receiver and back again.

In other words, the sender measures the difference in time when it sent the Ping, and when it received the Pong.

RTT = Time_receive_pong - Time_send_ping

The figure below shows the relationship between Ping, Pong, and round-trip-time.

Round-trip-time. Micro:bit 1 sends a *Ping* message to Micro:bit 2 at *Time_send*. The Micro:bit 2 responds with a *Pong* message. Micro:bit 1 receives the *Pong* message at *Time_receive*. The difference between these two times, *Time_receive* and *Time_send* is the round-trip-time.

Figure 1: Round-trip-time. Micro:bit 1 sends a Ping message to Micro:bit 2 at Time_send. The Micro:bit 2 responds with a Pong message. Micro:bit 1 receives the Pong message at Time_receive. The difference between these two times, Time_receive and Time_send is the round-trip-time.

Besides round-trip-time (RTT), the Ping program reports statistical information. The figure below shows an example output as a result of using the command:

ping www.google.com

on the http://ping.eu/ping website. In this example, four Ping messages were sent to www.google.com. The round-trip-time for each message is given with the time value in each line. For example, for the first ping, the RTT is 10.2 ms (milliseconds). The program also reports ping statistics. For example, 4 packets were sent, 4 packets were received. This means 0% packet loss. The average RTT (shown as avg) is 10.184 ms.

The output of running ping to send four messages to *www.google.com*. The <http://ping.eu/ping> online program reports round-trip time and a statistical summary of the results.

Figure 2: The output of running ping to send four messages to www.google.com. The http://ping.eu/ping online program reports round-trip time and a statistical summary of the results.

When programming your micro:bit, you will use the running_time() function to calculate the round-trip-time of your messages. This function returns the number of milliseconds since the micro:bit was last switched on. To calculate the round-trip time, you will need to record the time when you first sent a message, and also when you received a response.

Programming: Ping

This activity is best done with a team of two. To program the Ping program, you will need to complete five tasks.

Task 1: Prepare for unicast

Description: Ping uses unicast between the sender and the receiver micro:bits. Look at your notes for the previous chapter, Unicast Communication: One to One and your unicast program to remember how to do unicast.

Instruction: Start with using the unicast program you have written for Unicast Communication: One to One as a basis. In this program, decide which micro:bit is going send the Pings, and which micro:bit is going to respond with Pongs. Set the address variables based on your decision. Design your message header, Ping packet, and Pong packet.

Task 2: Send a Ping

Description: The ping sender records the time before it sends out a Ping packet. It unicasts the Ping packet.

Instruction: Use running_time() to record the Ping sending time. Send a Ping packet to the receiver micro:bit.

Task 3: Receive a Ping

Description: The receiver micro:bit responds a Ping message with a Pong.

Instruction: Program the receiver micro:bit to unicast a Pong packet when it receives a Ping packet.

Task 4: Receive a Pong and calculate round-trip-time

Description: When the sender micro:bit receives the Pong, it calculates the round-trip-time.

Instruction: Program the sender to receive a Pong packet. When the Pong is received, record the time using running_time() function. Show the difference between receiving and sending times, i.e., RTT, on your display.

Task 5: Put everything together

Description: Combine all the code you have written from Task 1 to Task 4 so that a single micro:bit can run the Ping program both as a Ping sender and a receiver.

Instruction: Use one of your micro:bits to send a Ping 5 times, and write down the round-trip-times that you see in your display. Answer these two questions:

  1. What are the minimum and maximum round-trip-time (RTT)?

  2. What is the average RTT?

Exercises

Exercise 1

Extend your Ping program to send more than one Ping message automatically. Test it with 10 Pings. Calculate the average round-trip time of these Ping messages.

Exercise 2

The Ping program reports the round-trip-time. What if you wanted to calculate the time the message took one way? Is it possible to calculate one-way times? In other words, is it possible to calculate how long it takes to send a message from the sender to the receiver? How long the messages take from the receiver to the sender?

Problems

  1. In the example ping figure from the http://ping.eu/ping site, what is 172.217.23.228?

  2. What is round-trip-time, and how is it calculated?

  3. Think about the following scenario: micro:bit 1 sends a Ping to micro:bit 2 at time 5. If the round-trip-time is 10, at what time did the micro:bit 1 receive the Pong message?

  4. In the example ping figure from the http://ping.eu/ping site, what are the minimum and maximum round-trip-times (RTTs)?

  5. In the example ping figure from the http://ping.eu/ping site, the packet loss 0% loss. What is the loss percentage, if 2 Ping messages were lost out of 5?

Solutions

Solutions for this chapter can be found under the Github directory.

Resources

Video: What is a Ping? - https://youtu.be/N8uT7LNVJv4