Morse code network

Look out for safety precautions like this.

This indicates an enquiry opportunity for students to discuss and figure out a solution.

This gives key knowledge and points to further resources.

In the 1980s and early 1990s, most home computers were alone. They had no connection to other computers or devices, except peripherals like printers.

Why do we network computers now?

• Communicating (eg messaging, emailing)
• Sharing files/data
• Sharing hardware (eg common printers/copiers)
• Sharing and accessing software (eg Google Docs and other cloud services)

Coding the micro:bits is an opportunity to address the Creating Digital Solutions strand (Victorian Curriculum) or the Processes and Production Skills strand (Australian Curriculum).

To save time, code can be copied from this lesson sequence.

What does our transmit program need to do?

• REPEAT FOREVER ...
• • IF button B is being pressed ...
  • • Digital write 1 (high) on Pin 1.
    • Draw the transmit arrow.
  • ELSE
  • • Digital write 0 (low) on Pin 1.
    • Clear the transmit arrow.
  • END IF
• END REPEAT

Diagram 2: Transmit Program

What does our receive program need to do?

• REPEAT FOREVER ...
• • IF digital read on Pin 0 is 1 (high) ...
  • • Draw the receive arrow.
  • ELSE
  • • Clear the receive arrow.
  • END IF
• END REPEAT

Diagram 3: Receive Program

Always read product safety warnings before using classroom electronics hardware.

Never connect a 3V (power) pin directly to a GND pin.

Can we combine the two programs so that a micro:bit can transmit and receive?

Let’s say that transmitting takes priority over receiving.

• REPEAT FOREVER ...
• • IF button B is being pressed ...
  • • Do the usual transmit code.
  • ELSE
  • • Do the usual clear transmit code.
    • Do the receive code here.
  • END IF
• END REPEAT

Diagram 5: Combine two programs so that a micro:bit can transmit and receive

What do computers use for network signals, instead of Morse code?

• It is the same system they use for storing letters and numbers: binary.

Binary is covered in the Data and Information strand (Victorian Curriculum) or the Knowledge and Understanding strand (Australian Curriculum). Data is stored in bits – 1s or 0s.

To send a binary message, the 1s and 0s are pushed out as high or low voltages.

A small whole number might be transmitted as a single byte of 8 bits (eg decimal number 57 = binary 00111001).

A letter or other character might be encoded using a system like ASCII (eg letter ‘g’ = ASCII code 01100111).

What are the challenges for humans trying to send messages to each other with binary?

• Too many codes to remember
• Not natural to convert binary in our heads (we use decimal for a reason!)
• How fast should the signal go? What if the transmitter and receiver are thinking at different speeds?

In our simulation, both transmitter and receiver already know they are using the same system – Morse code.

When computers and devices talk, they must also establish a common way of talking. This is called a network protocol.

What usually happens at the beginning of a phone call?

• Caller dials number.
• Caller hears a tone and waits for receiver to pick up.
• Receiver hears the phone ring and picks up.
• Receiver speaks: ‘Hi, this is Betty’.
• Caller speaks: ‘Hi Betty, it’s Ernie here’.
• Receiver speaks: ‘Hi Ernie!’
• Sometimes, there may be even more small talk before the real conversation begins, the reason for the call.

Why do humans do this?

• To set up a back-and-forth dialogue. Imagine if the caller just started asking for something as soon as the receiver picked up!
• To establish that the right person has picked up.
• To agree on the manner of the conversation (eg how much formality, how fast).

This is a protocol. Networked devices must also establish how they will communicate, by sending and receiving special signals at the beginning.

How do we measure how ‘fast’ a network connection is? The bitrate is the number of bits arriving per second (bps), not bytes per second.

• 1 000 bps = 1 kbps (kilobits per second)
• 1 000 000 bps = 1 Mbps (megabits per second)

What is the speed of your internet connection at home?

• Typical NBN download speeds range from 12 Mbps to 100 Mbps.

At 20 Mbps, how long would it take to receive a 2 GB movie file?

• First let’s convert the file size into bits:
• • A 2 GB movie is about 2 billion bytes¹.
  • 2 000 000 000 × 8 = 16 000 000 000 bits.
• Let’s convert the bitrate 20 Mbps into bps:
• • 20 × 1 000 000 = 20 000 000 bps.
• Time to download = ^{16 000 000 000} ⁄ _{20 000 000} = 800 seconds (about 13 minutes).

¹ Actually, it is 2 147 483 648 bytes (1024 Bytes = 1 kB, 1024 kB = 1 MB, 1024 MB = 1 GB).

A local area network (LAN) is the name given to a network of computers in the same geographical location. Connections may be wired or wireless.

Can you think of some examples of a LAN?

• A home network is a LAN, connecting:
• • computers, laptops, tablets and phones
  • smart TVs, game consoles
  • a modem or NBN box for access to the internet.
• A school campus has a LAN, connecting:
• • computer lab PCs
  • student or teacher laptops
  • copiers and printers
  • file servers, which store files and data that may be used by multiple people.
• A factory or office building usually has a LAN.

Our LAN works fine when one person is talking.

What do you think will happen if two people try to talk at once?

• The voltage on the wire will be forced up and down by whoever is talking.
• Messages will be garbled.

When two signals try to use the same medium at the same time, it is called a collision.

How do you avoid collisions when you are talking in a group?

• Wait for someone else to finish before talking.
• Raise hand to talk.
• Have a ‘talking stick’ that is passed around. Only the person with the stick can talk.

The problem of collisions was addressed as part of the ethernet protocol, a popular network protocol used around the world.

The method is called Carrier-Sense Multiple Access with Collision Detection (CSMA/CD).

• First, devices could sense when other signals were present.
• If, by chance, two devices began talking at the same time, each would wait a random amount of ‘backoff’ time before talking again. (This could be microseconds; that’s how fast computers can talk to each other.)
• Because the two devices each waited a different amount of time, there was a good chance that the collision would be avoided the next time.
• However, the more devices sharing the same connection, the more chance of collisions, which meant more attempts. It was possible to have gridlock.

(The metaphor of the ‘talking stick’ was used in a competing protocol called Token Ring.)

What does our hub program need to do?

The hub will operate completely automatically, without any pushbuttons needed.

• REPEAT FOREVER ...
• • IF digital read on Pin 0 is 1 (high) ...
  • • Digital write 1 (high) on Pin 1.
    • Digital write 1 (high) on Pin 2.
    • Draw a ‘H’ symbol.
  • ELSE
  • • Digital write 0 (low) on Pin 1.
    • Digital write 0 (low) on Pin 2.
    • Clear the ‘H’.
  • END IF
• END REPEAT

Diagram 9: Hub program

Hubs were often used in the early days of networks to allow more devices to connect.

Today, they are still used occasionally, eg to quickly add more network ports in a room.

Pros

Cheap

Make more ports available quickly

Cons

Broadcast the same signal to everyone

Exacerbate the collision problem

Suppose you are building a house. Your supplier needs to send lots of bricks, wood and other materials, but there isn’t room for it all in one truck.

What does your supplier do?

• Split the materials onto different trucks, and send them one after the other.

How does your supplier ensure that all the trucks get to your building site?

• Each truck is given the address of your building site.

In a modern network, messages are split into small parts called packets.

Packets are part of a protocol called internet protocol (IP). This protocol enables the internet, by assigning addresses to packets, so they can be delivered to devices around the world.

Each packet includes the data to be sent, but it also includes an IP address. This address specifies a device to go to, even on the other side of the world.

Jumper leads between desks may become a trip hazard. If possible, arrange desks together in the centre of the room, or in a U shape.

In this simulation, everyone would have received all the packets.

What’s the problem with everyone receiving the packets?

• Security: What if the packets contain sensitive information?
• Collisions: Too much unnecessary network traffic.

Clearly, we need something smarter than hubs for our internet to succeed.

Enter routers. Routers have many advantages over hubs:

Pros

They can direct a packet out of one port, instead of all ports. This is called packet switching.

They actively learn which ports lead to which destinations, by communicating with their immediate neighbours.

They also support security software (firewalls) to help secure a LAN.

Cons

Expensive compared to hubs.

What does our router program need to do?

Our router program will not automatically transmit an incoming message like the hub.

Instead, the student with this micro:bit will need to:

1. note down the incoming packet, including the destination
2. decide whether to transmit on Pin 1 or Pin 2
3. press Button A to toggle which pin to use
4. use Button B to transmit the Morse code.

Our program will need a variable to remember whether to transmit on Pin 1 or Pin 2. We’ll set it to Pin 1 at the start.

• transmissionPin ← 1
• REPEAT FOREVER ...
• • IF button A is being pressed...
  • • Swap value of transmissionPin.
    • Display a confirmation.
  • END IF
  • IF button B is being pressed...
  • • Transmit on transmissionPin.
  • ELSE
  • • Do usual clear transmit code.
    • Do the receive code here.
  • END IF
• END REPEAT

Diagram 11: Router program

You can follow a real packet passing through routers on the internet by using the traceroute command.

Try using the command to trace the route to an address that is definitely hosted overseas, such as www.thescotsman.co.uk. You may see routers in Sydney, California, New York and Britain.

Note: the traceroute command sometimes fails, and may be blocked by settings at your school.

Our micro:bit addresses are simple, but real IP addresses are made up of four numbers, each between 0 and 255, eg 172.16.254.1

How many unique addresses can be made from those four numbers?

• 256 × 256 × 256 × 256 = 4 294 967 296 (about 4 billion).
• Several million of those are reserved for special purposes.

How many networked devices do you think there are in the world today? Don’t forget, they could be computers, TVs, fridges, phones, tablets, etc.

• Cisco estimates 50 billion by 2020.

How is it possible that there are more unique devices than IP addresses?

Routers are able to assign any IP addresses within a LAN, and remember them when packets come in from outside. All addresses within a LAN must still be unique to each other.

Also, a new version of IP address (IPv6) contains many more digits, allowing for 3.4×10³⁸ unique addresses.

We can type names like www.google.com to get to a device, rather than an actual IP address.

This is thanks to Domain Name Servers (DNS), which keep registered domain names (like google.com) and match them to IP addresses.

When you type in www.google.com, your web browser queries a DNS and finds the actual IP address.

Hang on, so what’s a switch? To save complicating things, we’ve brushed over switches.

Switches are used extensively in large LANs like office buildings and schools, because they can have many more ports more cheaply than a router. At home, a single router is usually enough.

A switch is smarter than a hub, but not as smart as a router. Switches are able to see the Media Access Control (MAC) addresses of devices. They can learn to send packets to specific devices within a LAN, but they cannot communicate to other networks. That requires a router.

Last lesson, we began with a question about sending building materials to a construction site.

The supplier splits the materials onto different trucks. She gives the address of the construction site to each truck, but the trucks may all take different routes to the address.

Why might the trucks take different routes?

• A road might become closed.
• There might be heavy traffic on one route.

What problem might this cause at the construction site?

• Materials may arrive in the wrong order for construction.

The same problem can occur with our packets. Routers may send them down very different routes to the destination. Unlike building a house, the receiver usually won’t know in which order to reassemble the packets of a message.

What could be the solution?

• When you first split the message into packets, number each packet in order.

Numbering the packets for order is the job of another protocol called transport control protocol (TCP). TCP does its work of numbering the packets before IP adds the destination address to each one.

TCP also adds bits to help determine if a packet became corrupted on its way. If so, a resend can be requested.

TCP and IP work so closely together that they are often referred to as TCP/IP.

Suppose a hacker wanted to spy on the packets that arrive at the destination. How could they do that in our micro:bit network?

• Tap into the signal by clipping onto Pin 0 on the destination, or even onto one of the routers along the way.

Spying on packets is called packet sniffing. It’s not only used by hackers, but also by network analysts trying to improve network performance.

This form of tapping a network requires physical access to the hardware, but there are a number of more sophisticated ways to sniff packets, even in wireless networks.

How can a packet sniffer be thwarted? How can information be kept secure?

• Physical security (locks, security codes, surveillance)
• Software security: many routers come with firewall software, which can prevent common hacking techniques
• Encryption

Encryption is a focus in the years 9–10 Digital Technologies curriculum. Here, we will do a simple example only.

Is Morse code already a form of encryption?

• Not really. Although it is a form of encoding letters and numbers like ASCII, everyone already knows the key – how to decode it.

How could we encrypt the letters in our message?

• Move all letters up by 1. (A becomes B, B becomes C, etc.)
• Use a more complex algorithm based on the positions of the letters in the alphabet.

Encryption by simply moving letters is very weak. Why?

• Humans can often see the pattern quite quickly. Computers can discover the pattern even more quickly.
• A trick is to look for small words, like ‘a’ or ‘the’. The famous Enigma machine from WWII was easier to crack because Nazi correspondents often included the same information or phrases, eg ‘Heil Hitler’.

Our micro:bit network simulation has been very simple compared to real digital networks.

To finish this course, let’s compare some real technologies used for wired and wireless communication.

Some questions to research:

• What is the maximum bitrate you can expect from twisted pair? (Note: there are different versions.)
• What is the common name for the connectors on ends of twisted-pair cables?
• What advantage does optical fibre have over twisted pair?
• What is the Australian connection with Wi-Fi’s invention?
• What is the maximum range and bitrate of Wi-Fi (Note: there are different versions.)