Year 10 · Computer Science · Autumn 2

Data transmission and Boolean logic · Lesson 1 of 5

Packets and packet switching

Theory2.1 Types and methods of data transmission

Year 10 · Computer Science · Data transmission and Boolean logic · Lesson 1 of 5

You will learn to

  • Describe the three parts of a packet: header, payload and trailer.
  • State what the header holds: the sender's IP address, the receiver's IP address and the packet number.
  • Explain how packet switching sends packets along different routes, using routers.
  • Explain why packets can arrive out of order and how they are put back in order.

Year 10 · Computer Science · Data transmission and Boolean logic · Lesson 1 of 5

Success criteria

You have got it when you can say:

  • I can draw a packet and label its three parts.
  • I can say what the packet number (sequence number) is for.
  • I can describe packet switching in four steps.

Year 10 · Computer Science · Data transmission and Boolean logic · Lesson 1 of 5

Retrieval starter

Questions from before. Write your answers, then show them.

  1. Last unit

    Adding 11001100 and 01011010 in an 8-bit register gives…

    A 00100110 with no problemB 11111111C An overflow error

    C. An overflow error

  2. Last unit

    Shifting 00001010 (10) one place left gives…

    A 00001011 (11)B 00000101 (5)C 00010100 (20)

    C. 00010100 (20)

  3. Last unit

    A 10 × 10 image at 8 bits per pixel takes…

    A 80 bytesB 8,000 bitsC 800 bits = 100 bytes

    C. 800 bits = 100 bytes

Year 10 · Computer Science · Data transmission and Boolean logic · Lesson 1 of 5

Key words (1 of 4)

Packet
A small piece of data sent across a network, with a header, payload and trailer.
Header
The start of a packet: destination address, source address and packet number.
Payload
The actual data carried inside a packet.
Trailer
The end of a packet: error-checking data and an end marker.

Year 10 · Computer Science · Data transmission and Boolean logic · Lesson 1 of 5

Key words (2 of 4)

Packet switching
Sending each packet separately, possibly by different routes, then reordering them at the destination.
Router
A device that forwards packets towards their destination address.
Serial transmission
Sending data one bit at a time down a single wire.
Parallel transmission
Sending several bits at the same time down several wires.

Year 10 · Computer Science · Data transmission and Boolean logic · Lesson 1 of 5

Key words (3 of 4)

Skewing
When parallel bits arrive out of step because they travel at slightly different speeds.
Simplex
Data travels in one direction only.
Half-duplex
Data travels in both directions, but only one direction at a time.
Full-duplex
Data travels in both directions at the same time.

Year 10 · Computer Science · Data transmission and Boolean logic · Lesson 1 of 5

Key words (4 of 4)

Interference
Unwanted outside signals that can corrupt data on the way.
Checksum
A value calculated from a block of data, recalculated by the receiver to check for errors.
Check digit
A digit calculated from the other digits of a number, used to catch typing errors.

The explanation · step 1 of 7 · part 1 of 2

Data is split into packets

A packet is a small, fixed-size piece of data sent across a network.

A big file, like a photo, is cut into many packets before it is sent.

Small pieces are easier to send. If one piece is damaged, only that piece is sent again, not the whole file.

e.g.Example
A 3 MB photo sent over WhatsApp might be split into about 2,000 packets of around 1,500 bytes each.

The explanation · step 1 of 7 · part 2 of 2

Data is split into packets (continued)

?Why?

Splitting data lets many people share the same cables at the same time. Your packets and your neighbour's packets take turns on the line.

The explanation · step 2 of 7 · part 1 of 2

The three parts of a packet

The header holds the control information: the destination address, the originator's (source) address and the packet number.

The payload is the actual data being carried, such as part of the photo.

The trailer holds error-checking data (for example a checksum) and marks the end of the packet.

The explanation · step 2 of 7 · part 2 of 2

The three parts of a packet (continued)

PartWhat it containsWhy it is needed
HeaderDestination address, source address, packet numberRouters know where to send it; the receiver can reply and put packets in order
PayloadThe data itselfThis is the part the user actually wants
TrailerError-checking data and an end-of-packet markerThe receiver can check the packet arrived without errors
!Watch out

Students often put the error check in the header. In Cambridge answers, the error-checking data goes in the trailer.

There is a diagram for this step in the IGCSE library.

The explanation · step 3 of 7 · part 1 of 3

Packet switching

In packet switching, each packet is sent on its own across the network.

A router is a device that reads the destination address and forwards each packet along the best route it can find at that moment.

So packets from the same file can take different routes. They can arrive out of order.

At the destination, the computer uses the packet numbers to put them back in the right order.

The explanation · step 3 of 7 · part 2 of 3

Packet switching (continued)

e.g.Example
Try it: type a short message and press Send. Watch the packets take different routes and arrive mixed up. Then press Reorder and see the packet numbers fix the order.
?Why?

If one route is busy or broken, packets go another way. The network keeps working, and no single line is blocked by one big file.

The explanation · step 3 of 7 · part 3 of 3

Packet switching (continued)

!Watch out

If a packet is lost, the receiver asks for it again. Packet switching does not need every packet to use the same path.

There is something to try for this step in the IGCSE library.

The explanation · step 4 of 7 · part 1 of 3

Serial and parallel transmission

Serial transmission sends data one bit at a time down a single wire (or channel).

Parallel transmission sends several bits at the same time, each down its own wire. Eight wires can send a whole byte at once.

Over long distances, parallel bits can arrive at slightly different times. This is called skewing, and it corrupts the data.

Serial bits always arrive in the order they were sent, so serial is reliable over long distances.

The explanation · step 4 of 7 · part 2 of 3

Serial and parallel transmission (continued)

SerialParallel
How1 bit at a time, 1 wireMany bits at once, many wires
Speed over a short distanceSlowerFaster
Long distanceReliable: no skewingBits can skew and get out of step
Cost of cableCheaper (fewer wires)More expensive (more wires)
InterferenceLessWires close together can interfere with each other (crosstalk)
ExampleUSB cable, phone line to the internetInside a computer, e.g. the buses to memory; old printer cables

The explanation · step 4 of 7 · part 3 of 3

Serial and parallel transmission (continued)

?Why?

Today most cables are serial. Serial links can run at very high speeds without skew, so they beat old parallel cables even for printers.

The explanation · step 5 of 7 · part 1 of 2

Extra: how long will it take to send?

This calculation is not in the syllabus. It helps you feel why speed matters.

Speed is measured in bits per second (bps). File sizes are usually given in bytes, so first multiply by 8 to get bits.

Then divide the number of bits by the speed.

The explanation · step 5 of 7 · part 2 of 2

Extra: how long will it take to send? (continued)

e.g.Example
A photo is 1,500,000 bytes. It is sent over a serial link at 12,000,000 bits per second. Step 1: bits = 1,500,000 × 8 = 12,000,000 bits Step 2: time = 12,000,000 ÷ 12,000,000 = 1 second If eight wires send at the same rate in parallel: 12,000,000 ÷ (8 × 12,000,000) = 0.125 seconds.
!Watch out

Remember to change bytes into bits first (× 8). Forgetting this makes the answer 8 times too small.

The explanation · 2 more steps

More on the lesson page

The lesson page has the rest of the explanation, step by step:

  • Simplex, half-duplex and full-duplex
  • USB: universal serial bus

Open the lesson page

Year 10 · Computer Science · Data transmission and Boolean logic · Lesson 1 of 5

Try it: Send a message in packets

Year 10 · Computer Science · Data transmission and Boolean logic · Lesson 1 of 5

Quick quiz · 1 of 3

A packet's header holds…

  1. A Only the data
  2. B The whole file
  3. C The sender's and receiver's addresses and the packet number

C. The sender's and receiver's addresses and the packet number

The header is the label. The payload is the data.

Year 10 · Computer Science · Data transmission and Boolean logic · Lesson 1 of 5

Quick quiz · 2 of 3

Sending one bit at a time down one wire is…

  1. A Duplex
  2. B Serial
  3. C Parallel

B. Serial

Parallel sends several bits at once down several wires.

Year 10 · Computer Science · Data transmission and Boolean logic · Lesson 1 of 5

Quick quiz · 3 of 3

A walkie-talkie, where only one side can talk at a time, is…

  1. A Half-duplex
  2. B Full-duplex
  3. C Simplex

A. Half-duplex

Both ways, but not at once. Full-duplex is both at once, like a phone.

Year 10 · Computer Science · Data transmission and Boolean logic · Lesson 1 of 5

Exit ticket

Answer on paper before you go. Then check.

  1. With even parity, the 7 data bits 0110100 need the parity bit…

    A 1B It cannot be worked outC 0

    A. 1 0110100 has three 1s, which is odd. A 1 makes the count even.

  2. Parity can…

    A Detect and fix any errorB Detect two flipped bitsC Detect one flipped bit but not fix it

    C. Detect one flipped bit but not fix it Two flips keep the count even, so they slip through, and parity never says which bit flipped.