Year 7 · Autumn 2

Networks · Lesson 2 of 5

Signals as bits: why computers use binary

UnpluggedTopic 2 Bits and bytes

Year 7 · Networks · Lesson 2 of 5

You will learn to

  • Explain why computers use only two states: on and off.
  • Work out how many patterns 1 to 8 bits can make, by doubling each time.
  • Show any number from 0 to 255 using eight bit cards (128, 64, 32, 16, 8, 4, 2, 1).

Year 7 · Networks · Lesson 2 of 5

Success criteria

You have got it when you can say:

  • I can say that a bit is one 0 or 1, and a byte is 8 bits.
  • I can show 5 and 200 with the bit cards.
  • I can explain why 8 bits stop at 255.

Year 7 · Networks · Lesson 2 of 5

Retrieval starter

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

  1. Last lesson

    Which turns www.bbc.co.uk into an IP address?

    A SMTPB DNSC HTTP

    B. DNS

  2. Last unit

    Copyright in the UK…

    A Is automaticB Must be registered

    A. Is automatic

  3. Last lesson

    A 400 MB file on a 100 Mbps line takes at best…

    A 32 secondsB 40 secondsC 4 seconds

    A. 32 seconds

Year 7 · Networks · Lesson 2 of 5

Key words

Bit
One binary digit, 0 or 1. One switch.
Byte
8 bits. 256 possible patterns.
Nibble
4 bits. One hex digit.

The explanation · step 1 of 3

The bit

  • Bit is short for binary digit. Binary means 'made of two'.
  • One bit = one switch = one yes/no answer.
  • One bit alone can only say 2 things: 0 or 1.

The explanation · step 2 of 3

More bits = more patterns

BitsEvery patternHow many
10, 12
200, 01, 10, 114
3000, 001, 010, 011, 100, 101, 110, 1118
40000 … 111116
800000000 … 11111111256
?Why?

Why does it double each time?

Take all the patterns you had. Put a 0 in front of each. Then put a 1 in front of each. Same list, twice.

The explanation · step 2 of 3

The maths: How many patterns can n bits make?

The explanation · step 2 of 3

The maths: The biggest number n bits can hold

The explanation · step 3 of 3

Bits vs bytes in internet speeds

!Watch out

File sizes use bytes (big B: MB). Internet speeds use bits per second (small b: Mbps). A 100 Mbps connection gives at most 100 ÷ 8 = 12.5 MB per second.

The explanation · step 3 of 3

The maths: Internet speed into file speed

Year 7 · Networks · Lesson 2 of 5

Try it: How many patterns?

Year 7 · Networks · Lesson 2 of 5

Try it: Flip a switch

Year 7 · Networks · Lesson 2 of 5

Quick quiz · 1 of 3

A '1 TB' drive shows 931 GB in Windows because…

  1. A Some space is broken
  2. B Windows divides by 1,024 but still writes GB
  3. C The drive is fake

B. Windows divides by 1,024 but still writes GB

Same number of bytes, measured with a different ruler.

Year 7 · Networks · Lesson 2 of 5

Quick quiz · 2 of 3

5 KB into bytes. You should…

  1. A Multiply by 1000
  2. B Divide by 1000

A. Multiply by 1000

Bytes are smaller, so you need more of them: multiply. 5 KB = 5,000 bytes.

Year 7 · Networks · Lesson 2 of 5

Quick quiz · 3 of 3

A microphone hearing your voice is…

  1. A Input
  2. B Output
  3. C Process

A. Input

Information is coming in to the computer.

Year 7 · Networks · Lesson 2 of 5

Exit ticket

Answer on paper before you go. Then check.

  1. Why do computers use 2 states instead of 10?

    A On and off are easy to tell apart reliablyB Ten states would be too cheapC Two is the biggest number

    A. On and off are easy to tell apart reliably Wobbly signals make 10 levels easy to misread. On/off is reliable.

  2. A game you download is…

    A HardwareB Software

    B. Software You can't touch it. It's instructions stored as 0s and 1s.