Year 10 · Computer Science · Autumn 2

Data transmission and Boolean logic · Lesson 2 of 5

Serial, parallel, duplex and USB

Theory2.1 Types and methods of data transmission

Year 10 · Computer Science · Data transmission and Boolean logic · Lesson 2 of 5

You will learn to

  • Compare serial and parallel transmission for speed, distance and reliability.
  • Describe simplex, half-duplex and full-duplex, with a real example of each.
  • Explain why USB is widely used, giving its benefits and drawbacks.

Year 10 · Computer Science · Data transmission and Boolean logic · Lesson 2 of 5

Success criteria

You have got it when you can say:

  • I can explain why parallel transmission is only used over short distances.
  • I can match a radio, a walkie-talkie and a phone call to the right mode.
  • I can give two benefits and one drawback of USB.

Year 10 · Computer Science · Data transmission and Boolean logic · Lesson 2 of 5

Retrieval starter

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

  1. Last lesson

    A packet's header holds…

    A Only the dataB The whole fileC The sender's and receiver's addresses and the packet number

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

  2. Last unit

    How many bits are in a byte?

    A 4B 8C 16

    B. 8

  3. Last lesson

    Sending one bit at a time down one wire is…

    A SerialB ParallelC Duplex

    A. Serial

Year 10 · Computer Science · Data transmission and Boolean logic · Lesson 2 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 2 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 2 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 2 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 2 of 5

Quick quiz · 1 of 3

In asymmetric encryption the message is locked with the…

  1. A Same key that unlocks it
  2. B Public key
  3. C Private key

B. Public key

Anyone can lock with the public key. Only the private key unlocks.

Year 10 · Computer Science · Data transmission and Boolean logic · Lesson 2 of 5

Quick quiz · 2 of 3

ARQ stands for…

  1. A Address resolution question
  2. B Average rate query
  3. C Automatic repeat request

C. Automatic repeat request

The receiver acknowledges; if no acknowledgement arrives before the timeout, the sender resends.

Year 10 · Computer Science · Data transmission and Boolean logic · Lesson 2 of 5

Quick quiz · 3 of 3

XOR gives 1 when…

  1. A Both inputs are 1
  2. B Neither input is 1
  3. C Exactly one input is 1

C. Exactly one input is 1

Exclusive OR: one or the other, not both.

Year 10 · Computer Science · Data transmission and Boolean logic · Lesson 2 of 5

Exit ticket

Answer on paper before you go. Then check.

  1. A truth table for 3 inputs has…

    A 3 rowsB 8 rowsC 6 rows

    B. 8 rows 2 to the power of 3 combinations.

  2. NAND is…

    A AND followed by NOTB OR followed by NOTC NOT followed by AND

    A. AND followed by NOT Not-AND: the opposite output of AND.