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Year 10 · Computer Science · Autumn 2 · Data transmission and Boolean logic

Serial, parallel, duplex and USB

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What this lesson covers

2.1 Types and methods of data transmission

Cambridge IGCSE Computer Science 0478 · Topic 2: Data transmission · Paper 1

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.

Success criteria

  • 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.
The syllabus points for this section
  • Describe the structure of a packet: header, payload and trailer.
  • Explain how packet switching sends data across a network.
  • Compare serial and parallel transmission.
  • Compare simplex, half-duplex and full-duplex, with advantages and disadvantages.
  • Explain why USB is used, with its benefits and drawbacks.

Key words

Say each word, then read what it means. Every one is in the glossary too.

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.
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.
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.
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

Read it here, or open the study page below to go one step at a time with things to try.

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.
?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 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.

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.

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.

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.

!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.

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.

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
?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.

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.

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.

Simplex, half-duplex and full-duplex

These words describe the direction data can travel.

Simplex: data travels in one direction only. Example: a computer sending to a printer that never replies, or a TV broadcast.

Half-duplex: data can travel in both directions, but only one way at a time. Example: walkie-talkies, where you press to talk.

Full-duplex: data travels in both directions at the same time. Example: a phone call, or a broadband internet connection.

MethodAdvantageDisadvantage
SimplexCheapest: needs only one channel. No data collisions.The receiver cannot reply, so it cannot ask for missing data again.
Half-duplexTwo-way, but cheaper than full-duplex. Less chance of collisions than full-duplex.Slower: devices must wait their turn. Delays when the direction changes.
Full-duplexFastest: both sides send at once, no waiting.Most expensive: needs two channels (or a more complex link).
!Watch out

Do not mix these up with serial and parallel. Serial/parallel is about how many bits at once. Simplex/half/full is about which direction. A connection is described by both, e.g. serial full-duplex.

USB: universal serial bus

USB is a standard way to connect devices to a computer. It uses serial transmission.

When you plug in a USB device, the computer detects it and loads the right driver (a small program that lets the computer control the device).

USB can also carry power, so a phone can charge from the same cable.

BenefitsDrawbacks
Devices are detected automatically and the driver is loadedCable length is limited, about 5 metres for standard USB
Only fits one way (or is reversible for USB-C), so it is hard to connect wronglyOlder USB versions are slow, e.g. USB 2.0 compared with USB 3
An industry standard: works with almost every computer and deviceVery old devices may not support newer USB standards
Supplies power to the device, so it may not need its own power supplyPower supplied is limited, so big devices need their own supply

Study it

Open the study page and go one step at a time.

Computer Science topic 2

Data transmission

Step by step, with exam questions and a quiz.

Computer Science topic 10

Boolean logic

Step by step, with exam questions and a quiz.

Topic 15 · Year 7

Networks and the internet

A network is computers connected to share data. The internet is a network of networks. Data travels in small packets, following rules called protocols.

Check yourself

One question at a time. The reason shows after each answer.

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