Learning Objectives
- Describe serial transmission and parallel transmission.
- Explain how the number of data lines differs between the methods.
- Compare the advantages and disadvantages of each method.
- Explain why serial transmission is suitable over longer distances.
- Select a suitable method for a stated scenario.
Key Terms
- Serial transmission
- Sending bits one after another along a single data channel.
- Parallel transmission
- Sending several bits at the same time along separate data channels.
- Data line
- A physical or logical path carrying transmitted bits.
- Skew
- A timing difference that causes parallel bits sent together to arrive at different times.
- Interference
- Unwanted electrical or electromagnetic effects that can alter transmitted data.
How Serial Transmission Works
In serial transmission, the bits of a byte or data item are sent one after another. A single data line can carry the sequence 1, then 0, then 1 and so on. The receiving device collects the bits in the correct order and rebuilds the original value.
Serial transmission uses fewer wires or channels than parallel transmission. This normally makes the connection cheaper, smaller and easier to use over long distances. Because there are fewer adjacent lines carrying signals at the same time, problems caused by interference between lines are reduced.
A common mistake is to assume that serial must always be slow because it sends one bit at a time. Modern serial interfaces can operate at very high signalling rates. In syllabus questions, compare methods using the stated scenario rather than treating one method as universally faster.
How Parallel Transmission Works
In parallel transmission, multiple bits are sent simultaneously. To transmit an 8-bit value in one group, eight data lines can each carry one bit. The receiving device reads the values from all lines and combines them.
Sending several bits at once can make parallel transmission suitable for short internal connections where the lines are kept close and their timing can be controlled. However, parallel connections require more wires and a larger connector. This increases cost and physical complexity.
Over longer distances, signals on separate wires may not arrive at exactly the same time. This timing problem is called skew. Interference between nearby lines can also affect reliability. These disadvantages are why serial transmission is generally preferred for longer external links.
Comparing Speed Correctly
At a simple conceptual level, parallel transmission sends several bits during the time that serial sends one bit at the same signalling rate. This is the source of the traditional speed advantage of parallel transmission over a short distance.
However, a real comparison also depends on the signalling rate, distance, design and interference. The examination typically expects advantages and disadvantages rather than an absolute claim. A strong answer states the conditions: parallel can transfer several bits simultaneously over a short distance, while serial needs fewer lines and remains reliable over a longer distance.
Choosing For A Scenario
For a long cable between two devices, serial transmission is usually the more suitable answer because it uses fewer wires, costs less and is less affected by skew and cross-line interference.
For a short internal connection where several bits need to move together and extra lines are practical, parallel transmission may be selected. The explanation should use facts from the scenario, such as distance, cost, connector size or required data transfer.
When the question asks for a method and justification, naming the method earns only part of the credit. The reason must explain why the method fits the context.
Serial And Parallel Compared
| Feature | Serial | Parallel |
|---|---|---|
| Bits sent | One after another | Several at the same time |
| Data lines | Usually one data line for the stream | Several separate data lines |
| Wiring | Fewer wires; smaller connector | More wires; larger connector |
| Distance | Suitable for longer distances | More suitable for short distances |
| Timing | No between-line skew | Bits can arrive at different times |
| Interference | Less cross-line interference | More risk between adjacent lines |
Scenario Decisions
| Scenario | Likely Choice | Reason |
|---|---|---|
| Long external cable | Serial | Fewer wires and reduced skew/interference. |
| Short internal bus | Parallel | Several bits can be transferred simultaneously. |
| Small portable connector | Serial | Fewer pins are needed. |
Worked Examples
Selecting A Printer Connection
Question: A printer is five metres from a computer. Choose serial or parallel transmission.
- The distance is longer than a short internal connection.
- A parallel cable would need many wires and could suffer timing differences.
- Serial uses fewer wires and is more reliable over the distance.
Answer: Serial transmission is more suitable.
Internal Register Transfer
Question: Eight bits must move a very short distance between components at the same time. Which method is suitable?
- The distance is short.
- Several physical lines can be used without a large external cable.
- All eight bits can be transferred simultaneously.
Answer: Parallel transmission can be suitable.
Explaining Skew
Question: Why can a parallel byte be read incorrectly after travelling a long distance?
- Each bit travels on a separate line.
- The signals may experience slightly different delays.
- The receiver may sample before all bits from the same group have arrived.
Answer: Skew can cause bits intended to arrive together to be received at different times.
Examination Guidance
- Define the methods using the way bits are sent.
- Compare cost, number of wires, distance, interference and skew.
- Do not write that serial is always slower.
- Link the choice directly to the distance and physical conditions in the scenario.
Common Mistakes
- Confusing serial transmission with simplex transmission.
- Saying parallel uses one wire.
- Claiming skew affects a single serial data line in the same way as multiple parallel lines.
- Giving a method without a reason.
Knowledge Check
1. How are bits sent in serial transmission?
2. How are bits sent in parallel transmission?
3. Why is serial suitable over a long distance?
4. What is skew?
5. Give one disadvantage of parallel transmission.
