AQA A-Level Computer Science (7517)

## Learning Outcomes

- Define serial transmission, parallel transmission, simplex, half-duplex and full-duplex accurately
- Explain why serial transmission is usually preferred over longer distances
- Distinguish clearly between bit rate, baud rate, bandwidth and latency
- Apply the formula for calculating bit rate from baud rate and bits per symbol
- Explain synchronous and asynchronous transmission, including the role of start and stop bits
- Describe the purpose of handshaking and why protocols are needed in communication

## AQA A-Level Computer Science (7517) Syllabus

For the AQA A-Level Computer Science (7517), you are expected to understand communication basics, with a focus on the following syllabus points:

- Know the difference between **serial transmission** and **parallel transmission**
- Explain why parallel transmission is generally less reliable over longer distances
- Identify and compare **simplex**, **half-duplex** and **full-duplex** communication
- Define **bit rate**, **baud rate**, **bandwidth** and **latency** precisely
- Understand the relationship between bit rate, baud rate and bits per symbol
- Explain the difference between **synchronous** and **asynchronous** transmission
- Describe the purpose of **start bits**, **stop bits** and **handshaking**
- Understand that **protocols** provide standard rules for communication between devices

## Test Your Knowledge

_Attempt these questions before reading this article. If you find some difficult or cannot remember the answers, look more closely at that area during your revision._

1. What is the difference between serial and parallel transmission?
2. Why can serial transmission achieve a higher overall throughput than parallel transmission?
3. What is the difference between bit rate and baud rate?
4. How does asynchronous transmission work without a shared clock?
5. What is the purpose of a communication protocol?

## Introduction

Communication in computer systems means sending data from one device to another in a form both devices can understand. That can happen inside a computer, between a computer and a peripheral, or across a network between computers. For AQA, this topic is not just about definitions. You need to explain how communication works, why some methods are better in some situations, and how measures such as bandwidth and latency affect performance.

Students often lose marks here by mixing up terms that sound similar. The big examples are **bit rate vs baud rate** and **bandwidth vs latency**. In exam answers, precision matters. If a question says "define", you need the exact meaning. If it says "explain", you need to show the mechanism.

> **Key Term: communication**
> The transfer of data between devices using agreed methods, signals and rules.

### Transmission methods: serial and parallel

The first distinction is between **serial** and **parallel** transmission.

> **Key Term: serial transmission**
> A method of transmission in which data is sent one bit at a time over a single channel or wire.
>
> **Key Term: parallel transmission**
> A method of transmission in which multiple bits are sent at the same time over multiple channels or wires.

At first glance, parallel transmission seems better. If 8 bits are sent together, it looks faster than sending 1 bit at a time. That is true in a very limited sense: **more bits are transferred in one clock cycle**. But exam questions often want you to explain why this does not automatically make parallel transmission better overall.

The problem is that each wire in a parallel link is slightly different. Over distance, the signals on different wires may not arrive at exactly the same moment. This is called **skew**. If one bit arrives late, the receiving device may read the wrong combination of bits.

A second problem is **crosstalk**. When wires are close together, the signal in one wire can interfere with signals in nearby wires. As frequency rises, this gets worse. So parallel transmission becomes less reliable when you try to use it over greater distances or at very high speeds.

That is why parallel transmission is mostly useful over **short distances**, especially inside a computer system, such as communication across internal buses or within RAM.

Serial transmission sends bits one after another along a single line. Each cycle carries less data, but there are major advantages:

- fewer wires, so connectors and cables are cheaper and simpler
- less interference because there is only one channel
- no skew between multiple data lines
- more reliable over longer distances
- can run at much higher frequencies

This last point is easy to miss. Even though serial sends fewer bits per cycle, it can use a much faster clock rate. As a result, **modern serial links can outperform parallel links** in total data transfer rate.

A good exam answer does not just say "serial is slower" or "parallel is faster". That is too simple and often wrong. The better comparison is:

- **Parallel may transfer more bits per cycle**
- **Serial can achieve higher overall throughput because it supports higher frequencies and suffers less from skew and crosstalk**
  > **Key Term: skew**
  > A timing problem in parallel transmission where bits sent at the same time arrive at slightly different times.
  >
  > **Key Term: crosstalk**
  > Interference caused by signals in one wire affecting signals in nearby wires.

A cautious set of examples is useful. Peripherals and network links commonly use serial communication. Internal buses have traditionally used parallel communication over short distances. The example matters less than the principle.

> **Test Tip:** In a compare question, give both sides. Do not say only that parallel sends more bits at once. Add why serial is usually preferred in practice: fewer wires, less interference, less skew, and better long-distance reliability.

Communication can also be classified by direction.

- **Simplex**: data travels in one direction only
- **Half-duplex**: data can travel both ways, but not at the same time
- **Full-duplex**: data can travel both ways at the same time
  > **Key Term: simplex**
  > Communication in one direction only.
  >
  > **Key Term: half-duplex**
  > Communication in both directions, but only one direction at a time.
  >
  > **Key Term: full-duplex**
  > Communication in both directions at the same time.

The distinction between half-duplex and full-duplex is tested often because students blur them together. A walkie-talkie is a common half-duplex example because only one person speaks at a time. A telephone call is a common full-duplex example because both people can speak and hear at once.

### Measuring transmission: bandwidth, bit rate, baud rate and latency

These four terms are related, but they are not the same.

> **Key Term: bit rate**
> The number of bits transmitted per second, measured in bits per second (bps).
>
> **Key Term: baud rate**
> The number of signal changes or symbols transmitted per second, measured in baud.
>
> **Key Term: bandwidth**
> The range of signal frequencies or the capacity of a transmission medium to carry data.
>
> **Key Term: latency**
> The delay between data being sent and the first part of it being received.

Start with **bit rate**. This is the amount of data transferred each second. If a connection sends 2,000,000 bits each second, its bit rate is 2,000,000 bps or 2 Mbps.

Now compare that with **baud rate**. Baud rate counts signal changes, not bits. If one signal change represents one bit, then baud rate and bit rate are equal. But if one signal change can represent more than one bit, then bit rate is higher than baud rate.

The key formula is:

bit rate = baud rate × bits per symbol

A symbol is one distinct signal state. If a system can use 4 distinct signal states, each symbol can represent 2 bits because 2 bits give 4 combinations: 00, 01, 10, 11.

Worked example 1:

A line has a baud rate of 1200 baud. Each symbol represents 1 bit.

- bit rate = baud rate × bits per symbol
- bit rate = 1200 × 1
- bit rate = **1200 bps**

Worked example 2:

A line has a baud rate of 3000 baud. Each symbol represents 2 bits.

- bit rate = baud rate × bits per symbol
- bit rate = 3000 × 2
- bit rate = **6000 bps**

Worked example 3:

A transmission system uses 8 distinct signal states.

- bits per symbol = log2(8)
- bits per symbol = 3

If the baud rate is 5000 baud:

- bit rate = 5000 × 3
- bit rate = **15,000 bps**

Why does this matter? Because exam questions may ask why bit rate can be greater than baud rate. The answer is not "because the signal is faster". The real reason is that **one signal change may encode multiple bits**.

Bandwidth is different again. It refers to the capacity of the channel. In simple A-level terms, greater bandwidth allows a higher possible bit rate because the medium can carry a wider range of signals. So the relationship is:

- more bandwidth usually allows a higher maximum bit rate
- bandwidth is not itself the number of bits currently being transmitted

Latency is separate from both. Latency is the delay before the first bit arrives. A connection can have:

- high bandwidth and high latency
- high bandwidth and low latency
- low bandwidth and low latency

For example, a long-distance link might carry huge amounts of data once the transfer begins, but still have a noticeable delay before the first signal arrives. So latency is about **delay**, not throughput.

Common student errors in this area:

- saying baud rate is bits per second
- saying bandwidth is the same as bit rate
- saying latency means slow transfer speed in general
- assuming parallel is always faster than serial
  > **Test Tip:** If the question asks for a definition, keep it tight. "Baud rate is the number of signal changes per second" scores better than a vague answer such as "how fast the signal goes".

### Timing and framing: synchronous and asynchronous transmission

For communication to work, the receiver must know **when** to read each bit. That is the main issue in synchronous and asynchronous transmission.

> **Key Term: synchronous transmission**
> A transmission method in which sender and receiver use synchronised clocks so data can be sent in timed blocks.
>
> **Key Term: asynchronous transmission**
> A transmission method in which sender and receiver do not share a clock, so each unit of data is framed with start and stop bits.

In **synchronous transmission**, both devices are synchronised. That means they agree exactly when each bit begins and ends. Data is often sent in blocks. Because fewer extra control bits are needed, synchronous transmission is efficient. The receiver knows when to sample the incoming data because timing is shared.

In **asynchronous transmission**, there is no shared clock signal. Instead, each item of data is wrapped in extra bits so the receiver can detect its boundaries.

> **Key Term: start bit**
> A bit placed at the beginning of a data unit in asynchronous transmission to tell the receiver to begin reading.
>
> **Key Term: stop bit**
> A bit placed at the end of a data unit in asynchronous transmission to show that the transmission unit has ended.

Here is the logic:

1. The line is idle.
2. A **start bit** signals that a new data unit is beginning.
3. The receiver uses its own internal clock to sample the incoming bits at agreed intervals.
4. The data bits are read.
5. A **stop bit** marks the end of that unit.

This makes asynchronous transmission simpler because devices do not need continuous clock synchronisation. But there is a trade-off: the extra start and stop bits add overhead, so it is less efficient than synchronous transmission for large volumes of data.

Some systems may also include a parity bit for error checking, but if a question is about timing, keep the focus on framing and clocking.

A short comparison:

- **Synchronous**: shared timing, block transmission, efficient, less overhead
- **Asynchronous**: no shared clock, uses start/stop bits, simpler, more overhead

Students often miss the "why". The reason start and stop bits are needed is not just "to show the beginning and end". It is because without a shared clock, the receiver otherwise would not know when to start sampling the signal.

### Handshaking and protocols

Communication is not only about electrical signals. Devices also need agreement about the rules being used.

> **Key Term: handshaking**
> The process by which two devices establish that they are ready to communicate and agree how the communication will proceed.
>
> **Key Term: protocol**
> A set of rules and standards that devices follow to communicate correctly.

Handshaking happens before or during transmission to make sure both devices are ready. One device may send a signal equivalent to "I want to send". The other replies "ready to receive". Only then does transmission begin.

Why is this necessary?

- the receiving device may not be ready
- devices may need to agree transmission speed
- devices may need to coordinate error checking or flow control
- data could be lost if one side starts before the other is prepared

This is particularly useful when devices operate at different speeds. A fast sender and a slower receiver need some way to avoid overflow.

Protocols go further than handshaking. A protocol defines the standard rules for communication so that devices from different manufacturers can still exchange data properly. A protocol may specify:

- data format
- addressing
- timing
- error detection and correction
- transmission speed
- how a connection starts and ends

Without protocols, two devices might both send bits successfully at the electrical level but still fail to understand one another because they package or interpret the data differently.

At this level, you should know that **TCP/IP**, **HTTP** and **FTP** are examples of protocols or protocol sets, but for communication basics the most important point is the general definition: a protocol is a set of rules.

A strong exam answer often explains the consequence of having no protocol. For example, if sender and receiver used different data formats or different error-checking methods, the message might be corrupted, rejected or misunderstood.

## Key Point Checklist

_This article has covered the following key knowledge points:_

- Serial transmission sends data one bit at a time over a single wire or channel
- Parallel transmission sends multiple bits simultaneously over multiple wires or channels
- Parallel transmission is more suitable over short distances because skew and crosstalk reduce reliability over longer distances
- Serial transmission is usually cheaper, simpler and more reliable over long distances
- Simplex is one-way only, half-duplex is two-way but not simultaneous, and full-duplex is two-way simultaneous
- Bit rate is the number of bits transmitted per second, measured in bps
- Baud rate is the number of signal changes or symbols per second, measured in baud
- Bit rate = baud rate × bits per symbol
- Bit rate can be higher than baud rate when one symbol represents more than one bit
- Bandwidth describes channel capacity and greater bandwidth usually allows a higher bit rate
- Latency is the delay before the first part of the data reaches its destination
- Synchronous transmission uses shared timing, while asynchronous transmission uses start and stop bits instead of a shared clock
- Handshaking checks that devices are ready to communicate
- Protocols are standard rules that allow devices to communicate accurately

## Key Terms and Concepts

- communication
- serial transmission
- parallel transmission
- skew
- crosstalk
- simplex
- half-duplex
- full-duplex
- bit rate
- baud rate
- bandwidth
- latency
- synchronous transmission
- asynchronous transmission
- start bit
- stop bit
- handshaking
- protocol


Communication and Networking - Communication basics