4.1.4 - Wired and wireless performance
In specification point 4.1.4, you will learn why two connections to the same network can feel very different. You will separate bandwidth from actual speed and latency, then compare the range and performance of copper, fibre, Wi-Fi, Bluetooth, Zigbee, RFID and NFC. By the end, you will be able to justify a wired, wireless or mixed choice from the requirements in a scenario.
Four measures of performance
Network performance is not one measurement. A connection can have plenty of capacity but still respond slowly, or it can respond quickly while being unable to carry much data at once. Use the four measures precisely:
| Measure | Precise meaning | Common unit | Better performance usually means |
|---|---|---|---|
| Bandwidth | The theoretical amount of data a connection can carry in a given time: its capacity. | bits per second (bit/s or bps) | Higher bandwidth |
| Speed | The actual rate at which data is transferred at a particular time. | bits per second (bit/s or bps) | Higher speed |
| Range | The maximum distance over which the signal can be received well enough for the connection to work. | metres (m) | Enough range for the required distance |
| Latency | The delay between data being sent and received. | milliseconds (ms) | Lower latency |
Bandwidth and speed use the same kind of unit, but they are not the same idea. Think of bandwidth as the link's capacity and speed as how much data is actually getting through each second. Actual speed is normally below the theoretical bandwidth because capacity may be shared and transmissions use some capacity for control information. Interference, waiting and retransmission can reduce it further.
Latency is separate again. Increasing bandwidth may reduce waiting when a busy link is the cause, but it does not remove every delay. A connection can therefore have high bandwidth and high latency at the same time.
Worked example
A wireless link has these supplied measurements:
- theoretical capacity:
600 Mb/s - actual transfer rate during a download:
180 Mb/s - send-to-receive delay:
24 ms - greatest usable distance in the test area:
30 m
| Supplied value | Performance measure | Reason |
|---|---|---|
600 Mb/s | bandwidth | It is the theoretical capacity. |
180 Mb/s | speed | It is the achieved transfer rate at that time. |
24 ms | latency | It measures delay, so a lower value would be more responsive. |
30 m | range | It is the maximum usable distance in this test. |
The difference between 600 Mb/s and 180 Mb/s is not evidence that either value is necessarily wrong: one is capacity and the other is achieved speed under the stated conditions.
Bandwidth is capacity, speed is the achieved rate, range is usable distance, and latency is delay.
Copper and fibre connections
A wired connection carries data along a physical cable. Copper cable represents data using electrical signals; fibre-optic cable represents data using pulses of light. Both can provide fast, low-latency connections, but their characteristics make them suitable for different distances and environments.
| Performance feature | Copper cable | Fibre-optic cable |
|---|---|---|
| Bandwidth and speed | Can provide high bandwidth and high actual speed over suitable local cable runs. The cable category and connected equipment set limits. | Can support very high bandwidth and maintain high speeds over much longer links. The connected equipment can still be the bottleneck. |
| Range | Has a shorter practical range than fibre for common network cabling. Electrical signals weaken as cable length increases. | Has a much longer practical range because light can travel through fibre with low signal loss. |
| Latency | Usually low and consistent on a correctly installed local link. | Also usually low and consistent. It is not correct to claim that the medium removes all end-to-end delay. |
| Environment | Electrical interference can disturb signals, although suitable cable and shielding reduce this. | The light signal is not affected by electromagnetic interference, so fibre is useful near powerful electrical equipment. |
A cable usually gives a device a stable physical link to the next network device. It is not competing with nearby radio transmitters for that short connection, so its actual speed and latency tend to be more consistent than a busy or weak wireless link. This is a tendency, not a guarantee: damaged, old or unsuitable cable and slow connected equipment can still limit performance.
Worked choice: a design studio
The studio has fixed desktop computers in one room and a media server in another building.
- Desktops in the room: copper is a sensible choice. The distance is short, the computers do not move, and a suitable copper link can provide high speed with low, predictable latency.
- Link between buildings: fibre is a stronger choice. Its greater range and high bandwidth suit a long link carrying large media files, and electromagnetic interference will not disturb the signal in the fibre.
Notice the response shape: it names the medium, identifies a relevant characteristic and links that characteristic to the context. Saying only "fibre is better" does not explain which performance requirement it meets.
Wireless media and their environment
A wireless connection sends data through the air using electromagnetic signals rather than a cable to the device. This allows movement and avoids a cable run, but distance, obstacles, interference and other users can change its performance.
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