Provider offers talk almost exclusively about speed: 1 gigabit, 2.5 gigabit, 10 gigabit. Yet it happens that a very fast line gives you choppy video calls, or a modest connection works perfectly well for work. The reason is that "speed" is only one of the quantities that describe a network. To understand how it will really behave you need at least four.
The four quantities
Bandwidth
Bandwidth (or capacity) is the amount of data the line can carry per second, expressed in bits per second: Mbit/s or Gbit/s. Mind the unit: a 100 megabyte (MB) file contains 800 megabits (Mb). On a 100 Mbit/s line you download it, ideally, in about 8 seconds, not one.
Upload bandwidth matters a lot too, and on asymmetric technologies it is often much lower than download. It is what you use to send video in a call, upload files to the cloud or run remote backups.
Latency
Latency is the time a packet takes to go and come back (round-trip time), measured in milliseconds. It depends on:
- physical distance: light in fibre travels about 200 km per millisecond, so a round trip between Milan and New York cannot drop below several tens of milliseconds;
- the number and load of devices crossed;
- the access technology: fibre to the home has lower latency than a radio network or geostationary satellite;
- queues in network devices, which can add a great deal of delay when the line is saturated.
Jitter
Jitter is the variability of latency. If one packet takes 20 ms, the next 80 and the third 25, jitter is high. For real-time applications it is often more harmful than high but steady latency, because it forces data to be buffered to smooth out the irregularities.
Packet loss
Packet loss is the percentage of packets that never arrive. Even a constant 1% is a serious problem: TCP slows down dramatically to retransmit, while voice and video show dropouts and artefacts. Typical causes are a congested Wi-Fi channel, a damaged cable, a line with physical faults or an overloaded device.
What matters for each activity
| Activity | Bandwidth | Latency | Jitter and loss |
|---|---|---|---|
| Video streaming | Important | Barely relevant | Barely relevant (there is a buffer) |
| Video calls | Moderate, especially upload | Important | Very important |
| Online gaming | Low | Very important | Very important |
| Large downloads | Very important | Barely relevant | Loss matters |
| Web browsing | Moderate | Important | Moderately |
Browsing is an interesting case: a page requires many successive exchanges between browser and server, each paying the latency cost. Beyond a certain threshold, doubling bandwidth helps little, while reducing latency is felt immediately.
How to read a speed test
A speed test measures download and upload bandwidth and, usually, idle latency. To make it meaningful:
- run it from a computer connected by cable, not over Wi-Fi, if you want to assess the line rather than the wireless network;
- close other activities using the network;
- repeat it at different times, for example in the evening, when the provider's network is busiest;
- also look at latency under load, which many modern tests report: if it rises by hundreds of milliseconds during the download, you have a bufferbloat problem.
Remember that the test measures the path to one specific server. An excellent result to a nearby server does not guarantee the same performance to a service on the other side of the world.
Measuring with system tools
pingto the router shows the quality of the local network: on cable you should see under 1 ms, on Wi-Fi a few milliseconds with no loss.pingto an external server shows latency, jitter (from the variation in times) and loss towards the Internet.mtron Linux and macOS, orpathpingon Windows, combine ping and traceroute and show on which segment delay and loss appear.
How to improve
- Wire the fixed devices that benefit from it: work PCs, consoles, TVs.
- Improve Wi-Fi coverage rather than raising transmit power: more well-placed access points reduce latency and loss.
- Enable smart queue management (SQM) on the router to avoid bufferbloat.
- If the line shows loss even over cable, collect mtr results and report them to the provider: hard data is far more effective than a vague "it is slow".
In short
- Bandwidth, latency, jitter and loss describe different aspects of network quality.
- For calls and gaming, stable latency and no loss matter more than bandwidth.
- Tell megabits from megabytes, and do not neglect upload bandwidth.
- Run speed tests over cable, at different times, watching latency under load.
- With ping and mtr you can locate where delay and loss originate.
