Standards and Concepts · Topic 2 of 6
Bandwidth, Throughput, Latency and Speed Tests
Understand the difference between bandwidth and throughput, what latency and delay mean, and how an online speed test differs from an Iperf test.
- 4 min
- Bandwidth is the maximum capacity of a link. Throughput is the real data rate.
- Throughput is normally lower than bandwidth.
- Latency is the time data takes to travel, measured in milliseconds.
What it is
Bandwidth, throughput and latency are the three numbers people use to describe how well a network performs.
Bandwidth is the maximum amount of data a link can carry in a given time. Throughput is the amount that really gets through. Latency is how long data takes to travel.
Think of a road. Bandwidth is the number of lanes, throughput is the number of cars that actually pass each minute, and latency is the journey time.
How it works
Bandwidth is a limit set by the link, while throughput and latency depend on what happens on the path.
Bandwidth
Bandwidth is measured in bits per second. Common units are kilobits (Kbps), megabits (Mbps) and gigabits (Gbps) per second.
A Fast Ethernet port is rated at 100 Mbps, and a Gigabit Ethernet port at 1000 Mbps. These are the best case figures. A network card or cable that is faulty may run slower.
Note that network speeds use bits, while file sizes use bytes. One byte is eight bits, so a 100 Mbps link moves at most about 12.5 megabytes each second.
Throughput
Throughput is the real data rate that you measure. It is almost always lower than the bandwidth.
Several things reduce it:
- Other users sharing the same link.
- Headers and acknowledgements, which are overhead and not user data.
- Lost packets that have to be sent again.
- A slow device or a slower link along the path.
The slowest link on the path sets the upper limit for the whole path. This is called the bottleneck.
Latency and delay
Latency is the time it takes for data to get from the source to the destination. It is measured in milliseconds (ms). The round trip time, or RTT, is the time for a packet to go there and come back.
Delay has several parts:
| Type of delay | Cause |
|---|---|
| Propagation delay | The time a signal takes to travel along the cable or through the air |
| Serialisation delay | The time to put all the bits of a frame onto the link |
| Queuing delay | Time spent waiting in a buffer when a device is busy |
| Processing delay | Time a device takes to read the header and decide where to send the packet |
More bandwidth does not remove propagation delay. A satellite link can have high bandwidth and still have high latency because of the distance.
Latency matters most for voice, video calls and online games. File downloads care more about throughput.
Speed test versus Iperf
A speed test is usually a website or app. It measures the path between your device and a server run by the test provider on the internet. It shows download speed, upload speed and often the latency.
Iperf is a tool that you run yourself. One computer runs it as a server and another runs it as a client. It measures the throughput between those two computers, so you can test your own LAN or a link you control.
iperf3 -s
iperf3 -c 192.168.1.10The first command starts the server. The second runs a client test against the server at that address.
What to remember for the exam
Six facts cover most questions on this topic.
- Bandwidth is the maximum capacity of a link. Throughput is the real data rate.
- Throughput is normally lower than bandwidth.
- Latency is the time data takes to travel, measured in milliseconds.
- Delay comes from propagation, serialisation, queuing and processing.
- A speed test measures towards a server on the internet. Iperf measures between two devices you choose.
- The slowest link on a path limits the throughput of the whole path.
If a question says "maximum", think bandwidth. If it says "actual" or "measured", think throughput.
Common mistakes
These four errors appear again and again.
Treating bandwidth and throughput as the same thing. One is the capacity of the link, and the other is what you really get.
Confusing speed with latency. A link can be fast and still have high latency. Adding bandwidth does not shorten the distance.
Mixing up bits and bytes. Link speeds are in bits per second, and file sizes are in bytes.
Using a speed test to check a local link. A speed test also includes your internet connection, so use Iperf to test between two local computers.
A low speed test result does not prove the LAN is at fault. The cause may be the internet connection or the test server.
Check yourself
Work through these three questions.
A link is rated at 1 Gbps but file copies reach only 600 Mbps. Which number is the throughput?
The 600 Mbps is the throughput, because it is the real rate measured. The 1 Gbps is the bandwidth, which is the maximum.
Which kind of delay is caused by a frame waiting in a busy device's buffer?
That is queuing delay.
When would you choose Iperf instead of a web based speed test?
Use Iperf when you want to measure the throughput between two devices on your own network. A speed test measures the path to a server on the internet.
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