WiFi – and the strange letters

Introduction

People often ask: how fast is my network-connection?
The answer is: it depends.

It depends on how new and advanced your access point/router and device are. It depends on the distance between your access point and your device. It depends on physical barriers such as walls or furniture. It depends on your other devices and maybe even on your neighbors’ devices – which again depends on the chosen channels on your and your neighbor’s networks. If you are addressing something on the internet it depends on your wired internet-connection. Finally, it also depends on the server at the other end. This is something we sometimes forget, but in some cases, this can actually be your problem.

Brief History

In this “blog” I am focusing on the WiFi part. The following table is a reference for the whole page.

New NameLetterBandFeatures
(1)802.11a5 GHzNever really used
(2)802.11b2.4 GHzFirst mainstream technology.
Max speed 11 Mbps.
3802.11g2.4 GHzLong time popular.
Max speed 54 Mbps
4802.11n2.4 + 5 GHzHuge step forward. HT and MIMO.
Much Faster encodings.
Brought 5 GHz to the people.
5802.11ac5 GHzVHT.  MU-MIMO.
6802.11ax2.4 + 5 GHzUpcoming.

The above table introduces the main WiFi generations. Note the “New Name” column that uses the new generation numbers from the WiFi-alliance. These are meant to simplify matters. They basically start at “3” – I have guessed what 1 and 2 are. There are a number of intermediary “sub-generations” that I have skipped here.

The first really used WiFi networks were based on 802.11b – with theoretical speeds up to 11 Mbps. Later came 802.11g with speeds up to 54 Mbps. This lasted for a long time with a lot of vendor-specific enhancements. These networks all worked on 2.4 GHz.

Then came 802.11n which was the first practical introduction of 5 GHz networks. On top of this, 802.11n also vastly improved in the 2.4 GHz band. Now we saw speeds above 300 Mbps.

Current Status

Basically, we now have two bands: 2.4 GHz and 5 GHz. The 2.4 GHz band is very crowded. Some of the old WI-Fi equipment only works here, there are few channels to choose between, and it might even be disturbed by microwave ovens, movement detectors and other electronics. So why use it at all?

The simple answer is that radio-waves at 2.4 GHz reaches further out and penetrates walls etc. much better than at 5 GHz. In my house I have Chromecast devices in the basement and on the second floor – and my access point on the ground floor – between the Chromecasts. My Netflix works fine when I use a 2.4 GHz channel, but stops all the time if I use a 5 GHz channel. And yes – I really ought to put up those wires.

Most modern access points have two transmitter and receiver sets: one works in the 2.4 GHz band and another one works in the 5 GHz band. You may have an SSID for each of these bands, or you may use the same SSID on both bands. Using separate SSIDs give you more control. From your device you can select which SSID to use – and thereby which band. On the other hand – using the same SSID gives you more flexibility, and access points are getting better at handing over a device from one to the other band as your device moves. Theoretically the access point may keep devices in the roomy 5 GHz band until the reception at either end becomes to bad, in which case the access point may move you to 2.4 GHz. This doesn’t work very well in my installation.

Various info on networks and channels on 2.4 GHz.

Access points may be configured as to which channel to use in either band – or you can select “auto”. In any case the access point stays pretty fixed on the chosen channel. Even though the access point only has a single channel in each band in use, this does not mean that it cannot be shared among the devices. In fact, the devices are “time-sharing” the channel and to the users it seems like parallel connections.

In the 2.4 GHz band most countries allow 11 channels – numbered from 1 to 11. The gap between each of these is 5 MHz, but as a normal transmission occupies 20 MHz there is only room for 3 simultaneous channels in the air. Until a few years ago you could agree with your neighbors that one of you stay on channel 1, another on channel 6 and the last on channel 11. That was fine with 802.11b or 802.11g. However, with 802.11n this changed as this standard introduced HT (High Throughput). An important part of HT is “HT channels” that are 40 MHz wide. In this case there is only room for one HT channel in the 2.4 GHz band. 802.11n also introduced much more efficient physical encodings of the data. This makes the transmissions faster. You could also say that a given transmission bandwidth occupies less air time. A single 802.11b device can severely damage the efficiency of your whole network – and even your neighbors’ network. This is why you (and your neighbors) should get rid of any 802.11a/b/g devices.

802.11n revolutionized both the 2.4 GHz band and the 5 GHz band with the wider channels, the better physical encodings and with MIMO. The latter is the use of several antennas (multiple input – multiple output)– giving several “paths”. MIMO even takes advantage of reflections from e.g. walls. In older technologies, reflections were just seen as noise that might inhibit the reception. With 802.11n the reflections can be treated as a “free retransmission” – allowing the advanced DSP algorithms to dig out some data that might otherwise have been lost.

802.11ac is currently the latest affordable widespread technology. This only relates to the 5 GHz band. It introduces even wider channels (80MHz and 160MHz) and is thus called VHT for Very High Throughput. This is the case for “Elkbase5” in the figure above.

802.11ac also introduces MU-MIMO – multi-user MIMO. This is the first time we actually see a single access point truly work with more than one device at a time – at the same frequency. This is done through “beamforming”, where the beam from the antennas is directed against the relevant receiver by delaying the signal to one (or more) antenna slightly.

802.11ax is the next technology. This is not discussed here yet.

Connection Dynamics

All the listed WiFi technologies are backward compatible within the given band. So, to get back to the original question: “How fast is my network connection?”, it is clear that it depends on your devices. Newer devices are faster, but may be slowed down by older devices on the same network – or even in neighboring networks close to your network.

Even if you only have an access point and a single device, it can be difficult to predict the speed. When a device connects to an access point, there is a phase where they each inform the other about their capabilities. This can be features as described above, but also a number of minor features from the standard that is supported/enabled. The access point now selects the best parameter-subset that both parties support. There are also broadcasts and multicasts (please see my book “Embedded Software for the IoT” for details). For these the access point must select something that all participating devices support.

In many scenarios we also have CTS and RTS frames. These are negotiations needed before each actual transmission of a single block of data, in case there are older “b” or “g” devices present. They are short in terms of bytes, but as they need to be sent at low speeds in order for the slower devices to understand them, they “block the air” for an unproportionally long time. You can turn this “b/g support” off if you are sure there are no “b” or “g” devices in the vicinity. This is sometimes known as “greenfield” mode.

Finally, there may be small disturbances from e.g. electrical engines. This will cause the access point and the device to select slower, but more robust coding forms. Whenever it is safe, the speed goes up again. Thus, when a file is transmitted in many blocks, this may be at very different speeds.

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