How to design a Wi-Fi system – coverage

There are three main concerns when designing a Wi-Fi system: coverage, throughput, and features. In this post we consider coverage.

First a note on coverage as a distinct concern: It is not possible to completely separate issues of coverage from throughput. Better coverage will very often lead to better throughput. We will find out more about this in the post on throughput. Also some features are dependent on coverage. Again we will find out why in that post.

Terminology: A devices that provides Wi-Fi coverage is called an access point (AP). In commercial deployments APs are usually attached to ceilings or are fixed high up on walls, but they can be placed on surfaces. Most home and small business ‘routers’ now contain an AP, along with several other items of network technology. Any Wi-Fi enabled device that connects to an AP is called a station (STA). They includes all modern laptops, tablets, mobile phones, most e-readers, many games consoles, and increasingly TVs and other consumer electronics. In fact an AP is a special kind of STA which provides access to a distribution network for the STAs connected to it. A Wi-Fi system is an example of a wireless local area network (WLAN). Attenuation of a signal is weakening of it.

Coverage is about having a Wi-Fi signal where it is required. This has three main considerations: the number of APs that serve a location, the band of radio frequencies used, and obstacles to coverage.

The first consideration is the number of APs that serve a location. Even though most Wi-Fi systems try to ensure each location is covered by an AP, inevitably AP ranges overlap. So many locations will be covered by more than one AP. Indeed, in some cases we deliberately ensure that this is the case. This can be a kind of insurance policy against problems with APs. If your Wi-Fi coverage is critical or very important in some locations then you may wish to ensure they are covered by more than one AP. However, for Wi-Fi to work well APs with overlapping ranges should work on different radio frequencies. For a number of technical reasons this makes Wi-Fi more efficient, but for now it is enough to say we are trying to avoid interference. The most commonly used frequencies are in what is referred to as the 2.4 GHz band, which is available for use without a radio operator’s license. Consequently it is popular with many kinds of transmitting devices, which can be a problem; more on this later. The 2.4 GHz band is divided into 13 channels in the UK and Europe. Unfortunately each Wi-Fi channel is wider than one of these 13 channels, but four Wi-Fi channels can be well enough separated for tolerable interference levels by centring them on channels 1,5,9,13. In the US the situation is worse as they only have 11 channels in the 2.4 GHz band, so they can use only three Wi-Fi channels centred on channels 1,6,11. Much equipment is designed in America and they set defaults to suit their own market you will often see their pattern of channels 1,6,11 used in the UK too. Anyway, try to arrange things so that the APs that are closest together are not centred on the same channel. As your neighbours will also likely have Wi-Fi you may have to take this into account as well. So it may be best to start planning your channels by looking at what you are receiving from your neighbours at your boundary if coverage there is important.

The second consideration is the band of radio frequencies to use. In fact Wi-Fi can work in two radio frequency bands. The less commonly used higher frequency band is referred to as the 5 GHz band, which is also available for use without a radio operator’s license. In the UK it has 19 channels, although for arcane reasons only 16 are typically available. The 5 GHz band channels are a little complicated so we won’t discuss them here, but importantly 5 GHz Wi-Fi channels fit into them without interfering with their neighbours. These 5 GHz channels have two properties that are important to us that are due to the laws of physics for radio waves at higher frequencies. Firstly they don’t travel as far as 2.4 GHz band radio waves. Secondly it is easier to get them to transmit more information in an amount of time. For many the reduced range is the more important property. You will need to fit more APs working at 5 GHz for the same coverage so the cost of the system is higher. If however higher throughput and/or stability is important, then consider using the 5 GHz band. There is another important point to make about the 5 GHz band. Currently it is much less used so it is easier to get a signal free from interference. If your 2.4 GHz band is a mess with all sorts of transmissions you should consider using the 5 GHz band. The last thing we will say about the 5 GHz band is that the latest and greatest version of Wi-Fi – so called 5G Wi-Fi (802.11ac) – only works in this band. So if you are planning to upgrade from a 2.4 GHz system to 5G Wi-Fi, unless your existing system was designed for 5 GHz you will have more APs requiring extra cabling, almost certainly requiring existing cables to be repositioned, and probably requiring replacement of switches they connect to or extra switches.

The third consideration is that everything that Wi-Fi signals pass through attenuates them. Two things are particularly problematic: water and metal. Water absorbs Wi-Fi signals. As people are about 60% water they are a significant problem. That is why we prefer to place access points high up, so their signals pass more through air and less through people. This is particularly important where many people are close together, such as in locations that host events like conferences and social gatherings. Metal reflects Wi-Fi signals. So access points are generally best positioned away from metal. Unfortunately most buildings contain many metal parts. For example supporting columns contain steel, concrete is usually reinforced with steel, doorways may have a steel support above them, and separate rooms that have been made into one by removing a wall will have steel supporting the span. Some buildings use steel reinforced concrete for walls and floors. Stairs in non-domestic buildings are usually made of steel. Electrical, plumbing, and air conditioning infrastructure is mostly metal. In larger buildings you may need to consider where water and metal infrastructure is as they become significant sized obstructions. As a very rough guide, in the UK a 2.4 GHz signal will be usable in the next room, but weak in the room after that, so corridors can be good locations for access points to cover multiple rooms. Distance also weakens Wi-Fi signals, so downgrade your expectations with larger than average size rooms. Also downgrade coverage expectations significantly for 5 GHz. If it is important to get coverage right or you have doubts get one access point and test it in possible locations. There are many test tools, from free mobile phone apps through to very expensive professional equipment. Lastly, if your job or reputation will be damaged by getting it wrong you should probably call us. The small amount we charge for installing it is partly offset by the better prices we get for equipment, the more appropriate selection of equipment we will make, our more accurate and efficient installation and configuration, and your time saved.

Myrmidon Access Points

To make affordable WLANs capable of handling the large numbers of connections and high throughput most of us anticipate, I would like two classes of access point. The smart access points that we already have would work with many more much simpler and therefore much cheaper myrmidon access points that make most of the connections and shift most of the data. These two classes of access point must occupy the same space so that the advantages of each are always available.
To be cheap enough to be deployed in large numbers myrmidons must be very simple, specialising only in high numbers of connections and / or high throughput. Ideally they should also be small and use little power, but importantly they should require no individual human configuration or attention. As they need to coexist in the same space as smart Wi-Fi based APs they would be advantaged in using out-of-band wireless technologies like 802.11ad / WirelessHD / WiGig, DASH7, Zigbee, and Li-Fi. The sophistication they need but lack is delegated to specialised proximate controller devices. Each controller will orchestrate the configuration and behaviour of large numbers of myrmidons according to localised conditions and usage patterns, and in anticipation of events.
Right now I would like to be installing myrmidons. Desks are an obvious place, but the lower price of this capability enables it to be installed in many more locations. For example, it would be much more affordable to fit out large venue halls, sports stadiums, and outdoor locations such as car parks and playgrounds. It would also help low margin businesses such as mainstream hotels and high street shops to offer a better wireless connectivity experience. As the internet-of-things, low cost robotics, WPANs, BANs, wearables, and wireless sensors become more common so we will need this kind of WLAN.



This Wilocity chips sounds like a potential candidate to enable the client side connection to myrmidons