How many access points does a school need: sized by density, not floor area

An access point shares its capacity among all the devices

An access point does not just have to provide coverage: it also has to share its capacity among all the connected devices. And in a classroom with many devices, that sharing becomes the real limit long before the signal runs short. That is why, to work out how many access points a school needs, how many devices will connect at once matters far more than the building's square metres.

Wi-Fi runs over a shared medium. Devices connected to the same access point and the same channel have to take turns to transmit. That resource, known as airtime, is limited. The more devices compete for it, the less time each one has available and the more likely delays and slowness become.

This gives rise to one of the most common mistakes when designing a Wi-Fi network. An access point placed in a corridor can give a good signal to three classrooms in a row, but coverage and capacity are different things. All three classrooms can show excellent coverage, yet all their devices are still sharing the capacity of the same access point.

Ninety devices sharing that capacity behave very differently from thirty, even if the coverage map is identical. So a Wi-Fi survey can show an excellent signal and the real experience in the classroom can still be poor. The type of device matters too. A six-year-old tablet at the back of the classroom may need more time to transmit the same information than a modern laptop close to the access point.

Add to this the moments when connections are most concentrated. At the start of a lesson, dozens of devices can wake from sleep, connect and authenticate almost simultaneously. That first-minute rush, which can repeat at every change of lesson, can cause problems long before the speed of the internet connection becomes the limit.

• The practical limit of a classroom depends to a large extent on the airtime available, not just on the power or range of the signal.
• A correct coverage map does not show how many devices will have to share the capacity of each access point.
• Problems can appear when many devices connect and authenticate simultaneously, even before they start downloading content.

The first figure to know: concurrent devices

Sizing a network by density starts with a very specific question: how many devices will be connected at the same time in each space at the busiest moments? The total number of pupils in the school does not answer that question. What matters is knowing how many devices will be connected simultaneously in each classroom or space.

On top of the pupils' devices come the teacher's computer, the interactive panel, wireless projection systems and other networked devices the school has added over the years. That count must also take into account how the devices are used, because checking an educational platform does not place the same demand on the network as making a video call or sitting an online exam.

The figure we need, therefore, is how many devices will be connected simultaneously in each space, at what times and for what purpose. The total number of devices in the school, on its own, is no use for sizing a classroom.

Why there is no universal figure

The number of devices an access point can serve with good performance depends on many factors: what those devices are doing, their age and capabilities, the bands available, the characteristics of the building and interference from nearby access points. That is why there is no formula that works for every school, and no reliable figure can be given without knowing how the network is really used.

Nor does it help to go just by the maximum number of clients on the manufacturer's datasheet. That figure reflects how many devices the access point is capable of accepting, but not how many can work simultaneously with good performance. It is one thing for an access point to be able to accept many devices and quite another for them all to work smoothly at the same time. So the maximum number of clients stated by the manufacturer is not enough on its own to size the network.

In schools where every pupil works with their own device, a common rule of thumb is to install one access point per classroom. But it is only a starting point: the final figure will depend on the number of devices, the type of use and the characteristics of the building.

Installing more access points than necessary is not a good idea either. If they are not well planned, they can interfere with each other, increase costs and fail to improve network performance.

• There is no fixed number of devices per access point: it depends on usage, equipment and the characteristics of the building.
• The maximum number of clients stated by the manufacturer is not the same as the number of devices that can work simultaneously with good performance.
• Installing more access points does not necessarily improve the network if there is no proper planning of channels, power and placement.

Why the access point usually goes inside the classroom

Placing access points in the corridor can seem a simple solution because it makes cabling easier and allows several classrooms to be covered from one spot. The problem is that Wi-Fi communication works both ways. The access point can transmit with enough power to get through a wall and reach a device inside the classroom, but that device normally transmits with considerably less power and has to get its signal to make the return journey.

That is why a device can show good coverage and yet have trouble communicating properly with the access point. What is more, a unit in the corridor can end up serving devices in several classrooms at once. This increases the number of devices sharing its capacity and can lead some devices to connect to an access point that is not the most suitable one.

The building's own walls, although they hinder coverage, can also help to separate classrooms from each other and reduce interference between nearby access points. So where the equipment goes has to be decided with an eye both to coverage and to capacity, and to how channels are distributed within the building.

The bands, and the spaces that are not classrooms

The three Wi-Fi bands have different characteristics and do not play exactly the same role. The 2.4 GHz band reaches further and gets through walls better, but has few channels and shares frequencies with other devices. In high-density environments it is used mainly to maintain compatibility with devices that still need it.

The 5 GHz band currently carries much of the traffic from the school's devices. The 6 GHz band, available with Wi-Fi 6E and Wi-Fi 7, offers more spectrum and has the advantage that older Wi-Fi devices do not use it. It can be particularly useful in high-density classrooms, provided the school's devices are compatible.

Channel width also has to be decided. Using wider channels can increase a device's maximum speed, but it reduces the number of independent channels available. In a building with many classrooms it usually pays to strike a balance between speed and capacity so that channels can be reused without causing interference.

And a school is not made up of classrooms alone. An assembly hall can go from empty to holding hundreds of people and devices within a few minutes. The dining hall has a more open floor area, different times of occupancy and may have other sources of interference.

In a sports hall, mounting an access point very high up can make communication with devices at floor level more difficult. A library may keep a moderate number of devices connected for many hours. And the playground needs a specific approach, with equipment designed for outdoor use and taking into account how its signal may affect nearby classrooms.

• 2.4 GHz: greater range and compatibility, but few channels available and more chance of interference.
• 5 GHz: carries much of the school's everyday Wi-Fi traffic.
• 6 GHz: offers more spectrum for high-density environments when devices are compatible.
• Assembly hall, dining hall, sports hall, library and playground need to be looked at individually because each space has a different usage pattern.

The figure comes from the school itself

The answer to how many access points a school needs does not come from square metres alone. The first step is to carry out a study on the school's floor plans that takes into account the layout of each floor, the materials, the number of devices expected in each space and the type of use.

From there, the initial placement of the access points and the band and channel planning can be worked out. Then the actual behaviour of the signal in the building has to be checked, because a floor plan does not reflect every constraint that may come up during an installation.

A partition wall that looked light may be much thicker than expected. A metal cabinet can alter how the signal propagates. And two wings built at different times can behave completely differently as far as Wi-Fi coverage is concerned.

All of that can change the final number and placement of the access points. On top of that, each access point needs other infrastructure: a switch port, PoE power, cabling to its location and enough capacity on the network links. That is why installing more equipment than needed increases the investment and can create interference, while installing fewer access points than needed can lead to performance problems when the classrooms are in full use.

At PenwinEdu, with some basic information about the school and the experience we have gained across hundreds of schools and colleges, we can size the network and prepare a proposal adapted to the real needs of each space. To do so, we look at questions such as these:

• How many devices will be connected simultaneously in each classroom or space?
• What will the network be used for in each area: browsing, video, online exams, video calls, educational apps…?
• What is the building like, and which materials may affect coverage?
• Where should each access point go to ensure coverage and capacity?
• Which areas need specific treatment, such as playgrounds, sports halls, dining halls or assembly halls?
• What network infrastructure is already in place, and which part of it can be reused?
• Are there enough PoE ports, available power on the switches and capacity on the network links?
• What additional cabling will be needed to bring each access point to its location?
• What checks and measurements will need to be carried out at the school itself to fine-tune the final design?