Why school Wi-Fi is one of the hardest environments to cover

Many clients, close together and at the same moment

A classroom with one device per pupil packs between twenty-five and thirty-five Wi-Fi clients into fifty or sixty square metres, and a building with twenty classrooms can have more than six hundred devices associated at once in the same time slot. That density is nothing like an office's or a hotel's: it means many clients, close together, asking for service at the same moment and sharing a resource limited by physics: the airtime of each radio channel.

Airtime is the part of the conversation that almost never makes it into the quote. A wireless network does not share out capacity the way a switch does, where each port has its own cable. All the devices associated with the same channel take turns to talk and only one transmits at any given instant. The busier the channel, the longer each device spends waiting for its turn, and that wait is exactly what a pupil experiences as a page that will not load.

That is why coverage and capacity are different things. An access point can light up several adjoining classrooms with a good signal and still leave all three of them slow, because the signal gets there but the turns to speak have to be shared among ninety devices. A classroom can show five bars of signal and still not work.

The start of every lesson triggers a very particular connection peak

The second problem is the usage pattern. At the start of every lesson, dozens of tablets and laptops wake from sleep at the same time, associate with the access point, request an address over DHCP, authenticate against the directory and start syncing with the cloud.

What gets jammed at that point is the burst of association and authentication packed into sixty or ninety seconds, which can repeat at every change of lesson. The average contracted internet bandwidth has little to do with it. A network sized on bandwidth alone fails right there, and the teacher experiences it as ‘the Wi-Fi isn't working today’.

An office building also has its rush hour, but arrivals are spread across a wide window. The devices then stay associated for the rest of the working day. In a school, by contrast, many of those peaks coincide with the start of lessons, repeat several times over the day and can affect a large part of the school at once.

On top of that, an online exam can have two hundred devices opening the same platform in the same minute. So when a network issue at a school is being investigated, the useful question is not how much speed the line delivers but what happens in the minutes when connections are most concentrated: how many associations complete, how many DHCP requests get an answer and how long authentication takes to come back with a yes.

A building that was not built for this

Add to that the building itself. Many schools occupy buildings that are several decades old, with thick load-bearing walls, concrete floor slabs, wings added in successive refurbishments and, sometimes, restrictions on drilling or running trunking. The same access point model that covers a large area in an open-plan warehouse gets no further than a classroom and a half when there is reinforced concrete in between. That is not something you find out from the manufacturer's datasheet.

A mix of buildings is the norm, and it is common for a single school to have a block from the sixties, an extension from the nineties and a recent early years building, linked by corridors and playgrounds, each with its own radio behaviour and with distances between racks that dictate where the cable can run. A design done only on the floor plan, without setting foot in the school, goes wrong precisely at those joins.

The service does not end at the classrooms, either. Playgrounds, sports courts, the sports hall, the dining hall, the assembly hall, reception and after-school activity spaces also need coverage, with equipment, power and design criteria that differ from those used indoors.

Five conditions not found in other buildings

In short, these are the constraints that any network project in a school runs into, and the ones that explain why a design intended for an office does not carry over as it is.

1. High density: twenty-five to thirty-five devices per classroom in one-to-one models, well above what a design intended for offices assumes.
2. Simultaneous peaks: of association, DHCP and authentication at the start of lessons and at certain points in the day.
3. Attenuation: from load-bearing walls and floor slabs, and from buildings of different periods joined together.
4. Outdoor areas: no power points nearby, and equipment that has to withstand the weather.
5. Limited installation windows: the work has to fit into school holidays, public holidays or time outside school hours.

The playground, the sports courts and the dining hall: the problem outside

Open spaces are the part that is most often left until later and most often solved badly. The temptation is to turn up the power on the equipment already inside the building, and it does not work. The access point can transmit at higher power, but the devices connecting to it still reply with their own. Wi-Fi is two-way communication: there is little point in the access point reaching further if the client device cannot reply under the same conditions.

Add to that the fact that a playground offers no walls to mount anything on and no ceilings to hang anything from, that the weather rules out equipment designed for indoors, and that whatever is transmitted outside comes in through the windows of the classrooms overlooking that same playground.

That last condition is the one most often overlooked. An outdoor access point at full power pointing at the building injects noise into the classrooms on that façade, so the playground solution becomes the classrooms' problem. And power is no minor detail, because a playground rarely has a cabinet nearby, so the cable run, its route and the point the equipment is powered from are decided at the design stage, long before installation day.

The dining hall and the assembly hall deserve a separate mention. They are treated as high-density areas. They are large spaces, with high ceilings and reflective surfaces, that go from empty to holding hundreds of people within a few minutes. The design approach there is closer to that of a full classroom than to that of an area people simply pass through.