What a range extender actually does

An extender is a small computer with a wireless radio and a mains plug. To the router it looks like a laptop: it joins the network with the same name and key any other device would use. To a phone in the back bedroom it looks like a router: it publishes a network of its own, or the same one under the same name, and accepts connections.

Between those two faces it does one thing. Whatever it receives from one side it sends out again on the other. It does not amplify a signal, does not create one, and does not change what the router is capable of. It relays.

Wireless equipment can play one of two parts in a link. A station is a device that joins a network: a phone, a laptop, a television. An access point is the equipment that offers the network for stations to join. A router in a home contains an access point; an extender contains both parts at once.

On the side facing the router, the extender is a station. It associates with the router, proves it knows the key, and is handed an address on the household network like any other device. On the side facing the rest of the house, it is an access point. It advertises a network name, accepts stations, and forwards what they send.

The cheapest units carry one radio, and that single radio has to serve both roles. It listens to the router on a channel, and it talks to the phone on the same channel. A radio cannot transmit and receive at the same moment, so it alternates: a frame arrives from the router, is held briefly, then is sent on to the phone; the reply from the phone arrives, is held, then is sent to the router.

Almost every puzzling thing about extenders is a consequence of the relay model rather than a fault in a particular unit.

The speed drops

Every frame crosses the air twice, on the same channel, from the same radio. The channel is busy for twice as long, so the most the extender can pass on is roughly half of what it receives. Why a repeated signal carries less sets out the arithmetic and the arrangement that avoids most of it.

Its position sets its ceiling

The extender can only repeat what reaches it. Placed where the router’s signal is already poor, it repeats a poor signal faithfully, and the halving applies on top. Placed where the signal is still strong, it has something worth relaying. This is why the placement question matters more than the specification.

A second network appears, or seems to

Because the extender acts as an access point, it advertises a network. Some units give it a distinct name with a suffix; others copy the router’s name and key so that the household sees one network served from two places. The second network name explains both arrangements and what each changes.

The device, not the network, chooses

Nothing about the extender forces a phone to use it. A phone decides for itself which signal to hold and when to let go, and it is famously reluctant to let go. How a device decides which signal to use covers that behaviour.

It also adds no security of its own. Traffic between phone and extender is scrambled with the key the extender publishes, and traffic between extender and router with the router’s. Both keys sit in the extender, which re-encrypts in the middle. Security on a repeated network follows that through.

Most extenders carry a wired network socket. Connected by cable to the router, or to a wired point elsewhere in the building, the same unit can work as a plain access point: the station role is switched off, the cable carries traffic back to the router, and the radio is left entirely to the phone. In that arrangement none of the halving applies, because nothing is being repeated over the air.

A unit in access-point mode is a different proposition from the same unit as a repeater, and the difference is the cable. Four ways compared puts it alongside the alternatives.