The box that turns your wired network into Wi-Fi so phones and laptops can join.
Like a ceiling-mounted radio tower for one office: the more floor you cover, the more towers you need, all tuned to the same station.
A six-person clinic ran the whole office on the ISP's modem Wi-Fi. Patients joined the same network as the practice server. One patient's infected laptop scanned the network and found the server's open file share.
Network covers the firewall, switch and on-premises authentication. Access points sit on that same network, typically with a staff Wi-Fi and a guest Wi-Fi that cannot see each other.
Example scenario: A small cafΓ©-bookshop had one consumer router behind the counter. The point-of-sale tablet dropped offline every time the shop filled with customers on the free Wi-Fi. Two ceiling APs with a separate guest network fixed the drops, and the till never shared airtime with a customer's video call again.
Can't I just use the Wi-Fi on my internet provider's modem?
For a one-person shop, perhaps. Once you have staff, a server or customers on the Wi-Fi, you need APs that separate guests from the business and cover the whole space.

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