@BFrazer 

Thanks for reaching out. I understand that you are setting up a Deco Mesh system and want to know if all units need to communicate through the 'main' Deco, and whether you need to purchase a third unit for proper coverage.

In the Deco Mesh system, including your Deco XE75, one unit must be designated as the main Deco during setup. This main Deco connects directly to your modem/router (in your case, the Xfinity gateway) to provide Internet access to the entire mesh network. All other Deco units (satellites) connect to the main Deco, either wirelessly or via Ethernet, and extend coverage throughout your home.

Key points:
• All Deco units in the box are identical; any can be chosen as the main Deco. The main Deco must be physically connected to your modem/router via Ethernet to access the Internet.
• The satellite Deco units do not connect directly to your modem/router. Instead, they communicate with the main Deco, which then routes all data traffic to and from the Internet.
• In your planned setup, if the main Deco is on the 2nd floor and the satellite is in the basement, the satellite will send its data to the main Deco upstairs, which then passes it to the router on the main floor. This could result in less optimal performance if the signal between units is weak or obstructed.
• You are not required to purchase a third Deco for the system to function. However, adding a third unit on the main floor (closer to the router) could improve coverage and performance, especially in larger or multi-story homes.

Recommendation:
• For best results, place the main Deco as close as possible to the center of your home or near the modem/router, and position the satellite Deco(s) so that they have a strong signal to the main Deco. Avoid placing Deco units too far apart or with multiple floors/walls between them.
• If coverage or performance is not satisfactory with two units, you can add a third Deco unit to further extend your mesh network.

If you have any further questions or need help optimizing your Deco Mesh setup, feel free to ask!