The design requirements are smooth roaming and redundant coverage from at least two access points. Secondary coverage is the term for the wireless coverage provided by the second-strongest access point in any given location. Ensuring adequate secondary coverage directly fulfills the requirement for redundancy (coverage from a second AP). It is also a prerequisite for smooth roaming, as it guarantees that a client device can detect and transition to a new AP before losing the signal from its current one, preventing connection drops.

A. co-channel interference: This is a negative phenomenon that degrades network performance and is something to be minimized through proper channel planning.

C. primary coverage: This refers to coverage from only the single strongest access point, which does not satisfy the requirement for redundancy.

D. data rate: This is a performance metric or result of a good network design, not a foundational design principle that ensures coverage redundancy.

1. Juniper Networks TechLibrary, Mist Cloud Documentation, "View Coverage on a Floorplan." This document describes the tool used in the Mist platform to design and validate wireless networks. It explicitly defines and provides visualization for "Primary Coverage" (signal from the strongest AP) and "Secondary Coverage" (signal from the second strongest AP). Network designers use this feature to ensure that requirements for redundancy and seamless roaming are met by verifying sufficient secondary coverage across the floor plan.

2. Juniper Mist, "Validated Solution Guide: Campus Wired and Wireless LAN Deployment with Mist Cloud," Page 11, "WLAN Design Considerations." This guide discusses the importance of AP placement for providing overlapping coverage. It states, "Proper AP placement is critical to providing the required coverage... for high-availability and seamless roaming." This overlapping coverage is precisely what constitutes secondary coverage.