HGCAL detector performance and system validation with particle beams

The High-Granularity Calorimeter (HGCAL) is the new endcap calorimeter of CMS for operation in the High-Luminosity LHC (HL-LHC) era. Featuring 6 million silicon pads and 280'000 scintillating tiles, for an overall area of 620 m$^{2}$ and 370 m$^{2}$ respectively, HGCAL will deliver precise spatial, energy, and timing measurements of particles in its active volume. This level of granularity poses significant challenges for the real-time processing and transmission of trigger primitive data and event data over $O(10^{4})$ optical fibres. Over nearly a decade, multiple aspects of the detector technologies and of the front-end and back-end electronics have been progressively validated in beam tests of increasing complexity. These systems have provided the opportunity to construct vertically integrated end-to-end readout chains using final pre-production components. This contribution presents the measured performance of detector prototype chains, and discusses the studies guiding the system-level design and integration of the optical fibre plant that connects the front-end and back-end electronics.