  We present a comprehensive characterization of the interconnections between
single-mode, phaseinsensitive Gaussian Bosonic Channels resulting from channel
concatenation. This characterization enables us to identify, in the parameter
space of these maps, two distinct regions: low-ground and high-ground. In the
low-ground region, the information capacities are smaller than a designated
reference value, while in the high-ground region, they are provably greater. As
a direct consequence, we systematically outline an explicit set of upper bounds
for the quantum and private capacity of these maps, which combine known upper
bounds and composition rules, improving upon existing results.
