Volume 1 of 10 in the Engineering-to-Research Monograph Series. Connected systems, including wireless sensor networks, Internet-of-Things and cyber-physical systems, and the virtualized network infrastructure that carries their traffic, are usually studied as separate security domains. This report unifies the three into one layered attack surface: it develops a common threat taxonomy across the physical-device, network-routing, and orchestration-virtualization strata; reviews the cryptographic and protocol-level defenses available at each layer, from classical public-key cryptography through lightweight and post-quantum schemes; and synthesizes the findings into a layered defense reference model. The analysis is grounded in three implementation studies, namely a from-scratch RSA cryptosystem, an NS-3 investigation of TCP behavior under congestion, and an encrypted microcontroller client-server link including the SensoryPi cyber-physical system, and is situated against the 2023 to 2026 literature.The paper and figures are licensed CC BY 4.0; companion code is released under the MIT License. This work contains no confidential or proprietary employer information.