The rapid proliferation of Internet of Things (IoT) devices across healthcare, industrial automation, smart
agriculture, smart grid, and smart-city domains has magnified concerns surrounding data integrity, device
authentication, and network resilience. Conventional client-server and cloud-centric IoT security models suffer from
single points of failure, limited transparency, and poor scalability as the number of connected endpoints grows into
the tens of billions. Blockchain technology, with its decentralized, immutable, and cryptographically verifiable ledger
structure, has emerged as a promising complement to traditional IoT security mechanisms. This paper presents a
comprehensive study of blockchain-based security for IoT networks from an Electronics and Communication
Engineering (ECE) perspective. We review the layered architecture of IoT systems, identify dominant threat surfaces
at the perception, network, edge, and application layers, and propose an integrated five-layer blockchain-IoT security
architecture suited to resource-constrained embedded hardware. We present a comparative evaluation of consensus
mechanisms — Proof of Work (PoW), Proof of Stake (PoS), Practical Byzantine Fault Tolerance (PBFT), Delegated
Proof of Stake (DPoS), Proof of Authentication (PoAh), and Proof of Authority (PoA) — with respect to energy
consumption, latency, and scalability for constrained IoT nodes. A threat model mapping common blockchain-IoT
attack vectors to mitigation strategies is developed, along with case-study discussion of healthcare (IoMT), supply
chain, and smart-grid deployments. The paper concludes with open research challenges — including scalability,
interoperability, quantum-resistance, and regulatory alignment — and outlines future research directions relevant to
hardware-software co-design in the ECE domain