Internet of Things (IoT) is the new paradigm to the scaling nature of things and their elements, interconnected, exchanging data over a network supported with nodes. The Ubiquitous use of tiny devices and embedded sensor frameworks has pushed IoT to the forefront of emerging technologies used in many applications like peer-to-peer networks, smart energy grids, home and building automation, vehicle to vehicle communication, and wearable computing devices. This massive growth and extensive use brought forth security risks that could hinder its commencement in many novel applications. The number of interconnected devices leads the way to several entry points for intruders and, along with it, security risks. The sensitive nature of the IoT applications such as health, automation, and energy grids cannot afford security risks. Traditional security mechanisms will not design or develop to secure such an emerging technology as IoT. Existing technologies have to be relied on with the non-existence of security mechanisms for this purpose. Distributed Ledger Technology (DLT) is one such technology that can reduce the security risks in IoT. The central node vulnerability that can compromise the whole system can be mitigated by eliminating the need for a central node by using the distributed ledger. Blockchain, a distributed ledger technology, has attracted tremendous attention and harnessed in itself a real-world value. However, computationally costly with limited scalability is not entirely suited for the IoT environment. IoT Application (IOTA) technology is the distributed ledger technology that can provide unlimited scalability specifically suitable for the IoT industry. This survey provides an in-depth introduction to how blockchain performs and its constraints in its nature as a generic platform for DLT. In contrast, IOTA is introduced as the technology for IoT, the next-generation blockchain overcoming blockchain's limitations for its use in IoT.INDEX TERMS IoT, security, IOTA, blockchain.
I. INTRODUCTIONThe ''things'' in IoT have reached household appliances and entertainment to wearables, all connected through the internet. The internet of Things (IoT) has had a striking impact on the way we associate with our surroundings [1]-[3]. According to Gartner's forecast, by 2020, we will have about 20.6 billion connected things or more [4]. The idea that all things are super connected to everything, through which information is collected, monitored, and controlled via the internet to interact and interconnect with users, is the central concept of this ubiquitous technology [5], [6]. With the overestimated number of things connecting the IoT to the world, there is no doubt we are already inside the IoT world.The associate editor coordinating the review of this manuscript and approving it for publication was Yanli Xu .