## 2 A Conceptual Model for Adaptive _IoT_ Communications  
This section briefly summarizes the adaptation opportunities for tackling the challenges in _IoT_. Identified opportunities are attributed to certain layers with respect to the the OSI reference model [8, 9] and illustrated in the _Component Stack_ of Figure 1. The term \"component\" is used in a sense that adaptive communications are considered a composition of these opportunities. It should be noted that Figure 1 does not intend to reflect the actual implementation; the purpose of the figure (component stack, in particular) is to summarize points of interest and illustrate the organization of discussions in later sections. An opportunity for adaptation may be realized by one or more techniques. Figure 1: Adaptation Opportunities and Techniques: The Component Stack to the left highlights the adaptive opportunities identified at different layers of a typical low-power _IoT_ product such as wireless sensors. The rest of the figure demonstrates an exemplar scenario of interactions between traditional Internet hosts and adaptive wireless networks. The integration is achieved via the adoption of a compact version of IPv6 called 6 LoWPAN in the power constrained wireless devices. A lightweight adaptation between the 6 LoWPAN and standard IPv6 is performed at the gateway. Techniques at different layers may or may not have intra-layer or inter-layer dependency and some of them are not expected to work together at present. Furthermore, the differences between node types are purely to illustrate the diversity of their roles and do not imply any hardware or software variety. In fact, it is common that network nodes have identical physical constructions with the same software implemented. For example, a _Router Node_ is merely a node that happens to sit between the _End Node_ and the destination, and in the meantime can certainly be an end node itself. The physical layer (PHY) provides the most fundamental infrastructure for adapting to all spectrum-related events due to its direct contact with the wireless medium. The importance of PHY is twofold. First, it enables the acquisition of spectral conditions via a group of techniques known as \"spectrum sensing\", which is essential for any well-informed adaptation that is to be made. Second, a great part of spectrum-related adaptations are achieved by altering PHY parameters, including operating frequency and signal strength. Hence the reconfigurability of PHY is an indispensable component of adaptive communication. The medium access control (MAC) layer directly controls the PHY layer adaptation. On one hand, it regulates and coordinates spectrum-sensing activities to ensure that spectral information is efficiently collected whilst considering a number of tradeoffs. On the other, it makes certain adaptation decisions based on obtained knowledge, such as switching operating frequency, which are, in turn, put into practice by the reconfigurable PHY. Two adaptation opportunities are identified at the network layer. The first one is the incorporation of spectrum information in routing processes. Routing protocols for wireless networks usually measure the desirability of paths based on distance or number of hops. However, as recognized earlier, the spectral condition has a major influence on the quality of transmission, especially for low-power _IoT_ devices. Consequently, spectrum-aware routing protocols that take into account spectrum-related events as depicted in Figure 1 can be adopted to reduce the probability of routing decisions that are in conflict with the actual conditions of wireless medium. The other opportunity for adaptation at this layer concerns the formation of an integrated network of heterogeneous _things_.