Rapid advances in wireless technology have taken us towards a pervasively connected world in which a vast array of wireless devices, from mobile phones to environmental sensors, seamlessly communicate with each other. In some of these systems the freshness of the transmitted information is of high importance. Characterization of time-critical information can be done through the so called real-time status updates that are messages, in the form of packets, carrying a timestamp of their generation. Status updates track time-varying content that needs to be transmitted from the generation point to a remote destination within a network. To quantify the freshness of information in networked systems a novel metric, different than delay or latency, termed as "age of information" (AoI) has been introduced. In this thesis, we focus on the communication systems' adaptability to meet stringent timeliness constraints.

In Paper I, we expand the concept of information ageing by introducing the cost of update delay (CoUD) metric to characterize the cost of having stale information at the destination. Furthermore, we introduce the value of information of update (VoIU) metric that captures the degree of importance of the update received at the destination. A theoretical analysis and insights into the relation of cost and value are provided.

Paper II investigates AoI in relation with throughput in a shared access setup with heterogeneous traffc. More specifically, we consider a shared access system consisting of a primary link and a network of secondary nodes, with multipacket reception (MPR) capabilities. To study the joint throughput-timeliness performance, we formulate two optimization problems considering both objectives and provide guidelines for the design of such a multiple access system fulfilling both timeliness and throughput requirements.

Finally, in Paper III we investigate the AoI performance in various multiple access (MA) schemes, including scheduling and random access. We present analysis of the AoI with and without packet management at the transmission queue of the source nodes, considering that packet management is the capability to replace unserved packets at the queue whenever newer ones arrive. We incorporate the effect of channel fading and network path diversity in such a system and provide simulation results that illustrate the impact of network operating parameters on the performance of the different access protocols.