 waiting than a given router is able to store or buffer. Effects similar to prioritization can be achieved by caching (storing frequently used static data close to the user) and by replication (where the same dynamically gener- ated results can be produced in more than a single location in the network - cloud services can represent an example of this kind of distribution or replication function). The use of caching CDNs represents an increasingly common and important means for Error tolerance Conversation VolP Video streaming, and video telephony/ audio streaming conferencing Web browsing, transactions, file Messaging, background Critical loT upload/download Massive loT Time (s) loT - Internet of Things VolP - Voice over IP Figure 7.1 User-centric QoS. 7.1 QoS, QoE, and Application Needs improving the QoE. For example, locating YouTube video servers closer to end-users reduces delay and jitter, and hence gives higher QoE. The same approach refers to all services which are based on principle user to content (i.e. Internet native client-server networking approach). How common is delay-sensitive application traffic? The delay is the most important QoS parameter regarding the real-time traffic. In fact, the real-time services are defined on the basis of low delay requirements, or in other end, non-real-time traffic is more flexible regarding the delays. Hence, typical (now legacy) classification of IP traffic is to real-time conversational and streaming services, and non-real-time interactive (e.g. Web is the most prominent example in this group) and background services (email, file download or upload, etc.). Real-time conversational voice services benefit from limited delay, which can be accomplished in two ways: (i) guaranteed QoS end-to-end (e.g. NGN), and (ii) ultra-broadband access speeds which provide low delay for aggregate bitrates in the access part significantly below the traffic capacity (e.g. below 60% of the ultra-broadband link capacity). The load associated with VoIP is negligible. Video tele- phony is