 The concept of quality of experience is not new to us. It is related to the quality of service. In fact, the overall concept and the description of quality of service remains incomplete without quality of experience. So far, we know quality of experience as a rather abstract concept, something that is perceived by the user. But in this module, we are going to look at it in more detail and we will understand that it is not that abstract. In fact, it can be qualitatively measured. We will start with the definition first. The quality of experience is a formal concept defined by the ITUT. It is an overall acceptability of an application or a service. Since it involves users, so there is some level of subjectivity involved because a user may have personal likes or dislikes. A user's equipment may be constrained or may have certain value added features that modifies the user perception. From the network perspective, it is speed and speed alone that influences the quality of experience. Speed essentially is the availability of bandwidth or throughput and as little latency as possible. The concept of QoE is crucial to the overall business of companies because a dissatisfied or sometimes a disgruntled customer can have a backlash on the overall commercial viability of a service or an application. The features which affect quality of experience either directly or indirectly can be categorized into an exhaustive list. But for the sake of gravity, we will consider few which are somewhat more important. The first one is availability. We know that availability is a concept that is a numeric value that determines the total uptime of a system. Ideally speaking, 5-9's availability is what the networks and the service providers seek but attaining this availability means that the downtime should not be more than mere 5 minutes per annum. Then we have reliability. Reliability is the confidence to a certain user that the service provider is not going to make the service unavailable even if the connectivity is there. It means sometimes because of some physical layer or upper layer protocol issues, the traffic cannot be delivered though the physical infrastructure as such is available. So in that case, the organizations which provide such services have to have a rerouting plan to basically deliver the traffic to the user and requests from the user to the network in case of a failure or a network performance degradation. Sometimes reliability is also considered important from the user perspective in terms of the self-organization of the network. The user is asked to have minimum interaction with the network with regards to configuring it. It means the network is somewhat self-adaptable. This is a relief and a very comfortable feeling to the user. Then the user is also interested in understanding how many other users are getting this service. Having this feeling that this service is also being used by other users is quite relieving and satisfying. But at the same time, a user should not get a feeling that this service or this particular feature is provided to an excessively large number of users which cannot be accommodated by the network. This is known as the planning and dimensionality adjustment. And then the user is also interested in the cost that he or she is supposed to pay upfront and the regular monthly bill that they are going to get. There is a very interesting qualitative measure which is determined through questionnaires, telephone calls from the customer support centers known as the mean opinion score. It's a scale from 1 to 5 that is provided by the user through an interaction. It is somewhat similar to the Likert scale which determines the from totally disagreeing to absolutely agreeing. Now this opinion score was initially meant for voice circuits for voice calls. But now with the emergence of NGN services, this MOS has also been extended to non-voice apps as well. So we'll see how this helps the network and the service provider to determine the overall satisfaction of a user. Here you see that the quality is given a certain number starting from excellent to bad. And then something is known as high quality because of the overall user experience that is made while a call is entertained. For instance, in terms of excellent, the impairment is actually almost non-existent. In fair, it gets slightly annoying and for very annoying call experience, for instance, there is background noise, there is frequent outage, the call suddenly drops. So the user is going to give a main opinion score of just one or maybe the user considers that it deserves a zero. From the network perspective, if a service provider is giving services to the user for which the main opinion score turns out to be around four, it is considered to be somewhat an acceptable and sustainable service and it is expected that the user is going to stay loyal to the service and to the service provider.