 There are different perspectives to which quality of service can be defined from the service provider's viewpoint, from network's viewpoint, from customer's viewpoint, this is very intuitive. We have already covered the QoS perspectives from the ITUT perspective, from traffic engineering perspective, from network and user perspective. Now we are going to look at it in a more formal manner, that is the QoS management views. The QoS management views actually means that starting from the user to the network, from the service to the user again, what has been the overall quality of service experience that has been taken out of the network. So ITUT actually defines four viewpoints. These viewpoints are interrelated and we can actually understand the relationship established between one viewpoint and the other. It's a cyclic relationship. It starts with the customer's QoS requirement which is a static definition. It is the QoS level required by the subscriber. This is usually defined in the form of a service level agreement or contract. The network also makes a certain offering. Now this offering in terms of QoS is what the network provides or plans to provide or targets to provide. It includes specifics of QoS that are provided either by the service provider itself, by the network provider or by both. So this offering actually helps the user to become aware of the services it is going to get. Which in turn allows the customer to have a service level agreement or a contract with the service provider. Then it is also a declaration which is known as a public offering. Public offering is when the government is a witness to a contract that is established between a service provider and a service user. So this public offering is actually a QoS declaration that such kind of QoS is going to be expected by the subscribers. Then it also helps the organization or the service provider to plan a certain QoS and maintain that QoS. So this is once it has been declared then it becomes more of a compulsion for the service provider. And then for the service users or subscribers it also provides a means to compare the services coming from different service providers. So that if one service provider provides better QoS then the user can opt to stay on with that service provider and leave or terminate the contract with other service providers. Then there is another perspective, another view is the QoS actually can be achieved or delivered by the service provider. Now what offering a certain organization made to what actual QoS it was able to achieve or deliver to the customer. Now this is a measurement so it means it's a dynamic thing unlike the QoS that was declared by the service provider. Using that static declaration and this QoS measurement it can be used to see if the desired service is being provided to the user which was promised in the SLA. So this is basically a check which helps the corrective actions to be triggered once there is some discrepancy or deviation between the QoS that was offered and the QoS that is currently being provided. Then there is a customer perception of QoS. The customer perception actually is the post service delivery experience of the user in terms of approval ratings or the overall ranking or scoring of the service. Again it is a dynamic thing because a user might experience good service at one point in time and not a very good service at another. So it can again be used for comparison purposes. Now this helps the customer to make up his or her mind to stick on with the same service provider or move on to a new one. And this again basing upon the customer perception as in approval ratings helps the organization or a service provider to take corrective measures. Now these four viewpoints of the QoS are very well linked up in the form of a cycle that starts from top left that is customers QoS requirements and terminates with the customers perception of the QoS. It means the customer is eventually going to declare if it has realistic expectations or requirements from the service providers or if it needs to readjust or calibrate its requirements according to the experiences that it gets from a service provider or multiple service providers.