QoS support is provided by the following mentioned four pillars:
Packet Classification: - To classify the packets based on their class of traffic.
Isolation: scheduling and policing: - To manage the traffic flow is such a way that if there are some issues in a traffic flow, the other traffic will not be affected by the same.
High resource utilization: - To use the resources in an optimum manner while ensuring "isolation for traffic flows".
Call admission: - To check the network environment of a traffic flow, for satisfaction of QoS requirements, by following some validation mechanisms. Based on the outcome, it will be whether the traffic flow is allowed or blocked.
Let us discuss the background of this topic before proceeding further. At the time internet was designed, developers were not aware about the latest applications we have these days like "audio/video streaming, IP telephony". The initial design approach is based on a fair system, without any kind of biasing or priority given to any request; all requests are treated at the same level. So you cannot implement base level QoS directly which is required by some particular applications. "An application will receive whatever level of performance (e.g., end-end packet delay and loss) that the network is able to provide at that moment". (Kurose, 2004)
There is no direct way to implement the QoS for applications which requires it because of the infrastructure of internet. There is no scope for the direct implementation, but again with the help of the technology, there are some workarounds available which can help in achieving the required configurations.
The four pillars discussed above are the "new architectural components that can be added to the Internet architecture to shield an application from such congestion and thus make high-quality networked multimedia applications a reality" (Kurose, 2005). QoS can be achieved without leveraging the pillars only in some exceptional cases. One of the pillar "High resource utilization" can be ignored for those networks which have a sufficient infrastructure base as per the user's requirement. If the infrastructure is sufficient enough, then ignoring this pillar will have almost no effect on QoS.
PBX stands for "Private Branch Exchange". It is basically a switch station, used to switch and link various phone lines with each other. So we do not have to pay for individual phone lines, this acts as an extension system, user will only pay for one external phone line, and all the employees of the company can use their individual internal phone lines.