Elements of VoIP QoS
The funny thing about QoS is that it tends to be known only after the fact;  you use some application and you experience the QoS as you use it. The same principle applies to VoIPpoor QoS shows itself in excessive delay and possibly lost packets. In a more general sense, we can say that the major determinants of QoS are as follows:
Throughput: Amount of available bandwidth between two network points of interest (in Figure 1, for example, between the headquarters site and the branch office)
Availability: Fraction of time during which service is available between two points
Delay: Length of time taken to transmit data from one point to another
Delay variation: Difference in the delay for two packets
Loss: Ratio of packets received to packets sent
These simple elements are sufficient to put together very complex service level agreements (SLAs). Some service providers now supply stringent SLAs with proactive tariff rebatesall part of the increased importance of service differentiation. As time goes on, we can expect to see this enlarged role of SLAs emerge increasingly within enterprise networks.