The MK-N approximation analyzes the mean delay of class 
jobs
in an M/M/
queue with 
priority classes
by aggregating all the higher priority classes (classes 1 to 
).
The job size
distribution of the aggregated class is then approximated with an
exponential distribution by matching the first moment of the
distribution.
In MK-N, the job size distribution of the aggregated higher priority classes is approximated by an exponential distribution, since the exact or nearly exact analysis of a multiserver system with two priority classes are known only for exponential distributions. However, DR that we have developed in Chapter 2 allows us to analyze the multiserver system with two priority classes for the case where job sizes have PH distributions, and the moment matching algorithm that we have developed in Chapter 2 allows us to approximate the job size distribution of the aggregated higher priority classes by a PH distribution matching the first three moments. This motivates us to extend the MK-N approximation to PH distributions: DR-A.