Mixed queueing network models are those in which some classes are open and some are closed.
In the example network illustrated above, classes A and B are open and depart from the network after CPU queue with some probability, Classes C and D are closed and loop over CPU queue which is M/M/1.
I already know that if O is the set of open classes that enter CPU with $\lambda_i$ rate for each i $\in$ O and get service with rate $\mu'$ (which is affected by closed classes and not equal to the actual service rate of the queue), the stability condition if $\rho < 1$:
$\sum_{i \in O}\frac{\lambda_i}{\mu'} < 1$,
meaning that closed classes do not affect the instability of the queues they enter.
I don't remember where I saw it first and need solid proof or reference to a book or paper to back it up. Can anyone help me with that?