7.22 Consider the network shown in Figure 7.23, which has three exponential service stations with rates 1;

7.22 Consider the network shown in Figure 7.23, which has three exponential service stations with rates μ1; μ2, and μ3, respectively. External customers arrive at the station labeled Queue 1 according to a Poisson process with rate η. Let N1 denote the steadystate number of customers at Queue 1, N2 the steady-state number of customers at Queue 2, and N3 the steady-state number of customers at Queue 3. Find the joint PMF pN1N2N3 ðn1; n2; n3Þ.