Poisson arrival rate. Little's formula states the queue size equals the arrival rate times the average time in the system. Both the service time and the arrival rate are variable and exponentially distributed. For some systems the service time is always the same. When there are N servers, we assume that each server is identical.
All customers wait in a single queue and use the first available server.
The utilization is . For the calculations, an intermediate value, K is useful. Identify the server. Identify the queued items. Once the item is defined, convert all times and rates into these units.