  This paper considers a modified Erlang loss system for cognitive wireless
networks and related applications. A primary user has preemptive priority over
secondary users and the primary customer is lost if upon arrival all the
channels are used by other primary users. Secondary users cognitively use idle
channels and they can wait at an infinite buffer in cases idle channels are not
available upon arrival or they are interrupted by primary users. We obtain
explicit stability condition for the cases where arrival processes of primary
users and secondary users follow Poisson processes and their service times
follow two distinct arbitrary distributions. The stability condition is
insensitive to the service time distributions and implies the maximal
throughout of secondary users. For a special case of exponential service time
distributions, we analyze in depth to show the effect of parameters on the
delay performance and the mean number of interruptions of secondary users. Our
simulations for distributions rather than exponential reveal that the mean
number of terminations for secondary users is less sensitive to the service
time distribution of primary users.
