All actual telecommunication systems are loss systems, since all the buffers have finite, and hence limited capacity. By system with delay, we mean a system in which the dimensioning is such that the loss caused by the overflow is negligible, and for which the relevant criterion for assessing the performances, is the waiting time.

Before we start a detailed study of these systems, we first introduce a well-known, general and very useful relation called *Little's Formula*.

We consider a system with delay, in which the customers arrive at times (*T _{n}*,

THEOREM 8.1 (Little's Formula).– *We assume that N is asymptotically linear, i.e. there exists* λ > 0 *such that*

*and that the sequence W is ergodic, i.e.*

*Under these assumptions, we have*

*The existence of the latter limit is shown in the following proof.*

*Proof.* Let us fix ...

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