The links between nodes have finite capacity and thus some calls arriving may find no route available to their destination.
[1] The loss network was first studied by Erlang for a single telephone link.
When a call arrives if there is sufficient capacity remaining on all the required links the call is accepted and occupies the network for an exponentially distributed length of time with parameter 1.
If there is insufficient capacity on any individual link to accept the call it is rejected (lost) from the network.
Then the continuous-time Markov process n(t) has unique stationary distribution[5] where and From this result loss probabilities for calls arriving on different routes can be calculated by summing over appropriate states.