The traffic equations are the basis for the exact analysis of product
form queueing networks, and the approximate analysis of non-product fo
rm queueing networks. Conditions characterising the structure of the n
etwork that guarantees the existence of a solution for the traffic equ
ations are therefore of great importance. This note shows that the new
condition stating that each transition is covered by a minimal closed
support T-invariant, is necessary and sufficient for the existence of
a solution for the traffic equations for batch routing queueing netwo
rks and stochastic Petri nets.