For the unobservable queueing system, customers do not know the system queue length information before making behavior decisions, which also increases the uncertainty of the game between customers. Edelson and Hildebrand (Econometrica, 43, 81–92, 1975) consider the Naor’s model under the assumption that an arriving customer does not know the number of customers in the system, i.e., the FU case mentioned in Sect. 1.3 .