A buffering or temporary storage method in which the entity that first exits is the one that first entered. Thereby, the entity served (e.g., processed or switched) is the one that waited the longest period of time. FIFO is commonly used in message switches such as automatic call distributors (ACDs), PBXs, switches, and routers in the absence of a priority mechanism employed to establish quality-of-service (QoS) differentiation between different types of calls, packets, or other message entities. See also LIFO and queue.