A list, string, or stack of things constructed so that items are added to one end and relieved from one end or the other. Generally speaking, items are added to one end, known as the tail, and relieved from the other end, known as the head. In the absence of some priority mechanism for purposes of establishing and maintaining quality-of-service (QoS) differentiation, items are relieved from the head of the queue in the order they entered the tail.This approach is known as first-in-first-out (FIFO). Incoming call centers employ automatic call distributors (ACDs) that queue incoming calls, serving them to agents as they become available. Fax servers can queue documents for transmission during non-prime time hours, when international calling costs are lowest. PBX systems commonly have the capability to queue outgoing calls for expensive long distance circuits. Switches and routers queue packets in buffers until internal resources are available to process them or until bandwidth is available to forward them. Systems may support multiple queues for different types of calls or packets. Priority mechanisms can cause a call or packet to move up in the queue or even advance to the head of the queue in order that it can be served more quickly. See also ACD, call, facsimile, packet, PBX, QoS, router, and switch.