Q.920/Q.921 ITU-T specifications for the ISDN UNI data link layer. See also UNI. Q.922A ITU-T specification for Frame Relay encapsulation. Q.931 ITU-T specification for signaling to establish, maintain, and clear ISDN network connections. See also Q.93B. Q.93B ITU-T specification signaling to establish, maintain, and clear BISDN network connections. An evolution of ITU-T recommendation Q.931. See also Q.931. QLLC Qualified Logical Link Control. Data link layer protocol defined by IBM that allows SNA data to be transported across X.25 networks. QOS Quality of service. Measure of performance for a transmission system that reflects its transmission quality and service availability. QOS parameters Quality of service parameters. Parameters that control the amount of traffic the source router in an ATM network sends over an SVC. If any switch along the path cannot accommodate the requested QOS parameters, the request is rejected, and a rejection message is forwarded back to the originator of the request. Qualified Logical Link Control See QLLC. quality of service See QOS. quartet signaling Signaling technique used in 100VG-AnyLAN networks that allows data transmission at 100 Mbps over four pairs of UTP cabling at the same frequencies used in 10BASE-T networks. See also 100VG-AnyLAN. query Message used to inquire about the value of some variable or set of variables. queue 1.) Generally, an ordered list of elements waiting to be processed. 2.) In routing, a backlog of packets waiting to be forwarded over a router interface. queuing delay Amount of time that data must wait before it can be transmitted onto a statistically multiplexed physical circuit. queuing theory Scientific principles governing the formation or lack of formation of congestion on a network or at an interface.