ips. PVC (permanent virtual circuit) A defined path that provides essentially a dedicated pri- vate line between users in a packet-switching network. The network is aware of a fixed associa- tion between two stations, permanent logical channel numbers are assigned exclusively to the permanent circuit, and devices do not require permission to transmit to each other. PWM (pulse width modulation) PWM (pulse width modulation) The process of encoding information based on varia- tions of the duration of carrier pulses. Also known as pulse duration modulation (PDM). Px64 The ITU-T H.320 standard for interoperability in videoconferencing over ISDN. QAM (Quadrature Amplitude Modulation) A single-carrier scheme that modulates both the amplitude and phase to yield higher spectral efficiency. Various levels of QAM exist, and they are referred to as nn-QAM, where nn indicates the number of states per Hertz (Hz). The number of bits per symbol time is k, where 2k = NN. So, 4 bits/Hz is equivalent to 16-QAM, 6 bits/Hz is equivalent to 64-QAM, and 8 bits/Hz is equivalent to 256-QAM. QoS (quality of service) In networking, a concept by which applications can indicate their specific requirements to the network before they actually start transmitting information data. Implicit QoS means that the application chooses the required levels of QoS. With explicit QoS, the network manager controls that decision. QPSK (Quadrature Phase-Shift Keying) A single-carrier modulation scheme that sup- ports 2 bits per symbol time. Q.Sig The Ecma International standard for common-channel signaling between PBXs. quadruple-play services A service provider's bundle of voice, data, video/TV, and wireless/ mobile services, delivered over the same infrastructure. quantization noise Signal errors caused by the process of digitizing a continuously variable slope. query A request for information that is entered while a computer system is processing. queue An ordered accumulation of data or transactions stored for later processing. queuing A process in which c