0-9  A  B  C  D  E  F  G  H  I  J  K  L  M  N  O  P  Q  R  S  T  U  V  W  X  Y  Z quadrature amplitude modulation (quadrature sideband amplitude modulation) (QAM) A sophisticated modulation technique, using variations in signal amplitude and phase, that allows data-encoded symbols to be represented as any of 16 to 256 different states. Quadrature is a synonym for phase when talking about modulation techniques. quality of service (QoS) The ability to define a level of performance in a data communications system. For example, ATM networks specify modes of service that ensure optimum performance for traffic such as realtime voice and video. QoS has become a major issue on the Internet as well as in enterprise networks, because voice and video are increasingly traveling over IP-based data networks.