In a broadband system architecture, acoustic echo is created by the acoustic properties of the areas where a VoP phone is used. Acoustic echo canceller is to reduce the level of acoustic echos.
Line echo is an electrical form and is produced by a network hybrid (a 2-wire to 4-wire converter). Line echo canceller is to reduce the level of line echos.
Network echo canceller is to reduce the level of echoes that leak from paths within the gateway between the two networks.
