variable bit rate A QoS class defined by the ATM Forum for Asynchronous Transfer Mode (ATM) networks. VBR is subdivided into a real time (RT) class and non-real time (NRT) class. VBR (RT) is used for connections in which there is a fixed timing relationship between samples. VBR (NRT) is used for connections in which there is no fixed timing relationship between samples, but that still need a guaranteed QoS. Compare with ABR (available bit rate), CBR, and UBR.