Variable bit rate (VBR) is an asynchronous transfer mode (ATM) quality of service category, defined by the ATM Forum. Both real-time variable bit rate (rtVBR) and non real-time variable bit rate (nrtVBR) are defined. Apart from the traffic parameters for peak cell rate (PCR) and sustainable cell rate (which defines the average bit rate required by the application), additional quality-of-service parameters such as maximum cell transfer delay, cell delay variation and maximum burst size must be agreed upon. The typical application for which it is used is compressed voice and videoconferencing for rtVBR, and response-time-sensitive data such as Systems Network Architecture (SNA) for nrtVBR.