In asynchronous transfer mode (ATM), a best-effort class of traffic.Traffic parameters include peak cell rate (PCR) and cell delay variation tolerance (CDVT). No quality of service (QoS) commitment is made.ATM also defines available bit rate (ABR), constant bit rate (CBR), non realtime Variable Bit Rate (nrt-VBR), real-time Variable Bit Rate (rt-VBR), and variable bit rate (VBR) traffic classes. UBR traffic includes traditional computer applications such as file transfer and e-mail. See also ABR, ATM, CBR, CDVT, compression, nrt-VBR, PCR, QoS, real-time, rt-VBR, and VBR.