Variable bit rate (VBR)
RT-VBR:  Specified by the ATM Forum for services with stringent timing requirements (”real-time applications”), like CBR but for variable bit rate services, e.g. compressed speech.
NRT-VBR:  Specified by the ATM Forum for variable bit rate services without stringent timing requirements (”non-real-time applications”).
In both VBR service categories, we need to specify the following traffic parameters:
-  Peak Cell Rate (PCR)
-  Sustainable Cell Rate (SCR)
-  Maximum Burst Size (MBS)