Unspecified bit rate (UBR)
No QoS requirements (i.e. ”best effort” service).
The only traffic parameter of interest is the PCR which the user is not allowed to exceed. UBR supports a high degree of statistical multiplexing.
Typical applications:
-  File transfer (non-critical applications)
-  E-mail
(This is a new service category defined in the ATM Forum Traffic Management Specification Version 4.1)
(Guaranteed Frame Rate = GFR)