Unspecified Bit Rate (UBR) is one of five classes of service defined by the ATM Forum
. The UBR service class supports data traffic with variable bit rates up to a maximum transmission rate at which a tolerable delay can occur. A UBR link has no cell loss ratio (CLR) or cell transfer delay
(CTD) commitment. In terms of transmission capabilities, UBR is only suitable for burst traffic, i.e. for data transmission and LAN data traffic.