What is Radio Bearer in UMTS?

In mobile telephony a bearer service is a link between two points, which is defined by a certain set of characteristics. Whenever user equipment is being provided with any service , the service has to be associated with a Radio Bearer specifying the configuration for layer 2 and physical layer in order to have its QoS clearly defined. Radio bearers are channels offered by Layer 2 to higher layers for the transfer of either user or control data. In other words, Layer 2 offers to the upper layers the service of information transmission between the UE and the UTRAN by means of the Radio Bearers and Signaling Radio Bearers. Therefore, the service access points between Layer 2 and upper layers are RBs.