/IP to carry the user-plane PDUs between the NG-RAN node and the UPF. 124 Chapter 1 1.2.1.2 Bearers and Identifiers 1.2.1.2.1 Radio Bearers and Packet Data Unit Sessions In LTE, IP connection between a UE and a PDN is established via a PDN connection or EPS session. An EPS session delivers the IP packets that are labeled with UE IP address through logical paths between the UE and the PDN (UE-PGW-PDN). An EPS bearer is a logical pipe through which IP packets are delivered over the LTE network, that is, between a UE and a PGW (UE-eNB-SGW-PGW) A UE can have multiple EPS bearers concur- rently. Thus different EPS bearers are identified by their EPS bearer IDs, which are allo- cated by the MME. An EPS bearer in reality is the concatenation of three underlying bearers: (1) A DRB between the UE and the eNB, which is set up upon user-plane establish- ment between the UE and the access node; (2) an S1 bearer between the eNB and SGW, which is set up upon establishment of EPS connection between the UE and the EPC; and (3) an S5/S8 bearer between the SGW and PGW, which is set up upon establishment of PDN connection between the UE and the PGW. An E-URAN random access bearer (E- RAB) is a bearer with two endpoints at the UE and at the SGW, which consists of a DRB and an S1 bearer. As such, E-RAB is a concatenation of a DRB and an S1 bearer, which logically connects the UE to the SGW. LTE QoS architecture describes how a network operator could create and configure different bearer types in order to map various IP traffic categories (user services) to the appropriate bearers according to the service QoS require- ments. In that context, a bearer is an encapsulation mechanism or a tunnel that is created per user for transporting various traffic flows with specific QoS requirements within the LTE access and core networks. In LTE and NR, there are two types of radio bearers, namely signaling radio bearers (SRB) and DRB. The SRBs are radio bearers that are only used for the transmission of RRC and NAS messages, whereas the DRBs are r