adio bearers that are used to transport user-plane traffic. The RRC messages are used as signaling between UE and the access node (i.e., eNB or gNB). The NAS messages are used for signaling between the UE and the core network. The RRC messages are used to encapsulate the NAS messages for transfer between the UE and the core network through the access node (transparent to the access node). The SRBs are further classified into three types: SRBO, SRB1, and SRB2, where SRBO is used to transfer RRC messages which use common control channel, SRB1 is used to transfer RRC messages which use dedicated control channel, and SRB2 is used to transfer RRC messages which use dedicated control channel and encapsulate NAS messages. The SRB1 can be used to encapsulate NAS message, if SRB2 has not been configured. The SRB2 has lower priority than SRB1 and it is always configured after security activation. The SRBO uses RLC transparent mode, while SRB1 and SRB2 use RLC acknowledged mode. In LTE, upon connection establishment and at the beginning of an EPS session, a default EPS bearer, with no guaranteed bit rate and best effort QoS characteristics, is established. 5G Network Architecture 125 Access node Radio Interface Encapsulated IP IP flow IP flow 2 Radio bearer User-plane tunnel IP flow N - QoS flow(s) PDU session Figure 1.58 High-level illustration of 3GPP bearer architecture [16]. Dedicated EPS bearers with guaranteed bit rate and specific QoS attributes for various ser- vices can be established through negotiation and connection reconfiguration for active users. In LTE, the QoS control is performed per EPS bearer such that EPS bearers and radio bearers have a one-to-one relationship. There are two types of end-to-end bearers. The default bearer is established during the attach procedure and after an IP address is allocated to the UE and has best effort QoS characteristics. The dedicated bearer is typically estab- lished during the call setup after transition from the idle mode but can also be established during the attach pr