Radio Bearers

Signaling and data radio bearers

Signaling radio bearers are the bearers to transmit RRC and NAS messages. There are broadly three types of signaling bearers defined –

• SRB0 is used for transport of RRC messages associated with the common control logical channel.
• SRB1 is used for transport of RRC messages including piggybacked NAS messages as well as NAS messages prior to the establishment of SRB2 where all are associated with a dedicated control logical channel.
• SRB2 is used for transport of NAS messages using a dedicated control logical channel, has a lower priority than SRB1, and is always configured by E-UTRAN after security activation.

The SRB types and their usage are summarized in Table. The UE in the RRC_IDLE state does not have any allocated SRB or DRB. The UE requires (dedicated) SRBs and DRBs to exchange data/messages with the network. In particular, for transfer of NAS messages, SRBs are established. They carry registration and signaling for user-plane setup. The user-plane application data is transported on DRBs. An E-UTRAN radio access bearer (E-RAB) uniquely identifies the concatenation of an S1 bearer and the corresponding DRB. When an E-RAB exists, there is a one-to-one mapping between this E-RAB and an EPS bearer of the NAS. To establish, modify, or release DRBs, the E-UTRAN applies the RRC connection reconfiguration procedure.

DRBs are established by the eNB in the RRC_CONNECTED state based on the relevant information from the MME. This information includes the desired QoS for the radio bearer, which enables the eNB to appropriately configure the PDCP, RLC, and MAC parameters. In the RRC_CONNECTED state, the DRBs can be modified and released based on the signaling information received from the MME. All DRBs are implicitly released when the RRC connection is released. When establishing a DRB, the E-UTRAN decides how to transfer the packets of an EPS bearer across the radio interface. An EPS bearer is uniquely mapped to a DRB, a DRB is then mapped to a dedicated traffic logical channel, all logical channels are mapped to a downlink or uplink shared transport channel, which are mapped to the corresponding physical downlink or uplink shared channel. A UE in the RRC_CONNECTED state, for which security has been activated, may initiate the procedure in order to continue the RRC connection. The connection reestablishment succeeds only if the serving cell is prepared, i.e., it has a valid UE context. In the case where the E-UTRAN accepts the connection reestablishment, SRB1 operation resumes while the operation of other radio bearers remains suspended. If the AS security has not been activated, the UE does not initiate the procedure, but rather directly transitions to the RRC_IDLE state.

Before concluding this section, it would be useful if we summarize the EPS bearer architecture and understand the termination points of each bearer in the LTE system. The EPS bearer service architecture is illustrated in Figure 5.6, in which the following bearers are defined-

An uplink traffic flow template (TFT) binds a service data flow to an EPS bearer in the uplink direction, where multiple service data flows can be multiplexed into the same EPS bearer by including multiple uplink packet filters2 in the uplink TFT.

A downlink TFT in the P-GW binds a service data flow to an EPS bearer in the downlink direction, where multiple service data flows can be multiplexed into the same EPS bearer by including multiple downlink packet filters in the downlink TFT.

An E-RAB transports the packets of an EPS bearer between the UE and the EPC. When an E-RAB exists, there is a one-to-one mapping between the E-RAB and an EPS bearer.

A DRB transports the packets of an EPS bearer between a UE and the eNB. When a DRB exists, there is a one-to-one mapping between this DRB and the EPS bearer/E-RAB.

An S1 bearer transports the packets of an E-RAB between an eNB and an S-GW.

An S5/S8 bearer transports the packets of an EPS bearer between an S-GW and a P-GW.

A UE stores the mapping between an uplink packet filter and a DRB to create the binding between the service data flow and the DRB in the uplink.

The P-GW stores the mapping between the downlink packet filter and S5/S8a bearer to create the binding between a service data flow and the S5/S8a bearer in the downlink.

The eNB stores a one-to-one mapping between the DRB and the S1 bearer to create the binding between the DRB and the S1 bearer in both the uplink and downlink.

The S-GW stores a one-to-one mapping between the S1 bearer and the S5/S8a bearer to create the binding between the S1 bearer and the S5/S8a bearer in both the uplink and downlink.

In the downlink, the piggybacked (or encapsulated) NAS messages are used for bearer establishment, modification, or release. In the uplink, the piggybacked NAS messages are used for transferring the initial NAS messages during connection setup. The NAS messages transferred via SRB2 are also contained in RRC messages, which do not include any RRC protocol control information. Once security is established, all RRC messages on SRB1 and SRB2, including those containing NAS or non-3GPP messages, are integrity-protected and ciphered by PDCP sublayer. The NAS independently applies integrity protection and ciphering to the NAS messages.

Random access is specified entirely in the MAC sublayer including initial transmission power estimation. The RRC connection establishment includes establishment of SRB1. The E-UTRAN completes the RRC connection establishment prior to completing the establishment of the S1 connection, i.e., prior to receiving the UE context from the EPC. Therefore, the AS security is not activated during the initial phase of the RRC connection. During the initial phase of the RRC connection, the E-UTRAN may configure the UE to perform measurement reporting. However, the UE only accepts handover messages when security has been activated. Upon receiving the UE context from the EPC, the E-UTRAN activates ciphering and integrity protection using the initial security activation procedure. The RRC messages to activate security are integrity-protected, while ciphering starts only after completion of the procedure, i.e., the response to the message used to activate security is not ciphered, while the subsequent messages used to establish SRB2 and DRBs are both integrity-protected and ciphered.

Following the activation of the initial security, the E-UTRAN establishes SRB2 and user DRBs, i.e., the E-UTRAN may perform this function prior to receiving the confirmation of the initial security activation from the UE. The E-UTRAN applies both ciphering and integrity protection for the RRC connection reconfiguration messages used to establish SRB2 and user DRBs. The E-UTRAN should release the RRC connection, if the initial security activation and/or the radio bearer establishment fails (i.e., security activation and user DRB establishment are triggered by a joint S1 procedure, which does not support partial success). For SRB2 and user DRBs, the security is always activated from the beginning, i.e., the E-UTRAN does not establish these bearers prior to activating security. The release of the RRC connections is initiated by the E-UTRAN. The procedure may be used to redirect the UE to another frequency or radio access technology (RAT). In certain cases, the UE may discontinue the RRC connection, i.e., it may transition to RRC_IDLE without notifying the E-UTRAN.

In the cases where the UE fails to establish a connection to the network (e.g., RLF, handover failure, RLC unrecoverable error, or reconfiguration compliance failure), the UE initiates the RRC connection reestablishment procedure, if security is active. If security is not active when one of the above failures occurs, the UE transitions to the RRC_IDLE state. In order to re-establish the RRC connection, the UE starts a timer known as T311 and performs cell selection. In this case, the UE should prioritize searching of LTE frequencies. However, no requirements are specified to prevent the UE from searching for other RATs. Upon finding a suitable cell on an LTE frequency, the UE stops timer T311, starts timer T301 and initiates a contention-based random-access procedure, and sends an RRC Connection Reestablishment Request message. In the RRC Connection Reestablishment Request message, the UE includes the UE identity used in the previous cell, the identity of that cell, a short message authentication code, and a cause. The E-UTRAN uses the reestablishment procedure to continue SRB1 and to reactivate security without changing algorithms. A subsequent RRC connection reconfiguration procedure is used to resume operation on radio bearers other than SRB1 and to reactivate measurements. If the cell in which the UE initiates the reestablishment is not prepared (i.e., no UE context is available), the E-UTRAN will reject the procedure, causing the UE to transition to the RRC_IDLE state.

Reference : Sassan Ahmadi, LTE-Advanced, 2014