Prepared by Riya Ri yazz A. Kha Khan n
[email protected]
SRBs are associated with the control plane protocol stack. The control plane protocol stack includes RRC, RLC, MAC, Frame Protocol and Physical layers.
The concept of Signaling Radio Bearer (SRB) is used to define the logical signaling connection between the RLC layer in the UE and the peer RLC layer in the RNC.
An SRB is a control plane version of a Radio Bearer (RB). Every radio bearer has an identity and radio bearer identities 1 to 4 are reserved for SRB. It is possible to configure more than a single SRB to simultaneously link a UE to an RNC. Each SRB has its own logical channel between the RLC and MAC layers. The MAC layer is used to multiplex the multiple SRB onto a single transport channel which is then processed by the physical layer. This multiplexing operation requires the MAC layer to include a header to specify which SRB is using the transport channel at any point in time.
3GPP TS 25.331 specifies that all messages sent on the Common Control Channel (CCCH) logical channel make use of SRB0. SRB0 is always encapsulated by the RACH transport channel in the uplink direction and the FACH transport channel in the downlink direction.
UE in RRC Idle mode are limited to using only SRB0. This SRB is used when establishing an RRC Connection and making the transition from RRC Idle mode to RRC Connected mode. SRB0 uses Transparent Mode (TM) RLC in the uplink direction and Unacknowledged Mode (UM) RLC in the downlink direction. Both of these RLC modes rely upon re-transmissions being provided by layer 3 rather than layer 2, i.e. by the RRC layer rather than the RLC layer. This is in contrast to Acknowledged Mode (AM) RLC which allows re-transmissions from the RLC layer. The use of TM and UM RLC means that entire RRC messages have to be re-transmitted rather than only the individual transport blocks which have been received in error. Re-transmissions may be required if the air-interface conditions are relatively poor.
Concept of Signaling Radio Bearer
3GPP TS 25.331 specifies that SRB1, 2, 3 and 4 make use of Dedicated Control Channel (DCCH) logical channels. If all four of these SRB are used then four DCCH logical channels are multiplexed onto a single transport channel within the MAC layer. These DCCH SRB are configured during RRC Connection establishment within the RRC Connection Setup message. They can also be configured when making RRC Connected mode state changes, e.g. within a Radio Bearer Reconfiguration message when moving from CELL_DCH to CELL_FACH.
When a UE is in RRC Connected mode SRB1, 2 and 3 are configured as a minimum while SRB4 is optional.
SRB1 is used for all messages sent on the DCCH which use Unacknowledged Mode (UM) RLC. SRB2 is used for all messages sent on the DCCH which use Acknowledged Mode (AM) RLC and which do not contain NAS messages. SRB3 and SRB4 are used for all messages sent on the DCCH which contain NAS messages. SRB3 and 4 use AM RLC in the same way as SRB2. SRB3 is used rather than SRB 4 either when the NAS indicates that a message has high priority, or when SRB4 has not been configured. SRB4 is used when it has been configured and the NAS indicates that a message has low priority. The priority levels are used within the MAC layer when multiplexing the set of SRB onto a single transport channel, i.e. the priority determines which messages are sent first.
Summary of signaling radio bearers
RRC messages associated with each SRB
Always SRB0: Cell Update, RRC Connection Request, RRC Connection Setup, RRC Connection Reject, URA Update
Always SRB1: None
Always SRB2: Active Set Update Complete, Active Set Update Failure, Assistance Data Delivery, Cell Change Order From UTRAN, Cell Change Order from UTRAN Failure, Counter Check, Counter Check Response, Handover from UTRAN Command, Handover from UTRAN Failure, Handover to UTRAN Complete, Measurement Control, Measurement Control Failure, Paging Type 2, Physical Channel Reconfiguration Complete, Physical Channel Reconfiguration Failure, Radio Bearer Reconfiguration Complete, Radio Bearer Reconfiguration Failure, Radio Bearer Release Complete, Radio Bearer Release Failure, Radio Bearer Setup Complete, Radio Bearer Setup Failure, RRC Connection Setup Complete, RRC Status, Security Mode Command, Security Mode Complete, Security Mode Failure, Signaling Connection Release, Signaling Connection Release Indication, Transport Channel Reconfiguration Complete, Transport Channel Reconfiguration failure, Transport Format Combination Control Failure, UE Capability Information, UTRAN Mobility Information Confirm, UTRAN Mobility Information Failure
Always SRB3:Initial Direct Transfer Always SRB4: None SRB0 or SRB1: Cell Update Confirm, RRC Connection Release, URA Update Confirm SRB1 or SRB2: Active Set Update, Measurement Report, Physical Channel Reconfiguration, Radio Bearer Reconfiguration, Radio Bearer Release, Radio Bearer Setup, RRC Connection Release Complete, Transport Channel Reconfiguration, Transport Format Combination Control, UE Capability Enquiry, UE Capability Information Confirm, UTRAN Mobility Information SRB3 or SRB4: Downlink Direct Transfer, Uplink Direct Transfer
The SRB associated with each RRC message can be deduced from the logical channel type, RLC mode and whether or not the RRC message includes a NAS message. The majority of RRC messages make use of SRB2, i.e. the DCCH logical channel with AM RLC and without including a NAS message. There are no messages which always use SRB1 or SRB4. The RRC Initial Direct Transfer message is always sent with high priority whereas the priority of the Uplink and Downlink Direct Transfer messages depends upon the type of encapsulated NAS message.
SRB 1 to 4: The RB represents the logical connection between the UE and the RNC. The concept of a user plane RB is the same as the concept of a control plane SRB. RB identities 1 to 4 are reserved for SRB and so user plane RB have identities which are greater than 4.
