Section: 5.4.5 Buffer Status Reporting
Content: The Buffer Status reporting procedure is used to provide the serving eNB with information about the amount of data available for transmission in the UL buffers associated with the MAC entity. RRC controls BSR reporting by configuring the three timers periodicBSR-Timer, retxBSR-Timer and logicalChannelSR-ProhibitTimer and by, for each logical channel, optionally signalling logicalChannelGroup which allocates the logical channel to an LCG, as specified in TS 36.331[ Evolved Universal Terrestrial Radio Access (E-UTRA); Radio Resource Control (RRC); Protocol specification ]  [8]. For the Buffer Status reporting procedure, the MAC entity shall consider all radio bearers which are not suspended and may consider radio bearers which are suspended. For NB-IoT the Long BSR is not supported and all logical channels belong to one LCG. A Buffer Status Report (BSR) shall be triggered if any of the following events occur: - UL data, for a logical channel which belongs to a LCG, becomes available for transmission in the RLC entity or in the PDCP entity (the definition of what data shall be considered as available for transmission is specified in TS 36.322[ Evolved Universal Terrestrial Radio Access (E-UTRA); Radio Link Control (RLC) protocol specification ]  [3] and TS 36.323[ Evolved Universal Terrestrial Radio Access (E-UTRA); Packet Data Convergence Protocol (PDCP) specification ]  [4] or TS 38.323[ NR; Packet Data Convergence Protocol (PDCP) specification ]  [17] respectively) and either the data belongs to a logical channel with higher priority than the priorities of the logical channels which belong to any LCG and for which data is already available for transmission, or there is no data available for transmission for any of the logical channels which belong to a LCG, in which case the BSR is referred below to as "Regular BSR"; - UL resources are allocated and number of padding bits is equal to or larger than the size of the Buffer Status Report MAC control element plus its subheader, in which case the BSR is referred below to as "Padding BSR"; - retxBSR-Timer expires and the MAC entity has data available for transmission for any of the logical channels which belong to a LCG, in which case the BSR is referred below to as "Regular BSR"; - periodicBSR-Timer expires, in which case the BSR is referred below to as "Periodic BSR". For Regular BSR: - if the BSR is triggered due to data becoming available for transmission for a logical channel for which logicalChannelSR-Prohibit is configured by upper layers: - start or restart the logicalChannelSR-ProhibitTimer; - else: - if running, stop the logicalChannelSR-ProhibitTimer. For Regular and Periodic BSR: - if more than one LCG has data available for transmission in the TTI where the BSR is transmitted: report Long BSR; - else report Short BSR. For Padding BSR: - if the number of padding bits is equal to or larger than the size of the Short BSR plus its subheader but smaller than the size of the Long BSR plus its subheader: - if more than one LCG has data available for transmission in the TTI where the BSR is transmitted: report Truncated BSR of the LCG with the highest priority logical channel with data available for transmission; - else report Short BSR. - else if the number of padding bits is equal to or larger than the size of the Long BSR plus its subheader, report Long BSR. For NB-IoT or BL UEs: - if rai-Activation is configured, and a buffer size of zero bytes has been triggered for the BSR, and the UE may have more data to send or receive in the near future: - cancel any pending BSR. If the Buffer Status reporting procedure determines that at least one BSR has been triggered and not cancelled: - if the MAC entity has UL resources allocated for new transmission for this TTI: - instruct the Multiplexing and Assembly procedure to generate the BSR MAC control element(s); - start or restart periodicBSR-Timer except when all the generated BSRs are Truncated BSRs; - start or restart retxBSR-Timer. - else if a Regular BSR has been triggered and logicalChannelSR-ProhibitTimer is not running: - if an uplink grant is not configured or the Regular BSR was not triggered due to data becoming available for transmission for a logical channel for which logical channel SR masking (logicalChannelSR-Mask) is setup by upper layers; or - if sr-WithHARQ-ACK-Config is configured and there is valid resource for SR together with acknowledgement of the data in this TTI: - a Scheduling Request shall be triggered. A MAC PDU shall contain at most one MAC BSR control element, even when multiple events trigger a BSR by the time a BSR can be transmitted in which case the Regular BSR and the Periodic BSR shall have precedence over the padding BSR. For EDT, the MAC entity shall not generate a BSR MAC control element if new transmission is for Msg3. For CP-PUR, the MAC entity shall not generate a BSR MAC control element if new transmission is intended for preconfigured uplink grant. The MAC entity shall restart retxBSR-Timer upon indication of a grant for transmission of new data on any UL-SCH. All triggered BSRs shall be cancelled in case the UL grant(s) in this TTI can accommodate all pending data available for transmission but is not sufficient to additionally accommodate the BSR MAC control element plus its subheader. All triggered BSRs shall be cancelled when a BSR is included in a MAC PDU for transmission. The MAC entity shall transmit at most one Regular/Periodic BSR in a TTI. If the MAC entity is requested to transmit multiple MAC PDUs in a TTI, it may include a padding BSR in any of the MAC PDUs which do not contain a Regular/Periodic BSR. All BSRs transmitted in a TTI always reflect the buffer status after all MAC PDUs have been built for this TTI. Each LCG shall report at the most one buffer status value per TTI and this value shall be reported in all BSRs reporting buffer status for this LCG. NOTE 1: A Padding BSR is not allowed to cancel a triggered Regular/Periodic BSR, except for NB-IoT. A Padding BSR is triggered for a specific MAC PDU only and the trigger is cancelled when this MAC PDU has been built. NOTE 2: If UL HARQ operation is autonomous for the HARQ entity and if the BSR is already included in a MAC PDU for transmission by this HARQ entity, but not yet transmitted by lower layers, it is up to UE implementation how to handle the BSR content.