Buffer Status Report (BSR) is a kind of Medium Access Control- Control Element (MAC-CE) from UE to Network, carrying the information on how much data is in UE buffer to be send out. Putting it another way, it is a kind of MAC layer message from UE to Network (eNodeB) saying "I have something to transmit, would you give me a Grant to send this data ?
A BSR is triggered if any of the following event occurs:
- Uplink data for a logical channel becomes available for transmission, and either the data belongs to a logical channel with higher priority than the priorities of the logical channels for which data is already available for transmission,or there is no data available for transmission for any of the logical channels.In this case, the BSR is referred to as“regular BSR.”
- Uplink resources are allocated and the number of padding bits is equal to or larger than the size of the BSR MAC-control element, in which case the BSR is referred to as“padding BSR.”
- A serving cell change occurs,in which case the BSR is referred to as“regular BSR.”
- There transmission BSR timer expires and the UE has data available for transmission, in which case the BSR is referred to as“regular BSR.”
- The periodic BSR timer expires, in which case the BSR is referred to as“periodic BSR.”
