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BSR (buffer status report)

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Introduction:

Buffer Status Report (BSR) is a message used in Long Term Evolution (LTE) networks for the exchange of buffer status information between the User Equipment (UE) and the Evolved Node B (eNodeB). It is a feedback message generated by the UE and sent to the eNodeB, informing it about the buffer status of the UE.

BSR in LTE:

In an LTE network, data is transmitted from the eNodeB to the UE in the form of Transport Blocks (TBs). These TBs are buffered at the UE, and the UE needs to inform the eNodeB about the buffer status to ensure efficient transmission of data. This is where BSR comes into the picture. BSR is used to inform the eNodeB about the amount of data waiting to be transmitted from the UE.

BSR Procedure:

The BSR procedure is initiated by the UE when it has data to be transmitted to the eNodeB. The UE generates a BSR message that contains information about the amount of data waiting to be transmitted. The BSR message is then sent to the eNodeB using the uplink (UL) channel. The eNodeB uses the information in the BSR message to allocate resources for transmission of data from the UE.

The BSR procedure can be initiated by the UE in two ways:

  1. Periodic BSR: The UE generates a BSR message periodically, even if there is no data waiting to be transmitted. The period of BSR generation is determined by the network and is indicated to the UE through a Radio Resource Control (RRC) message.
  2. Event-triggered BSR: The UE generates a BSR message when there is data waiting to be transmitted, and the buffer reaches a threshold level. The threshold level is also determined by the network and is indicated to the UE through an RRC message.

BSR Formats:

The BSR message can be of two formats:

  1. Short BSR: The Short BSR message is used when the amount of data waiting to be transmitted is less than or equal to 64 bytes. The Short BSR message consists of a single octet that indicates the amount of data waiting to be transmitted.
  2. Long BSR: The Long BSR message is used when the amount of data waiting to be transmitted is greater than 64 bytes. The Long BSR message consists of multiple octets, and each octet indicates the amount of data waiting to be transmitted in a particular logical channel.

The UE can send multiple BSR messages in a single transmission, depending on the amount of data waiting to be transmitted.

BSR Fields:

The BSR message contains the following fields:

  1. BSR index: The BSR index indicates the size of the buffer status information field in the BSR message. It is a two-bit field and can take the values 0, 1, 2, or 3.
  2. Buffer status information: This field contains the actual buffer status information. It can be either a single octet in the case of a Short BSR message or multiple octets in the case of a Long BSR message.

BSR Index Values: The BSR index field in the BSR message can take the following values:

  1. 0: The UE has no data waiting to be transmitted.
  2. 1: The UE has data waiting to be transmitted, and the buffer status information field is 1 octet long.
  3. 2: The UE has data waiting to be transmitted, and the buffer status information field is 2 octets long.
  4. 3: The UE has data waiting to be transmitted, and the buffer status information field is 3 octets long.

BSR and Radio Resource Control (RRC):

The BBSR message is used by the RRC layer to make decisions about the allocation of radio resources for uplink transmission from the UE. The RRC layer uses the information in the BSR message to determine the amount of resources needed for uplink transmission from the UE. This information is used to decide the grant size and duration for uplink transmission.

The RRC layer also provides configuration information to the UE regarding the periodicity of the BSR message and the threshold for event-triggered BSR. The UE uses this information to generate BSR messages and inform the eNodeB about the buffer status.

BSR and Quality of Service (QoS):

The BSR message is an important tool for maintaining Quality of Service (QoS) in LTE networks. The eNodeB uses the information in the BSR message to allocate resources for uplink transmission from the UE. This ensures that the UE can transmit data to the eNodeB in a timely manner, maintaining QoS for the user.

The BSR message also helps in managing congestion in the network. When the UE generates a BSR message, it informs the eNodeB about the amount of data waiting to be transmitted. The eNodeB can then use this information to allocate resources in a way that minimizes congestion in the network.

Conclusion:

In conclusion, the Buffer Status Report (BSR) is a message used in LTE networks for the exchange of buffer status information between the UE and the eNodeB. The BSR message is generated by the UE and sent to the eNodeB, informing it about the buffer status of the UE. The BSR message is an important tool for maintaining QoS in LTE networks and managing congestion. The BSR procedure is initiated by the UE either periodically or when the buffer reaches a threshold level, and the BSR message can be of two formats: Short BSR and Long BSR. The BSR message contains the BSR index field and the buffer status information field, and the RRC layer uses the information in the BSR message to make decisions about the allocation of radio resources for uplink transmission from the UE.