What triggers a BSR in 5G NR?

A Regular BSR is triggered when uplink data arrives in a logical channel group that previously had no data (new data arrival), or when data arrives for a logical channel with higher priority than any existing buffered data. A Periodic BSR is triggered by a configurable timer (periodicBSR-Timer, 1 to 2560 ms). A Padding BSR is included when a UL grant has space remaining after all pending data and MAC CEs are accommodated. The UE maintains a retxBSR-Timer that re-triggers BSR if the previous report was not followed by a sufficient uplink grant.

What BSR formats exist?

Short BSR (1 byte): reports the buffer size for one logical channel group (LCG) using a 5-bit buffer size index (0 to 31) with logarithmic quantization from 0 to over 150,000 bytes. Long BSR (variable): reports buffer sizes for all 8 LCGs using 8 bits each for finer quantization. Short Truncated BSR: same as Short BSR but used when insufficient UL resources exist for the full report. Long Truncated BSR: reports a subset of LCGs when space is limited. In NR, buffer size tables use an expanded 8-bit index with values ranging from 0 bytes to over 81 million bytes to accommodate high-throughput 5G data rates.

How does the gNB use BSR for scheduling?

The gNB scheduler uses BSR information along with QoS requirements, channel quality indicators (CQI), and fairness policies to determine uplink grant sizes and timing. A high-priority LCG with large buffer triggers immediate large grants to minimize latency (for URLLC traffic). A best-effort LCG receives grants based on available capacity after priority traffic is served. Without BSR, the gNB has no visibility into UE buffer status and cannot efficiently allocate uplink resources, leading to either wasted grants (over-allocation) or excessive latency (under-allocation).

5G NR & LTE

Buffer Status Report

/buf-ur stay-tus ree-port/ — abbr. BSR

A MAC Control Element sent by the UE to inform the gNB of uplink data buffered across logical channel groups (LCGs). The gNB uses BSR to allocate appropriate uplink grants, balancing resource efficiency against latency. BSR formats range from Short (1-byte, single LCG) to Long (all 8 LCGs), with logarithmic buffer size quantization supporting 5G data rates up to 81+ million bytes per LCG.

Category: 5G NR MAC Layer

Formats: Short, Long, Truncated

LCGs: Up to 8

Understanding Buffer Status Reports

In cellular networks, the base station controls all uplink resource allocation. The UE cannot transmit without an uplink grant. But the gNB has no direct visibility into the UE's transmit buffer. The BSR bridges this information gap: when new data arrives at the UE, it triggers a BSR that tells the gNB how much data is waiting and which logical channel group it belongs to, enabling the scheduler to allocate appropriately sized grants with correct priority.

NR supports 8 logical channel groups (LCGs 0 to 7), each mapping to one or more logical channels with similar QoS requirements. The buffer size is reported using logarithmic quantization tables: 5-bit (Short BSR, 32 levels) or 8-bit (Long BSR, 256 levels) indices that map to byte ranges. This compact representation conveys buffer state with minimal overhead while covering the enormous dynamic range from 0 bytes to over 81 million bytes needed for 5G peak rates.

BSR Timing Parameters

BSR Triggers:
Regular: new data in empty LCG or higher-priority data arrival
Periodic: periodicBSR-Timer (1 to 2560 ms, RRC configured)
Padding: spare space in existing UL grant

Buffer Size Index (NR Long BSR):
Index 0: 0 bytes | Index 1: 10 bytes | ... | Index 255: > 81,338,368 bytes
Logarithmic quantization for wide dynamic range

Scheduling Response:
URLLC LCG: immediate grant, < 1 ms target
eMBB LCG: proportional fair scheduling

BSR Format Comparison

Format	Size	LCGs Reported	Resolution	Trigger
Short BSR	1 byte	1 LCG	5-bit (32 levels)	Single LCG has data
Long BSR	3+ bytes	All 8 LCGs	8-bit (256 levels)	Multiple LCGs have data
Short Truncated	1 byte	1 LCG (highest)	5-bit	Insufficient UL resources
Long Truncated	Variable	Subset	8-bit	Partial report fits
Padding BSR	Varies	1 or all	5 or 8-bit	Spare space in grant

Common Questions

Frequently Asked Questions

What triggers a BSR?

Regular: new data in empty LCG or higher-priority arrival. Periodic: configurable timer (1 to 2560 ms). Padding: spare space in existing grant. retxBSR-Timer re-triggers if previous BSR wasn't followed by sufficient UL grant.

What formats exist?

Short BSR: 1 byte, single LCG, 5-bit index. Long BSR: all 8 LCGs, 8-bit indices. Truncated variants when UL resources are insufficient. NR expanded table covers 0 to 81M+ bytes for high-throughput 5G rates.

How does the gNB use BSR?

Combines BSR with CQI, QoS requirements, and fairness policies. High-priority LCG with large buffer gets immediate large grants (URLLC). Best-effort LCG gets capacity-based scheduling. Without BSR, gNB over-allocates (waste) or under-allocates (latency).

Related Terms

← Buffer Layer Buffer Status Report (Wi-Fi) →