QCI (QoS Class Identifier) is a scalar value (1 to 9 for standardized classes) that defines the quality of service treatment for an EPS bearer in 4G LTE networks. Each QCI maps to specific priority, delay budget and packet loss rate parameters that determine how traffic is handled.
What is QCI?
QoS Class Identifier: the framework that ensures different traffic types receive appropriate network treatment.
Detailed explanation
In LTE, every data flow is carried over an EPS bearer, and each bearer is assigned a QCI value that defines its QoS characteristics. The QCI determines whether the bearer gets guaranteed bit rate (GBR) or operates on a best-effort basis (non-GBR), along with the scheduling priority, packet delay budget and acceptable packet error loss rate.
The 3GPP specification (TS 23.203) defines 9 standardized QCI values. QCI 1 through 4 are GBR bearers for real-time services like voice (QCI 1) and video (QCI 2). QCI 5 through 9 are non-GBR bearers for services like IMS signalling (QCI 5), web browsing (QCI 8) and default data (QCI 9).
Operators can also define custom QCI values beyond the standardized 9 for specific services. The QCI value is carried in NAS signalling during bearer establishment and modification. The eNodeB scheduler uses QCI parameters to prioritize traffic and enforce delay budgets.
In 5G, the QCI concept evolves into 5QI (5G QoS Identifier), which maintains the same framework but adds new classes for URLLC (ultra-low latency), V2X (vehicle communication) and other 5G-specific use cases. The 5QI system also supports reflective QoS, where the network can mirror uplink QoS rules from downlink flows.
Standardized QCI classes
| QCI | Type | Delay | Example |
|---|---|---|---|
| 1 | GBR | 100 ms | VoLTE voice |
| 2 | GBR | 150 ms | Video call |
| 5 | Non-GBR | 100 ms | IMS signalling |
| 9 | Non-GBR | 300 ms | Default data |
How HiCellTek monitors QCI
The L3 Decoder module in HiCellTek extracts the QCI value from NAS bearer establishment messages, showing which QoS class is assigned to each active bearer. Combined with the VoLTE QoE module, engineers can verify that voice calls receive the expected QCI 1 treatment and diagnose bearer setup failures.
Frequently asked questions
What is QCI in LTE?
What QCI is used for VoLTE?
What is the 5G equivalent of QCI?
HiCellTek decodes QCI values, bearer parameters and QoS configuration directly on Android smartphones.