Quality of Service Class Identifier

Description

The Quality of Service Class Identifier (QCI) is a fundamental mechanism within the 3GPP Evolved Packet System (EPS) for managing and enforcing Quality of Service (QoS). It is a standardized integer value, ranging from 1 to 9 in its initial definition and later extended, that maps to a pre-configured set of QoS characteristics. These characteristics are not signaled on a per-bearer basis but are instead node-specific parameters that are pre-provisioned within network elements like the eNodeB, Serving Gateway (S-GW), and Packet Data Network Gateway (P-GW). When a bearer is established or modified, it is associated with a specific QCI. This QCI value acts as a reference pointer, instructing each network node on how to handle the packets belonging to that bearer.

Each QCI value is linked to a specific resource type (Guaranteed Bit Rate - GBR or Non-GBR), priority level, Packet Delay Budget (PDB), and Packet Error Loss Rate (PELR). The priority is an integer where a lower value indicates a higher priority for scheduling. The PDB defines an upper bound for the time a packet may be delayed between the UE and the P-GW (or UE and the RAN node in 5G). The PELR defines an upper bound for a rate of non-congestion related packet losses. For GBR bearers, the QCI also implies the need for admission control based on the guaranteed bit rate. The network uses these parameters to make scheduling, queue management, and link layer configuration decisions to meet the service requirements.

Architecturally, QCI is a core part of the EPS bearer model. It is used in the S5/S8 interface between the S-GW and P-GW, the S1 interface between the eNodeB and the MME/S-GW, and over the radio Uu interface. In the control plane, the MME receives the authorized QCI for a bearer from the P-GW (via the S-GW) and communicates it to the eNodeB during bearer setup. The eNodeB then uses this QCI, along with its locally configured mapping tables, to apply the appropriate radio resource scheduling (e.g., in the MAC layer). In 5G, the QoS model evolved with the 5G QoS Identifier (5QI), which is a direct conceptual successor to QCI, though with an expanded range of standardized values and more flexible parameters for new service types.

Purpose & Motivation

QCI was introduced to solve the critical problem of traffic differentiation and guaranteed service performance in all-IP mobile networks. Prior to 3GPP Release 8 and the EPS, circuit-switched and packet-switched domains were separate, with QoS often tied to specific, rigid bearer services. The move to a flat IP architecture required a new, scalable, and efficient method to manage diverse traffic—from voice and video streaming to web browsing and background file downloads—over a shared infrastructure. QCI provides this by standardizing a limited set of well-understood QoS profiles, enabling multi-vendor interoperability and simplifying network configuration and policy management.

Its creation was motivated by the need to support IMS-based services like Voice over LTE (VoLTE) which demand low latency and guaranteed bandwidth, alongside best-effort internet traffic. Without a mechanism like QCI, all packets would be treated equally, leading to poor user experience for real-time applications. QCI allows operators to create a virtual 'pipe' (the bearer) with specific characteristics for a service or application, ensuring that network resources are allocated appropriately. It abstracts complex per-flow QoS parameters into a simple integer, reducing signaling overhead and enabling fast, consistent policy enforcement across the entire network path from the core to the radio interface.