Best Effort Quality of Service

Description

Best Effort QoS is the lowest of all QoS traffic classes in 3GPP networks. It represents a service model where the network makes no guarantees about the delivery of packets in terms of throughput, delay, jitter, or packet loss. Instead, the network delivers the QoS that can be achieved under current conditions, which is also called best effort QoS. This is in contrast to guaranteed QoS classes (e.g., conversational, streaming) that provide specific performance bounds.

In 3GPP, QoS is managed through bearers (in 4G) or QoS flows (in 5G). Each bearer or flow is associated with a QoS Class Identifier (QCI) in 4G or a 5G QoS Identifier (5QI) in 5G. Best effort QoS typically corresponds to QCI 9 (or 5QI 9) for non-GBR (Guaranteed Bit Rate) traffic. Non-GBR bearers do not reserve network resources; they share available bandwidth with other traffic. This means that during congestion, best effort traffic may experience reduced throughput, increased latency, and higher packet loss.

The best effort model is used for applications that are tolerant to delay and loss, such as web browsing, email, file downloads, and background updates. It is also the default for internet traffic when no specific QoS is requested. The network treats all best effort traffic equally, without prioritization, unless other mechanisms (e.g., traffic shaping, policing) are applied. In 5G, best effort QoS is supported by non-GBR QoS flows, which are established without a guaranteed flow bit rate.

Best effort QoS is implemented through the packet scheduler in the radio access network (e.g., eNodeB, gNB) and core network (e.g., SGW, PGW, UPF). The scheduler allocates radio resources among users based on various factors, such as channel conditions, traffic load, and fairness. For best effort traffic, the scheduler does not reserve resources but rather uses leftover capacity after serving higher-priority traffic. This ensures that guaranteed QoS traffic is protected, while best effort traffic fills in the gaps.

In terms of architecture, best effort QoS is supported by the QoS framework defined in 3GPP TS 23.107 (for UMTS) and TS 23.501 (for 5G). The QoS profile includes parameters such as allocation and retention priority (ARP), which determines whether a bearer can be established or modified during congestion. For best effort, ARP is typically set to a low priority. The QoS flow is mapped to a Data Radio Bearer (DRB) in the RAN, and the core network enforces QoS via the user plane function (UPF) and policy control function (PCF).

Best effort QoS is essential for providing ubiquitous connectivity without over-provisioning. It allows operators to offer affordable, high-speed internet access while reserving guaranteed QoS for premium services. However, it also means that during peak hours, users may experience degraded performance. To mitigate this, operators use techniques such as traffic offloading, caching, and capacity expansion. In 5G, network slicing can be used to create dedicated slices with guaranteed QoS, while best effort traffic can be relegated to a shared slice.

Purpose & Motivation

Best Effort QoS exists to provide a simple, scalable, and cost-effective way to deliver data services without the need for complex resource reservation. In the early days of mobile data (e.g., GPRS, UMTS), most applications were not delay-sensitive, and users were willing to accept variable performance. Guaranteed QoS mechanisms were complex and expensive to implement, requiring admission control, resource reservation, and policing. Best effort QoS offered a pragmatic alternative: deliver data as fast as possible, with no guarantees, and let higher-layer protocols (e.g., TCP) handle reliability.

The concept was formalized in 3GPP Rel-4 as part of the UMTS QoS framework, which defined four traffic classes: conversational, streaming, interactive, and background. Best effort QoS corresponds to the background class, which is the lowest priority. It addresses the limitations of circuit-switched data, which required dedicated resources and was inefficient for bursty traffic. By using packet-switched best effort, operators could multiplex many users onto the same resources, improving efficiency and reducing costs.

Over time, best effort QoS has remained relevant because the vast majority of internet traffic (web, video streaming, file sharing) can tolerate some delay and loss. It is also the foundation for over-the-top (OTT) services that do not require network guarantees. In 5G, best effort QoS continues to be the default for non-GBR flows, but it is complemented by network slicing and edge computing to provide differentiated services. The purpose remains to offer a baseline level of connectivity that is affordable and ubiquitous, while reserving guaranteed QoS for applications that truly need it.