Visible Interruption Length

Description

Visible Interruption Length (VIL) is a quality of service (QoS) metric defined in 3GPP Release 19, appearing in specifications TS 31.122 (Universal Subscriber Identity Module (USIM) conformance) and TS 38.133 (NR requirements for support of radio resource management). VIL measures the duration of interruptions that are visible to the user in services such as video streaming, augmented reality, or interactive applications. It is a key performance indicator (KPI) for assessing the continuity and reliability of user experiences in 5G and beyond.

In the context of 3GPP, VIL is used to evaluate the performance of mobility procedures, such as handovers, and radio resource management (RRM) functions. When a user moves between cells or experiences network changes, there may be brief interruptions in the data flow. If these interruptions exceed a certain threshold, they become visible to the user, causing a degraded experience. VIL quantifies the length of such interruptions, typically in milliseconds, and sets requirements to ensure that they remain below a perceptible level. For example, in video streaming, an interruption longer than 100 ms might cause a visible freeze or stutter.

The term is closely related to other QoS metrics like interruption time, latency, and packet loss. However, VIL specifically focuses on the user-visible aspect, meaning that it accounts for buffering and error concealment mechanisms that might hide short interruptions. VIL is measured at the application layer or at the interface between the network and the user equipment, depending on the specification. In TS 38.133, VIL requirements are likely defined for NR UEs during handover or other mobility events, ensuring that the interruption does not exceed a specified duration.

VIL is particularly important for services that require high reliability and low latency, such as mission-critical communications, cloud gaming, and extended reality (XR). In these applications, even short interruptions can significantly impact the user experience. By defining VIL requirements, 3GPP ensures that network equipment and UEs are designed to minimize visible interruptions, thereby improving overall QoS. The metric also helps operators benchmark their networks and identify areas for improvement.

From a technical perspective, VIL is influenced by factors such as handover execution time, radio link failure recovery, and data forwarding during mobility. Techniques like make-before-break handover, dual connectivity, and packet duplication can reduce VIL. The specifications TS 31.122 and TS 38.133 provide test procedures and requirements for VIL, enabling conformance testing of UEs and network nodes. As 5G evolves towards 6G, VIL will remain a critical metric for ensuring seamless user experiences.

In summary, VIL is a user-centric QoS metric that quantifies the duration of visible interruptions in data services. It is defined in 3GPP Release 19 and is used to specify and test the performance of mobility and RRM procedures in NR networks.

Purpose & Motivation

VIL was created to address the need for a user-centric QoS metric that captures the actual impact of network interruptions on the user experience. Traditional metrics like handover interruption time or packet loss ratio are network-centric and may not directly reflect what the user perceives. For example, a 50 ms interruption might be invisible in a voice call due to buffering, but could cause a visible glitch in a video stream. VIL fills this gap by defining the maximum duration of an interruption that remains imperceptible to the user.

Previous approaches to QoS often focused on average throughput or latency, which do not capture the episodic nature of interruptions. Users are more sensitive to brief but frequent interruptions than to consistently lower throughput. VIL provides a standardized way to measure and limit these interruptions, ensuring that services like video streaming, gaming, and XR remain smooth even during mobility. It also aligns with the industry trend towards experience-centric KPIs, such as those defined by ITU-T and other standards bodies.

The motivation for VIL came from the growing demand for seamless connectivity in 5G, especially for applications that require high reliability and low latency. As networks become more complex with densification and multi-connectivity, ensuring uninterrupted service during handovers and other events is challenging. VIL provides a clear target for network designers and equipment vendors, enabling them to optimize mobility procedures and buffering strategies to keep interruptions below the visible threshold.