in-band announcement

Description

An in-band announcement is a service capability defined by 3GPP that allows the network to send an announcement to a user equipment (UE) using the bearer established for an ongoing communication. Unlike out-of-band signaling, which uses separate control channels, in-band announcements are transmitted within the user-plane data stream of the communication session. This approach ensures that the announcement is delivered reliably and in synchronization with the media flow, even when the UE is roaming or when the control plane is congested.

The primary use cases for in-band announcements include supplementary services such as call waiting, call hold, advice of charge (AoC), and multi-party conferencing. For example, when a user is engaged in a call and receives a second incoming call, the network may inject an in-band announcement to inform the user of the waiting call. Similarly, for advice of charge, the network can periodically send charging information as an in-band announcement during the call. These announcements are typically generated by the network node responsible for the service, such as the MSC (Mobile Switching Center) or the IMS (IP Multimedia Subsystem) application server, and are sent through the media gateway or media resource function.

The technical implementation of in-band announcements relies on the bearer established for the communication, which could be a voice channel in circuit-switched (CS) networks or a Real-time Transport Protocol (RTP) stream in packet-switched (PS) networks like VoLTE. In CS networks, the announcement is often played as an audio message over the voice channel. In PS networks, the announcement may be delivered as an audio or video stream using the same RTP session as the ongoing media. The UE must be capable of mixing or temporarily replacing the media stream with the announcement, and then resuming the original communication. This requires support in the UE's media handling and user interface to alert the user appropriately.

3GPP specifications 24.428, 24.528, and 24.628 define the procedures for in-band announcements in the context of supplementary services. These specifications outline the signaling flows, the conditions under which announcements are triggered, and the interaction with other services. The announcements are typically triggered by network events, such as an incoming call while the user is busy, or by a request from the user via Unstructured Supplementary Service Data (USSD) or other means. The network ensures that the announcement does not disrupt the ongoing communication and that the user can continue the original session after the announcement.

In-band announcements are particularly important in scenarios where out-of-band signaling is not feasible or reliable, such as during handovers or when the UE is in a limited-service state. They also provide a seamless user experience by integrating the announcement into the media stream, avoiding the need for the user to switch between different channels or applications. As networks evolve towards all-IP architectures, in-band announcements continue to be relevant for IMS-based services, where they are delivered using the same IP bearer as the voice or video call. The 3GPP specifications have been updated over releases to ensure compatibility with new technologies, but the fundamental concept remains unchanged.

Purpose & Motivation

In-band announcements exist to provide a reliable and efficient way for the network to deliver service-related information to users during an active communication session. Before the introduction of in-band announcements, such information was often conveyed through out-of-band signaling, which could be unreliable in certain conditions, such as when the control plane is congested or when the UE is roaming. Out-of-band signaling also required additional resources and could introduce delays or synchronization issues. In-band announcements solve these problems by using the already established bearer, ensuring that the announcement is delivered in a timely manner and in sync with the media.

The motivation for in-band announcements was to enhance the user experience for supplementary services. For example, call waiting notifications needed to be delivered without interrupting the ongoing call, and advice of charge information needed to be presented in real-time. By using the media bearer, the network can inject announcements seamlessly, and the UE can handle them as part of the media stream. This approach also simplifies the network architecture, as it does not require separate channels for announcements, reducing complexity and cost.

Historically, in-band announcements were introduced in 3GPP Release 7 as part of the evolution of supplementary services in both CS and PS domains. They have since been adopted in later releases for IMS-based services, where they are used for similar purposes. The concept has proven robust and continues to be specified in the latest releases, demonstrating its ongoing relevance in modern networks.