No Audio-alerting capability

Description

No Audio-alerting capability (NA) is a supplementary service feature within the 3GPP circuit-switched and IP Multimedia Subsystem (IMS) telephony specifications. It is a subscriber or terminal capability that indicates the inability to produce an audible ringing tone or alert for an incoming voice call. This capability is registered with the network, typically in the Home Subscriber Server (HSS) or a service control function. When a call is routed to a subscriber marked with NA, the network modifies standard call establishment procedures. Instead of sending alerting messages to trigger the terminal's ringer, the network may immediately divert the call to voice mail, present a busy tone to the caller, or attempt alternative notification mechanisms as defined by the service logic.

The technical operation involves the service control logic within the Mobile Switching Center (MSC) for circuit-switched calls or the Serving-Call Session Control Function (S-CSCF) in IMS. During call termination, the network checks the subscriber's profile for the NA indicator. If active, the network suppresses the normal 'Alerting' signal (SIP 180 Ringing in IMS) that would be sent to the calling party and the called terminal. Instead, it may invoke a service such as Call Completion to Busy Subscriber (CCBS) or connect the call directly to an announcement server or a messaging system. The called party might receive a non-audible notification, such as a flash on the screen, a text message, or a data push indicating a missed call, depending on the terminal's capabilities and service subscription.

Architecturally, NA interacts with other supplementary services like Call Forwarding and Message Waiting Indication (MWI). Its implementation is detailed across multiple 3GPP specifications covering service requirements (22-series), protocol details (24-series for CS, 29-series for IMS Diameter and SIP), and management (32-series). The feature is crucial for accessibility, allowing users who are deaf or hard of hearing to use telephony services effectively. It also has applications in scenarios where audible alerts are undesirable, such as in hospitals, libraries, or for on-call devices that use vibration-only modes. The network's handling ensures that the calling party receives appropriate feedback, and the called party is notified through accessible means, maintaining the utility of the telephony service while accommodating the lack of audio alerting.

Purpose & Motivation

The No Audio-alerting capability was created to ensure telephony services are accessible and functional for all users, including those with hearing impairments. Its primary purpose is to modify the standard call alerting process to accommodate terminals or users that cannot perceive an audible ring. Without this service, a call to a deaf user would result in the phone ringing unheard, eventually timing out and forwarding to voice mail, providing no immediate, accessible notification to the called party. NA solves this by allowing the network to intervene and provide an alternative, accessible form of call notification or immediate call handling.

Historically, as telephony evolved from basic circuit-switched services to intelligent network features and later to IMS, the need for inclusive design became a regulatory and ethical imperative. The NA feature, introduced in early 3GPP releases, addresses limitations of the traditional alerting model. It enables service providers to offer compliant accessibility services, often mandated by national regulations. The feature also has practical applications beyond accessibility, such as in machine-to-machine (M2M) communication where devices have no speaker, or in specialized user environments where sound must be minimized.

Furthermore, NA facilitates the integration of telephony with other notification systems. It allows calls to trigger visual signals on specialized terminals, text messages to pagers or secondary devices, or updates to message waiting indicators. This bridges the gap between pure voice telephony and multimodal communication, aligning with the convergence trend in modern telecommunications. By solving the basic problem of alerting without sound, it upholds the principle of universal service and enables innovative call-handling applications for diverse user needs and device types.