Outgoing Call Timer

Description

The Outgoing Call Timer (OCT) is a subscriber-specific timer stored within the Universal Subscriber Identity Module (USIM) application, as defined in 3GPP TS 31.102. It is a proactive UICC (Universal Integrated Circuit Card) feature, meaning the USIM can initiate actions independently of the Mobile Equipment (ME). The timer's primary function is to limit the duration of a mobile-originated call setup procedure. When a user initiates a call, the ME sends a SETUP message to the network. Concurrently, the USIM starts the OCT. If the call is successfully established (e.g., the user answers), the timer is stopped. However, if the call setup does not complete within the pre-configured OCT duration—due to network congestion, the called party not answering, or other failures—the USIM proactively commands the ME to terminate the call attempt. This is typically done by sending a terminal response with a specific result, such as 'Call control by USIM,' which instructs the ME to release the call.

The OCT value is a parameter defined in the USIM's Elementary File (EF) under the Telecom service table. It is configured by the operator and can be personalized for different subscribers or subscription plans. The timer mechanism is part of the Call Control (CC) service provided by the USIM, which interacts with the ME's call control layer via the Application Protocol Data Unit (APDU) interface defined by ETSI/3GPP. The USIM monitors the call state by processing envelope commands from the ME that carry call status information.

Architecturally, the OCT operates within the secure environment of the UICC, ensuring the timer logic and value are tamper-resistant. Its enforcement is independent of the network core, providing a reliable fallback mechanism even if network signaling experiences delays or anomalies. By imposing a finite time limit on call setup, the OCT helps prevent scenarios where a mobile device remains stuck in a prolonged call establishment state, which could drain battery, occupy radio resources unnecessarily, and lead to a poor user perception of service reliability. It is a fundamental element of the USIM's role in managing basic telephony services securely and efficiently.

Purpose & Motivation

The Outgoing Call Timer was introduced to address the problem of indefinite or excessively long call setup attempts in mobile networks. Prior to its standardization, if a call setup failed due to network issues or an unresponsive called party, the mobile device might remain in a call setup state for an unpredictable duration, waiting for a network response or timeout. This wasted valuable radio resources (like the traffic channel) and device battery life, and could cause user confusion as the device appeared to be 'stuck.' The OCT provides a standardized, subscriber-controlled mechanism to bound this time.

Its creation was motivated by the need for improved network efficiency and a consistent user experience. By allowing operators to define a maximum call setup duration, networks can more quickly free up resources from failed attempts, improving overall capacity and stability. From a user perspective, it ensures that a failed call attempt concludes in a predictable timeframe, allowing the user to retry or attempt other actions. The timer's implementation within the secure USIM also means its operation is trusted and consistent across different mobile devices, as it is governed by the operator-provisioned SIM card rather than variable device software.