Barred Dialling Number

Description

Barred Dialling Number (BDN) is a standardized service feature within the 3GPP framework, specifically defined as part of the USIM (Universal Subscriber Identity Module) application toolkit. Its primary function is to allow the barring (blocking) of outgoing voice calls, and potentially other communication attempts like SMS or data sessions, to a predefined list of telephone numbers or number ranges. The BDN list is stored as a dedicated file (EF_BDN) on the USIM card, which is a secure, tamper-resistant element. This file contains entries specifying the barred numbers, which can be full international numbers (e.g., +441234567890) or partial numbers using wildcard characters to block number ranges (e.g., blocking all premium rate numbers starting with a specific prefix). The enforcement of the barring is performed locally by the Mobile Equipment (ME), i.e., the handset or device, not by the network core. When a user initiates a call, the device's protocol stack consults the BDN list stored on the USIM before proceeding with the call setup signaling. If the dialed number matches an entry in the BDN list, the device aborts the call attempt locally and typically informs the user that the call is barred. This local enforcement is a key architectural aspect, as it does not consume network resources for a call that will ultimately be blocked, and it provides immediate feedback to the user.

The management of the BDN list can occur through several mechanisms. It can be pre-provisioned by the network operator onto the USIM, often for business or security reasons (e.g., barring international dialing on a corporate phone). Alternatively, it can be configured by the subscriber via the handset's user interface if the operator permits such user control. The USIM Application Toolkit (USAT) provides the standardized commands for secure interaction between the device and the USIM to read, update, or verify the BDN list. The feature's operation is integrated with other USIM-based services and the device's call control logic. It acts as a first-line check before any radio resource connection is established for a mobile-originated call.

From a network perspective, BDN is a subscriber-centric service management tool. While the core network (MSC/VLR in circuit-switched domains) is unaware of the BDN check happening on the device, the feature directly impacts the service experience and billing. It helps prevent unintended usage, such as calls to expensive premium-rate numbers or international numbers, which can lead to bill shock. For enterprises, it is a tool for enforcing telecom policies on company-provided devices. The specification of BDN across documents like 3GPP TS 31.102 (USIM characteristics) and TS 31.121 (UICC-terminal interface) ensures interoperability between USIMs from different vendors and mobile devices, making it a universally supported feature in 3GPP networks from 3G onwards. Its role is complementary to network-based barring services like Barring of All Outgoing Calls (BAOC) or Barring of Outgoing International Calls (BOIC), offering a more granular, number-specific control mechanism.

Purpose & Motivation

The Barred Dialling Number feature was introduced to address the need for granular, subscriber-specific control over outgoing calls in mobile networks. Prior to its standardization, call barring was primarily a network-centric feature (like BAOC) controlled by the operator via the HLR or served MSC. These network-based barring services were broad-brush, applying to entire categories of calls (e.g., all international calls) and were not easily modifiable by the subscriber. There was a growing demand, especially from corporate customers and cost-conscious consumers, for the ability to block calls to specific, known expensive numbers (e.g., certain premium-rate services) or unauthorized numbers, without restricting all calls of a certain type.

BDN solves this by moving the control point to the USIM and the handset. This decentralization allows for a personalized list of barred numbers that travels with the subscriber's SIM card. The purpose is twofold: first, to empower users or their administrators (e.g., corporate IT) with direct control over call permissions, enhancing security and cost management; second, to reduce customer service issues and disputes related to unauthorized calls, as the barring is proactively enforced by the device itself. Historically, the rise of premium SMS and voice services in the early 2000s, which sometimes led to 'subscription traps' or fraudulent charges, motivated the need for such user-accessible barring tools.

The creation of BDN was motivated by the increasing programmability and intelligence of the SIM card, evolving into the USIM. The USIM application platform provided a secure and standardized environment to host such a feature. By storing the barred list on the USIM, it becomes independent of the handset; if a user changes phones, their barring list remains intact. This solves the limitation of handset-only barring features, which were not portable. Furthermore, it provides a standardized interface that all compliant handsets can use, ensuring a consistent user experience across different device manufacturers and network operators, which was a key goal of 3GPP interoperability efforts.