Backward Call Indicators

Description

Backward Call Indicators (BCI) are a collection of parameters transmitted in the backward direction (from the called party's network toward the calling party's network) within ISDN User Part (ISUP) and Bearer Independent Call Control (BICC) signaling messages. These indicators are defined in 3GPP TS 29.163 as part of the Interworking between the IP Multimedia (IM) Core Network (CN) subsystem and Circuit Switched (CS) networks specification. The BCI parameters are carried in specific information elements that provide essential details about the call's termination side to the originating network.

During call setup procedures, when a call traverses from a CS network to an IP Multimedia Subsystem (IMS) network or vice versa, the BCI parameters are extracted from incoming ISUP messages and mapped to appropriate SIP headers or BICC parameters. This mapping occurs at the Media Gateway Control Function (MGCF) or other interworking functions. The BCI includes critical information such as the nature of the called party's line, whether the called party is a payphone, whether echo control devices are present, and other network-specific characteristics that affect call handling.

Key components of BCI include parameters like the Called Party's Status Indicator, which indicates whether the called party is free, busy, or has an unknown status; the Charge Indicator, which provides information about charging arrangements; and the Echo Control Device Indicator, which signals whether echo control is required. These parameters are encoded as bit fields within specific octets of the signaling messages, allowing for efficient transmission while maintaining backward compatibility with legacy systems.

The role of BCI in the network is crucial for maintaining service quality and proper call routing across heterogeneous network environments. When a call originates in a CS network and terminates in an IMS network, the BCI parameters ensure that the IMS network can apply appropriate policies and treatments based on the characteristics of the originating network. Similarly, when calls flow in the opposite direction, BCI helps the CS network understand the capabilities and requirements of the IMS network, enabling seamless service delivery across network boundaries.

Purpose & Motivation

BCI was created to address the challenges of interworking between circuit-switched and packet-switched networks during the transition to all-IP networks. As telecommunications evolved from traditional PSTN/ISDN networks to IP-based IMS architectures, there was a critical need to preserve call-related information that was essential for proper call handling, billing, and service quality. Without BCI, important parameters about the called party's line characteristics and network conditions would be lost when calls crossed network boundaries, leading to degraded service quality and incorrect billing.

Historically, ISUP signaling in CS networks contained numerous parameters that provided detailed information about call characteristics, but these parameters were not natively supported in IP-based signaling protocols like SIP. The creation of BCI provided a standardized mechanism to map these critical parameters between different signaling domains. This was particularly important during the early phases of IMS deployment when most calls still originated or terminated in legacy CS networks, requiring robust interworking solutions.

BCI solves the problem of information loss during network transitions by ensuring that essential call parameters are preserved and properly interpreted across different network generations. It addresses limitations of previous approaches that either dropped important signaling information or used proprietary mapping solutions that hindered interoperability between equipment from different vendors. By standardizing the backward call indicators in 3GPP specifications, network operators could deploy multi-vendor solutions while maintaining consistent call handling and service quality across hybrid network environments.