Bearer Independent Call Control

Description

Bearer Independent Call Control (BICC) is a signalling protocol architecture standardized by 3GPP that decouples call control from the underlying bearer technology. It was developed to allow traditional narrowband ISDN User Part (ISUP) services to be provided over a variety of transport networks, including ATM and IP. BICC is defined primarily in 3GPP Technical Specifications 23.153 (out-of-band transcoder control), 23.205 (bearer-independent circuit-switched core network), 23.231 (SIP-I and BICC interworking), and 24.259 (BICC interworking with SIP-I). The protocol stack consists of a call control layer (BICC CS) and a bearer control layer (BICC BC). The call control layer is based on ISUP messages but is extended to support bearer-independent operation, while the bearer control layer uses protocols such as IPBCP (IP Bearer Control Protocol) or AAL2 signalling to set up and manage the actual transport bearers. This separation allows the call control function to be located in a Media Gateway Controller (MGC) or Softswitch, while the bearer is terminated in a Media Gateway (MGW). The MGC and MGW communicate via the H.248/Megaco protocol, which is used to control the media gateway resources.

The BICC architecture introduces the concept of a Serving Node (SN) and a Gateway Node (GN). The SN handles the call control and service logic, while the GN provides the bearer interworking between different transport technologies. The BICC protocol supports the Nc interface between MGCs and the Mc interface between MGC and MGW. The Nc interface uses BICC signalling to establish, modify, and release calls, and to negotiate bearer characteristics. The Mc interface uses H.248 to control the media gateway, including bearer establishment, modification, and release. BICC also defines the Nb interface between media gateways for bearer transport. The protocol supports both ATM and IP transport for the bearer, and it can interwork with legacy ISUP networks through a gateway function.

One of the key features of BICC is its ability to support the full set of ISUP supplementary services, such as call forwarding, call waiting, and conference calling, even when the bearer is packet-based. This is achieved by carrying the ISUP parameters in BICC messages and mapping them to the appropriate bearer control actions. BICC also supports the separation of call and bearer control, which allows for more efficient use of network resources and enables the deployment of a distributed architecture. The protocol includes mechanisms for bearer establishment, such as the 'forward bearer establishment' and 'backward bearer establishment' procedures, which allow the bearer to be set up either before or after the called party answers. BICC also supports the negotiation of codecs and other bearer characteristics through the use of the BICC Bearer Control Protocol (BCP).

In a typical BICC call flow, the calling party's MGC receives an ISUP IAM message from the access network. The MGC then sends a BICC IAM message to the called party's MGC over the Nc interface. The called party's MGC may then instruct its MGW to set up a bearer using H.248. Once the bearer is established, the called party's MGC sends a BICC ANM message back to the calling party's MGC, and the call is connected. The BICC protocol also supports mid-call bearer modification, which allows the bearer characteristics to be changed during a call, for example to support codec changes or to add media streams. BICC is an important enabler for the migration of traditional circuit-switched networks to packet-switched networks, as it allows operators to reuse their existing ISUP-based services while taking advantage of the cost and flexibility benefits of packet transport.

BICC has been widely deployed in 3GPP networks, particularly in the core network for voice services. It is used in the IMS (IP Multimedia Subsystem) for interworking with circuit-switched networks, and it is also used in the CS domain of 3GPP networks for call control between MSCs and MGWs. The protocol continues to be relevant in the context of 5G, where it can be used for voice over NR (VoNR) interworking with legacy networks. BICC is specified in a number of 3GPP releases, and its latest enhancements are included in Release 19. The protocol is also used in other standards bodies, such as ITU-T, where it is known as BICC (Bearer Independent Call Control) and is specified in Q.1902 series recommendations.

Purpose & Motivation

BICC was created to address the limitations of traditional circuit-switched networks, which were based on TDM (Time Division Multiplexing) technology and were inflexible and expensive to scale. As operators began to deploy packet-switched networks based on ATM and IP, they needed a way to provide the same voice services over these new transport technologies without having to redesign their entire service infrastructure. BICC solves this problem by separating call control from bearer control, allowing the call control to remain based on ISUP while the bearer can be any packet technology. This separation also enables a distributed architecture where the call control and bearer functions can be located in different network elements, leading to more efficient use of resources and easier scalability.

Another motivation for BICC was the need to support the full set of ISUP supplementary services over packet networks. Previous approaches, such as simply carrying ISUP over IP (ISUP over IP), did not provide a clean separation of call and bearer control and were not well-suited for a distributed architecture. BICC provides a standardized way to carry ISUP parameters and to map them to bearer control actions, ensuring that services like call forwarding, call waiting, and conference calling work seamlessly over packet transport. BICC also supports the interworking of different transport technologies, such as ATM and IP, which was important during the transition period when operators had a mix of transport technologies in their networks.

Finally, BICC was designed to be extensible and to support future services. It includes mechanisms for negotiating bearer characteristics, such as codecs and bandwidth, and it supports mid-call bearer modification. This makes it suitable for a wide range of services, including voice, video, and multimedia. BICC has been adopted by 3GPP as the standard for bearer-independent call control in the core network, and it continues to be enhanced in subsequent releases to support new requirements, such as interworking with IMS and 5G networks.