Service Specific Coordination Function – Network Node Interface

Description

The Service Specific Coordination Function – Network Node Interface (SSCF-NNI) is a protocol component within the Signalling System No. 7 (SS7) and ATM (Asynchronous Transfer Mode) signalling stacks, specifically defined for use at the Network Node Interface (NNI). It is part of the Service Specific Coordination Function (SSCF) family, which provides adaptation between the generic signalling transport layers and the specific upper-layer signalling protocols used in core networks.

SSCF-NNI operates at the layer above the SAAL (Signalling ATM Adaptation Layer) and below the higher-layer signalling protocols such as MTP3 (Message Transfer Part Level 3) or BICC (Bearer Independent Call Control). Its primary role is to coordinate the establishment, maintenance, and release of signalling connections across the NNI, ensuring that the service-specific requirements of the upper layers are met. This includes managing signalling link states, handling error conditions, and providing flow control and congestion management.

In the context of 3GPP, SSCF-NNI is referenced in TS 21.905, the vocabulary specification, indicating its relevance to the core network signalling architecture. It was originally developed for ATM-based transport networks, where it provided a standardized way to carry SS7 signalling over ATM. The NNI designation distinguishes it from the User-Network Interface (UNI) variant, SSCF-UNI, which is used between a user and the network.

SSCF-NNI functions include the mapping of upper-layer signalling messages onto the underlying ATM connections, the coordination of link alignment and activation, and the handling of failures and recovery procedures. It ensures that the signalling transport is reliable and that the upper layers can operate transparently over the ATM infrastructure. The protocol is defined in ITU-T recommendations (such as Q.2140) and adopted by 3GPP for its core network interfaces.

Although ATM-based transport has largely been replaced by IP-based transport in modern 3GPP networks, SSCF-NNI remains a historically important protocol and is still listed in 3GPP vocabulary specifications. Its concepts of service-specific coordination and reliable signalling transport have influenced the design of later protocols such as SCTP and M3UA, which now carry SS7 signalling over IP.

In summary, SSCF-NNI is a legacy but foundational protocol that illustrates how 3GPP adapted SS7 signalling to ATM transport, providing a template for subsequent signalling transport evolution.

Purpose & Motivation

SSCF-NNI was created to enable the reliable transport of SS7 signalling messages over ATM networks, which were widely deployed in core networks during the 1990s and early 2000s. At the time, ATM offered high-speed, connection-oriented transport with quality-of-service guarantees, making it attractive for carrying critical signalling traffic. However, SS7 and ATM had different paradigms, so an adaptation layer was needed.

The SSCF-NNI provides the coordination between the ATM signalling transport and the upper-layer SS7 protocols, ensuring that signalling links are properly established, monitored, and released. It solves the problem of mapping connection-oriented ATM virtual circuits to the connectionless or message-oriented SS7 signalling, while also handling error and congestion conditions.

As networks migrated to IP transport, the need for SSCF-NNI diminished, but its design principles influenced the development of IP-based signalling transport protocols. It remains a reference point in 3GPP vocabulary for understanding the evolution of core network signalling.