Service Specific Co-ordination Function

Description

The Service Specific Co-ordination Function (SSCF) is a sublayer within the control plane protocol stack of the UMTS terrestrial interfaces, primarily the Iu interface (between RNC and Core Network) and the Iur interface (between two RNCs). It is defined in 3GPP TS 25.412 (Iu), TS 25.422 (Iur), and related specifications. The SSCF sits between the Service Specific Connection Oriented Protocol (SSCOP) above it and the underlying network service layer below it, which could be the Message Transfer Part level 3 Broadband (MTP3-B) for ATM-based transport or the Signaling ATM Adaptation Layer (SAAL). Its core function is to adapt the service primitives and requirements of SSCOP to the specific characteristics and service primitives offered by the lower layer.

Architecturally, SSCF is part of the ATM adaptation layer for signaling (SAAL), which is split into the Common Part (SSCS) and the Service Specific Part. The SSCF is the Service Specific Part. It does not perform segmentation/reassembly or error correction itself—those are handled by SSCOP. Instead, SSCF handles coordination tasks like local status management, traffic management, and mapping of SSCOP's connection-oriented, assured data transfer service onto the potentially different service model of MTP3-B. For example, SSCOP provides a reliable, sequenced data pipe, while MTP3-B provides a connectionless, message-oriented service with its own routing and management. The SSCF translates between these two worlds, ensuring that SSCOP's establishment, release, and maintenance procedures are properly triggered based on the status of the MTP3-B signaling link.

How it works: The SSCF receives service primitives from SSCOP (e.g., request to establish an SSCOP connection, data transfer request). It then issues corresponding primitives to the lower layer to achieve that goal, such as initiating an MTP3-B signaling link setup or packaging data into MTP3-B messages. Conversely, it receives indications from the lower layer (e.g., link failure, congestion) and translates them into appropriate indications to SSCOP, which can then take recovery actions. This coordination is vital for maintaining the integrity of the signaling connection, especially during network failures or congestion events. The SSCF for the Network-to-Network Interface (SSCF-NNI) is used on Iu and Iur interfaces, while a different version (SSCF-UNI) is defined for the User-Network Interface but is not typically used in 3GPP RAN interfaces.

Purpose & Motivation

The SSCF was created to resolve the mismatch between the service requirements of higher-layer signaling protocols and the services provided by the underlying transport technology in UMTS networks, which initially relied heavily on ATM. Protocols like RANAP (on Iu) and RNSAP (on Iur) require a reliable, connection-oriented data transfer service for their signaling messages. SSCOP was designed to provide exactly that service over an ATM network. However, ATM's native signaling layer (SAAL and MTP3-B) did not directly offer an interface that SSCOP could use.

The SSCF's purpose is to act as the essential glue or adaptation layer. It solves the problem of interworking between SSCOP's abstract service interface and the concrete primitives and procedures of MTP3-B within the SAAL. Without the SSCF, SSCOP could not function over an ATM-based signaling network. This was a critical design choice in early UMTS (R99) to leverage the QoS and reliability guarantees of ATM for the crucial control plane signaling in the RAN.

Its creation was motivated by the need for a robust, standardized signaling transport for the new, split RAN-CN architecture of UMTS. The Iu and Iur interfaces required a more sophisticated transport than the TDM-based SS7 used in GSM A-interface. The SSCF, as part of the SAAL stack, enabled this by providing the necessary coordination to build a reliable signaling bearer on top of ATM virtual connections, ensuring that control plane messages for mobility management, call control, and radio resource management were delivered accurately and in sequence, which is fundamental to network stability and performance.