Service Specific Convergence Sublayer

Description

The Service Specific Convergence Sublayer (SSCS) is a component of the ATM Adaptation Layer (AAL), as defined in the ITU-T I.363 series and adopted by 3GPP for UMTS transport. The AAL is divided into two sublayers: the Convergence Sublayer (CS) and the Segmentation and Reassembly (SAR) sublayer. The CS is further split into the Common Part Convergence Sublayer (CPCS) and the Service Specific Convergence Sublayer (SSCS). The SSCS is, as the name implies, service-specific—it provides functions tailored to the requirements of a particular higher-layer service or protocol, such as signaling or data transfer.

In the context of 3GPP, the SSCS is primarily used in the transport network layer for control plane signaling across the Iu and Iur interfaces when ATM is employed. For signaling transport, the SSCS typically contains the Service Specific Connection Oriented Protocol (SSCOP) and possibly a Service Specific Coordination Function (SSCF). The SSCF maps the services of SSCOP to the requirements of the upper-layer service, such as the Signaling Connection Control Part (SCCP). The SSCS operates above the AAL5 Common Part Convergence Sublayer (CPCS), which handles functions like padding and error detection via a CRC trailer. The SSCS adds protocol-specific capabilities like connection management, error recovery, and assured data transfer that the CPCS does not provide.

The role of the SSCS is to adapt the needs of the higher-layer signaling protocol to the generic services offered by AAL5 and the underlying ATM layer. It ensures that signaling messages receive the appropriate quality of service, reliability, and sequencing. By encapsulating SSCOP, the SSCS delivers a reliable, in-order, flow-controlled data pipe for critical control plane communications between network elements like the RNC and MSC or SGSN. This layered approach allows for flexibility; different SSCS implementations can be defined for different services, though in 3GPP UMTS, the SSCOP-based SSCS for signaling is the most prominent.

Purpose & Motivation

The SSCS exists to bridge the gap between the generic transport services provided by ATM and the specific requirements of upper-layer applications, particularly signaling protocols. ATM's AAL5 offers a basic, connection-oriented service but is primarily designed for data segmentation and reassembly with minimal error checking. Signaling protocols, however, demand reliable, error-free, and in-sequence delivery to function correctly. The SSCS provides this service-specific adaptation without requiring every higher-layer protocol to implement its own complex adaptation logic.

In 3GPP Release 99, ATM was the mandated transport for the Iu and Iur interfaces. The SSCS, housing SSCOP, was essential to meet the stringent reliability requirements for control plane signaling in the nascent UMTS network. It solved the problem of how to carry signaling over an ATM network that was optimized for bulk data but not for the small, critical messages of control protocols. By standardizing the SSCS with SSCOP, 3GPP ensured interoperability between RAN and CN equipment from different vendors, providing a consistent and reliable signaling transport foundation that supported mobility management, call establishment, and other vital network functions.