Signalling ATM Adaptation Layer

Description

The Signalling ATM Adaptation Layer (SAAL) refers to the protocol stack used to adapt signalling plane messages for reliable transport over an Asynchronous Transfer Mode (ATM) network. In the 3GPP context, SAAL was primarily used in the UMTS terrestrial radio access network (UTRAN) for the Iu (RNC-CN), Iur (RNC-RNC), and Iub (RNC-Node B) interfaces in Releases 99 through to Release 7. SAAL is not a single protocol but a combination of ATM Adaptation Layer type 5 (AAL5) along with higher-layer service-specific components defined by ITU-T for signalling: the Service Specific Coordination Function (SSCF) and the Service Specific Connection Oriented Protocol (SSCOP).

SAAL operates by segmenting signalling messages from higher-layer protocols (like Radio Access Network Application Part - RANAP on Iu, or Node B Application Part - NBAP on Iub) into ATM cells. AAL5 performs the segmentation and reassembly (SAR) of variable-length protocol data units (PDUs) into the 48-byte payload of ATM cells, adding a trailer for error detection. However, AAL5 itself only provides unassured, connection-oriented service. For signalling, which requires guaranteed delivery and sequence integrity, the SSCF and SSCOP layers are added on top of AAL5. SSCOP provides reliable data transfer over the ATM connection. It includes mechanisms for sequence numbering, error correction by selective retransmission, flow control, and connection keep-alive. The SSCF layer acts as an adaptation layer between SSCOP and the specific signalling user (e.g., MTP3-B for broadband signalling or directly a 3GPP-specific adaptation).

In the UTRAN architecture, SAAL formed the data link layer for the control plane. For example, on the Iu-CS interface between the Radio Network Controller (RNC) and the Circuit-Switched Core Network (MSC), the RANAP messages were carried over a SAAL/ATM connection. The SAAL connection was established over a permanent virtual circuit (PVC) or switched virtual circuit (SVC) configured between the endpoints. This provided a reliable, in-sequence signalling pipe that was independent of the user plane traffic, which also used ATM but with different AAL types (like AAL2 for voice). The use of ATM and SAAL was a defining characteristic of early 3G networks, offering high bandwidth efficiency and quality of service (QoS) differentiation through ATM's traffic management capabilities.

Purpose & Motivation

SAAL was created to meet the stringent reliability requirements of telecommunications signalling over the emerging high-speed, cell-based ATM transport technology. Traditional TDM-based signalling systems like Signalling System No. 7 (SS7) used the Message Transfer Part (MTP) layers over dedicated timeslots. As networks moved towards broadband ISDN (B-ISDN) and ATM-based infrastructure in the 1990s, a new adaptation method was needed to carry signalling messages with equivalent reliability. SAAL, standardized by ITU-T, filled this gap by providing a connection-oriented, error-corrected service over the inherently connection-oriented but potentially lossy ATM layer.

For 3GPP UMTS, SAAL was adopted to leverage the benefits of ATM in the access network. ATM offered statistical multiplexing, efficient bandwidth usage for bursty data, and built-in QoS classes—advantages over pure TDM for the mixed traffic of early 3G. SAAL provided the trusted, reliable transport layer needed for the critical control signalling between network elements (RNC, Node B, MSC, SGSN). It solved the problem of how to run complex, transaction-oriented signalling protocols like RANAP and NBAP over a fast packet-switched network without compromising the robustness expected from telecom networks. It served as a transitional technology, enabling the feature-rich UTRAN architecture until the industry-wide shift towards all-IP transport in later 3GPP releases.