Signalling ATM Adaptation Layer – Network Node Interface

Description

The Signalling ATM Adaptation Layer for the Network Node Interface (SAAL-NNI) is a critical protocol stack component defined within the 3GPP UMTS specifications for the transport of signalling messages over Asynchronous Transfer Mode (ATM) networks. It operates specifically at the interface between core network nodes, such as between an MSC and an RNC or between two MSCs. SAAL-NNI is not a single protocol but a structured adaptation layer that sits between the higher-layer signalling protocols (like BSSAP or RANAP) and the underlying ATM layer. Its primary function is to provide a reliable, connection-oriented data transfer service that masks the complexities and potential errors of the raw ATM cell-based transport from the upper layers.

The architecture of SAAL-NNI is based on the ITU-T Q.2100 series recommendations and typically comprises several sub-layers. The core sub-layer is the Service Specific Connection Oriented Protocol (SSCOP), defined in Q.2110, which provides error correction, sequence integrity, flow control, and connection management. SSCOP ensures that signalling messages are delivered reliably and in order over the inherently connection-oriented but potentially lossy ATM Virtual Channel Connections (VCCs). Below SSCOP, the Service Specific Coordination Function (SSCF) maps the services of SSCOP to the requirements of the specific interface (NNI in this case), as defined in Q.2140. This SSCF-NNI adapts the SSCOP primitives to the needs of the MTP-3b (Message Transfer Part level 3 broadband) signalling network layer or directly to the upper layer signalling application.

In operation, when a signalling association needs to be established between two network nodes, the SAAL-NNI layer manages the setup, maintenance, and release of the underlying ATM signalling link. It segments large signalling messages from upper layers into sequences of Protocol Data Units (PDUs) for SSCOP, which are then further segmented into cells by the ATM layer. At the receiver, SAAL-NNI performs reassembly, error detection via sequence numbers, and retransmission of any lost PDUs using a selective-repeat ARQ mechanism. This guarantees the integrity of critical signalling messages for call control, mobility management, and session management. Its role was fundamental in 3G UMTS Release 99 and Release 4 circuit-switched core networks, providing the robust signalling transport foundation before the full migration to IP-based transport (IPoATM and later pure IP) in later releases.

Purpose & Motivation

SAAL-NNI was created to address the need for a standardized, reliable signalling transport mechanism for the emerging 3G UMTS networks, which initially adopted ATM as the preferred transport technology for both user plane and control plane. Prior to 3G, GSM networks used Time Division Multiplexing (TDM) and the SS7 signalling suite with MTP layers over 64 kbit/s timeslots. The shift to broadband ATM for 3G required a new adaptation method to carry traditional telephony signalling protocols over cell-switched, connection-oriented virtual circuits. The purpose of SAAL-NNI was to provide this adaptation, ensuring that the essential properties of signalling—reliability, in-sequence delivery, and flow control—were maintained over the potentially lossy ATM network.

The motivation stemmed from the limitations of using raw ATM Adaptation Layer 5 (AAL5) alone, which provides a simple, non-assured frame relay service suitable for data but lacks the robust error recovery and connection management needed for mission-critical signalling. SAAL-NNI, with its SSCOP sub-layer, filled this gap. It solved the problem of how to integrate the existing and new 3GPP signalling application protocols with the high-speed, packet-oriented ATM backbone. Its creation was driven by the industry's consensus around ATM as the unifying transport for broadband ISDN and early 3G networks, necessitating a standardized NNI signalling adaptation layer to ensure interoperability between equipment from different vendors in the core network.

Key Features

• Provides reliable, connection-oriented data transfer service for signalling
• Based on the ITU-T Q.2100 series protocol stack for broadband signalling
• Utilizes the Service Specific Connection Oriented Protocol (SSCOP) for error correction and flow control
• Includes Service Specific Coordination Function for NNI (SSCF-NNI) for protocol adaptation
• Ensures in-sequence delivery and integrity of signalling messages over ATM VCCs
• Manages establishment, maintenance, and release of signalling links between network nodes

Evolution Across Releases

Defining Specifications