Description
The Access Signalling Gateway (A-SGW) is a critical functional entity within the 3GPP core network architecture, specifically for the Circuit-Switched (CS) domain. It was introduced as part of the network evolution towards an all-IP transport infrastructure. Architecturally, the A-SGW is positioned between the Radio Access Network (RAN), such as a Base Station Controller (BSC) or Radio Network Controller (RNC), and the core network's Mobile Switching Centre (MSC). Its primary function is to terminate and relay signalling protocols. On the RAN side, it interfaces using traditional circuit-switched signalling protocols, typically based on SS7 (Signalling System No. 7) with protocols like BSSAP (Base Station System Application Part) or RANAP (Radio Access Network Application Part). On the core network side, it converts and transports these messages over an IP-based network using signalling transport protocols like SIGTRAN (Signalling Transport), specifically the M3UA (MTP3 User Adaptation) or SUA (SCCP User Adaptation) layers. This allows the MSC to be physically separated from the RAN and connected via a standard IP network, decoupling the service logic from the underlying transport technology.
Operationally, the A-SGW works by acting as a signalling proxy. When a BSC or RNC needs to communicate with the MSC—for instance, to perform a location update, establish a voice call, or handle handover—it sends a signalling message. The A-SGW receives this message, encapsulates the application-layer signalling (BSSAP/RANAP) into a SIGTRAN adaptation layer packet, and routes it over an IP network to the appropriate MSC. The process is reversed for messages from the MSC to the RAN. The A-SGW is responsible for managing the signalling associations, ensuring reliable delivery, and providing a degree of signalling traffic management and security at the transport layer. It does not interpret the higher-layer call control or mobility management messages; its role is strictly focused on the reliable transport and protocol adaptation of these messages.
Key components of the A-SGW functionality include the signalling interface handlers for SS7 links (e.g., using MTP1, MTP2, MTP3), the SIGTRAN adaptation layer stack (M3UA/SUA over SCTP/IP), and connection management logic. Its role in the network is foundational for enabling a split architecture where the MSC can be centralized or even virtualized, while RAN elements can be distributed. This separation reduces costs, increases scalability, and simplifies network deployment and maintenance. The A-SGW is a cornerstone of the 'MSC Server' split architecture defined in 3GPP Release 4, where the call control (MSC Server) is separated from the media gateway (MGW). The A-SGW handles all the signalling path to the MSC Server, while the user plane traffic (voice) takes a separate path through Media Gateways (MGW).
Purpose & Motivation
The A-SGW was created to address the limitations of traditional monolithic MSC architectures in 2G and early 3G networks. In those systems, the MSC was a single, integrated node handling both call control signalling and circuit-switched voice switching, directly connected to the RAN via dedicated TDM (Time-Division Multiplexing) links for both signalling and user plane. This architecture was inflexible, costly to scale, and geographically constrained, as the MSC had to be physically near its served RAN nodes to maintain acceptable signalling delays and link costs.
The primary motivation for introducing the A-SGW in Release 4 was to enable the transition to an IP-based core network transport layer. By inserting an A-SGW, operators could begin using cost-effective, scalable, and ubiquitous IP networks (like IP backbones) to connect distributed RAN equipment to centralized MSCs. This solved the problem of expensive and rigid TDM trunking. The A-SGW specifically solves the signalling transport problem by providing protocol conversion between legacy SS7-based RAN signalling and IP-based SIGTRAN protocols, allowing the MSC Server to be deployed independently of the underlying transport technology. This separation was a key step towards network softwarization and the eventual virtualization of network functions (NFV). It allowed for more efficient resource utilization, simplified network expansion, and paved the way for later architectural advancements like IMS (IP Multimedia Subsystem) by establishing IP as the default transport for all core network domains.
Key Features
- Signalling protocol interworking between SS7-based RAN protocols (BSSAP/RANAP) and IP-based SIGTRAN protocols (M3UA/SUA)
- Enables IP transport for circuit-switched core network signalling, decoupling it from traditional TDM links
- Supports the 3GPP Release 4 split MSC architecture (MSC Server and MGW) by providing the signalling path to the MSC Server
- Acts as a signalling relay and proxy, managing SCTP associations for reliable signalling transport over IP networks
- Facilitates geographical flexibility by allowing centralized deployment of MSC Servers independent of RAN location
- Provides a foundation for network consolidation and cost reduction by leveraging shared IP infrastructure
Evolution Across Releases
Introduced the Access Signalling Gateway (A-SGW) as a new functional entity. It defined its role in the split architecture of the Circuit-Switched (CS) core network, enabling the separation of the MSC into an MSC Server (for control) and Media Gateway (for user plane). The A-SGW's initial capability was to provide signalling interworking between the BSS/RAN using SS7 protocols and the MSC Server using SIGTRAN over IP, establishing IP as a viable transport for legacy CS signalling.
Defining Specifications
| Specification | Title |
|---|---|
| TS 21.905 | 3GPP TS 21.905 |