Network Attachment Subsystem

Description

The Network Attachment Subsystem (NASS) is a collection of functions defined by the European Telecommunications Standards Institute (ETSI) and adopted within the 3GPP IP Multimedia Subsystem (IMS) architecture for fixed broadband access (e.g., xDSL, fiber). It is specified in the context of Next Generation Networks (NGN) and provides a standardized way for user equipment (UE), often called Customer Premises Equipment (CPE) in fixed networks, to attach to an IP network and obtain the necessary configuration for accessing services, particularly IMS-based services like Voice over LTE (VoLTE) or fixed-line VoIP. NASS operates in the transport layer, below the service layer where IMS resides, and provides critical attachment information upwards.

The core function of NASS is to manage the initial network access procedure. When a CPE/router powers on, it interacts with the NASS to perform authentication (e.g., using PPP, DHCP, or 802.1X), authorization, and IP address allocation. Key logical functions within NASS include the Network Access Configuration Function (NACF), which assigns IP configuration parameters to the UE; the Access Management Function (AMF), which handles the access session; and the Connectivity Session Location and Repository Function (CLF), which is a central database. The CLF stores the binding between the user's physical line identifier (e.g., DSLAM port), the assigned IP address, and the user's profile. This binding is crucial for providing location information for emergency services.

NASS interacts with the IMS core via the `e2` and `e4` reference points. The most important interaction is between the CLF within NASS and the IMS core's Breakout Gateway Control Function (BGCF) or Emergency Call Session Control Function (E-CSCF). When an IMS application, such as an emergency call, is initiated, the P-CSCF or E-CSCF can query the CLF (via the `e2` interface) to retrieve the subscriber's access network location information based on the source IP address of the SIP signaling. This allows the IMS network to determine the correct Public Safety Answering Point (PSAP) to route the emergency call to, even if the user is nomadic (using the fixed line from different physical sockets).

Furthermore, NASS enables access transfer and service continuity scenarios. For example, in a Voice Call Continuity (VCC) or Single Radio Voice Call Continuity (SRVCC) context involving a fixed-mobile handover, the NASS can provide information about the characteristics of the fixed access leg to the IMS application server managing the handover. In summary, NASS acts as the 'glue' between the physical/transport layer of a fixed broadband network and the service-aware IMS layer. It abstracts the details of the access network and provides a standardized interface for service layers to retrieve essential attachment context, which is fundamental for regulatory services like emergency calling and for advanced service continuity features.

Purpose & Motivation

The Network Attachment Subsystem was created to solve a specific problem in converging fixed and mobile service delivery, particularly for IMS-based services. In traditional telephony (PSTN), the physical line identifier (telephone number/line) directly and immutably indicated a geographic location, which was straightforward for routing emergency calls. With the advent of IP-based fixed broadband (like VoIP over DSL), this relationship broke down. A user could obtain an IP address via DHCP from a provider's network, but that IP address was not statically tied to a physical location, especially if the user moved their router (nomadism). This posed a major challenge for emergency services, which require accurate location information.

NASS was developed within the ETSI TISPAN NGN standards to reintroduce this location and attachment awareness into packet-switched networks. Its creation was motivated by the need for fixed network operators to offer carrier-grade VoIP and other IMS services with the same reliability and regulatory compliance (especially for emergency calls - E112) as the PSTN. Before NASS, IMS architectures primarily assumed mobile or managed access where location was derived from the cellular network (e.g., cell ID). For fixed access, a standardized subsystem was needed to perform the attachment, authentication, and—critically—to maintain the dynamic binding between IP address, user identity, and physical access point location.

By providing a standardized set of functions and interfaces (like `e2`), NASS allows the IMS service layer to remain largely agnostic of the underlying fixed access technology (xDSL, FTTH, Cable). It addresses the limitation of having service logic directly interact with access-specific protocols like DHCP or PPP. Instead, IMS functions query the NASS CLF via a uniform interface to get the necessary location and access characteristics. This separation of concerns enables fixed network operators to deploy IMS for multimedia services while meeting regulatory requirements for emergency caller location, and it facilitates the development of unified services that work seamlessly across fixed and mobile access networks.

Key Features

• Manages IP address allocation and configuration for fixed access UEs/CPE
• Maintains a dynamic binding database (in the CLF) linking IP address, line ID, and user profile
• Provides location information to IMS for emergency call routing (E-CSCF query)
• Supports user authentication and authorization at the network attachment level
• Enables access-aware services and access transfer scenarios in IMS
• Defines standardized reference points (e2, e4) between transport and IMS layers

Evolution Across Releases

Defining Specifications