Access Unit

Description

The Access Unit (AU) is a network functional entity specified within the context of 3GPP's early work on interworking with fixed networks and services. Its definition is rooted in the CCITT (now ITU-T) X.31 recommendation, which details the procedures for supporting packet-mode terminal equipment via an ISDN. Architecturally, the AU is positioned between the non-ISDN terminal equipment (TE) and the ISDN network. Its primary role is to provide the necessary adaptation functions to allow a TE, which uses protocols like X.25, to access packet-switched services over the ISDN's digital bearer channels (B-channels) or signaling channels (D-channels).

Operationally, the AU works by implementing protocol conversion and mapping. It terminates the protocol stack used by the terminal (e.g., the X.25 protocol over a physical interface like X.21 or V-series) and maps the user data and control signaling onto the appropriate ISDN layers. This involves adapting the link-layer procedures (LAPB for X.25) to the ISDN's LAPD protocol used on the D-channel for signaling and possibly for packet data. For actual data transfer, the AU manages the establishment and teardown of B-channel connections as requested by the terminal's X.25 call setup packets, effectively acting as a gateway between the X.25 and ISDN domains.

Key components of the AU functionality include the physical interface adaptation for the TE, the X.25 protocol termination point, the ISDN protocol stack (Q.931 for call control, Q.921/LAPD for data link), and the interworking logic that maps between the two service domains. In the 3GPP network context, its role was primarily defined for scenarios where mobile network services needed to interwork with or provide access to legacy public packet-switched data networks (PSPDNs) that were prevalent at the time. It formed part of the broader interworking function (IWF) between a PLMN and external networks.

The AU's significance in early 3GPP specifications lies in its role as a bridge between the evolving mobile digital standards and the existing, widely deployed world of X.25-based data communications. It ensured backward compatibility and service continuity, allowing mobile users and services to connect to corporate networks, online databases, and other value-added services that operated over public packet networks. Its specifications in documents like 23.043 and 23.044 detail the scenarios and procedures for this interworking.

Purpose & Motivation

The Access Unit was created to solve the fundamental problem of network interworking between new digital ISDN-based networks (including early digital cellular networks like GSM) and the vast installed base of non-ISDN data terminal equipment, particularly those using the X.25 protocol suite. Prior to ISDN and mobile digital networks, data communication for terminals often relied on dedicated leased lines or dial-up modems using analog telephone networks (PSTN) with X.25 running over them. The limitations of these approaches included low speeds, analog line noise, and inefficient use of infrastructure.

The advent of ISDN offered digital end-to-end connectivity with higher reliability and speed via Bearer (B) channels. However, the existing X.25 terminals could not natively use the ISDN protocols. The AU was the technological solution to this incompatibility, motivated by the need to protect investments in existing terminal equipment and network services while migrating to modern digital networks. It allowed network operators to offer packet data services to their subscribers without forcing a wholesale replacement of customer premises equipment.

In the historical context of 3GPP (starting from Release 4), the inclusion of the AU concept reflects the standards body's effort to define comprehensive interworking capabilities for the GSM/GPRS and UMTS core networks. It addressed the requirement for mobile stations or network interworking functions to connect to external PSPDNs, ensuring that mobile data services could integrate seamlessly with the global data communications infrastructure of the late 20th century, which was heavily reliant on X.25 for applications like credit card verification, airline reservations, and corporate network access.

Key Features

• Protocol conversion between X.25 and ISDN Q.931/Q.921
• Support for X.31 Case A and Case B operation modes
• Termination of LAPB link-layer from the terminal
• Mapping of X.25 virtual calls onto ISDN circuit-switched B-channels or packet-mode D-channels
• Physical interface adaptation for non-ISDN terminals (e.g., X.21, V.24/V.28)
• Part of the Interworking Function (IWF) between PLMN and PSPDN

Evolution Across Releases

Defining Specifications