Access Stratum

Description

The Access Stratum (AS) is a fundamental architectural concept in 3GPP systems, representing the collection of protocols and functions that are specific to the radio access technology. It encompasses the lower layers of the protocol stack that manage the physical transmission of data over the air interface, as well as the control plane protocols that govern radio resource management and mobility. The AS is contrasted with the Non-Access Stratum (NAS), which handles communication between the UE and the core network and is independent of the access technology.

In the UE, the AS includes the physical layer (L1), the Medium Access Control (MAC) layer, the Radio Link Control (RLC) layer, the Packet Data Convergence Protocol (PDCP) layer, and the Radio Resource Control (RRC) layer. These layers work together to provide reliable data transfer, radio resource allocation, and mobility management. The physical layer handles modulation, coding, and transmission over the radio interface; MAC manages scheduling and multiplexing; RLC provides segmentation, retransmission, and flow control; PDCP performs header compression, ciphering, and integrity protection; and RRC handles connection establishment, radio bearer configuration, and handover procedures.

On the network side, the AS is implemented in the base station (eNodeB in LTE, gNodeB in 5G NR) and includes the corresponding protocol layers. The base station communicates with the UE using AS protocols to manage the radio link, allocate resources, and ensure reliable delivery of user data. The AS also interacts with the core network through the S1/NG interfaces, but the core network itself is not part of the AS; it is part of the NAS.

The AS is responsible for several critical functions: radio bearer establishment and maintenance, mobility management (including handovers and cell reselection), security functions (ciphering and integrity protection of user and control data), and QoS enforcement at the radio level. It also handles measurement reporting, which is essential for mobility decisions and network optimization.

In 5G NR, the AS has evolved to support new features such as beamforming, massive MIMO, and flexible numerology. The AS architecture in 5G also includes a split between the Central Unit (CU) and Distributed Unit (DU), which allows for flexible deployment options. The AS remains a key concept for understanding how UEs connect to and communicate with the radio access network, and it is defined in 3GPP TS 21.905 as a functional grouping specific to the access technique.

Purpose & Motivation

The Access Stratum exists to separate the radio-specific aspects of communication from the core network functions, enabling modularity and independence between the access technology and the core network. This separation allows different radio access technologies (e.g., LTE, 5G NR, Wi-Fi) to connect to the same core network, facilitating interworking and evolution. It also simplifies the design of the core network, as it does not need to be aware of the specifics of the radio interface.

Historically, before the clear separation of AS and NAS, early mobile networks had more tightly integrated radio and core functions, which made it difficult to introduce new radio technologies or evolve the core network independently. The AS/NAS split was formalized in 3GPP to provide a clean interface between the radio access network and the core network, allowing each to evolve separately. This has been crucial for the success of 3GPP technologies, as it enabled the smooth migration from 2G to 3G to 4G and now to 5G, with each generation introducing new AS capabilities while reusing or evolving the NAS.

The AS also provides a standardized framework for radio protocol design, ensuring interoperability between UEs and base stations from different vendors. It defines the procedures for connection establishment, radio bearer configuration, mobility, and security, which are essential for reliable and secure communication. Without the AS concept, the complexity of managing radio resources and mobility would be unmanageable, and the ability to introduce new services and features would be severely limited.