NAS (Non access stratum)

The Non-Access Stratum (NAS) is an essential part of the overall architecture of mobile communication systems, such as GSM (Global System for Mobile Communications), UMTS (Universal Mobile Telecommunications System), and LTE (Long-Term Evolution). It plays a crucial role in providing various services and functionalities to mobile devices, ensuring reliable and secure communication between the user equipment (UE) and the core network.

NAS is responsible for handling signaling and control functions that are necessary for the establishment, maintenance, and release of connections between the UE and the network. It operates on top of the Access Stratum (AS), which is responsible for managing the physical layer and radio link control functions. The AS and NAS together make up the protocol stack that allows communication between the UE and the network.

The NAS layer is primarily concerned with the management of mobility, session management, and security functions. Let's explore each of these aspects in more detail:

Mobility Management: Mobility management involves handling the mobility of UEs as they move within the coverage area of different network cells. It ensures that the UE can access the network regardless of its location and facilitates seamless handover between cells as the UE moves. NAS handles various mobility-related procedures, including registration, location updating, paging, and authentication.

Registration: When a UE first enters a network or changes its location area, it needs to register with the network. NAS initiates the registration process, where the UE provides its identity and other relevant information to the network.

Location Updating: During location updating, the UE informs the network about its new location area. This procedure ensures that the network knows the current location of the UE for delivering incoming calls or data.

Paging: Paging is used to alert the UE when there are incoming calls or messages. The network initiates paging procedures through NAS, notifying the UE to establish a connection.

Authentication and Security: NAS manages the authentication process to ensure that only authorized UEs can access the network. It also handles security-related functions such as key agreement, ciphering, and integrity protection to protect user data and signaling messages.

Session Management: Session management involves the establishment, maintenance, and release of communication sessions between the UE and the network. It provides the necessary mechanisms to initiate voice calls, establish data connections, and manage the associated resources.

Call Setup: NAS handles the procedures for initiating voice calls, including call establishment, call routing, and call termination.

Data Connection Establishment: NAS facilitates the establishment of data connections for services such as internet access or multimedia streaming. It sets up the necessary protocols and parameters for data transmission.

Quality of Service (QoS): NAS manages QoS parameters to ensure that the required quality of service is maintained for different types of applications and services. It prioritizes resources and manages their allocation based on the requested QoS.

Security Management: Security is a critical aspect of mobile communication systems to protect user data and prevent unauthorized access. NAS incorporates various security mechanisms to ensure the integrity, confidentiality, and authenticity of signaling messages and user data.

Security Associations: NAS establishes security associations between the UE and the network entities to enable secure communication. It manages the key agreement process and ensures that both ends have the necessary keys and security parameters.

Ciphering: NAS handles the encryption and decryption of user data and signaling messages to prevent eavesdropping and unauthorized access. It applies cryptographic algorithms to protect the confidentiality of the transmitted information.

Integrity Protection: NAS ensures the integrity of signaling messages and user data by applying integrity protection mechanisms. It uses cryptographic algorithms to detect any tampering or modification of the transmitted information.

In addition to the above functionalities, NAS also manages supplementary services such as call forwarding, call waiting, and short message service (SMS). It handles the necessary signaling procedures for activating and managing these supplementary services, allowing users to customize their communication experience and take advantage of additional features provided by the network.

Furthermore, NAS plays a crucial role in supporting emergency services. It includes procedures and protocols to prioritize emergency calls and ensure their swift delivery to the appropriate emergency service providers. NAS also assists in location tracking during emergency calls to help responders locate the caller accurately.

The NAS layer operates through a set of protocols and messages that enable communication between the UE and the network. These protocols include the Mobility Management Entity (MME) in LTE, the Serving GPRS Support Node (SGSN) in UMTS, and the Mobile Switching Center (MSC) in GSM. These network entities interact with the UE through NAS signaling messages, which carry the necessary information for establishing and managing communication sessions.

NAS signaling messages are exchanged between the UE and the network using various interfaces, such as the radio interface between the UE and the base station (e.g., eNodeB in LTE), and the interface between the base station and the core network (e.g., S1-MME interface in LTE). These interfaces allow the exchange of NAS messages to control and manage the UE's mobility, session establishment, security, and other related functions.

It is worth noting that NAS operates independently of the underlying radio access technology. This means that NAS protocols and procedures remain consistent across different generations of mobile communication systems. Therefore, the concepts and functionalities described here are applicable to GSM, UMTS, LTE, and potentially future systems as well.

In conclusion, the Non-Access Stratum (NAS) is a crucial component of mobile communication systems that handles signaling and control functions necessary for establishing, maintaining, and releasing connections between user equipment (UE) and the core network. It manages mobility-related procedures, session establishment, and security functions to ensure reliable and secure communication. Through NAS, mobile users can access various services, including voice calls, data connections, supplementary services, and emergency services. The NAS layer relies on protocols and signaling messages exchanged between the UE and the network, operating independently of the specific radio access technology in use. Its functionalities play a vital role in enabling seamless and secure communication for mobile users in diverse network environments.