NAS Network access stratum

Network Access Stratum (NAS) is the protocol layer in the mobile communication system that is responsible for the communication between the mobile device and the core network. The NAS protocol provides the control plane for user authentication, mobility management, and session management. In this article, we will discuss NAS in detail, including its architecture, functions, and protocols.

NAS Architecture

The NAS architecture is divided into two layers: the upper layer, which is responsible for signaling and mobility management, and the lower layer, which is responsible for radio resource control. The upper layer is responsible for the communication between the mobile device and the core network, while the lower layer is responsible for the communication between the mobile device and the radio access network (RAN).

The upper layer of the NAS protocol is divided into two sub-layers: the Non-Access Stratum (NAS) signaling layer and the Mobility Management Entity (MME) signaling layer. The NAS signaling layer is responsible for the communication between the mobile device and the core network for signaling purposes, such as user authentication and session management. The MME signaling layer is responsible for mobility management, such as location update, tracking area update, and handover management.

The lower layer of the NAS protocol is divided into two sub-layers: the Radio Resource Control (RRC) signaling layer and the Packet Data Convergence Protocol (PDCP) layer. The RRC signaling layer is responsible for the communication between the mobile device and the RAN for radio resource control, such as channel allocation and power control. The PDCP layer is responsible for the compression and decompression of user data for transmission over the air interface.

NAS Functions

The NAS protocol provides various functions for the mobile communication system, such as user authentication, mobility management, and session management. In this section, we will discuss these functions in detail.

User Authentication

User authentication is the process of verifying the identity of the mobile device user. The NAS protocol provides mutual authentication between the mobile device and the core network. The authentication process is initiated by the mobile device when it sends an attach request message to the core network. The attach request message contains the International Mobile Subscriber Identity (IMSI) of the mobile device. The core network verifies the identity of the mobile device by requesting authentication information from the Home Subscriber Server (HSS). The HSS sends a response message to the core network, which contains the authentication information. The core network sends a challenge message to the mobile device, which generates a response message using the authentication information and sends it back to the core network. The core network verifies the response message and sends an attach accept message to the mobile device, indicating that the authentication process is successful.

Mobility Management

Mobility management is the process of tracking the location of the mobile device and managing its mobility in the network. The NAS protocol provides various mobility management functions, such as location update, tracking area update, and handover management.

Location Update

Location update is the process of notifying the core network of the current location of the mobile device. The mobile device sends a location update message to the core network when it moves to a new location area. The location update message contains the location information of the mobile device, such as the Cell ID and Tracking Area Code (TAC). The core network updates the location information of the mobile device in the Location Management (LM) database.

Tracking Area Update

Tracking area update is the process of notifying the core network of the current tracking area of the mobile device. The mobile device sends a tracking area update message to the core network when it moves to a new tracking area. The tracking area update message contains the new tracking area information of the mobile device. The core network updates the tracking area information of the mobile device in the LM database.

Handover Management

Handover management is the process of transferring the ongoing communication of the mobile device from one cell to another cell without interrupting the communication. The NAS protocol provides various handover management functions, such as handover preparation, handover decision, and handover execution.

Handover Preparation

Handover preparation is the process of preparing for a handover. The RAN initiates the handover preparation process when it detects that the mobile device is moving out of its coverage area. The RAN sends a handover request message to the core network, which contains the handover parameters, such as the target cell and the handover type. The core network selects the appropriate target cell and sends a handover command message to the mobile device, indicating that it should start the handover process.

Handover Decision

Handover decision is the process of deciding whether to perform a handover or not. The core network makes the handover decision based on various criteria, such as the quality of the radio signal, the available radio resources, and the load on the cells. The core network sends a handover command message to the mobile device, indicating whether the handover should be performed or not.

Handover Execution

Handover execution is the process of performing the handover. The mobile device starts the handover process by synchronizing with the target cell and transmitting a handover request message to the target cell. The target cell confirms the handover by sending a handover complete message to the mobile device. The mobile device completes the handover by releasing the radio resources allocated by the source cell and allocating new radio resources from the target cell.

Session Management

Session management is the process of establishing, maintaining, and releasing a communication session between the mobile device and the core network. The NAS protocol provides various session management functions, such as session establishment, session modification, and session release.

Session Establishment

Session establishment is the process of establishing a communication session between the mobile device and the core network. The mobile device initiates the session establishment process by sending a service request message to the core network, which contains the type of service requested. The core network selects the appropriate service and sends a service accept message to the mobile device, indicating that the session establishment process is successful.

Session Modification

Session modification is the process of modifying the parameters of an ongoing communication session. The mobile device or the core network initiates the session modification process by sending a session modification request message to the other party, which contains the modified parameters. The other party responds with a session modification response message, indicating whether the session modification is successful or not.

Session Release

Session release is the process of releasing an ongoing communication session. The mobile device or the core network initiates the session release process by sending a service request message to the other party, which contains the reason for releasing the session. The other party responds with a service reject message or a service release message, indicating whether the session release is successful or not.

NAS Protocols

The NAS protocol uses various protocols for signaling and data transmission over the air interface. In this section, we will discuss some of the important NAS protocols.

Authentication and Key Agreement (AKA)

Authentication and Key Agreement (AKA) is a protocol used for user authentication and key agreement between the mobile device and the core network. AKA is a challenge-response protocol that uses symmetric encryption and mutual authentication. The AKA protocol is based on the 3rd Generation Partnership Project (3GPP) specifications and is used in both 3G and 4G networks.

Mobile Application Part (MAP)

Mobile Application Part (MAP) is a protocol used for signaling between the core network elements, such as the Mobile Switching Center (MSC) and the HSS. MAP is a message-based protocol that uses a set of predefined message types for various signaling purposes, such as location updating, call setup, and SMS delivery. MAP is based on the Signaling System 7 (SS7) protocol suite and is used in both 2G and 3G networks.

Radio Resource Control (RRC)

Radio Resource Control (RRC) is a protocol used for controlling the radio resources, such as frequency, power, and time, between the mobile device and the RAN. RRC is responsible for setting up, maintaining, and releasing the radio bearers, which are the logical channels used for data transmission over the air interface. RRC is based on the 3GPP specifications and is used in both 3G and 4G networks.

Non-Access Stratum (NAS)

Non-Access Stratum (NAS) is a protocol used for signaling and data transmission between the mobile device and the core network over the radio access network (RAN). NAS is responsible for various functions, such as user authentication, session management, and mobility management. NAS is based on the 3GPP specifications and is used in both 3G and 4G networks.

Conclusion

The Network Access Stratum (NAS) is a protocol used for signaling and data transmission between the mobile device and the core network over the radio access network (RAN). NAS is responsible for various functions, such as user authentication, session management, and mobility management. The NAS protocol provides various handover management functions, such as handover preparation, handover decision, and handover execution. The NAS protocol uses various protocols for signaling and data transmission over the air interface, such as Authentication and Key Agreement (AKA), Mobile Application Part (MAP), and Radio Resource Control (RRC). The NAS protocol is an important component of the cellular network architecture and plays a crucial role in providing seamless communication services to mobile users.