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AS (Access Stratum)

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Introduction

The Access Stratum (AS) is a layer of the protocol stack that resides in the User Equipment (UE) and is responsible for controlling the radio interface between the UE and the network. The AS provides functions such as radio resource control, radio bearer control, and security. The AS is divided into two sub-layers: the Radio Resource Control (RRC) sub-layer and the Medium Access Control (MAC) sub-layer. In this article, we will discuss the Access Stratum and its sub-layers in detail.

Radio Resource Control (RRC) sub-layer

The RRC sub-layer is responsible for controlling the establishment, maintenance, and release of the radio bearers (RBs) between the UE and the network. The RRC sub-layer performs functions such as:

  1. System information acquisition: The UE acquires system information from the network to obtain information about the network and its capabilities.
  2. Establishment and release of radio bearers: The RRC sub-layer establishes and releases radio bearers between the UE and the network based on the user's data requirements and the network's resource availability.
  3. Radio bearer modification: The RRC sub-layer can modify the attributes of an existing radio bearer, such as Quality of Service (QoS) parameters.
  4. Handover: The RRC sub-layer initiates the handover process when the UE moves from one cell to another.
  5. Paging: The RRC sub-layer receives paging messages from the network and notifies the upper layers of the UE about incoming calls or messages.
  6. Connection setup and release: The RRC sub-layer initiates and terminates the connection between the UE and the network.

Medium Access Control (MAC) sub-layer

The MAC sub-layer is responsible for managing the access to the shared radio resources and ensuring efficient utilization of the available resources. The MAC sub-layer performs functions such as:

  1. Radio resource allocation: The MAC sub-layer allocates the radio resources to the UE based on the user's data requirements and the network's resource availability.
  2. Multiplexing and demultiplexing: The MAC sub-layer multiplexes multiple data streams into a single transmission burst and demultiplexes the received transmission burst into multiple data streams.
  3. Scheduling: The MAC sub-layer schedules the transmission of data based on the priority and QoS requirements of the data.
  4. Retransmission control: The MAC sub-layer performs retransmissions of lost or corrupted packets to ensure reliable data transmission.
  5. Hybrid Automatic Repeat Request (HARQ): The MAC sub-layer implements HARQ, which is a combination of ARQ (Automatic Repeat reQuest) and Forward Error Correction (FEC), to improve the reliability of data transmission.
  6. Power control: The MAC sub-layer performs power control to optimize the transmission power based on the channel conditions and the QoS requirements of the data.
  7. Random access: The MAC sub-layer handles the random access procedure when the UE initiates a new connection with the network.

Security

The Access Stratum provides security functions to protect the UE and the network from unauthorized access, data interception, and data modification. The security functions are implemented in the RRC sub-layer and the MAC sub-layer and include the following:

  1. Authentication: The RRC sub-layer performs authentication to verify the identity of the UE before allowing access to the network.
  2. Key agreement: The RRC sub-layer and the MAC sub-layer perform key agreement to generate and exchange keys for use in encryption and decryption of the data.
  3. Encryption and decryption: The RRC sub-layer and the MAC sub-layer encrypt and decrypt the data to ensure the confidentiality of the data during transmission.
  4. Integrity protection: The RRC sub-layer and the MAC sub-layer provide integrity protection to ensure that the data has not been modified during transmission.
  5. Radio bearer security: The RRC sub-layer and the MAC sub-layer provide security for the radio bearers by applying encryption, integrity protection, and key agreement to the RBs.

Conclusion

In conclusion, the Access Stratum is a critical layer in the protocol stack that resides in the UE and is responsible for controlling the radio interface between the UE and the network. The Access Stratum is divided into two sub-layers: the Radio Resource Control (RRC) sub-layer and the Medium Access Control (MAC) sub-layer. The RRC sub-layer is responsible for controlling the establishment, maintenance, and release of the radio bearers, while the MAC sub-layer is responsible for managing the access to the shared radio resources and ensuring efficient utilization of the available resources. Additionally, the Access Stratum provides security functions to protect the UE and the network from unauthorized access, data interception, and data modification. Understanding the Access Stratum is crucial for the design, optimization, and troubleshooting of wireless networks.