LTE Protocol Stack: User Plane vs. Control Plane Explained

The LTE protocol stack is divided into two essential planes: the user plane and the control plane. These layers manage the transmission of data and signaling information across the LTE network, playing a critical role in ensuring smooth communication between devices and the network.

In this guide, we will explore the LTE protocol stack layers, including the functions of both the user plane and control plane, providing a clear understanding of their importance in LTE operations.

The LTE User plane layer consists of upper layers, NAS, PDCP, RLC, MAC, PHY, and RF. The LTE Control plane layer covers upper layers, NAS, RRC, PHY, and RF. The figure below depicts the LTE protocol stack with the main functions of each layer. NAS is not shown in the figure, it sits above RRC in the control plane (on the left side) and above PDCP in the user plane (on the right side). Above NAS, upper layers exist.

LTE User Plane

The user plane LTE protocol stack consists of upper layers, NAS, PDCP, RLC, MAC, PHY, and RF. The functions of each are outlined below:

• NAS (Non-Access Stratum): In the uplink, it performs packet filtering.
• PDCP (Packet Data Convergence Protocol):
  • Uplink: Sequence number addition, handover data handling, integrity protection, ciphering, and header compression.
  • Downlink: In-sequence delivery, duplicate packet detection, integrity validation, deciphering, and header decompression.
• RLC (Radio Link Control):
  • Uplink: Buffer status report, segmentation and concatenation, ARQ (for AM mode).
  • Downlink: Re-ordering, assembly, and ARQ (for AM mode).
• MAC (Medium Access Control):
  • Uplink: Channel mapping, multiplexing, handling control elements, random access procedure, logical channel priority, HARQ, and sending BSRs.
  • Downlink: Channel mapping, de-multiplexing, DRX, Handling control elements, HARQ.
• PHY (Physical Layer):
  • CRC attachment
  • Coding block
  • Scrambling/descrambling
  • Modulation/de-modulation
  • Measurement
  • Resource element mapping/demapping
  • HARQ
  • MIMO
• RF (Radio Frequency): Radio Transmission and Reception

LTE Control Plane

The control plane protocol stack layers in LTE consist of upper layers, NAS, RRC, PHY, and RF. The functions of each are outlined below:

• Upper layer: Provide interfacing between upper layer information with lower layers (NAS).
• NAS (Non-Access Stratum):
  • Mobility Management
  • Session Management
  • Bearer Management
  • Paging Control
  • Security Management
• RRC (Radio Resource Control):
  • Configuration Management
  • Connection Management
  • Paging control
  • Security Management
  • Broadcast
  • Measurement configuration
  • Measurement Reporting
  • Cell selection and reselection
  • Mobility Management
• PHY (Physical Layer) and RF (Radio Frequency): Functions are the same as mentioned above in the User Plane section.

LTE RRC States: IDLE/Connected

There are two radio resource control (RRC) states defined: RRC IDLE and RRC CONNECTED.

• RRC IDLE state:
  • UE is known in EPC and has an IP address but is not known in E-UTRAN/eNB.
  • UE can receive broadcast/multicast data, monitors a paging channel to detect incoming calls, performs neighbor cell measurements and cell selection/reselection, and acquires system information.
  • In the RRC IDLE state, a UE-specific DRX (discontinuous reception) cycle may be configured by upper layers to enable UE power savings.
• RRC CONNECTED state:
  • UE is known in EPC and E-UTRAN/eNB.
  • UE location is known at the cell level.
  • Mobility is UE-assisted, network-controlled.
  • UE also monitors control channels associated with the shared data channel to determine if data is scheduled for it, provides channel quality feedback information, performs neighbor cell measurements and measurement reporting, and acquires system information.

Conclusion

The user plane and control plane in the LTE protocol stack work together to facilitate efficient data transfer and signaling in the network. Understanding these layers is key to optimizing LTE network performance and maintaining seamless communication.