PDCP Layer in LTE

The Packet Data Convergence Protocol (PDCP) is an important component of the LTE protocol stack. As a sublayer within the LTE architecture, PDCP plays a critical role in enabling efficient and secure data transmission. Situated between the Radio Resource Control (RRC) layer and the Radio Link Control (RLC) layer, PDCP handles tasks such as header compression, ciphering, and integrity protection.

3gpp Specification number for PDCP Layer is 36.323.

In the context of LTE, which is designed to support high-speed data communication, PDCP's functions are essential for optimizing the network's performance and maintaining the security of user data.

Overview of PDCP Architecture

The PDCP layer's architecture can be divided into two primary components: the PDCP structure and the PDCP entities. These components work together to manage the flow of data and signaling messages within the LTE network.

PDCP Structure

The PDCP structure consists of several PDCP entities, each associated with a specific Radio Bearer (RB). These entities are responsible for managing the data flow for their respective bearers. The PDCP layer is designed to be flexible, allowing it to handle different types of bearers, including split bearers and LTE-WLAN Aggregation (LWA) bearers.

For each RB, there is a corresponding PDCP entity, which interacts with one or more RLC entities depending on the direction of data flow (uplink or downlink) and the mode of operation (acknowledged or unacknowledged mode). The PDCP entities are configured by the RRC layer, ensuring that they operate in accordance with the overall network configuration.

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PDCP Entity:

• The PDCP entity is the central component in the PDCP sublayer. Each PDCP entity is associated with a specific radio bearer. The PDCP entity is responsible for functions like header compression, ciphering, integrity protection, and more.
• In the figure, two PDCP entities are shown, each linked to a different type of RLC SAP (Service Access Point).

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Service Access Points (SAPs):

• PDCP-SAP: This is the interface between the PDCP sublayer and the RLC sublayer. It handles the transfer of PDUs (Protocol Data Units) from PDCP to RLC.
• C-SAP (Control SAP): The C-SAP is used for control signalling between the upper layers and the PDCP layer.
• RLC UM-SAP (Unacknowledged Mode Service Access Point): This SAP connects the PDCP entity to the RLC entity operating in unacknowledged mode (UM), where data is sent without requiring acknowledgment from the receiving side.
• RLC AM-SAP (Acknowledged Mode Service Access Point): This SAP connects the PDCP entity to the RLC entity operating in acknowledged mode (AM), where data transmission is acknowledged by the receiver to ensure reliable delivery.

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Radio Bearers:

• Radio bearers are the logical channels that carry data between the UE (User Equipment) and the eNB. Each PDCP entity is associated with a specific radio bearer.

PDCP-PDU and RLC-SDU:

• PDCP-PDU: This represents the data units processed by the PDCP layer. These PDUs are passed down to the RLC sublayer through the PDCP-SAP.
• ?RLC-SDU: This represents the data units received by the RLC sublayer from the PDCP sublayer. The RLC layer processes these SDUs for transmission over the air interface.

Consolidated procedure:

• Data from the upper layers is passed down to the PDCP entity through the control SAP (C-SAP) or directly to the PDCP entity.
• The PDCP entity processes the data according to its functions (e.g., header compression, encryption) and prepares PDCP PDUs.
• These PDCP PDUs are then passed to the RLC sublayer via the PDCP-SAP.
• The RLC sublayer receives the PDCP PDUs as RLC SDUs. Depending on whether the RLC entity is in Unacknowledged Mode (UM) or Acknowledged Mode (AM), the data is processed differently:
• In RLC UM: Data is sent without acknowledgment, useful for real-time services where timely delivery is more critical than reliability.
• In RLC AM: Data is sent with acknowledgments, ensuring reliability by retransmitting lost or erroneous data.

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PDCP Entities and Functions

The PDCP layer's entities and functions are designed to manage the various tasks involved in data transmission within the LTE network. These functions are essential for ensuring that data is transmitted efficiently, securely, and in the correct order.

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Control Plane PDCP Entities

Control plane PDCP entities are responsible for handling signaling messages, which are critical for maintaining the connection between the UE and the eNB. These entities provide functions such as integrity protection and ciphering, ensuring that signaling messages are transmitted securely.

The integrity protection function ensures that signaling messages have not been tampered with during transmission, while the ciphering function protects the confidentiality of these messages. Together, these functions ensure the secure and reliable transmission of control plane data within the LTE network.

User Plane PDCP Entities

User plane PDCP entities manage the transmission of user data, performing tasks such as header compression, ciphering, and sequence number management. These entities are crucial for optimizing the use of network resources and ensuring the efficient transmission of data.

The header compression function reduces the overhead associated with transmitting IP packets over the LTE air interface, while the ciphering function protects the confidentiality of user data. Sequence number management ensures that data packets are transmitted in the correct order, preventing issues such as data duplication or loss.

Functions of PDCP

The PDCP layer performs several key functions that are essential for the efficient operation of the LTE network. These functions include header compression and decompression, ciphering and deciphering, integrity protection and verification, sequence number handling, in-sequence delivery and reordering, duplicate elimination, timer-based discard, and routing and reordering for split and LWA bearers.

• Header Compression and Decompression:

1. Uses Robust Header Compression (ROHC) protocol to compress IP packet headers.
2. Reduces data transmission size over the air, important for small-packet applications like VoIP.
3. Decompression at the receiving end restores headers before passing data to upper layers, ensuring efficiency and data integrity.

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• Ciphering and Deciphering:

1. Encrypts user and control plane data before transmission to ensure security.
2. Deciphering at the receiving end decrypts the data, maintaining its confidentiality and integrity.
3. Utilizes a key provided by upper layers to protect against security threats.

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• Integrity Protection and Verification:

1. Provides integrity protection for control plane data to prevent tampering during transmission.
2. Integrity verification checks data integrity at the receiving end; data is discarded if verification fails.

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• Sequence Number Handling:

1. Assigns unique sequence numbers (SNs) to each data packet for tracking and reordering.
2. Ensures packets are transmitted and received in the correct order, critical for scenarios with delays or retransmissions.
3. Manages ciphering by using SNs to encrypt packets with unique keys.

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• In-sequence Delivery and Reordering:

1. Ensures data packets are delivered to upper layers in the correct order.
2. Reorders packets received out of sequence before passing them on, preventing data loss or duplication.

• Duplicate Elimination:

1. Identifies and eliminates duplicate packets to ensure only unique packets reach the upper layers.
2. Prevents issues such as data duplication, particularly in retransmission scenarios.

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• Timer-based Discard:

1. Discards data packets not transmitted within a configured time frame, preventing unnecessary retransmissions and buffer overflows.
2. Ensures network efficiency by controlling the timing of packet discard.

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• Routing and Reordering for Split and LWA Bearers:

1. Manages the routing and reordering of data packets across multiple paths in split bearer or LTE-WLAN Aggregation (LWA) scenarios.
2. Uses sequence numbers to ensure packets are delivered in order, even when distributed across multiple RLC entities or technologies.

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PDCP Procedures

The PDCP layer is crucial in managing data transmission within the LTE network, ensuring data is processed, transmitted, and received securely and efficiently.

• Uplink (UL) Data Transfer Procedures

1. Header Compression: Utilizes the ROHC protocol to compress IP packet headers, reducing data size for transmission.
2. Ciphering: Encrypts data before transmission to prevent unauthorized access, using a key provided by the upper layers.
3. Sequence Number Assignment: Assigns unique sequence numbers to each data packet, ensuring correct order during transmission.

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• Downlink (DL) Data Transfer Procedures

1. Deciphering: Decrypts received data to maintain confidentiality and integrity.
2. Header Decompression: Restores compressed headers at the receiving end before passing data to upper layers.
3. Reordering: Rearranges out-of-order packets using sequence numbers to ensure correct data delivery.

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• PDCP Re-establishment Procedures

1. Uplink and Downlink Considerations: Manages re-establishment differently for UL and DL; UL may involve retransmission; DL may require reordering.
2. Handling PDCP Re-establishment after Handover: Ensures seamless data transmission during handovers by resetting state variables and reconfiguring operations.

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• PDCP Status Report

1. Transmit Operation: Compiles and sends status reports, indicating missing packets and the status of subsequent packets.
2. Receive Operation: Processes received status reports to confirm data delivery, retransmitting packets if necessary.

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• PDCP Discard Procedures

1. Uses a discard timer to remove packets not transmitted within a set timeframe, preventing buffer overflows and unnecessary retransmissions.

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• Handling Unknown, Unforeseen, and Erroneous Protocol Data

1. Discards corrupted or unrecognized packets and may generate error reports for upper layers.

PDCP Parameters

• Sequence Numbers (SNs): Used to track and maintain the correct order of data packets during transmission.

• COUNT Parameter: Combines PDCP SN with Hyper Frame Number (HFN) to ensure secure ciphering and deciphering of data.

• MAC-I (Message Authentication Code for Integrity): Provides integrity protection for control plane data, ensuring data has not been tampered with during transmission.
• ROHC Configuration Parameters:

1. MAX_CID: Defines the maximum context ID for managing the compression of multiple IP flows.
2. PROFILES: Determine the specific ROHC profiles used for header compression, configured by upper layers.

• Timers:

discardTimer:

?? Started on the transmitter side upon reception of an SDU from the upper layer.

?? Duration is configured by upper layers.

t-Reordering:

?? Used on the receiver side to detect loss of PDCP PDUs when the reordering function is active.

?? Only one t-Reordering timer runs per PDCP entity at a time.

?? Duration is configured by upper layers.

t-StatusReportType1:

?? Used on the receiver side for LWA bearers to trigger status report transmission.

?? Duration is configured by upper layers (statusPDU-Periodicity-Type1).

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t-StatusReportType2:

?? Also used on the receiver side for LWA bearers to trigger status report transmission.

?? Duration is configured by upper layers (statusPDU-Periodicity-Type2 and statusPDU-Periodicity-Offset).

?? The first run of the timer after (re)configuration considers both periodicity and offset.

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