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5G PDCP Uplink Architecture Explained: Functions and Workflow

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5G PDCP Uplink Architecture Explained: Functions and Workflow
5G PDCP Uplink Architecture Explained: Functions and Workflow
5G & 6G Prime Membership Telecom

In 5G NR (New Radio), the Packet Data Convergence Protocol (PDCP) plays a vital role within Layer 2. This sublayer is responsible for maintaining security, reliability, and efficiency in the transmission of user-plane and control-plane data between User Equipment (UE) and the gNB (Next Generation NodeB).

While downlink communication focuses on sending data to users, the uplink—depicted in the accompanying diagram—ensures that data created by the UE securely reaches the gNB and arrives in the correct order. This blog will detail the uplink architecture of PDCP, breaking down its functions step by step.

Where Does PDCP Fit in 5G?

Before diving into the specifics of uplink architecture, let's position PDCP within the 5G protocol stack:

Layer 3: RRC (Radio Resource Control)

Layer 2: * PDCP (top sublayer) * RLC (Radio Link Control) * MAC (Medium Access Control)

Layer 1: Physical Layer

PDCP acts as a bridge between the RRC/SDAP (control and QoS adaptation) and the RLC, which guarantees reliable delivery across the radio link.

The diagram shows PDCP_UL_gNB (PDCP in the uplink direction). Here, data flows from the UE to the gNB through PDCP, where several key functions take place:

Reordering

Integrity Verification / Protection

ROHC (Robust Header Compression)

Ciphering (Encryption)

Duplicate Discarding

Moreover, the PDCP layer accommodates different radio bearers:

SRB0 (Control Plane Data) → For RRC messages over PCCH, BCCH, CCCH.

SRB1-3 (Signaling Radio Bearers) → Carry DCCH signaling messages.

DRBs (Data Radio Bearers, SRB1-29) → Transport user plane traffic (UP data).

Each category has a slightly different PDCP processing flow, depending on whether the bearer is for control or user data.

Let’s break down the functions of PDCP in uplink, as shown in the image:

  1. Reordering

Purpose: Ensures PDCP packets stay in sequence.

In the uplink, reordering makes sure that packets are aligned before moving forward in processing.

This is particularly important when using dual connectivity or carrier aggregation, as packets could come in through different paths.

  1. Integrity Verification (for SRBs)

Primarily applied to Signaling Radio Bearers (SRBs) that carry DCCH messages.

It confirms that signaling messages between the UE and gNB haven’t been tampered with.

Uses integrity keys derived from the 5G security context.

📌 Example: Connection reconfiguration messages from RRC are verified here, blocking any malicious control signals from getting through.

  1. Integrity Protection (for DRBs)

Used for Data Radio Bearers (DRBs).

Guarantees that the user-plane traffic is trustworthy from end to end.

Blocks replay or tampering attempts on data packets.

  1. ROHC (Robust Header Compression)

This is applied only to user-plane traffic (UP Data).

It compresses large IP headers (which can be as big as 60 bytes for IPv6).

This reduction in overhead is crucial in wireless channels, which often have limited bandwidth.

📌 Example: VoIP packets, which have small payloads but substantial headers, gain a lot from ROHC, leading to better bandwidth use.

  1. Ciphering (Encryption)

Ensures both user and control data remain confidential.

Ciphering is applied before data is sent over the air interface.

This prevents eavesdropping on sensitive information.

Algorithms used include AES and SNOW 3G, in line with 3GPP standards.

  1. Duplicate Discarding

In the uplink, duplicates of PDCP PDUs can occur in cases like multi-connectivity.

PDCP ensures only one copy reaches RLC, discarding the rest.

This helps avoid unnecessary retransmissions and enhances efficiency.

The diagram points out three types of bearers:

  1. SRB0 (Control Plane Data)

Carries initial RRC signaling (like connection setup).

Functions: Reordering only (no ciphering or integrity at this stage).

  1. SRB1-3 (Signaling Radio Bearers)

Carry DCCH signaling messages.

Functions: Reordering + Integrity Verification + Ciphering.

These are crucial for secure and reliable control signaling.

  1. SRB1-29 (User Plane Data – DRBs)

Handle user application data (voice, video, IoT packets).

Functions: Reordering + ROHC + Integrity Protection + Ciphering + Duplicate Discarding.

This set of operations is the most comprehensive, as both performance and security are key here.

Configuration via RRC

The RRC (Radio Resource Control) layer configures PDCP functions.

For example, it determines which DRBs need integrity protection or ROHC.

RRC sends configuration messages to tell PDCP how to handle data across different bearers.

📌 Key Point: PDCP operations aren’t set in stone—they can be dynamically adjusted by RRC based on network conditions and UE capabilities.

Bearer Type | Traffic Type | PDCP Functions Applied

SRB0 | Initial control (CP data) | Reordering only

SRB1-3 | Control signaling (DCCH) | Reordering, Integrity Verification, Ciphering

SRB1-29 (DRBs) | User data (DTCH) | Reordering, ROHC, Integrity Protection, Ciphering, Duplicate Discarding

  1. Security

Provides user privacy and secure signaling through integrity protection and ciphering.

  1. Efficiency

ROHC helps save bandwidth, especially important for VoIP and real-time applications.

  1. Reliability

Reordering and duplicate discarding ensure data gets to its destination correctly without wasting resources.

  1. Flexibility

Different configurations for SRBs and DRBs allow for tailored quality of service and security.

Real-World Example

Picture a smartphone user on a 5G call while streaming video:

Voice packets (VoIP) → Sent via DRBs with ROHC, ciphering, and integrity protection for efficient, low-latency transmission.

Video streaming data → Sent via DRBs with ciphering and duplicate discarding for optimal throughput.

Control signaling (like handover commands) → Sent via SRBs with integrity protection and ciphering to ensure reliability.

This combination guarantees a seamless user experience while keeping things secure and efficient.

Increased Complexity: Managing multiple bearers and processes makes PDCP management tricky.

Processing Overhead: ROHC and ciphering require computing power, potentially affecting UE battery life.

Latency Concerns: In ultra-low latency scenarios (URLLC), some PDCP functions might need to be fine-tuned.

Conclusion

The uplink architecture of PDCP in 5G NR is crafted to find a balance between security, efficiency, and reliability. As outlined in the diagram:

SRB0 manages basic control data with minimal processing.

SRB1-3 guarantee secure signaling through integrity verification and ciphering.

DRBs (SRB1-29) offer the most extensive PDCP functions, supporting real-time and high-throughput applications with ROHC, ciphering, and duplicate discard.

For telecom professionals, grasping PDCP uplink architecture is key to enhancing 5G performance. For tech enthusiasts, it highlights the complex processes that make 5G fast, secure, and reliable. Though the PDCP layer may go unnoticed by end users, it's the unseen force enabling secure, efficient, and robust uplink communication in 5G.