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5G NR Layer 2 – Packet Data Convergence Protocol (PDCP) Overview

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Introduction

5G NR PDCP is a Layer 2 protocol and overall functionality is almost same as it was in 4G LTE PDCP. PDCP layer resides between RRC on the upper side and RLC on the lower side of the control plane and between SDAP and RLC in user plane. Following figure shows the same.

Key Pointer for PDCP :

PDCP Services

The PDCP layer provides following services to upper layer i.e.RRC or SDAP :

The PDCP layer expect following services from lower layer i.e. RLC 

PDCP Functionality

Following is the list  functions performed by PDCP Layer/sub-layer

PDCP Layer Structural View:

The structural view of PDCP layer given in below diagram, which will give a very high level idea about PDCP layer.

Now, on seeing above diagram, there are some questions are coming in minds like “What is the PDCP entity?”, “What is the PDCP SAP?”, “What is the C-SAP?”, “What is the RLC UM-SAP?”,  “What is the RLC AM-SAP?”,  “What is the PDCP SDU/PDU?” and “What is the RLC SDU/PDU?”. Don’t worry about these questions,I’ll will explain.

The functional diagram for PDCP layer:

This diagram will give high level idea about PDCP layer functionality. On seeing, this diagram, there are some questions coming into minds like –

Don’t worry about these questions, I’ll explain. But first I want to tell you about PDCP sub-layer functional flow, it means what kinds of functionality will be performed during TX (Downlink) / RX (Uplink). Here I’m explaining w.r.t. gNB and almost same functional flow applicable for UE as well.

Let’s follow the diagram from TX (Downlink):

  1. PDCP SDU coming into PDCP sub-layer firstly stored in transmission buffer, then goes through the “Sequence Numbering” functionality. It means PDCP adds a sequence number to each of incoming PDCP SDU, then it will go for further PDCP functionality.Once it adds sequence number, it has to maintain the sequence number on the receiver end. With the help of sequence number, we can figure out many things like “Is the data getting in order or not?”, “Is there any duplicate data”,“How can I combine the multiple chunks of data block into an original big chunk of data”.
  2. After going through Sequence Numbering procedure, it goes through header compression procedure, but it is applicable only for User plane data. It means control plane data i.e. signaling message not going through header compression procedure. We can disable header compression/decompression procedure if it is not implemented at UE.
  3. After header compression, there are two paths, i) Going through Integrity/Ciphering ii) Directly going to Add PDCP Header.
  4. Integrity protection applies only to signaling message i.e. control plane data (It means RRC/NAS message i.e. DCCH data, not DTCH data). Here, you may disable Integrity protection by applying a signaling message.
  5. After this, it goes through ciphering, which is applies on both user plane data and control plane data. Here, you may disable ciphering by applying a signaling message.
  6. Then,it goes to Add PDCP Header, it adds PDCP header to the data.
  7. If split bearer is enabled, then PDCP routing function routes the PDCP PDU to intended bearer and inside the routing duplication procedure comes into picture.

Note : We need to perform reverse operation at receiver end to get original data.

Article is submitted by Manoj Kumar 

About Manoj 
Manoj Kumar is having a Master of Science ( Computer Science )  from J.K. Institute of Applied Physics and Technology, University of Allahabad and Post Graduate Diploma in Advanced Computing from C-DAC Hyderabad, India. IYou may reach him on  linkedin https://www.linkedin.com/in/itsmanoj

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