How does DL data travel in 5G-NSA mode with Split Bearer?

As we know, 5G system has a CU (Centralised Unit) and DU (Distributed Unit) split type architecture. As per the 5G specification, split can happen from any layer. But most of the industries are following - EGTPU & PDCP at CU side and NR-RLC and lower layers at DU side. Below diagram is based on the same understanding. Let's have a look and try to run into our mind that how the Downlink Data will pass through the gNB stack in NSA mode and reaches to UE.

SPLIT Data in 5G NSA mode at gNB

We will explore the above diagram based on the functionality of each of the layers/modules of 5G stack. Also we will see how the packet looks like. So, the incoming packet from (S1U or X2U) will look like this:

[IP Packet]

The incoming packets which looks like the above IP packet lands to UDP interface (generally Sockets). UDP interface will remove the IP & UDP headers and give the packets UDP payload to EGTPU protocol.

[GTP packet]

Now, the packet will reach to EGTPU layer, where GTP header will be removed and the data PDU will be passed on to the PDCP layer to process to the downlink data. 

[Data Unit]

The data unit is the pure data which came for the user from the outer network. Now PDCP will associate a sequence number with the packet and send it to encryption and integrity engine, where encryption and integrity algorithms will apply. We will discuss NR-PDCP in details very soon. The PDCP PDU with encrypted data will look like this:

[PDCP PDU]

The PDCP PDU will enter to the NRUP protocol where it will add a NRUP extension header. It will add a header as Downlink User Data (DUD). If you don't know what is DUD, Please Click here and read. The packet will look like the below. 

[NRUP PDU]

This packet will be the input to egtpu again, which will add it's own egtp header and then UDP header.

[UDP packet]

Now, You will ask me - "Are you serious? You have just increased the size of the packet by adding PDCP & NRU headers and again added back GTP & UDP header. Why do we need UDP & GTP header now?"

The answer is simple, This is a split architecture, where our CU & DU does NOT resides in the same hardware. Hence, it has to transmit the received data from CU (where PDCP resides) to DU (where NR-RLC is present) or to eNB side RLC. So, to again transmit these packets to RLC side (NR/LTE), it has to go via internet, so we need UDP & GTP header. Hope this is clear, If not, read it once again, if still not clear, please comment below. I will try to help you.

So, the question is how will you then control the flow of packets transmitted from CU to DU or CU to LTE-RLC? What will happen if some packets are lost in between transmission?

The answer is : NRUP will take care. It has two main procedure:

        a. Downlink User Data

        b. Downlink Data Delivery Status 

You can click on these procedure to learn more about them.

Now, the UDP packet reached to DU/LTE side. We will start removing UDP header and GTP header, as shown below:

[NRUP SDU at receiver]

This packet will be the input to NRUP protocol and it will remove NRUP header and keep sending periodic DDDS. The packet will now be sent to RLC. Let's suppose this time it is at NR side RLC.

[RLC SDU]

Now, the RLC will check, if segmentation is needed, it will do the segmentation and it will add the RLC header with each segment of the packet.

[NR-RLC Segmentation]

These packets would be the input to the MAC. Now, the MAC layer will create Transport Block based on the Downlink resource available in the particular TTI, communicated by PHY based on CSI. in LTE and in NR the transport block creation is little different. Let's have look on how will our transport block looks like:

Whereas in LTE, transport block will look like this:

The transport block will be associated with a HARQ process of the UE and then will be given to PHY layer. These packets will be now transmitted by PHY by applying modulation techniques.

Now, after transmitting the packets from both the stack (viz. NR & LTE), how UE will manage to combine & retrieve the data? We will read it in my next post.