Radio Protocol Stack Architecture
NR Radio Protocol Stack Architecture is almost same as LTE Radio Protocol Stack Architecture. If you are already familiar with LTE protocol stack or general concept of radio protocol stack, you would not need to spend too much time in reading this page. Just take a brief look at the various figures / diagrams shown in this page would be enough. If you are new to the concept of LTE/NR radio protocol stack, I would suggest you to go through this page whenever you have chance and try to form your own big picture.
I would not describe much details on each component of the protocol stack in this page. It is too much to describe everything in a single page. The purpose of this page is to provide you with some big picture or intuitive understanding of the radio protocol stack. Most of the fundamental idea in this page comes from 3GPP 38.300.
As in LTE / WCDMA, NR radio protocol stack has two different stack depending on the type of data that is processed by the stack. If the data is Signaling message, it goes through the C-plane stack and if it is user data, it goes through U-Plane stack. Both U-Plane and C-Plane is made up of a common structure : PHY <-> MAC <-> RLC <-> PDCP, but the components sitting on top of PHY/MAC/RLC/PDCP gets different between C-Plane and U-Plane. In case of U-Plane, a layer called SDAP is sitting at the top of the radio stack and the SDP is connected to UPF (User Plane Function). In case of C-Plane, the two layers RRC and NAS are sitting at the top of the stack. NAS layer gets connected to AMF (Access and Mobility Management Function).
What I've mentioned can be described in a block diagram as shown below.
Now let's look just one step further into the protocol stack. Take a look at the L2 (layer 2) structure of the NR U-Plane radio protocol. The structure of L2 downlink stack can be illustrated as shown below. Except the new layer called SDAP, you would notice that the overall structure is almost identical to LTE L2 structure. NR support carrier aggregation from the beginning, data for each carrier is processed separately for each carrier in SDAP, PDCP, RLC and multiplexed/scheduled in the common MAC layer. This is also same as LTE Rel 10 or higher.
Following is L2 structure of NR U-Plane Uplink radio protocol. Basic structure is same as downlink structure except that Uplink does not support carrier aggregation.