# 5G NR Logical Architecture and its Split Function

## A. 5G NEW FUNCTIONAL REQUIREMENTS
- **Modularization**
-In previous networks: monolithic network functions corresponding to physical network elements while in **5G networks** modular network functions (NFs) for both control plane (CP) and user plane (UP) for both access network (AN) and core network (CN)
- **Network Slicing**
-Network Functions provides specific network capabilities for service verticals
-Resource abstractions by utilizing software-based Network Functions
- **Network Softwarization**
-**“Network programmability”** by extending SDN to all control and data layers
-Virtual Network Functions deployed closely to the users to reduced latency
- **Multi-Access Edge Computing (MEC)**
⎯ Native support of MEC → reduced E2E latency and increased network efficiency
- **Higher frequency operation and spectrum flexibility**
- **Ultra-Lean Design**
-Minimizing “always-on” type control signals in order to improve energy efficiency and reduced overall inter-cell-interference
- **Forward Compatibility**
-Maximizing the amount of time/frequency resources that can be flexibly utilized or that can be left blank; and confining signals and channels for physical layer functionalities within a configurable/allocabl time/frequency resource
- **Duplex Schemes**
-Common frame structure for both duplex scheme; Dynamic TDD for better utilization
- **Low-Latency Support**
-Use of “front-loaded” reference and control signaling;Transmission over a fraction of slot; “mini-slot”; Only “asynchronous” mode to reduce HARQ acknowledgement down to one slot

## B. 5G Protocol Stack and its Architecture
- **5G NR Radio Access Network Basic Protocol**
- 
- where The gNB is responsible for all radio-related functions in one or several cells
- Xn interface connecting gNBs directly to support activemode mobility and DC functionalities
- Standard Fs interface for connecting gNB-CU and gNB-DU
- **5G NR Protocol Stack**
- **User Plane Protocol Stack**
- Downlink UP Protocol Stack
- **Service Data Application Protocol (SDAP)** ⎯ Map QoS based bearers to radio bearers according to their quality of service requirements; New in 5G NR due to new mechanism for handling QoS
- **Packet Data Convergence Protocol (PDCP)** ⎯ IP header compression, ciphering and integrity protection ⎯ Routing and duplication for DC with split bearers
- Header compression based on Robust Header Compression (RoHC)
- **RoHC**is for IP packets only and not apply to SDAP header
- **Radio Link Control (RLC)** ⎯ Segmentation and retransmission ⎯ No in-sequence delivery support of data to higher protocol layers to reduce latency
- **Operates in three modes depending on application requirements:**
- Transparent mode (TM): for BCCH, CCCH and PCCH
- Unacknowledged mode (UM): supports segmentation but not retransmission when error free delivery is not required (VoIP)
- Acknowledged mode (AM): Segmentation, duplication removal and retransmission are all supported (DL -SCH and UL -SCH)
- **Medium Access Control (MAC)** ⎯ Multiplexing and mapping of logical channels into transport channels; HARQ and all scheduling related functions
- **Physical Layer (PHY)** ⎯ Coding/decoding, modulation/demodulation, multi -antenna mapping and other PHY related functions;Mapping transport channels to physical channels
- **5G NR PROTOCOL STACK USER PLANE DATA FLOW**
- 
- **5G MAPPING BETWEEN LOGICAL, TRANSPORT AND PHYSICAL CHANNELS**
- 
- **5G QoS Architecture**
- Comparison

## C. 5G FUNCTIONAL SPLITS
- **Control Plane/User Plane split (Vertical split)**
⎯ First step for introduction of SDN in the RAN
⎯ Allows separate optimization of CP and UP
⎯ Consistent CP in multi-vendor networks
⎯ More challenges for lower layer splits
- **Central Unit/Distributed Unit split (Horizontal split)**
⎯ Obtain centralization gain, both in terms of performance gains and economy of scale
⎯ Shift functionalities to deferent locations
based on morphologies and transport availabilities
⎯ Make overall RAN more future proof and less costly for future generation upgrades
- Not a single split is going to fit all
- Different morphologies require different splits
- Only a software based RAN can support dynamic and fluid split options
- Control plane splits (vertical) are as important as user plane splits (horizontal)
-