September 30, 2023
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Telecom Standards & SEPs

Technical Analysis – 5G and 4G architecture

The 5G core network architecture is at the heart of the new 5G specification and enables the increased throughput demand that 5G must support. The new 5G core, as defined by 3GPP, utilizes cloud-aligned, service-based architecture (SBA) that spans across all 5G functions and interactions including authentication, security, session management and aggregation of traffic from end devices. The 5G core further emphasizes NFV as an integral design concept with virtualized software functions capable of being deployed using the MEC infrastructure that is central to 5G architectural principles.

The 5G Core Access and Mobility Management Function (AMF) receives all connection and session-related information from the User Equipment (UE) (N1/N2) but is responsible only for handling connection and mobility management tasks. All messages related to session management are forwarded over the N11 reference interface to the Session Management Function (SMF).

AMF is in charge of managing handovers between gNodeB’s (gNB’s), within the Next Generation Radio Access Network (NG-RAN). Formally referred to as an X2 handover, in 5G it is termed an Xn handover.

AMF     Access and Mobility Management Function

AUSF    Authentication Server Function

DN       Data Network

NEF      Network Exposure Function

NRF      Network Repository Function

NSSF    Network Slice Selection Function

PCF      Policy Control Function

(RAN)  (Radio) Access Network

SMF     Session Management Function

UDM     Unified Data Management

UPF      User Plane Function

SMSF   SMS Function

5G/NR – RAN Architecture     

Overall Architecture of NR RAN (Radio Access Network) would not look much different from LTE RAN Architecture. However, getting into details, you would start seeing some differences as well. You see different names of each node and interface. MME/S-GW in LTE is replaced by AMF/UPF in NR and X2/S1 in LTE is replaced by Xn/NG-C/U in NR. Different names would mean different protocols and implementations. Among all of these differences, the most outstanding one would be that the gNB internal structure is split into two parts called CU (Central Unit) and DU (Distributed Unit) as shown below and these two entities are connected by a new interface called F1.

Split between CU and DU

Packet Data Convergence Protocol (PDCP

 It provides its services to the RRC and user plane upper layers, e.g., IP at the UE or to the relay at the base station. The following services are provided by PDCP to the upper layers:

  • transfer of user plane data;
  • transfer of control plane data;
  • header compression;
  • ciphering;
  • integrity protection.


Radio link control (RLC) is a layer 2 Radio Link Protocol used in UMTS, LTE and 5G on the Air interface. This protocol is specified by 3GPP in TS 25.322[1] for UMTS, TS 36.322[2] for LTE and TS 38.322[3] for 5G New Radio (NR). RLC is located on top of the 3GPP MAC layer and below the PDCP-layer. The main tasks of the RLC protocol are:

  • Transfer of upper layer Protocol Data Units (PDUs) in one of three modes: Acknowledged Mode (AM), Unacknowledged Mode (UM) and Transparent Mode (TM)
  • Error correction through ARQ(only for AM data transfer)
  • Concatenation, segmentation and reassembly of RLC SDUs (UM and AM)
  • Re-segmentation of RLC data PDUs (AM)
  • Reordering of RLC data PDUs (UM and AM);
  • Duplicate detection (UM and AM);
  • RLC SDU discard (UM and AM)
  • RLC re-establishment
  • Protocol error detection and recovery


The MAC sublayer provides two main services to upper layers viz. data transfer and radio resource allocation. The other functions of 5G NR MAC are as follows.

  • Mapping between logical and transport channels (both Downlink and Uplink).
  • Multiplexing of MAC SDUs onto TBs (Transport Blocks) (in Uplink), SDUs belong to logical channels and transport blocks belong to transport channels.
  • Demultiplexing of MAC SDUs from TBs (in Downlink)
  • Scheduling information reporting (in Uplink)
  • Error correction through HARQ (in Downlink and Uplink)
  • Logical Channel Prioritization (in Uplink)


The physical layer provides the following services to the MAC sublayer.

  • Data Transfer
  • HARQ feedback signaling
  • Scheduling Request signaling
  • CQI (Channel Quality Indication) measurements


Differences in 4G Architecture

Changes at the core level are among the myriad of architectural changes that accompany the shift from 4G to 5G, including the migration to millimeter wave, massive MIMO, network slicing and essentially every other discrete element of the diverse 5G ecosystem. The 4G Evolved Packet Core (EPC) is significantly different from the 5G core, with 5G core leveraging virtualization and cloud-native software design at unprecedented levels.


Among the other changes that differentiate the 5G core from its predecessor 4G are User Plane Function (UPF) to decouple packet gateway control and user plane functions and Access; and Mobility Management Function (AMF) to segregate session management functions from connection and mobility management tasks.




LTE Network Architecture

LTE MME | Mobility Management Entity

Following are the functions of MME (Mobility Management Entity):

  • It keeps track of UEs which are registered on LTE network.
  • It takes care of user requests for network access. It helps in setting up and tearing down the data sessions.
  • It authenticates and authorizes users on the network.
  • It does a selection of S-GW and P-GW to be used for a given session.

LTE PGW | PDN Gateway | Packet Data Network Gateway

Following are the functions of PGW (Packet Data Network Gateway):

  • It provides access to external PDNs (Packet Data Networks).
  • If UE has multiple data sessions with multiple PDNs, in this case, UE can be connected with multiple PGWs but it will still be served by only one SGW.
  • It is also known by the name PCEF as it is responsible for dictating QoS and BW parameters for subscriber’s session.
  • PGW acts as an IP router with support for mobile-specific tunneling and signaling protocols.

LTE SGW | Serving Gateway

Following are the functions of SGW (Serving Gateway):

  • One or more SGWs will serve as a given group of eNBs for user plane data.
  • Single UE is served by one S-GW at any time.
  • It receives instructions from MME to set up and tear down sessions for particular UE.
  • It acts as an interface module for signaling between PGW and MME.
  • It takes care of user IP packets between P-GW and eNB.
  • SGW functions as an IP router with GTP support and charging functionality


Note: At Expertlancing, our team of experts has expertise in 5G/4G architecture and other 5G/4G – related technologies. Please reach out to us for any kind of technical analysis related to 5G/4G.

Written by Anusha & Ankit Puri 

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