The fifth generation of mobile networks (5G) is capable of offering higher speeds and lower latencies compared to its predecessors. Its development is based on various use cases ranging from manufacturing, automotive, energy, healthcare, agriculture, all the way to the entertainment sector. Based on these use cases, the desired characteristics and requirements for the operation of 5G networks have been established. The ITU-R (International Telecommunication Union – Radiocommunication Sector) highlights the most important features of the new network as Enhanced Mobile Broadband (eMBB), Ultra-Reliable and Low Latency Communications (URLLC), and Massive Machine Type Communications (mMTC). Additionally, the 3GPP (Third Generation Partnership Project) adds Network Operation and Enhancement of Vehicle-to-Everything (eV2X) to the previously mentioned three.
The 5G network is composed of four basic elements: the user equipment, the radio access and the core network, and the data network. The 5G network distinguishes between two deployment options: standalone (SA) and non-standalone (NSA). While the standalone option uses only one type of base station (LTE or 5G), the non-standalone option uses a combination of both. Furthermore, each of these options can use either the LTE (Evolved Packet Core – EPC) or the 5G core.
The protocol stack in the 5G radio access network is divided into the user plane and the control plane. The physical layer is the same on both planes. The data link layer on the control plane includes the MAC (Medium Access Control), RLC (Radio Link Control), and PDCP (Packet Data Convergence Protocol) protocols, while on the user plane, it also includes the SDAP (Service Data Adaptation Protocol) protocol. The network layer appears in the radio access network only on the control plane, consisting of the RRC (Radio Resource Control) protocol and the NAS (Non-Access Stratum) sublayer.
The functions of the physical layer include error detection and correction, mapping between transport and physical channels, data rate synchronization, radio resource allocation, and more. Within 5G, 3GPP has introduced a higher frequency range (FR2) in the centimeter and milimeter wavelengths. The radio frame in the 5G network is divided into subframes, each containing a different number of slots, depending on the subcarrier spacing (SCS). The 15 kHz spacing was already used in the LTE network, while the 5G introduced additional larger values, which make OFDM (Orthogonal Frequency Division Multiplexing) symbols shorter and significantly contribute to lower latencies.
The data link layer on the user plane consists of four sublayers. The MAC protocol is the lowest, with its main functions being mapping between logical and transport channels, (de)multiplexing service data units into transport blocks, error correction using the HARQ (Hybrid Automatic Repeat Request) mechanism, and more. MAC frames in the 5G network consist of smaller subframes, each with an associated subheader.
The RLC protocol can operate in three different transmission modes: Transparent Mode (TM), Unacknowledged Mode (UM), and Acknowledged Mode (AM). In transparent mode, messages are merely forwarded through the layer, unacknowledged mode supports segmentation, reassembly, and discarding of erroneous/duplicate messages, while acknowledged mode, in addition to all UM mode functions, also provides error correction using the ARQ (Automatic Repeat Request) mechanism, re-segmentation of corrupted/lost messages, and detection of duplicate messages. RLC in 5G no longer performs concatenation, and it also omits in-order message delivery, further meeting the high network demands for low latency.
The main functions of the PDCP protocol include numbering data radio bearers, header compression, integrity protection on the control plane, ciphering, and the detection and removal of duplicate messages. In 5G, PDCP supports out-of-order message delivery and message duplication, thereby increasing reliability by transmission on different links.
The SDAP protocol is new in the 5G network, as it was not present in the LTE network. The main function of this protocol is the Quality of Service (QoS) flow to data radio bearers mapping.
The network layer on the control plane contains the RRC protocol and the NAS sublayer. The main function of the RRC protocol is to establish and maintain connection between user equipments and the base station. It accomplishes this through functions such as broadcasting system information, transmitting paging messages, mobility and quality of service management, providing security services, and more.
The NAS sublayer connects the user equipment with the AMF (Access and Mobility Management Function) element in the 5G core network, hence is located beyond the radio access network protocols. The AMF element handles numerous processes such as authentication, authorization, mobility management, and more. The NAS sublayer contains the 5GSM (5G System Session Management) protocol for session management and 5GMM (5G System Mobility Management) for mobility management.
The protocol stack in the 5G core network is divided into the user plane and the control plane. The user plane manages user data transmission, message routing, traffic control, provision of the required quality of service, and recording of billing data, while the control plane handles authentication, authorization, compliance with the specified principles and policies, and mobility management. The biggest change compared to the LTE network is the introduction of Service-Based Architecture (SBA), which consists of numerous network functions.
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