5G NR Protocols

Article by Abhijeet Kumar

What Are Protocols? Protocols are rules and standards that define how data is transmitted and managed across network connections. In telecommunications, protocols ensure that hardware and software components operate harmoniously across diverse devices and infrastructures. They govern everything from how data packets are formed, transmitted, and received, and devices connect and communicate securely and efficiently.

Why We Need Protocols in Telecommunications:

• Interoperability: Protocols standardize communications between different systems and devices, ensuring they can interact without confusion.
• Efficiency: Optimized protocols make better use of network resources, reducing costs and improving service quality.
• Security: Protocols incorporate security measures to protect data integrity, confidentiality, and authenticity.
• Scalability: Standardized protocols support the expansion of network capabilities without significant changes to the core network structure.

The Layered Architecture in 5G: L1, L2, L3

The architecture of 5G networks is built on a layered protocol structure, commonly referred to as L1, L2, and L3. This structure helps in organizing network functions modularly, simplifying design, implementation, and troubleshooting. Here’s why each layer is crucial:

L1: Physical Layer

• Purpose: The physical layer is responsible for the transmission and reception of raw bit streams over a physical medium. It converts the digital bits into signals and vice versa.
• Functions in 5G:Waveform Generation: Uses OFDM (Orthogonal Frequency-Division Multiplexing) which allows high-speed data transmission that is efficient and resistant to interference.Modulation and Demodulation: Determines how signals are formed, including the modulation scheme (e.g., QPSK, QAM) based on network conditions.Error Correction: Employs techniques like forward error correction to improve data integrity without needing retransmission.

L2: Data Link Layer

• Purpose: The data link layer ensures that the data is reliably transmitted across the physical network. It organizes data into frames and detects/resolves errors that occur at the physical layer.
• Sublayers in 5G:MAC (Medium Access Control): Manages and maintains the control of the radio channels and multiplexes data streams from various sources.RLC (Radio Link Control): Enhances reliability through segmentation and reassembly of packets, and manages error correction through ARQ (Automatic Repeat Request).PDCP (Packet Data Convergence Protocol): Compresses headers and provides encryption and integrity checks to ensure the security of user data.

L3: Network Layer

• Purpose: The network layer is responsible for the delivery of packets from the source host to the destination host based on their addresses. It defines the paths that the data packets take from the sender to the receiver.
• Key Functions in 5G:IP Routing and Transfer: Manages packet forwarding including addressing, routing, and traffic control.Session Management: Manages the setup and maintenance of network connections.Mobility Management: Handles the operations required when devices move between different sectors or networks while maintaining ongoing sessions..

TS 38.331 - RRC Protocol Specification- L3 Layer

• Connection Management: Manages the signaling connections between the mobile device and network, facilitating communications and mobility management.
• Broadcast Information: Distributes system information related to network and cell configurations, essential for device operations within the network.
• Bearer Control: Manages the setup, modification, and release of radio bearers, aligning network resources with current data traffic and service needs.
• Mobility Handling: Oversees mobile device movements between cells and different radio access technologies, ensuring seamless service continuity.

TS 37.324 - SDAP Specification- L2

• QoS Flow Mapping to DRBs: Maps QoS flows to Data Radio Bearers (DRBs), aligning bearer resources with the QoS requirements of different traffic streams.
• QoS Enforcement: Applies QoS rules to ensure traffic handling adheres to specific service level agreements and user expectations.
• Reflector QoS Flow Mapping: Facilitates dynamic adjustment of QoS flow mappings based on observed data flow directions and requirements.
• Service Continuity across Handovers: Ensures ongoing sessions maintain their QoS characteristics even when the network topology changes due to user mobility.

TS 38.323 - PDCP Protocol Specification

• Header Compression: Implements Robust Header Compression (ROHC) to reduce the overhead of IP packets, enhancing data transfer efficiency over the air interface.
• Encryption and Integrity Protection: Provides security functions, encrypting user data and ensuring the integrity of control plane data.
• In-sequence Delivery: Ensures the sequential delivery of upper layer packets, even across handovers between different network cells.
• Duplicate Elimination: Removes duplicated packets received in the PDCP layer, improving the efficiency of data transmissions.

TS 38.322 - RLC Protocol Specification

• Segmentation and Reassembly: Efficiently divides larger packets into smaller ones for transmission and reassembles them at the receiver end.
• Error Correction: Utilizes automatic repeat request (ARQ) mechanisms in acknowledged mode to ensure error-free data delivery.
• Protocol Data Unit (PDU) Handling: Manages the packaging and interpretation of data units, including sequence numbering to support ordering and reassembly.
• Buffer Management: Manages data buffers for storing PDUs awaiting transmission or retransmission, optimizing memory and throughput performance.

TS 38.321 - MAC Protocol Specification

• Multiplexing/Demultiplexing of Logical Channels: Handles the multiplexing of data from different logical channels onto transport channels for transmission and demultiplexing of data from transport channels to logical channels upon reception.
• Priority Handling and Channel Access: Manages the priority and ordering of data transmission based on QoS parameters, ensuring that high-priority traffic receives timely access to the radio interface.
• Error Correction through HARQ: Implements Hybrid Automatic Repeat Request (HARQ) processes that combine error detection and correction capabilities with retransmission mechanisms. This dual approach enhances data integrity and reduces latency in data delivery.
• Link Adaptation: Dynamically adjusts modulation and coding schemes based on the radio link quality to optimize data throughput and maintain link reliability under varying channel conditions.
• Buffer Management: Oversees the storage of data waiting for transmission or retransmission in HARQ processes, optimizing resource use and response times.
• Scheduling Information Reporting: Gathers and reports channel quality information and buffer status to assist the scheduling decisions made by the base station, enhancing the overall efficiency of resource allocation.

What is SDU and PDU in protocols

Service Data Unit (SDU)

• Definition: The Service Data Unit (SDU) refers to a unit of data that has been passed down from an upper layer to a lower layer in the network protocol stack. The SDU contains the payload or the data that needs to be transmitted, which the upper layer expects the lower layer to deliver.
• Role in Communication: An SDU is essentially the data that a service (application or process) wishes to transmit using the underlying network. When an SDU is passed to a lower protocol layer for transmission, it may be combined with other information such as headers or trailers, transforming it into a Protocol Data Unit (PDU) appropriate for that layer.

Protocol Data Unit (PDU)

• Definition: A Protocol Data Unit (PDU) is a combination of the SDU and the protocol-specific control information (like headers and trailers). Each layer in a network may add or remove its own PDU headers or trailers, thus encapsulating or decapsulating the SDUs as they pass through the layers.
• Role in Communication: The PDU represents a packet of data that includes both the original service data (SDU) and the control information needed for the network to handle the data correctly. This control information can include error checking, segmentation, identification, and other control mechanisms that ensure the data can be properly routed and delivered

In 5G networks, the use of SDUs and PDUs is crucial in ensuring that data is appropriately formatted and handled at different layers, including:

• PDCP Layer in 5G: Handles PDCP PDUs, which encapsulate upper layer SDUs (from the RRC or user data) with control information like sequence numbers and header compression for efficient transmission.
• RLC Layer in 5G: Manages RLC PDUs, segmenting and reassembling RLC SDUs to ensure reliable data delivery across the network.
• MAC Layer in 5G: Utilizes MAC PDUs, which are formatted data units containing MAC headers and payloads that ensure data is scheduled and transmitted efficiently by the physical layer.

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