Protocol Stack in LTE

LTE is a standard for wireless broadband communication for mobile devices and data terminals, designed to increase the capacity and speed of mobile networks. The LTE protocol stack, which forms the backbone of the LTE technology, is essential for understanding how data transmission occurs across the network. It is divided into different layers that perform specific functions, each contributing to efficient and reliable communication.

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The LTE protocol stack consists of three major layers, which are split between the User Plane and Control Plane:

User Plane: Concerned with actual data transmission (e.g., voice, video, internet browsing).

The figure 4.3.1-1 as per 3gpp illustrates the protocol stack for the user-plane, where the PDCP, RLC, and MAC sublayers (terminated at the eNB on the network side) handle a variety of key functions

§? Header Compression: Efficiently compresses IP headers to reduce overhead and optimize bandwidth usage.

• Ciphering: Encrypts user data for secure transmission and protection against unauthorized access.
• Scheduling: Allocates resources dynamically to ensure efficient data transmission based on network conditions.
• ARQ (Automatic Repeat Request): Provides error detection and correction through retransmission requests for lost or corrupted data.
• HARQ (Hybrid Automatic Repeat Request): Enhances error correction with soft combining, ensuring higher reliability and faster recovery from transmission errors.
• Segmentation and Reassembly: Splits large data packets into smaller segments for transmission and reassembles them at the receiver side.
• Prioritization: Ensures that high-priority data, such as voice or real-time applications, receives preferential treatment during transmission.

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Control Plane: Manages signaling (e.g., call setup, mobility management, session management).

The diagram below outlines the control-plane protocol stack,where various layers perform key functions:

• PDCP sublayer: Terminated at the eNB, responsible for control-plane tasks such as ciphering and integrity protection, ensuring secure communication.
• RLC and MAC sublayers: Also terminated at the eNB, these layers handle the same functions as in the user plane, including data segmentation, retransmissions, and resource multiplexing.
• RRC (Radio Resource Control): At the eNB, RRC manages key control-plane activities such as system information broadcast, paging, RRC connection management, radio bearer control, mobility (handover), and UE measurement reporting.
• NAS control protocol: Terminated at the MME, it handles EPS bearer management, authentication, idle-mode mobility, paging, and security control.

Note: The NAS control protocol is beyond the scope of this specification and is included for informational purposes only.

?Layer Functions and Interaction Across UE, eNB, and MME

The LTE protocol stack involves communication between the UE, eNodeB, and the MME.

1. UE (User Equipment): This represents the mobile device used by the end-user. The LTE protocol stack in the UE includes the Non-Access Stratum (NAS), Radio Resource Control (RRC), Packet Data Convergence Protocol (PDCP), Radio Link Control (RLC), Medium Access Control (MAC), and Physical (PHY) layers.
2. eNB (evolved NodeB): This is the base station that manages communication between the UE and the LTE network. The eNB contains the same set of protocol layers as the UE except for the NAS layer. The eNB is responsible for managing the air interface and radio resources.
3. MME (Mobility Management Entity): The MME is part of the LTE core network responsible for signalling and managing UE mobility and session states. The NAS layer in the UE communicates directly with the NAS layer in the MME, bypassing the eNB.

Detailed Layer-by-Layer Interaction:

NAS (Non-Access Stratum):

The NAS layer is responsible for managing communication between the UE and the core network, specifically the Mobility Management Entity (MME) in LTE. It handles mobility and session management, operating outside the radio access network (RAN).

Key Responsibilities of the NAS Layer:

·???????? Authentication and Security: Ensures that users are authenticated and data transmissions are secure.

·???????? Session Management: Manages data sessions and establishes EPS (Evolved Packet System) bearers for data communication.

·???????? Mobility Management: Handles tracking area updates and manages UE mobility when the UE is idle.

·???????? Bearer Management: Establishes and modifies EPS bearers for data transfer.

The NAS layer operates above the RRC layer and is critical for establishing secure connections between the UE and the network.

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RRC (Radio Resource Control):

The RRC layer manages all aspects of radio resource control, ensuring efficient use of radio spectrum and handling all signaling between the mobile device (UE) and the network.

Key Responsibilities of the RRC Layer:

·???????? Connection Setup and Release: Establishes, maintains, and releases radio bearers and connections.

·???????? Mobility Management: Manages handovers, ensuring seamless connectivity as a user moves from one cell to another.

·???????? Security Control: Manages encryption keys and other security-related functions.

·???????? Measurement Reporting: Collects and reports signal strength measurements for neighboring cells.

·???????? Paging: Manages the paging process to notify devices about incoming data or calls.

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The RRC layer operates in either RRC_IDLE or RRC_CONNECTED states, depending on whether a connection is established. It interacts with the NAS layer and passes instructions down to the lower layers (PDCP, RLC, MAC, PHY) for execution.

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PDCP (Packet Data Convergence Protocol):

The PDCP layer is responsible for header compression and ciphering, ensuring efficient and secure transmission of data. It plays a crucial role in both the User Plane and the Control Plane.

Key Responsibilities of the PDCP Layer:

·???????? Header Compression: Compresses IP headers using protocols like ROHC (Robust Header Compression) to save bandwidth, especially for real-time services like VoIP.

·???????? Ciphering: Encrypts data to provide confidentiality and integrity, using algorithms such as AES (Advanced Encryption Standard) and SNOW 3G.

·???????? Integrity Protection: Ensures that control plane messages are authentic and not tampered with during transmission.

·???????? In-Sequence Delivery: Ensures that the PDCP SDUs are delivered to higher layers in the correct sequence.

PDCP Functions in the Control and User Plane:

·???????? Control Plane: Responsible for ciphering and integrity protection of RRC and NAS signalling messages.

·???????? User Plane: Handles ciphering and header compression for user data, like voice or video.

The PDCP layer passes data to and from the RRC layer in the control plane and the GTP-U in the user plane.

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RLC (Radio Link Control):

The RLC layer manages the flow of data between the MAC and PDCP layers. It ensures reliable data transfer and can operate in three modes: Transparent Mode (TM), Unacknowledged Mode (UM), and Acknowledged Mode (AM).

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Key Responsibilities of the RLC Layer:

·???????? Segmentation and Reassembly: Breaks down large SDUs into smaller segments for transmission (segmentation) and reassembles them at the receiving end.

·???????? Error Detection and Correction: In AM, it provides error correction by requesting retransmissions for lost packets.

·???????? Concatenation: Combines multiple SDUs into one Protocol Data Unit (PDU) for efficient transmission.

·???????? Reordering: Ensures packets arrive in sequence, reordering them if necessary.

RLC Modes:

1. Transparent Mode (TM): No header, used for broadcast or multicast messages.
2. Unacknowledged Mode (UM): No retransmission; used for applications where delay is critical (e.g., live video).
3. Acknowledged Mode (AM): Provides retransmissions for error correction, ensuring reliable delivery.

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The RLC layer interacts with the MAC layer for transmitting PDUs and with the PDCP layer for receiving SDUs.

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MAC (Medium Access Control):

The MAC layer, sitting above the PHY layer, is responsible for scheduling and multiplexing data streams into logical channels. It manages how and when the physical layer should transmit data, ensuring efficient use of resources.

Key Responsibilities of the MAC Layer:

·???????? Scheduling: Allocates resources to different users based on scheduling algorithms. Scheduling decisions are dynamic and based on the needs of users and available resources.

·???????? HARQ (Hybrid Automatic Repeat Request): Handles retransmissions to ensure reliable communication. It uses soft combining to improve error correction.

·???????? Priority Handling: Ensures high-priority traffic, such as voice, gets resources ahead of less critical traffic.

·???????? Multiplexing: Maps data from higher layers (RLC) into transport channels and forwards them to the PHY layer.

The MAC layer works closely with the RLC layer, receiving Service Data Units (SDUs) and breaking them down into smaller units for transmission.

PHY (Physical Layer):

The physical layer is responsible for actual transmission and reception of data over the air interface. It transforms digital data into electromagnetic waves and vice versa. LTE employs OFDMA (Orthogonal Frequency Division Multiple Access) for the downlink and SC-FDMA (Single Carrier Frequency Division Multiple Access) for the uplink, ensuring high data rates and spectral efficiency.

Key Responsibilities of the PHY Layer:

·???????? Modulation and Demodulation: Converts bits into signal waveform (modulation) for transmission and back into bits (demodulation) for reception.

·???????? Error Detection and Correction: Uses mechanisms like CRC (Cyclic Redundancy Check) to detect errors and retransmit corrupt data.

·???????? Resource Allocation: Manages frequency and time domain resources based on instructions from higher layers (e.g., MAC).

·???????? Power Control: Adjusts transmission power to improve efficiency and reduce interference.

The PHY layer interfaces directly with the MAC layer, receiving Transport Blocks for transmission.

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