LTE Protocol Stack

User plane and control plane protocol stack in LTE.

1. The Language of the Network: Understanding Protocols and Layers

Before we can understand the specific "stack" of protocols used in LTE, we first need to understand what a protocol is and why communication systems are organized into layers. At its core, a is simply a set of rules for communication. Just as humans use grammatical rules and shared vocabulary to understand each other, network devices use protocols to ensure that data sent from one point can be correctly received and interpreted by another.

Why Use a Layered Architecture? The Protocol Stack

A single, monolithic protocol to manage every aspect of communication from radio waves to application data would be incredibly complex and impossible to manage. Instead, modern networks use a layered architecture, often called a protocol stack. This approach divides the immense challenge of communication into a series of smaller, more manageable problems, with each layer responsible for a specific set of tasks.

Each layer in the stack provides services to the layer directly above it and uses services from the layer directly below it. This creates a modular and flexible system with several key advantages:

Simplicity and Modularity: Each layer can be designed and updated independently without affecting the others. For example, engineers can improve the physical radio technology (Layer 1) without having to rewrite the software that manages web browsing (Layer 7).
Standardization: Layering allows for the creation of clear standards for each function. This means equipment from different manufacturers can work together seamlessly, as long as they adhere to the same protocol rules for each layer.
Troubleshooting: When a problem occurs, it can be isolated to a specific layer, making it much easier to diagnose and fix.

2. The Two Worlds of LTE: User Plane and Control Plane

A crucial concept in the LTE architecture is the strict separation of the protocol stack into two distinct "planes." This division optimizes the network by separating the actual flow of your data from the signaling and management messages that control the network.

The User Plane (U-Plane)

Think of the User Plane as the data highway. Its one and only job is to transport the user's actual data packets (IP packets) as quickly and efficiently as possible. This is the path your video stream, web pages, and file downloads travel. The protocols in this plane are optimized for high throughput, low latency, and efficient data handling. The U-Plane deals with the "what" you are sending.

The Control Plane (C-Plane)

If the User Plane is the highway, the Control Plane is the entire traffic management system. It is responsible for setting up, maintaining, and tearing down the data highway. It handles all the signaling messages needed to manage the connection, such as establishing the radio link, authenticating the user, managing mobility (handovers), and setting up the data bearers. No user data travels here, only network management instructions. The C-Plane deals with the "how" and "where" your data should be sent.

This separation ensures that a massive surge in signaling messages (for example, many users connecting to a cell at once) does not slow down the actual data flow for already connected users, and vice versa. It is a cornerstone of LTE's robust and efficient design.

3. The LTE User Plane Protocol Stack

The User Plane stack is designed for speed and efficiency. It is a lean set of protocols that prepares user IP packets for their journey over the airwaves. Let us examine the layers from the bottom up.

Layer 1: PHY (The Physical Layer)

This is the lowest layer, responsible for the actual transmission and reception of raw bits over the radio interface. It takes the data prepared by the upper layers and converts it into radio waves.

Functions: Modulation and demodulation (e.g., QPSK, 16-QAM, 64-QAM), channel coding for error correction (FEC - Forward Error Correction), and implementing the OFDMA (downlink) and SC-FDMA (uplink) transmission schemes.
In essence: The PHY layer is the hardware and signal processing that makes wireless communication physically possible. It deals with frequencies, power levels, and timings.

Layer 2: The Data Link Layer (Subdivided)

In LTE, Layer 2 is more complex and is subdivided into three distinct sublayers, each with specific tasks: MAC, RLC, and PDCP.

MAC (Medium Access Control) Sublayer

The MAC layer acts as the primary traffic controller for the air interface. It manages which users get to transmit and when.

Scheduling: The MAC scheduler in the eNodeB is one of the most critical components of the entire LTE system. It dynamically allocates radio resources (blocks of subcarriers and time slots) to different users based on their data needs, QoS requirements, and current radio conditions.
Hybrid ARQ (HARQ): HARQ is a very fast and efficient error correction mechanism. If a packet received over the air is corrupted, the HARQ process at the MAC layer immediately requests a retransmission from the other end. Doing this at the lowest possible layer is much faster than waiting for a higher-layer protocol like TCP to notice the error and request a retransmission from a server on the internet.
Multiplexing: It takes data from different logical channels (e.g., voice, video, web browsing for the same user) and combines them into transport blocks to be sent to the Physical Layer.

RLC (Radio Link Control) Sublayer

The RLC layer acts as a reliable data delivery service for the layers above it. Its primary job is to handle segmentation and reassembly, and to provide different levels of reliability.

Segmentation and Reassembly: An IP packet arriving from the top can be too large to fit into a single transmission block determined by the MAC layer. The RLC layer chops the large packet into smaller RLC Protocol Data Units (PDUs) and adds sequence numbers. The RLC layer at the receiving end uses these sequence numbers to reassemble the segments back into the original IP packet.
Operational Modes: The RLC layer can operate in three modes to suit different types of services:
- Transparent Mode (TM): No overhead. Data is passed through without segmentation or error correction. Used for services that manage their own reliability, like some voice transmissions.
- Unacknowledged Mode (UM): Provides segmentation and reassembly but does not guarantee delivery. It doesn't ask for retransmissions of lost segments. This is ideal for real-time services like VoIP or online gaming, where receiving slightly corrupted data late is worse than not receiving it at all.
- Acknowledged Mode (AM): Provides full reliability with error detection and retransmission requests for lost or corrupted RLC PDUs. This mode is essential for services that cannot tolerate any data loss, such as file downloads and web browsing.

PDCP (Packet Data Convergence Protocol) Sublayer

PDCP is the topmost sublayer of Layer 2. It is primarily responsible for processing IP packets and making them more efficient and secure for radio transmission.

IP Header Compression: The headers of IP packets (especially for TCP and UDP) are quite large, often 40-60 bytes. For small data packets (like in VoIP), this overhead is significant. PDCP uses protocols like to shrink these headers to just a few bytes, dramatically improving the efficiency of the radio interface.
Ciphering and Integrity Protection: The PDCP layer is where user data is encrypted (ciphered) for security. It also adds an integrity check to ensure that the data has not been maliciously altered in transit.
Handover Support: During a handover, the PDCP layer plays a key role in ensuring a seamless transition. It reorders packets that may arrive out of order during the handover and manages the forwarding of data from the old eNodeB to the new one to prevent data loss.

4. The LTE Control Plane Protocol Stack

The Control Plane stack is responsible for managing the connection. Its lower layers are similar to the User Plane, but at the top, it has unique protocols designed for signaling and control.

Lower Layers (PHY, MAC, RLC, PDCP)

The Control Plane also uses the Physical, MAC, RLC, and PDCP layers. However, instead of carrying user IP packets, these layers in the C-Plane are dedicated to carrying signaling messages. For instance, the PDCP layer in the C-Plane provides integrity protection for critical signaling messages but does not perform header compression (as signaling messages don't have IP headers to compress).

RRC (Radio Resource Control) Layer

The RRC is the master controller of the radio interface between the UE and the eNodeB. It is the most important protocol in the Control Plane at the radio access level.

Key RRC functions include:

Broadcasting System Information: The eNodeB constantly broadcasts essential system information (in messages called SIBs, System Information Blocks) that allows devices to connect to the cell.
Connection Management: Handles the establishment, maintenance, and release of the RRC connection between the UE and the eNodeB.
Paging: Notifies idle devices of incoming calls or data.
Mobility Management: Manages the entire handover procedure, including instructing the UE to take measurements of neighboring cells and commanding it to switch to a new cell.
Security Management: Configures the encryption and integrity protection in the lower layers (PDCP).

NAS (Non-Access Stratum) Protocols

The NAS layer handles signaling communication directly between the User Equipment (UE) and the Core Network (specifically, the MME). The crucial aspect of NAS is that its messages are transparent to the E-UTRAN (the eNodeB). The eNodeB simply acts as a "postman" for these messages, passing them along without reading or interpreting them.

NAS protocols are responsible for functions that need to be maintained even as the user moves between eNodeBs, such as:

Mobility Management (EMM - EPS Mobility Management): This includes procedures for attaching to the network, authenticating the user with the HSS, and managing tracking area updates.
Session Management (ESM - EPS Session Management): This handles the creation and management of user data sessions, such as requesting a default IP bearer to get connected to the internet.

5. The Flow of Data: Encapsulation

To understand how these layers work together, it is useful to follow the journey of a single piece of data as it travels down the protocol stack at the transmitting end. This process is called encapsulation. Each layer adds its own control information, in the form of a header, to the data it receives from the layer above.

IP Packet: The journey begins with an IP packet generated by an application on your device (e.g., your web browser requesting a page).
PDCP Layer: The PDCP layer takes the IP packet. It performs header compression (if configured) and encrypts the packet. It then adds a PDCP header containing a sequence number. The result is a PDCP PDU.
RLC Layer: The RLC layer receives the PDCP PDU. If the PDU is too large, the RLC layer segments it into multiple smaller chunks. It adds an RLC header to each chunk, containing information for reassembly and error correction. The result is one or more RLC PDUs.
MAC Layer: The MAC layer receives the RLC PDUs. The scheduler decides when to transmit this data. It multiplexes RLC PDUs from different logical channels and adds a MAC header. The MAC header includes scheduling information and details for the HARQ process. The final unit is a MAC PDU, also known as a Transport Block.
PHY Layer: The MAC PDU (Transport Block) is passed to the Physical Layer, which adds channel coding (FEC), modulates the data onto the subcarriers, and transmits it as a radio signal over the Uu interface.

At the receiving end, the process is reversed in a process called decapsulation. Each layer strips off its corresponding header, processes the information, and passes the remaining data up to the layer above it, until the original IP packet is finally delivered to the application. This orderly process ensures that complex communication can happen reliably and efficiently.