LTE Protocol Stack | Teleinf Edu

1. LTE protocols and layers

The LTE "stack" is a set of cooperating protocols. In LTE, a is a set of rules for communication. Just as humans use grammar and shared vocabulary, network devices use protocols so that data sent from one point can be received and interpreted correctly at another point.

Why use a layered architecture?

A single, monolithic protocol to manage every aspect of communication from radio waves to application data would be complex and difficult to maintain. Modern networks use a layered architecture, often called a protocol stack. This approach divides the communication problem into a series of smaller, manageable tasks, with each layer responsible for a specific function.

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. LTE user plane and control plane

LTE separates the protocol architecture into two distinct "planes." The user plane carries application traffic, while the control plane carries the signalling and management messages that establish, maintain, and release the resources used by that traffic. NAS sits above the radio access protocols and terminates in the EPC MME, whereas RRC terminates at the eNodeB.

The User Plane (U-Plane)

The User Plane carries the user's IP packets between the UE and the packet core. Video, web pages, and file transfers use this path. Its radio protocols are arranged for efficient throughput and controlled latency, while the surrounding bearers provide the required QoS. The U-Plane therefore describes the data being transported.

The Control Plane (C-Plane)

The Control Plane carries the signalling needed to establish, maintain, and release that data path. It handles radio-connection setup, authentication, mobility procedures such as handovers, and bearer configuration. Application IP packets do not use this plane. The C-Plane describes how the network prepares the resources and where the user traffic should be delivered.

The separation gives signalling and user traffic distinct protocol responsibilities and makes each path easier to provision and troubleshoot. It does not remove contention for shared radio or transport resources, but it prevents control procedures from being confused with the payload they manage.

3. The LTE User Plane Protocol Stack

The User Plane stack prepares user IP packets for transmission over the LTE radio interface. Its layers are described from the bottom up.

Layer 1: PHY (The Physical Layer)

This is the lowest layer, responsible for transmitting and receiving coded symbols over the radio interface. It maps transport blocks to physical resources, applies modulation and coding, and recovers the bits delivered by the upper layers.

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.
Role: 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 controls transmission grants on the radio interface. It determines which users transmit and which resources they use.

Scheduling: The MAC scheduler in the eNodeB 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 fast retransmission and error-recovery mechanism at the MAC/physical boundary. The receiving side reports whether a transport block was decoded, and the transmitting side can send a redundancy version again. This local loop is faster than waiting for a higher-layer protocol such as TCP to detect loss through an end-to-end timeout or duplicate acknowledgement.
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 adapts data delivery between PDCP and MAC. Its primary jobs are segmentation and reassembly, ordering, and the selection of one of three delivery modes with different reliability properties.

Segmentation and Reassembly: An RLC service data unit (SDU) from the upper layer can be too large for the transmission opportunity selected by MAC. RLC segments or concatenates SDUs into RLC Protocol Data Units (PDUs), adding sequence information in UM and AM. The receiving RLC entity uses that information to reorder and reassemble the SDUs.
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, reassembly, and reordering without RLC retransmission. Missing or late data is discarded according to the configured procedures, which suits delay-sensitive services such as VoIP and interactive media.
- Acknowledged Mode (AM): Provides acknowledged delivery, error detection, and retransmission requests for lost or corrupted RLC PDUs. This mode is used for services that require data delivery, 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 minimum IPv4 header is 20 bytes, while the IPv6 base header is 40 bytes. A base TCP header is 20 bytes and UDP is 8 bytes, so the IPv4/TCP, IPv6/TCP, IPv4/UDP, and IPv6/UDP combinations are at least 40, 60, 28, and 48 bytes respectively. The base RTP header adds 12 bytes before any CSRC identifiers and extensions, so IPv4/UDP/RTP is at least 40 bytes and IPv6/UDP/RTP is 60 bytes. For a stream with repeated fields, PDCP can use . It sends shorter compressed headers, whose size depends on the profile, changing fields, and extensions.
Ciphering and Integrity Protection: PDCP applies ciphering to user-plane data. In LTE, integrity protection is principally applied to control-plane signalling bearers, where it helps detect unauthorised modification of messages.
Handover Support: During a handover, the PDCP layer helps maintain continuity. 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 manages the connection. Its lower layers resemble the User Plane, while its upper protocols carry radio and core-network signalling.

Lower Layers (PHY, MAC, RLC, PDCP)

The Control Plane also uses PHY, MAC, RLC, and PDCP. In the C-Plane these layers carry signalling messages instead of user IP packets. PDCP provides ciphering and integrity protection for control-plane bearers, while ROHC header compression is associated with user-plane data and is not applied to ordinary RRC signalling.

RRC (Radio Resource Control) Layer

RRC controls the radio interface between the UE and the eNodeB. At the radio-access level, it carries the control procedures needed to configure and maintain that interface.

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

NAS handles signalling between the User Equipment (UE) and the Core Network, specifically the MME in EPC. NAS messages are transparent to E-UTRAN at the protocol level. The eNodeB transports them inside the appropriate RRC and S1 procedures without terminating the NAS protocol.

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. Data encapsulation in LTE

To understand how these layers work together, consider the path of a single data unit as it moves 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: Processing begins with an IP packet generated by an application on the device, such as a 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 for the selected transmission opportunity, RLC segments it into smaller pieces and adds the header information needed for sequence handling and reassembly. In acknowledged mode, status procedures and retransmission provide the error-recovery function. The result is one or more RLC PDUs.
MAC Layer: The MAC layer receives RLC PDUs. Its scheduler selects the transmission opportunity, MAC multiplexes data from logical channels, and a MAC header identifies the contained subheaders and control elements. The resulting MAC PDU is mapped to a transport block for the physical layer, which adds its own CRC and coding.
PHY Layer: The MAC PDU is placed in a transport block and passed to the Physical Layer. PHY adds the transport-block CRC and channel coding (FEC), maps the coded bits to subcarriers, modulates them, and transmits the signal over the Uu interface.

At the receiving end, the process is reversed in decapsulation. PHY decodes the transport block, MAC demultiplexes the logical channels, RLC reorders and reassembles its SDUs, and PDCP restores the packet delivered to the IP layer. Each stage removes or interprets its own control information before passing the remaining data upward.