LTE Architecture | Teleinf Edu

1. Evolution of cellular network architecture

LTE architecture emerged from the shift of mobile networks from voice-first services to packet data. Each generation increased capability and changed how users communicated, while the arrival of smartphones made a direct, efficient data path more important than another voice-only upgrade.

From voice to data: service evolution

First generation (1G): Analog voice systems. They were limited to voice calls, had poor security, and provided inconsistent quality.
Second generation (2G): Digital voice and data services. Technologies like GSM introduced digital voice calls, which were clearer and more secure. 2G also brought the first mobile data services, such as SMS (text messages), and later GPRS and EDGE, which offered a low data rate suitable for basic text-based emails or simple web pages.
Third generation (3G): Growth of mobile Internet access. With standards like UMTS, 3G was designed to provide faster data speeds, making mobile web browsing, video calls, and music streaming possible at wider scale. However, 3G architecture was complex. It was an evolution of the 2G system that added high-speed data capabilities to a network originally designed for voice calls. This led to higher latency and inefficiencies that became a bottleneck as demand for mobile data rose sharply with the advent of the smartphone.

The Goals of LTE (Long Term Evolution)

LTE designers treated data as the primary service requirement. Applications, video streaming, online gaming, and interactive services needed a network that was fast, responsive, and efficient. The answer was a redesigned architecture called Long Term Evolution, or LTE, designed as a packet-based development of mobile communication.

Higher Data Rates: A significant increase in download and upload speeds to support high-definition video and fast file transfers.
Lower Latency: Reducing the delay between a user action (like clicking a link) and the network's response. This is critical for real-time applications like online gaming and VoIP.
All-IP Network: One of the main architectural changes was designing LTE from the ground up as an . Older systems had separate paths for voice and data. The packet-based model simplifies the network, reduces costs, and improves efficiency.
Flattened Architecture: Reducing the number of network nodes that data has to pass through, which helps lower latency and operational costs.

2. LTE domains: E-UTRAN and EPC

The LTE architecture is logically divided into two main parts. E-UTRAN handles the radio connection to the UE, while EPC handles data processing, routing, and session management. The two domains exchange control and user-plane traffic through standardized interfaces, so their responsibilities can be scaled separately.

The Evolved UTRAN (E-UTRAN): The Radio Access Network.
This part of the network handles the radio connection to the UE. In the basic LTE architecture its main and only node type is the eNodeB. The 'E' stands for 'Evolved', marking its relationship to the 'UTRAN' used by 3G UMTS.
The Evolved Packet Core (EPC): The Core Network.
The EPC is responsible for managing subscribers, routing data packets to and from the internet, ensuring quality of service, and handling user mobility. MME and HSS provide control and subscriber functions, SGW anchors user-plane mobility, and PGW connects to external packet networks. It is a completely packet-switched, All-IP core.

This division allows network operators to evolve the radio part (E-UTRAN) and the core part (EPC) independently. For example, they can introduce new radio technologies without having to completely replace the core network. Modularity is part of the LTE architecture model.

3. E-UTRAN and eNodeB functions

The E-UTRAN has a comparatively flat architecture. Unlike 2G and 3G systems, which separated base stations (BTS/NodeB) from base-station controllers (BSC/RNC), E-UTRAN places the corresponding radio-control functions in the eNodeB.

The eNodeB (Evolved Node B)

The eNodeB is the radio-access node of E-UTRAN. It manages the radio interface directly with the UE and terminates the E-UTRA user-plane and control-plane protocols on the network side. The user-plane terminations include PDCP, RLC, MAC, and PHY, while RRC carries radio control.

Key responsibilities of the eNodeB include:

Radio Resource Management (RRM): The eNodeB is in complete control of the radio resources. It decides which frequencies and time slots to allocate to each user, manages interference between users, and makes decisions about when a user moves from one cell to another.
Radio Bearer Control: Establishes, maintains, and releases the radio connections that carry user data.
Header Compression: To use radio spectrum efficiently, the eNodeB compresses the IP packet headers before sending them over the air.
Ciphering/Deciphering: It encrypts (ciphers) and decrypts user data to ensure the privacy and security of the communication over the radio interface.
Forwarding user data: The eNodeB forwards packets destined for the internet over S1-U towards the Serving Gateway (SGW) in the EPC.

Moving radio-control functions into the eNodeB removes the separate controller hop used in 3G. That shorter path contributes to LTE's lower latency, although transport distance, scheduling, and core-network processing still affect the end-to-end delay.

4. Evolved Packet Core nodes

The EPC is the packet core of the LTE system. It handles subscriber access, session control, mobility, policy, charging, and forwarding beyond the radio connection. Its principal nodes divide those responsibilities between control and user-plane functions.

MME (Mobility Management Entity)

The MME is the main control node in the EPC. It does not handle user data itself; its role covers signalling and session management.

Session Management: It handles the processes of attaching and detaching a user's device to and from the network.
Bearer Management: It is responsible for activating, deactivating, and managing logical that carry user traffic.
Tracking Area Management: It keeps track of the location of idle devices to efficiently page them when there is an incoming call or data.
Authentication and Security: The MME communicates with the HSS to authenticate the user and establish security keys for communication.
Gateway Selection: When a user attaches to the network, the MME selects the appropriate Serving Gateway (SGW) and Packet Data Network Gateway (PGW) for them.

SGW (Serving Gateway)

The SGW is the traffic anchor point for the user plane during inter-eNodeB handovers. All user IP packets are routed through the SGW.

Packet Routing and Forwarding: It routes and forwards data packets between the E-UTRAN (the eNodeBs) and the PGW.
Mobility Anchor: During a handover between cell towers managed by the same SGW, the SGW remains an anchor for the data session and helps preserve it. It is also the mobility anchor during handovers to other technologies like 2G/3G.
Buffering: During paging procedures, when a device is in an idle state, the SGW buffers any incoming data packets for the user until the device is located and activated.

PGW (Packet Data Network Gateway)

The PGW connects the LTE network to external packet data networks, most notably the public internet. It is the entry and exit point for user data.

IP Address Allocation: The PGW is responsible for allocating an IP address to the user's device. This is what the user's device uses to communicate on the internet.
Policy Enforcement: It enforces policy rules provided by the PCRF. This includes Quality of Service (QoS) management, ensuring that different types of traffic get the priority they need.
Packet Filtering: It applies packet-filtering and traffic-handling rules associated with the selected policy. The exact inspection and filtering services depend on the operator's deployment.
Charging Support: It collects charging data (e.g., amount of data used) and sends it to the billing system.

HSS (Home Subscriber Server)

The HSS is the master database for all subscriber information. It is a central repository that holds all the essential data about the network's users.

Subscriber Data: It contains the user's profile, including unique identifiers such as the .
Authentication Information: Stores security keys and authentication vectors used to verify the user's identity.
Location Information: Keeps track of which MME the user is currently registered with, allowing the network to find them.
Service Profiles: Contains information about the services the user is subscribed to (e.g., data limits, call forwarding settings).

PCRF (Policy and Charging Rules Function)

The PCRF is the part of the network that makes policy decisions and sets charging rules. It enables service differentiation and tiered data plans.

QoS Policy: It decides the Quality of Service for each data flow based on the user's subscription, the application being used, and current network conditions. For example, it can prioritize a VoIP call over a background file download.
Charging Rules: It determines how the user's data usage should be billed (e.g., per-megabyte, flat rate, zero-rated for certain services).
Flow-Based Charging: The PCRF enables operators to charge differently for different types of data, such as charging more for video streaming than for simple web browsing.

5. LTE interfaces and protocols

The communication between these different network components is defined by standardized interfaces. Each interface has a specific name and uses specific protocols to exchange information.

Uu Interface: The air interface between the User Equipment (UE) and the eNodeB. This is where radio protocols like OFDMA and SC-FDMA are used.
S1 Interface: This interface connects the E-UTRAN to the EPC. It is split into two logical parts:
- S1-MME: The control plane interface between the eNodeB and the MME for signaling messages.
- S1-U: The user plane interface between the eNodeB and the SGW for carrying user data packets. This separation of control and user data is a key design principle.
X2 Interface: The interface that connects eNodeBs directly to each other. For an X2-based handover, the source eNodeB can exchange signalling and forward user-plane data to the target eNodeB over X2 while the core nodes retain the session context. This shortens the handover path and helps limit interruption and packet loss.
S6a Interface: Connects the MME to the HSS. It uses the Diameter protocol to allow the MME to fetch subscriber data for authentication and authorization.
S11 Interface: The control plane interface between the MME and the SGW. It is used to manage bearers and sessions.
S5/S8 Interface: Connects the SGW and the PGW. It carries user plane data and some control signaling related to bearer management. The interface is called S5 when both gateways are in the same operator's network (home network). It is called S8when the user is roaming, connecting an SGW in the visited network to a PGW in the user's home network.
SGi Interface: The interface between the PGW and external packet data networks, such as the public internet. This is the final gateway for user traffic.