LTE Architecture

The Big Shift

LTE is fundamentally different from GSM/3G. It’s an all-IP network.

Everything - voice, video, messages - becomes data packets. No more circuit-switching.

This is why 4G feels like “mobile broadband” rather than a “phone network with data added on.”

Two Planes

LTE separates all traffic into two distinct paths:

Data Plane (User Plane)

• Carries your actual content - videos, web pages, files
• The highway for user traffic
• Optimized for speed and throughput

Control Plane

• Carries signaling - authentication, mobility, session management
• The management layer
• Optimized for reliability and security

Think of a postal system: the data plane is trucks carrying packages. The control plane is the logistics system deciding where trucks go.

Why Separate Them?

The Network Nodes

LTE has six key nodes. Let’s meet them one by one.

User Equipment (UE)

Your phone, tablet, or any LTE device.

• Connects to the network over radio
• Contains a SIM card with your identity
• Handles all the radio complexity (modulation, MIMO, etc.)

eNodeB (Evolved Node B)

The cell tower - but smarter than before.

In GSM, towers were “dumb” - controlled by a separate BSC. In LTE, the eNodeB makes decisions locally. This reduces latency.

S-GW (Serving Gateway)

The data router inside the LTE network.

• Routes user data packets between eNodeB and P-GW
• Anchor point when you move between towers
• Buffers data during handovers

When you’re driving and switch towers, the S-GW ensures your YouTube video doesn’t stop. It’s the mobility anchor for data.

P-GW (PDN Gateway)

The door to the internet.

Every packet leaving or entering LTE passes through the P-GW. It’s the boundary between “LTE world” and “internet world.”

The Two Gateways

Why two gateways? They have different jobs:

MME (Mobility Management Entity)

The brain of the control plane.

The MME never touches user data. It only handles signaling. This separation is key to LTE’s scalability.

HSS (Home Subscriber Server)

The database of all subscribers.

• Stores your identity (IMSI)
• Stores your subscription details (what services you can use)
• Stores security keys for authentication

When you turn on your phone, the MME asks the HSS: “Is this user legitimate? What are they allowed to do?”

How Everything Connects

The Data Path

When you watch a YouTube video:

1. Video packets come from the internet
2. Enter LTE through the P-GW
3. Route through the S-GW
4. Reach your eNodeB (tower)
5. Transmitted over radio to your UE

The MME and HSS are not involved in this flow. They only handled the initial setup.

The Control Path

When you first connect to LTE:

1. UE sends attach request to eNodeB
2. eNodeB forwards to MME
3. MME asks HSS to authenticate you
4. HSS confirms your identity and subscription
5. MME sets up security and creates your session
6. MME tells S-GW and P-GW to prepare your data path

Then data can flow.

Flat vs Hierarchical

GSM Architecture (Hierarchical)

UE → BTS → BSC → MSC → Gateway

Many layers, many hops, higher latency.

LTE Architecture (Flat)

UE → eNodeB → Gateways

Fewer layers, fewer hops, lower latency.

Why Flat Matters

Real-time apps (video calls, gaming) need low latency. The flat architecture makes this possible.

Key Takeaways

Remember

• Data plane = user traffic (S-GW, P-GW)
• Control plane = signaling (MME, HSS)
• eNodeB = the only node that touches both planes
• All-IP = everything is packets, no circuit switching