Behind every mobile data session lies an extraordinarily sophisticated network architecture—a layered system of radio access, transport, and core network elements working in precise coordination. When you stream video on your smartphone, your data traverses multiple network domains: the radio access network converting bits to radio waves, the transport network moving data across distances, and the core network routing your traffic to the internet while managing your identity, security, and mobility.

The architectural evolution from 3G to 4G represents one of the most significant transformations in telecommunications history. 3G's hierarchical, circuit-based architecture gave way to 4G's flat, all-IP design—a fundamental reimagining that reduced latency, simplified operations, and enabled the cloud-connected services we take for granted today.

Understanding this architecture is essential for network engineers, systems designers, and anyone working with mobile services. The patterns established in 3G/4G directly inform 5G architecture and continue influencing next-generation system design.

UMTS architecture comprises three distinct domains: User Equipment (UE), UTRAN (UMTS Terrestrial Radio Access Network), and Core Network (CN). This hierarchical design evolved from GSM, maintaining backward compatibility while adding 3G capabilities.

User Equipment (UE):

The mobile device comprises two logical components:

The USIM separation enables:

• Subscriber portability across devices
• Secure storage of cryptographic keys (IMSI, Ki, authentication algorithms)
• Personal data storage (contacts, SMS)
• Network-independent authentication

UTRAN Architecture:

The radio access network provides the air interface and radio resource management:

Node B (Base Station):

• Radio transmission/reception (RF processing)
• Physical layer processing (spreading, scrambling)
• Inner-loop power control
• Timing synchronization
• Soft handoff combining

RNC (Radio Network Controller):

• Radio resource management (admission, congestion control)
• Outer-loop power control
• Handover decisions and execution
• Ciphering and integrity protection
• Macro-diversity combining across Node Bs
• RLC/MAC processing

Core Network - Circuit-Switched Domain:

Handles voice and legacy circuit services:

Core Network - Packet-Switched Domain:

Handles data services (evolved from GPRS):

Dual Domain Complexity:

This parallel structure required:

• Separate signaling for voice (MSC) and data (SGSN)
• Dual registration procedures
• Coordinated mobility (PS and CS domains tracked separately)
• Complex interworking for services like CS Fallback

UMTS defines precise interfaces between network elements, each with specific protocols for control plane (signaling) and user plane (data).

UTRAN Interfaces:

Core Network Interfaces:

Radio Interface Protocol Stack (Uu):

┌────────────────────────────────────────────────────────────┐
│                     Non-Access Stratum (NAS)               │
│              MM/GMM, CC/SM                                 │
│            (Terminates at MSC/SGSN)                        │
├────────────────────────────────────────────────────────────┤
│                        RRC                                  │
│              Radio Resource Control                         │
│            (Terminates at RNC)                              │
├────────────┬───────────────────────────────────────────────┤
│    PDCP    │      Packet Data Convergence Protocol          │
│            │      Header compression, ciphering             │
├────────────┼───────────────────────────────────────────────┤
│    RLC     │      Radio Link Control                        │
│            │      Segmentation, ARQ                         │
├────────────┼───────────────────────────────────────────────┤
│    MAC     │      Medium Access Control                     │
│            │      Scheduling, multiplexing                  │
├────────────┴───────────────────────────────────────────────┤
│                     Physical Layer                          │
│              WCDMA (spreading, modulation)                  │
└────────────────────────────────────────────────────────────┘

Key Protocol Functions:

RRC (Radio Resource Control):

• Connection establishment/release
• Radio bearer configuration
• Measurement reporting and handover control
• Paging and broadcast
• Security mode configuration

NBAP (Node B Application Part):

• Node B configuration
• Radio link setup/modification/release
• Common transport channel management
• Measurement reporting

RANAP (Radio Access Network Application Part):

• RAB (Radio Access Bearer) management
• SRNS relocation (inter-RNC handover)
• Security mode control
• Paging
• Location services

The Evolved Packet System (EPS) represents a fundamental architectural departure from 3G. Gone is the hierarchical RNC layer; eliminated is the circuit-switched domain. What remains is an elegant, flat, all-IP architecture designed for packet data efficiency.

EPS Architecture Overview:

EPS comprises two primary components:

• E-UTRAN: The evolved radio access network (eNodeBs only)
• EPC (Evolved Packet Core): The all-IP core network

E-UTRAN (Evolved UTRAN):

The radio access network consists solely of eNodeBs (evolved Node B)—intelligent base stations that absorb most RNC functions:

The X2 Interface:

Direct eNodeB-to-eNodeB communication via the X2 interface enables:

• Fast handover without core network involvement
• Inter-cell interference coordination
• Load balancing
• Resource status exchange

Evolved Packet Core (EPC) Elements:

MME (Mobility Management Entity):

The control plane anchor for the access network:

• NAS signaling termination (EMM, ESM)
• Authentication and security (via HSS/AuC)
• Idle-mode mobility (tracking area management, paging)
• Bearer management (establishment, modification, release)
• Selection of S-GW and P-GW
• Inter-MME handover support
• Lawful interception signaling

S-GW (Serving Gateway):

The user plane anchor for intra-LTE mobility:

• Local mobility anchor during handover
• Downlink data buffering while UE is paged
• Packet routing and forwarding
• Transport level marking for QoS
• Accounting functions for charging

P-GW (PDN Gateway):

The anchor point for external network connectivity:

• IP address allocation (static or dynamic)
• Policy enforcement (via PCRF)
• Charging support
• Per-user packet filtering
• Lawful interception
• QoS enforcement
• Interface to external PDN (Internet, IMS)

HSS (Home Subscriber Server):

Evolution of HLR/AuC:

• Master subscriber database
• Authentication vector generation
• Subscription data (QoS profile, APN, restrictions)
• Location information storage

PCRF (Policy and Charging Rules Function):

• Policy decision point
• QoS authorization
• Gating control (allow/deny flows)
• Usage monitoring
• Charging rule provision

LTE defines a comprehensive set of interfaces with standardized protocols for control and user plane communication.

E-UTRAN Interfaces:

EPC Interfaces:

S1-AP Protocol:

S1 Application Protocol manages the E-UTRAN/EPC interface:

Elementary Procedures:

X2-AP Protocol:

Enables direct eNodeB coordination:

• X2 Setup: Neighbor relationship establishment
• Handover Preparation/Cancel: X2 handover execution
• SN Status Transfer: PDCP sequence number synchronization
• Load Information: Inter-cell load sharing
• Resource Status: Interference coordination data exchange

GTP (GPRS Tunneling Protocol):

GTP is fundamental to EPS user plane operation:

GTP-C (Control Plane):

• Session management (create/modify/delete session)
• Bearer management
• Path management
• Uses UDP port 2123

GTP-U (User Plane):

• User data encapsulation
• Tunnel endpoint identification (TEID)
• Sequence numbering
• Uses UDP port 2152

GTP Header Structure:

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Version| PT|   |E|S|PN| Message Type |        Length           |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                         TEID (32 bits)                        |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|    Sequence Number (16 bits)    |N-PDU Number|Next Extension |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

TEID (Tunnel Endpoint Identifier):

The 32-bit TEID uniquely identifies a GTP tunnel endpoint. Each bearer has separate uplink and downlink TEIDs, exchanged during bearer establishment. The receiving entity uses TEID to identify which bearer/user the packet belongs to.

LTE implements QoS through the bearer concept—end-to-end logical channels with guaranteed quality characteristics.

Bearer Types:

Bearer Classification:

QoS Parameters:

Bearer Establishment Flow:

1. UE Attach (Default Bearer):

UE → eNB → MME: Attach Request (with PDN Connectivity Request)
MME → HSS: Authentication Info Request
HSS → MME: Authentication Vectors
MME ↔ UE: Authentication and Security Mode
MME → S-GW: Create Session Request
S-GW → P-GW: Create Session Request
P-GW: Allocates IP, creates default bearer
P-GW → S-GW: Create Session Response
S-GW → MME: Create Session Response
MME → eNB: Initial Context Setup Request
eNB → UE: RRC Connection Reconfiguration
UE ↔ PDN: Data flow established

2. Dedicated Bearer Establishment (Network-Initiated):

PCRF → P-GW: Policy/QoS rules for service
P-GW → S-GW: Create Bearer Request
S-GW → MME: Create Bearer Request
MME → eNB: Bearer Setup Request
eNB → UE: RRC Connection Reconfiguration
UE → eNB: RRC Reconfiguration Complete
eNB → MME: Bearer Setup Response
MME → S-GW → P-GW: Create Bearer Response

Traffic Flow Template (TFT):

TFTs define packet filters mapping traffic to specific bearers:

• Source/destination IP and port
• Protocol type (TCP, UDP, etc.)
• ToS/DSCP
• Flow label (IPv6)

Without TFT match, traffic uses default bearer.

Mobility management ensures continuous service as users move across cells, tracking areas, and networks.

Mobility States:

Tracking Area (TA):

A TA is a group of cells where an idle UE can move without updating location:

• UE registered in one or more TAs (TA list)
• When crossing TA boundary not in list: TAU (Tracking Area Update)
• Network pages across all TAs in list when needed
• TA size balances paging load vs. update signaling

Handover Types:

X2 Handover Procedure (Most Common):

1. UE sends Measurement Report to Source eNB
   (Serving and neighbor cell signal quality)

2. Source eNB decides handover based on measurements
   (A3 event: neighbor better than serving by offset)

3. Source eNB → Target eNB: Handover Request (X2)
   (UE context, bearer info, security context)

4. Target eNB admits UE, prepares resources

5. Target eNB → Source eNB: Handover Request Ack
   (Allocated resources, new C-RNTI)

6. Source eNB → UE: RRC Connection Reconfiguration
   (Target cell config, security key)

7. Source eNB → Target eNB: SN Status Transfer
   (PDCP sequence numbers for seamless handover)

8. UE synchronizes to Target eNB (random access)

9. UE → Target eNB: RRC Reconfiguration Complete

10. Target eNB → MME: Path Switch Request
    (Update S-GW downlink path)

11. MME → S-GW: Modify Bearer Request
    (New eNB address)

12. S-GW → MME: Modify Bearer Response

13. MME → Target eNB: Path Switch Ack

14. Target eNB → Source eNB: UE Context Release (X2)
    (Cleanup resources at source)

Key Performance Metrics:

The architectural journey from 3G to 4G established patterns that continue shaping mobile network evolution. Let's consolidate the key transformations and their implications:

Forward-Looking: 5G Architecture Evolution

The patterns established in LTE directly inform 5G architecture:

• Service-Based Architecture (SBA): Core functions become services with HTTP/2-based interfaces
• Network Function Virtualization (NFV): Network elements run as software on commodity hardware
• Control/User Plane Separation (CUPS): Formalized separation for flexible deployment
• Edge Computing: User plane functions deployed closer to users for ultra-low latency
• Network Slicing: Isolated virtual networks with distinct characteristics on shared infrastructure

What's Next:

The final page explores the Evolution from 3G through 4G to 5G—examining the technological transitions, the migration challenges operators faced, and the continuous improvement that keeps cellular networks advancing.