LTE Co-existence with 2G/3G Networks

📚 Introduction

As mobile networks evolve, Long Term Evolution (LTE) must co-exist with legacy 2G (GSM/GPRS/EDGE) and 3G (UMTS/HSPA) networks during the transition period. This study guide covers the fundamental concepts, architectures, and mechanisms enabling seamless interworking between these technologies.

Key Learning Objectives:

Understand the architectural differences between LTE and legacy networks
Master Inter-RAT (Radio Access Technology) mobility mechanisms
Analyze CS Fallback (CSFB) and SRVCC voice continuity solutions
Evaluate network interfaces and protocol stacks for interworking

Evolution Context

The migration from GSM/UMTS to LTE represents a fundamental shift from Circuit-Switched (CS) based networks to all-IP purely Packet-Switched (PS) networks [^12^]. During this transition, operators deploy LTE in various scenarios:

📊

Data-Only LTE

LTE deployed initially for data-only devices (dongles, tablets) with fallback to 2G/3G for voice and SMS.

📱

CS Fallback (CSFB)

Voice calls fall back to 2G/3G CS domain while data remains on LTE. Uses SGs interface between MME and MSC.

🔄

VoLTE + SRVCC

Voice over LTE via IMS with Single Radio Voice Call Continuity for handover to 2G/3G when leaving LTE coverage.

🏗️ Network Architecture

The Evolved Packet System (EPS) consists of E-UTRAN (access) and EPC (core). For interworking with 2G/3G, specific architectural components and interfaces are required [^3^].

Core Network Elements

eNodeB

S1-MME

MME

S6a

HSS

S-GW

S5/S8

P-GW

SGi

PDN

NodeB

Iub

RNC

Iu-PS

SGSN

BTS

Abis

BSC

SGSN

MSC

SGs

MME

(CSFB Interface)

Key Insight: The Serving Gateway (S-GW) acts as the mobility anchor for interworking with other 3GPP technologies such as GSM and UMTS, while the PDN Gateway (P-GW) serves as an anchor allowing seamless mobility to non-3GPP networks [^3^].

Interworking Options

TS 23.401 Annex D Method:

MME appears as an SGSN to the legacy 2G/3G network
All adaptations for interworking are made in the LTE/EPC side alone
No support for Idle Mode Signaling Reduction (ISR)
P-GW acts as the anchor point
Can interwork with pre-Release 8 SGSN

S4 SGSN Method:

S4 SGSN exposes S3 & S4 interfaces to the LTE core
Support for ISR with S-GW as the anchor point
Support for direct tunneling between RNC and S-GW (S12 interface)
Reduces signaling load during idle mode mobility

⚙️ Interworking Mechanisms

1. Idle Mode Signaling Reduction (ISR)

ISR is critical for reducing location update signaling when users move between LTE and 2G/3G coverage areas [^2^]:

UE registers in both RATs

Dual registration active

Only TA/LA changes trigger updates

UE paged on both technologies

Trade-off: ISR reduces location update signaling but increases paging load. Requires common user-plane anchor (S-GW) for dual paging.

2. Circuit Switched Fallback (CSFB)

CSFB enables voice services in LTE by falling back to 2G/3G CS domain [^7^]:

Aspect	CSFB Mechanism	Technical Details
Interface	SGs	Connects MME to MSC, based on Gs interface (TS 23.060)
Paging	MSC → MME	Voice call termination paging forwarded via SGs
SMS	Over SGs	SMS delivery over LTE without CSFB (optional)
Data Handover	S3 Interface	Active PS bearers handed over during CSFB
Call Setup	Retuning	UE retunes to 2G/3G for call setup (adds delay)

Challenge: CSFB increases call setup time due to retuning procedures in 2G/3G radios. Efficient deployment requires careful TAC → LAC mapping to avoid fallback to external MSC/LAC, which further increases setup time [^7^].

3. Single Radio Voice Call Continuity (SRVCC)

SRVCC ensures continuity of active VoLTE calls when moving to 2G/3G coverage [^5^][^6^]:

LTE signal degrades

eNodeB selects target cell

MME triggers SRVCC

Session transfer to IMS

Handover to CS domain

Sv Interface

New interface between MME and MSC Server running GTPv2-C protocol for SRVCC handover signaling.

SCC AS

Service Centralization and Continuity Application Server in IMS manages session transfer procedures.

STN-SR

Session Transfer Number for SRVCC - unique number generated by UE, stored in HSS, used to anchor calls.

SRVCC vs CSFB: CSFB is a service handover procedure (happens before call setup), while SRVCC is a coverage handover procedure (transfers active call). CSFB is used when IMS is not available; SRVCC requires IMS infrastructure [^7^].

🔗 Key Interfaces

Control Plane Interfaces

Interface	Between	Protocol	Purpose
S3	MME ↔ SGSN	GTPv2-C	Idle/active mobility, bearer context transfer
S4	S-GW ↔ SGSN	GTP-U	User plane between EPC and legacy PS core
S5/S8	S-GW ↔ P-GW	GTP	Serving gateway relocation, user plane anchor
SGs	MME ↔ MSC	SGs-AP	CSFB paging, SMS, location updates
Sv	MME ↔ MSC	GTPv2-C	SRVCC handover signaling
S12	RNC ↔ S-GW	GTP-U	Direct tunneling (bypass SGSN for user plane)

User Plane Handling

The S-GW serves as the local mobility anchor for data bearers during inter-RAT handovers. For SRVCC, voice bearers (QCI=1) are separated from non-voice bearers and handed over to the CS domain, while data bearers may continue on PS domain or be suspended [^8^].

📡 Deployment Scenarios

Scenario Analysis Tool

Select deployment scenario to view characteristics:

Quality of Service Interworking

QoS parameters must be mapped between different RATs [^2^]:

EPS Bearers → PDP Contexts

QCI (QoS Class Identifier) mapped to legacy QoS parameters (Traffic Class, Reliability Class)

Guaranteed Bit Rate

GBR bearers in LTE mapped to streaming/interactive traffic classes in UMTS/GPRS

Policy Control

PCRF policies enforced via P-GW must be compatible with legacy policy nodes

Optimization Tip: Efficient TAC (Tracking Area Code) planning is crucial. TAC boundaries should align with LAC (Location Area Code) boundaries to minimize inter-RAT location updates and reduce signaling load during CSFB procedures.

📝 Knowledge Check

1. What is the primary purpose of the SGs interface in LTE-2G/3G interworking?

A) Transfer user plane data between LTE and 3G networks B) Enable paging from MSC to MME for CS Fallback voice calls C) Provide direct tunneling for PS data handover D) Authenticate users in the HSS

2. Which interface is specifically introduced for SRVCC handover signaling?

A) S3 interface B) S4 interface C) Sv interface D) S12 interface

3. What is the main advantage of using ISR (Idle Mode Signaling Reduction)?

A) Reduces location update signaling during spotty LTE coverage B) Increases data throughput during handover C) Eliminates the need for dual registration D) Provides encryption for user plane data

4. In CSFB, what happens to active PS bearers during a voice call?

A) They are immediately dropped B) They are transferred to WiFi C) They may be handed over via S3 interface or suspended D) They continue unchanged on LTE

5. What is the key difference between CSFB and SRVCC?

A) CSFB is for call setup fallback; SRVCC is for active call handover B) CSFB requires IMS; SRVCC does not C) CSFB uses S3 interface; SRVCC uses S4 interface D) CSFB is only for 2G; SRVCC is only for 3G

🎯 Summary

Architecture

S-GW acts as mobility anchor; P-GW as inter-technology anchor. MME interfaces with legacy core via S3/SGs/Sv.

Data Interworking

Inter-RAT handover maintains IP connectivity. ISR reduces signaling but increases paging. Two options: Gn/Gp or S4 SGSN.

Voice Solutions

CSFB for networks without IMS (fallback to CS domain). SRVCC for IMS networks (handover active VoLTE calls).

Key Standards

TS 23.401 (EPC), TS 23.272 (CSFB), TS 23.216 (SRVCC), TS 23.060 (GPRS) [^2^]

References & Further Reading

3GPP TS 23.401: GPRS Enhancements for E-UTRAN Access
3GPP TS 23.272: Circuit Switched (CS) Fallback in EPS
3GPP TS 23.216: Single Radio Voice Call Continuity (SRVCC)
3GPP TS 23.060: General Packet Radio Service (GPRS) Service Description
Afralti LTE Interworking Training Materials