Voice in LTE Networks

1. Introduction to Voice in LTE

Long Term Evolution (LTE) is a purely packet-switched network designed primarily for high-speed data services. Unlike its predecessors (2G and 3G), LTE does not natively support circuit-switched (CS) voice services. This architectural shift created the need for alternative solutions to provide voice communication over LTE networks.

                Key Challenge: LTE is an all-IP network with no circuit-switched domain. Traditional voice services require CS infrastructure, necessitating interworking solutions between the packet-switched (PS) LTE domain and legacy CS networks.
            

1.1 Evolution of Voice Solutions in LTE

Pre-Standard Simultaneous Voice-LTE (SV-LTE)

Early solution using dual radios (LTE for data, CDMA for voice). High battery consumption, not standardized by 3GPP.

3GPP Rel-8 Circuit-Switched Fall Back (CSFB)

Fallback to 2G/3G CS domain for voice calls. First standardized interim solution.

3GPP Rel-8/9 VoLGA & Basic SRVCC

VoLGA using Generic Access Network; SRVCC for handover continuity.

3GPP Rel-10+ VoLTE (Voice over LTE)

IMS-based voice service, the ultimate goal for LTE voice.

1.2 The Voice over LTE Landscape

Several approaches were proposed and standardized to address voice services in LTE:

Circuit-Switched Fall Back (CSFB): Fallback to legacy CS networks before call establishment
VoLGA (Voice over LTE via Generic Access): Tunneling CS services over LTE using GAN framework
SRVCC (Single Radio Voice Call Continuity): Handover mechanism for ongoing calls between LTE and legacy networks
VoLTE (Voice over LTE): Native IMS-based voice service over LTE

2. Circuit-Switched Fall Back (CSFB)

2.1 Overview

CSFB is a mechanism that allows LTE devices to fall back to 2G or 3G circuit-switched networks to originate or receive voice calls. The UE temporarily leaves the LTE network, camps on the legacy network, performs the voice call, and returns to LTE upon call completion.

Core Principle: CSFB enables voice services by redirecting the UE from LTE (PS-only) to GERAN/UTRAN (CS-capable) networks before establishing a circuit-switched call.

2.2 Technical Specification

Standard: 3GPP TS 23.272
Interface: SGs interface between MME and MSC Server
Supported Fallback Targets: GSM (GERAN), UMTS (UTRAN), CDMA2000 1x

2.3 Architecture

+-------------+ LTE/PS +------+ SGs +-------------+ | UE |<--------------->| MME |<-------------->| MSC Server | | (CSFB | +------+ | (2G/3G | | Capable) | | CS Core) | +-------------+ +-------------+ | | | CS Domain (Voice Call) | |<------------------------------------------------------>| | | +-------------+ | | GERAN/ |<-------------------------------------------+ | UTRAN | (Circuit-Switched Voice Path) +-------------+

2.4 CSFB Procedures

2.4.1 Mobile Originating Call (MO-CSFB)

UE camps on LTE network and performs combined EPS/IMSI attach via SGs interface
UE sends Extended Service Request to MME indicating CS fallback
MME initiates fallback procedure (cell selection or handover to 2G/3G)
UE moves to target cell (GERAN/UTRAN) and establishes RR connection
UE sends CM Service Request to MSC
MSC establishes call and voice path is established
Upon call completion, UE returns to LTE (if supported)

2.4.2 Mobile Terminating Call (MT-CSFB)

Incoming call arrives at MSC Server
MSC queries MME via SGs interface for UE location
MME pages UE on LTE network
UE responds with Extended Service Request
MME initiates fallback to CS domain
UE establishes connection in target cell
Call setup continues in CS domain

2.5 SMS over SGs

CSFB also supports SMS delivery without leaving LTE:

MO-SMS: UE sends SMS via SGs interface; MSC forwards to SMSC
MT-SMS: MSC receives SMS, queries MME via SGs, MME pages UE, SMS delivered over SGs

2.6 Fallback Mechanisms

Mechanism	Description	Latency Impact
Redirection	RRC Connection Release with redirection info; UE selects target cell	Higher (no measurement reports)
PS Handover	Handover command to target cell; faster transition	Lower (prepared target)
Cell Change Order (CCO)	Used for GERAN; network orders cell change	Medium

Limitations: CSFB introduces call setup delays (additional 1-3 seconds), requires LTE coverage overlap with 2G/3G, and cannot support concurrent voice and LTE data (unless Dual Transfer Mode is supported in GSM).

3. Voice over LTE via Generic Access (VoLGA)

3.1 Overview

VoLGA was an industry initiative (not standardized by 3GPP but based on 3GPP GAN standards) that aimed to provide CS voice and SMS services over LTE by tunneling CS signaling and media through the LTE packet network to a specialized gateway.

                Foundation: VoLGA is based on the existing 3GPP Generic Access Network (GAN) standard (TS 43.318/44.318), originally developed for WiFi-Cellular convergence (formerly UMA - Unlicensed Mobile Access).
            

3.2 Architecture

The VoLGA architecture introduces a VoLGA Access Network Controller (VANC) that interworks between the LTE packet domain and the CS core network.

+-------------+ LTE +------+ IPsec +------+ | UE |<------------>| eNB |<--------------->| VANC | | (VoLGA | +------+ (Secure |(VoLGA| | Client) | Tunnel) | ANC) | +-------------+ +------+ | | A-interface v +-------------+ | MSC Server | | (CS Core) | +-------------+

3.3 Key Components

VoLGA Client: Software in UE that encapsulates CS signaling (DTAP) and voice frames
VANC (VoLGA Access Network Controller): Gateway that terminates IPsec tunnels, extracts CS signaling, and interfaces with MSC via standard A/Iu interfaces
IPsec Tunnel: Secure tunnel between UE and VANC carrying encapsulated CS traffic

3.4 Operation

UE establishes LTE connection and attaches to EPC
UE discovers VANC address (via DNS or provisioning)
UE establishes IPsec tunnel to VANC (IKEv2 authentication)
VANC performs location update with MSC on behalf of UE
CS signaling (MM, CC, SMS) is tunneled over UP (User Plane) connection
Voice media (RTP/RTCP) is encapsulated and transported over LTE bearer
VANC converts VoLGA format to standard CS A/Iu interface format

3.5 Characteristics

Aspect	VoLGA Approach
Core Network Impact	Minimal - reuses existing MSC, no IMS required
Radio Access	Standard LTE, no modifications needed
Handset Requirements	VoLGA client software required
Service Continuity	Handover between LTE and 2G/3G not standardized
Service Set	Full CS services (Voice, SMS, USSD, Supplementary Services)

Industry Status: VoLGA was primarily promoted by the VoLGA Forum (operators and vendors) as a transitional solution. With the industry convergence on IMS-based VoLTE and the standardization of SRVCC, VoLGA saw limited commercial deployment but remains technically relevant as a case study in access-agnostic voice delivery.

4. Single Radio Voice Call Continuity (SRVCC)

4.1 Overview

SRVCC is a 3GPP-standardized feature that enables seamless handover of an ongoing voice call from the LTE packet-switched domain (VoIP over IMS) to the circuit-switched domain of legacy networks (2G/3G) when LTE coverage is lost. It ensures voice call continuity during mobility scenarios where LTE coverage is not ubiquitous.

Critical Distinction: SRVCC is not a voice service initiation mechanism like CSFB; it is a continuity mechanism for ongoing IMS voice calls that need to transfer to CS domain due to coverage limitations. It requires single-radio UE capability (cannot simultaneously transmit on LTE and 2G/3G).

4.2 Technical Specification

Standard: 3GPP TS 23.216, TS 23.237
Interface: Sv interface between MME and MSC Server
IMS Role: Service Centralization and Continuity Application Server (SCC AS) anchors the call

4.3 Network Architecture

Before SRVCC (VoLTE Call): UE-A (VoLTE) <--> eNB <--> MME <--> EPC (S/P-GW) <--> IMS (SCC AS) <--> UE-B | | (IMS Signaling Anchored at SCC AS) v +---------+ | SCC AS | <-- Session Anchor +---------+ After SRVCC (CS Domain): UE-A (CS) <--> BSS/RNS <--> MSC Server <--> IMS (SCC AS) <--> UE-B | | (Sv Interface) v MME

4.4 SRVCC Evolution (3GPP Releases)

Rel-8 Basic SRVCC

Voice call continuity from E-UTRAN to UTRAN/GERAN. Supports active voice call handover only.

Rel-9 Emergency SRVCC

Support for IMS emergency call continuity to CDMA2000 1xRTT and UTRAN/GERAN CS.

Rel-10 Enhanced SRVCC (eSRVCC & aSRVCC)

eSRVCC: MSC Server-assisted mid-call features (held calls, conference). aSRVCC: Alerting phase handover support.

Rel-11 Video SRVCC & rSRVCC

vSRVCC: Video call continuity. rSRVCC: Reverse SRVCC (CS to PS handover when returning to LTE coverage).

4.5 SRVCC Handover Procedure (E-UTRAN to GERAN/UTRAN)

UE measures neighbor cells and reports to eNodeB; LTE signal weakens
eNodeB decides SRVCC handover needed (based on QCI=1 bearer and neighbor list)
eNodeB sends Handover Required to MME with SRVCC HO indication
MME splits voice bearer (QCI 1) from non-voice bearers
MME sends SRVCC PS to CS Request to MSC Server via Sv interface (includes STN-SR, C-MSISDN)
MSC Server initiates Session Transfer to IMS using STN-SR (Session Transfer Number for SRVCC)
SCC AS in IMS executes Access Transfer: prepares CS access leg, updates remote end
MSC Server prepares CS resources with target BSS/RNS
MSC Server responds to MME with CS handover command info
MME sends Handover Command to eNodeB; eNodeB commands UE to handover
UE tunes to target GERAN/UTRAN cell, establishes RR connection
UE sends Handover Complete; MSC Server connects CS bearer
IMS switches media path from PS to CS domain
Source IMS access leg released; call continues in CS domain

4.6 Key Identifiers

Identifier	Description	Usage
STN-SR	Session Transfer Number for SRVCC	Routing number used by MSC to reach SCC AS for session transfer
C-MSISDN	Correlation MSISDN	Associates different access legs to same UE in SCC AS
ATU-STI	Access Transfer Update - Session Transfer Identifier	Used for rSRVCC (CS to PS return)

4.7 Performance Requirements

Voice Interruption Time

< 300 ms

3GPP Requirement

SRVCC Success Rate

99%

Target Reliability

Call Drop Rate

< 1%

Maximum Acceptable

4.8 SRVCC to CDMA2000 1xRTT

For SRVCC to CDMA networks, the architecture differs:

No Sv interface; instead uses S102 tunnel between MME and 1xRTT IWS (Interworking Solution)
UE communicates directly with 1xRTT MSC via NAS message tunneling through MME
MME acts as signaling relay; IWS interworks 3GPP2 signaling

5. Voice over LTE (VoLTE)

5.1 Overview

VoLTE is the standardized IMS-based solution for delivering voice and communication services over the LTE packet network. It represents the ultimate architecture for voice in LTE, providing high-quality voice services while enabling rich communication services (RCS) and efficient network operation.

                GSMA Standard: VoLTE is defined by GSMA IR.92 (Voice over LTE) and IR.94 (Video over LTE) specifications, based on 3GPP IMS standards.
            

5.2 Architecture Components

VoLTE Architecture: +--------+ +-----+ +------+ +-----+ +--------+ | UE |<--->| eNB |<--->| EPC |<--->| IMS |<---> PSTN | | (VoLTE | +-----+ | +------+ +-----+ | /PLMN | | Client)| | | +--------+ | +------+ | | | | PCRF |<-----------+ | | +------+ | v | +---------+ | | P-CSCF | (Proxy) | +---------+ | | | +---------+ +---------->| S-CSCF | (Serving) +---------+ | +---------+ | TAS/ | (Telephony AS) | MMTel | +---------+ | +---------+ | SCC AS | (for SRVCC) +---------+

5.3 Key Technical Elements

5.3.1 IMS Core Network

P-CSCF (Proxy Call Session Control Function): Entry point for IMS signaling, handles security, compression
S-CSCF (Serving CSCF): Central brain of IMS, handles registration, routing, service triggering
I-CSCF (Interrogating CSCF): Contact point for external networks, queries HSS
TAS/MMTel (Telephony Application Server): Provides telephony services, call control

5.3.2 Voice Codec

Adaptive Multi-Rate Wideband (AMR-WB)
• Sample Rate: 16 kHz
• Bit Rates: 6.60 - 23.85 kbps
• Audio Bandwidth: 50 Hz - 7 kHz (HD Voice)
• 3GPP Mandatory: AMR-WB 23.85 kbps

5.3.3 Bearer Management (QoS)

QCI	Resource Type	Priority	Packet Delay Budget	Packet Error Loss Rate	Service
1	GBR	2	100 ms	10⁻²	Conversational Voice
2	GBR	4	150 ms	10⁻³	Conversational Video
5	Non-GBR	1	100 ms	10⁻⁶	IMS Signaling

5.4 VoLTE Call Flow

UE attaches to LTE network and establishes default bearer (QCI 9)
UE performs IMS registration (SIP REGISTER) via P-CSCF
S-CSCF authenticates user and downloads service profile from HSS
For MO call: UE sends SIP INVITE with SDP offer (AMR-WB codecs)
PCRF establishes dedicated bearer (QCI 1) via RAR/Re-Auth Request
Resource reservation completed; ringing indication sent
Call answered; RTP media flows on dedicated bearer
Upon call end: dedicated bearer released, IMS session terminated

5.5 VoLTE Advantages

Superior Voice Quality: HD Voice (AMR-WB) vs. narrowband in CS (AMR-NB)
Faster Call Setup: ~1-2 seconds vs. 3-6 seconds in CSFB
Spectral Efficiency: Voice packets statistically multiplexed, ~50% spectrum savings
Rich Communication: Native support for video, messaging, file transfer (RCS)
Simultaneous Services: Voice and LTE data concurrently (no fallback needed)
Cost Efficiency: Single all-IP network, no CS infrastructure maintenance

5.6 VoLTE and SRVCC Interworking

VoLTE and SRVCC are complementary technologies:

VoLTE provides the native voice service over LTE
SRVCC ensures continuity when moving out of LTE coverage to 2G/3G
SCC AS anchors the call in IMS, enabling seamless domain transfer
Return SRVCC (rSRVCC) allows handover back to VoLTE when returning to LTE

6. Comparative Analysis

6.1 Technical Comparison Matrix

Feature	CSFB	VoLGA	SRVCC	VoLTE
Domain	CS Domain (Fallback)	CS Domain (Tunnelled)	PS→CS Handover	PS Domain (IMS)
Standard	3GPP Rel-8 (23.272)	VoLGA Forum (GAN-based)	3GPP Rel-8/9/10/11	GSMA IR.92 / 3GPP
Core Network	MSC Server (legacy)	VANC + MSC	MSC + IMS (SCC AS)	IMS Core
Call Setup Time	High (3-6s fallback)	Medium	N/A (mid-call HO)	Low (1-2s)
Voice Quality	Narrowband (CS)	Narrowband (CS)	Maintains quality	HD Voice (AMR-WB)
Data During Voice	No (unless DTM)	Yes (PS)	Yes (if DTM/PS HO)	Yes (Concurrent)
Handset Impact	Single Radio	VoLGA Client	Single Radio	IMS Stack
Network Impact	SGs interface	VANC deployment	Sv interface, MSC upgrade	Full IMS deployment
Service Continuity	N/A (new call)	Limited	Seamless handover	With SRVCC
Use Case	Interim/Legacy	Transitional	Coverage continuity	Target Architecture

6.2 Decision Framework

                Operator Migration Path:

                1. Initial LTE Deployment: CSFB for quick voice launch

                2. IMS Preparation: Deploy IMS core, introduce VoLTE

                3. Coverage Continuity: Implement SRVCC for seamless mobility

                4. Full Migration: VoLTE as primary service, CSFB/SRVCC for gaps

                5. Legacy Sunset: Eventually retire CS domain

7. Summary and Key Takeaways

7.1 Core Concepts

LTE is PS-only: No native CS support requires alternative voice architectures
CSFB: Simple fallback to legacy CS for voice; introduces delay but widely deployed
VoLGA: Access-agnostic tunneling approach; limited commercial adoption
SRVCC: Critical for VoLTE deployment ensuring service continuity in non-ubiquitous LTE coverage
VoLTE: IMS-based target architecture offering superior quality and efficiency

7.2 Critical Success Factors

VoLTE Quality

MOS > 4.0

Mean Opinion Score

SRVCC Handover

< 300ms

Interruption Time

End-to-End

< 150ms

Latency Budget

7.3 Examination Checklist

Key Topics for Study:

Understand the SGs interface in CSFB and its role in combined attach/SMS
Differentiate between CSFB (call establishment) and SRVCC (mid-call handover)
Explain the role of SCC AS in SRVCC and session anchoring
Describe QCI=1 bearer characteristics for VoLTE
Calculate SRVCC interruption budgets and success rate requirements
Compare AMR-NB vs AMR-WB codecs and bandwidth requirements
Explain the Sv interface and STN-SR in SRVCC procedures

7.4 Standards Reference

Technology	Primary 3GPP Specifications
CSFB	TS 23.272, TS 24.301, TS 29.118
VoLGA	TS 43.318, TS 44.318 (GAN basis)
SRVCC	TS 23.216, TS 23.237, TS 29.280
VoLTE	TS 23.228, TS 24.229, TS 26.114, GSMA IR.92