An Introduction to Voice and SMS in LTE Networks

An Introduction to Voice and SMS in
LTE Networks
Date: Dec 2013

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High Level Architecture Of Mobile Network
Confidential – Restricted Distribution © eXplanoTech Ltd., 2013 3
BSC
BTS
MSC
Voice (PSTN)
Network
SGSN
Data (IP)
Network
RNC
Node BeNodeB
MME GGSN
Access
Network
Core
Network
Air
Interface
MS UEUE

Voice, SMS and LTE
• LTE is an ‘All-IP’ network and has no CS Domain
that can carry CS Voice and SMS.
– Even though in the legacy networks, SMS could be
carried over the PS domain, nearly all networks
always used the CS domain to carry the SMS
• There are two approaches to solve the Voice
and SMS problems:
– Deliver the Voice/SMS as a user plane data to the
network
– Use the legacy CS domain to deliver the Voice and
SMS

Why is Voice and SMS still important
• In 2012, its estimated that combined global
operator revenue was $1000 billion
• On an average, the Voice and SMS still accounts
for between 75-80% of the revenues

Over The Top (OTT) Voice Services
• 3rd party OTT voice services like Skype, Viber, Google
voice, Kakao Talk, etc., are readily available
• These kind of services are often referred to as Voice
over IP networks or VoIP.
• These services use the mobile network as a data pipe
– Can work on any access service like WiFi, etc.
• These are non standardised services, often closed,
using proprietary codecs
• Users are required to install the applications that can
run this service.
• Each system has its own registration and number
assignment that is not reachable using PSTN.

Over The Top (OTT) Messaging Services
• 3rd party OTT messaging services are more common and
easy to implement than voice services.
• Messaging service is no only used as a replacement for
International SMS that are generally expensive but also
as a replacement for MMS, etc. that are equally
expensive.
• Its estimated that OTT messaging costs operators around
$20 billion.
• The most popular service is WhatsApp that had over 250
million users as of Dec. 2012. It handles approx. 10
billion messages per day.
• Apple’s iMessage service announced in Oct. 2012 that
since its debut, 300 billion messages has been sent.

Voice over LTE Generic Access (VoLGA)
 VoLGA was a solution being proposed
before VoLTE was standardised and is not
supported by 3GPP
 It was promoted by an industry initiative
called the VoLGA forum.
 The technique was based on earlier 3GPP
specifications (3GPP TS 43.318), through
which a mobile can reach the 2G/3G core
network through a generic access network
(GAN) such as a wireless local area network.
 Using GAN, the LTE Network could be
viewed as a Generic RAN by 2G/3G network
and use its services
 After the release of VoLTE, this solution has
effectively been killed with nobody working
actively on this.
LTE
Packet
Switching
Service
UTRAN/HSPA
GERAN/EDGE
Circuit
Switching
Service

Simultaneous Voice and LTE (SVLTE)
 This is also known as the Dual Radio Concept
 The UE has two receivers so it can use one for LTE to do data
transfer and use another one for GSM/UMTS voice establishment
 Two radios in the UE increase the cost of the handset
 Having said that, this is a very good and simple solution and can also
allow LTE data rates when voice call is ongoing
 This is the default solution being used in case of LTE/C2K phones.
 Most Verizon phones are dual radio but Verizon has recently
announced that they will only support VoLTE from 2014
 iPhone 5 don’t support SVLTE so a voice call pre-empts data

SVLTE Operation
 Separate registration takes
place for the 2G/3G and LTE
networks
 Each network need not be
aware of what the UE is doing
with the other one.
 Since Voice and Data are
independent one of them does
not impact the other one.
LTE
2G/3G
• 2G/3G registration
• Paging for Voice call
• MO/MT Voice call
• LTE registration
• Paging for Data call
• Data callUE

Circuit Switched Fallback (CSFB)
 CSFB has been designed as an interim solution until the LTE
networks are rolled out completely.
 In case of CSFB, the UE falls back to 2G/3G network when a voice call
/ SMS needs to be initiated / sent or received.
 Since the UE has just one receiver, the data rates after CSFB would
be the data rates offered by the 2G/3G network.
LTE
2G/3G UE
CSFB

CSFB Approaches
Redirection
• When the UE starts the
Voice/SMS call while in
RRC_IDLE state then
Redirection is used to ask the
UE to release the RRC
Connection and Establish a
new RRC Connection using
2G/3G network
Handover
• When the UE is in
RRC_Connected state with
active data transfer on-going
then Handover is performed
to the 2G/3G network to
handover PS RAB’s to the
2G/3G network.
• Mobility from EUTRA
Command message is used to
inform the UE to perform
Handover/CCO

CSFB concept
UTRAN/
GERAN/
CDMA 2000
E-UTRAN
UE
UTRAN/
GERAN/
CDMA 2000
E-UTRAN
UE
Voice service triggered
No voice call

CSFB Architecture
UE
PSTN
MME
MSC
MGW
MSC
MGW
UTRAN
GERAN
HSS
Iu-CS Nb PSTN
PSTN
McMc
DSGs
A / Iu-CS
Traffic
Signalling

Combined Mobility Management for CSFB
TA LA MSC/VLR
TA1 LA1 MSC/VLR #1
TA2 LA2 MSC/VLR #2
TA3 LA2 MSC/VLR #2
TA1 TA2 TA3
LA1 LA2
MSC / VLR #1 MSC / VLR #2
MME

Combined EPS/IMSI Attach Procedure
Location update in CS domain
Attach procedure – part 2
Attach procedure – part 1
Attach Request (Combined EPS/IMSI attach)
SGs-AP: Location Update Request
SGs-AP: Location Update Accept
UE MME MSC HSS

MO Voice Call Setup - Handover
Handover to UTRAN with PS RAB
UL Information Transfer [EMM: Extended Service Request]
S1-AP: Information Transfer[EMM: Extended Service Request]
S1-AP: UE Context Modification Request(CSFB Indicator, LAI)
S1-AP: UE Context Modification Response
Mobility from EUTRA Command
UE MME MSCeNB

MT Voice Call Setup – Redirection
Approach 1
RRC Reconfiguration
AS Security activation
Handover to UTRAN
Measurement Report
S1-AP: Initial UE Message [EMM: Extended Service Request]
Paging and RRC Setup procedure
RRC Connection Release (Redirected Carrier Info)
UE MME MSCeNB

MT Voice Call Setup – Redirection
Approach 2
S1-AP: Initial UE Message [EMM: Extended Service Request]
Paging and RRC Setup procedure
RRC Connection Release (Redirected Carrier Info)
UE MME MSCeNB

System Information (SI) Tunneling
RRC Connection Release
Release Cause, Redirected Carrier Info (O), Idle Mode Mobility Control Info (O)
RRC Connection Release
Release Cause, Redirected Carrier Info (O), Idle Mode Mobility Control Info (O),
Cell Info List R9 - CHOICE {Geran, Utra-FDD, Utra-TDD} (O)
Release-8
Release-9
Physical Cell Id
UTRA BCCH Container
MIB
SIB1
SIB3
SIB5
SIB7
SIB11, SIB11bis, SIB12
UE eNB
Utra-FDD

CSFB Mobility Options
No Mobility Option (Related RRC Messages) Target RAT Release UE Capability
1 Redirection (RRCConnectionRelease) UTRAN Release 8 Mandatory for UEs
supporting CSFB to the UTRAN
2 Redirection with system information
(RRCConnectionRelease)
UTRAN Release 9 Indication of the
e-RedirectionalUTRA in the UE capability
3 Redirection (RRCConnectionRelease) GERAN Release 8 Mandatory for UEs
supporting CSFB to the GERAN
GERAN Release 9 Mandatory for UEs
UTRAN Release 8 Mandatory for UEs
supporting CSFB to the UTRAN
6 PS handover with DRB(s)
(MobilityFromEUTRACommand)
UTRAN Release 8 Indication of the
interRAT-PS-HO-ToGERAN in the UE
capability
7 Cell change order with the NACC
8 Cell change order without the NACC

IMS Based Voice over IP (VoIP)
VoIP bearer
Data bearer
eNB
UE
Internet

Comparison between IMS VoIP and CSFB
Comparison Class Comparison Factor IMS VoIP CSFB
Performance Call setup delay Comparable or shorter than
native call setup delay
Longer than native call setup
delay
Concurrent voice and data during
voice call
Yes Yes/No (dependence on target)
Maintaining LTE data rate during
voice service
Yes No
Service Support of advanced services Yes No
UE complexity Features to be supported Medium complexity
(IMS client, SRVCC)
Low complexity (CSFB-related
enhancement)
NW operational benefit/cost New network equipment upgrade Yes (IMS core) None, but small upgrades to
MME
Legacy network equipment
upgrade
Limited Yes (all MSCs overlaid on LTE
networks)
Coverage engineering between
LTE and legacy coverage
Not required Required
All IP structure Yes No
Interoperability issue Standard compliance 3GPP compliant 3GPP compliant

‘One Voice’ profile of IMS SIP
• In 2010 GSMA selected ‘One Voice Profile of IMS SIP’ for voice service and SMS
• The work scope includes:
– One Voice work + end-to-end voice and SMS ecosystem
– One Voice profile
– IMS basic capabilities and supplementary services for telephony
– Real-time media negotiation, transport, and codecs
– LTE radio and evolved packet core capabilities
– Roaming and interconnect interfaces also included as work scope
Bearers / QoS RoHC
TCP/IP – UDP/IP TCP/IP – UDP/IP TCP/IP – UDP/IP
Codecs
Suppl
services
SIP
HTTP/
XCAP
RTP / RTPC
Codecs
Suppl
services
SIP
HTTP/
XCAP
RTP / RTPC
Bearers / QoS RoHC
LTE
With VoIP optimizations
LTE
With VoIP optimizations
Mobile Device Radio Access Network IMS Servers

IP Multimedia Subsystem (IMS) Architecture
MME
E-UTRAN
PDN
S-GW P-GW
PCRF
Other
IMS
PSTN
HSS
LTE Signalling
IMS Signalling
IMS Traffic
S6a
SGi
SGi
SGi
S5 / S8
GxS11
S1-U
S1-MME

IP Multimedia Subsystem (IMS) Architecture
MME
E-UTRAN
PDN
S-GW P-GW
PCRF
Other
IMS
PSTN
HSS
LTE Signalling
IMS Signalling
IMS Traffic
S6a
SGi
SGi
SGi
Rx
S5 / S8
GxS11
S1-U
S1-MME

UICC and ISIM
2G SIM UMTS SIM (USIM)
IMSI MSISDN
SMS Data Address Book
IMSI
MSISDN
MSISDN
MSISDN
Authentication
Data and Keys
Multimedia
Messaging
Config Data
IMS SIM (ISIM)
Security Keys
Home Network Domain
Name (URI)
Private User Identity
y
Public User Identity
Administrative Data
Access Rule Reference
Address of P-CSCF

‘Attach Request’ including IMS
RRC Connection Setup Complete
S1-AP: Initial UE Message
(Attach Request + PDN Connectivity Request )
RRC Connection Request
RRC Connection Setup
(Attach Request + PDN Connectivity Request )
UE IP Address Requested
+ P-CSCF address
requested
UE MMEeNB

Attach Request Message

Attach Request Message
Voice Domain Preference

PDN Connectivity Request
VoLTE UE should request IPv4v6
Atleast 2 elements (P-CSCF IPv4
address request, P-CSCF IPv6
address request) and maybe DNS
Server IP addresses may be
requested

Attach Accept and Complete
New MME
UL Information Transfer
IMS CN
(Attach Complete + Activate Default EPS Bearer
Context Accept )
RRC Connection Reconfiguration
RRC Connection Reconfiguration Complete
N/w support for IMS
voice over PS session
indicated
(Attach Accept + Activate Default EPS Bearer Context Request )
UE IP address can be
provided + P-CSCF IP
address(es) provided
IMS REGISTRATION PROCEDURE
If IP address was not
assigned then UE has to
obtain it dynamically
here
UE eNB

Attach Accept

Attach Accept
EPS Network Feature Support
IMS voice over PS session
in S1 mode supported

Activate default EPS bearer context request
UE IP Address
IMS APN
QoS required for IMS SIP
signalling

IMS Registration
REGISTER
401 Unauthorized (Authentication Challenge)
Unprotected
Client port
Unprotected
Server port 5060
Unprotected
Server port 5060
Unprotected
Client port
Verify AUTN & compute RES
IPSec SA Establishment
REGISTER
200 OK (REGISTER)
Port_uc
Port_uc; spi_uc
Port_ps
Port_ps; spi_ps
SUBSCRIBE
200 OK (SUBSCRIBE)
Port_uc
Port_uc; spi_uc
Port_ps
Port_ps; spi_ps
NOTIFY
200 OK (NOTIFY)
Port_uc
Port_uc; spi_uc
Port_ps
Port_ps; spi_ps
IMS CNUE

Mobile Originated VoLTE call establishment (1)
(Activate Dedicated EPS Bearer Context Accept)
(Activate Dedicated EPS Bearer Context Request ) Network initiates adding
dedicated bearer for
voice on the existing IMS
PDN
UECalled UE
INVITE
100 Trying
INVITE
183 Session Progress
PRACK
PRACK
200 OK (PRACK)
200 OK (PRACK)
IMS CNUE eNB
Voice Call
Establishment

Radio Resource Config Dedicated
Should be UM bidirectional
Next Slide

PDCP-Config
‘rohc’ is selected
Profile1 = true for RTP/UDP/IP (to
compress the RTP packets)
Profile2 = true for UDP/IP (to
compress RTCP packets)

Activate Dedicated EPS Bearer Context Request
TFT would depend on media and
bandwidths agreed during voice
call setup.
QoS for the voice call. Generally
QCI 1. Bit rates would depend on
media and bandwidths agreed
during voice call setup.

Mobile Originated VoLTE call establishment (2)
180 Ringing
180 Ringing
PRACK
PRACK
200 OK (PRACK)
200 OK (PRACK)
200 OK (INVITE)
200 OK (INVITE)
ACK
ACK
Called UEIMS CNUE eNB
Right back
tone
Ringing
Answering
Voice call started

Mobile Originated VoLTE call release
(Deactivate EPS Bearer Context Accept)
(Deactivate EPS Bearer Context Request )
BYE
BYE
200 OK
200 OK (BYE)
Voice call Ongoing
Voice call release
triggered

Mobile Terminated VoLTE call establishment (1)
(Activate Dedicated EPS Bearer Context Accept)
INVITE
100 Trying
INVITE
PRACK
PRACK
200 OK (PRACK)
200 OK (PRACK)
Paging and Service Request
INVITE
(Activate Dedicated EPS Bearer Context Request )
Voice call
establishment

Mobile Terminated VoLTE call establishment (2)
180 Ringing
180 Ringing
PRACK
PRACK
200 OK (PRACK)
200 OK (PRACK)
200 OK (INVITE)
200 OK (INVITE)
ACK
ACK
Voice Call Started
Ringing
Answering
Ring-back
tone

Mobile Terminated VoLTE call release
(Deactivate EPS Bearer Context Accept)
(Deactivate EPS Bearer Context Request )
BYE
BYE
200 OK
200 OK (BYE)
Voice call Ongoing
Voice call release
triggered

VoLTE power consumption issues
• VoLTE is not yet in mass market deployments mainly due to the
power consumption being too high at the moment.
• The power consumption issues can be solved by two features:
– Semi persistent scheduling (SPS)
– TTI Bundling

Semi-Persistent Scheduling (SPS)
Dynamically Scheduled PRBs
BFNmod4=0 BFNmod4=1 BFNmod4=2 BFNmod4=03
0ms 10ms 20ms 30ms 40ms
HARQ
Process
UEID
PR
B
0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7
0
a
0
b
0
a
0
a
0
b
0
b
0
b
0
a
0
b
First Transmission
1st ReTx
2nd ReTX
3rd ReTx
4th ReTx
0
a

TTI Bundling
Retransmissions
RLC SDU
RLC PDU
Transport block
TTI #
HARQ
process #
CRC
Coding
RM
Not used for
this processNACK
ACK
Segmentation
RV0 RV1 RV2 RV3
RV0 RV1 RV2 RV3
0
0
1
0
2
0
3
0
4
4
5
5
6
6
7
7
8
0
9
1
10
2
10
2
11
3
12
4
13
5
14
6
15
7
16
0
17
0
18
0
19
0

SMS over IMS
E-UTRAN
UE
 Details in 3GPP TS 23.204

VoLTE and SRVCC
E-UTRAN
UTRAN GERAN
IMS
Client
UE
Telephony
server
Rich
Communication
Suites
SMS
server
MGW
IMS CORE
 Access Network
 IMS VoIP over LTE
 IMS VoIP over 2G/3G PS
 CS Voice after SRVCC
 IMS core and Applications

Single Radio Voice Call Continuity (SRVCC)
UE
E-UTRAN
UTRAN GERAN
HSS
S-GW PGW
MSC
MME
SGSN
SV
S3

Voice path before and after SRVCC
UE
E-UTRAN
UTRAN GERAN
HSS
S-GW P-GW
MSC
MME
SGSN
SV
S3
UE
E-UTRAN
UTRAN GERAN
HSS
MSC
SV
MME
SGSN
S3
SRVCC
PStoCS
S-GW P-GW

SRVCC evolution path
Functionality 3GPP Release Description
Basic SRVCC Rel-8 One-way Handover of a single active speech session from PS
to CS systems.
Supplementary Services Rel-9 Continuation of Supplementary Services (including mid-call
services) when the UE goes through the SRVCC Handover.
Emergency Rel-9 SRVCC Handover for IMS emergency sessions
aSRVCC Rel-10 SRVCC PS-CS access transfer of media of an IM session in
alerting phase
eSRVCC Rel-10 Various SRVCC enhancements in the network, no impact on
the UE
vSRVCC Rel-11 SRVCC for video call
rSRVCC Rel-11 reverse SRVCC HO from CS call to PS IMS

Summary of the LTE ‘Voice’ issues
2012 2013 2014 2015+ 2020+
IMS VoIP : Evolving to a Richer Carrier Grade Voice Service
2G/3G
Provides Voice
WCDMA
1X
WCDMA+
1X Advanced
VoLTE
VoIP over HSPA+ (EV-DO)Seamless
Transitions CSFB1 SRVCC2
Packet
Handover
Circuit Switched Voice: Evolving To Support More Data
1CSFB: Circuit Switched Fallback. 2SRVCC: Single Radio Voice Call Continuity to enable handover to WCDMA/1X/GSM CS voice outside VoLTE coverage.

17 possible official LTE Voice modes
1x
SVLTE
UMTS
CSFB
(FDD)
GSM
CSFB
(FDD)
1x
CSFB
(FDD)
UMTS
CSFB
(TDD)
GSM
CSFB
(TDD)
TDS
CSFB
(TDD)
VoLTE
(FDD)
VoLTE
UMTS
SRVCC
(FDD)
VoLTE
GSM
SRVCC
(FDD)
1x
CSFB
(TDD)
VoLTE1x
SRVCC
(TDD)
VoLTE
(TDD)
VoLTE1x
SRVCC
(TDD)
VoLTE
UMTS
SRVCC
(TDD)
VoLTE
GSM
SRVCC
(TDD)
VoLTE
TDS
SRVCC
(TDD)

An Introduction to Voice and SMS in LTE Networks

An Introduction to Voice and SMS in LTE Networks

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