VoLTE flows - basics

realtimecommunication.wordpress.com
VoLTE Flows
Introduction v2
UASUAC
SIP INVITE (SDP-O)
Terminating
Network
Originating
Network
P-CSCF
S-CSCF
I-CSCF
A-SBC
O-TAS IPSMGW
DNS
HSS
DNS
HSS
P-CSCF
S-CSCF
I-CSCF
T-TAS
A-SBC
AGWAGW
realtimecommunications.wordpress.com

VoLTE Deployments
https://www.gsma.com/futurenetworks/resources/all-ip-statistics/
3 0
12
33
56
16
0 0
1
16
35
3
8 20
12
7
3
3
0
10
20
30
40
50
60
70
80
90
100
2012 2013 2014 2015 2016 2017
ALL-IP DEPLOYMENTS
VoLTE VoWifi RCS
3 0
12
33
56
19
0
0
1
16
35
4
8
20
12
7
3
4
0
10
20
30
40
50
60
70
80
90
100
2012 2013 2014 2015 2016 2017
ALL-IP DEPLOYMENTS
VoLTE VoWifi RCS

What is VoLTE?
LTE
Tel: num1 Tel: num2
GSM
Tel: num3
• VoLTE stands for Voice over LTE. LTE is a new standard for wireless communication
of high-speed data for mobile phones. ViLTE means Video over LTE.

What is in the LTE?
4
LTE
• A packet-switched all-IP network which provides data connectivity and mobility.
• LTE access network doesn’t provide any voice, video or text services. This is to be handled by
IMS.
e-nodeB
e-nodeB
EPC
MME SGW
PGW
E-UTRAN
HSS
UE
• We will not discuss LTE RAN and EPC in this training.

IMS
5
LTE
• VoLTE is based on the IP Multimedia Subsystem (IMS) network – a framework designed for
delivering IP multimedia services.
• IMS is access independent, user-centric network providing services such as authentication,
session routing and management, QoS, roaming, provisioning, charging etc.
LTE
IMS

4G
IMS
P-CSCF
S-CSCF
I-CSCF
A-SBC
TAS IPSMGW
DNS
HSS
ALG/AGW
MRF
AP
SGW
PGW
DNS
HSS
ePDG
2G / 3G
AAA
PCRF
DSC
MGCF
MSC
MGW
TWAG
Wifi MME
Internet
ePC
WLAN
E-CSCF
BGCF
LRF

4G Ecosystem
• The Fourth Generation of Mobile Networks is based on IP protocol. From Operator point of view it is a
huge change
• New Radio Network
• New Enhance Packed Core
• New Service Network – IMS
• New Protocols, Flows, KPIs, Vendors, New Issues
• 4G is user centric and supports multiple access networks
• We can connect to IMS using any IP-based access network
• A-SBC provides connectivity, security, NAT/NAT traversal, media handling etc.
• IMS is integrated also with the legacy (Sigtran) network
• VoWifi (GSMA IR.51) provides the same set of services as VoLTE and is integrated using EPC.
• RCS UP is integrated with VoLTE/VoWifi.

IP-based Communication
• Separates Signaling (SIP, H.323, XMPP, REST, ..) and Media (RTP, MSRP, WS, HTTP, ..) flows.
• The purpose of Signaling is to
• Locate the recipient
• Share presence information
• Exchange the parameters of the multimedia session
• Binding between IP and User Identity is created during Registration.
• One user can use multiple different identities and devices/sw clients.
For VoLTE
• SIP is used for end-to-end signaling, RTP for Voice/Video
• Public User Identity (IMPU) is based on telephone number can be e.g.
• sip: +<number>@volte.ims.mnc012.mcc345.3gppnetwork.org
• tel: +<number>
• Binding between actual IP assigned by LTE (PGW) and IMPU is stored in IMS Network (S-CSCF)

What is Signalling?
LTE
IP2IP1
Tel: num1 Tel: num2
IMS
WLAN
num1

What is ?
• The standard is defined by GSMA in IR.92 and IR.94 as a minimum mandatory set of features for an interoperable,
high quality IMS-based telephony service and SMS over LTE network.
The scope includes:
• IMS basic capabilities and supplementary services for telephony
• Real-time media negotiation, transport, and codecs
• LTE radio and evolved packet core capabilities
• Functionality that is relevant across the protocol stack and subsystems
• Additional features that need to be implemented for the UEs and networks that wish to support concurrent Circuit
Switched (CS) coverage
Description of basic call flows and procedures can be found in VoLTE Service Description and Implementation Guide.

IMS
LTE
• VoLTE is based on the IP Multimedia Subsystem (IMS) network – a framework designed for
delivering IP multimedia services.
• IMS is access independent, user-centric network providing services such as authentication,
session routing and management, QoS, roaming, provisioning, charging etc.
IMS

3GPP IMS

IMS Simplified IMS
P-CSCF
S-CSCF
I-CSCF
A-SBC
TAS IPSMGW
DNS
HSS
ALG/AGW
MRF
2G / 3G
MGCF
MSC
MGW
ePC
BGCF

IMS Entities I
Proxy - Call Session Control Function (P-CSCF)
• First point of contact for SIP signaling within IMS network
• Can be located in a visited as well as in a home network
• Often deployed as a part of Access Session Border Controller (A-SBC)
• Authorizes media plane resources, interacts with access network via PCRF
• May encrypt communication using IPSec or TLS sessions
Interrogating-CSCF (I-CSCF)
• Contact point in the home network
• The most important functionality is to provide location service - localization of an S-CSCF based on received
name or capabilities from the HSS for a particular subscriber
Serving-CSCF (S-CSCF)
• SIP server responsible for registration and routing within IMS network
• Along with P-CSCF assigned to user for the whole duration of registration
• Based on service profile received from HSS triggers ASs, maintains AS associations

IMS Entities II
Telephony Application Server (TAS)
• Application server is responsible for telephony services which can be applied
• Provides address normalization, call diverting, call forwarding, barring, triggers playing of
tones/announcements, creates billing records, etc.
• TAS can retrieve subscriber profile from HSS, can communicate also with CS network elements over Camel.
• TAS is what makes the VoLTE enhancements on top of the pure VoIP
Multimedia Resource Function (MRF)
• Media Server which can play tones and announcements (early media).
• Supports conferencing services where acts as a mixer.
IP-SM-GW
• Application server which provides Transport Level Interworking for SMS sent over IP.
• SMS service still provided by SMSC in CS network.

IMS Databases
DNS
• Provides routing on SIP layer
• Important for load-sharing and failover scenarios
• Enables zero-configuration using service discovery
ENUM
• Used for Mobile Number Portability (MNP)
• Can be used for routing towards CS network
Home Subscriber Server (HSS)
• Enables routing on service layer
• Contains all information about subscribers and their services
• The only persistent storage, hence crucial for failover/restoration scenarios

LTE
How to Register?
UE
SIP REGISTER
P-CSCF
S-CSCF
I-CSCF
A-SBC
TAS IPSMGW
DNS
HSS
ALG/AGW

Why it is that complex?

IMS Registration
Purpose
• Authenticate Subscriber
• Encrypt communication channel
• Create a binding between IMPU and actual location (Contact address)
• Update soft-state presence information
• Learn capabilities of a particular device
• Create a linkage between S-CSCF and P-CSCF
Key Facts
• All the devices belonging to a particular subscriber are handled by one S-CSCF
• One S-CSCF and one P-CSCF are assigned to UE for the whole duration of registration
• Registration data is stored in HSS and can be used in Restoration Scenarios
• S-CSCF can provide “Implicit Registration” to other IMPUs which are listed in subscriber profile
• If Registration doesn’t work, other callflows don’t work either. Therefore the Registration is the most
important scenario!

Registration Call-Flow

Registration Routing
UAC
P-CSCF
S-CSCF
I-CSCF
A-SBC
O-TAS
DNS
HSS
ALG/AGW
DNS NAPTR/SRV/A
REGISTER sip:volte.mnc123.mcc456.3gppnetwork.org SIP/2.0
Route: <sip:10.100.100.1:5060;lr>
To: <sip:+123456789012@volte.mnc123.mcc456.3gppnetwork.org>
Path: <sip:12abc@sbc01.volte.mnc123.mcc456.3gppnetwork.org:5062;lr>
Cx UAR
Cx MAR, SAR
SIP/2.0 200 OK
Service-Route: <sip:xyz@scscf01. volte.mnc123.mcc456.3gppnetwork.org:5061;lr>

Registration Identities
UAC
P-CSCF
S-CSCF
I-CSCF
A-SBC
O-TAS
DNS
HSS
ALG/AGW
DNS NAPTR/SRV/A
Route: <sip:10.100.100.1:5060;lr>
To: <sip:+123456789012@volte.mnc123.mcc456.3gppnetwork.org>
SIP/2.0 200 OK
P-Associated-URI: <+123456789@volte.mnc123.mcc456.3gppnetwork.org >
Cx UAR
Cx MAR, SAR

SIP Session
IMS
RTP
UASUAC
SIP INVITE (SDP-O)
200 OK (SDP –A)
SIP ACK

How to Call?
RTP
UASUAC
SIP INVITE (SDP-O)
Terminating
Network
Originating
Network
P-CSCF
S-CSCF
I-CSCF
A-SBC
O-TAS IPSMGW
DNS
HSS
DNS
HSS
P-CSCF
S-CSCF
I-CSCF
T-TAS
A-SBC
ALG/AGWALG/AGW

IMS Session Setup
Purpose
• Set up a signaling path
• Exchange parameters of the multimedia session
• Allocate resources in access networks and on SBCs
• Trigger TAS to apply originating and terminating services
Key Facts
• There are always two TASes and two IMS Cores on the signaling path. Although physically they can be the
same servers, logically we have to distinguish the originating and terminating network elements
• Routing towards ASs is driven by IFC (part of the Subscriber Profile)
• Originating S-CSCF provides routing towards terminating network
• IMS Network (domain) is represented by I-CSCF. The translation (domain -> IP) is done by DNS. I-CSCF
implements Location Service
• Both TAS and S-CSCF can modify R-URI. Both can also trigger ENUM
• In case S-CSCF can’t find the routing in IMS (e.g. because of tel uri in R-URI), the message is routed to CS (via
BGCF and MGCF)
• Terminating S-CSCF updates R-URI with the Contact information
RTP
UASUAC
P-CSCF
S-CSCF
I-CSCF
O/T-TAS IPSMGW
DNS
HSS
ALG/AGW

UAC
P-CSCF
S-CSCF
I-CSCF
A-SBC
O-TAS
DNS
HSS
ALG/AGW
INVITE sip:+123456789012@operator.com;user=phone SIP/2.0
Route: <sip:context456@s-cscf.ims.operator.com:5061;lr>
Route: <sip:10.0.0.1:5060;lr>
Route: <sip:tas1.ims.operator.com;orig;transport=udp;lr>
How to Route Originating Call?

How to Route Terminating Call?
UAS
DNS
HSS
P-CSCF
S-CSCF
I-CSCF
T-TAS
A-SBC
ALG/AGW
INVITE sip:+123456789012@operator.com; SIP/2.0
Route: <sip:tas2.ims.operator.com;transport=udp;lr>
Route: <sip:context789@s-cscf2.ims.operator.com:5061;lr>
Route: <sip:context789@s-cscf2.ims.operator.com:5061;lr>
INVITE sip:2233445566@10.10.1.11:5060; SIP/2.0
Route: <sip:contextABC@sbc2.ims.operator.com:5070;lr;mtcall>
INVITE sip:2233445566@10.10.1.11:5060; SIP/2.0

Users’ Identities
UASUAC
P-CSCF
S-CSCF
I-CSCF
A-SBC
O-TAS IPSMGW
DNS
HSS
DNS
HSS
P-CSCF
S-CSCF
I-CSCF
T-TAS
A-SBC
ALG/AGWALG/AGW
P-Preferred-Identity: <sip:99123456789@operator.com>
P-Asserted-Identity: <sip:99123456789@operator.com>
P-Served-User: sip:99123456789@operator.com;
sescase=orig;regstate=reg
P-Served-User: sip: 123456789012@operator.com;
sescase=term;regstate=reg

Session Setup

SDP
realtimecommunication.wordpress.com UASUAC
P-CSCF
S-CSCF
I-CSCF
A-SBC
O-TAS IPSMGW
DNS
HSS
DNS
HSS
P-CSCF
S-CSCF
I-CSCF
T-TAS
A-SBC
ALG/AG
W
ALG/AG
W
o=BRAND-VoLTE-UE 12345 IN IP4 10.0.0.111
s=VoLTE
c=IN IP4 10.0.0.111
b=AS:44
b=RS:512
b=RR:1598
m=audio 1111 RTP/AVP 116 107 118 96 111 110
o=SBC-12345 789110 IN IP4 10.114.1.21
s=IMS
c=IN IP4 10.114.1.21
b=AS:80
b=RS:1000
b=RR:2000
m=audio 12034 RTP/AVP 116 107 118 96 18 8
o=SBC-12345 789110 IN IP4 10.114.1.21
s=IMS
c=IN IP4 10.114.1.21
b=AS:80
b=RS:1000
b=RR:3000
m=audio 12034 RTP/AVP 116 107 118 96
18 8

Media Routing
UASUAC
P-CSCF
S-CSCF
I-CSCF
A-SBC
O-TAS IPSMGW
DNS
HSS
DNS
HSS
P-CSCF
S-CSCF
I-CSCF
T-TAS
A-SBC
ALG/AGWALG/AGW
Offer
c=IN IP4 IP1
Offer
c=IN IP4 IP2
Offer
c=IN IP4 IP3
Answer
c=IN IP4 IP5
Answer
c=IN IP4 IP6
IP1
Answer
c=IN IP4 IP4
IP6
IP2
IP5
IP3
IP4

What is the difference betweem VoTE and
OTT calling?

Rx, Gx interfaces
Non 3GPP Access
Untrusted
ePDG
3GPP Access
Serving
Gateway
S5
S2b
SWn
SWx
A-SBC/P-CSCF
ATCF/ATGW
TAS IP-SM GW
Rx
I-CSCF/
S-CSCF
AAA
Presence
HSS
Non 3GPP Access
Trusted
Internet
HSS
WebRTC
GW
OAuth
IMS
Gx
Rx/Gx
Gxc
SWm SWx
S6b
Gxb
TWAN
Packet Data
Gateway
PCRF

QoS?

How to Interconnect?
IPX
RTP UASUAC
SIP INVITE (SDP-O)
Operator BOperator A
P-CSCF
S-CSCF
I-CSCF
A-SBC
O-TAS IPSMGW
DNS
HSS
DNS
HSS
P-CSCF
S-CSCF
I-CSCF
T-TAS
A-SBC
ALG/AGWALG/AGW
IBCF
I-SBC
IBGW/TrGW
IBCF
IBGW/TrGW
I-SBC

And Roaming?
RTP
UASUAC
SIP INVITE (SDP-O)
Operator C
Operator B
P-CSCF
S-CSCF
I-CSCF
A-SBC
O-TAS IPSMGW
DNS
HSS
DNS
HSS
P-CSCF
S-CSCF
I-CSCF
T-TAS
A-SBC
ALG/AGWALG/AGW
IBCF
I-SBC
IBGW/TrGW
IBCF
I-SBC
IBGW/TrGW
DNS IBGW/TrGW
I-SBC
IBCF
IBGW/TrGW
I-SBC
IBCF
Operator A
Operator D

CS related flows
Facts
• Not everywhere we have strong LTE signal
• Many devices still don’t support 4G
• Operators invested a lot in 3G infrastructure
• Not all the services are supported or transferred in
4G network
Solution
MSS
CS
IMS
P-CSCF
A-SBC
AGW
MGW
LTE
S-CSCF
I-CSCF
T-TAS
DNS
HSS
PGW
SGW
MSC
MGCF
IM-MGW
eMSC
• IMS supports interworking with legacy network via MGCF/MGW
• Routing towards particular MSC is performed by BGCF
• Our aim is to centralize services in IMS – that means even those subscribers who are currently in CS
network will be handled by IMS. IMS Centralization (IMC) is provided by a dedicated application
server – SCC AS
• SCC AS does also support Voice Call Continuity. That is Access Transfer during an active call.

And how does it work in practice?
Access Site
R4
Core Site A
P-CSCF
S-CSCF
I-CSCF
A-SBC
O-TAS IPSMGW
DNS
HSS
S-CSCF
I-CSCF
T-TAS
ATGW
I-SBC
Access site R1
Access Site
R5
Access Site
R2
Access Site
R3
RCS
MRF
BGCF
MGCF
MSC
MGW
PSXDMS
Core Site B
E-CSCF
DSC
MRF
BGCF
E-CSCF
IPSMGW
ATCF

IMS
P-CSCF
S-CSCF
I-CSCF
A-SBC
TAS IPSMGW
DNS
HSS + UDM
ALG/AGW
MRF
AP
PCRF
+ PCF
E-CSCF
LRF
realtimecommunications.wordpress.coms
E-UTRAN NG-RAN
5GS+EPC
S1-MME
S1-U
S11
S6a
N26
N2
N3
N3
N11S5-C
Rx
N8
N15
N4
Ut Gm
N10
SGW
PGW-U
+ UPF
DNS
HSS + UDM
MME AMF
S5-U
N7
PGW-C
+ SMF
5G?

Take Away
• VoLTE provides telecommunication services using IMS network.
• IMS provides a framework for basic services such as User Identities, Authentication, Session Routing and Session
Control, Service Triggering etc.
• Basic IMS entities are A-SBC, P-CSCF, CSCF, DNS, HSS, Application Servers.
• For Signaling IMS mandates SIP protocol. The purpose of signaling is to locate the recipient.
• In order to create a binding between actual point of presence (IP) and user identity (tel/sip uri) all users have to
register first.
• Multimedia data goes out of band using RTP.
• The GSMA/3GPP standards define the signalling Call Flows, along with the content and syntax of the protocol
headers, see: VoLTE Service Description and Implementation Guide or GSMA IMS Profile for Converged IP
Communications.

Where can I find more?
VoLTE and CS Network
IMS from 10.000 feet
VoLTE in IMS
SIP Illustrated: SIP by sip
VoLTE flows – close encounters

Thank you!

VoLTE flows - basics

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