424185963-Introduction-to-VoLTE.pdf

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Introduction to VoLTE
Reference Document

MPI0112-050-010 © Mpirical Limited, 2018 i
Reference Document

ii © Mpirical Limited, 2018 MPI0112-050-010
Mpirical classes have been developed in accordance with the technical specifications
published by the 3GPP. As such the 3GPP have granted Mpirical Limited the right to use the
3GPP logo to identify specifications, compliant products and services.
First published by Mpirical Limited in 2017
© Mpirical Limited, 2018
All rights reserved. No part of this book or accompanying software may be reproduced or
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information contained within.

MPI0112-050-010 © Mpirical Limited, 2018 iii
Contents
VoLTE Concepts and Drivers ................................................................................1
1.1 VoLTE Standardization..................................................................................3
VoLTE End to End Architecture.............................................................................4
2.1 LTE Access Network .....................................................................................4
2.2 IP Multimedia Subsystem..............................................................................5
2.3 Policy and Charging Control .........................................................................6
VoLTE Initial Procedures.......................................................................................7
3.1 Subscriber Provisioning ................................................................................7
3.2 Establishing IMS Connectivity.......................................................................8
3.3 IMS Registration............................................................................................8
3.4 Post Registration...........................................................................................9
VoLTE Call Procedures .......................................................................................10
4.1 VoLTE Media - Codecs ...............................................................................10
4.2 SIP Signalling Exchange............................................................................. 11
4.3 Establishing Dedicated Bearers..................................................................12
4.4 Tearing Down the Call.................................................................................12
Interworking.........................................................................................................13
5.1 Interworking with Legacy Voice Networks...................................................14
5.2 Handing Over to 2G/3G (SR-VCC).............................................................14
5.3 Wi-Fi Calling................................................................................................16
Glossary............................................................................................ 19

iv © Mpirical Limited, 2018 MPI0112-050-010

MPI0112-050-010 © Mpirical Limited, 2018 v
Figures
Figure 1 Key Drivers for VoLTE.....................................................................................1
Figure 2 No Native Support for Voice in LTE.................................................................1
Figure 3 Competition from OTT Players........................................................................2
Figure 4 HD Voice .........................................................................................................2
Figure 5 Efficient Data Transport...................................................................................2
Figure 6 Foundation for Next Generation Communication............................................3
Figure 7 Fixed Mobile Convergence .............................................................................3
Figure 8 VoLTE Standardization (GSMA IR.92) ............................................................3
Figure 9 Basic VoLTE Architecture................................................................................4
Figure 10 LTE Access Network .....................................................................................5
Figure 11 IP Multimedia Subsystem..............................................................................6
Figure 12 Policy and Charging Control .........................................................................7
Figure 13 VoLTE Initial Procedures...............................................................................7
Figure 14 Subscriber Provisioning – Key Elements......................................................7
Figure 15 Establishing IMS Connectivity.......................................................................8
Figure 16 Bearers Available Post LTE Attach................................................................8
Figure 17 IMS Registration Procedure..........................................................................9
Figure 18 Post Registration Procedures .......................................................................9
Figure 19 3rd
Party Registration.....................................................................................9
Figure 20 Registration Event Subscription....................................................................9
Figure 21 End to End VoLTE Call Establishment........................................................10
Figure 22 General Stages of a VoLTE Call Establishment..........................................10
Figure 23 VoLTE Media............................................................................................... 11
Figure 24 High Level SIP Signalling Exchange........................................................... 11
Figure 25 Establishing Dedicated Bearers..................................................................12
Figure 26 SIP Signalling Exchange for Call Tear Down..............................................13
Figure 27 Tearing Down the VoLTE Call .....................................................................13
Figure 28 Drivers for Interworking...............................................................................13
Figure 29 Breaking Out to the PSTN or 2G/3G Voice Network ..................................14
Figure 30 Soft Switch Components.............................................................................14

vi © Mpirical Limited, 2018 MPI0112-050-010
Figure 31 Handing Calls from 4G to 2G/3G (SR-VCC)...............................................15
Figure 32 VoLTE and VoWi-Fi.....................................................................................16
Figure 33 End to End VoLTE Architecture...................................................................17

MPI0112-050-010 © Mpirical Limited, 2018 1
VoLTE Concepts and Drivers
Although LTE has been widely deployed around the globe, voice services in
LTE are still relatively new. In early LTE networks, service providers relied on
their mature 2G/3G networks for voice services, utilizing CSFB (Circuit
Switched Fallback) to facilitate a voice call (essentially, when a subscriber on
4G wishes to make a call, they will leave the 4G network and conduct the call
as normal on 2G/3G). However, 2G and 3G networks will not be around in the
longer term; service providers understand that a new voice service
architecture is required and consequently, VoLTE has come to the fore as a
solution for providing voice services across a packet based access network
(including 5G’s NexGen and New Radio architecture).
The key drivers for VoLTE are outlined in Figure 1.
Key Drivers for VoLTE
Fixed Mobile
Convergence
HD Voice
Competition from OTT
Players
Efficient Data
Transport
No Native Support of
Voice in LTE
Foundation for Next
Generation
Communication
Figure 1 Key Drivers for VoLTE
No Native Support for Voice in LTE
Since LTE is a data only service, there was and still is no support for a
“traditional” voice service. As such, any voice service must run as a data
service, which is essentially VoIP (Voice over IP).
UE
LTE Network
Packet Data Network
eg. Internet
IP
IP
IP
IP
IP
IP
Figure 2 No Native Support for Voice in LTE
Competition from OTT Players
Most service providers have rolled out VoLTE a number of years after their
initial LTE rollout. In that interim period, subscribers have grown accustomed
to using OTT (Over the Top) apps such as Skype and WhatsApp for real time
communication (voice, video and instant messaging). Consequently, there is
an expectation from subscribers of what a real time communication service
should be, based on their OTT app experiences. Traditional 2G/3G voice
cannot provide many of the features that subscribers have grown accustomed
to eg HD voice, group chat, video call etc, therefore VoLTE offers service
providers the capability to compete.

2 © Mpirical Limited, 2018 MPI0112-050-010
Figure 3 Competition from OTT Players
HD Voice
Although strictly speaking HD voice is available on 3G, most new VoLTE
deployments offer HD voice as a base feature. That said, a new generation of
voice codecs that are VoLTE compliant are set to offer even clearer voice by
utilizing the bandwidth improvements of LTE and LTE Advanced Pro, coupled
with the greater processing power of today’s smartphones.
Figure 4 HD Voice
Efficient Data Transport
For a number of years, service providers have exploited the cost savings that
IP based transport networks can offer. LTE and VoLTE are both IP based and
can utilize the same underlying transport network that is already used by the
2G and 3G infrastructure.
SS7 Signalling
TDM Voice
Each call has a
dedicated timeslot (which
leads to underutilization)
Voice over IP
Voice packets share a
common transport
medium, improving
resource utilization
Common Packet
Transport Network
Figure 5 Efficient Data Transport
Foundation for Next Generation Communication
VoLTE is largely centred around voice and instant messaging. However,
initiatives such as RCS (Rich Communication Services) are attempting to
standardize a range of additional services, such as video calling, file transfer,
geolocation and social presence. VoLTE is just the start of what will be a new
generation of communication services.

Figure 6 Foundation for Next Generation Communication
Fixed Mobile Convergence
The ultimate goal for many service providers is to provide a particular service
such as voice, regardless of the access network type. Although the name
VoLTE suggests an LTE access network, the call control for VoLTE is based
on an IMS (IP Multimedia Subsystem). Therefore, it’s possible that other
packet based access networks such as Wi-Fi and 5G will be suitable as
access networks, bearing in mind that the IMS is IP-CAN (IP Connectivity
Access Network) agnostic. Indeed, many service providers who have made
the move to VoLTE have also deployed a Wi-Fi Calling service which
compliments the VoLTE service.
2G/3G/4G Data
Wi-Fi
DSL/Cable/Fibre
IMS Based Service Control
Figure 7 Fixed Mobile Convergence
1.1 VoLTE Standardization
VoLTE as a standard1 was introduced by the GSMA as “IMS Profile for Voice
and SMS” (in which the term “VoLTE” is actually used very infrequently). The
standard itself is a framework document which defines how an IMS based
telephony service can be supported over an LTE based RAN (Radio Access
Network). The document defines a profile which identifies a minimum
mandatory set of features that the mobile and the network must support. All of
the features mandated are defined in various 3GPP or IETF (Internet
Engineering Task Force) specifications. At a high level, they include (outlined
in Figure 8:
Figure 8 VoLTE Standardization (GSMA IR.92)
 IMS based capabilities and supplementary services for telephony.
 Real time media negotiation, transport and codecs.
 LTE radio and EPC (Evolved Packet Core) capabilities.
1
GSMA IR.92 – IMS Profile for Voice and SMS

 Functionality that is relevant across the protocol stack and subsystems.
VoLTE End to End Architecture
The basic architecture for VoLTE is outlined in Figure 9.
IP Multimedia Subsystem
Policy and Charging Control
LTE Access Network
(plus Wi-Fi, 5G, etc)
Call Signalling
Transport
Voice Packet
Transport
Registration
Call Control
including
Continuity
Resource
Reservation
Billing
Figure 9 Basic VoLTE Architecture
It should be noted that the end to end architecture for VoLTE is complex, with
lots of nodes involved in registration and call procedures. However, the key
components include:
2.1 LTE Access Network
The LTE access network performs the role of packet transport, ensuring that
the call signalling associated with setting up the call and then the subsequent
voice packets are handled appropriately. It is important to remember that
signalling and voice are two different traffic types:
 Call signalling is in the form of SIP (Session Initiation Protocol). A
series of SIP messages will be exchanged on an end to end basis in
order to establish and subsequently tear down the call. It is important
from both a user experience and billing perspective that these
messages are not delayed and are not dropped.
 Voice packets are in the form of RTP (Real time Transport Protocol).
Minimizing delay for voice packets is essential, however it is possible
to drop packets without degrading the call (as long as it is not a
consecutive group of packets). This is due to the fact that an individual
voice packet carries a very small sample (typically 20ms) of actual
voice.
Based on these differing transport requirements, during a call the LTE network
will provide two data bearers to the mobile:
 The Default EPS (Evolved Packet System) Bearer, which has a QoS
profile designed for SIP signalling; low latency, with a very good packet
error ratio. The Default Bearer will always be in place as long as the
phone is attached to the LTE network.
 The Dedicated EPS Bearer, which has a QoS profile designed for real
time voice; low latency, but the likelihood of dropped packets is higher
than the Default Bearer. The Dedicated Bearer will only be in existence
whilst a call is in progress, due to the fact that it is a GBR (Guaranteed

Bit Rate) bearer; this means resources are reserved across the
network in order to provide the correct data rate. As such, if the bearer
is not required i.e.there is no call taking place, the network will not want
to tie up resources which will not be used, hence the bearer is only set
up as and when a call takes place.
Figure 10 shows the key components of the LTE network, including the two
EPS bearer types that may be encountered.
Figure 10 LTE Access Network
 eNB (evolved Node B) – this is the main element of the LTE RAN
(Radio Access Network), responsible for the radio link between the
mobile and the network.
 S-GW (Serving Gateway) – this serves as the entry and exit point
between the RAN and the core; as the subscriber moves and is handed
from one eNB to another, the S-GW serves as the anchor point for the
subscriber’s uplink and downlink data.
 PDN-GW (Packet Data Network Gateway) – this serves as the entry
and exit point into and out of the LTE network. The PDN-GW provides
connectivity to different networks:
 Connectivity to the Internet to allow subscribers to browse the
web.
 Connectivity to the IMS to allow subscribers to make and receive
VoLTE calls.
 MME (Mobility Management Entity) – this provides control of data
bearers and will be involved in the creation and deletion of EPS
bearers.
 HSS (Home Subscriber Server) – this is a central repository for LTE
related subscriber information.
2.2 IP Multimedia Subsystem
The IMS (IP Multimedia Subsystem) provides call control services, in that
when a VoLTE subscriber wishes to make a call, they will send SIP signalling
(via the Default Bearer) towards the IMS. The IMS will process this SIP
signalling and ultimately ensure the call gets set up towards the correct
recipient.
 If the call is from a VoLTE user to a VoLTE user, all of the call control
will be handled by the IMS and call control signalling will be SIP on an
end to end basis.
 If the call is from a VoLTE user to a subscriber on legacy voice, the IMS
will interact with the legacy voice network, which may involve

converting the SIP signalling into a different call control protocol
appropriate to the legacy network.
Before making or receiving calls, VoLTE subscribers must first register with
the IMS. Figure 11 shows the main components found within the IMS,
including Application Servers which perform specific roles within the VoLTE
service.
Figure 11 IP Multimedia Subsystem
 CSCF (Call Session Control Function) – CSCFs are responsible for
analyzing and routing the SIP based call control signalling used to set
up a VoLTE call. CSCFs will take in SIP requests for calls and ensure
they are sent to the correct onward hop.
 HSS – this is a central repository for IMS related subscriber
information.
 SCC (Service Centralization and Continuity) Application Server –
supports mid call handovers to the 2G/3G network, termed SR-VCC
(Single Radio - Voice - Call Continuity).
 TAS (Telephony Application Server) – supports voice supplementary
services such as Calling Line ID Hiding, Call Divert, etc.
 PSTN (Public Switched Telephone Network) – the IMS provides the
interaction to allow calls to be set up to the fixed line PSTN.
 PLMN (Public Land Mobile Network) – the IMS provides the interaction
to allow calls to be set up to subscribers on the legacy 2G/3G voice
network.
2.3 Policy and Charging Control
In order to make or receive a call, SIP signalling must be sent to the IMS via
the Default Bearer. In turn, a Dedicated Bearer must then be established to
support the voice packets. The problem is that the LTE network does not read
the SIP signalling; from the perspective of LTE, SIP is just user data that the
LTE network will dutifully transport from the mobile to the IMS.
As such, there must be a mechanism which allows the IMS to inform the LTE
network that LTE resources (the GBR Dedicated Bearer) are required for the
call. This mechanism is PCC (Policy and Charging Control), which acts as the
gateway between the SIP based session signalling related to the IMS, and the
LTE based signalling which is used to establish bearers.
Figure 12 outlines this concept, whereby SIP signalling requesting a call to be
established passes through the LTE network without being processed (Step
1). Once the SIP reaches the IMS, a CSCF in the IMS will interact with the
PCRF (Policy and Charging Rules Function), highlighted in Step 2, in order to
trigger the Dedicated Bearer.
The PCRF will determine whether the Dedicated Bearer is allowed. Assuming
there is no reason why the VoLTE subscriber is not allowed to make the call,

the PCRF will instruct the PDN-GW to signal the LTE network in order to set
up the Dedicated Bearer (Step 3). Finally, during the call, the PCC framework
will ensure the call is billed for appropriately (Step 4).
IP Multimedia
Subsystem
UE
CSCF
LTE Access
Network
GW
PDN
Policy and
Charging Control
PCRF
1
2
3
4
Figure 12 Policy and Charging Control
VoLTE Initial Procedures
Before VoLTE call procedures can be conducted, the VoLTE device must
conduct a number of initial procedures (outlined in Figure 13).
Subscriber
Provisioning
Establishing IMS
Connectivity
IMS Registration Post Registration
Figure 13 VoLTE Initial Procedures
3.1 Subscriber Provisioning
Strictly speaking, subscriber provisioning must occur before the subscriber is
able to use their VoLTE service. This process is the responsibility of the
network, which must ensure that all of the relevant network elements are
configured to accommodate the VoLTE subscriber. The key elements which
must hold appropriate configuration data are outlined in Figure 14. If a
problem occurs during the provisioning process, the result will typically be a
service disruption to the subscriber.
HSS (LTE) HSS (IMS)
TAS and SCC
Application
Servers
PCC Database AAA Servers
Figure 14 Subscriber Provisioning – Key Elements
 HSS (LTE) – the subscriber profile must have the correct QoS and
connectivity profile for VoLTE, including roaming if supported.
 HSS (IMS) – the subscriber profile must include VoLTE as an active
and permitted service.
 TAS and SCC Application Servers – these elements must be aware of
the supplementary services and SR-VCC capability of the subscriber.
 PCC Database – the PCRF may refer to this as part of the decision
making process when a Dedicated Bearer is being established.

 AAA (Access, Authorization and Accounting) Server – if services such
as Wi-Fi Calling are supported, the AAA server may be required to
store subscriber data.
3.2 Establishing IMS Connectivity
IMS Connectivity is based on the establishment of a Default EPS Bearer
within the LTE access network. A realistic flow of events is outlined in Figure
15, which shows the mobile first of all establishing connectivity to the Internet.
Although Internet connectivity is not required for VoLTE, Internet connectivity
is established to help provide a good user experience (when a subscriber
attaches to LTE, they are just as likely to browse the Internet as they are to
make a voice call).
Figure 15 Establishing IMS Connectivity
 LTE Initial Attach is the name given to the procedure which allows the
mobile to attach to the LTE network and establish a Default EPS
Bearer to the Internet.
 PDN Connectivity Request is an LTE procedure which allows the
mobile to connect to an additional PDN (Packet Data Network), which
in the case of VoLTE is the IMS.
The end result in terms of the LTE network bearers available following the
LTE attach is outlined in Figure 16.
Figure 16 Bearers Available Post LTE Attach
If Wi-Fi Calling is available as a service, the difference would be that the
Radio Network would be Wi-Fi instead of LTE. The Core Network, specifically
the PDN-GW, would still be utilized.
3.3 IMS Registration
IMS Registration for VoLTE services is structured around the regular
procedure for IMS registration, with a number of minor modifications for
VoLTE specifics. If registration is not carried out, the user will not be able to
access any VoLTE services. Figure 17 outlines the IMS Registration
procedure.
 Step 1 – the mobile sends a registration request to the IMS. The P-
CSCF (Proxy CSCF) sits at the entry/exit point of the IMS network in
order to provide security and SIP message routing.
 Step 2 – the S-CSCF (Serving CSCF) is the IMS device that has
overall control of registration. It sends a request to the HSS to obtain
security information which allows authentication to take place.
 Step 3 – the S-CSCF sends an authentication challenge to the mobile.

 Step 4 – the mobile attempts to register again, but this time includes a
response to the authentication challenge received in Step 2.
 Step 5 – if the authentication challenged response is correct, the
mobile will be allowed access to the IMS and hence VoLTE services.
Figure 17 IMS Registration Procedure
3.4 Post Registration
Following the successful registration, additional post registration procedures
are conducted, as outlined in Figure 18.
Figure 18 Post Registration Procedures
3rd
Party Registration
During the registration process, the Application Servers such as the TAS
(Telephony Application Server) and the SCC (Service Centralization and
Continuity) Application Server will not be involved in the subscriber’s
successful registration. As such, post registration, a “3rd Party Registration”
will be sent from the S-CSCF to the relevant Application Servers, informing
them that subscriber’s VoLTE service is in an active state.
Figure 19 3rd
Party Registration
Registration Event Subscription
Figure 20 Registration Event Subscription

Although the mobile is able to initiate a deregister from the network as and
when required, SIP initially did not feature the ability for the network to initiate
a subscriber’s deregistration. To work around this, following the successful
registration, both the mobile and the application servers will inform the S-
CSCF that they wish to be kept updated as to the status of the subscriber’s
registration. If the registration status of the subscriber then changes for a
network related reason, the S-CSCF will inform any interested parties using
SIP signalling.
VoLTE Call Procedures
Making a call in VoLTE involves a complicated series of events, including key
procedures taking place within the LTE, IMS and PCC elements of the
network.
SIP and IMS
Diameter and
PCC
LTE Access
Network
Figure 21 End to End VoLTE Call Establishment
For reference, the complete end to end architecture involved in a VoLTE call
can be found in Figure 33. Although the details of the architecture will not be
discussed in this narrative, Figure 33 highlights how many “working parts” are
involved in a call establishment procedure.
The general stages of a VoLTE call establishment are highlighted in Figure
22.
Exchange
SDP Offer and
Answer (SIP)
PCC
Interaction
(Rx and Gx)
Establish
Dedicated
EPS Bearer
Finalise Call
Establishment
(SIP)
Figure 22 General Stages of a VoLTE Call Establishment
4.1 VoLTE Media - Codecs
During a VoLTE call, a bidirectional media stream must be established
between the Calling and Called Party. The role of the IMS is to facilitate the
transfer of SIP signalling which will ultimately establish this media flow.
The media itself, voice, is transported as a payload of RTP (Real time
Transport Protocol). RTP is used in conjunction with RTCP (Real time
Transport Control Protocol), which can provide feedback on the
delivery of the RTP packets for each end of the call.
Before the voice can be transported as an RTP packet, the voice must first be
digitized using an audio codec. For VoLTE, this codec is predominantly AMR
(Adaptive Multi Rate)2, although the EVS (Enhanced Voice Services)3 codec
is also an option.
2
3GPP TS 26.071 - Mandatory speech CODEC speech processing functions; AMR speech Codec;
General description
3
3GPP TS 26.441 – Codec for Enhanced Voice Services; General Description

Figure 23 VoLTE Media
4.2 SIP Signalling Exchange
Fundamentally, a VoLTE to VoLTE call requires the exchange of SIP signalling
on an end to end basis, mobile to mobile, but within the network a significant
number of other control protocols will be utilized to ensure the network is able
to establish the VoLTE service.
Figure 24 outlines the high level exchange of the SIP signalling which will
typically be involved in a VoLTE to VoLTE call establishment.
In the initial SIP exchange within Figure 24, the SIP carries SDP (Session
Description Protocol) as a payload. SDP is crucial to the VoLTE call
establishment because it contains all the information that PCC and the LTE
network needs with respect to the media bearer, such as:
 IP addressing and ports between which the voice packets will flow.
 Codecs utilized for the voice encoding.
 Bandwidth requirements, which are used to define the Guaranteed Bit
Rate that the Dedicated Bearer in the LTE network must support.
Figure 24 High Level SIP Signalling Exchange
In Figure 24, the SDP Offer and Answer are encountered twice. The first
instance is used to describe the media bearer and also set up “preconditions”.

Preconditions are used to delay the Called Party’s mobile alerting the user
until both the Calling and Called Party have their Dedicated Bearer
established. The second SDP Offer/Answer exchange is used after the
Dedicated Bearers are established, to enable the call procedure to continue
(at this point the Called Party’s mobile will alert the user).
4.3 Establishing Dedicated Bearers
The overall exchange of SIP signalling will, during the call establishment
process, trigger the establishment of the Dedicated Bearers. Remember that
in a VoLTE to VoLTE call, both subscribers will each require a Default Bearer
to carry their SIP signalling, plus a Dedicated Bearer to carry their voice
packets.
The Dedicated Bearer is established when the P-CSCF within the IMS sees
the SDP Offer and SDP Answer. It is at this point that the P-CSCF has enough
information to fully describe the media bearer (the Dedicated Bearer) that
needs to be put in place in the LTE network.
This information is passed to the PCRF (Policy and Charging Rules Function)
using the Diameter protocol. Here, the PCRF will make a decision as to
whether the Dedicated Bearer can be established, then use Diameter
signalling again to push a policy rule to the PCEF (Policy and Charging
Enforcement Function). The PCEF is usually contained within the PDN-GW.
Upon receipt of the policy rule from the PCRF, the PDN-GW will send the
appropriate signalling into the LTE network in order to establish the Dedicated
Bearer. This is initially based on GTPv2-C (GPRS Tunnelling Protocol version
2 – Control), but additional protocols are used in the LTE network as part of
the Dedicated Bearer Establishment process.
Figure 25 outlines the procedure that will happen on the Calling and Called
Party legs of the call.
Figure 25 Establishing Dedicated Bearers
4.4 Tearing Down the Call
Once the call is complete, any resources that are no longer required must be
torn down. Note that the Default Bearer carrying SIP signalling always stays in
place, regardless of whether the subscriber is in call or not. From the SIP
perspective, the signalling is simple (the high level message exchange is
shown in Figure 26.

BYE
200 OK
Shuts down the call at the
SIP level
Calling
Party
Called
Party
IMS
Figure 26 SIP Signalling Exchange for Call Tear Down
Figure 27 shows the main stages associated with tearing down the VoLTE
call. The flow of SIP signalling around the network is identical to the initial call
establishment. Moreover, tearing down the Dedicated Bearer looks almost
identical, apart from the fact that the PCRF will remove the policy rule from
the PCEF rather than installing it. This triggers the PDN-GW into sending the
GTP signalling into the network which will remove the Dedicated Bearer.
P
CSCF
PCRF
GW
PDN
PCEF
Diameter
Indication that the
VoLTE session has
ended for this
subscriber
SIP
Exchange of SIP BYE and 200 OK
between the mobile and the IMS network
1
2
Diameter
Instruction to the PCEF to remove the
policy rule
3
4 GTP
Dedicated Bearer
Deletion
Figure 27 Tearing Down the VoLTE Call
Interworking
For the foreseeable future, a huge amount of interworking will need to take
place between VoLTE capable networks and their older, legacy voice
counterparts. Drivers for interworking are outlined in Figure 28.
Legacy
Subscribers
2G/3G voice and the fixed line PSTN will be around for the
foreseeable future ; until those networks and phones can understand
SIP, interworking will be required
Patchy
Coverage
If a service provider has patchy LTE coverage , SR-VCC will need to
be used in order to avoid dropped calls
Utilising Other
Radio
Technologies
WiFi and HSPA networks can accommodate VoLTE , but interworking
between the radio network and the core network is required
Figure 28 Drivers for Interworking

5.1 Interworking with Legacy Voice Networks
If a VoLTE subscriber wishes to call somebody on the PSTN or the 2G/3G
voice network, an IMS “breakout” procedure needs to take place. Breakout is
simply a term used to describe the process of converting the SIP based
VoLTE signalling and also the RTP based voice packets into a format suitable
for whichever legacy network the call needs to be directed towards.
Figure 29 Breaking Out to the PSTN or 2G/3G Voice Network
Breakout procedures are facilitated by one or more soft switches within the
IMS. Soft switches are comprised of two main elements, as outlined in Figure
30.
Figure 30 Soft Switch Components
5.2 Handing Over to 2G/3G (SR-VCC)
Although 4G technology based on LTE (Long Term Evolution) is relatively well
established, for many 4G customers network coverage has yet to reach the
ubiquitous levels of 2G. In this interim period whilst service providers roll out
and enhance their 4G coverage, a mechanism must be available which allows
the network to hand a VoLTE call that was initiated in 4G over to a 2G/3G
network, if 4G coverage begins to fail. This mechanism is termed SR-VCC
(Single Radio - Voice Call Continuity), and assumes that although 4G
coverage may be failing, there is 2G/3G coverage available.
It is worth pointing out that there is another technology termed CSFB (Circuit
Switched Fallback) that sounds similar; however, the key difference is that
with CSFB, the call is handed to 2G/3G before the call is connected. With SR-
VCC, the call has already been established and voice is already being
exchanged when the handover occurs.

Figure 31 Handing Calls from 4G to 2G/3G (SR-VCC)
As 4G coverage diminishes, there will come a point whereby the 4G radio
network decides that a handover to 2G/3G must take place. Whether the
mobile actually goes to 2G or 3G is dependent on the measurements the
mobile takes at the time; regardless, from a high level SR-VCC perspective,
the choice does not influence the handover significantly.
Once the handover is triggered, three key elements to the SR-VCC handover
process are brought into play:
 MME (Mobility Management Entity) – this is the main device in the LTE
network that provides control of services and data connectivity. As
such, the MME will be informed by the 4G radio network that the
handover needs to take place and will then contact the 2G/3G voice
core network. In particular, the MME will inform the MSC-S that a
handover needs to take place.
 MSC-S (Mobile Switching Centre – Server) – this is the main controlling
entity for voice services in the 2G/3G voice core network. Any voice
calls that need to be established will be under the control of an MSC-S.
When the MME sends the handover request to the MSC-S, the MSC-S
will conduct two activities:
 The MSC-S will contact the 2G/3G radio network to ensure
resources for the voice call are set up in the network.
 The MSC-S will contact the SR-VCC controller in the IMS to
trigger the ongoing voice call to be switched away from 4G and
towards the 2G/3G network.
 SR-VCC Controller – with SR-VCC, the call is anchored in the IMS.
This means that when the call was established, all the call signalling
went through the IMS and specifically the SR-VCC controller. It also
means that if a change occurs related to the call, such as a change in
radio access (from 4G to 2G/3G for instance), the SR-VCC controller
will need to be informed. As such, when the MSC-S sends the request
to redirect the voice from 4G to 2G/3G, the SR-VCC controller will
coordinate this switchover.
All that remains is for the MSC-S to inform the MME that the 2G/3G network is
ready to accept the call, at which point the mobile will be instructed to join the
2G/3G network.

5.3 Wi-Fi Calling
Wi-Fi Calling, which may also be termed VoWi-Fi (Voice over Wi-Fi), is closely
related to VoLTE (Voice over LTE) in that it utilizes the same IMS telephony
client and supports mobility between Wi-Fi and LTE. In essence, traffic is
anchored at the PDN-GW (Packet Data Network – Gateway) regardless of the
radio access network in operation – LTE or Wi-Fi.
GW
PDN
eNB
S-GW
ePDG
Access Point
All traffic is anchored at
the PDN-GW regardless
of access network.
Figure 32 VoLTE and VoWi-Fi
In the case of VoLTE, voice traffic is carried using EPS (Evolved Packet
System) bearers which run from the mobile to the PDN-GW. However for
VoWi-Fi, these bearers only exist between the ePDG (Evolved Packet Data
Gateway) and the PDN-GW, with IPSec (IP Security) tunnels being used to
securely carry the voice traffic between the mobile and ePDG. Wi-Fi itself
simply carries the IP traffic across the final interface to the mobile.

PDN
S
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i
S
6
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PDN
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S
CSCF
I
CSCF
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PDN
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PCRF
PCRF
SCC
TAS
TAS
SCC
Figure 33 End to End VoLTE Architecture

Glossary
AAA (Access, Authorization and
Accounting)
AMR (Adaptive Multi Rate)
CSCF (Call Session Control Function)
CSFB (Circuit Switched Fallback)
eNB (evolved Node B)
EPC (Evolved Packet Core)
ePDG (Evolved Packet Data Gateway)
EPS (Evolved Packet System)
EVS (Enhanced Voice Services)
GBR (Guaranteed Bit Rate)
GTPv2-C (GPRS Tunnelling Protocol
version 2 – Control)
HSS (Home Subscriber Server)
IETF (Internet Engineering Task Force)
IMS (IP Multimedia Subsystem)
IP-CAN (IP Connectivity Access Network)
IPSec (IP Security)
MME (Mobility Management Entity)
PCC (Policy and Charging Control)
PCEF (Policy and Charging Enforcement
Function)
PCRF (Policy and Charging Rules
Function)
PDN (Packet Data Network)
PDN-GW (Packet Data Network –
Gateway)
PDN-GW (Packet Data Network Gateway)
PLMN (Public Land Mobile Network)
PSTN (Public Switched Telephone
Network)
RAN (Radio Access Network)
RTP (Real time Transport Protocol)
SCC (Service Centralization and
Continuity)
SDP (Session Description Protocol)
S-GW (Serving Gateway)
SIP (Session Initiation Protocol)
TAS (Telephony Application Server)
VoLTE (Voice over LTE)
VoWi-Fi (Voice over Wi-Fi)

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