2. VoLTE IMS Network Architecture tutorial

hi there this is a voltage architecture tutorial my name is Vikas and i am

subject matter expert in the telecom industry here I am going to simplify

volte mystery for you are you finding 3gpp specs extremely

complicated clumsy and hard to understand yeah

it's complex well not to worry at all this is a right place for you I am going

to simplify and make it easy for you so that you are going to remember it

forever I assure you you will feel much more comfortable with VoLTE

architecture within the next few minutes let's start our journey with VoLTE

Hi , Along with explaining the VoLTE architecture , I am also going to provide

you two bonus tips on Volte network in the end . You will not find these

practical tips anywhere else since I have learned these things over the time

during past few years . This video contains Gist or Summary of all my

learnings . I believe in building concepts without solid foundation you will

trouble creating anything of a value . I am going to work on building your

fundamentals in this particular video . This video will we will cover up high

level nodes in VoLTE and also covers deconstruction of telephony services

along with the integration of IMS functions . Let's quickly jump onto the

fundamentals here

Let's discuss about high level Volte connectivity

what is volte ? . As diagram suggests it's all about carrying voice over LTE Data Network

Since IMS is part of the Third Generation Partnership Project also called as 3GPP, this

makes IMS as Standard solution with Pre-Defined Interworking with underlying 3G & 4G Services.

Along with LTE Network, Operators can harness the true power of IMS which stands for IP

Multimedia Subsystem.

IMS is a standalone system which resides out of the LTE network and connected to PDN Gateway

or PGW through SGi Service .

Now , We can see this all IP Network which is LTE as access is being used for carrying voice calls

Now , Let's discuss something about IMS , Now IMS as it have been shown in center in the Orange color

You can clearly see like IMS is sitting out of LTE Network , This is standalone network which is sitting in the middle

Now , Let me tell you something about the IMS

IMS is a very old concept , It was was very very popular in Year 2003 - 2004

almost 12-13 years back, this used to be a failed Concept

What happened at that point of time

When IMS have came into the picture and when it was introduced in year 2003

Operators have conceived this as failed technology because it is too too complex

Operators have seen it as far too expensive and burdensome to be introduced and to maintain

With VoLTE Launch, This again got popular

Now , VoLTE requires IMS to be there and to be sitting on top of core LTE network

This have made IMS popular again and Its in Demand now a days again

OK , In coming slides , We are going to discuss the VoLTE Building blocks

As we discussed earlier , Its all about voice over LTE and it’s more or less exactly what it says on the tin . Rather

than the 2G or 3G connections which are usually used for voice calls , VoLTE offers voice

calls over all IP to IP based 4G Network . We tend to think of 4G as mostly being about

downloading, streaming and web browsing, and indeed that’s primarily what it’s been

used for so far, but it can also be used to next Level of Voice Services with IMS

Now , Let's quickly see what all Benefits which Users will get along with VoLTE Services

The First benefit which user is going to get is

Superior call Experience - The Greatest advantage of VoLTE is that Voice call quality is superior

from 2G or 3G connections . Essentially it’s an HD voice call and

It’s a much richer experience over for end user

Similarly , VoLTE also provides faster call setup

VoLTE can connect call 2wise or 3rice as fast as currently used in 2G or 3G Network

Our typically 2G or 3G call takes 4 to 6 seconds for maturing , While VoLTE requires only

a typically less than 1 seconds or 1-2 seconds for getting connected end to end

Other benefits of VoLTE for End users are better battery life

Better battery life - Anyone who currently uses 4G could also find their battery life

increased with VoLTE . For a Non-VoLTE call , Handset needs to switch from 4G to underlying

3G or 2G network to receive voice call . This is done using CSFB or Circuit Switch Fall

Back technology . This frequent switching of Mobile from 4G to 3G & Vice versa consumes

lots of Battery . In VoLTE call , Users Need not to Go to 2G or 3G Network to receive voice call

Now , That saves battery for end users , Similarly , There is one benefit which user is going to get

That one is the Video calling , Just like OTT Providers such as Whatsapp , Facebook ,Google Duos and Skype

VoLTE also gives you Video calling service , Now the benefit of this Video calling service is

Its available on the stock dialer from the main handset itself , You need not to download any 3rd party

Application from Google Play store or may be from the Apple Store , Its directly available from the handset itself

you put a SIM , If its VoLTE enabled handset , You can straight away make a Video Call .. Just as simple as that ..

As , next step , We are also going to De-Construct the entire network which is going to be used for

VoLTE, Where we are going to discuss about Handset , The LTE Network , The IMS Core and the voice PSTN in coming slides

On Left hand , We can see SIP Enabled User

Equipment . This Handset or Smartphone needs to be VoLTE Compliant where SIM should support

ISIM & SIP User Agent for making voice calls over VoLTE

The 2nd one is the LTE Network which typically shows the ENodeBs and the LTE core network , We will also discuss this in brief in coming slides

The 3rd one is the IMS Network which is sitting on top of underlying LTE Network

The last one is the PSTN Network which is used for Break-in and Break-out of calls from IMS

We are going to discuss in detail in the coming slides

Let's quickly jump to the VoLTE handset component , The "1"st one

We will start our Journey with User Handset . For VoLTE to work , User handset must support

two critical things : 1st is the SIM Support which is there on left hand side

and then the VoLTE Binary Application in Handset which is also called as user Agent

Which is on the Right hand side .. So , Let's start with the SIM piece

SIM plays the crucial role in the VoLTE Support

SIM is known by the name of UICC which stands for Universal Integrated Circuit . There are primarily 3 types

of SIM Cards :- The 1st one is typical SIM Card which is used for 2G or 3G

This is a traditional SIM , It securely stores the international mobile subscriber

identity (IMSI) and the related key used to identify and authenticate subscribers on mobile telephony

The 2nd type of SIM is USIM . The USIM was introduced with 3G and 4G Services

This is new Age SIM card which came to us

The last one is ISIM which is used for VoLTE Services .

ISIM is also known as IP Multimedia services identity module

This is used by IMS or VoLTE subsystem

For Authentication and rest of SIM functions . There are 2 Types of entities which are stored on this ISIM SIM :-

1st one is IMPI , IMPI stands for IP Multimedia Private Identity . IMPI is a global identity allocated by home network.

IMPI contains home operator’s domain information , The 2nd one is IMPU . IMPU stands for IP Multimedia Public Identity

IMPU acts like a telephone number which can either be a SIP URI

or a tel URI as defined in RFC 39664 , Now Let's see whats there on Right hand side

Its our SIP User Agent , which we are going to discuss . Unlike , 2G or 3G or LTE , Where all Handsets

are default supporting all available Network , VoLTE requires detailed Handset testing

and Certification for Every Handset Make & Model . This testing is required as handset manufacturers

and operators will be seeking to assure themselves that the underlying protocol features are

present and functioning correctly . Operator want to ensure that the underlying IMS and

protocol is functioning, and performing as intended, in order to deliver the expected

quality of service . SIP user agent is typically a Binary which resides inside the handset

The main function of this binary is to transmit & receive SIP

messages . It Provides basic telephony functionality & It is acting as a client or a Server for exchanging the SIP Messages withe the IMS core

Now , That's all about Handset . Now , Let's quickly jump to the Next slide where in like we are going to Cover

High level building blocks of LTE Network

Ok , Now we have covered the 1st Piece , Now quickly jump onto the 2nd one

OK , This is s LTE Architecture slide

I assume you already know LTE Architecture & will not spend too much time here . All

Yellow Nodes are control Plane & are responsible for User Authentication . Control plane nodes

include EnodeB , MME , HSS , OCS & PCRF

The Green ones are User Plane which includes

EnodeB , SGW & PGW . EnodeB is Radio cell site of LTE Network & is used in both Control

& User Plane . PGW is also used for both User & Control Plane traffic

Now , We have covered User Handset & LTE Core Network . Its time to demystify IMS Core network

. VoLTE Voice Call Traffic is originated by User Handset & Terminates on IMS Network . LTE

Network is just used as Bridge acting as IP Supporting Carrier for this communication

. Now , We will jump on to IMS Core without wasting any further time

This is 3GPP IMS VoLTE Architecture , I am going

to simplify this in another few Minutes & you will be able to understand every individual

component shown in this complicated diagram . The VoLTE IMS Architecture is covered under

@ 3GPP 23.228 . Similarly , You can also study GSMA Document : GSMA FCM.01 and GSMA IR.92

These 2 documents are really Good , If you want to further study VoLTE and want to Go in depth , You can refer to GSMA documents

We will start with High level concept of LTE Network & Going to Stitch it together with IMS

This is typical LTE Network which Provides high Speed Internet access . Traffic flows

all the way from Mobile handset to EnodeB to SGW to PGW to Internet Cloud

Signaling is controlled by MME and PGW with support of HSS , PCRF

You can see User Payload Traffic flow along with Arrow on Screen

LTE Network is part of Evolved Packet System as shown in diagram here .

Our IMS Core Integration begins here . IMS stands for IP Multimedia Subsystem

IMS Core is directly connected to PGW as shown on screen . This is connected over sGi Interface

For LTE Network , IMS or SIP traffic is just another Payload similar to Internet Traffic

. LTE PGW doesn’t Decode or Interfere this traffic .

LTE is used as Carrier for accessing IMS Services which provides IP Telephony or Voice Services

over IP Network , There could be multiple Access type for IMS Network such as Wifi , or

Fix Broadband or DSL Networks

SIP Server

From This Slide onwards , I will start breaking down IMS Cloud in various Bits & Pieces to

make it more clear to understand IMS Core consists of broadly Two Type of Network

Elements , First is SIP Infrastructure & Another type of Nodes are Media Gateways . These are

tightly coupled under IMS Core Network . Let’s understand SIP Server in detail

Users are going to register with SIP Server which is Part of Control Plane & Signaling

This SIP Server will setup Media Part or Payload of Voice Call with Media Gateway shown in

in Blue Color which is further connected to PSTN

SIP Server is anchoring point for Session set-up, session tear-down, session control

along with BCDR Generation SIP Server is consisting of various sub Modules

covered in upcoming Slides

Media Gateway

Now , Let’s Understand Media Gateway which is Highlighted in Green Color

Media Gateway is Responsible for carrying actual voice traffic & handing it over to

other Networks . These Media Gateways handles interworking of Voice traffic between IMS

& PSTN Networks . Any call coming from traditional 2G or 3G

or Fix line network is using this Media Gateway as Entry point to IMS Network .

Media Gateway is Interconnect point to circuit-switched networks and Other IMS Network . It also handles

Transcoding of the media plane . Broadly it the gateway between the LTE and other PSTN Networks

For the sake of easiness , I am using only One Box depicting LTE Network

This Consists of MME , SGW & PGW. There can be multiple access networks for

providing IMS Services . This could be Wifi , Fix Broadband , DSL & LTE Networks

As long as user handset supports SIP Device & has IP connectivity to reach IMS Network

, We can offer IMS Services to User IPCAN Stands for IP Connectivity Access Network

This is equivalent to Bearer of a IP Connectivity established from user towards IMS or Service Network

Now , I am going to discuss most critical Element of IMS Core Network . We call this as CSCF , You can see it here in central SIP Server

You can also see it here along with SIP Proxy in the diagram . It stands for Call Session Control Function & act as heart of the IMS system

The CSCF performs all the signaling operations, manages SIP sessions and coordinates with

other network entities for session control, service control and resource allocation.

It consists of three different entities: the Proxy-CSCF (P-CSCF), the Interrogating-CSCF which is also called as I-CSCF

and the last one is Serving-CSCF (S-CSCF).

Let’s start with First Entity : P-CSCF also known as Proxy-CSCF or Proxy Call Session

Control Function This SIP Proxy is sitting in Front of SIP

Server always SIP Proxy is first Point of Entry point to

IMS Network . This Proxy SIP Server is typically located

in Visitor PLMN Network ( I mean to say the roaming network ) . This SIP Proxy has several important functions

such as a) Validation of the correctness of SIP messages coming from the user b) It also ensure the security

It also does the function of Authentication

& asserts the identity of the UE In last , It also finds out Home SIP Server IP & Routes Traffic to same

The Interrogator

I am adding up two things here One is Interrogator & Another is Home Subscriber Server

Let's discuss the Interrogator .. I-CSCF or Interrogator CSCF performs below Critical functions :-

SIP Proxy forwards requests to Interrogator which in-turns finds out which SIP Server to Select

User may be served by Various SIP Servers

, While Only One SIP Servers serves customer While SIP Proxy is usually located in Roaming

or Visited Network , Interrogator sits in Home Network

So this is visited network , This is home network

All the main tasks of Interrogator is Select Correct SIP Server or Registrar which is going to

Server User All this communication between Proxy , Interrogator

& SIP is always on SIP Protocol between all these Nodes . I-CSCF queries the HSS using the DIAMETER

Cx interface to retrieve the user location and then routes the SIP request to its assigned

S-CSCF during Registration of a call.

It is also used during During MT Call routing

HSS ( Home Subscriber Server )

Now , Lets discuss HSS which is also know as Home Subscriber Server

The HSS is the database of all subscriber and service data

HSS contains the subscription-related information (user profiles), used by the control layer

HSS is the master user database that is used for Registration , Authentication in both

LTE Network , As well as in VoLTE IMS Network This allows the user to access the packet

and circuit domains of the network initially, via IMSI authentication.

HSS Communication is via Diameter protocol For super easy understanding, HSS is used

as profile or database of User where all information is saved . This HSS will decide whether you

are allowed to use VoLTE or Not , This will decide Whether You are even allowed to LTE Network or 4G Network or Not ? Etc..

That's the role of HSS, It's a significant stuff after LTE launch

Telephony App Server (TAS)

Also called MMTEL or TAS .. Till now , We discussed IMS Core which is great for handling SIP but this requires App

Servers to provide various supplementary services . For example , Voice calls in Mobile network

requires Telephony App Server (TAS) . We need similar App servers for other Multimedia services

such as Video , Gaming etc..

Application Servers host and execute services, and interface with the S-CSCF using SIP.

This allows third party providers to provide an easy integration and deployment of their value

added services to the IMS infrastructure.

IMS was never indented to be limited for voice services , Rather it was designed for multimedia

services & voice is only One part of same IMS is not a service but service facilitator

network . Its enables user to access various Applications such as Telephony service

App Server handles Telephony part While SIP Server is still controlling Session

such as Initiation , Maintain & Tear down of sessions .The TAS

the provides telephone brain to the App server

TAS is a telephony app server used for Telephony Service such as Call waiting, Call hold, Call pick up , Call forwarding, Call transfer , Call

blocking services, Malicious Caller Identification , Lawful interception , Announcements, Digit

collection , Conference call services etc..

We can check various other App Servers such as Video App Server , Conference Bridge Service etc..

Its an addon to the IMS just like TAS we can add on all these application servers onto the existing IMS Network

App Servers are not considered as part of Core IMS Service

Media Control

In this Slide , We have split Media functions into two part – Control & Payload function

The top one is the control function and lower one is user payload function

Media Control is performing critical control

plane function for VoLTE Call . Let;s discuss this one in detail . User can make call to PSTN Network or Fix

Line Network where Media Control & Gateway is supposed to Speak to PSTN Network over

ISUP / SS7 Protocol User can also make call to another IMS Networks

Media Control handles the parts of the call state that pertain to connection control for

media channels Media Control communicates with the SIP Server

and selects the appropriate node depending on the routing number for incoming calls from

legacy network Media Control also Performs protocol conversion

between ISUP and the IM subsystem call for control protocols

It also Controls the MGW resources with a H.248 interface

It does all the control function of voice call

Now , Let's discuss

Media Gateway Media Gateway is element responsible for Payload

Handling of VoLTE Call . Here is media Gateway

It Interacts with Control and terminates bearer channels from a switched circuit network and

media streams from a packet network (e.g., RTP streams in an IP network)

Interfaces the media plane of the CS network, by converting between RTP and PCM

It can also perform media transcoding, when the codecs used do not match (e.g. IMS might

use AMR, PSTN might use G.711) . These 2 are different codecs . Other than Media conversion , It also performs

bearer control and payload processing using codecs, echo cancellers and Managing other

on-board resources

Net , Net .. This is the guy who is going to ensure both the nodes , One side on LTE speaks to another side on PSTN

Here , I am going to further Expand Media Control & Media Gateway into actual Node functions

IMS Gateway which communicates with Other IMS Networks

3GPP Names of these Nodes are IBCF & TrGW . IBCF is control Plane Entity & TrGW is user Plane Entity

Similarly , PSTN Gateway is the node which communicates with PSTN Networks . 3GPP Names of these Nodes are MGCF and IMS-MGW

MGCF is control Plane Entity & IMS-MGW is user Plane Entity

ATCF stands for Access Transfer Control Function ATGW stands for Access Transfer Gateway

As name suggest , These both Access nodes reside in serving IMS network (i.e. visited

network in the roaming case) , They interact with LTE Network for Media Control & Payload

handling . This is the node who is going to speak to the LTE network down the line ATCF handles the Access side Media Control

for handing Voice calls with LTE PGW ATCF also plays vital role in enabling SRVCC

Session Transfer mechanisms The ATGW is a media function that is used

to anchor the media with LTE PGW . Net Net .. Its job is to interface with LTE and handover or receive the calls from the underlying 4G or LTE Network

Now , I am showing you all 3 Media entities

in One Screen While MGCF & IM-MGW is designed to communicate

to PSTN Networks such traditional Voice network running on SS7 or ISUP

IBCF & TrGW is designed to interwork with Other IMS Networks running on SIP

This is prime reason for Breaking them into various network Elements . You can also see

PCRF (Policy and Charging Rule Function)

PCRF : This is used in both IMS & LTE Network . The main purpose of PCRF is to apply Policy

to PDN or IPCAN Session . The PCRF provides priority of Bearers used for Making VoLTE

calls & SIP Signaling . The QOS Provided by PCRF provides Highest priority to VoLTE voice

traffic . This prioritization helps Voice call to pass thru even in case of Radio Congestion

. PCRF also anchors communication between PCSCF & PGW over Gm Protocol reference Point

Now , We are going to discuss .. MRF ( Media Resource Function )

MRF stands for Media resource function MRF is both Gateway & Control function

It deals with Call conferencing & things associated with RTP Mixing

It is also used for Announcement & Media related functionalities

MRF provide media plane processing independent of application types, e.g. transcoding, multiparty

conferencing, network announcements/tones, etc.

Now , Let's discuss our last component which is BGCF .. B Number routing Table

BGCF stands for Breakout Gateway Control Function Serving CSCF Needs to make decision where

to route SIP Message ( i.e.

To PSTN or to Other SIP Network ) , BGCF Simplifies this Job by making these decisions about routing

the call to destination Network . This is similar to

our B Table of MSC

BGCF is responsible for determining the next

hop for routing of SIP messages For PSTN terminations, the BGCF determines

the network in which CS domain breakout is to occur and selects the appropriate MGCF

For terminations in peer IMS networks, the BGCF selects the appropriate IBCF to handle

the interconnect to the peer IMS domain

This is the Guy who is going to take routing decisions and who is going to route the calls to respective IBCF and MGCFs

Ok , So Now We have covered almost everything out there on IMS , Let's see complete VoLTE IMS Picture

The IMS architecture involves a clear separation of three layers :-

This Green one is Transport layer

The blue one is Application and Services layer

And now , This red one is Session and Control layer

The entire IMS is divided into these 3 Layers

Now , let's see the Big picture . We have covered all the way from user handset to the LTE network to the IMS Core to the Voice PSTN

This is Complete network diagram for VoLTE or IMS services

Ok , Now let's see .. What happens when there is a congestion on Radio , Have you ever thought what will happen to

Voice Calls during these Congestion ? .. As General principal , Data Internet

can wait for some time , But Voice call Can’t . Voice has to be real-time

LTE QoS provide us flexibility to provide priority of Bearer or Connection during Radio

Congestions . Since Voice can’t be compromised and can’t afford delays , VoLTE is always

provided Highest Priority over Internet traffic .

with help of this QCI , We prioritize VoLTE over the generic Internet . We will be using typically 2 QCIs .

1st one is the QCI=5 is used for VoLTE IMS Signaling . The another one is QCI=1 which is used for voice IMS Call

Rest of QCIs which belongs from = 6 to 9 belongs to Internet . They are always low priority as compared to IMS

They never provide Guaranteed BIT rate , Instead they work on best effort basis .

Please refer to my another Video on VoLTE IMS Registration for detailed understanding

of this along with other concepts & Call flows of VoLTE Registration , IMS APN Establishment

& Default EPS bearer setup . I am going to cover all these concepts in detail in coming Videos .

Here is another Bonus tip for you

Now , Let's understand what all it takes to deploy VoLTE in existing 4G Network

Lets see what all is required on PGW or SGW or on existing LTE network

On PGW , The 1st piece is to enable default and dedicated bearers

The default and dedicated dedicated Bearer concept is required when we are making multiple bearers for having SIP Signalling for making a additional voice calls on existing LTE network

The 2nd Piece is IMS APN IP Pool obviously , We are going to new IP Pool for IMS APN , I am also going to connect my PGW to IMS

The last one is , We also need to disable Online charging of the SIP signalling and voice calls for Data Piece

I am going to charge , the voice calls as voice but not going to charge them for payload carried over data in terms of MB , I am going to do the charging

per minute basis , but not on per MB basis ,

Similarly , Let;'s see what all changes are required in MME , The 1st critical thing which we are going to do is SRVCC , I am going to cover SRVCC in coming Videos

The 2nd One is again the IMS APN has be created , The 3rd is paging policy

for VoLTE Calls , Every incoming call to the VoLTE customer require paging . We need to define high priority paging for VoLTE

then the last one is QCI definition in MMEs , This is something which we recently discussed in our previous slides

Now , Let's see what is required is voice core or MSS Network in order to support VoLTE .

The 1st one is SRVCC which is required to handover the in case customer goes of coverage area , I am going to cover this particular element in the coming Videos

The 2nd one is again the reach-ability between CS Core Network and IMS . The 3rd one is the configuration for breakin and breakout calls for IMS or vice versa

The last one is the Codec negotiation which is going to occur between MSS and IMS

Again .. ANOTHER BONUS TIP , Let's Assume there is operator who don't have ISIM

Do existing 3G or LTE customer using USIM need to go for SIM Swap for availing VoLTE Services ?

How do I launch VoLTE ? If i am having USIM type of SIMs available in the network , Do i Need to go for SIM Swap

Of all the entire base with ISIM , Well no , There is a workaround available .

USIM can be used to Avail VoLTE Services also , USIM with help of the binary with help of SIP user agent can drive IMPU and IMPI for authentication