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  • Writer's pictureVikas Shokeen

01. What is VoLTE – IMS Overview , Basics & Fundamentals

Updated: Jun 7, 2020

Fundamentals & Basics of VoLTE and IMS Technology , Covers VoLTE Overview & Explain IMS Network

  • Network Evolution to VoLTE

  • How to Enable VoLTE in handset

  • Differences between VoLTE & CSFB Call

  • Voice call in LTE & VoLTE Networks

  • Evolution of Voice Call

  • VoLTE – Benefits for Users

  • VoLTE – Benefits for Operators

  • VoLTE – Challenges

  • VoLTE – Handling Congestion Scenarios


Download PPT ( pdf ) - Click here


What is VoLTE ( Voice over LTE )

What is VoLTE

Pre-VoLTE Era – Challenges with Voice Traffic

Pre-Volte_Era - Data Usage

For understanding VoLTE , We need to back into History where VoLTE was not launched as there was 4G network along with 2G / 3G Network

4G came prior to VoLTE , 4G started getting commercial launched in 2009 and 2010 while VoLTE came very late in 2014 .Since , 4G is an all-IP technology and therefore cannot transport Circuit Switch services such as Voice Call . You can see on screen that so many data Packets are moving between Internet & User using 4G Network . Let’s see how Voice works in this scenario ( I will recommend you to see this Youtube Video for Animations used in illustration of Concepts )

CSFB as Voice Solution for Pre-VoLTE Era

Pre-Volte-Era - Voice Call

Since 4G can’t carry voice traffic , This is big problem for everyone . Circuit Switch Fall Back which is also known as CSFB helped Operators here , While 4G or LTE Network is used for Internet and Data Connectivity , User is thrown to underlying 3G or 2G Networks for voice calls . Existing 2G/3G mobile systems use “Circuit Switching” to carry voice traffic. Circuit Switching or CS is basically a dedicated, end-to-end, channel or circuit is established to carry the voice traffic for the entire duration of a transmission . While user is able to use Circuit Switch for voice services but there are many problems with this technology as user needs to Switch between 4G & 3G Network for every voice call . This also requires lot of Battery in handset and also Generates lots of Signaling traffic for Mobile Operator which is Bad for Network . Now , Every one started working on Better solution & they come up with VoLTE as solution to these problems


How VoLTE Works

VoLTE uses your 4G Data Connectivity to send data packets for Voice calls . Basically it’s like calling someone using internet say just like Whatsapp call . VoLTE enables the Voice on LTE Network or 4G Network

As LTE have given us All IP to IP Based Network , We need to have some technology to carry voice traffic using IP packets over data connection. For this , VoLTE solution is discovered . We also call this as Packet Switched Network as all traffic goes in form of Stream of IP Packets & It doesn’t require dedicated or Reserved Path .

VoLTE Benefits

What all benefits VoLTE brings to Life of Users

  1. Better User Experience : As user is not running pillar to post between 4G and 3G Radios , This improves overall experience of User and It also brings lot of relief for Mobile Operators

  2. The 2nd Benefit is HD Calling : As VoLTE is running on All IP to IP .. Packet Based Network , This provides opportunity for Mobile Operators to use Bigger Pipes and Better Codecs for carrying High Definition voice Calls . This gives us Crystal clear HD voice call on VoLTE which was not possible on Old 3G or 2G Networks

  3. 3rd Benefit is No drop in background data speeds : Without VoLTE , User needs to switch from 4G Network to 3G or 2G Networks for voice services which reduces speed of Data Connection running in Background . In few cases such as 2G Network , Data stops working while you are on call . In case of VoLTE Call , Since user remains on 4G network all the time , Speed of Data Connection never degrades here … even during voice calls

  4. Another Great benefit .. Faster calling : A VoLTE network connects calls faster than the legacy networks . A Normal 3G call takes 4-6 Seconds but VoLTE calls requires less than 1 Sec to connect

  5. Better battery life : Since your phone is not changing networks between 4G & 3G or 2G for voice calls  . Users saves couple of Amps & battery life with VoLTE

VoLTE - Benefits for Users

Evolution of GSM & Mobile technology

Evolution of Mobile Technology

History of VoLTE : Initially IMS was Opposed by various Operators due to multiple complexities involved . In the year 2009, Couple of Operators and Equipment Manufacturers such as AT&T, Verizon Wireless, Nokia , Alcatel-Lucent & many OEMs gathered and proposed a solution for IMS based VoLTE. This initiative was called “One Voice” and the proposed solution was supposed to provide a seamless and common solution for voice over IP networks . Further , GSMA (Global System for Mobile Communication) got attention on this Subject & they started to work towards standardizing this as an official solution to provide next generation voice over LTE networks along with 3GPP , Finally we have seen initial working trial of VoLTE happening in Year 2012 . Let’s Quickly run thru Evolution of Mobile Technology . You must have seen all these symbols on your handset . I have mapped them as per Name of their Technical Specification & Year of Introduction . We can clearly see Evolution from 2G to 2.5 G to 3G to 3.5 G to 4G Networks

  1. GPRS / Edge : This is normal 2G Network with minimal support of Low Speed Data Services , This is very very old technology and It will be shutted across Entire Globe in coming years

  2. 3G : This is 3G Network with Better Data Speeds and also provides voice support

  3. H / H+ : This is Super fast 3G network which is still in use Now a days

  4. 4G or LTE Network : This is latest Technology which is having Bright future … This will play significant role for next 5-6 until 5G Launch Happens

VoLTE Launch : Voice over LTE was available to End users in 2014 & SingTel Singapore became 1st Operator to Launch Commercial VoLTE Services . With VoLTE Networks , SingTel users were able to make or receive Voice calls on 4G Networks


Enable VoLTE on Handset – Android & Apple iOS

How to Enable VoLTE in handset

Now a days , Most of upcoming Smartphones are already supporting VoLTE , This also depends upon your Operator if particular handset is allowed for VoLTE

Unlike 2G or 3G or 4G which works on all the Smartphones by Default , VoLTE is bit complicated & doesn’t work until Mobile Operator perform Rigorous testing & Approve handset for Enabling VoLTE . For Example , If new Handset is getting Launched in Market , All the Operators needs to perform Validation of this new handset in their Network & Needs to allow that Make Model to work on VoLTE

This is complicated task & requires close coordination among Handset Manufacturer and Mobile Operator .

Apple iOS

How to enable VoLTE on iPhone : Go to Settings > Mobile Data > Mobile Data Options > Enable LTE

  1. 1st Option is OFF : This means , Both LTE / VoLTE are disabled

  2. 2nd Option is Voice and Data : This means , Both LTE & VoLTE are enabled , All Calls will be handled on 4G radio only

  3. 3rd Option is Data Only : This means , Both only LTE is supported but VoLTE is not enabled . This is as Good as VoLTE disabled where user will use CSFB Technology to make or receive calls on 3G radio

Android Device

How to enable VoLTE on Android : Under Mobile Network Settings , First Select “LTE” in Preferred network type . Then Select “Enhanced 4G LTE Mode” which is nothing but VoLTE Switch . Please enable this in order to start using VoLTE Services

In few smartphones , VoLTE can’t be enabled in Settings menu as Handset Manufacturer have not enabled same . However , In such devices , We can use secret menu *#*#4636#*#* to go to Hidden Settings & Enable VoLTE . This Trick usually works in most of Android devices


What all is required to avail VoLTE Services 

Requirements for VoLTE Service

VoLTE is not supported in all handset by default . Usually , Every Mobile Operator perform testing of handset and then Handset Manufacturer such as Xiaomi , Apple , Samsung etc.. Enables VoLTE Support in Software Build shipped along with Handset . This Software build & Operator will decide whether VoLTE will work for you or Not

Here , We are going to cover the mandatory things required for VoLTE

What Mobile Operator Needs to do for Enabling VoLTE Service

  1. Good 4G Coverage is mandatory where user can make or Receive Calls fluently . Good coverage will ensure that user will be getting required bandwidth or Radio Speeds for entire duration of call

  2. Provisioning of VoLTE Services : You Mobile Operator needs to Activate or Provision VoLTE Services in Profile of User stored in HSS . Few Operator Provision VoLTE by Default to all users , Other Operator provision VoLTE Services on basis of Handset Make / Model used by User selectively

What User Needs to do for availing VoLTE Service

  1. The handset hardware ( Make & Model ) used by customer should support VoLTE

  2. Also The Software Binary running on handset should also support VoLTE for corresponding Mobile Operator . This Software / or Firmware Build to allow or Disallow VoLTE is controlled by Handset manufacturer .

  3. ISIM or USIM : VoLTE will not work on traditional SIM used in 2G or 3G Era

  4. Most critical is VoLTE Switch in Handset where user can enable or Disable VoLTE Services himself

If Everything is done as mentioned above , User will be able to use VoLTE Service


Voice call Evolution & Life cycle of Voice 

Evolution of Voice Call

Here , We will see options available with User to make outgoing call on Network

  1. 2G : In the 2G legacy networks, voice calls are made practically only on circuits for each call on CS domain

  2. 3G : 3G Networks are similar to 2G Networks , No Major Change

  3. CSFB : Used in Operators where 4G have been launched without support of VoLTE Services

  4. VoLTE : Voice Over LTE

  5. SRVCC Extension : This is enhancement to VoLTE to allow handover of Ongoing call from LTE to 2G / 3G in case there are coverage Holes in 4G Network such Lifts , Basement etc..


Steps / Path followed by Mobile Operators to Evolve Voice Services

Types of Mobile Operators

Here , We can see options available with User to make outgoing call on Network

  1. Voice Call on 2G / 3G : Only Operators ( Without LTE )

  2. Voice Call on CSFB : Earlier Operators were using CSFB as Only Option with LTE Launch

  3. Voice Call on VoLTE / CSFB Both : Operators who have launched VoLTE uses both CSFB & VoLTE Approach

  4. Voice Call on VoLTE Only : There are some LTE only New Operators which works only on VoLTE & CSFB is not supported here


Why VoLTE is Good for Operators

VoLTE - Benefits for Operators

Increased Spectrum Efficiency

Operators have wasted lots of Spectrum of on Older technology such as 2G which is only used for Voice . If We can Re-Use this 2G Spectrum & Re-Farm it to LTE , By doing this , Operators will be able to use precious spectrum in better way . Operator can do this once majority of customers are moved to VoLTE Network which runs both Data & Voice on 4G Network . Ultimately, better use of spectrum makes for better quality, which in turn leads to higher subscriber satisfaction and retention . The same amount of spectrum if used in LTE as compared to 2G will give us 2x Numbers of Calls , 100% Incremental Data Capacity

Business Critical

Even in 2018 , Voice is still accounting for appx 50% of Total revenue coming for Mobile Operator . Providing better Voice Experience is now mandatory for all Operators in order to Survive . VoLTE is best way to cater Voice call requirements


With VoLTE , Now Both Voice & data are riding on same LTE Network which means we don’t need separate network for Voice such as 3G / 2G .. This helps in reducing Operational Cost Reduction

New Service Integration

VoLTE Enables Faster , Quicker Integration of new services such as Chat / Multimedia services to compete with OTT Players in better way


Challenges @ VoLTE growth

VoLTE Challenges ..

VoLTE is booming technology , Let’s understand few Challenges which are impacting VoLTE Penetration among End Users

  1. Not all Operator Supports VoLTE : Out of Total 850 Operators across Globe , LTE is supported by 530 Operators & VoLTE is launched by 127 Operators

  2. Not all Handset Supports VoLTE : Handset was Bottleneck 2 Years Back , Now a days , All Smartphones are supporting VoLTE

  3. Increased Complexity : VoLTE increases complexity multifold as there are many new Nodes which generates ton of Signaling traffic

  4. LTE Coverage is mandatory for VoLTE : VoLTE Requires Good Strength LTE to be mandatory available which should provide both Outdoor and Indoor Coverage

  5. No Inter-Operator Connectivity : Even after VoLTE Launch , Operators are still connected to each other via traditional Non-SIP PSTN Networks

  6. Every Operator needs to Test Every Device : Device is big mystery for VoLTE Support , There is lot of dependency on Device Validation prior to Launch by Every Operator


SRVCC – User Moving into 4G low coverage area during ongoing call

SRVCC (Single Radio Voice Call Continuity)

Have you ever wondered , What will happen when User makes 4G Voice Call & moves into Low coverage areas where 3G Network is available but 4G Network is not available ..The call will drop or continue ?

Example : You move into Lift of Building or Goes into Basement or Travel by Metro where Radio condition changes rapidly . In these cases , LTE may not be available . Here we use the Technology by Name of SRVCC to ensure Voice call continues and handed over to 3G Network without dropping the call . Name of this technique is single radio voice call continuity and this is developed on later stage as enhancement . In order to support SRVCC  , All the Network Elements should be connected to each Other .. Packet Core Serving Data , Voice Core Serving Voice & IMS Network handling VoLTE must be connected with each other .


VoLTE Architecture

VoLTE IMS Architecture

The LTE Network allows users to gets Signal on Handset helps in Latching . IMS Network be helping user to Make or receive VoLTE Calls . Refer Diagram .. There is Police Inspector sitting in between . His Name is PCRF & his role is to provide relevant priority to traffic  , For example , VoLTE Traffic will be given higher priority & Generic Internet traffic is Normal Priority

For VoLTE , The first thing happens here is Registration on Mobile operator Network to confirm our presence of Mobile Handset .. This is just like Putting Attendance in Register before starting anything serious . This happens with help of SIP Bearer which is always active between IMS Network & User Handset . Post Registration , User can make and receive MO / MT Calls


VoLTE Architecture as per 3GPP

IMS ( VoLTE Network )

In this IMS World , All Elements communicate with each other on SIP Protocol for everything .

While , This seems to be Quite difficult to see in 1st Look .. Well , I am going to simplify this for you


VoLTE Architecture at Glance

Simple IMS Architecture

I have some how tried to simplify the IMS Architecture shown in previous slide . This is all IP to IP Network which relies on SIP Protocol for voice calls

4G Network Integration

Voice calls made by user are carried on 4G Network and handed over to IMS or VoLTE Network

PSTN Integration

Communication with PSTN Networks happens via Media Gateway which is having specialty to carry Voice Call Traffic Over IP Traffic

IMS Network

IMS Network contains Two critical components .. 1st One is SIP Server Infrastructure which typically contains Three SIP Servers by Name of P-CSCF , I-CSCF & S-CSCF . These SIP servers are going to register User with SIP Server which is Part of Control Plane & Signaling Plane  . These SIP Server will setup Media Part or Payload of Voice Call with Media Gateway shown in Right hand side which is further connected to PSTN . SIP Server is responsible for Session set-up, session tear-down, session control

There is another component by the name of TAS Servers … which is Telephony App Server (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.. TAS is 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..


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