SRVCC (Single Radio Voice Call Continuity) in VoLTE & Comparison with CSFB


SRVCC stands for Single Radio Voice Call Continuity
SRVCC solution is used when the VoLTE calls cannot be continued due to Coverage Holes
of LTE network. In such cases SRVCC allows a Packet Switched VoLTE Call to transition
to a legacy Circuit Core network. It is a basically Handover of ongoing Voice
Calls from IMS Network to Circuit Switched legacy system such as 3G without dropping
the Call This is light Hearted easy to understand summary of 3GPP Specs TS 23.216 What is SRVCC ? 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 .. Example : You move into Lift of Building or
Goes into Basement or Travel by Metro where Radio coverage changes very very rapidly . In these cases , LTE may not be available . Here we use the Technology by Name of SRVCC to ensure Voice call continues and is handed over to 3G Network without dropping the call . Name of this technique is single radio voice call continuity and this VoLTE feature is developed
on later stage as a enhancement By using SRVCC , The Mobile operators are able to make the handovers while maintaining existing quality of service . The SRVCC requires only
a single active radio in the handset and requires some upgrades to the supporting network infrastructure
to support this feature Now , Let’s understand evolution of voice call Here , We will see options available with
User to make or receive call on Network On the Left hand side , We can see its a 2G legacy Network , Voice calls are made practically only on the circuits for each call on Circuit Switched domain
While we move to 3G , Its more or less are similar to 2G Networks No Major Change other than supporting some advanced Codecs . Now comes CSFB which is used in the Operators where LTE have been launched without support
of VoLTE Services The Next one is VoLTE Itself
Next one is SRVCC Extension where we allow handover of a call from LTE to 2G / 3G Network in case
there are coverage Holes in the 4G Network such Lifts and Basement etc.. as we discussed in the earlier Slide . The Last one is enhanced SRVCC & Alerting SRVCC which reduces call switching time when
the user moves from 4G to 2G or 3G Radio , This improves user experience to Greater Extentent Now , Lets understand the various approaches available with us for handling a Voice call in a Network Here , I am going to show you working & differences
between the CSFB & SRVCC On the Left hand side you can see its a CSFB, On the Right hand side , You can see Its VoLTE+SRVCC Now , Let’s start with CSFB Approach CSFB Stands for Circuit Switch Fall Back .. This
is applicable to Operators where LTE is launched but VoLTE is still not supported . Since LTE
itself doesn’t support voice & its all IP to IP Network It uses legacy 3G / 2G network
for providing voice services to user . There is SGs link used between MME & MSC for handling
Incoming & Outgoing call communication . As you can see in the diagram , Immediately after
Initiating Call , User is thrown in 3G Network where it can camp on RNC & Can make or receive Calls . This is temporary solution & will Eliminate over the time once all Operators upgrade to VoLTE Services Now , Let’s discuss VoLTE+SRVCC which is there on Right hand side , SRVCC stands for Single Radio Voice Call Continuity … Basically, SRVCC is a call transfer method or handover which is implemented in a simplified and in a reliable manner . When an LTE user has an active voice session in IMS and is
moving to areas which have legacy 2G/3G coverage and does not have LTE coverage. For Example
, You can see One LTE Coverage hole on top right corner of Screen . The user moving to
this Area will loose LTE coverage and uses SRVCC Now , This SRVCC will help this particular user to continuing voice calls using the VoLTE initiated in LTE Coverage area down the line in 2G / 3G Networks The main advantage of this solution is , The call will not
drop but will only be transferred to the CS domain of the legacy networks. However, the signaling required is very very complicated in such scenarios & There is new Interface which is introduced by 3GPP by the name of “Sv” Unlike CSFB , This is a Permanent solution
and this is going to stay along with VoLTE for longer duration Although CSFB & SRVCC can’t be compared because CSFB is a service based handover procedure while SRVCC is a coverage based handover procedure . I am still trying to convey some of the Benefits which VoLTE+SRVCC Approach brings to the users and Operators as compared to the CSFB The 1st One is the Call setup time: When operators uses CSFB, one of the biggest problems faced (and one of the major disadvantages of CSFB) is the
increase in call setup time due to retuning procedures required for 2G/3G radios . On Other Hand VoLTE provider a faster call setup time The Second One is Call quality: call quality in VoLTE is better due to specific QoS allocated to the IMS Call , And the Codecs which are bieng used in VoLTE or IMS , They are far far superior than Legacy Networks They can provide a better Quality which is not there in 2G or 3G Legacy Networks The Last one is the Resource benefits for VoLTE : Codec used in VoLTE requires much less resources and it consumes much lower data rate as compared to CSFB which is working on 2G or 3G networks using Dedicated circuits Now , Let’s understand the Evolution of the SRVCC These are various evolution of SRVCC which are introduced by 3GPP between Release 8 to Release 11 So at the End of the day , These Enhancements or Evolutions or the New things are introduced by 3GPP They are going to increase the user Experience to a greater Extent, So Let’s start our journey with Basic One 1st one is Basic SRVCC which came in Release 8 and this address basic issue of Call Continuity from LTE to 3G / 2G Networks
2nd / 3rd are aSRVCC and eSRVCC which help in improving user experience by reducing overall
Switching time , They will be covered in later part of this Video as I want to keep things
really simple at this point of time The 4th One is vSRVCC which is Video SRVCC
Last One is rSRVCC which is introduced in Rel 11 , This provides continuity from 3G
to LTE ( Its Reverse to the SRVCC which we discussed in the Begining ) You can also refer to 3GPP Document TS 23.216
for further study which covers all above Flows in Depth . aSRVCC & eSRVCC are widely used in many Mobile Operators & these helps to improve overall VoLTE user experience We are going to cover this particular portion in the End of the Video Now , Let’s see how SRVCC Works 3GPP have introduced a special Link … by
the Name of Sv Link between MME & MSC for enabling SRVCC functions . This Sv interface
runs on GTPv2 protocol With help of Sv Interface , Both MME & MSC
talk to each other & exchange information pertaining to user who is going to perform
SRVCC Let’s See , What happens in Life of User
.. We can see here that user is latched in LTE network , as visible on the screen Now , He makes MO Call to another IMS User , The Voice path used here is shown in Yellow Color Since user is moving into Low Coverage of
LTE Network , User Informs EnodeB that I am running out of LTE Signal MME in-turn uses Sv Link and Ask MSC to take
charge of this Call As next step , MSC align all the resources
& Establishes connectivity with IMS so that it can take full control of Call once user
latches on 3G radio. Once everything is aligned , MSC informs MME Now , MME Ask user to Release 4G Bearer and
Go to 3G Radio . Now User tune to 3G radio & Contact MSC which is already aligned to
receive call . Here MSC patches this ongoing call from 3G Radio with IMS Network without
dropping the call This is SRVCC call flow , This is extract from 3GPP Specs document # TS 23.216 . Here we can see complete Ladder flow diagram This Tells us , How SRVCC works & How user move from LTE Radio to underlying 3G Radio without dropping the call Just like normal data handover , This is also divided into 2 Parts The 1st Part is Handover Preparation flow where all the readiness & resource reservation is completed prior to Actual Handover of User
The 2nd One is Handover Execution Flow where Actual Handover happens & Call Gets switched to 3G
radio We are cover that flow once we understand the Handover preparation in nitty-gritty in detail Let’s Start our Journey with SRVCC Call
flow , We can see Customer Happily latched on 4G network here You can see this blue color is 2G / 3G Coverage and the Orange or Yellowish is LTE Coverage As visible , The 2G / 3G Coverage is widely available everywhere but 4G coverage is very very limited So , We will understand what will happen to life of user once this particular user moves out of LTE coverage to 2G / 3G Coverage We will understand how this SRVCC works , How this particular call gets handed over and switch to the 2G / 3G Networks As a 1st step they user make a VoLTE call on 4G Network which uses the Path of EnodeB to SAE
GW to IMS Core Now , UE Starts moving away from 4G coverage & started getting Good signals from 3G network As this point of time , Handset Realize that
4G signal level begins to decrease, the UE tells EnodeB about Current Radio Levels are declining and it does with help of Measurement reports Now , The EnodeB takes decision & identifies the best available network to receive the calls EnodeB sends the handover request to the MME with SRVCC Handover indication ( This will
tell MME that there is handover which is required for both CS and PS) And there is ongoing call which has to be handed over for SRVCC After receiving the SRVCC HO Indication, the
source MME , splits the voice bearer from all other PS bearers and initiates their relocation
towards MSC Server and SGSN This MME does two critical task here after
Bearer Splitting :- The 1st one is .. MME Initiates the PS-CS handover procedure for the voice bearer by sending a SRVCC PS to CS Request Message to the MSC Server . This
contains critical information such as IMSI, Target ID for Voice Call , Is also contains STN-SR , C-MSISDN etc.. We will understand the significance of these parameter such as STNSR & C-MSISDN in End of this Video In parallel , MME initiates relocation of
the PS bearers. MME sends a Forward Relocation Request to the target SGSN . This is required for handing over the Internet session which is running Earlier in 4G network Now , We are going to handover Both voice as well data session on underlying 2G or 3G Networks So this MME to MSC side communication on the Right hand side takes care of voice flow On the Left hand side , The communication which is happening from the MME to SGSN takes care of the Internet Now both MSC and SGSN will approach the RNC of 3G Network by requesting resource allocation for the CS & PS Target RNC acknowledges the PS & CS Relocation
Handover by sending the Relocation handover Request Acknowledgment to the respective SGSN & MSC here SGSN Acknowledge MME by Sending Forward Relocation response back to MME You can see this acknowledgment coming back in the Green Color Arrow Once MSC receives Relocation handover Request Ack from RNC , It need to setup CS Leg with IMS Network The MSC Server initiates the Session Transfer
by using the STN-SR with IMS Network here In actual , MSC Sends an New Call ISUP IAM
message toward the IMS with STNSR as destination This Newly establish Call between MSC & IMS Network is used to carry voice once handover is completed The Standard IMS Service Continuity procedures are applied for execution of the Session Transfer Here , There are Two critical task of Session transfer & Update Remote Leg is done to divert the flow of downlink flow of VoIP packets towards
CS Leg via MSC Earlier they were going via PGWs and via LTE Network to the user , Now they have been switched to the MSC via circuits switched domain The IMS Leg to Packet Core is now released
It is shown in Red Color color on left hand side Now , The traffic is diverted from Packet Switched Network to the Circuit Switch Network The MSC Server sends a SRVCC PS to CS message to the MME , You can see this in Blue Color Arrow from all the way MSC to MME The MME sends a Handover Command message to the ENodeB , In turn , The EnodeB sends a Handover Command message back to the UE which is direct Order that Goto 3G radio and Switch / Latch onto 3G Radio Please Goto 3G Radio and Camp onto that , Leave 4G radio Right away With this , We have completed Handover Preparation
flow where all resources have been reserved , Let’s move ahead 2nd Phase of Handover
Execution Inorder To complete Handover , Now User Camps onto 3G Network You can see user getting signal from 2G 3G radio in Blue color as mentioned on Screen This is Reference Ladder Diagram which is also extracted from 3GPP Document which explains us Handover Execution Flow where Actual Execution Handover happens Now , Let’s jump onto understanding
Flow Its fairly simple , As compared to the Earlier discussion and the Flow which we have gone thru Now , Let’s understand what happens to the user once the execution of handover happens The user tunes to the target UTRAN/GERAN cell . It means the user is now latching onto 2G or 3G radio down the line Handover Detection is happening on the
the target RNC or BSC The UE sends a Relocation Handover Complete message via the target RNC/BSC to the target MSC
At this stage, the UE re-establishes the connection with the network and can send/receive voice
data Now , MSC Server sends a SRVCC PS to CS Complete Notification message back to the MME . The MME acknowledges the information by sending a
SRVCC PS to CS Complete Acknowledge message to the MSC Server . Here , The MME deactivates
the voice bearer towards S-GW/P-GW The MSC Server performs other procedure such as TMSI reallocation , It also does MAP Location Update with HLR / HSS to complete the Entire Flow Now, all the steps are closed & Voice is flowing on new Path Similarly , For Data as You can see on the Left hand side , The Handover completion
& Bearer Updation is done between SGSN & PGW so that user can start Internet Session on
3G radio Now With this , We have already completed the Handover Execution flow and SRVCC overall call flows have been already explained Now , Let’s understand niitty-gritty of rest of Elements like STN-SR, Like eSRVCC like Other things which are actually required for day to day handling of SRVCC based stuff Now , First we discuss the SRVCC Identities which are being used on day to day basis in the Network We are going to discuss Identities
used for SRVCC functionality 1st One is STN SR , STN SR is Session transfer
number for SRVCC used by MGCF for routing of call towards IMS Network . This follows
the E.164 telecommunications number format, and is used by the MSC server for session
transfer of the media from the PS domain to the CS Domain. HSS provides the STN-SR to the MME that is provisioned for a user against its attach procedure . The MME sends this STN-SR which was received during attach down the line to MSC in the SRVCC PS to CS request The MSC initiates a request
for an access transfer of the active session from the IMS network and includes
this STN-SR in the request Let’s understand the flow HLR gives the STN-SR to MME , MME gives STN-SR back to MSC MSC Now use this STN-SR to route the particular call to the IMS so that the Ongoing can be switched on this particular path which is bieng created between MSC and IMS using STN-SR as a B-Number There is new element the SCC-AS eventually receives the INVITE with help of STN-SR which indicates the need for an access transfer of an active
session. We will discuss SCC-AS also on the later part of the slide The STN-SR is the Number that is used to facilitate all this coordination of a voice call The 2nd One as you can see on the Screen , is C-MSISDN , This is a Correlation
MSISDN also known by the name of C-MSISDN , It is used for correlation of sessions at access transfer
and to route a call from the IMS to the same user in the CS domain. Now , Let’s understand the Network changes which are required for enabling the SRVCC SRVCC started evolving from Rel 8 & It continued till Rel 10/11 . The network upgrades required for the mobile operators are needed in both
the LTE network and as well as in legacy Network Now , Let’s understand which all network elements do we need to touch if we are going to launch the SRVCC The 1st one is ATCF , ATCF is a signalling controller that facilitates the handover from LTE to the Circuit switched 2G / 3G Networks and updates the VCC application server
VCC stands for ( Voice call continuity) application server These ATCF/ATGW enhancements will be implemented
in the SBC The 2nd one is ATGW : It is Access Transfer Gateway: It acts as an anchor for the IMS media traffic to allow media traffic to be switched quickly from
the PS access network to the CS access network via the MSC . Now , These changes in ATCF and ATGW are required on the eSRVCC . The next one is the changes which are required in the MSC Server : MSC is a Centre of all communication for SRVCC , It handles the Relocation Preparation procedure with MME via Sv Interface , It also
handles session transfer procedure with IMS Network It should support STN-SR and SRVCC specific parameters The Next one is the E-UTRAN : E-UTRAN is a 4G radio which selects a target cell for SRVCC handover, it is required to sends an indication to MME that this handover procedure requires
SRVCC Next one the MME which should support the SRVCC functionality such as Bearer Splitting , Sv Interface & STN-SR communication with MSC and so-n-so-far Some changes are also required in HSS like it should provision STN-SR & It should also provision C-MSISDN and It should pass on these parameters back to MME while a user is attaching on to the Network Now comes there are 2 New elements which have been there , The 1st one is SR-VCC-IWF : This is Single Radio Voice Call Continuity – Interworking Function : This is new Node altogether which was introduced in IMS Network which eliminates the need for MSC upgrades I mean to say if you don’t want to touch MSC Infrastructure , You can simply put up a box with SRVCC-IWF functionality and Now this node can do all the Jugglery , All the communications which is required between MME and MSC down the line It connects MME on one side and It connect MSC on the other side of Network The last one is SCCAS : This is also a New Node which is introduced for SRVCC , The SCCAS is a entity which combines the SIP signaling and acts as a back-to-back user agent , This provides an anchor point for incoming and outgoing calls Now , Let;s understand the eSRVCC There are few advancements introduced in SRVCC in order to Improve overall user experience , In the plain Old SRVCC, the home network application server
.. SCC-AS anchors the call . This anchoring by home network tends to increase the handover
delay in case a home subscriber roams to a distant network and latches on to a roaming
partner . This effectively means that Call must be routed back to Home Network only for any SRVCC handover happening in the Network . This problem is very very significant to Countries where National Roaming prevails There is major change done here with eSRVCC .. In enhanced-SRVCC , An ATCF/ATGW is introduced to anchor media sessions. All session messages before
and after the handover must exchange through the ATCF/ATGW in Serving Network only . Only
Bearers between the calling UE and the ATCF/ATGW need to be updated, This shortens the speech gap Along with eSRVCC , We got another feature
of aSRVCC also known as SRVCC in alerting phase , This adds the ability to perform access
transfer of media of an IM session in PS to CS direction in alerting phase for access
transfers As more and more operators trying to roll out LTE and adopt VoLTE/IMS as platform for providing voice services, SRVCC will be an option for
addressing the gaps in LTE coverage while providing continuous coverage for voice service. Despite the fact that SRVCC is apparently
more complex than CSFB , This continues to be the choice of the Mobile Operators due
to benefits coming to us from VoLTE Services . SRVCC plays key role in providing uniform
services across EPC Domain You can go thru above Specs for further studies
on this domain We have completed the SRVCC tutorial here , In case of any Queries or Doubts , Please feel free to put your comments , I will revert Quickly on same If you want to download this PPT or Video
, Please visit my Website . Its http://telecomtutorial.info Hope this Video presentation is useful for
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