VoLTE_SRVCC_E2Erevised

VoLTE & SRVCCPresented by: Amit DeshmukhVersion 1.0 Delivered: 01 / 17 / 2014

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Introduction to IMS

What is IMS?• IMS - IP Multimedia Subsystems

• It is a network architecture established to provide seamless mobility and service delivery and ensure that subscriber services can be delivered to them irrespective of their terminal, access and core n/w connectivity within a home or visited network.

• Also provides service delivery for fixed network.

• In IMS, the signaling sessions are independent of underlying bearer network which is made possible by providing consistent interface between terminal and application server (TAS) and also various functional nodes and the application server.

Introduction to IMS

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Introduction to IMS

• IMS is facilitated through the introduction of a service layer that extends across all functions involved in the delivery of services.

• The service layer encompasses:AS – Provides service and session control aka. TAS (Telephony Application Server)CSCF (Call Session Control Function) – Provides SIP Proxy Function

CSCF has three statesS-CSCF (Serving Call Session Control Function)I-CSCF (Interrogating Call Session Control Function)P-CSCF(Proxy Call Session Control Function)

HSS (Home Subscriber Server) – Provides subscription, security and mobility information.

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Introduction to IMS

What does the IMS provide that the internet can’t offer the subscriber, particularly when service is free on the internet

Answer: Provision for QoS is an important factor. Internet is a collection of IP networks across which end-to-end QoS is very difficult to provide and maintain.The richness of service is difficult to offer with the internet. IMS networks and services are based on IP networks which are managed and reliable.

Example: An IMS subscriber engaged in Voice or Video call can seamlessly switch between terminals and access networks whilst still on call and the service is also providing presence, redirect, voicemail etc.

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Introduction to IMS

IMS Services• Voice and Video calling including multiparty and multiple session

support.• Enhanced redirect services and call prioritization• Ability to split incoming media eg. – Incoming video call has video

split to the TV and audio split to the phone.• Multiplayer gaming services• IPTV• Presence information for all services• Location based services• Voice/Video mail• Prepaid services• Hi-Definition audio and video services• Instant messaging• Push to talk

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Introduction to IMS

Some Acronyms• IMPI - IP Multimedia Private Identity (sip:IMSI@host)• IMPU - IP Multimedia Public Identity (sip:1MSISDN@host)• MIME - Multipurpose Internet Mail Extensions• UAR – User Authorization Request• UAA – User Authorization Answer• SAR – Server Assignment Request• SAA – Server Assignment Answer• LIR – Location Information Request• LIA – Location Information Answer• MAR – Media Authorization Request• MAA – Media Authorization Answer• UDR – User Data Request• UDA – User Data Answer• PUR – Profile Update Request• PUA – Profile Update Answer

Diameter

SIP

sip:IMSI@host

sip:1MSISDN@host

VoLTE - Registration

(1) REGISTER (IMPU, IMPI)(2) REGISTER (STN-SR Added)

(3) UAR (IMPU, IMPI)

(4) UAA – SERVER CAPABILITIES (S-CSCF IP ADDRESS)

(5) REGISTER (IMPU, IMPI) (6) MAR (IMPU, IMPI)

(7) MAA (IMPU, IMPI, auth vectors)(8) 401 Unauthorized(9) 401

Unauthorized)(10) 401 Unauthorized)

(11) REGISTER (IMPU, IMPI, auth resp) (12)(13) UAR (IMPU, IMPI)

(14) UAA – S-CSCF NAME (FQDN)

(15) REGISTER (IMPU, IMPI, auth resp)

(16) SAR (IMPU, IMPI, S-CSCF Name)

(17) SAA (IMPU, User Profile)

VoLTE UE SAEGWeNodeB MME P-CSCF /ATCF I-CSCF S-CSCF TAS-

SCCAS HSS

VoLTE - Registration

VoLTE UE

(18) 200 OK(19) 200 OK

(20) 200 OK(21) REGISTER (IMPU, ServiceInfo)

(22) 200 OK(23) SUBSCRIBE (Event: Reg)

(24) SUBSCRIBE (Event: Reg)

(25) NOTIFY (Event: Reg, State: Active)

(26) NOTIFY (Event: Reg, State: Active) (27) SIP INFO MESSAGE WITH ATCF MGMT URI (C-MSISDN)

(28) 200 OK

SAEGWeNodeB MME P-CSCF /ATCF I-CSCF S-CSCF TAS-

SCCAS HSS

VoLTE – Registration with Sh Interface Enabled

Authentication

REGISTER SIP: IMPU Authorization: Username: IMPI

REGISTER STN-SR, ATCF-mgmt-URI

UAR

UAAREGISTER

SAR

SAA200 OK

200 OK

200 OK

MIME body (orig. Register including contact)


SCCAS HSS

VoLTE & SRVCC - Registration

Sh UDR (IMPU, IRS)

Sh UDA (IMPU List)

Sh UDR (IMPU , MSISDN)

Sh UDA (C-MSISDN)

Sh PUR (STN-SR)

Sh PUA (IMPU list)Insert Subscriber Data – Request (STN-SR)

Insert Subscriber Data – Answer

200 OK

MESSAGE (ATU-STI, C-MSISDN)



200 OK200 OK

200 OK


SCCAS HSS

VoLTE Call Flow

(1) INVITE sip:msisdn@host (SDP)

Originating Services

(2) LIR (IMPU =sip:msisdnA@host

(3) LIA (S-CSCF Name)

(4) INVITE sip:msisdn@host (SDP)

Terminating Services

(5) 183 Session Progress (SDP)

UE - O MME P-GW S-CSCF I-CSCF HSS S-CSCF UE-T

eNodeB S-GW ATCF / P-CSCF TAS-O PCRF TAS-T ATCF /

P-CSCF

sip:msisdn@host

sip:msisdn@host

VoLTE Call Flow Cont.

(8) AAA

(9) RAR

(10) RAA

(11) Create Bearer Request(Via S-GW- QCI-1)

(12) Activate Dedicated Bearer Context Request (eRAB Setup)

(13) eRAB Setup Request QCI-1

(14) eRAB Setup response

183 Session Progress (SDP)

(6) AAR(7) 183 Session Progress



P-CSCF

VoLTE Call Flow Cont.

(15) Activate Dedicated Bearer Context Accept

(16) Create Bearer Response

(17) PRACK

PRACKPRACK

PRACK

PRACK

180 Ringing



P-CSCF

SRVCC From LTE to 3GPP UTRAN/GERAN

• SRVCC – Single Radio Voice Call Continuity

• SRVCC 3gpp based architecture is designed to provide seamless voice call continuity when a VoLTE device with active VoIP call, transitions from LTE to UTRAN or GSM only coverage.

• The SRVCC nodes on the LTE side are eNodeB, MME.

• The SRVCC nodes on the target CS domain is MSC.

• The SRVCC nodes in the IMS domain are ATCF and SCC AS (TAS)

• The SRVCC nodes on the target CS network and IMS network are enhanced with additional capabilities to support smooth transition of the UE from LTE to 3GPP UTRAN/GERAN based networks (e.g. 2G/3G).

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• The UE sends measurement reports to E-UTRAN.

• Based on UE measurement reports, the source E UTRAN decides to trigger an SRVCC handover to the CS Domain.

• Source E UTRAN sends Handover Required message having Target ID, generic Source to Target Transparent Container, SRVCC HO indication tothe source MME. The E UTRAN places the “old BSS to new BSS information IE” for the CS domain in the generic Source to Target Transparent Container. The SRVCC HO indication indicates to the MME that the target is only CS capable hence this is a SRVCC and over operation only toward the CS domain.

• Based on the QCI associated with the voice bearer (QCI 1) andthe SRVCC HO indication, the source MME splits the voice bearer from the non-voice bearers and initiates the PS-CS handover procedure for the voice bearer only toward the MSC Server.

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EPS Reference Architecture for SRVCC to UTRAN as Destination Network


Some Terminologies• SCC AS- Session Centralization and Continuity Application Server• ATCF – Access Transfer Control Function• STN-SR – Session Transfer Number for SRVCC• C-MSISDN- Correlation Mobile Subscriber Integrated Services Digital Network-

Number.• PS – Packet Switched• CS – Circuit Switched• PAI – P-Asserted Identity

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Role of STN-SR• The STN-SR is configured for an SRVCC subscriber at the time of service provisioning (TS 23.237).• The HSS provides the STN-SR to the MME that is provisioned for a UE during its attach

procedure.• If the user has an active context at the MME, the HSS also updates the MME if the STN-SR has

been modified for that UE.• The MME sends the STN-SR in the SRVCC PS to CS request to the MSC over the Sv interface.• The MSC initiates a request for an access transfer of the active session from the IMS network

and includes the STN-SR in the request.• Refer to section 6.2.2.1 of TS 23.216• The SCC AS eventually receives an INVITE with the STN-SR or C-MSISDN which indicates the

need for an access transfer of an active session.• The SCC AS is the entity in the IMS domain that handles management of the access transfer of

an active session.• The SCC AS coordinates the proper media and signaling exchange to the access network of

where the UE is present. The STN-SR is the number that is used to facilitate that • Refer to section 6.3.2.1.4 of TS 23.237

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Use of C-MSISDN in SRVCC• C-MSISDN is one of the variables that is used to identify an anchored voice session in IMS• Below is a list of events that take place to clarify the use of the C-MSISDN during a PS-to-CS handover

for a VoLTE session:1) The SCC AS is the anchor of the call in the IMS domain.2) Per TS 23.216, during the Attach process, the HSS sends the C-MSISDN to the MME as part of the UE’s subscription data.3) Per TS 24.237, the SCC AS obtains registration state information from the HSS which includes the C-MSISDN that is bound to the IMS private user id per TS 23.237.4) Per TS 23.237, the SCC AS provides the C-MSISDN to the ATCF upon successful IMS registration.5) As the UE moves out of LTE coverage, the MME requests the MSC to transfer the call to legacy technologies such as 3G.6) Per TS 23.216, the MME sends the C-MSISDN to the MSC as part of the SRVCC PS to CS handover.7) Per TS 23.856, “The MSC Server correlates the incoming PS to CS Handover request with the anchored session using the C-MSISDN obtained when anchoring the session”.8) MSC sends an INVITE to the IMS side which includes the C-MSISDN (as a P-Asserted-Identity) representing the calling party number.9) Upon receiving the INVITE, the SCC AS determines that an access transfer is being requested. The SCC AS uses the STN-SR or the C-MSISDN to identify the anchored session.


eNodeB MSC P-GW S-CSCF I-CSCF HSS S-CSCF UE-T

MME S-GW ATCF / P-CSCF TAS-O PCRF TAS-T ATCF /

P-CSCFUE - O


MeasurementReport

HO Req.

SRVCC PS to CS Req.

Ongoing VoIP Session with IMS Established over LTE

MSC initiates Bearer establishment in the CS domain (Prep HO Req. to Target MSC

SRVCC PS to CS Resp.



P-CSCFUE - O


HO Command

HO from EUTRANCommand

INVITE tel:+STN-SR

INVITE C-MSISDN in PAI

Session Transfer Update Remote End. Release Resource IMS Access Leg

SRVCC PS to CS Complete Notification

SRVCC PS to CS CompleteAck.

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P-CSCFUE - O


Delete Bearer Command

Delete Bearer Command

Delete Bearer Request

Delete Bearer Response

Delete Bearer Response



P-CSCFUE - O


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CS to IMS/VoLTE Call Flow

UE-B ATCF S-CSCF TAS I-CSCF HLR/STP USD HSS MGCF/GMSC

SRI [msisdn, GMSCE164] ADDR

SRI [msisdn, GMSCE164] ADDR

LDAP: Search Request (IMPU/msisdn,imsi)

LDAP: SearchResEntry, SearchResDoneIf subscriber profile is not present,but the entry is present in the TAS DB

SRI Ack [IMRN=<IMRN prefix>MSISDN-B]INVITE tel:IMRN,c1, SDP1

INVITE sip:MSISDN-B, c2, SDP2

183 Session in Progress, SDP2

PRACK, 200 OK not shown, If preconditions are used, a subsequent UPDATE (SDP offer) and 200 OK (SDP Answer) will occur end to end here

180 Ringing

200 OK

ACK

sip:MSISDN-B

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TAS And Its Interfaces

Thank You

VoLTE_SRVCC_E2Erevised

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