VoLTE Flows and legacy CS network. Basic call routing to and from CS network using BGCF, MGCF, MGW. ENUM role in routing. IMS Cetralized Services (IMC) and SRVCC scenarios.
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What is ?
• The standard is defined by GSMA in IR.92 and IR.94 as a minimum mandatory set of features for an interoperable,
high quality IMS-based telephony service and SMS over LTE network.
The scope includes:
• IMS basic capabilities and supplementary services for telephony
• Real-time media negotiation, transport, and codecs
• LTE radio and evolved packet core capabilities
• Functionality that is relevant across the protocol stack and subsystems
• Additional features that need to be implemented for the UEs and networks that wish to support concurrent Circuit
Switched (CS) coverage
Description of basic call flows and procedures can be found in VoLTE Service Description and Implementation Guide.
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CS related flows
Key Facts
• Not everywhere isa strong LTE signal
• Many devices still don’t support 4G
• Operators invested a lot of money in their 3G infrastructure
• Not all the services are supported or have been transferred in 4G network yet
(e.g. voice mail, media servers for ring-back-tones, etc.)
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IMS Entities III
Breakout Gateway Control Function (BGCF)
• In case S-CSCF is not able to find the routing towards terminating network within IMS, the request is sent to
BGCF
• Based on prefix the BGCF selects MGCF and forwards the message
• Can communicate with BGCF in a peer network
• Can be part of IMS Core or SIP router
Media Gateway Control Function (MGCF)
• Provides signaling interworking between CS and IMS
• Controls MGW
Media Gateway (MGW)
• Provides media interworking between CS and IMS
• MGW and MGCF are collocated, mostly a part of MSC.
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How to Call?
RTP
UASUAC
SIP INVITE (SDP-O)
Terminating
Network
Originating
Network
P-CSCF
S-CSCF
I-CSCF
A-SBC
O-TAS IPSMGW
DNS
HSS
DNS
HSS
P-CSCF
S-CSCF
I-CSCF
T-TAS
A-SBC
ALG/AGWALG/AGW
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IMS Session Setup
Purpose
• Set up a signaling path
• Exchange parameters of the multimedia session
• Allocate resources in access networks and on SBCs
• Trigger TAS to apply originating and terminating services
Key Facts
• There are always two TASes and two IMS Cores on the signaling path. Although physically they can be the
same servers, logically we have to distinguish the originating and terminating network elements
• Routing towards ASs is driven by IFC (part of the Subscriber Profile)
• Originating S-CSCF provides routing towards terminating network
• IMS Network (domain) is represented by I-CSCF. The translation (domain -> IP) is done by DNS. I-CSCF
implements Location Service
• Both TAS and S-CSCF can modify R-URI. Both can also trigger ENUM
• In case S-CSCF can’t find the routing in IMS (e.g. because of tel uri in R-URI), the message is routed to CS via
BGCF and MGCF
• Terminating S-CSCF updates R-URI with the Contact information
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IMS Databases II
Electronic Number Mapping System (ENUM)
• Part of DNS, provides translation for telephone numbers
• Can be used for routing towards CS network.
• Triggered either by TAS or by S-CSCF.
• In case TAS receives ENUM response, it includes npdi tag in the route header
• Used for Mobile Number Portability (MNP), MNP Discovery allows its users to customize the information
returned in the query response, including:
• PSTN or Tel URI (user@domain format)
• E.212 MCC/MNC
• Service Provider Network (SPN)
• Alternate SPN
• Number Portability Dip Indicator
• Standardized in GSMA PRD IR.67
• GSMA mandates PathFinder services that covers 240 countries and territories and thus powers access to
95 % of world’s phone numbers
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IMS Entities IV
Service Centralization and Continuity Application Server (SCC AS)
• Supports centralization of services in IMS
• SCC AS performs CS Breakout or CS Retry
• The SCC AS performs CS Breakout for a terminating session setup request that is anchored in IMS and
when subscriber is currently located in CS for media.
• The SCC AS performs CS Retry for a terminating session setup request that is anchored in IMS when
the session termination in PS fails
• Supports Access Transfer
• The SCC AS keeps a special number required for Access Transfer up-to-date in (ePC)HSS
• Handles Access Transfer requests and updates the remote leg when required
• Provides Terminating Access Domain Selection (T-ADS)
• The SCC AS selects the domain (PS or CS) for a terminating session setup request that is anchored in
IMS and performs then actual session termination in the selected domain
• Often collocated with TAS
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ICS Routing Numbers
• IP Multimedia Routing Number (IMRN)
• An IP Multimedia Routeing Number (IMRN) is a routable number that points to an SCC AS in the IMS
• In a roaming scenario, the IMRN has the same structure as an international ISDN number
• IMRN is retrieved from SSC AS via CAMEL (SCC AS implements Service Control Point (SCP)) or via I1 reference
point (ICS enhanced UEs)
• See TS 23.003 for more information
• CS domain Routing Number (CSRN)
• Is a routable number that is used to route a call from IMS to the UE in the CS domain.
• Retrieved from HSS
• See TS 23.003 for more information
• Mobile Station Routing Number (MSRN)
• Is a routable number that is used to route a call in the CS domain
• Retrieved from HLR
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SRVCC
• Single Radio Voice Call Continuity (SRVCC)
• Access transfer without media anchoring – no ATCF/ATGW, SCC AS addressed by STN-SR (R9)
• Emergency SRVCC
• Support of access transfer for Emergency Calls (R9)
• eSRVCC
• Enhanced SRVCC – Media anchoring (+ mid-call feature support) (R10)
• aSRVCC
• Alerting SRVCC – Support of SRVCC PS-CS transfer of a call in alerting (ringing) phase (R10)
• vSRVCC
• Video Call support (R11)
• rSRVCC
• Reverse SRVCC – Support of access transfer from GSM/UTMS to LTE (R11)
• bSRVCC
• Before ringing SRVCC – Support of SRVCC PS-CS transfer of a call in pre-alerting phase (R12)
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IMS Entities V
• Access Transfer Control Function (ATCF)
• Acts as a SIP signalling anchor
• Located in the SIP signalling path between P-SCSF and S-CSCF
• Controls the ATGW, where the media plane is anchored
• During the session transfer, the ATCF establishes a new session with the SCC AS. This new session substitutes
the old session between the ATCF and the SCC AS
• Often it is a part of the SBC
• Access Transfer Gateway (ATGW)
• Anchors the media session
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SRVCC Identifiers
• Session Transfer Number for SRVCC (STN-SR)
• It is a TEL URI of ATCF
• Used by MGCF for routing of the call towards IMS
• ATCF can support multiple STN-SRs
• Correlation MSISDN (C-MSISDN)
• MSISDN of the user in CS Network
• Used as P-Asserted-Identity in the new session
• Helps to identify the session to be transferred
• Access Transfer Update – Session Transfer Identifier (ATU-STI)
• PSI of SCC-AS
• Used by ATCF during session transfer
• Target-Dialog (tdialog)
• Identifies the session to be transferred
• Sent from ATCF to SCC-AS
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Take Away
• IMS supports interworking with legacy network via MGCF/MGW
• Routing towards particular MSC is performed by BGCF
• ENUM dip can be used for decision whether user is IMS or SC subscriber.
• Long-term goal is to centralize services in IMS – that means even those subscribers who are
currently in CS network will be handled by IMS. IMS Centralization (IMC) is provided by a dedicated
application server – SCC AS
• SCC AS does also support Voice Call Continuity – SRVCC scenarios
