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Overview

The E-CSCF and EATF may reside within the P-CSCF to support Emergency and related SRVCC procedures. Sonus provides the P-CSCF, E-CSCF, and EATF functions on the SBC and partner with IMS core and LRF or ECS vendors for complete solution.

Emergency Session Transfer Number for SRVCC (E-STN-SR) isused in the session transfer procedure for emergency calls, pointing toward the serving (visited if roaming) IMS EATF, which handles the IMS emergency session transfer from PS to CS access. The same E-STN-SR is used for all the emergency session access transfers within one PLMN.

As per standards, either E-STN-SR is configured locally in MME and transferred to MSC or MME sends an Emergency indication to MSC and lets MSC to utilize its local configured E-STN-SR. This means in an IMS core network we need to have a single EATF running which is serving that PLMN so that all the MSC servers in that region are going to point to same EATF for PS to CS handover.

At a high level, the functions performed by the EATF are:

• Serves as a VLR while roaming in IMS network.
• Provides the procedures for IMS emergency session anchoring and PS to CS Access Transfer.
• Acts as a routing B2BUA which invokes third party call control (3pcc) to enable Access Transfer.
• Performs the session continuity when the Access Transfer request indicated by the E-STN-SR is received.

Call Flows

The following call-flow depicts the call anchoring at EATF when the initial call is made over PS domain:

A brief description of the call flow is as follows:

1. UE sends an INVITE with Emergency URN to P-CSCF.
2. P-CSCF detects the call as Emergency call based on Emergency URN (SoS URN) and forwards the call to E-CSCF.
3. E-CSCF forwards the call EATF. E-CSCF adds two Route headers – one pointing to EATF and another pointing itself (E-CSCF).
4. EATF anchors the call and acts as a B2BUA. Sonus EATF deletes the top-most Route header pointing to itself and anchors the emergency session.
5. Sonus EATF loops back the call towards E-CSCF based on the top route header (that points E-CSCF) from the remaining route header list.
6. E-CSCF forwards it to PSAP.
   In the remaining steps, the 200 OK SIP response is forwarded as appropriate.

The following call-flow depicts the PS to CS handover of an emergency call that was anchored at EATF:

A brief description of the call flow is as follows:

1. Initial call is established over PS domain and the call is anchored at EATF.
2. After the UE moves from LTE to 2G/3G domain, based on the UE measurement reports, E-UTRAN decides to trigger PS to CS handover procedures.
3. MSC sends an INVITE with Request-URI pointing to E-STN-SR.
4. I-CSCF routes the INVITE with E-STN-SR to EATF.
5. EATF handles the INVITE with E-STN-SR locally and initiates re-INVITE towards the remote peer. The re-INVITE contains the SDP pointing to MSC so that a media path is established between remote peer and calling party (via MSC).

Global Configuration

set addressContext default zone ZONE_CORENW disableZoneLevelLoopDetection enabled

EATF Configuration

set global signaling srvcc eStnSr +1-237-555-5555 callLingerTimer 32

Emergency Profile

set profiles services emergencyCallProfile EMG-ECSCF sosInContactOfRegister enable
commit

Configuring SBC Towards E-CSCF

set addressContext default zone ZONE_CORENW sipTrunkGroup TG_TOWARDS_ECSCF services emergencyCallProfile EMG-ECSCF
commit

The following profiles are configured:

• Transparency Profile  
• Zone Index profile  
• Attaching EATF Profile to IPSP
  
• Ip Signaling Profile
• Sip Trunk Group 
  

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Transparencyprofileecscf Transparencyprofileecscf

Transparency Profile

set addressContext default zone ZONE_ECSCF sipTrunkGroup TG_TOWARDS_ECSCF services transparencyProfile TRANS_PROF_ECSCF
commit

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ZoneIndexProfileecscf ZoneIndexProfileecscf

Zone Index Profile

The following fields must be configured:

• Zone Index Profile
• Description
• Zone Index

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AttachingEATFProfiletoIPSP AttachingEATFProfiletoIPSP

Attaching EATF Profile to IPSP

1. Select EATF profile from the Access Transfer Policy Profile option.

   Caption
   0 Figure 1 Selecting EATF Profile
   Image Modified

2. Attach the EATF profile to the IP SIGNALING PROFILE of a Trunk Group.

   Caption
   0 Figure 1 Attaching EATF profile to the IPSP Trunk group
   Image Modified

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IpSignalingProfileecscf IpSignalingProfileecscf

Ip Signaling Profile

• Common Ip Attributes
•  Relay Flags   
• Transparency Flags
  
• Egress Ip Attributes 
• Ingress Ip Attributes
  

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Common Ip Attributes_ecscf Common Ip Attributes_ecscf

Common Ip Attributes

The following Common IP Attributes must be enabled in PSX under IP SIGNALING PROFILE:

• Disable Host Translation
• End to End BYE
• End to End RE-INVITE
• End to End UPDATE
• End to End PRACK
• Insert Peer Address As Top Route Header
• No Port Number 5060
• ReQuery PSX on REGISTER Refresh
• Store P-Charging-Vector
• Validate Access Nw Info Header

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Relay Flags_ecscf Relay Flags_ecscf

Relay Flags

The following Relay Flags must be enabled under IP SIGNALING PROFILE:

• Conference Event Package
• Dialog Event Package
• Info
• Message
• Options
• PUBLISH
• REFER
• Reg Event Package
• Status Code 4XX-6XX
• Select Reject the REFER request if no match is found from Refer To Header Relay option.

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Transparency Flags_ecscf Transparency Flags_ecscf

Transparency Flags

The following Transparency Flags must be enabled under IP SIGNALING PROFILE:

• Request-URI

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Egress Ip Attributes_ecscf Egress Ip Attributes_ecscf

Egress Ip Attributes 

The following Egress IP Attributes must be enabled:

• Select SIP Only from IP Protocol Type.
• Disable 2806 Compliance (under Flags)
• Transit PAI From Unregistered Peer (under Flags)
• Add Loop Back Route Header (under Flags)
• Select Include npdi from NPDI Options (under Number Portability Attributes option).
• Select P-Preferred-ID from Privacy Information (under Privacy option).
• Merge Received Contacts with Existing Contacts (under Redirect)
• Select Include None from Include Charge Information option (under SIP Headers and Parameters)
• Select Original Called Number (OCN) from SIP TO Header Mapping option (under SIP Headers and Parameters)
• Include CPC Information (under SIP Headers and Parameters > Flags)
• Transparency For Destination Trunk Group Parameter (under SIP Headers and Parameters > Flags)
• End To End Ack (under SIP Headers and Parameters > Flags)
• Diversion (under SIP Headers and Parameters > Call Forwarding)
• Select Retry Without ETS from the Action For ETS 400 Responses With 417 Reason Code drop-down box (under SIP RPH ETS)
• Set ETS Default Priority Value as 0  (under SIP RPH ETS)

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Ingress Ip Attributes_ecscf Ingress Ip Attributes_ecscf

Ingress Ip Attributes

The following Ingress IP Attributes must be enabled:

• 181 Supported (under Flags option)

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SipTrunkGroupecscf SipTrunkGroupecscf

Sip Trunk Group

The following profiles/fields must be enabled:

• Zone Index Profile
• IP Signaling Profile
• Feature Control Profile
• Signaling Profile
• SIP Domain
• IP Version Preference
• Use IPTG Routing (Hop By Hop Routing) For Ingress (Ingress > Flags)
• Do Not Use For Fallback Bearer Capability (Egress > Flags)
• IP Peer Supported (Egress > IPTG)
• Egress IP Signaling Profile (Egress > IPTG)
• Select Traffic Management Options from Trunk Group Reserved Level 1 and Trunk Group Reserved Level 2.
• Select Services as Not Screened.

Configuring SBC Towards MSC

set addressContext default zone ZONE_CORENW sipTrunkGroup TG_TOWARDS_MSC state enabled mode inService media mediaIpInterfaceGroupName LIF2
commit 

The following profiles are configured:

• Transparency Profile
• Zone Index Profile
• IP Signaling Profile
• SIP Trunk Group

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TransparencyProfile TransparencyProfile

Transparency Profile

set addressContext default zone ZONE_CORENW sipTrunkGroup TG_TOWARDS_IMSCORE services transparencyProfile TRANS_PROF_MSC
commit

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ZoneIndexProfile ZoneIndexProfile

Zone Index Profile

The following flags must be configured:

• Zone Index Profile
• Description
• Zone Index

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IPSignalingProfile IPSignalingProfile

IP Signaling Profile

The following parameters must be configured under "Ip Signaling Profile".

• Configuring Common IP Attributes
• Configuring Relay Flags
• Configuring Transparency Flags
• Configuring Egress IP Attributes
• Configuring Ingress IP Attributes

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Common IP Attributes Common IP Attributes

Common IP Attributes

The following must be enabled in PSX under IP SIGNALING PROFILE:

• Add P-Charging Function Addr
• No Port Number 5060
• P-Called-Party-Id-Support
• Create P-Charging-Vector
• Create P-Visited-Network Id
• Create Path Header
• Disable Host Translation
• Restrict History Info Header
• End To End BYE
• End To End RE-INVITE
• End To End UPDATE
• End To End PRACK
• Enable Default PUI Procedures
• Enable Dial String Handling
• Store P-Charging Vector
• Store Service -Route Header
• Validate Access Nw Info Header

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Relay Flags Relay Flags

Relay Flags

The following Relay Flags must be enabled under IP SIGNALING PROFILE:

• Dialog Event Package
• Info
• Options
• Select Reject the REFER request if no match is found from Refer to Header Relay

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Transparency Flags Transparency Flags

Transparency Flags

The following Transparency Flags must be enabled under IP SIGNALING PROFILE:

• Pass Complete Contact Header

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Egress IP Attributes Egress IP Attributes

Egress IP Attributes

The following parameters are to enabled :

• Select SIP Only from IP Protocol Type
• BCI Interwork Encountered (in BCI)
• Select Use IP Signaling Peer Domain In R-URI (in Domain Name)
• Select Include npdi from NPDI Options (in Number Portability Attributes option).
• Select Include Privacy and Do Not Include Tel URI in PAI Header(in Privacy)
• Select Merge Received Contacts with Existing Contacts from Contact Handling(under Redirect)
• Select Include None from Include Charge Information (in SIP Headers and Parameters option).
• Select None from SIP TO Header Mapping (under SIP Headers And Parameters option)
• Select Include CPC Information(under Flags option)
• Select Transparency For Destination Trunk Group Parameter(under Flags option)
• Select End to End Ack (under Flags option)
• Select Diversion from Redirect Information (under Call Forwarding)

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Ingress IP Attributes Ingress IP Attributes

Ingress IP Attributes

The following parameter is to be enabled:

• Map Called Party Category In P-Sig-Info Header

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SIPTrunkGroup SIPTrunkGroup

SIP Trunk Group

The following profiles/fields must be enabled:

• Feature Control Profile
• IP Signaling Profile
• Signaling Profile
• SIP Domain
• Zone Index Profile
• IP Version Preference

• Use IPTG Routing (Hop by Hop Routing) For Ingress (Ingress > Flags)

• Do Not Use For Fallback Bearer Capability (Egress > Flags)
• IP Peer Supported (Egress > IPTG)
• Egress IP Signaling Profile (Egress > IPTG)
• Select Traffic Management Options from Trunk Group Reserved Level 1 and Trunk Group Reserved Level 2.

• Select Services as Not Screened.