OMR and TRF Support

Optimal Media Routing Support

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Voice over LTE (VoLTE) is a technology for providing existing voice and SMS services over LTE which does not have a circuit-switched domain. It is implemented using IP Multimedia Subsystem (IMS), which is a standard 3GPP technology for IP-based multimedia services. One of the key requirements of the Roaming Architecture for Voice over LTE with Local Breakout (RAVEL) as described in 3GPP TS 23.228, 3GPP TS 24.229, and as proposed in 3GPP TR 23.850, is that the  Interconnection Border Control Function (IBCF) supports Optimal Media Routing (OMR) procedures and allows bypassing the Transition Gateways (TrGWs).

The 3GPP TS 29.079 describes the procedures for optimizing the media path for call within IP Multimedia Subsystem. The purpose of OMR is to identify and remove unnecessary media functions from the media path for each media stream associated with a session. The OMR procedures identify and name the IP address realm used for each media path segment among UAs and TrGWs. An IP realm name is associated with each set of IP endpoints that are mutually reachable through IP routing and without address translation. Endpoints in different IP realms usually require allocation of a TrGW between those IP realms for connectivity and possibly for NAT.

Prior to OMR procedures, the

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supported direct media between endpoints residing in the same media zone. With the implementation of OMR procedures, the

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is enhanced to perform media path optimization even when direct media is not possible between end points by allowing intermediate nodes to be bypassed for media anchoring.

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supports OMR procedures in IBCF and P-CSCF roles.

supports secondary realm if it is present in the OMR SDP attributes. It does not add secondary-realm while forwarding SDP offer.

When the endpoints in different IP realms are mutually reachable without allocation of a TrGW, then OMR procedures may use provisioned information about such connected IP realms to determine possible optimal media paths through these connected realms. An

can be bypassed if:

• No transcoding is required
• Protection of a network resource is unnecessary
• When there are multiple SBCs, the successive
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  in a path is reachable by the previous
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  on the same path through a common IP realm.

The OMR procedure is applied before sending out the offer and re-evaluated based on the answer received in 18x. The re-evaluation of OMR procedure during answer processing is based on certain conditions:

• Local SBC was in media path while sending offer
• Egress peer supports SIP UPDATE
• Pass-through based call outcome on answer received in 18x 

Four possible outcomes exist when applying OMR procedures:

• Bypass local MR and bypass previous MR
• Bypass no local MR, and bypass previous MR
• Bypass local MR (SBC) and bypass no previous MR (SBC)
• Bypass no local MR and bypass no previous MR (same as normal call without OMR)

remains in the path (local policy) and disables OMR when any of the following features are active:

• Multiple M-Lines in SDP
• Video Only Call
• LRBT
• Two Stage Calls

For the following features, "bypass no local MR, and bypass previous MR" is attempted, with no visited-realm sent in egress INVITE.

• Lawful Intercept
• SIP Rec
• Trans-code Only
• ATCF Call

OMR Bypass Rules

Four possible bypass rules (outcomes) exist when applying OMR procedures as shown in the table below:

Support for Insertion and Transparency of TRF Attribute as per RAVEL Architecture

supports Transit and Roaming Function (TRF) and applies TRF procedures defined for a looped back RAVEL call. The TRF procedures include the following:

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   as PCSCF uses Roaming Agreement from Router Controller Bundle (RCB). If UE is identified as roaming, the Roaming Agreements flag Support RAVEL, is accessed from RCB.
   
   
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   as PCSCF adds configured preferred URIs.
   
   1. A new configuration Preferred_TRF_URI of type "string" is introduced in IPSP level in PSX. If the incoming request is identified as roaming (calling Party), SBC adds "Feature-Caps" header with configured TRF-URI in "+g.3gpp.trf" parameter value.
   2. A new configuration Preferred_MRB_URI of type "string" is introduced in IPSP level in PSX. If the incoming request is identified as roaming (calling Party), SBC adds "Feature-Caps" header with configured MRB-URI in "+g.3gpp.mrb" parameter value.
      
      

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   as IBCF adds configured preferred URIs.
   1. A new configuration Preferred_TRF_URI of type "string" is introduced in IPSP level in PSX. If the incoming request is identified as roaming (calling Party) and call is originating, SBC adds "Feature-Caps" header with configured TRF-URI in "+g.3gpp.trf" parameter value. If the received request already contains a "+g.3gpp.trf" parameter field in "Feature-Caps" header, SBC does not overwrite the header field value with the configured Preferred_TRF_URI value.

   2. A new configuration Preferred_MRB_URI of type "string" is introduced in IPSP level in PSX. If the incoming request is identified as roaming (calling Party) and call is originating, SBC adds "Feature-Caps" header with configured MRB-URI in "+g.3gpp.trf" parameter value. If the received request already contains a "+g.3gpp.mrb" parameter field in Feature-Caps header, SBC does not overwrite the header field value with the configured Preferred_MRB_URI value.
      
      

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   as IBCF/TRF identifies TRF role and Mark Looped-back call. If the SIP INVITE contains "Feature-Caps" header field “+g.3gpp.loopback”, the call is marked as a looped back call. If the top route header points to self, SBC considers itself as the TRF. As part of TRF Processing, SBC performs:
   
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      as IBCF/TRF supports to remove loopback from "Feature-Caps".
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      removes “+g.3gpp.loopback" when copying "Feature-Caps" header to egress.
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      as IBCF/TRF supports to handle IOI-Parameter (Inter-Operator-ID) in P-Charging Vector.
      
      1. SBC stores any orig-IOI parameter received in P-Charging-Vector header.
      2. SBC uses a type-2 orig-IOI parameter in the P-Charging-Vector header of egress request, which identifies the network that SBC (TRF) belongs to (IOI Configured in Ingress SG).
      3. SBC stores the term-IOI in P-Charging-Vector and uses a type-2 term-IOI, which identifies the TRF’s network  in the P-Charging-Vector header of the response  generated for corresponding 1xx/2xx responses received.
      4. SBC removes any transit-IOI parameter from P-Charging-Vector sent in egress request.
         
         
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   as IBCF to add TRF. SBC checks if "+g.3gpp.trf=<trf_uri>" exists in "Feature-Caps" header. if not, SBC supports to send bottom "Route Header" in PSX Query to fetch Roaming Agreement. If Roaming Agreement is present and TRF-URI/MRB_URI is configured in egress IPSP (in PSX), SBC is supported to send "+g.3gpp.trf=<trf_uri>"/"+g.3gpp.mrb=<mrb_uri>" in "Feature-Caps" header.