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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).
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For ATCF calls, local SBC always stays in media path and can only try OMR procedure to bypass the previous MR.
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OMR Bypass Rules
Four possible bypass rules (outcomes) exist when applying OMR procedures as shown in the table below:
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| 0 | Table |
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| 1 | OMR Bypass Rules |
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| 3 | OMR Bypass Rules |
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In general, codecs received in main SDP should survive NRMA's intersection with both route PSPs (augmentation also if applicable). | |
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| Bypass Current and Previous Node | Objective: Bypass local media routing (SBC), and bypass previous media routing (SBC) Preconditions: - Egress Realm is present in OMR VR (except in last VR Instance)
- At least one codec in corresponding OMR PSP is present/allowed by egress Route PSP.
Note: - Codec attributes (DTMF, ptime, SS) is considered only if “transcode conditional” for that attribute is enabled.
- Codec attributes are matched for all the codec entries that are present in egress route PSP. For the ones that are not present in egress route, SBC’s behavior is don’t care* - i.e., neither the basic codec nor their attributes are taken in to account for this scenario.
- During rule re-evaluation based on answer in 18x, Codec attributes shall not be considered even if "transcode conditional" is enabled.
| | Bypass Previous Node Only | Objective: Bypass NO local media routing, and bypass previous media routing Preconditions: - Ingress Realm is present in OMR VR (except in last VR Instance)
- Codecs in corresponding OMR PSP survives the NRMA's intersection with both route PSPs (augmentation also if applicable)
| | Bypass Current Node Only | Objective: Bypass local media routing, and bypass NO previous media routing – same as the current DM feature that SBC supports Preconditions: - Both Ingress and Egress Realms are identical (or configured connected realms are matching)
- Maximum VR Instance also matches the realms
- At least one codec in corresponding OMR PSP is present/allowed by egress Route PSP.
NOTE: - Codec attributes (DTMF, ptime, SS) are considered only if “transcode conditional” for that attribute is enabled.
- Codec attributes are matched for all codec entries present in egress route PSP. For the ones that are not present in egress route, SBC’s behavior is *don’t care* - i.e., neither the basic codec nor their attributes are taken in to account for this rule.
- During rule re-evaluation based on answer in 18x, Codec attributes are considered even if "transcode conditional" is enabled.
| | Bypass Current Node Only | Objective: Bypass NO local media routing, and bypass NO previous media routing – same as normal call without OMR Preconditions: - c=0.0.0.0 is received in OFFER
- At least one codec in corresponding OMR PSP is present/allowed by egress Route PSP.
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Support for Insertion and Transparency of TRF Attribute as per RAVEL Architecture
SBC 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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