What it does

Reorigination enables CS calls to be processed in the SCC-AS. SS7 triggers received by the Rhino VoLTE TAS are forwarded to a number within a pre-defined range, which causes the call to be rerouted into the IMS. The SCC-AS then acts as a B2BUA between the I-CSCF and the subscriber’s assigned S-CSCF.

Routing through the I-CSCF

The SCC-AS will add a CSCF address to the route header of the outgoing INVITE message. This address can be either the calling party’s S-CSCF address retrieved from the HSS, or it can be the I-CSCF address configured in i-cscf-uri.

Anti-fraud measures

To guard against fraud, the P-Asserted-Identity from the incoming INVITE can be compared to the normalized calling party number from the InitialDP. If they do not match, the request is rejected with a 403 response.

The P-Visited-Network-Information header

The SCC-AS will add a P-Visited-Network-Information header to the outgoing INVITE. It is recommended that this is set according to IR.65, section 6.2.1, that is, that the format for this is epc.ims.mnc<MNC>.mcc<MCC>.3gppnetwork.org if the version followed is 12 or less, or ims.mnc<MNC>.mcc<MCC>.3gppnetwork.org if a later version is used. The values of <MCC> and <MNC> are extracted from the triggering SS7 message.

The IP Multimedia Routing Number pool

The number used as the interchange between the SS7 and IP networks (the IMRN) must belong to a pre-allocated, reserved range of values. These numbers must be reserved to prevent overlap with numbers in use by subscribers. The pre-allocated range must be configured within the Rhino VoLTE TAS.

The pool of values does not need to be overly large, as values will only be reserved for the time between the receipt of the SS7 trigger and the processing of the INVITE. As an approximation, with normal error rates and a 20 ms interval between IMRN reservation (on the SS7 end) and IMRN return-to-pool, a range of 1000 values will accommodate 50,000 calls per second. Adjust the pool sizings to meet network requirements accordingly.

The simple-imrn-pool defines the digit string, with the cdma-imrn-formation and gsm-imrn-formation sections defining the protocol-specific digit formatting details.

Bypass reorigination for unregistered subscribers

If a subscriber is not registered in the IMS network, reorigination from SS7 to IMS can be skipped to avoid unnecessary processing. This bypass can be enabled for all unregistered subscribers, or for only non-roaming subscribers (to support terminating charging of roaming subscribers through the IMS).

Interactions with other services

The reoriginated call will be subjected to standard SCC call processing.

Configuration

The example for sentinel-volte-gsm-config.yaml and example for sentinel-volte-cdma-config.yaml show example configuration relevant to reorigination in the sentinel-volte/scc/service-centralisation section.

What you need

  • ❏ The expected format of the P-Visited-Network-Information header.

  • ❏ The range of numbers available to be used as the IMRN.

  • ❏ The expected format of the IMRN.

Setting up reorigination

I want to…​

Use the configured I-CSCF address and prevent the HSS from being queried

To include the configured I-CSCF address in the route header of the outgoing INVITE, in the service-centralisation section, set use-direct-icscf-routing:

            use-direct-icscf-routing: true

Related section: Routing through the I-CSCF

Set the format of the PVNI header to the later standard

To set the P-Visited-Network-Information header template to the later format, in the service-centralisation section, set generated-pvni-template:

            generated-pvni-template: "ims.mnc<MNC>.mcc<MCC>.3gppnetwork.org"

Related section: The P-Visited-Network-Information header

Reject messages sent to an IMRN where the P-Asserted-Identity does not match the recorded calling party digits

To enable comparing the P-Asserted-Identity header to the normalized calling party number, in the service-centralisation section, set police-originating-requests:

            police-originating-requests: true

Related section: Anti-fraud measures

Set a 1000 element IMRN range

To create a 1000 element range with 10-digit numbers from 5550000000 to 5550000999, in the service-centralisation section, set the values in simple-imrn-pool as follows:

            simple-imrn-pool:
                minimum-correlation-id: 5550000000
                maximum-correlation-id: 5550000999
                number-of-digits-in-correlation-id: 10

Related section: The IP Multimedia Routing Number pool

Configure the CDMA responses sent in error conditions

In CDMA networks, the responses sent in error conditions are configured in the scc-cdma-actions section.

                scc-cdma-actions:
                    action-on-unsupported-trigger: accessDeniedReason_terminationDenied
                    action-on-failed-to-allocate-routing-number: accessDeniedReason_busy
                    default-failure-action: accessDeniedReason_serviceDenied

To configure the error response:

Set the format of the GSM IMRN

In GSM networks, the non-digitstring section of the IMRN is configured in the scc-gsm-service-centralisation, gsm-imrn-formation section.

                gsm-imrn-formation:
                    routing-to-internal-network-number-allowed: true
                    nature: NATIONAL
                    numbering-plan: ISDN

To configure the GSM IMRN information:

Set the format of the CDMA IMRN

In CDMA networks, the non-digitstring section of the IMRN is configured in the scc-cdma-service-centralisation, cdma-imrn-formation section.

                cdma-imrn-formation:
                    imrn-type-of-digits: DIALED_OR_CALLED_PARTY_NUMBER
                    imrn-nature-of-number: INTERNATIONAL
                    imrn-numbering-plan: TELEPHONY

To configure the CDMA IMRN information, set:

Bypass terminating reorigination for unregistered subscribers on GSM networks

In GSM networks, the terminating reorigination trigger can be bypassed for subscribers that are not registered in the IMS.

To enable this, in the scc-gsm-service-centralisation section, set bypass-terminating-forwarding-if-served-user-not-ims-registered:

                bypass-terminating-forwarding-if-served-user-not-ims-registered: true
Bypass terminating reorigination for unregistered, non-roaming subscribers on GSM networks

In GSM networks, the terminating reorigination bypass can be enabled for all unregistered subscribers, or for only non-roaming subscribers.

To bypass reorigination for unregistered subscribers while still reoriginating roaming traffic, in the scc-gsm-service-centralisation section, set bypass-terminating-forwarding-if-served-user-not-ims-registered and always-term-reoriginate-if-served-user-is-roaming:

This will result in only unregistered, non-roaming subscribers bypassing reorigination. Registered subscribers and roaming subscribers will still reoriginate to the IMS.

                bypass-terminating-forwarding-if-served-user-not-ims-registered: true
                always-term-reoriginate-if-served-user-is-roaming: true
Bypass reorigination for unregistered subscribers on CDMA networks

In CDMA networks, to bypass reorigination for subscribers that are not registered in the IMS, in the scc-cdma-service-centralisation section, set bypass-forwarding-if-served-user-not-ims-registered:

                bypass-forwarding-if-served-user-not-ims-registered: true
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Rhino VoLTE TAS Version 4.0.0