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IMPORTANT

The Transparency Profile is the recommended method of configuring transparency on the SBC Core for new deployments as well as when applying additional transparency configurations to existing deployments. Do not use IP Signaling Profile flags in these scenarios because the flags will be retired in upcoming releases.

Refer to the SBC SIP Transparency Implementation Guide for additional information.

Overview

The SBC Core supports Dialog Transparency which involves passing through "Call-ID", "From" tag, and "To" tag transparently in all the requests and responses. With a call traversing through multiple paths in an IMS VoLTE network, UEs and intermediate IMS entities need to correlate a call based on correct dialog information under various scenarios such as call transfer.

The dialogTransparency flag under the Zone object passes dialogs transparently for SIP messages processed in that specified zone. If dialog transparency is disabled, the SBC translates the dialog identifiers in the Target-Dialog header received in INVITE and REFER messages and sends the Target-Dialog header with translated dialog identifiers on the other leg. If dialog transparency is enabled, Target-Dialog header translation is not performed. However, the header is transparently passed if configured in transparency profile.

The dialogTransparency flag must be enabled on both ingress and egress zones because if the flag is only enabled on one zone, all calls and messages are rejected. In this situation, the sonusSbxDialogTransparencyMisConfiguredNotification alarm is raised on the zone in which the flag is disabled.

Note

The following features are not supported by the SBC when Dialog Transparency is enabled:

  • Support of local handling of REFER (REFER relay is supported)
  • Late media support for downstream forking (when both downstream forking and dialog transparency are enabled 
  • Dialog Transparency support in REGISTER
  • NICE recording call flows
  • GW-GW support

The SBC implements the SIP Dialog Identification internally based on certain SIP parameters. Manipulation of these parameters using the dialog scope SIP Message Manipulation (SMM) affects the intergrity of the dialog scope SMM functionality. The parameters to be considered differs based on whether the Dialog Transparency is enabled or not.

The SBC supports both dialog transparency and spiral calls. To support these features, dialog stateful SMM must support multiple dialogs that have the same dialog identifiers. Dialog stateful SMM variables also support dialog transparency, which is enabled with INVITE and SUBSCRIBE dialogs.

The SBC supports dialog stateful SMM variables to modify inbound or outbound SIP messages and dialog transparency to pass the dialog identifiers (call-id and tags) transparently. Similarly, for spiral calls, the SBC handles multiple legs with same dialog identifiers (call-id and tags). TheSBC distinguishes between the multiple legs using the proprietary parameter call-info. The parameter call-info is inserted in requests and responses outbound from the SBC in such a way that it must present in almost every subsequent SIP requests and responses of a dialog.

The SBC also supports different SMM dialog for multiple call legs with same SIP dialog identifiers (call-id and tags) as well as dialog stateful SMM variables for INVITE and out-of-dialog SUBSCRIBE messages.

For more information on dialog stateful SMM variables, refer to SMM (Advanced) Examples.

Dialog Scope SMM without Dialog Transparency

If the advanced SMM is enabled in any adapter profile which applies to Trunk Group, manipulation of "From tag", "To tag" and "Call-ID header" is not supported for that Trunk Group. In rare cases where there is no other solution, Sonus Technical support can help with testing and implementing the "To Tag".

Note

This functionality only applies to Basic Call Flows.

Dialog Scope SMM with Dialog Transparency

If the advanced SMM is enabled in any adapter profile applying to a Trunk Group under a zone where the Dialog Transparency is enabled, manipulation of "branch parameter", "Call-Info parameter" or "Call-ID header" is not supported for that Trunk Group.

For example (no dialog transparency): consider a configuration where dialog scope variable is used in the inbound adapter profile applicable to Trunk Group TG_1 via Zone or Trunk Group. The SMM rules for an action specifying modification of "From-tag", "To tag" or "Call-ID" cannot be present in that inbound adapter as well as in the outbound adapter if one applies to "TG_1" through Trunk Group or Zone.

Figure : Dialog Transparency Call Flow

Note

Only relevant headers are shown in this call flow.

Note

When dialog transparency is enabled and "Advanced SMM" functionality is required:

  • If both inputAdapterProfile and outputAdapterProfile are provisioned, enable the advancedSMM flag on both SIP Adapter Profiles (inputAdapterProfile and outputAdapterProfile).
  • If only one SIP Adaptor Profile is provisioned (either inputAdapterProfile or outputAdapterProfile), enable the advancedSMM flag only for the provisioned SIP Adaptor Profile. 

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