Reserving Network Resources

Preconditions Interworking

The SBC Core provides a preconditions support feature to reserve network resources as a preventative measure to minimize chances of session failure due to lack of resources before alerting the called party. This prevents a "ghost ring" where the called party is alerted before the call is set up.

The SBC Core supports preconditions interworking functionality only when one leg of a call supports preconditions and other does not. The SBC supports preconditions functionality on either of the legs (egress or ingress). With this feature, preconditions interworking is decided based on the precondition profile configuration, present in the Common IP Attributes of the IP Signaling Profile (IPSP).

This feature supports ISUP-SIP and SIP-SIP preconditions interworking, and provides transparency by dropping/adding of preconditions attributes between supporting or non-supporting peers. The preconditions are configured from the SBC at the SIP Trunk Group level using the parameters “preconditions” (for incoming requests) and “transmitPreconditions” (for outgoing requests).

The SBC Core completes the ingress preconditions when the call terminates at the PSX script. Users can configure the list of script names (in the psxScriptProfile) for which the SBC performs ingress preconditions interworking. The ingress preconditions trigger when one of the configured script names matches with the PSX returned script name. The script execution continues after the ingress preconditions are met.

NOTE: The SBC allows a maximum of three script names to configure. The configurable script name is the same as the script name that the PSX returns.

The pemInterworking flag inhibits sending the P-Early Media header in the first 18x response for the ingress preconditions interworking scenario.

The SBC adds a P-Charging-Vector header with the termIOI parameter to the 18x response sent to the ingress network. The termIOI parameter sends the terminating networking identifier to the ingress network. The SBC creates and sends the termIOI parameter when there is no egress network, which occurs when the call is terminated in the script. When there is an egress network, the termIOI parameter is received from the egress network.

NOTE: The termIOI parameter is configured in the Signaling SIP Trunk Group.

The CLI syntax is shown below:

% set addressContext <addressContext_name> zone <zone_name> sipTrunkGroup <sipTrunkGroup_name> services preconditions [none | required | supported]
% set addressContext <addressContext_name> zone <zone_name> sipTrunkGroup <sipTrunkGroup_name> services transmitPreconditions [none | required | supported]

Preconditions Interworking Properties

The preconditions interworking is provided based on a new Preconditions Profile (PP), and the interpretation of what IPTG configuration to apply is driven by the ingress/egress PP configuration. This is required to provide flexibility for environments where mixed combinations of functionality is required as shown in the following table:

Table 1: Preconditions Interworking Properties

The PP is present in  the "Common IP Attributes" of the IP Signaling Profile (IPSP) for the PSX/ERE.

Interworking functionality is triggered, and ingress/egress PP is evaluated only under the following two circumstances:

• Ingress session offer has precondition attributes and egress TG PP SupportIfEgressIPTG is disabled.
• Ingress session offer has no precondition attributes and egress TG PP SupportIfEgressIPTG is enabled.

Precondition Profile Flags Interaction and Outcome

Refer to the table below for flags interactions between the ingress and egress trunk groups. For a description of the flags, go to 

 Case 1:

Ingress session offer has precondition attributes and egress TG PP SupportIfEgressIPTG is disabled.

Table 2: Precondition Profile Flags Interaction and Outcome

Case 2:

 Ingress session offer has no precondition attributes and egress TG PP “Preconditions Support If Egress IPTG” = Enabled.

Table 3: Precondition Profile Flags Interaction and Outcome

NOTE:

• If Preconditions interworking is configured as “Use UPDATE”, configure the Allow UPDATE flag as well.
• If Preconditions interworking is configured as “Use UPDATE” but a 183 from the peer does not have UPDATE in the Allow header, the SBC will not send an UPDATE and does not take any other action, thus a call timeout timer generates to terminate the call.
• Do not set Preconditions to "Required" for interworking scenarios.

• During an ingress precondition interworking scenario, if the remote precondition is met in PRACK, the SBC processes it and sends the INVITE towards the egress.

Preconditions Interworking Using 183 Response

The SBC supports preconditions interworking for generic call flows based on the ingress IPTG or on the message content of the same IPTG. A new SIP Message Manipulation (SMM) variable is used to determine the applicable IPTG configurables for an initial INVITE, SUBSCRIBE, REGISTER, or an Out-Of-Dialog request.

If support of an IPTG prefix-based Call Admission Control (CAC) together with SMM-based ingress IPTG selection is desired, use IPTGs which are selected based on the SMM. The SMM rules in the same profile are used for the selection, and each profile is associated with an IP prefix (by virtue of being attached to an IPTG which is selected based on IP prefix). A Parent IPTG is defined covering all such SMM-based IPTGs, and the parent IPTG CAC corresponds to the IP prefix-based IPTG.

The message content policy selection is supported through Flexible Policy.

The SBC Flexible Policy supports the selection of the IPTG for the ingress leg though SIP SMM rules. The IPTG that is selected using SMM configuration is applied to all the incoming initial INVITE, initial SUBSCRIBE, initial REGISTER, and/or Out-Of-Dialog request. The CAC is applied to both the IP-prefix based IPTG and the SMM based IPTG.

Support of a IPTG prefix based CAC, along with SMM based ingress IPTG selection, it is accomplished as follows:

1. IPTGs selected based on the SMM, uses the SMM rules in the same profile for selection and each profile is associated with an IP prefix (since it is attached to an IPTG, which is selected based on an IP prefix).
2. A parent IPTG is defined as covering all the SMM based IPTGs, and the parent IPTG CAC corresponds to the IP prefix based IPTG.

The SBC also provides preconditions support for 183 responses using following CLI syntax, (This flag is for specific call flows and is not intended for generic use)

% set addressContext <addressContext_name> zone <zone_name> sipTrunkGroup <sipTrunkGroup_name> services preconditionIntwkUsing183 <disable|enable>

NOTE:

• If both Preconditions Profile and the flag preconditionIntwkUsing183 are configured, the Preconditions Profile takes a higher precedence. Settings of “preconditions=transparent”/”100rel support=yes” on both legs is the only supported configuration for proper precondition interworking functionality.
• Do not enable preConditionIntwkUsing183 when End-to-End Prack is configured.

P - Early Media Preconditions Interworking

The parameter pemInterworking is added to the Trunk Group Signaling configuration to control when to send the PEM header during ingress Preconditions interworking for SIP-SIP calls.

For SIP - SIP / SIP-SIP-T  with preconditions interworking

The SBC Core does not add P-early media in the first 183 which is sent to ingress as a part of preconditions interworking. The SBC relays P-early media received from Egress peer except for the case where the SBC provides Ringback Tone (RBT). Note that the SBC must create and send 180 ringing with PEM sendrecv before providing the RBT. If P-early media is not received in 180 ringing coming from the Egress peer, the SBC needs to create and add P-early media sendrecv to the 180 ringing sent to ingress. 

For SIP–ISUP with preconditions interworking

The SBC does not add P-early media in the first 183 which is sent to ingress as a part of preconditions interworking. The SBC needs to create and add P-early media sendrecv to the 180 ringing sent to ingress upon receipt of ACM/CPG from Egress ISUP network before providing the RBT. 

Call Flows

This feature supports the following call flows:

Mobile — “SIP” — SBC — “GWGW” — GSX — “ISUP” — PSTN
Mobile — “SIP” — SBC — “GWGW” — SBC — “SIP-I/T” — C5IMS
Mobile — “SIP” — SBC — “SIP-I/T” — C5IMS