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High Probability of Completion (HPC) features comprise a set of functionalities that provide an enhanced probability of call completion to authorized Government Emergency Telecommunications Service (GETS) and Wireless Priority Service (WPS) users during network stress and/or congestion.

There are two methods in HPC call classification:

  • Early HPC classification—An HPC call is recognized based on the received SIP RPH content in the incoming INVITE or the code point in Calling Party's Category (CPC) parameter of the ISUP-MIME. For ANSI, the CPC parameter is National Security/Emergency Preparedness (NS/EP) and for ITU, the CPC parameter isInternationalEmergency Preference Scheme (IEPS).
  • Late HPC classification—An HPC call can also be recognized by the PSX during analysis of the Called Party Number.


The SBC must be configured to use the external PSX for this feature. For more information on using the PSX for GETS / WPS, refer to the PSX documentation Government Emergency Telecommunications Service.

The SBC supports the use of different Differentiated Services Code Point (DSCP) marking for signaling traffic that is associated with HPC and non-HPC calls. To support different DSCP marking for HPC and non-HPC calls in a SIP trunk group, the dscpValue parameter in the HPC profile configuration marks HPC calls and the DSCP configuration in the SIP signaling port marks non-HPC calls. The DSCP value configured in the HPC profile takes precedence over the DSCP value configured in the SIP signaling port and applies to all outbound traffic (IPv4 and IPv6) associated with HPC calls.

The outbound SIP signaling messages associated with HPC and non-HPC calls are marked with the DSCP value configured in the SIP signaling port when the hpcCallProfile is not configured in a SIP trunk group.


Setting the DSCP on the UDP or TCP network socket is a system call, which impacts the system performance if the DSCP frequently switches between HPC and non-HPC calls. 


This feature only applies to HPC calls over UDP transport. This feature does not apply to TCP transport since TCP is byte stream oriented and does not preserve message boundaries.

When the dscpValue parameter is modified, the new value applies only to new HPC calls while the existing HPC calls use the previously configured value.

The following call flow illustrates the DSCP marking for HPC calls.

Figure : DSCP Marking for HPC Call



The SIP 100 Trying provisional response is handled as an exception. The SBC sends this provisional response with a DSCP value that is configured in the SIP signaling port because the provisional response is sent before an incoming call is determined as an HPC or non-HPC call (see the preceding call flow).

The following call flow illustrates the DSCP marking for non-HPC calls.

Figure : DSCP Marking for Non-HPC Call

HPC capable switches such as GSX9000 enable HPC calls to receive additional network support to increase the chances of successful call completion.

For more information on GSX9000’s GETS and WPS feature, refer to GSX documentation Support for Government Emergency Telecommunications Service and Wireless Priority Service.

The SBC Core supports a subset of GSX9000's HPC capabilities and the capabilities are as follows:

  • Early or late HPC call classification
  • Call Admission Control (CAC) treatment: ingress CAC for early HPC classified calls and egress CAC treatment for early and late HPC classified calls
  • Resource-Priority header (RPH) which is used for treating and marking SIP calls as HPC calls.
  • Network-Wide HPC (NW-HPC) License enforcement

The Service profile, hpcCallProfile, is configurable to classify a call as HPC and add RPH for HPC calls. Also, the parameter hpcOversubscription can be added to various CAC configuration objects to support the HPC CAC logic.

Preferential servicing of HPC calls is conditional on the installation of NW-HPC license on the EMS. For more information on EMS licenses, refer to the EMS documentation Licensed Applications and Features.