How it Works

This section describes how WPS prioritization works in UPF.

Priority Handling

The session-priority-profile maps the session-priority values from 0 to 15 to precedence values from 1 to 4. It is used internally in modules like RCM and data-queue distribution.

SR and ICSR

When the feature is not enabled, any session with a valid session-priority received over N4/SX will be prioritized for SR/ICSR as per earlier design.

When the feature enabled, there is a change such that only the Emergency and WPS sessions will be prioritized for SR/ICSR. During UPF downgrade, the priority values are recovered with an offset of +1.

Data Queue

The following table describes the traffic distribution over data queues based on session precedence:

Queue #

Details

3 (Highest priority)

Reserved for Sx

2

Used for Precedence 1,2 sessions

1

Used for Precedence 3,4 sessions

0 (Lowest priority)

Used for Data/Non-prioritized sessions

The default precedence values will be used when:

  • Only session-type is configured and precedence is not defined for session-priority

  • No priority value is configured in the session-priority profile

  • WPS feature is enabled with the default session-priority profile

Note
  • If the feature is disabled, UPF uses N-1 support prioritization.

  • For session recovery of combo calls, the highest priority value received on Sxa or N4 will be used for both sessions.

Mapping to Session Type

UPF maps the session priority values (WPS, Emergency, and IMS) received over N4 to the session types using the "associate session-priority-profile" CLI configuration.

For VoLTE non-active sessions, mapping is not available. For VoLTE active sessions, the priority value 'ims' will be used, while the next priority value (+1) is implicitly used for VoLTE non-active sessions. Hence, "ims+1" priority should not be configured in the session-priority profile.

Version Compatibility

  1. For the latest version (X) of RCM and a prior version (X-1) of UPF:

    1. RCM will support the previous UPF version (X-N), as the priority set is extended without affecting the previously supported set of priorities (1, 2, 3).

    2. During switchover, the flush sequence will be completely based on the priority received from the UPF, which was not the case in earlier versions.

  2. For the prior version (X-1) of RCM and latest version (X) of UPF:

    1. RCM will not support the extended set of priorities, and the checkpoints received for priority 4 will be dropped.

    2. During switchover, calls for priority 1, 2, and 3 can be restored.

Control-Plane Compatibility

To enable WPS prioritization, upgrade the UPF to the new configuration. Prioritization will not be guaranteed in the following scenarios when:

  • SMF/CP is not upgraded

  • Priority values are sent based on N-1 support

  • WPS prioritization is enabled on UPF

Based on the priority values received from the Control Plane, the following combinations will work:

S. No.

SMF / cnSGW / PGW-C / SGW-C Version

UPF Version

WPS prioritization enabled on UPF

UPF Behavior

1

N-1

Or

N without config

N-1

N/A

No WPS prioritization - same as previous behavior.

2

N-1

Or

N without config

N

No

No WPS prioritization - same as previous behavior.

3

N-1

Or

N without config

N

Yes

Session prioritization will not be guaranteed.

4

N (Config present)

N-1

N/A

This is not recommended as session prioritization will not be guaranteed. RCM checkpoints will be dropped for priority>3.

5

N (Config present)

N

Yes

WPS prioritization works.

6

N (Config present)

N

No

This is not recommended as session prioritization will not be guaranteed.