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full Contents for  TS 23.503  Word version:   16.4.1

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6.2.2  Session Management Function (SMF)
6.2.2.1  General
The SMF is responsible for the enforcement of the policy decisions related to service data flow detection, authorized QoS, charging, gating, traffic usage reporting, packet routing and forwarding and traffic steering. The SMF controls the policy and charging enforcement which includes the binding of service data flows to QoS Flows (as described in clause 6.1.3.2.4) as well as the interaction with the CHF. The SMF interacts with the UPF(s), the RAN and the UE to achieve the appropriate treatment of the user plane traffic.
The SMF control of the UPF(s) is described in TS 23.501 as well as the interaction principles between SMF and RAN and between SMF and UE. The procedures for the interaction between SMF and UPF, SMF and RAN as well as SMF and UE are described in TS 23.502.
The SMF is enforcing the Policy Control as indicated by the PCF in two different ways:
  • Gate enforcement. The SMF shall instruct the UPF to allow a service data flow, which is subject to policy control, to pass through the UPF if and only if the corresponding gate is open;
  • QoS enforcement:
  • 5QI corresponding with 5G QoS Characteristics. The SMF shall be able to convert a 5QI value to 5G QoS Characteristics values.
    • PCC rule QoS enforcement. The SMF shall instruct the UPF to enforce the authorized QoS of a service data flow according to the active PCC rule (e.g. to enforce DSCP marking).
    • QoS Flow QoS enforcement. The SMF controls the QoS that is provided to a combined set of service data flows. The policy enforcement function ensures that the resources which can be used by an authorized set of service data flows are within the "authorized resources" specified by the PCF. The authorized QoS provides an upper bound on the resources that can be reserved (GFBR) or allocated (MFBR) for the QoS Flow. During QoS Flow QoS enforcement, if packet filters are provided to the UE, the SMF shall provide packet filters with the same content as that in the SDF template filters received from the PCF.
The SMF is enforcing the charging control in the following way:
  • For a service data flow (defined by an active PCC rule) that is subject to charging control, the SMF shall allow the service data flow to pass through the UPF if and only if there is a corresponding active PCC rule with and, for online charging, the CHF has authorized credit for the charging key. The SMF may let a service data flow pass through the UPF during the course of the credit re-authorization procedure.
For a service data flow (defined by an active PCC rule) that is subject to both Policy Control and Charging Control, the SMF shall allow the service data flow to pass through the UPF if and only if the right conditions from both policy control and charging control happen. i.e. the corresponding gate is open and in the case of online charging the CHF has authorized credit for its charging key.
For a service data flow (defined by an active PCC rule) that is subject to policy control only and not charging control, the SMF shall allow the service data flow to pass through the UPF if and only if the conditions for policy control are met.
A SMF may be served by one or more PCF nodes. The SMF shall contact the appropriate PCF as described in clause 6.3.7.1 of TS 23.501.
The operator may configure an indicator in UDM which is delivered to the SMF within the Charging Characteristics and used by the SMF to not establish the SM Policy Association during the PDU Session establishment procedure.
NOTE 1:
The decision to not establish the SM Policy Association applies for the life time of the PDU Session.
NOTE 2:
The indicator in the UDM is operator specific, therefore its value is understood within the HPLMN and can be used in both non-roaming or home routed roaming cases.
The SMF shall, on request from the PCF, modify a PCC rule, using the equivalent SMF behaviour as the removal of the old and the activation of the new (modified) PCC rule. The SMF shall modify a PCC rule as an atomic operation. The SMF shall not modify a predefined PCC rule on request from the PCF.
The SMF should support predefined PCC rules.
The SMF shall gather and report QoS Flow usage information according to clause 6.1.3.3. The SMF may have a pre-configured Default charging method. Upon the initial interaction with the PCF, the SMF shall provide pre-configured Default charging method if available.
At PDU Session establishment the SMF shall initiate the SM Policy Association Establishment procedure. If no PCC rule was activated for the PDU Session, the SMF shall reject the PDU Session establishment.
If there is no PCC rule active for a successfully established PDU Session at any later point in time, e.g., through a PCF initiated SM Policy Association Modification, the SMF shall initiate a PDU Session termination procedure. If the PCF terminates the SM Policy Association, the SMF shall initiate a PDU Session termination procedure.
If there is no PCC rule active for a successfully established QoS Flow at any later point in time, e.g., through a PCF triggered SM Policy Association Modification, the SMF shall initiate a PDU Session Modification procedure an terminate the QoS Flow.
If the PDU Session is modified, e.g. by changing the characteristics for an QoS Flow, the SMF shall first use the Policy Control Request Trigger to determine whether to request the PCC rules for the modified PDU Session from the PCF; afterwards, the SMF shall use the re-authorisation triggers, if available, in order to determine whether to require re-authorisation for the PCC rules that were either unaffected or modified. If the SMF receives an unsolicited update of the PCC rules from the PCF, the PCC rules shall be activated, modified or removed as indicated by the PCF.
The SMF shall inform the PCF about the outcome of a PCC rule operation. If a QoS Flow cannot be established or modified to satisfy the QoS Flow binding, then the SMF shall reject the activation of a PCC rule.
The SMF shall inform the PCF about any removal of a PCC rule, that the PCF has activated, that occurs without explicit instruction from the PCF.
When QoS Flow resources are released, i.e. at SM Policy Association termination or SMF-initiated SM Policy Association modification notifying that PCC Rules are removed, the SMF shall also provide, if available, the reason why resources are released, i.e. RAN/NAS Release Cause.
NOTE 3:
In the case of a rejection of a PCC rule activation the PCF may e.g. modify the attempted PCC rule, de-activate or modify other PCC rules and retry activation or abort the activation attempt and, if applicable, inform the AF that transmission resources are not available.
The SMF forwards the Maximum Packet Loss Rate for UL and DL, if received from PCF for the PCC rule bound to a 5QI=1 QoS Flow. In the case multiple PCC Rules share one 5QI=1 QoS Flow and the SMF received multiple Maximum Packet Loss Rates, the SMF chooses the lowest value per direction related to these PCC rules.
When the PCF provides updated PCC rules for the PDU Session to the SMF, and the PCC rules were not enforced due to that the UE is in suspend state, e.g. due to SRVCC to GERAN without DTM support as specified in clause 6.2.2.1 in the TS 23.216 or CSFB to UTRAN without PS Handover as specified in clause 6.5 in the TS 23.272, the SMF shall indicate to the PCF that the PCC Rules were not enforced with the reason that the UE is in suspend state. Upon reception of the failure indication, the PCF may subscribe to UE resumed from suspend state event trigger.
NOTE 4:
This above description applies in the case of EPC interworking.
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6.2.2.2  Service data flow detectionWord-p. 70
The Service Data Flow detection uses the service data flow template included in a PCC Rule provide by the PCF. The service data flow template defines the data for the service data flow detection as a set of service data flow filters or an application identifier referring to an application detection filter. The SMF maps the service data flow template in the PCC Rule into the detection information in a Packet Detection Rules to the UPF as described in TS 23.501.
The application detection filters provided to the SMF may be extended with the PFDs provided by the NEF (PFDF). How the SMF uses the service data flow detection capabilities in the UPF is described in TS 23.501, clause 5.8.2.
For IP PDU Session type and Ethernet PDU Session type, the service data flow filters that may apply for traffic on a PDU Session are defined in TS 23.501, clause 5.7.6. The following specifics apply:
  • Each service data flow template may contain any number of service data flow filters;
  • Each service data flow filter is applicable uplink, downlink or both uplink and downlink;
  • NOTE 1:
    Service data flow filters that apply in both uplink and downlink should be used whenever possible.
  • Each service data flow filter may contain information about whether the explicit signalling of the corresponding traffic mapping information to the UE is required.
  • NOTE 2:
    This information enables e.g. the generation/removal of traffic mapping information for the UE as well as the usage of PCC rules with specific service data flow filters on the QoS Flow associated with the default QoS rule without the need to generate traffic mapping information.
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6.2.2.3  Measurement
The SMF shall ensure that the UPF supports data volume, duration, combined volume/duration and event based measurement for charging. The Measurement method indicates what measurement type is applicable to the PCC rule.
NOTE 1:
Event based charging is only applicable to predefined PCC rules and PCC rules using an application detection filter (i.e. with an application identifier).
The SMF shall ensure that the UPF measurement measures all the user plane traffic, except traffic that PCC causes to be discarded.
The SMF shall ensure that the UPF maintains a measurement per QoS Flow, and Charging Key combination.
If Service identifier level reporting is mandated in a PCC rule, the SMF shall ensure that the UPF maintains a measurement for that Charging Key and Service Identifier combination, for the QoS Flow.
NOTE 2:
In addition, the SMF may instruct the UPF to maintain QoS Flow level measurement if required by the operator.
For usage monitoring, the SMF shall ensure that the UPF supports volume and time measurement for a PDU Session and maintains a measurement for each PDU Session for which the PCF has requested the Usage report trigger and provided threshold values on a PDU Session level. The SMF shall ensure that the UPF is able to support volume and time measurements simultaneously for a given PDU Session.
The SMF shall ensure that the UPF supports volume and time measurement per Monitoring key and maintain a measurement for each Monitoring key if the PCF has requested the Usage report trigger and provided threshold values on Monitoring key level. The SMF shall ensure that the UPF is able to support volume and time measurements simultaneously for a given Monitoring Key.
The SMF shall ensure that the UPF supports simultaneous volume and time measurement for usage monitoring on PDU Session level and Monitoring key level for the same PDU Session.
Volume and time measurements for usage monitoring purposes on PDU Session level and on Monitoring key level shall be performed independently of each other. If the PCC rule is associated with an indication of exclusion from session level monitoring, the SMF shall ensure that the UPF does not consider the corresponding service data flow for the volume and time measurement on PDU Session level.
If the Usage report reached Policy Control Request Trigger is set and a volume or a time threshold is reached, the SMF shall report this event to the PCF. The SMF shall continue to perform volume or time measurement after the threshold is reached and before a new threshold is provided by the PCF. At PDU Session termination or if the conditions defined in clause 6.4 for continued monitoring are no longer met, or if the PCF explicitly requests a usage report, the SMF shall inform the PCF about the resources that have been consumed by the user since the last usage report for the affected Monitoring keys, including the resources consumed before and after the Monitoring time was reached, if provided according to clause 6.2.1.1.
If combined volume and time measurements are requested by the PCF, then the reporting shall be done for both together. For example, if the volume threshold is reached, the consumed time shall be reported as well and, in order to continue combined volume and time measurements, the PCF shall provide a new time threshold along with a new volume threshold. The SMF shall continue to instruct the UPF to perform volume and time measurement after the threshold is reached and before a new threshold is provided by the PCF. If new threshold is provided only for time or volume, then the measurements shall continue only for that provided type and the SMF shall ensure that the accumulated usage for the non provided type is discarded by the UPF.
When the PCF requests to report usage, the SMF shall report the accumulated usage to the PCF according to the provided usage threshold, i.e. the SMF reports accumulated volume when the volume threshold was provided by the PCF, accumulated time when the time threshold was provided by the PCF and both accumulated volume and accumulated time when volume threshold and time threshold were provided by the PCF.
If the Usage thresholds for a Monitoring key are not provided to the SMF in the acknowledgement of a PDU Session modification where its usage was reported, then the usage monitoring shall not continue in the SMF for that Monitoring key.
When the Monitoring time occurs, the accumulated volume and/or time usage shall be recorded by the UPF and reported to the SMF, and:
  • If the subsequent usage threshold value is provided, the usage threshold shall be reset to this value by the SMF.
  • Otherwise, the usage threshold shall be set by the SMF to the remaining value of the threshold previously sent by the PCF (i.e. excluding the accumulated usage).
The first usage report after the Monitoring Time was reached shall indicate the usage up to the Monitoring time and usage after the Monitoring time.
In order to support time based usage monitoring, the PCF may optionally indicate to the SMF, along with other usage monitoring information provided, the Inactivity Detection Time. This value represents the time interval after which the time measurement shall stop for the Monitoring key, if no packets are received belonging to the corresponding Monitoring Key during that time period. Time measurement shall resume on receipt of a further packet belonging to the Monitoring key.
Time measurement for a Monitoring key shall also be stopped when time based usage monitoring is disabled, if this happens before the Inactivity Detection Time is reached.
If an Inactivity Detection Time value of zero is provided, or if no Inactivity Detection Time is present within the usage monitoring information provided by the PCF, the time measurement shall be performed continuously from the point at which it was started until time based usage monitoring is disabled.
The SMF instructs the UPF to provide usage reports to the SMF as described in TS 23.501, clause 5.8.2.6.
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6.2.2.4  QoS controlWord-p. 72
The SMF receives the authorized QoS for a service data flow in the PCC rule. The SMF derives the QoS parameters for a QoS Flow (other than the QoS Flow associated with the default QoS rule) based on the PCC rule information of the PCC rule(s) bound to this QoS Flow:
  • The SMF shall set the QoS Flow parameters 5QI and ARP to the values of the corresponding PCC rule parameters.
  • For the QoS Flow parameters QNC, Priority Level, Averaging Window and Maximum Data Burst Volume, the SMF shall use the corresponding PCC rule parameters if they are available in the PCC rule.
  • For GBR QoS Flows, the SMF should set the GFBR to the sum of the GBRs of all PCC rules that are active and bound to that QoS Flow and the MFBR to the sum of the MBRs of all PCC rules that are active and bound to that GBR QoS Flow. If a set of PCC Rules is subject to resource sharing as specified in clause 6.1.3.13 the SMF should use, for each applicable direction, the highest GBR from the set of PCC Rules sharing resources as input for calculating the GFBR and may use, for each applicable direction, the highest MBR from the set of PCC Rules as input for calculating the MFBR.
  • For GBR QoS Flows, the SMF shall set the QoS Flow parameter Maximum Packet Loss Rate for UL and DL if the corresponding PCC rule parameters are available in the PCC rule. In the case multiple PCC Rules are bound to the QoS Flow and the SMF received multiple Maximum Packet Loss Rates, the SMF chooses the lowest value per direction in all these PCC rules.
  • If the PCC rule contains a non-standardized 5QI, the SMF shall also provide the corresponding 5G QoS characteristics parameters (as received in the PDU Session related information Explicitly signalled QoS Characteristics) for the QoS Flow.
  • If the PCC rule contains Alternative QoS Parameter Sets, the SMF shall provide their attributes as Alternative QoS Profile(s) (see TS 23.501, clause 5.7.1.2a) in the same prioritized order (in which they occur in the PCC rule) in addition to the QoS parameters for the QoS Flow. The QoS Flow parameters ARP and QNC in the Alternative QoS Profile(s) are set to the values of the PCC rule.
The SMF shall set the QoS parameters of the QoS Flow associated with the default QoS rule to:
  • the PCC rule parameters contained in the PCC rule that is bound to this QoS Flow (in the way it is described above) if the PCC rule attribute Bind to QoS Flow associated with the default QoS rule and apply PCC rule parameters is present; or otherwise
  • the Authorized default 5QI/ARP received in the PDU Session related information. If the Authorized default 5QI contains a non-standardized 5QI, the SMF shall also provide the corresponding 5G QoS characteristics parameters (as received in the PDU Session related information Explicitly signalled QoS Characteristics) for the QoS Flow associated with the default QoS rule.
The SMF receives the authorized Session AMBR in the PDU Session related information. The SMF ensures that the authorized Session AMBR for a PDU Session is enforced for bandwidth policing at the UPF(s) as described in TS 23.501, clause 5.7.1.
The SMF generates QoS rule(s) as described in TS 23.501. For a PDU session of unstructured type, only one PCC Rule allowing all packets is to be activated in the SMF and only the QoS Flow associated with the default QoS rule exists as described in TS 23.501, clause 5.7.1.
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6.2.2.5  Application detection
The SMF shall instruct the UPF to detect the Start and Stop of the application traffic for the PCC rules used for application detection (i.e. with application identifier) that the PCF has activated at the SMF.
If the PCF has subscribed to the event and notification is not muted for the specific PCC Rule, the SMF shall also instruct the UPF to report the Start/Stop of application, as described in the TS 23.501.
When receiving the application detection report from UPF, the SMF shall forward the application identifier, the start/stop indication and, when service data flow descriptions were deduced, the application instance identifier(s) and the service data flow description(s), to the PCF.
NOTE:
The PCF can make policy decision when receiving the application detection report.
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6.2.2.6  Traffic steeringWord-p. 73
The SMF shall support traffic steering control as defined in Clause 6.1.3.14.
The SMF may be configured with the traffic steering policy IDs related to the mechanism enabling traffic steering to the N6-LAN, DN and/or DNAIs associated with N6 traffic routing requirements.
Upon receiving a PCC rule which contains the traffic steering control information, the SMF shall provide the information to the UPF for the enforcement. The traffic steering control information in the PCC rule may include a set of DNAI(s) and for each DNAI a traffic steering policy ID and/or N6 traffic routing information dynamically provided by the AF.
Based on the received traffic steering policy ID(s), the UPF may remove or insert VLAN tags on N6 interface for downlink and uplink frames, respectively. The details of the scenario is defined in clause 5.6.10.2 of TS 23.501.
NOTE:
The UPF can, for example, perform marking packets in order to indicate a certain type of traffic to the DN side of the N6 reference point which enables those packets to be steered in the DN. As another example the UPF can forward, i.e. offload, traffic identified by the traffic descriptor to a local tunnel.
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6.2.2.7  Access Traffic Steering, Switching and Splitting [R16]
The SMF may support functionality for traffic steering, switching and splitting within a MA PDU Session, as described in TS 23.501.
Upon receiving a PCC rule which contains the MA PDU Session Control information, the SMF shall instruct the UPF accordingly and shall also create and provide applicable ATSSS rules to the UE (the details for both SMF actions are described in TS 23.501).
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6.2.3  Application Function (AF)
The Application Function (AF) is an element offering applications that require dynamic policy and/or charging control over the user plane behaviour and/or an element requesting non-session based network capability exposure. The AF shall communicate with the PCF to transfer dynamic session information, required for PCF decisions as well as to receive access network specific information and notifications about events related to the PDU Session or the QoS Flow transferring the application traffic. One example of an AF is the P CSCF of the IM CN subsystem.
An AF may communicate with multiple PCFs. The mechanism for an AF to select the PCF associated to a PDU Session based on the UE address is described in clause 6.1.1.2.
AF may contact the PCF via the NEF for network capability exposure as defined in clause 4.3.6.
The AF may receive an indication that the service information is not accepted by the PCF together with service information that the PCF would accept. In that case, the AF rejects the service establishment towards the UE. If possible, the AF forwards the service information to the UE that the PCF would accept.
For certain events related to policy control, the AF shall be able to give instructions to the PCF to act on its own, i.e. based on the service information currently available as described in clause 6.1.3.6.
NOTE 1:
The QoS authorization based on incomplete service information is required for e.g. IMS session setup scenarios with available resources on originating side and a need for resource reservation on terminating side.
The AF may request the PCF to report events related to the PDU Session or the QoS Flow transferring the application traffic as defined in clause 6.1.3.18. The AF may use the access network specific information and notifications about events in the AF session signalling or to adjust the event reporting related to the PDU Session or the QoS Flow transferring the application traffic.
The AF may contact the PCF via the NEF to request a time window and related conditions for future background data transfer. Details of the AF behaviour to support future background data transfer are defined in clause 6.1.2.4.
To support sponsored data connectivity the AF may provide the PCF with the sponsored data connectivity information, including optionally a usage threshold, as specified in clause 6.2.1.1. The AF may request the PCF to report events related to sponsored data connectivity.
NOTE 2:
Annex D describes the scenario for sponsored data connectivity.
The AF may receive a request to terminate an AF session. The PCF may include an indication that the transmission resources are lost due to PS to CS handover.
NOTE 3:
The AF action upon reception of the indication that the transmission resources are lost due to PS to CS handover is application specific. The IMS uses the indication to prevent a termination of an ongoing session as specified in TS 24.229 and TS 24.237.
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6.2.4  Unified Data Repository (UDR)Word-p. 74
The Unified Data Repository (UDR) is defined in TS 23.501.
6.2.5  Charging Function (CHF)
The Charging Function is specified in TS 32.240.
6.2.6Void
6.2.7  Network Exposure Function (NEF)
The Network Exposure Function (NEF) is defined in TS 23.501 and additionally supports the following policy related functionalities:
  • Service specific policy and charging control;
  • Management of packet flow descriptions;
  • Sponsor data connectivity including usage monitoring (as defined in clause 6.2.1.1);
  • Negotiations for future background data transfer.
6.2.8  Access and Mobility Management Function (AMF)
The Access and Mobility Management Function (AMF) is defined in TS 23.501 and additionally supports the following policy related functionalities:
  • Enforcement of access and mobility related policies received from the PCF.
  • Transfers of UE access selection and PDU Session selection policies received from the PCF to the UE via N1 interface.
  • Reporting of events to the PCF that the PCF has subscribed to.
6.2.9  Network Data Analytics Function (NWDAF)
NWDAF is defined in TS 23.288.

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