E.2 QoS related concepts

23.2283GPPIP Multimedia Subsystem (IMS)Release 18Stage 2TS

Tools: ARFCN - Frequency Conversion for 5G NR/LTE/UMTS/GSM

E.2.1 Application Level Signalling for IMS

E.2.1.0 General

When the UE uses GERAN/UTRAN-access for IMS services, it shall be able to establish a dedicated signalling PDP-Context for IM CN Subsystem related signalling or utilize a general-purpose PDP context for IM Subsystem signalling traffic.

When the UE uses E-UTRAN access for IMS services, it shall be able to request establishment of a default or dedicated EPS bearer for IM CN Subsystem related signalling.

E.2.1.1 QoS Requirements for Application Level Signalling

It shall be possible to request prioritised handling over the GERAN/UTRAN radio for IM CN Subsystem related signalling by including the Signalling Indication in the QoS IE of the PDP Context to be used for this traffic as described in clause E.2.1a.1.

It shall be possible to request prioritised handling over the E-UTRAN radio for IM CN Subsystem related signalling by including the appropriate QCI value for signalling traffic as specified in TS 23.203 [54], clause 6.1.7, Standardized QCI Characteristics, Table 6.1.7.

E.2.1.2 Requirements for IM CN subsystem signalling flag

The IM CN Subsystem Signalling flag is used to indicate the dedicated signalling PDP context for IMS signalling. If the network operator does not support a dedicated signalling PDP context or the UE does not include the IM CN Subsystem Signalling flag, the network will consider the PDP context as a general purpose PDP context.

The IM CN Subsystem Signalling flag is used to indicate EPS bearer dedicated for IMS signalling. If the network operator does not support a dedicated signalling EPS bearer or the UE does not include the IM CN Subsystem Signalling flag, the network will consider the EPS bearer as a default or dedicated bearer according to TS 23.401 [70].

A PDP context/EPS bearer dedicated for IM CN Subsystem signalling provides dedicated IP-Connectivity Access Network bearers for IM CN subsystem signalling traffic, hence architectural requirements described in clause 4.2.6 for the usage of dedicated bearer resources shall be applied. The UE is not trusted to implement these restrictions, therefore the restrictions are enforced in the GGSN/P‑GW by the operator of the GGSN/P‑GW.

If the PDP context request/EPS bearer is initiated by the IP‑CAN, then the GGSN/P‑GW may provide a set of UL filters for the PDP context/EPS bearer used for IM CN Subsystem Signalling. The UL filters provide the UE with the rules and restrictions applied by the GGSN/P‑GW for the dedicated IM CN Subsystem signalling IP‑CAN bearer. The GGSN/P‑GW may in addition provide the IM CN subsystem signalling flag to explicitly indicate to the UE the intention of using the PDP context/EPS bearer for IM CN Subsystem related signalling.

Policy and Charging Control functionality can be used to provide additional charging capabilities for dedicated signalling PDP context/EPS bearer dedicated to be used for IMS signalling (as well as for a general-purpose PDP context) as described in clause 4.2.6.

Whether the network is configured to support IM CN signalling flag or Policy and Charging Control functionality or both, is dependent on the operator configuration policy.

The requirements described above also apply in the case of E-UTRAN access and for GERAN/UTRAN access using EPS supporting dedicated bearer for IM CN Subsystem Signalling traffic and where appropriate filters are configured in the P‑GW and PCRF as applicable.

E.2.1.3 Application Level Signalling support for IMS services

In order to receive different level of support for application level signalling in a PDP context/EPS bearer, the UE may choose one of the following options:

– Include both the IM CN Subsystem Signalling Flag in the PCO IE and the Signalling Indication in the QoS. This indicates to the network (radio & core) the requirement of using the PDP context/EPS bearer for application level signalling after it has been negotiated with the networks, to provide prioritised handling over the radio interface (as described in clause E.2.1.1), with rules and restrictions applied in the network (as described in clause E.2.1.2).

– For GERAN/UTRAN access the UE includes the IM CN Subsystem Signalling Flag in the PCO IE and the Signalling Indication in the QoS IE in the PDP context activation or the Secondary PDP context activation procedure.

– For E-UTRAN access the UE includes both the IM CN Signalling Flag in the PCO IE and the appropriate QCI value for signalling traffic in the UE Requested Bearer resource Modification procedure.

NOTE 1: When the UE Requested Bearer Resource Modification procedure is used the IM CN Subsystem Signalling Flag in the PCO IE should be sufficient to trigger the network to provide UL packet filters to the UE, i.e. the UE is not required to provide any meaningful filter information related to the IMS signalling.

– Include the IM CN Subsystem Signalling Flag in the PCO IE in the PDP context activation or the Secondary PDP context activation procedure for GERAN/UTRAN access and for E-UTRAN access in the Attach, PDN Connectivity request or the UE requested bearer resource modification procedure. This indicates to the GPRS/EPS network the requirement of using PDP context/EPS bearer for application level signalling with restricted handling as described in clause E.2.1.2, after it has been negotiated with the networks.

NOTE 2: If the PDN connection is not limited to IMS based services only and the Default EPS bearer is used to support application level signalling for IMS, the UE request for establishment of a general purpose EPS bearer (i.e. a Dedicated non-GBR EPS bearer with a filter set appropriately for a general purpose EPS bearer) might be rejected by the network.

– Utilize a general purpose PDP Context/default EPS bearer with a negotiated QoS profile (this includes the possibility of having the Signalling Indication in the QoS IE for GERAN/UTRAN and the QCI for E-UTRAN.

In the case of E-UTRAN access, when referring to the appropriate QCI for the signalling traffic, the functions described above are fulfilled as specified in TS 23.203 [54] using EPS bearers.

The IM CN Subsystem signalling flag is used to reference rules and restrictions on the PDP context/EPS bearer used for application level signalling, as described in clause E.2.2.

The Signalling Indication in the QoS IE or the appropriate QCI for signalling traffic provides prioritised handling over the radio interface. The Signalling Indication in the QoS IE is detailed in TS 23.107 [55] and clause E.2.1a.1 and the appropriate QCI for signalling traffic is detailed in TS 23.203 [54].

Depending on the operator’s policy, one or more of the above combinations may be allowed in the GPRS/EPS network.

E.2.1a PDP context/EPS Bearer procedures for IMS

E.2.1a.1 Establishing PDP Context/EPS bearer for IM CN Subsystem Related Signalling

It shall be possible for the UE to convey to the network the intention of using the PDP context/EPS bearer for IM Subsystem related signalling. For this purpose it uses the mechanism described in this clause and Application Level Signalling in clauses E.2.1.1, E.2.1.2 & E.2.1.3.

When the bearer establishment is controlled or a bearer establishment is requested (in the case of EPS) by the UE, in order to establish a PDP context/EPS bearer for IM CN Subsystem related signalling, the UE shall be able to include the IM CN subsystem signalling flag in the PDP context activation/UE Requested Bearer Resource Modification procedure. This indicates to the network the intention of using the PDP context/EPS bearer for IM CN Subsystem related signalling.

For GERAN/UTRAN access:

To establish a PDP context for IM CN Subsystem related signalling with prioritised handling over the radio interface, the UE shall be able to set the Signalling Indication in the QoS IE in the PDP context activation procedure and the Secondary PDP context activation procedure. The Signalling indication in the QoS IE indicates to the radio and core networks the requirement for enhanced handling over the radio interface, once it has been negotiated with the networks.

A request for a general purpose PDP context having the "signalling indication" within the QoS IE may be accepted or downgraded according to operator policy configured at the GGSN using the usual QoS negotiation mechanisms described in TS 23.060 [23]. It shall not be possible to modify a general purpose PDP context into a dedicated PDP context for IM CN Subsystem related signalling and vice versa.

For E-UTRAN access:

The (default or dedicated) EPS bearer for IMS signalling may be established from network side at Attach/UE Requested PDN connectivity Request time, in which case the appropriate QCI for signalling traffic and the packet filters will provide the necessary QoS and any restrictions applicable on packets sent over this EPS bearer.

A request for an EPS bearer having the appropriate QCI for signalling traffic according to TS 23.203 [54], may be either accepted or rejected according to operator policy configured at the P‑GW i.e. there is no QoS negotiation mechanism used in EPS.

In order to establish a Dedicated EPS bearer for IM CN Subsystem related signalling, the UE shall be able to request the appropriate QCI for signalling traffic as specified in TS 23.203 [54] in the UE Requested Bearer Resource Modification procedure. This indicates to the radio and core network the requirement for enhanced handling over the radio interface, once it has been accepted by the network. It shall not be possible to modify an existing EPS bearer in order to convert it to be dedicated for IM CN Subsystem related signalling and vice versa.

For all 3GPP accesses:

The IM CN Signalling Flag in the PCO IE is used to reference rules and restrictions on the PDP context/EPS bearer used for application level signalling, as described in clause 4.2.6. Based on operator policy the "Signalling Indication" in the QoS IE or the appropriate QCI for signalling traffic may be allowed only if the "IM CN Subsystem Signalling" flag is present in the PCO IE.

The IM CN subsystem signalling flag and the Signalling Indication in the QoS IE or the appropriate QCI for signalling traffic may be used independently of each other.

E.2.1a.2 Deletion of PDP Context/EPS bearer used to transport IMS SIP signalling

If the GPRS subsystem deletes the PDP Context used to transport IMS SIP signalling, then according to clause 5.10.3.0 the UE or GGSN shall initiate a procedure to re-establish (or modify where possible) a PDP Context for IMS signalling transport. If there are any IMS related PDP contexts active, the re-establishment of the PDP context to transport IMS signalling shall be performed by using the Secondary PDP Context Activation Procedure (or the Network Requested Secondary PDP Context Activation Procedure if initiated by the GGSN) as defined in TS 23.060 [23].

If the EPC system deletes the Dedicated EPS bearer used to transport IMS SIP signalling, then according to clause 5.10.3.0 the UE or PDN Gateway shall initiate a procedure to re-establish (or modify where possible) an EPS bearer for IMS signalling transport. If there are any IMS related EPS bearers active, the re-establishment of the EPS bearer to transport IMS signalling shall be performed by the UE using the UE Requested Bearer Resource Modification procedure or the PDN Gateway using the Dedicated bearer activation procedure as defined in TS 23.401 [70].

The failure in re-establishing the ability to communicate towards the UE results also in the P‑CSCF/PCRF being informed that the IMS SIP signalling transport to the UE is no longer possible which shall lead to a network initiated session release (initiated by the P‑CSCF) as described in clause 5.10.3.1 if any IMS related session is still ongoing for that UE. Additionally, the P‑CSCF shall reject subsequent incoming session requests towards the remote endpoint indicating that the user is not reachable, until either:

– the registration timer expires in P‑CSCF and the user is de-registered from IMS;

– a new Register message from the UE is received providing an indication to the P‑CSCF that the PDP Context/EPS bearer used for IMS SIP Signalling transport for that user has become available again and session requests can be handled again.

E.2.2 The QoS requirements for an IM CN subsystem session

E.2.2.0 General

The selection, deployment, initiation and termination of QoS signalling and resource allocation shall consider:

– the general requirements described in clause 4.2.5.

for E-UTRAN access, the QoS handling is described in TS 23.401 [70], TS 23.203 [54].

– for GERAN/UTRAN access, the requirements described in this clause so as to guarantee the QoS requirement associated with an IM CN subsystem session for IMS services.

1. QoS Signalling at Different Bearer Service Control Levels

During the session set-up in a IM CN subsystem, at least two levels of QoS signalling/negotiation and resource allocation should be included in selecting and setting up an appropriate bearer for the session:

a. The QoS signalling/negotiation and resource allocation at the IP Bearer Service (BS) Level:

The QoS signalling and control at IP BS level is to pass and map the QoS requirements at the IP Multimedia application level to the UMTS BS level and performs any required end-to-end QoS signalling by inter-working with the external network. The IP BS Manager at the UE and the GGSN is the functional entity to process the QoS signalling at the IP BS level.

b. The QoS signalling/negotiation and resource allocation at the UMTS Bearer Service Level:

The QoS signalling at the UMTS BS Level is to deliver the QoS requirements from the UE (received from the GGSN in the case of IP‑CAN Bearer Control) to the RAN, the CN, and the IP BS manager, where appropriate QoS negotiation and resource allocation are activated accordingly. When UMTS QoS negotiation mechanisms are used to negotiate end-to-end QoS, the translation function in the GGSN shall co-ordinate resource allocation between UMTS BS Manager and the IP BS Manager.

Interactions (QoS class selection, mapping, translation as well as reporting of resource allocation) between the QoS signalling/control at the IP BS Level and the UMTS BS Level take place at the UE and the GGSN which also serve as the interaction points between the IM CN subsystem session control and the UMTS Bearer QoS control.

UMTS specific QoS signalling, negotiation and resource allocation mechanisms (e.g. RAB QoS negotiation and PDP Context set-up) shall be used at the UMTS BS Level. Other QoS signalling mechanisms such as RSVP at the IP BS Level shall only be used at the IP BS Level.

It shall be possible to negotiate a single resource allocation at the UMTS Bearer Service Level and utilise it for multiple sessions at the IP Bearer Service Level.

E.2.2.1 Relation of IMS media components and PDP contexts/EPS bearers carrying IMS media

All associated media flows (such as e.g. RTP / RTCP flows) used by the UE to support a single media component are assumed to be carried within the same PDP context/EPS bearer.

E.2.3 Interaction between GPRS/EPS QoS and session signalling

E.2.3.0 General

The generic mechanisms for interaction between QoS and session signalling are described in clause 5.4.7, the mechanisms described there are applicable to GERAN/UTRAN/E-UTRAN-accesses as well.

This clause describes the GERAN/UTRAN/E-UTRAN-access-specific concepts.

At PDP context/EPS bearer setup the user shall have access to either GPRS/EPS without Policy and Charging Control, or GPRS/EPS with Policy and Charging Control. The GGSN/P‑GW shall determine the need for Policy and Charging Control, possibly based on provisioning and/or based on the APN of the PDN connection.

For the GPRS/EPS without Policy and Charging Control case, the bearer is established according to the user’s subscription, local operator’s IP bearer resource based policy, local operator’s admission control function and GPRS/EPS roaming agreements.

For the GPRS/EPS with Policy and Charging Control case, policy decisions (e.g., authorization and control) are also applied to the bearer.

The GGSN/P‑GW contains a Policy and Charging Enforcement Function (PCEF).

E.2.3.1 Resource Reservation with Policy and Charging Control

Depending on the Bearer Control Mode, as defined in TS 23.060 [23], selected for the GPRS IP‑CAN session, resource reservation shall be initiated either by the UE or by the IP‑CAN itself. IMS media which require resource reservation is always mapped to a dedicated bearer, i.e. a dedicated EPS bearer or a PDP context activated using the Secondary PDP Context Activation Procedure. For IP‑CAN initiated resource reservation, the PCRF has the responsibility to ensure that a dedicated bearer is used for media which require resource reservation.

For GERAN/UTRAN the UE initiates the activation or the modification of an existing PDP Context for the media parameters negotiated over SDP using the procedures for Secondary PDP-Context Activation and MS-Initiated PDP Context Modification respectively as defined in TS 23.060 [23] subject to policy control.

Otherwise, the GGSN/P‑GW within the GPRS IP‑CAN initiates the activation or the modification of an existing PDP Context for the media parameters negotiated over SDP using the procedures for Network Requested Secondary PDP Context Activation and GGSN/P‑GW-Initiated PDP Context Modification respectively as defined in TS 23.060 [23].

For E-UTRAN, the UE initiates the resource reservation request for the media parameters negotiated over SDP using procedure UE Requested Bearer Resource Modification procedure as defined in TS 23.401 [70] subject to policy control.

Otherwise, the P‑GW within the EPS IP‑CAN initiates the activation or the modification of an existing Dedicated EPS bearer for the media parameters negotiated over SDP using the procedures for Dedicated bearer activation and PDN GW initiated bearer modification with or without bearer QoS update as specified in TS 23.401 [70].

The request for GPRS/EPS QoS resources may be signalled independently from the request for IP QoS resources by the UE. At the GPRS/EPS BS Level, the PDP Context activation / UE Requested Bearer Resource Modification shall be used by the UE for QoS signalling. At the IP BS Level, RSVP may be used for QoS signalling.

E.2.4 Network initiated session release – P‑CSCF initiated

E.2.4.0 General

In the event of loss of coverage for GERAN/UTRAN access, TS 23.060 [23] defines the Iu or RAB Release procedures. In the case of PDP context/EPS bearer with streaming or conversational class the maximum bitrate of the GTP tunnel between SGSN and GGSN or between SGSN and S‑GW/P‑GW is modified to 0 kbit/s in up- and downlink direction. This is indicated to the P‑CSCF/PCRF by performing an IP‑CAN session modification procedure (see TS 23.203 [54]) as shown in Figure E.3. This procedure also applies to PDP Contexts/EPS bearer used for IMS SIP Signalling transport. For loss of coverage in the case of other PDP contexts/EPS bearer (background or interactive traffic class), the PDP context/EPS bearer is preserved with no modifications and therefore no indication to the P‑CSCF/PCRF.

In the event of loss of coverage for E-UTRAN access, TS 23.401 [70] defines the S1 release Procedure. This procedure releases the EPS bearers. This is indicated to the P‑CSCF/PCRF by performing an IP‑CAN session modification procedure (see TS 23.203 [54]) as shown in figure E.3. The UE will become aware of the release of the GBR bearer the next time it accesses the E-UTRAN network via the procedures as described in the clauses 5.3.3 and 5.3.4 of TS 23.401 [70].

E.2.4.1 Network initiated session release – P‑CSCF initiated after loss of radio coverage

Figure E.3: Network initiated session release – P‑CSCF initiated after loss of radio coverage

1. In the case of GERAN/UTRAN access, in the event of loss of radio coverage for a PDP context with streaming or conversational class the maximum bitrate of the GTP tunnel between SGSN and GGSN and between SGSN and S‑GW /P‑GW is modified to 0 kbit/s in up- and downlink direction. The P‑CSCF/PCRF receives an indication of PDP context/EPS bearer modification or EPS bearer removal. This also applies to PDP Contexts/EPS bearer used for IMS SIP Signalling transport.

In the case of E-UTRAN access, loss of radio coverage causes the GBR bearers to be released in the network and P‑CSCF/PCRF is notified appropriately.

2. It is optional for the P‑CSCF/PCRF to deactivate the affected bearer and additional IP bearers (e.g. an IP bearer for chat could still be allowed). If the P‑CSCF decides to terminate the session then the P‑CSCF/PCRF removes the authorization for resources that had previously been issued for this endpoint for this session (see TS 23.203 [54]).

3. The P‑CSCF decides on the termination of the session. In the event of the notification that the signalling transport to the UE is no longer possible, the P‑CSCF shall terminate any ongoing session with that specific UE. If the P‑CSCF decides to terminate the session then the P‑CSCF/PCRF removes the authorization for resources that had previously been issued for this endpoint for this session. (see TS 23.203 [54]).

The following steps are only performed if the P‑CSCF/PCRF has decided to terminate the session.

When receiving an indication that bearer resources are not available for a voice media negotiated in a multimedia session that is in pre-alerting phase e.g. due to weak E-UTRAN coverage, the P-CSCF performs the procedures according to clause 6.2.1.3a or clause 6.2.2.3a in TS 23.237 [67].

4. The P‑CSCF generates a Hangup (Bye message in SIP) to the S‑CSCF of the releasing party.

5. The S‑CSCF invokes whatever service logic procedures are appropriate for this ending session.

6. The S‑CSCF of the releasing party forwards the Hangup to the S‑CSCF of the other party.

7. The S‑CSCF invokes whatever service logic procedures are appropriate for this ending session.

8. The S‑CSCF of the other party forwards the Hangup on to the P‑CSCF.

9. The P‑CSCF/PCRF removes the authorization for resources that had previously been issued for this endpoint for this session. This step also results in a release indication to the GPRS/EPS system to confirm that the IP bearers associated with the session have been deleted for UE#2.

10. The P‑CSCF forwards the Hangup on to the UE.

11. The UE responds with an acknowledgement, the SIP OK message (number 200), which is sent back to the P‑CSCF.

12. The IP network resources that had been reserved for the message receive path to the UE for this session are now released. Depending on the Bearer Control Mode selected for the IP‑CAN session, the release of previously reserved resources shall be initiated either by the UE or by the IP‑CAN itself. The UE initiates the release of the IP‑CAN bearer resources as shown in figure E.3. Steps 12 and 13 may be done in parallel with step 11. Otherwise, the GGSN/P‑GW within the GPRS/EPS IP‑CAN initiates the release of the bearer PDP context/EPS bearer deactivation after step 9 instead.

13. The GPRS/EPS system releases the PDP context/EPS bearer. The IP network resources that had been reserved for the message receive path to the UE for this session are now released. This is initiated from the GGSN/P‑GW. If RSVP was used to allocated resources, then the appropriate release messages for that protocol would invoked here.

14. The SIP OK message is sent to the S‑CSCF.

15. The S‑CSCF of the other party forwards the OK to the S‑CSCF of the releasing party.

16. The S‑CSCF of the releasing party forwards the OK to the P‑CSCF of the releasing party.