GB2614113A - Improvements in and relating to user equipment behaviour - Google Patents

Abstract

A method of operating a User Equipment in communication with a telecommunication network wherein after an inter-system change from S1 mode to N1 mode, the UE verifies if it has already successfully performed a Protocol Data Unit, PDU, session modification procedure. Optionally if the UE is unable to verify this then the UE performs a PDU session modification procedure to report its 5GSM capabilities to the network. The PDU session modification may have not been successfully performed due to a back-off timer running or because of congestion. The UE may store information regarding if the PDU session modification procedure has been performed successfully or not and this information may be stored in the form of a flag. A UE arranged to perform the method is also claimed.

Description

Improvements in and relating to User Equipment Behaviour The present invention relates to improvements in User Equipment, UE, behaviour, in relation to management of unstructured Protocol Data Unit, PDU, Session Types.

In a telecommunication network, the UE may first register on S1 mode (i.e. Evolved Packet System, EPS) and establish a Packet Data Network, PDN, connection. At a later time, the UE may perform its first inter-system change from S1 mode to N1 mode (5GS) after which the UE is mandated to perform a PDU session modification in order to report its 5GSM capabilities to the Session Management Function, SMF as described in 3GPP TS 24.501 V17.3.0.

The UE's 5GSM message (e.g. the PDU Session Modification Request message) may be rejected by the network due to congestion at the 5GSM level. When this happens, the PDU session modification which is required after the first inter-system change will not be successful and, as such, the UE is later required to perform the procedure (i.e. to send the PDU Session Modification Request message) after the back-off timer expires.

The following is specified in 3GPP TS 24.501 V17.3.0 in line with the description above: "The purpose of the UE-requested PDU session modification procedure is: [SKIP] ...

e) to indicate to the network the relevant 5GSM parameters and capabilities (e.g. the UE's 5GSM capabilities, whether the UE supports more than 16 packet filters, the maximum data rate per UE for user-plane integrity protection supported by the UE for uplink, the maximum data rate per UE for user-plane integrity protection supported by the UE for downlink and whether the UE requests the PDU session to be an always-on PDU session in the 5GS) for a PDN connection established when in 51 mode, after the first inter-system change from S1 mode to N1 mode, if the UE is a UE operating in single-registration mode in a network supporting N26 interface; [ SKIP] If the UE needs to initiate the UE-requested PDU session modification procedure to indicate to the network the relevant 5GSM parameters and capabilities (e.g. the UE's 5GSM capabilities, whether the UE supports more than 16 packet filters, the maximum data rate per UE for user-plane integrity protection supported by the UE for uplink, the maximum data rate per UE for user-plane integrity protection supported by the UE for downlink and whether the UE requests the PDU session to be an always-on PDU session in the 5GS) for a PDN connection established when in 81 mode, after the first inter-system change from S1 mode to N1 mode, the UE is a UE operating in single-registration mode in the network supporting N26 interface and timer T3396, T3584, T3585 or the back-off timer is running, the UE shall initiate the UE-requested PDU session modification procedure after expiry of timer T3396, T3584 or T3585 or after expiry of the back-off timer." Service gap control is (SGC) is a feature that enables the network to control the frequency of mobile originated (MO) requests for the purpose of sending data. The network uses a SGC timer which determines the duration of time between consecutive MO requests. For example, after the UE registers with the network, the network may provide a SGC timer to the UE. The UE starts the timer after it transitions from 5GMM-CONNECTED mode to 5GMM-IDLE mode, after which the UE is not permitted to perform MO requests unless for certain exceptions. However, when the timer expires, the UE can transition from 5GMM-IDLE mode to 5GMMCONNECTED mode for MO data transfer.

The following is specified in 3GPP TS 24.501 V17.3.0 about SGC: "Service gap control (SGC) only applies to 3GPP access.

The network may control the frequency with which UEs can transition from 5GMM-IDLE mode to 5GMM-CONNECTED mode via the SGC as specified in 3GPP TS 23.501 [8] and 3GPP TS 23.502 [9]. If the network supports SGC and the service gap time value i.e. T3447 value is available in the 5GMM context of the UE, the AMF shall consider SGC as active for the UE.

The UE and the network negotiate usage of SGC during the registration procedure for initial registration and registration procedure for mobility and periodic registration update: - the UE supporting SGC indicates its support in the REGISTRATION REQUEST message. If the UE supports SGC and the SGC is active for the UE, the AMF includes T3447 value IE in the REGISTRATION ACCEPT message (see subclause 5.5.1.2 and subclause 5.5.1.3). The UE stores the T3447 value; and for UEs that do not support SGC when the network rejects mobility management signalling requests because SGC is active in the network, the mechanism for general NAS level mobility management congestion control as specified in subclause 5.3.9 applies.

The network can provide a new T3447 value to the UE to be used next time it is started or stop timer T3447 in the UE if running with the Generic UE configuration update procedure as specified in subclause 5.4.4.

The UE shall start timer T3447 when the N1 NAS signalling connection is released and if: the UE supports SGC, and the T3447 value is available in the UE and does not indicate zero; and the N1 NAS signalling connection released was not established for: * paging; * registration procedure for initial registration with Follow-on request indicator set to "No follow-on request pending"; * registration procedure for mobility and periodic registration update with Follow-on request indicator set to "No follow-on request pending" and without Uplink data status IE included; * requests for emergency services; or * requests for exception data reporting.

If the SGC is active in the network, after the UE transitions from SGMM-CONNECTED mode to 5GMM-IDLE mode except when the UE was in 5GMM-CONNECTED mode due to: paging; registration procedure for initial registration with Follow-on request indicator set to "No follow-on request pending"; registration procedure for mobility and periodic registration update with Follow-on request indicator set to "No follow-on request pending" and without Uplink data status IE included, requests for emergency services; or requests for exception data reporting, the network shall start timer T3447 if not already running: with the T3447 value available in the SGMM context minus 4 minutes, if the UE supports SGC and the T3447 value has been sent to the UE with a non-zero value; or - with the T3447 value available in the 5GMM context if the UE does not support SGC.

When timer T3447 is running, the network allows: requests for emergency service; requests for emergency services fallback; requests for high priority access; requests for exception data reporting; registration procedure for initial registration with Follow-on request indicator set to "No follow-on request pending"; registration procedure for mobility and periodic registration update without Uplink data status IE included and with Follow-on request indicator set to "No follow-on request pending"; or service request procedure or registration procedure for mobility and periodic registration update triggered by paging and subsequent MO signalling or MO data, if any, until the UE enters 5GMM-IDLE mode.

The UE or the network with a running T3447 timer keeps the timer running when the UE transits from 5GMM-IDLE mode to SGMM-CONNECTED mode." From the above, it can be concluded that when the SGC timer (i.e. T3447) expires and the UE goes to connected mode for MO data or signaling, the UE will start T3447 upon transition to idle mode again.

A problem with the prior art is that in some cases, the UE will not be able to perform a PDU session modification procedure to report its 5GSM capabilities after an inter-system change from Si mode to N1 mode.

In more detail, the problem will be described using the following scenario: 1. The UE starts in EPS and has at least one PDN connection 2. The UE performs its first inter-system change from S1 mode to N1 mode 3. As required, the UE performs the PDU session modification procedure in N1 mode so as to report its 5GSM capabilities after its first inter-system change from Si mode to N1 mode 4. The SMF rejects the procedure due to congestion and provides a 5GSM back-off (BO) timer to the UE 5. The UE starts the 5GSM BO timer 6. The UE performs an inter-system change from N1 mode to Si mode 7. The 5GSM BO timer expires (optionally, the 5GSM BO timer is still running) 8. The UE performs an inter-system change from Si mode to N1 mode (optionally the 5GSM BO timer expires after the inter-system change is performed) When the timer expires (either while in Si mode or after the inter-system change from Si mode to N1 mode) and the UE is in N1 mode (i.e. the UE has performed its second inter-system change from Si mode to N1 mode), then the UE is now its second inter-system change.

The problem identified herein is as follows: the current specification requires the UE to perform a PDU session modification procedure after the first inter-system change to report its 5GSM capabilities. However, as explained above, the UE may not be able to perform the procedure after the first inter-system change and the UE can then perform a subsequent inter-system change. At this stage, the condition to perform a PDU session modification will no longer be met i.e. the condition of the inter-system change being a first inter-system change. And therefore, the objective of the UE performing the PDU session modification to report its 5GSM capability cannot be achieved and so the SMF cannot know the UE's capabilities.

This can result in uncertain performance and it is an aim of an embodiment of the present invention to address this and other issues.

Another problem with the prior art is the issue of a prolonged gap in the UE's ability to get normal service. The UE may have a SGC timer that guards the period of time before a subsequent MO request can be made. When the timer expires, the UE can transition to connected mode to request the establishment of user-plane (UP) resources for the purpose of sending (or receiving) data. However, in some cases, the network may not be able to establish UP resources due to lack of resources at the User Plane Function, UPF. As such, the UE's attempt is unsuccessful, although the SGC timer has expired. Moreover, when the UE transitions back to idle mode, the UE would restart the SGC timer which means the UE has to wait for another round of gap in the service, where this period is determined by the length of the SGC timer, before the UE can initiate an MO request. This means that the UE would need to wait for a period of at least twice the length of the SGC timer before it can initiate an MO request.

Note that when the network cannot establish UP resources due to lack of resources at the UPF, the Access and Mobility Management Function, AMF, returns the 5GMM cause #92 to the UE as shown below from 3GPP TS 24.501 V17.3.0: "If the user-plane resources cannot be established for a PDU session, the AMF shall include the PDU session reactivation result IE in the SERVICE ACCEPT message indicating that user-plane resources for the corresponding PDU session cannot be reestablished, and: a) if the user-plane resources cannot be established because the SMF indicated to the AMF that the UE is located out of the LADN service area (see 3GPP TS 29.502 [20A]), the AMF shall include the PDU session reactivation result error cause IE with the 5GMM cause set to #43 "LADN not available"; b) if the user-plane resources cannot be established because the SMF indicated to the AMF that only prioritized services are allowed (see 3GPP TS 29.502 [20A]), the AMF shall include the PDU session reactivation result error cause IE with the 5GMM cause set to #28 "restricted service area"; or c) if the user-plane resources cannot be established because: 1) the SMF indicated to the AMF that the resource is not available in the UPF (see 3GPP TS 29.502 [20A]); or 2) the UE is in NB-N1 mode and the result will lead to user-plane resources established for more than two PDU sessions (see 3GPP TS 23.502 [9]) the AMF shall include the PDU session reactivation result error cause IE with the 5GMM cause set to #92"insufficient user-plane resources for the PDU session" NOTE 3: For a UE that is not in NB-NI mode, it is up to UE implementation when to re-send a request for user-plane re-establishment for the associated PDU session after receiving a PDU session reactivation result error cause IE with a 5GMM cause set to #92 "insufficient user-plane resources for the PDU session"." From the above, it can be seen that the UE can retry the request at another time which may be e.g. one minute after receiving the 5GMM cause #92. This is, however, based on UE implementation.

However, if the UE starts the SGC timer, then the ability to retry is no longer possible unless the SGC timer expires. This is problematic, as it can delay the UE's ability to use the service for a much longer time, whereas the concept of the SGC should enable the UE to use the service at least once after the expiry of the SGC timer and before the restart of the SGC timer again.

Note that a similar problem arises if the UE's MO request is rejected due to congestion at the network e.g. general mobility management congestion or congestion related to the sending of data over the control plane. As such, it is a further aim of an embodiment of the present invention to address this issue and others.

According to the present invention there is provided an apparatus and method as set forth in the appended claims. Other features of the invention will be apparent from the dependent claims, and the description which follows.

Regarding the PDU session modification procedure, optionally after an inter-system change from S1 mode to N1 mode: * The UE no longer verifies if the PDU session modification procedure should be done based on the inter-system change being a first inter-system change from S1 mode to N1 mode. Instead, the UE optionally needs to locally store a flag to indicate whether the procedure needs to be performed or not regardless of the number of inter-system changes done so far. The value of the flag is updated based on the success of the procedure and the determination to perform the procedure requires verification of the value of the flag under different cases such as after the expiry of a back-off timer Regarding the start of a service gap timer: * The UE and/or the network should additionally verify if the user plane resources could not be established due to congestion at the UPF when determining to start the service gap control timer. The UE and/or the network should only start the timer if the network did not indicate congestion at the UPF According to a first aspect of the present invention, there is provided a method of operating a User Equipment in communication with a telecommunication network wherein after an intersystem change from S1 mode to N1 mode, the UE verifies if it has already successfully performed a Protocol Data Unit, PDU, session modification procedure.

In an embodiment, if the UE is unable to verify if it has already successfully performed a Protocol Data Unit, PDU, session modification procedure, then the UE performs the PDU session modification procedure to report its 5GSM capabilities to the network and if the UE determines that it has already successfully performed a Protocol Data Unit, PDU, session modification procedure, then the UE does not initiate the procedure.

In an embodiment, the 5GSM capabilities comprise one or more of: * whether the UE supports more than 16 packet filters; * the maximum data rate per UE for user-plane integrity protection supported by the UE for uplink; * the maximum data rate per UE for user-plane integrity protection supported by the UE for downlink; and * whether the UE requests the PDU session to be an always-on PDU session in the 5GS In an embodiment, the UE additionally verifies if at least one of the following conditions are true before verifying if it has already successfully performed a Protocol Data Unit, PDU, session modification procedure: * operating in single registration mode; * operating in a network for which the N26 interface is available; and * at least one Packet Data Network, PDN, connection was established in S1 mode.

In an embodiment, if a 5GSM Back Off, BO, timer expires while the UE is in N1 mode, the UE initiates the PDU session modification procedure.

In an embodiment, the PDU session modification procedure was not successfully performed due to one of: the BO timer was started by the UE and then, in N1 mode, the UE is unable to perform the PDU session modification procedure, because the BO timer is running and, as such, the PDU session modification procedure was not successfully performed, and the UE came from S1 mode and the PDU session modification procedure was rejected in N1 mode due to congestion and so the UE started the BO timer.

In an embodiment, the UE stores information regarding if the PDU session modification procedure has been performed successfully or not.

In an embodiment, the information is stored in the form of a flag stored in the UE.

In an embodiment, the UE verifies if it has already successfully performed a Protocol Data Unit, PDU, session modification procedure by inspecting the stored information.

In an embodiment, the stored information is updated in accordance with an outcome of the PDU session modification procedure.

According to a second aspect of the present invention, there is provided a User Equipment arranged to perform the method of the first aspect.

Although a few preferred embodiments of the present invention have been shown and described, it will be appreciated by those skilled in the art that various changes and modifications might be made without departing from the scope of the invention, as defined in the appended claims.

For a better understanding of the invention, and to show how embodiments of the same may be carried into effect, reference will now be made, by way of example only, to the accompanying diagrammatic drawings in which: Figure 1 shows a flowchart illustrating a first embodiment of the invention; Figure 2 shows a flowchart illustrating a second embodiment of the invention; Figure 3 shows a flowchart illustrating a third embodiment of the invention; and Figure 4 shows a flowchart illustrating a fourth embodiment of the invention.

Note that throughout this application, reference to the UE determining if a certain act has been performed, should be interpreted as the UE determining if the certain act has been successfully performed, rather than merely attempted. It may also mean determining whether a certain act was not performed due to any condition that might not have permitted the act to be performed.

A first embodiment of the invention provides a new condition for performing a PDU session modification after an inter-system change from S1 mode to N1 mode.

This embodiment provides for a UE that is optionally operating in single registration mode, and/or optionally for a network for which the N26 interface is available.

For a UE that is optionally operating in single registration mode, and/or optionally for a network for which the N26 interface is available, the UE should perform a PDU session modification procedure after any inter-system change from S1 mode to N1 mode if the UE has not already done so in a previous inter-system change from S1 mode to N1 mode. As such, when the UE performs the inter-system change from S1 mode to N1 mode, the UE may verify any of the following conditions in any order or combination: * Any or all of the conditions above have been met e.g. optionally operating in single registration mode, and/or optionally for a network for which the N26 interface is available * For at least one PDN connection that was established in S1 mode * The UE has not yet performed a PDU session modification procedure, optionally given this inter-system change is not the first inter-system change (or optionally given this inter-system change is at least the second inter-system change) Wien any one or combination of the above are met, the UE should perform the PDU session modification procedure to report its 5GSM capability to the network e.g. to the SMF.

Note that in another embodiment, the UE may keep/store a flag to indicate if the PDU session modification procedure has been performed or not. The flag may be any indication such as a one-bit value or another value pertaining to another type. The UE may set the flag (or a local stored indication) to the appropriate or corresponding value if the UE has performed a PDU session modification procedure (or not). As such, when the UE performs an inter-system change from S1 mode to N1 mode, optionally irrespective of whether the inter-system change is the UE's first one or second one, etc, the UE should verify, optionally using a flag (or a local stored indication) as set out herein, whether a PDU session modification procedure has been performed or not (optionally in addition to any of the other conditions as listed above in any order or in any combination).

If the UE determines that a PDU session modification has not yet been performed, or performed but not successfully executed/completed (e.g. using a locally stored indication such as but not limited to a flag e.g. as set out herein), then the UE should initiate or perform the PDU session modification procedure to report its 5GSM capability. On the other hand, if the UE determines that a PDU session modification procedure, optionally to report the UE's 5GSM capability, has been performed successfully by the UE (e.g. due to a previous inter-system change from S1 mode to N1 mode), then the UE does not need to perform the procedure in order to report its 5GSM capability.

Note that the UE may still determine to perform the PDU session modification procedure e.g. when other triggers occur, where the triggers may not be related to the UE reporting its 5GSM capability after an inter-system change from S1 mode to N1 mode.

It should be noted that the conditions listed above are not exhaustive and there may be other conditions that the UE may also verify. In this case, the detail set out herein would still apply. Note that the conditions verified by the UE may be in any order or in any combination.

The procedure above may be used/applied after a back-off (BO) timer expires in the UE, where the BO timer may be for any of: a BO timer related to DNN congestion (e.g. timer T3396), a BO timer related to slice congestion (e.g. T3585), or a BO timer related to congestion that is associated with a combination of slice and DNN (e.g. T3584). Note that any of these timers may optionally be associated with a PLMN identity.

Figure 1 shows a flowchart which illustrates the aforementioned embodiment. Note that the figure is just an example illustration and is not intended to be limiting. Note that the steps shown in Figure 1 can occur in a different order to that shown.

At step SO, which is optional, the UE stores a local flag/indication whose value reflects whether or not a PDU session modification has been successfully performed (optionally after an inter-system change from S1 mode to N1 mode e.g. to report at least one 5GSM capability).

When the UE moves from S1 mode to N1 mode (as shown in Step S1), the UE uses its local flag or indication to determine whether a PDU session modification procedure has been performed successfully as shown in Step S2. Note that this verification and/or determination is irrespective of the UE's inter-system change being a first one, a second one, or after an Nth inter-system change where N is a positive integer that is optionally greater than 1. Based on this verification, if the UE determines that the procedure has not been successfully performed then the UE initiates the PDU session modification procedure as shown in Step S4 and the UE should then update its local flag/indication to reflect the result of the procedure (e.g. to indicate that the procedure has been performed if indeed it succeeds), otherwise if the procedure has been performed then the UE need not initiate the procedure for the purpose of reporting its 5GSM capability (Step S3).

Note that when the UE is in N1 mode, optionally after the Mth inter-system change from S1 mode to N1 mode, where M is a positive integer, the UE may perform a PDU session modification procedure which is unsuccessful due to congestion in the network which then leads to a BO timer being started in the UE, or due to an existing BO timer that is already running in the UE which disables the UE from performing the SGSM procedure. When the UE performs an inter-system change from S1 mode to N1 mode and while the UE is in N1 mode, if at least one of the UE's 5GSM BO timer expires, then the UE needs to verify whether it has already performed the PDU session modification procedure successfully, optionally for reporting its 5GSM capability to the network (e.g. SMF), where this verification may be done irrespective of the number of inter-system changes that have been performed thus far. If the UE determines that it has not yet performed the procedure even when the number of intersystem changes from S1 mode to N1 mode is more than one, then the UE should perform the PDU session modification procedure to report its 5GSM capability to the network and the UE should then update its local flag/indication accordingly (e.g. to reflect that the procedure has been done) if the procedure is successful.

In other words, after the BO timer expires, the UE verifies if it has come from S1 mode and has not successfully performed the PDU session modification procedure. If it determines that it has not successfully performed the procedure, then it initiates it.

Note that the verification may be done before the expiry of the 5GSM BO timer and, as such, the UE may, based on a prior determination to perform a PDU session modification procedure e.g. based on the local flag described herein, initiate the SGSM procedure after the expiry of the 5GSM BO timer. As such, all of the steps described herein can be applied in any order and in any combination.

Figure 2 shows another embodiment of the present invention, showing how the UE determines to perform a 5GSM procedure.

The UE may have a 5GSM BO timer running that prevents the initiation of the procedure or the success of the procedure after the inter-system change from S1 mode to N1 mode (e.g. Step S16). While a 5GSM BO timer is running, the UE may either remain in N1 mode or go back to S1 mode. If the UE goes to S1 mode and then comes back to N1 mode (Step S18 and Step S11 respectively), then the UE should verify if it has performed the PDU session modification procedure regardless of the number of inter-system changes done so far. Based on this, the UE should then determine to perform the 5GSM procedure or not. For example, if there is no 5GSM BO timer running, then the UE should perform the procedure and updates its local flag/indication as shown in Step S15. Alternatively, if a 5GSM BO timer is running, the UE may perform the determination after the expiry of the BO timer. Otherwise, the UE may also determine if the procedure needs to be performed and take the necessary action after the expiry of the timer, where the necessary action is associated with the determination that was done while the timer was running.

In summary, the determination on whether to perform a PDU session modification procedure after an inter-system change from S1 mode to N1 mode should not be based on the number of times that the inter-system change has been performed but, by whether or not the procedure has been performed after any inter-system change, optionally where this determination may be achieved using a local flag/indication whose value reflects this accordingly.

In a further embodiment, the UE and the network check for an additional condition before starting the SGC timer (T3447). In a particular embodiment, the UE needs to check for an additional condition before determining whether or not to start the SGC timer i.e. T3447.

The UE should verify if any of the following conditions have occurred: * The UE's NAS message was rejected due to congestion, where optionally the UE received a NAS reject message (e.g. Service Reject) with the 5GMM cause #22 "congestion" and/or with the BO timer T3448 included in the message * The network indicates that UP resources cannot be established due to lack of resources in the network, where optionally the lack of resources is at the UPF. Optionally, the network indicates, or the UE receives, the 5GMM cause #92 "insufficient user-plane resources for the PDU session" in a NAS message (e.g. Service Accept).

For any of the above, in any combination, the UE does not start the SGC timer T3447 after the UE transitions from 5GMM-CONNECTED mode to 5GMM-IDLE mode.

As such, when the UE transitions from 5GMM-CONNECTED mode to 5GMM-IDLE mode, the UE starts the SGC timer T3447 if: the UE supports SGC, and the T3447 value is available in the UE and does not indicate zero: and the N1 NAS signalling connection released was not established for: paging; registration procedure for initial registration with Follow-on request indicator set to "No follow-on request pending"; registration procedure for mobility and periodic registration update with Follow-on request indicator set to "No follow-on request pending" and without Uplink data status IE included; - requests for emergency services; requests for exception data reporting: the UE did not receive the 5GMM cause #22 "congestion", optionally for a NAS signalling connection that was initiated by the UE (i.e. not in response to paging), where optionally the UE did not receive the BO timer T3448 in the NAS message; or the UE did not receive the 5GMM cause #92 "insufficient user-plane resources for the PDU session".

Figure 3 shows a flowchart illustrating a further embodiment of the present invention. Note that this is to be taken as an example only that the steps can be implemented in any order.

At step 521 the UE transitions from 5GMM-CONNECTED to 5GMM-IDLE mode. At step S22, optionally, the UE verifies if any of the following was received: 5GNN cause #22, T3448 or 5GMM cause '92. If yes, then at step S22 the UE does not start T3447. Else, if no, then at step S24, the UE starts T3447.

Note that the steps above also apply to the network e.g. the AMF. As such, the AMF should not start the timer T3447 if the AMF provides any of the following: 5GMM cause #22 "congestion", timer T3448, 5GMM cause #92 "insufficient user-plane resources for the PDU session".

If the SGC is active in the network, after the UE transitions from 5GMM-CONNECTED mode to 5GMM-IDLE mode except when the UE was in 5GMM-CONNECTED mode due to: paging; registration procedure for initial registration with Follow-on request indicator set to "No follow-on request pending"; registration procedure for mobility and periodic registration update with Follow-on request indicator set to "No follow-on request pending" and without Uplink data status IE included; requests for emergency services; - requests for exception data reporting, the network shall start timer T3447 if not already running. However, the network does not start T3447 if the network (e.g. the AMF) sends (or sent, optionally to the UE) any of the following to the UE: 5GMM cause #22 "congestion", timer T3448, 5GMM cause #92 "insufficient user-plane resources for the PDU session", in any NAS message.

Figure 4 shows an embodiment from the perspective of the network (e.g. AMF), noting that this an example only and that the steps can be implemented in any order. Steps S31-S34 mirror steps S21-S24, but relate to the network, e.g. the AMF, taking the steps, rather than the UE.

According to one or more of the embodiments set out above, there is provided at least one solution to one or more of the problems set out previously.

By operating according to the prior art referred to above, it would be impossible for a UE to report its SGSM capability in some cases, as explained earlier. Similarly, operation according to the prior art can lead to service delays in the UE, as the SGC timer is started without considering certain conditions. Embodiments of the present invention address and at least partly overcome these and other issues.

At least some of the example embodiments described herein may be constructed, partially or wholly, using dedicated special-purpose hardware. Terms such as 'component, 'module' or unit used herein may include, but are not limited to, a hardware device, such as circuitry in the form of discrete or integrated components, a Field Programmable Gate Array (FPGA) or Application Specific Integrated Circuit (ASIC), which performs certain tasks or provides the associated functionality. In some embodiments, the described elements may be configured to reside on a tangible, persistent, addressable storage medium and may be configured to execute on one or more processors. These functional elements may in some embodiments include, by way of example, components, such as software components, object-oriented software components, class components and task components, processes, functions, attributes, procedures, subroutines, segments of program code, drivers, firmware, microcode, circuitry, data, databases, data structures, tables, arrays, and variables. Although the example embodiments have been described with reference to the components, modules and units discussed herein, such functional elements may be combined into fewer elements or separated into additional elements. Various combinations of optional features have been described herein, and it will be appreciated that described features may be combined in any suitable combination. In particular, the features of any one example embodiment may be combined with features of any other embodiment, as appropriate, except where such combinations are mutually exclusive. Throughout this specification, the term "comprising" or "comprises" means including the component(s) specified but not to the exclusion of the presence of others.

Attention is directed to all papers and documents which are filed concurrently with or previous to this specification in connection with this application and which are open to public inspection with this specification, and the contents of all such papers and documents are incorporated herein by reference.

All of the features disclosed in this specification (including any accompanying claims, abstract and drawings), and/or all of the steps of any method or process so disclosed, may be combined in any combination, except combinations where at least some of such features and/or steps are mutually exclusive.

Each feature disclosed in this specification (including any accompanying claims, abstract and drawings) may be replaced by alternative features serving the same, equivalent or similar purpose, unless expressly stated otherwise. Thus, unless expressly stated otherwise, each feature disclosed is one example only of a generic series of equivalent or similar features.

The invention is not restricted to the details of the foregoing embodiment(s). The invention extends to any novel one, or any novel combination, of the features disclosed in this specification (including any accompanying claims, abstract and drawings), or to any novel one, or any novel combination, of the steps of any method or process so disclosed.

Claims (11)

1. CLAIMS1. A method of operating a User Equipment in communication with a telecommunication network wherein after an inter-system change from S1 mode to N1 mode, the UE verifies if it has already successfully performed a Protocol Data Unit, PDU, session modification procedure.
2. 2. The method of claim 1 wherein if the UE is unable to verify if it has already successfully performed a Protocol Data Unit, PDU, session modification procedure, then the UE performs the PDU session modification procedure to report its 5GSM capabilities to the network and if the UE determines that it has already successfully performed a Protocol Data Unit, PDU, session modification procedure, then the UE does not initiate the procedure.
3. 3. The method of claim 2 wherein the 5GSM capabilities comprise one or more of: * whether the UE supports more than 16 packet filters; * the maximum data rate per UE for user-plane integrity protection supported by the UE for uplink; * the maximum data rate per UE for user-plane integrity protection supported by the UE for downlink; and * whether the UE requests the PDU session to be an always-on PDU session in the 5GS
4. 4. The method of any preceding claim wherein the UE additionally verifies if at least one of the following conditions is true before verifying if it has already successfully performed a Protocol Data Unit, PDU, session modification procedure: * operating in single registration mode; * operating in a network for which the N26 interface is available; and * at least one Packet Data Network, PDN, connection was established in S1 mode.
5. 5. The method of any preceding claim wherein if a 5GSM Back Off, BO, timer expires while the UE is in N1 mode, the UE initiates the PDU session modification procedure.
6. 6. The method of claim 5 wherein the PDU session modification procedure was not successfully performed due to one of: the BO timer was started by the UE and then, in N1 mode, the UE is unable to perform the PDU session modification procedure, because the BO timer is running and, as such, the PDU session modification procedure was not successfully performed, and the UE came from S1 mode and the PDU session modification procedure was rejected in N1 mode due to congestion and so the UE started the BO timer.
7. 7. The method of any preceding claim wherein the UE stores information regarding if the PDU session modification procedure has been performed successfully or not.
8. 8. The method of claim 6 wherein the information is stored in the form of a flag stored in the UE.
9. 9. The method of claim 7 or 8 wherein the UE verifies if it has already successfully performed a Protocol Data Unit, PDU, session modification procedure by inspecting the stored information.
10. 10. The method of claim 9 wherein the stored information is updated in accordance with an outcome of the PDU session modification procedure.
11. 11. A User Equipment arranged to perform the method of any preceding claim.