Improvements in and relating to ciot optimisations in a telecommunication network

Abstract

Disclosed is a method of operating a UE in NB-N1 mode wherein the UE informs a serving network about a number of Data Radio Bearers, DRBs, that the UE is able to support, according to its capabilities. The method involves controlling the establishment of User Plane resources for the UE; if a Payload container type IE is set to "NI SM information", the Request type IE is set to "initial request"; an AMF in the network verifies how many DRBs are already present for the UE and if the AMF determines that there are User Plane resources that are equal to a maximum number of DRBs that the UE supports, then the AMF either: (a) sends a message back to the AMF; or (b) establishes the session as a control plane only session. The method relates to improvements in Cellular Internet of Things (CIoT) optimisation.

Description

“Upon reception of a UL NAS TRANSPORT message, if the Payload container type IE is set to "N1 SM information", the Request type IE is set to "initial request", and a) the UE is in NB-N1 mode; b) the UE has indicated preference for user plane CloT 5GS optimization; c) the network accepted the use of user plane CloT 5GS optimization; and d) the AMF determines that there are user-plane resources established for two other PDU sessions for this UE (see 3GPP TS 23.501); the AMF shall either: a) send back to the UE the message which was not forwarded as specified in in subclause 5.4.5.3.1 case hi); or b) proceed with the PDU session establishment and include the Control Plane CloT 5GS Optimisation indication or Control Plane Only indicator to the SMF.” It should be noted again that these restrictions are only for NB-loT devices i.e. UEs in NB-N1 mode in the case of 5GC. Similar limitations exist for a UE in NB-loT that is using CloT optimizations in EPS i.e. a UE in NB-S1 mode. The main difference however, between EPC and 5GC, is that EPC does not support selective user plane activation for a UE in connected mode (EMM-CONNECTED mode). As such, while in S1 mode, if the UE is in connected mode for the purpose of data then the DRBs and UP resources will be set up for all Packet Data Network (PDN) connections that are active in the UE. So for example, if the UE has established 3 PDN connections in EPS, when the UE sends the Service Request message, the MME will establish the UP resources for all 3 PDN connections. However, for NB-loT devices there is a limitation regarding the maximum number of DRBs that can be supported in S1 mode. The UE can support 1 or 2 DRBs, where 2 is the maximum number of DRBs that can be supported as described in 3GPP TS 24.301 V16.4.0: “For a UE in NB-S1 mode, the UE's implementation-specific maximum number of active user plane radio bearers is 2 (as defined in 3GPP TS 36.300) when the UE sets the Multiple DRB support bit to "Multiple DRB supported" during attach or tracking area update procedures, and 1 otherwise.” The UE in S1 mode indicates in the UE network capability IE whether it supports 2 DRBs by setting the “Multiple DRB support” bit to “Multiple DRB supported”. It should also be noted that for NB-S1 mode, the support of DRB is limited to default EPS bearers and dedicated EPS bearers are not supported on NB-S1 mode. This is similar to 5GS in this regard. The following is specified in 3GPP TS 24.301 V16.4.0 regarding this: “In NB-S1 mode, the dedicated EPS bearer contexts activation procedure is not used. Upon an inter-system mobility from WB-S1 mode to NB-S1 mode in EMM-IDLE mode, if the UE has at least one dedicated EPS bearer context in ESM state BEARER CONTEXT ACTIVE, the UE shall locally deactivate any such dedicated EPS bearer context and shall include the EPS bearer context status IE in TRACKING AREA UPDATE REQUEST message.” When the UE moves to NB-S1 mode, from WB-S1 mode, the UE will deactivate any dedicated EPS bearer and include the EPS bearer context status IE in TRACKING AREA UPDATE REQUEST message as specified in 3GPP TS 24.301 V16.4.0: “The UE shall include the EPS bearer context status IE in TRACKING AREA UPDATE REQUEST message: for the case g; for the case s; for the case zb; if the UE has established PDN connection(s) of "non IP" or Ethernet PDN type; and if the UE locally deactivated at least one dedicated EPS bearer context upon an intersystem mobility from WB-S1 mode to NB-S1 mode in EMM-IDLE mode.” It should be noted that the EPS bearer context status IE indicates which EPS bearer identity is active in the UE and the IE contains a bitmap each bit of which corresponds to a well known EPS bearer identity (see section 9.9.2.1 of 3GPP TS 24.301 V16.4.0). A UE that supports both S1 mode and N1 mode can be redirected by a core network (CN) e.g. 5G CN with which the UE is registering, to a target CN e.g. EPC, or vice versa. The following describes the overall concept as specified in the aforementioned NAS specification: “The network that supports CloT optimizations can redirect a UE between EPC and 5GCN as specified in subclause 5.31.3 of 3GPP TS 23.501. The network can take into account the UE’s N1 mode capability or S1 mode capability, the CloT network behaviour supported and preferred by the UE or the CloT network behaviour supported by the network to determine the redirection. NOTE:     It is assumed that the network would avoid redirecting the UE back and forth between EPC and 5GCN. The network redirects the UE to EPC by rejecting the registration request with the 5GMM cause #31 "Redirection to EPC required" as specified in subclause 5.5.1.2.5 and 5.5.1.3.5. Upon receipt of reject message, the UE disables the N1 mode capability for 3GPP access as specified in subclause 4.9.2 and enables the E-UTRA capability if it was disabled in order to move to EPC. The network that supports CloT optimizations can also redirect a UE from EPC to 5GCN as specified in subclause 5.3.19.2 of 3GPP TS 24.301.” Currently, in 5GS, the AMF can redirect the UE to EPC by rejecting a Registration Request and including the 5GMM cause value #31. Currently, in EPS, the MME redirect the UE to 5GC by rejecting the Attach Request, Tracking Area Update (TAU) Request, or the Combined TAU Request message, and including the EMM cause value #31 "Redirection to 5GCN required" (see 3GPP TS 24.301 V16.4.0). Note that redirection of the UE is described in [4] which uses the term “steering” instead of redirection. The following is described in 3GPP TS 23.501 V16.4.0: “The UE selects the core network type (EPC or 5GC) based on the broadcast indications for both EPC and 5GC, and the UE's EPC and 5GC Preferred Network Behaviour. Networks that support NB-loT shall broadcast an indication whether N3 data transfer is supported or not in system information. When the UE performs the registration procedure it includes its Preferred Network Behaviour (for 5G and EPC) in the Registration Request message and the AMF replies with the 5G Supported Network Behaviour in the Registration Accept message. If the UE supports any of the CloT 5GS Optimisations included in 5GC Preferred Network Behaviour, then when the UE performs an Attach or TAU procedure and the UE includes its EPC Preferred Network Behaviour then the UE shall also include its 5GC Preferred Network Behaviour. In networks that support CloT features in both EPC and 5GC, the operator may steer UEs from a specific CN type due to operator policy, e.g., due to roaming agreements, Preferred and Supported Network Behaviour, load redistribution, etc. Operator policies in EPC and 5GC are assumed to avoid steering UEs back and forth between EPC and 5GC. To redirect a UE from 5GC to EPC, when the UE sends a Registration Request, the AMF sends a Registration Reject with an EMM cause value indicating that the UE should not use 5GC. The UE disables N1 mode and re-enables S1 mode, if it was disabled. The UE then performs either an Attach or TAU in EPC as described in clause 5.17.2. To redirect a UE from EPC to 5GC, when the UE requests an Attach or TAU procedure, the MME sends a reject message with an EMM cause indicating the UE should not use EPC. The UE disables S1 mode and re-enables N1 mode, if it was disabled. The UE then registers with 5GC as described in clause 5.17.2. When determining whether to redirect the UE, the AMF / MME takes into account the UE support of S1 / N1 mode, respectively, and the UE's Preferred Network Behaviour and the Supported Network Behaviour of the network the UE is being redirected towards. If after redirection the UE cannot find a cell supporting connectivity, the UE may re-enable the disabled N1 / S1 mode and then perform Registration, Attach or TAU.” As can be seen, the prior art solution for redirection or steering of the UE depends only on rejecting specific NAS messages i.e. by sending a Registration Reject message in 5GS, and Attach Reject or TAU Reject message in EPS. There are a few indications that the UE can send in 5GSM messages. For example, during PDU session establishment procedure, the UE sends the Maximum number of supported packet filters IE in the PDU SESSION ESTABLISHMENT REQUEST message if the UE supports more than 16 packet filters. Similarly, the UE should indicate if it supports Reflective QoS (RQoS) during PDU session establishment as described below from the aforementioned NAS specification: “The UE should set the RQoS bit to "Reflective QoS supported" in the 5GSM capability IE of the PDU SESSION ESTABLISHMENT REQUEST message if the UE supports reflective QoS and: a) the UE requests to establish a new PDU session of "IPv4", "IPv6", "IPv4v6" or "Ethernet" PDU session type; b) the UE requests to transfer an existing PDN connection in the EPS of "IPv4", "IPv6", "IPv4v6" or "Ethernet" PDN type or of "Non-IP" PDN type mapping to "Ethernet" PDU session type, to the 5GS; or c) the UE requests to transfer an existing PDN connection in an untrusted non-3GPP access connected to the EPC of "IPv4", "IPv6" or "IPv4v6" PDN type to the 5GS.” Similarly, when the UE comes from EPS (i.e. from S1 mode) into 5GS (i.e. into N1 mode), if the UE has a PDN connection that was first established in S1 mode, the UE shall perform a PDU session modification procedure to report some of its capabilities such as: whether the UE supports more than 16 packet filters, or whether the UE supports RQoS. The following from the aforementioned NAS specification describes this: “For a PDN connection established when in S1 mode, after the first inter-system change from S1 mode to N1 mode, if the UE is operating in single-registration mode in the network supporting N26 interface, the PDU session is of "IPv4", "IPv6", "IPv4v6", or "Ethernet" PDU session type, and: a) the UE is performing the PDU session modification procedure to indicate the support of reflective QoS, the UE shall set the RQoS bit to "Reflective QoS supported" in the 5GSM capability IE of the PDU SESSION MODIFICATION REQUEST message; or b) the UE is performing the PDU session modification procedure to indicate that reflective QoS is not supported, the UE shall set the RQoS bit to "Reflective QoS not supported" in the 5GSM capability IE of the PDU SESSION MODIFICATION REQUEST message. For a PDN connection established when in S1 mode, after the first inter-system change from S1 mode to N1 mode, if the UE is operating in single-registration mode in the network supporting N26 interface, the PDU session is of "IPv4", "IPv6", "IPv4v6", or "Ethernet" PDU session type, and the UE supports more than 16 packet filters for this PDU session, the UE shall indicate the maximum number of packet filters supported for the PDU session in the Maximum number of supported packet filters IE of the PDU SESSION MODIFICATION REQUEST message.” The following is specified in the aforementioned NAS specification regarding interworking and the transfer of PDU sessions between N1 mode and S1 mode. During interworking, the UE has to handle specific parameters as defined by the following rules from the aforementioned NAS specification: “Interworking with EPS is supported for a PDU session, if the PDU session includes the mapped EPS bearer context(s) or has association(s) between QoS flow and mapped EPS bearer after inter-system change from S1 mode to N1 mode. The SMF shall not include any mapped EPS bearer contexts associated with a PDU session for LADN and with a PDU session which is a multi-homed IPv6 PDU session. See coding of the Mapped EPS bearer contexts IE in subclause 9.11.4.8. In an MA PDU session, the UE shall have one set of the mapped EPS bearer contexts. The network can provide the set of the mapped EPS bearer contexts of the MA PDU session via either access of the MA PDU session. In an MA PDU session, the UE shall support modification or deletion via an access of a mapped EPS bearer context of the MA PDU session created via the same or the other access. Upon inter-system change from N1 mode to S1 mode, the UE shall create the default EPS bearer context and the dedicated EPS bearer context(s) based on the parameters of the mapped EPS bearer contexts or the associations between QoS flow and mapped EPS bearer in the PDU session, if available. The EPS bearer identity assigned for the QoS flow of the default QoS rule becomes the EPS bearer identity of the default bearer in the corresponding PDN connection. If there is no EPS bearer identity assigned to the QoS flow of the default QoS rule of a PDU session associated with 3GPP access, the UE shall perform a local release of the PDU session. If there is no EPS bearer identity assigned to the QoS flow(s) of a PDU session associated with 3GPP access which is not associated with the default QoS rule, the UE shall locally delete the QoS rules and the QoS flow description(s). The UE uses the parameters from each PDU session for which interworking with EPS is supported to create corresponding default EPS bearer context and optionally dedicated EPS bearer context(s) as follows: a)    the PDU session type of the PDU session shall be mapped to the PDN type of the default EPS bearer context as follows: 1) the PDN type shall be set to "non-IP" if the PDU session type is "Unstructured"; 2)    the PDN type shall be set to "IPv4" if the PDU session type is "IPv4"; 3)    the PDN type shall be set to "IPv6" if the PDU session type is "IPv6"; 4)    the PDN type shall be set to "IPv4v6" if the PDU session type is "IPv4v6"; 5)    the PDN type shall be set to "non-IP" if the PDU session type is "Ethernet", and the UE, the network or both of them do not support Ethernet PDN type in S1 mode; and 6) the PDN type shall be set to "Ethernet" if the PDU session type is "Ethernet" and the UE and the network support Ethernet PDN type in S1 mode; b) the PDU address of the PDU session shall be mapped to the PDN address of the default EPS bearer context as follows: 1) the PDN address of the default EPS bearer context is set to the PDU address of the PDU session, if the PDU session type is "IPv4", "IPv6" or "IPv4v6"; and 2) the PDN address of the default EPS bearer context is set to zero, if the PDU session type is "Ethernet" or "Unstructured"; c) the DNN of the PDU session shall be mapped to the APN of the default EPS bearer context; d) the APN-AMBR and extended APN-AMBR received in the parameters of the default EPS bearer context of the mapped EPS bearer contexts shall be mapped to the APN-AMBR and extended APN-AMBR of the default EPS bearer context; e) for each PDU session in state PDU SESSION ACTIVE, PDU SESSION MODIFICATION PENDING or PDU SESSION INACTIVE PENDING the UE shall set the state of the mapped EPS bearer context(s) to BEARER CONTEXT ACTIVE; and f) for any other PDU session the UE shall set the state of the mapped EPS bearer context(s) to BEARER CONTEXT INACTIVE. Additionally, for each mapped EPS bearer context or the association between QoS flow and mapped EPS bearer in the PDU session: a) the EPS bearer identity shall be set to the EPS bearer identity received in the mapped EPS bearer context, or the EPS bearer identity associated with the QoS flow; b) the EPS QoS parameters shall be set to the mapped EPS QoS parameters of the EPS bearer received in the mapped EPS bearer context, or the EPS QoS parameters associated with the QoS flow; c) the extended EPS QoS parameters shall be set to the mapped extended EPS QoS parameters of the EPS bearer received in the mapped EPS bearer context, or the extended EPS QoS parameters associated with the QoS flow; and d) the traffic flow template shall be set to the mapped traffic flow template of the EPS bearer received in the mapped EPS bearer context, or the stored traffic flow template associated with the QoS flow, if available. After inter-system change from N1 mode to S1 mode, the UE shall associate the PDU session identity, the S-NSSAI, and the session-AMBR with the default EPS bearer context, and for each EPS bearer context mapped from one or more QoS flows, associate the QoS rule(s) for the QoS flow(s) and the QoS flow description(s) for the QoS flow(s) with the EPS bearer context. After inter-system change from N1 mode to S1 mode, the UE and the SMF shall maintain the PDU session type of the PDU session until the PDN connection corresponding to the PDU session is released if the UE supports non-IP PDN type and the PDU session type is "Ethernet" or "Unstructured". After inter-system change from N1 mode to S1 mode, the UE and the SMF shall maintain the following 5GSM attributions and capabilities associated with the PDU session until the PDN connection corresponding to the PDU session is released: the always-on PDU session indication; the maximum number of supported packet filters; the support of reflective QoS; the maximum data rate per UE for user-plane integrity protection supported by the UE for uplink and the maximum data rate per UE for user-plane integrity protection supported by the UE for downlink; and the support of multi-homed IPv6 PDU session. After inter-system change from N1 mode to S1 mode, the UE shall deem that the following features are supported by the network on the PDN connection corresponding to the PDU session: PS data off; and Local address in TFT. If there is a QoS flow used for IMS signalling, after inter-system change from N1 mode to S1 mode, the EPS bearer associated with the QoS flow for IMS signalling becomes the EPS bearer for IMS signalling.” The text above describes that the UE, at inter-system change from N1 mode to S1 mode, will create mapped EPS bearer context for the default bearer and the dedicated bearer, specifically “the UE shall create the default EPS bearer context and the dedicated EPS bearer context(s) based on the parameters of the mapped EPS bearer contexts or the associations between QoS flow and mapped EPS bearer in the PDU session, if available”. For example, when the UE moves from N1 mode to S1 mode, then for “each PDU session in state PDU SESSION ACTIVE, PDU SESSION MODIFICATION PENDING or PDU SESSION INACTIVE PENDING the UE shall set the state of the mapped EPS bearer context(s) to BEARER CONTEXT ACTIVE”, i.e. this means that the UE considers the EPS bearer context to be active and hence can be transferred. And “after inter-system change from N1 mode to S1 mode, the UE shall associate the PDU session identity, the S-NSSAI, and the session-AMBR with the default EPS bearer context, and for each EPS bearer context mapped from one or more QoS flows, associate the QoS rule(s) for the QoS flow(s) and the QoS flow description(s) for the QoS flow(s) with the EPS bearer context.” i.e. the QoS rule(s) and the QoS flow descriptions^) are maintained and are associated with the EPS bearer context. There are several issues in the prior art referred to above and it is an aim of embodiments of the present invention to address these. In particular, as described earlier, the AMF can only redirect the UE during a registration procedure. This means that if the UE is already in connected mode or transitions to connected mode with the service request procedure, then the AMF will not be able to redirect the UE for a relatively long time. Similarly, the MME may not be able to redirect the UE that is in connected mode for a relatively long time if the UE continues to transition to connected mode with the service request procedure. This is illustrated, in particular, in Figure 3 which shows the steps involved in the registration procedure and the issue whereby the AMF keeps waiting, not being able to redirect the UE, because the UE does not send a Registration Request. Further, the UE in EPS indicates how many DRBs it can support which is dependent on the UE’s NB-loT capabilities. For 5G CloT, the UE’s RAT is the same but simply connected to the 5G CN. As such, it may be the case that the NB-loT UE actually supports only 1 DRB and if so, the AMF does not know about it. In fact, the AMF, in the prior art, assumes that the NB-loT UE always supports 2 DRBs. This can lead to system-wide issues and unexpected UE and network behaviour e.g. when the network tries to setup more than one DRB for a UE that supports only one DRB. This is illustrated in Figure 4, which shows the signalling between UE, RAN and AMF respectively. Still further, a UE that first establishes a PDN connection in EPS may then transfer the session to 5GS. The UE may support user plane but the connection may be a connection for control plane only. Accordingly, such connections do not use packet filters. In the prior art, when the UE moves from S1 mode into N1 mode for the first time and the UE has a PDN connection that was established in S1 mode, the UE is mandated to report how many packet filters it supports if the UE does support more than 16 packet filters. Alternatively, the UE should report whether it supports RQoS. However, since this is a control plane only connection, this information is useless for the network, as it will never be used for such a connection or session. Therefore, sending this information only consumes resources unnecessarily. Figure 5 illustrates this issue and shows the signalling between the UE, MME, AMF and SMF respectively. It should be noted that step 5A / 5B in Figure 4 is always performed by the UE upon the first inter-system change from S1 mode to N1 mode if the UE does indeed support more than 16 packet filters. There was no other condition / exception that would have prevented the UE from taking this step. The PDN connection that is established in S1 mode can be a connection for sending data over the control plane when the UE is using control plane (CP) CloT optimization. Such a PDN connection may be only used for control plane data i.e. the user plane resources will never be set up for the PDN connection. In this case, the UE will receive the “control plane only indication” in the session management message. Therefore, for any PDN connection that is associated with the control plane only indication the UE can only send data over the control plane (i.e. over NAS) and the user plane will never be used. Hence packet filters will never be used for such a PDN connection. This will lead to extra signalling and an increase in power consumption especially for NB-loT devices that can be power limited. Embodiments of the present invention aim to address shortcomings in the prior art, whether mentioned herein or not. 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. According to a first aspect of the present invention, there is provided a method of redirecting a User Equipment, UE, from a serving network to a target network, whereby the serving network rejects a service request message. In an embodiment, the serving network is a 5GC and the target network is an EPC whereby an AMF of the serving network sends a Service Reject message and includes 5GMM cause #31 "Redirection to EPC required”. In an embodiment, wherein when the UE, in N1 mode, receives a Service Reject with 5GMM cause #31, the UE takes the following actions: a) the UE set the 5GS update status to 5U3 ROAMING NOT ALLOWED b) The UE resets the service request attempt counter and enter the state 5GMM-REGISTERED.LIMITED-SERVICE c) The UE enable E-UTRA capability if it was disabled and disables the N1 mode capability for 3GPP access d) The UE, operating in single-registration mode, handles the EPS parameters, the EMM parameters, EMM state, and EPS update status e) If 5GMM cause #31 is received by a UE that has not indicated support for CloT optimizations, or received by a UE over non-3GPP access, or from a cell belonging to an SNPN, this is considered as an abnormal case f) The UE discards the Service Reject message with cause #31 if the message is received without integrity protection In an embodiment, the serving network is an EPC and the target network is a 5GC whereby an MME of the serving network sends a Service Reject message and includes EMM cause #31 "Redirection to 5GCN required”. In an embodiment, when the UE in S1 mode receives a Service Reject with EMM cause #31, the UE takes the following actions: a) The UE set the EPS update status to EU3 ROAMING NOT ALLOWED b) The UE resets the service request attempt counter and enter the state EMM-REGISTERED.LIMITED-SERVICE c) The UE enables N1 mode capability for 3GPP access if it was disabled and disable the E-UTRA capability d) The UE, operating in single registration mode, handles the 5GMM parameters, 5GMM state, 5GS update status e) If EMM cause #31 is received by a UE that has not indicated support for CloT optimizations, this is considered as an abnormal case. According to a second aspect of the present invention, there is provided a method of managing a PDN connection, wherein if the PDN connection is established in S1 mode, the UE verifies if an associated PDU session is associated with a Control Plane only indication. In an embodiment, the PDN connection is established after a first inter-system change from S1 mode to N1 mode and the UE is operating in single-registration mode in a network supporting N26 interface and the PDU session is one of "IPv4", "IPv6", "IPv4v6", or "Ethernet" PDU session type, and the UE supports more than 16 packet filters for this PDU. In an embodiment, if the UE determines that the PDU session is not associated with a control plane only indication, then the UE shall include the Maximum number of supported packet filters IE in a PDU SESSION MODIFICATION REQUEST message. In an embodiment, wherein the PDU SESSION MODIFICATION REQUEST message is not sent if a reason to otherwise send the message is to indicate the number of packet filters that the UE supports. In an embodiment, wherein the PDU SESSION MODIFICATION REQUEST message is not sent if a reason to otherwise send the message is to indicate that the UE supports RQoS. According to a third aspect of the present invention, there is provided a method of managing a PDN connection in a UE, wherein after a system change from S1 mode to NB-N1 mode, if the UE determines that a PDU session modification procedure should performed for the purpose of indicating that the UE supports RQoS, the UE nevertheless, does not send PDU SESSION MODIFICATION REQUEST message In an embodiment, the method of the third aspect is performed regardless of a Control Plane only indication or not. According to a fourth aspect of the present invention, there is provided a method of operating a UE in NB-N1 mode wherein the UE informs a serving network about a number of Data Radio Bearers, DRBs, that the UE is able to support, according to its capabilities. In an embodiment, the UE informs by means of a bit in a message to the network wherein a first value of the bit means that the UE supports 1 DRB or that multiple DRBs are not supported, and a second value of the bit means that the UE supports more than one DRB, which may be a maximum of M DRBs, where M is an integer. In an embodiment, the bit is bit 3, octet 5 of 5GMM Capability IE. According to a fifth aspect of the present invention, there is provided a method of a UE requesting establishment of User Plane resources for a number of PDU sessions, wherein the number of PDU sessions is not greater than a maximum number of DRBs that the UE can support. In an embodiment, when the UE requests Use Plane resources, the UE, by means of an Uplink data status IE does not indicate a total number of User Plane resources that is greater than a number of DRBs that the UE can support. According to a sixth aspect of the present invention, there is provided a method, in a network, of establishing User Plane resources in response to a request from a UE, wherein the User Plane resources are established if the total number of User Plane resources does not exceed a maximum number of DRBs that are supported by the UE. In an embodiment, an AMF in the network requests an SMF in the network to establish the User Plane resources. In an embodiment, the AMF verifies if a new DRB can be established based on how many DRBs the UE can support. According to a seventh aspect of the present invention, there is provided a method, in a network, of controlling establishing User Plane resources for a UE, wherein if a Payload container type IE is set to "N1 SM information", the Request type IE is set to "initial request", an AMF in the network verifies how many DRBs are already present for the UE and if the AMF determines that there are User Plane resources that are equal to a maximum number of DRBs that the UE supports, then the AMF either: (a) sends a message back to the UE; or (b) establishes the session as a control plane only session According to an eighth aspect of the present invention, there is provided apparatus arranged to perform any one of the previous aspects. 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 signalling according to the prior art; Figure 2 shows a message format according to an embodiment of the invention; and Figures 3 to 5 show messaging formats illustrating various issues in the prior art. According to a first embodiment, when a current serving (core) network (EPC or 5GC, and hence MME or AMF respectively) determines to redirect a UE to a target system or (core) network (5GC or EPC), the current service network may take the following actions: • If the UE is in connected mode (i.e. EMM-CONNECTED mode for S1 mode, or 5GMM-CONNECTED mode for N1 mode) o If the UE is in N1 mode, the AMF sends the DEREGISTRATION REQUEST message to the UE and includes the 5GMM cause #31 "Redirection to EPC required" o If the UE is in S1 mode, the MME sends the DETACH REQUEST message to the UE and includes the EMM cause #31 "Redirection to 5GCN required" • If the UE is in idle mode (i.e. EMM-IDLE mode for S1 mode, or 5GMM-IDLE mode for N1 mode), and the UE transitions to connected mode (i.e. EMM-CONNECTED mode for S1 mode, or 5GMM-CONNECTED mode for N1 mode) with the Service Request message, Control Plane Service Request message, or Extended Service Request message (only applicable to S1 mode), and the core network (e.g. MME or AMF) determines to redirect the UE to a target core network (CN), then o If the current CN node is an AMF (i.e. UE is in N1 mode), the AMF shall send the Service Reject message and include #31 "Redirection to EPC required" o If the current CN node is an MME (i.e. UE is in S1 mode), the MME shall send the Service Reject message and include #31 "Redirection to 5GCN required" If the UE is in N1 mode and receives the Deregistration Request message with 5GMM cause #31 "Redirection to EPC required": • If 5GMM cause #31 is received by a UE that has not indicated support for CloT optimizations or received by a UE over non-3GPP access is considered as an abnormal case and the behaviour of the UE is specified in subclause 5.5.1.2.7 of TS 24.501 • If the 5GMM cause #31 cause value is received from a cell belonging to an SNPN is considered as an abnormal case and the behaviour of the UE is specified in subclause 5.5.1.2.7. • The UE shall set the 5GS update status to 5U3 ROAMING NOT ALLOWED (and shall store it according to subclause 5.1.3.2.2) and shall delete any 5G-GUTI, last visited registered TAI, TAI list and ngKSI. Additionally, the UE shall reset the registration attempt counter and enter the state 5GMM-DEREGISTERED. • The UE shall enable the E-UTRA capability if it was disabled and disable the N1 mode capability for 3GPP access (see subclause 4.9.2 of TS 24.501). • If the message was received via 3GPP access and the UE is operating in singleregistration mode, the UE shall handle the EMM parameters EMM state, EPS update status, 4G-GUTI, TAI list, eKSI and attach attempt counter as specified in 3GPP TS 24.301

[15] for the case when the EPS attach procedure is rejected with the EMM cause with the same value. If the UE is in N1 mode and receives the Service Reject message with 5GMM cause #31 "Redirection to EPC required", the UE shall take the same actions as set out above (for receiving Deregistration Request in N1 mode). Optionally in addition to some of the actions set out above, the UE shall set the 5GS update status to 5U3 ROAMING NOT ALLOWED (and shall store it according to subclause 5.1.3.2.2). The UE shall reset the service request attempt counter and enter the state 5GMM-REGISTERED.LIMITED-SERVICE. The UE may enter this this state instead of 5GMM-DEREGISTERED. For a UE in N1 mode, if the Deregistration Request message with 5GMM cause #31, or the SERVICE REJECT message with 5GMM cause #31 is received without integrity protection, then the UE shall discard the message. If the UE is in S1 mode and receives the Detach Request message with EMM cause #31 "Redirection to 5GCN required": • If the EMM cause #31 received by a UE that has not indicated support for CloT optimizations is considered as an abnormal case and the behaviour of the UE is specified in subclause 5.5.1.2.6. The UE shall set the EPS update status to EU3 ROAMING NOT ALLOWED (and shall store it according to subclause 5.1.3.3) and shall delete any GUTI, last visited registered TAI, TAI list and eKSI. Additionally, the UE shall reset the attach attempt counter and enter the state EMM-DEREGISTERED. • The UE shall enable N1 mode capability for 3GPP access if it was disabled and disable the E-UTRA capability (see subclause 4.5 of TS 24.301). • If the UE is operating in single-registration mode, the UE shall in addition handle the 5GMM parameters 5GMM state, 5GS update status, 5G-GUTI, last visited registered TAI, TAI list and ngKSI as specified in 3GPP TS 24.501 for the case when the initial registration procedure performed over 3GPP access is rejected with the 5GMM cause with the same value If the UE is in S1 mode and receives the Service Reject message with EMM cause #31 "Redirection to 5GCN required", the UE shall take the same actions as set out above (for receiving Detach Request in S1 mode). Optionally in addition to some of the actions set out above, the UE shall set the 5GS update status to EU3 ROAMING NOT ALLOWED (and shall store it according to subclause 5.1.3.2.2). The UE shall reset the service request attempt counter and enter the state EMM-REGISTERED.LIMITED-SERVICE. The UE may enter this this state instead of EMM-DEREGISTERED. For a UE in S1 mode, if the DETACH REQUEST message or SERVICE REJECT message with EMM cause #31 is received without integrity protection, then the UE shall discard the message. The UE in NB-N1 mode (i.e. in 5GS) should inform the network (e.g. AMF) about the number of data radio bearers (DRBs) that the UE can support based on its capabilities. To do so the UE can send a new indication to the AMF in any 5GMM NAS message. This indication may be implemented in a new IE or an existing IE, known in the prior art. Furthermore, the indication may be in the form of a number i.e. to indicate that the UE supports X DRBs, where X is an integer and where the indication enables the UE to signal X. Alternatively, a new bit can be defined (in a new or existing IE), where the new bit can have one of two values as follows: If the bit is set to zero, then this means that the UE supports 1 DRB or it means that multiple DRB is not supported. Alternatively, if the bit is set to one (i.e. 1), then this means that the UE supports more than one DRB, and this may be a maximum of M, where M is an integer. For example, M can be the integer 2, hence by setting the bit to ‘1 ’, the UE indicates that it supports 2 DRBs. Or this value (i.e. 1) can be interpreted to mean "Multiple DRB supported". For example, a new bit can be used in the 5GMM capability IE bit. For example, bit 3 of octet 5 of this prior art IE can be defined to mean be the Multiple DRB support bit (‘multipleDRB’ bit). This is shown in the table in Figure 2 Note that the 5GMM capability IE is used as an example, but another IE may be used instead. As such, the UE should indicate its capability for the multipleDRB bit by setting the appropriate value in the bit when sending the Registration Request. If the UE supports N3 data transfer (i.e. user plane data transfer) and multiple user plane (data) radio bearers, then the UE shall set the Multiple DRB support bit to "Multiple DRB supported" in the 5GMM capability IE of the REGISTRATION REQUEST message. In the prior art, the AMF verifies if the UE has user plane resources established for a certain number of PDU sessions, where this number is currently 2. However, this is incorrect since the UE may actually support 1 DRB. Therefore, the UE cannot have user plane resources established for more than the maximum number of DRBs that the UE can support, where this maximum number can be an integer M. For example, the UE in NB-N1 mode can at most support 1 DRB or 2 DRBs. The UE shall not use the service request procedure to request the establishment of user plane resources for a number of PDU sessions that is greater than the maximum number of DRBs that the UE can support. Hence, if the UE sends a Service Request (SR) message or the Control Plane Service Request (CPSR) message, the UE shall ensure that: • The Uplink data status IE shall not be set such that user plane resources are requested for a number of PDU sessions that is bigger or larger than the maximum number of DRBs that the UE supports. Hence, the total number of PDU sessions for which user plane resources are requested shall not exceed the total number of DRBs that the UE supports • The Allowed PDU session status IE shall not be set such that user plane resources are requested for a number of PDU sessions that is bigger or larger than the maximum number of DRBs that the UE supports. Hence, the total number of PDU sessions for which user plane resources are requested (to be transferred to 3GPP access) shall not exceed the total number of DRBs that the UE supports • The Uplink data status IE and the Allowed data status IE shall both not be used and set such that user plane resources are requested (in both lEs) for a number of PDU sessions that is bigger or larger than the maximum number of DRBs that the UE supports. Hence, the total number of PDU sessions for which user plane resources are requested (in both lEs) shall not exceed the total number of DRBs that the UE supports As such, when the AMF receives a SR message, or CPSR message that contains: • The Uplink data status IE, the AMF shall verify if the number of PDU sessions for which UP resources will be established based on the IE (and possibly based on any PDU session that already has UP established for the UE) is bigger than the maximum number of DRBs that the UE supports based on the indication from the UE as proposed earlier. If yes, then the AMF shall: o Either not request the SMF to establish UP resources for the UE; or o shall choose to request the SMF(s) to establish UP resources for the UE such that the total number of PDU sessions for which UP resources will be established does not exceed the maximum number of DRBs that the UE supports based on the indication from the UE as set out earlier • The Allowed data status IE, the AMF shall verify if the number of PDU sessions for which UP resources to be established based on the IE (and possibly based on any PDU session that already has UP established for the UE) is bigger than the maximum number of DRBs that the UE supports based on the indication from the UE as proposed earlier. If yes, then the AMF shall: o Either not request the SMF to establish UP resources for the UE; or o shall choose to request the SMF(s) to establish UP resources for the UE such that the total number of PDU sessions for which UP resources will be established does not exceed the maximum number of DRBs that the UE supports based on the indication from the UE as set out earlier For a UE in NB-N1 mode, if the Uplink data status IE (or Allowed PDU session status IE) is included in the SR message or CPSR message, and does not indicate a request to have userplane resources established for a number of PDU sessions that is bigger / larger / more than the maximum number of DRBs that the UE can support (as has been set out earlier i.e. based on the UE’s indication in the 5GMM capability IE), then the AMF shall indicate to the SMF to reestablish the user-plane resources for the corresponding PDU sessions. For a UE in NB-N1 mode, if the Uplink data status IE (or Allowed PDU session status IE) is included in the SR message or CPSR message, and indicates a request to have user-plane resources established for a number of PDU sessions that is bigger / larger / more than the maximum number of DRBs that the UE can support (as has been set out earlier i.e. based on the UE’s indication in the 5GMM capability IE), then the AMF shall not indicate to the SMF to reestablish the user-plane resources for the corresponding PDU sessions. For a UE in NB-N1 mode, if the Uplink data status IE and the Allowed PDU session status IE are included in the SR message or CPSR message, and indicate a request to have user-plane resources established for a number of PDU sessions that is bigger / larger / more than the maximum number of DRBs that the UE can support (as has been set out earlier i.e. based on the UE’s indication in the 5GMM capability IE), then the AMF shall not indicate to the SMF to reestablish the user-plane resources for the corresponding PDU sessions. An AMF should take the following actions when the UE requests to establish a PDU session and the UE is in NB-N1 mode and is using user plane CloT 5GS optimization. Upon reception of a UL NAS TRANSPORT message, if the Payload container type IE is set to "N1 SM information", the Request type IE is set to "initial request", and a) the UE is in NB-N1 mode; b) the UE has indicated preference for user plane CloT 5GS optimization; c) the network accepted the use of user plane CloT 5GS optimization; and d) the AMF determines that there are user-plane resources established for a total number of PDU sessions for this UE and this number is equal to the maximum number of DRBs that the UE supports (or has indicated that is supports as proposed earlier); the AMF shall either: a) send back to the UE the 5GSM message in a DL NAS TRANSPORT message and include the 5GMM cause 5GMM cause #92 "insufficient user-plane resources for the PDU session". Note that another existing 5GMM cause can also be used; or b) proceed with the PDU session establishment and include the Control Plane CloT 5GS Optimisation indication or Control Plane Only indicator to the SMF. Note that the procedure set out above applies if the Request type is set to "existing PDU session". Based on the above, the AMF shall not assume that the UE always supports 2 DRBs and so it cannot assume that if the UE has UP resources established for 1 PDU session, then the network can establish UP resources for another new session. If the UE supports only 1 DRB, then the AMF shall either reject a new request for a PDU session (by sending the 5GSM message in a DL NAS TRANSPORT message and include the 5GMM cause 5GMM cause #92 "insufficient user-plane resources for the PDU session"), or proceed with the PDU session establishment and include the Control Plane CloT 5GS Optimisation indication or Control Plane Only indicator to the SMF. Therefore, the determination to setup a new PDU session with user plane resources for a UE in NB-N1 mode should be based on what the UE supports in terms of number of DRBs. Hence, the AMF should verify if the UE has UP resources established for a total number of PDU sessions and whether this total number is the same as the maximum number of DRBs that the UE supports: • If the UE currently has UP resources established for a number of PDU sessions and this number is less than the maximum number of DRBs that the UE can support (as has been set out earlier i.e. based on the UE’s indication in the 5GMM capability IE), then the AMF can accept the request from the UE to establish a PDU session and establish the corresponding UP resources • If the UE currently has UP resources established for a number of PDU sessions and this number is equal to the maximum number of DRBs that the UE can support (as has been set out earlier i.e. based on the UE’s indication in the 5GMM capability IE), then the AMF shall either reject the request (by sending the 5GSM message in a DL NAS TRANSPORT message and include the 5GMM cause 5GMM cause #92 "insufficient user-plane resources for the PDU session"), or proceed with the PDU session establishment and include the Control Plane CloT 5GS Optimisation indication or Control Plane Only indicator to the SMF. When the UE has a PDN connection that is first established in S1 mode, and the UE performs the first inter-system change from S1 mode to N1 mode, if the PDN connection was established as a Control plane only connection (i.e. the UE received the Control plane only indication IE in the in the ACTIVATE DEFAULT EPS BEARER CONTEXT REQUEST message), the UE shall not send the Maximum number of supported packet filters IE of the PDU SESSION MODIFICATION REQUEST message even if the UE supports more than 16 packet filters for this PDU session. If the PDU session modification procedure (e.g. after the first inter-system change from S1 mode to N1 mode and optionally the UE is operating in single registration mode, and optionally the network supports the N26 interface) is to be performed (optionally solely) for the purpose of indicating the number of packet filters that the UE supports (optionally where / when this number is more than 16), then the UE should not perform the PDU session modification procedure, optionally where not performing the PDU session modification procedure implies that the UE does not send the PDU Session Modification Request message. However, if the UE determines to perform the PDU session modification procedure (e.g. the UE determines to send the PDU Session Modification Request message) for other reasons than indicating the number of packet filters that the UE supports, then the UE should not include the Maximum number of supported packet filters IE in the PDU SESSION MODIFICATION REQUEST message if the UE PDU session is determined to be associated with a control plane only indication (even when the UE indeed can support more than 16 packet filters). Otherwise, if the PDU session is not associated with a control plane only indication and the UE does support more than 16 packet filters, then the UE can include the Maximum number of supported packet filters IE in the PDU SESSION MODIFICATION REQUEST message if the UE determines that the message is to be sent. In general, the UE should determine whether a transferred session is a control plane only session or not, and: • If the UE determines that the session is a control plane only session, and if the UE supports more than 16 packet filters, then the UE shall not send the Maximum number of supported packet filters IE of the PDU SESSION MODIFICATION REQUEST message. However, if the message is to be sent for (optionally solely for) the purpose of indicating the number of packet filters that the UE supports (optionally where / when this number is more than 16), then the UE should not perform the PDU session modification procedure, optionally where not performing the PDU session modification procedure implies that the UE does not send the PDU Session Modification Request message • If the UE determines that the session is not a control plane only session, and if the UE supports more than 16 packet filters, then the UE shall not send the Maximum number of supported packet filters IE of the PDU SESSION MODIFICATION REQUEST message For a PDN connection established when in S1 mode, after the first inter-system change from S1 mode to N1 mode, if the UE is operating in single-registration mode in the network supporting N26 interface, the PDU session is of "IPv4", "IPv6", "IPv4v6", or "Ethernet" PDU session type, and the UE supports more than 16 packet filters for this PDU session: • If the PDU session is not a control plane only session, the UE shall indicate the maximum number of packet filters supported for the PDU session in the Maximum number of supported packet filters IE of the PDU SESSION MODIFICATION REQUEST message; • Otherwise (i.e. if the PDU session is a control plane only session) the UE shall not indicate the maximum number of packet filters supported for the PDU session in the Maximum number of supported packet filters IE of the PDU SESSION MODIFICATION REQUEST message The above can also apply in the general case for an inter-system change from S1 mode to NB-N1 mode when user plane CloT optimization is being used since only one default QoS rule is supported per PDU session in NB-N1 mode. As such, when the UE moves from NB-N1 mode to WB-N1 mode, the UE can then send the PDU Session Modification Request message to indicate that it supports more than 16 packet filters (if this is the case) by including the Maximum number of supported packet filters IE of the PDU SESSION MODIFICATION REQUEST message. Another option is that the UE, after an inter-system change from S1 mode to NB-N1 mode, should not indicate that is supports RQoS. For example, if for any of the following combination of conditions (e.g. that should be verified by the UE): • after an inter-system change from S1 mode to NB-N1 mode, • the UE is operating in single registration mode • the network supports N26 interface • the UE supports RQoS then if the UE determines that a PDU session modification procedure needs to be performed for (optionally solely for) the purpose of indicating that the UE supports RQoS, then the UE should not perform the PDU session modification procedure i.e. the UE should not send the PDU Session Modification Request message. If the UE determines to perform the PDU session modification procedure (i.e. the UE determines to send the PDU Session Modification Request message e.g. due to other reasons), then the UE may indicate that it does not support RQoS (or the UE may not indicate that it supports RQoS) even if the UE actually supports RQoS. However, after an inter-system change from NB-N1 mode to WB-N1 mode, the UE should send a PDU Session Modification Request message to indicate if (whether) it supports RQoS. Note that the above also applies to any inter-system change from S1 mode to N1 mode for a PDU session for which the UE determines that the session is associated with a control plane only session. As such, for any such PDU session, the UE need not indicate that it supports RQoS as it simply will not be applies for control plane only PDU sessions. As such, if the UE determines that a PDU session modification procedure is required for the purpose of indicating the support of RQoS (e.g. under the conditions listed above), then the UE may further determine to not perform the PDU session modification procedure if the PDU session is associated with a control plane only indication, or for any other case in which the UE performs an inter-system change from S1 mode to N1 mode (optionally if the UE is operating in single registration mode, and / or the network supports N26 interface). The above may also apply for other new conditions such as, but not limited to, the type of PDU session e.g. unstructured PDU session, or any other new PDU session may be defined in the future for which such capabilities (or any other capabilities) would not apply. As such, for any capability that does not apply under certain conditions, the UE may determine to not perform the necessary 5GSM procedure when the procedure is to be performed for the purpose of indicating the capability in question. On the other hand, if the UE sends the 5GSM message then it may not indicate the capability in question if the capability would not apply in the mode in which the UE is currently operating in. As such, the UE may send the 5GSM message when it changes its mode of operation (e.g. enters a new mode or performs an inter-system change to a new mode) where the capability would then apply in the new mode. 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

1. A method, in a network, of controlling establishing User Plane resources for a UE, wherein if a Payload container type IE is set to "N1 SM information", the Request type IE is set to "initial5 request", an AMF in the network verifies how many DRBs are already present for the UE and if the AMF determines that there are User Plane resources that are equal to a maximum number of DRBs that the UE supports, then the AMF either: (a) sends a message back to the UE; or (b) establishes the session as a control plane only session10   2. Apparatus arranged to perform the method of claim 1.

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