WO2022066070A1 - Control plane function associating with and performing control plane functions for one or more user equipments - Google Patents

Abstract

Methods and apparatus are provided. In an example aspect, a method in a first base station control plane function of performing control plane functions for one or more User Equipments (UEs) is provided. The method comprises determining that a second base station control plane function associated with one or more UEs is unavailable, determining one or more UE contexts for the one or more UEs, and performing control plane functions for the one or more UEs.

Description

CONTROL PLANE FUNCTION ASSOCIATING WITH AND PERFORMING

CONTROL PLANE FUNCTIONS FOR ONE OR MORE USER EQUIPMENTS

Technical Field

Examples of the present disclosure relate to a control plane function associating with one or more User Equipments (UEs), and causing the control plane function to associate with one or more UEs. Examples also relate to a control plane function performing control plane functions for one or more UEs.

Background

The 5th generation (5G) New Radio (NR) cellular telecommunication system aims at supporting Ultra Reliable Low Latency Communication (URLLC). Examples of features of URLLC include PDCP duplication functionality to increase reliability, as well as make before break handover procedures to ensure minimal interruption time at handover, where a UE switches from one cell or beam to another.

Typical deployments of 5G NR systems are expected in the future to utilize cloud infrastructure that supports 5G Core Network and upper Radio Access Network (RAN) functionality such as the 3GPP defined gNodeB-Central unit-Control Plane (gNB-CU-CP) functionality. The cloud infrastructure differs in reliability from typical dedicated or specialized hardware which has traditionally been used to support cellular network functions. One such difference is the availability of the underlying hardware, which may be significantly less reliable for generic off the shelf cloud hardware compared to traditional dedicated hardware. Therefore, cloud deployment of telecommunication functions may introduce mechanisms for dealing with failures of the cloud infrastructure. Examples of such mechanisms include using distributed databases to provide persistent storage even in the case of failure one or more hardware nodes supporting this database.

Methods for ensuring reliable connections to UEs using URLLC may include setting up parallel user plane connections to the same UE, so that if one of the user plane connections is lost, it may be possible to continue data transmission via the other connection. Multiple user plane connections may be supported either using a single control plane connection (e.g. RRC, NAS connection), or with two independent control plane connections, which in turn requires that the device has dual radio capability and is able to set up two independent radio connections, which typically increases the cost of the UE. The latter solution also increases network complexity, as the network would need to both understand that the two control plane connections are related, and to ensure that these devices are steered to using different network resources to avoid single point of failure.

Figure 1 illustrates an example scenario 102 for end to end redundant User Plane paths using Dual Connectivity. For example, Figure 1 illustrates user plane resource configuration of dual PDU sessions when redundancy is applied. One Protocol Data Unit (PDU) Session 102 spans from the UE via Master NG-RAN node to a first User Plane Function (UPF1) acting as the PDU Session Anchor, and the other PDU Session 104 spans from the UE via Secondary NG-RAN node to UPF2 acting as the PDU Session Anchor. As described in 3GPP Technical Specification (TS) 37.340, which is incorporated herein by reference, a NG- RAN may realize redundant user plane resources for the two PDU sessions with two NG- RAN nodes (i.e. Master NG-RAN and Secondary NG-RAN as shown in Figure 1) or a single NG-RAN node. In both cases, there is a single N2 interface towards AMF from the Master NG-RAN node.

Based on these two PDU Sessions 102 and 104, two independent user plane paths are set up. UPF1 and UPF2 connect to the same Data Network (DN), even though the traffic via UPF1 and UPF2 may be routed via different user plane nodes within the DN.

The 5G RAN architecture 200 is described in 3GPP TS 38.401, which is incorporated herein by reference, and is illustrated in Figure 2. The NG architecture can be further described as follows:

• The NG-RAN consists of a set of gNBs connected to the 5GC through the NG interface.

• A gNB can support FDD mode, TDD mode or dual mode operation.

• gNBs can be interconnected through the Xn interface.

• A gNB may consist of a gNB-CU (central unit) and one or more gNB-DUs (distributed units).

• A gNB-CU and a gNB-DU are connected via F1 logical interface.

• A gNB-DU is connected to only one gNB-CU.

NG, Xn and F1 are logical interfaces. For NG-RAN, the NG and Xn-C interfaces for a gNB consisting of a gNB-CU and gNB-DUs terminate in the gNB-CU. For E-UTRAN-New Radio Dual Connectivity (EN-DC), the S1-U and X2-C interfaces for a gNB consisting of a gNB-CU and gNB-DUs terminate in the gNB-CU. The gNB-Cll and connected gNB-DUs are only visible to other gNBs and the core network (5GC) as a gNB.

The NG-RAN is layered into a Radio Network Layer (RNL) and a Transport Network Layer (TNL). The NG-RAN architecture, i.e. the NG-RAN logical nodes and interfaces between them, is defined as part of the RNL. For each NG-RAN interface (NG, Xn, F1) the related TNL protocol and functionality are specified. The TNL provides services for user plane transport and signalling transport. In NG-Flex configuration, each gNB is connected to all Access and Mobility Management Functions (AMFs) within an AMF Region. The AMF Region is defined in 3GPP TS 23.501, which is incorporated herein by reference.

An open interface has been suggested between the control plane (CU-CP) and the user plane (CU-UP) parts of the central unit (CU). The related specification is 3GPP TS 38.463, which is incorporated herein by reference. The open interface between CU-CP and CU-UP is named E1. The architecture is shown in Figure 3, which illustrates a split gNB architecture. Three deployment scenarios for the split gNB are shown in 3GPP Technical Report (TR) 38.806, which is incorporated herein by reference:

Scenario 1 : CU-CP and CU-UP centralized;

Scenario 2: CU-CP distributed and CU-UP centralized;

Scenario 3: CU-CP centralized and CU-UP distributed.

The E1 application protocol (E1AP) is defined in TS 38.463. The E1AP defines the messages that are exchanged between the CU-CP and the CU-UP for the sake of providing user-plane services to the UE.

In LTE and NR, the Radio Resource Control (RRC) protocol is used to setup, configure and maintain the radio connection between the UE and an eNB or gNB. When the UE receives an RRC message from the eNB or gNB, it will apply or “compile” the configuration, and if this succeeds the UE generates an RRC complete message that indicates the transaction ID of the message that triggered this response.

Since LTE-release 8 (rel-8), three Signaling Radio Bearers (SRBs), namely SRB0, SRB1 and SRB2, have been available for the transport of RRC and Non Access Stratum (NAS) messages between the UE and the eNB. A new SRB, known as SRBIbis, was also introduced in rel-13 for supporting DoNAS (Data Over NAS) in narrowband-internet of Things (NB-loT). SRB0 is for RRC messages using the CCCH logical channel, and is used for handling RRC connection setup, RRC connection resume and RRC connection reestablishment. Once the UE is connected to the eNB (i.e. RRC connection setup or RRC connection reestablishment/resume has succeeded), SRB1 is used for handling RRC messages (which may include a piggybacked NAS message) as well as for NAS messages prior to the establishment of SRB2, all using the DCCH logical channel. SRB2 is for RRC messages which include logged measurement information as well as for NAS messages, all using the DCCH logical channel. SRB2 has a lower priority than SRB1, because logged measurement information and NAS messages can be lengthy and could cause the blocking of more urgent and smaller SRB1 messages. SRB2 is always configured by E-UTRAN after security activation.

In rel-15, for the case of dual connectivity (LTE as the master node while NR is the secondary node, or vice versa, as well as dual connectivity using two NR nodes), split SRBs are introduced, where SRB1/2 can be transported via the radio resources of the master node or the secondary node (with the protocol being terminated at the master node). Additionally, a new SRB called SRB3 is introduced (if the secondary node is NR), that is used to directly transfer RRC messages from the secondary node to the UE for those messages that do not require co-ordination with the master node.

The majority of the RRC procedures are initiated by the network, e.g. to re-configure some behavior of the UE such as measurement reporting configuration, lower layer configuration, radio bearer configuration, etc. Typically the UE acknowledges the RRC message from the network with an RRC reply message indicating that the new configuration has been adopted. Some RRC procedures are also initiated by the UE, such as:

Measurement reports (e.g. used for UE mobility) RRC re-establishment in case of connection failure Initial RRC connection setup (e.g. if the UE is IDLE mode and need to transition to connected)

RRC relies on PDCP/RLC/MAC protocols to guarantee lossless, in-order, duplication free delivery of RRC messages. The in-order delivery is ensured by the PDCP sequence number, meaning that PDCP only delivers an RRC message (e.g. delivers to a later above the PDCP layer) if it has a PDCP sequence number which is the next expected PDCP sequence number (i.e. the number is one more than the PDCP sequence number of last delivered message). This means that if the network loses knowledge of the next PDCP sequence number that is expected by a UE, it would not be able to send any RRC message to the UE since if the network does not guess the exact sequence number correctly the message will not be delivered by the PDCP layer in the UE to a higher layer (e.g. the RRC layer) in the UE.

In addition to the RRC protocol, the UE is also controlled by the NAS protocol which is between the UE and the core network. The NAS protocol is delivered embedded within RRC messages.

Figure 4 illustrates an example of a control plane (CP) protocol stack 400 in NR and Figure 5 illustrates an example of a user plane (UP) protocol stack 500 in NR, where the CU/DU split architecture is employed (where the CU is also split into CU-UP and CU-CP). Note that the CU-CP/CU-UP split is a functional split and as such the two functionalities can reside either in the same node or different node or even one of the functionalities, i.e. the CU-UP or CU- CP, for a given gNB can be physically realized in several physical nodes/entities can be distributed physically. For example, several instances of the CU-UP/CU-CP for a given gNB can exist in an operator’s cloud, for redundancy or for load balancing purposes.

If a connection with a UE fails, e.g. due to radio link failure, handover failure (T304 timer expiry) or inability to comply with RRC message, integrity verification failure etc., the UE will initiate an RRC re-establishment procedure. In NR, this is captured in TS 38.331 , which is incorporated herein by reference, in section 5.3.7. Figure 6 shows an example of a RRC connection re-establishment (successful) procedure 600, and Figure 7 shows an example of a RRC re-establishment with fallback to RRC establishment (successful) procedure 700.

The purpose of the re-establishment procedure is to re-establish the RRC connection. A UE in RRC_CONNECTED state, for which AS security has been activated with SRB2 and at least one DRB setup, may initiate the RRC re-establishment procedure in order to continue the RRC connection. The connection re-establishment succeeds if the network is able to find and verify a valid UE context for the UE or, if the UE context cannot be retrieved, the network responds with an RRCSetup message as shown in Figure 7 for example.

The network applies the procedure for example as follows:

When AS security has been activated and the network retrieves or verifies the UE context: re-activate AS security without changing algorithms; re-establish and resume SRB1 ; When UE is re-establishing an RRC connection, and the network is not able to retrieve or verify the UE context: discard the stored AS Context and release all RBs; fallback to establish a new RRC connection.

If AS security has not been activated, the UE shall not initiate the procedure but instead moves to RRCJDLE state directly, with release cause 'other'. If AS security has been activated, but SRB2 and at least one DRB are not setup, the UE does not initiate the procedure but instead moves to RRCJDLE directly, with release cause 'RRC connection failure'.

The UE can re-establish to a node different from the one it was connected to before detecting a problem. When the target network node receives the RRCReestablishmentRequest message (e.g. illustrated in Figure 6 or 7), it can use the ReestabUE-ldentity that is included in the request message to attempt to locate the old UE context for the UE. If the target network node can locate the context, it can transmit the RRCReestablishment message to the UE and the UE will restore the old configurations. If the target network node cannot find the UE context, it has to perform a NAS recovery and instead sends an RRCSetup message to the UE as illustrated in Figure 7 for example.

The target network node will transmit the RETRIEVE UE CONTEXT REQUEST to the source node (as defined in TS 38.423/36.423, which are incorporated herein by reference), i.e. the node to which the UE was connected before the connection failure, and the source node can respond with the RETRIEVE UE CONTEXT RESPONSE (if the UE identification and verification is successful). If successful, the RETRIEVE UE CONTEXT RESPONSE will contain the HandoverPreparationlnformation message, defined in TS 38.331, that contains the UE context. Once the UE has received the RRCReestablishment message, restored some configurations (e.g. SRB1) and reestablished the connection, it will transmit the RRCReestablishmentComplete message to the network. The network will then send an RRCReconfiguration message to the UE which will restore and possibly reconfigure the rest of the configurations in the UE.

In the context of CU/DU split, the re-establishment procedure is shown in Figure 8 (from TS 38.401, section 8.7), which shows an example of a RRC re-establishment procedure 800 in the case of CU/DU split architecture. The steps in this procedure are as follows:

1. UE sends preamble to the gNB-Dll. 2. The gNB-Dll allocates new C-RNTI and responds UE with Random Access Response (RAR).

3. The UE sends RRCReestablishmentRequest message to the gNB-DU, which contains old C-RNTI and old Physical Cell ID (PCI).

4. The gNB-DU includes the RRC message and the corresponding low layer configuration for the UE in the INITIAL UL RRC MESSAGE TRANSFER message and transfers to the gNB-CU. The INITIAL UL RRC MESSAGE TRANSFER message should include C-RNTI.

5. The gNB-CU includes RRCReestablishment message and transfers to the gNB-DU. If the UE requests to re-establish RRC connection in the last serving gNB-DU, the DL RRC MESSAGE TRANSFER message shall include old gNB-DU UE F1AP ID.

6. The gNB-DU retrieves UE context based on old gNB-DU UE F1AP ID, replaces old C-RNTI/PCI with new C-RNTI/PCI. It sends RRCReestablishment message to UE.

7-8. The UE sends RRCReestablishmentComplete message to the gNB-DU. The gNB- DU encapsulates the RRC message in UL RRC MESSAGE TRANSFER message and sends to gNB-CU.

9-10. The gNB-CU triggers UE context modification procedure by sending UE CONTEXT MODIFICATION REQUEST, which may include DRBs to be modified and released list. The gNB-DU responses with UE CONTEXT MODIFICATION RESPONSE message.

9'-1 O'. The gNB-DU triggers UE context modification procedure by sending UE CONTEXT MODIFICATION REQUIRED, which may include DRBs to be modified and released list. The gNB-CU responses with UE CONTEXT MODIFICATION CONFIRM message.

NOTE: Here it is assumed that UE accesses from the original gNB-DU where the UE contexts are available for that UE, and either step 9-10 or step 9’ and 10’ may exist or both could be skipped.

NOTE: If UE accesses from a gNB-DU other than the original one, gNB-CU should trigger UE Context Setup procedure towards this new gNB-DU.

11-12. The gNB-CU includes RRCReconfiguration message into DL RRC MESSAGE TRANSFER message and transfers to the gNB-DU. The gNB-DU forwards it to the UE.

13-14. The UE sends RRCReconfigurationComplete message to the gNB-DU, and gNB-DU forwards it to the gNB-CU.

The signaling radio bearers (SRBs) used for delivery of RRC messages provide lossless, duplication free, in-order delivery of RRC messages. It is the responsibility of the PDCP layer to ensure in order and duplication free delivery to layer(s) above the PDCP layer. It does this by assigning a PDCP sequence number to every message. The sequence number is transferred in the PDCP header of a PDCP message. The receiving PDCP entity (e.g. in the UE) will only deliver a RRC message within a PDCP packet to the RRC layer if the PDCP sequence number of the packet that contains the RRC message is the next expected PDCP sequence number (which is the PDCP sequence number last used plus one). If the network uses a PDCP sequence number which is smaller than the PDCP sequence number last used before the failure, the UE will consider the packet a duplicate and discard it. If the network uses a PDCP sequence which is larger than the next expected PDCP sequence number in the UE, the UE will store the message in the PDCP layer and wait for the missing packet(s) with the missing sequence number(s). For example, if the last received RRC message before the CP failure was within a PDCP packet with sequence number (SN) x, and a RRC message is received with SN x+2, the PDCP layer will wait for a PDCP packet with SN x+1 before forwarding a RRC message in the PDCP packet with SN x+2 to the RRC layer. That is, if a packet with SN x+1 is not received, the packet with SN x+2 will be stored indefinitely in the UE and its contents will not be delivered to a higher layer. Since the PDCP sequence number wraps around, the determination if an out of sequence packet is a duplicate or future out of sequence packet is based on if the packet is within our outside of the PDCP receive window.

During the initiation of the re-establishment procedure (according to section 5.3.7.2 in TS 38.331), the UE: resets the MAC (i.e. any pending data at the MAC level will be flushed) both carrier aggregation (CA) and dual connectivity (DC) are released all DRBs and SRBs (except SRB0, which is used for sending the re-establishment message) are suspended

The UE then starts timer T311 and performs cell re-selection, which could select the same primary cell that it was previously connected to, or another cell. If the timer expires before a suitable cell is found, NAS recovery has to be performed via IDLE mode. If a suitable cell is found, the UE applies default MAC and control channel configurations, starts timer T301 , and sends the RRC re-establishment request message to the chosen cell (including UE identity, a security checksum called short MAC-I that is computed using the old RRC integrity that it was using before re-establishment is triggered, and a re-establishment cause, which can take one of the following values: radio link failure, reconfiguration failure, handover failure, or other failure). If timer T301 expires before the UE receives the RRCReestablishment message from the network, UE has to perform NAS recovery via IDLE mode. On receiving the RRCReestablishment message, which contains the NCC (Next hop Chaining Count), the UE uses the NCC value to derive new user plane and control plane encryption and integrity protection keys to be used for the connection, including for verification of the integrity of the received Reestablishment message. The network always sends an RRC Reconfiguration message after the Reestablishment message (referred to as “first reconfiguration after re-establishment” in the specifications). This reconfiguration message contains a full configuration flag and thus resets the whole radio configuration (except the C-RNTI and the access stratum security configuration associated with the master key) and replaces it with a new one. Additionally, the reestablishPDCP field will be included for each SRB/DRB, which will re-establish the PDCP entity of each SRB/DRB. Reestablishment of the PDCP will result in the reconfiguration of the PDCP entities to use the new ciphering and integrity protection keys calculated on the reception of the Reestablishment message, and for the transmitting PDCP entities, to transmit any pending PDCP packets after protecting them with the new keys (“pending” here means those packets which were already sent, protected using the old security keys, but the acknowledgement of their reception was not received). Also, for SRBs, re-establishment of the PDCP means the reset of the sequence numbers to initial values.

The main reason for the use of the NCC and update of the security keys is that from a security perspective re-use the same sequence number (Count) and security key for different data should be avoided, which could otherwise happen during re-establishment as the sequence numbers of the SRBs are re-set. This is due to the ciphering principle in 3GPP networks (as shown in 3GPP TS 33.501, which is incorporated herein by reference, in figure D2.1.1-1) being based on XORing the data (plaintext block) with a security bitstream (keystream block) generated by the ciphering algorithm based on a key, sequence number etc. It could be possible to remove this security bitstream by XORing two messages which has been encrypted with the same bitstream. The remaining bits will then be just an XOR of the two original messages. If an attacker could then guess one of the messages it would automatically also be able to decode the other message. Figure D2.1.1-1 of 3GPP TS 33.501 is shown in Figure 9.

The RRC connection re-establishment procedure can be used to resynchronize the network and the UE. It does however always lead to a reset of the user plane, meaning for instance the user plane Data Radio Bearers (DRBs) are suspended, and any packets in the lower layer state machines (e.g. RLC/MAC) are flushed. Studies show that a typical RRC connection re-establishment procedure will cause a user plane service interruption of 100ms or more. This could be considerably longer in reality, as the re-establishment is also controlled by several timers that have configurable values (e.g. T310, which is started when detecting physical layer problems can be up to 2 seconds long, and UE will not initiate the re-establishment procedure until this timer expires). Typically, this is not a problem for normal mobile broadband (MBB) users or other users since the overall performance of connection will not be significantly affected by such a service interruption. For connections requiring LIRLLC however this service interruption could mean LIRLLC requirements, such as for example latency and/or reliability, are not fulfilled.

In LIRLLC communications, the survival time is the time that an application consuming an LIRLLC communication service may continue without an anticipated message. If communication service recovery (e.g. RRC re-establishment) is not completed before survival time expires, the end user application considers the communication service as unavailable and may for example begin taking emergency actions to recover. The survival time also puts limits on the allowed user plane interruption time (e.g. at UE mobility). The survival time is a key requirement since it puts limits on how fast the system needs to recover from a failure to avoid application downtime. If the recovery time after a communication failure is shorter than the survival time, the failure may pass unnoticed by the application. Examples of use cases with a demanding survival time include public safety drones, which may have a survival time of approximately 100ms; and industrial automation use cases in which traffic is cyclic, with frequent small packets, typically using Industrial Ethernet. The survival time in these cases may allow loss of a single packet or a few consecutive packets. For example, Motion Control may have a survival time of 0-2ms, PLC to PLC communication 8-48ms and Automated Guided Vehicle (AGV) 40-500 ms.

Summary

One aspect of the present disclosure provides a method in a first base station control plane function of performing control plane functions for one or more User Equipments (UEs). The method comprises determining that a second base station control plane function associated with one or more UEs is unavailable, determining one or more UE contexts for the one or more UEs, and performing control plane functions for the one or more UEs.

Another aspect of the present disclosure provides a method in a network function in a network of causing a first base station control plane function to associate with one or more User Equipments (UEs). The method comprises determining that a second base station control plane function associated with one or more UEs is unavailable, and sending a message to a first base station control plane function to cause the first base station control plane function to associate with the one or more UEs.

A further aspect of the present disclosure provides apparatus in a first base station control plane function for associating with one or more User Equipments (UEs). The apparatus comprises a processor and a memory. The memory contains instructions executable by the processor such that the apparatus is operable to determine that a second base station control plane function associated with one or more UEs is unavailable, determine one or more UE contexts for the one or more UEs, and perform control plane functions for the one or more UEs.

A still further aspect of the present disclosure provides apparatus in a network function in a network for causing a first base station control plane function to associate with one or more User Equipments (UEs). The apparatus comprises a processor and a memory. The memory contains instructions executable by the processor such that the apparatus is operable to determine that a second base station control plane function associated with one or more UEs is unavailable, and send a message to a first base station control plane function to cause the first base station control plane function to associate with the one or more UEs.

An additional aspect of the present disclosure provides apparatus in a first base station control plane function for associating with one or more User Equipments (UEs). The apparatus is configured to determine that a second base station control plane function associated with one or more UEs is unavailable, determine one or more UE contexts for the one or more UEs, and perform control plane functions for the one or more UEs.

Another aspect of the present disclosure provides apparatus in a network function in a network for causing a first base station control plane function to associate with one or more User Equipments (UEs). The apparatus is configured to determine that a second base station control plane function associated with one or more UEs is unavailable, and send a message to a first base station control plane function to cause the first base station control plane function to associate with the one or more UEs.

Brief Description of the Drawings For a better understanding of examples of the present disclosure, and to show more clearly how the examples may be carried into effect, reference will now be made, by way of example only, to the following drawings in which:

Figure 1 shows an example scenario for end to end redundant User Plane paths using Dual Connectivity (DC);

Figure 2 illustrates an example of a 5G Radio Access Network (RAN) architecture;

Figure 3 illustrates a split gNodeB architecture;

Figure 4 illustrates an example of a control plane (CP) protocol stack;

Figure 5 illustrates an example of a user plane (UP) protocol stack;

Figure 6 shows an example of a RRC connection re-establishment (successful) procedure;

Figure 7 shows an example of a RRC re-establishment with fallback to RRC establishment (successful) procedure;

Figure 8 shows an example of a RRC re-establishment procedure in case of CU/DU split architecture;

Figure 9 shows Figure D2.1.1-1 of 3GPP TS 33.501 ;

Figure 10 is a flow chart of an example of a method in a first base station control plane function of performing control plane functions for one or more User Equipments (UEs);

Figure 11 is a flow chart of an example of a method in a network function in a network of causing a first base station control plane function to associate with one or more User Equipments (UEs);

Figure 12 shows a particular example of communications within a network according to an embodiment of this disclosure; Figure 13 is a schematic of an example of apparatus 1300 in a first base station control plane function for associating with one or more User Equipments (UEs); and

Figure 14 is a schematic of an example of apparatus 1400 in a network function in a network for causing a first base station control plane function to associate with one or more User Equipments (UEs).

Detailed Description

The following sets forth specific details, such as particular embodiments or examples for purposes of explanation and not limitation. It will be appreciated by one skilled in the art that other examples may be employed apart from these specific details. In some instances, detailed descriptions of well-known methods, nodes, interfaces, circuits, and devices are omitted so as not obscure the description with unnecessary detail. Those skilled in the art will appreciate that the functions described may be implemented in one or more nodes using hardware circuitry (e.g., analog and/or discrete logic gates interconnected to perform a specialized function, ASICs, PLAs, etc.) and/or using software programs and data in conjunction with one or more digital microprocessors or general purpose computers. Nodes that communicate using the air interface also have suitable radio communications circuitry. Moreover, where appropriate the technology can additionally be considered to be embodied entirely within any form of computer-readable memory, such as solid-state memory, magnetic disk, or optical disk containing an appropriate set of computer instructions that would cause a processor to carry out the techniques described herein.

Hardware implementation may include or encompass, without limitation, digital signal processor (DSP) hardware, a reduced instruction set processor, hardware (e.g., digital or analogue) circuitry including but not limited to application specific integrated circuit(s) (ASIC) and/or field programmable gate array(s) (FPGA(s)), and (where appropriate) state machines capable of performing such functions.

It is desirable to support continuous user plane transmission for a wireless device, even if some part of the control plane connection fails on the network side, e.g. due to failed cloud infrastructure hardware and/or software. This is particularly useful for wireless devices requiring URLLC performance with low survival time. Thus it is desirable for example to support continuous user plane transmission for one or more UEs in the event that all or part of a control plane function associated with the one or more UEs fails or is unavailable. In examples of this disclosure, a control plane function associated with one or more UEs means that the control plane function serves those UEs, for example for performing control plane functions for those UEs.

Examples of this disclosure may provide functionality to handle the case where a RAN control plane entity associated with a UE (or multiple UEs) becomes available, for example due to failure of the control plane entity (also referred to as a control plane function). In example embodiments, a control plane entity or function is able to take over functionality of an unavailable control plane function that is serving one or more UEs, without any impact on the user plane for the UE(s).

Advantages of example embodiments may include that Quality of Service (QoS) requirements for bearers and services with stringent latency and survival time requirements (e.g. URLLC) may be fulfilled at a UE, even if the control plane function serving the UE becomes unavailable, such as for example due to hardware or software failure of a processing entity running the control plane function application. Example embodiments may be implemented without any additional requirements or changes to UEs.

Example embodiments of this disclosure provide network side solutions, whereby the failure of a control plane entity (e.g. function or node) may have no impact on the user plane functionality of one or more UEs served by the failed control plane entity. Example embodiments may allow another control plane entity to take over the control plane functionality for the UE(s). Some examples make use of a UE context database that is external to the control plane entity (e.g. CU-CP) that is currently serving the UE(s) and that may become unavailable. The database may be located in some examples within a Distributed Unit (DU) serving the UEs, or at any node or function external to the control plane entity that may become unavailable. Some examples may also provide a UE Context Failure Detection function. In a similar way as for the UE Context Database, the UE Context Failure Detection function may be located within the DU or at any node external to the control plane function that becomes unavailable.

Figure 10 is a flow chart of an example of a method 1000 in a first base station control plane function of performing control plane functions for one or more User Equipments (UEs). The first base station control plane function may be for example a base station central unitcontrol plane (CU-CP), a function within a CU-CP, or a function within any suitable network node. The method 1000 comprises, in step 1002, determining that a second base station control plane function associated with one or more UEs is unavailable, for example due to hardware and/or software failure of a data processing node that is implementing the second base station control plane function. The second base station control plane function may be for example a base station central unit-control plane (CLI-CP), a function within a CLI-CP, or a function within any suitable network node.

The method 1000 also comprises, in step 1004, determining one or more UE contexts for the one or more UEs, and in step 1006, performing control plane functions for the one or more UEs. Determining the one or more UE contexts may for example allow the first control plane function to take over responsibility of control plane functions from the second control plane functions, and become associated and begin to serve the one or more UEs. The first control plane function may do this for example without any impact on user plane connections or functionality of the one or more UEs. Thus the first control plane function will thus for example be able to perform control plane functions for the one or more UEs (e.g. serve the one or more UEs). This may comprise for example generating and sending control plane messages to the one or more UEs, and/or receiving control plane messages from the one or more UEs.

Each UE context may in some examples comprise one or more of a UE RRC context, a UE PDCP context for a control plane, a UE NG-C interface context, a UE control plane context, a UE user plane context and one or more pending RRC procedures. Additionally or alternatively, the one or more contexts may comprise for example one or more UE contexts used by the second base station control plane function.

In some examples, the method 1000 may comprise sending a message to a base station distributed unit (DU) associated with the one or more UEs (e.g. serving the one or more UEs) that the first base station control plane function is associated with the one or more UEs. Thus the DU will then be aware of the first control plane function that is to be serving the one or more UEs for control plane functions for the UE(s), and will thus for example know the appropriate function to which control plane messages should be forwarded.

Each context referred to above in respect of step 1004 of the method 1000 may in some examples include respective a user plane UE context. The method 1000 may thus comprise maintaining the one or more user plane UE contexts for the one or more UEs. Thus for example the user plane contexts may not be reset, reconfigured or flushed. In particular, for example, maintaining the one or more user plane UE contexts for the one or more UEs may comprise maintaining the one or more user plane UE contexts without performing a RRC reestablishment or RRC setup procedure. Thus for example the user plane functionality of the one or more UEs may avoid any interruption or disruption. In some examples, determining that the second base station control plane function is unavailable in step 1002 comprises receiving an instruction to associate with the one or more UEs. This instruction may be received for example from a base station distributed unit (DU) associated with (e.g. serving) the one or more UEs or from a network function. More generally, in some examples, the instruction may be received from any suitable network node or function that determines that the second base station control plane function is unavailable, and knows or selects the first control plane function for taking over control plane functionality for the one or more UEs (or for the DU).

In some examples, determining that the second base station control plane function is unavailable in step 1002 comprises determining that one or more expected messages or acknowledgements have not been received from the second base station control plane function. For example, the first control plane function may send periodic messages to the second control plane function, for which acknowledgements are expected, or the second control plane function may send periodic messages to the first control plane function to indicate that the second control plane function is still operational. If any of these messages or acknowledgements are not received by the first control plane function, for example, this may indicate that the second control plane function has failed, and the first control plane function may then take steps to take over control plane function responsibility for the one or more UEs as described herein.

Determining the one or more UE contexts for the one or more UEs may comprise for example retrieving the one or more UE contexts from a database. For example, the one or more UE contexts may be retrieved from a database via a base station distributed unit (DU) associated with the one or more UEs. Alternatively, in some examples, the first base station control plane function includes the database. Thus for example the first control plane function may operate as a “backup” control plane function for the case where the second control plane function becomes available, and thus may store the UE contexts on an ongoing basis. Thus, in some examples, the method 1000 may comprise, before determining unavailability of the second base station control plane function, receiving one or more indications of the one or more contexts and/or one or more updates to the one or more contexts, and storing the contexts or updating the contexts based on the one or more indications. In some examples, the method 1000 may comprise updating the database to indicate that the first base station control plane function is associated with the one or more UEs. Determining the one or more UE contexts for the one or more UEs in step 1004 may alternatively comprise for example receiving an indication of the one or more UE contexts from a base station distributed unit (DU) associated with the one or more UEs. For example, the first base station control plane function may request the context(s) from the DU after determining that the second control plane function is unavailable in step 1002, e.g. after receiving an instruction to take over control plane functionality as suggested above.

In some examples, there may be pending RRC procedures. For example, there may be one or more RRC procedures or messages that have not been sent by the second control plane function when it became unavailable, and/or one or more RRC procedures or messages that have been sent to the one or more UEs but have not been acknowledged (or acknowledgements have not been received) at the second control plane function when it became unavailable. The method 1000 may in some examples thus comprise, if there are any pending or unacknowledged (by the UE) RRC procedures in the one or more UE contexts, performing the RRC procedures. This may ensure for example synchronization of a RRC configuration state or other configuration between the one or more UEs and the network.

Each UE context may in some examples comprise a most recently acknowledged UE context for each UE. For example, where a RRC message or procedure is sent to a UE, and an acknowledgement is received that indicates that any configuration change has been implemented or was successful, the UE context (that may indicate for example the configuration such as the RRC configuration of the UE) may be considered as the most recently acknowledged UE context. Acknowledgements may be received in some examples at the second control plane function before it becomes unavailable. Thus synchronization of a configuration state (e.g. RRC configuration) may be synchronized between the one or more UEs and the network in some examples.

The method may comprise for example sending a message to one or more further base station distributed units (DUs) associated with one or more further UEs that are associated with the second base station control plane function that the first base station control plane function is associated with the one or more further UEs (e.g. is to serve the one or more further UEs, at least for control plane functionality). This may be the case for example where the second control plane function was associated with multiple DUs before it became unavailable. However, in some examples, the DUs may be associated with different control plane functions after the second control plane function becomes available and the different control plane functions have taken over responsibility for the UE(s) served by the second control plane function.

Figure 11 is a flow chart of an example of a method 1100 in a network function in a network of causing a first base station control plane function to associate with one or more User Equipments (UEs). The network function may be any suitable function or node in the network. Examples include a DU as suggested above, or any function or node that monitors availability of a second base station control plane function. The method comprises, in step 1102, determining that a second base station control plane function associated with (e.g. serving) one or more UEs is unavailable, e.g. due to hardware and/or software failure. The method 1100 also comprises, in step 1104, sending a message to a first base station control plane function to cause the first base station control plane function to associate with the one or more UEs. In some examples, the first base station control plane function may receive the message in some examples of step 1002 of the method 1000 described above. In some examples, the first and/or second base station control plane function may comprise a base station CU-CP.

In some examples, sending the message to the first base station control plane function to cause the first base station control plane function to associate with the one or more UEs in step 1104 comprises sending the message to the first base station to cause the first base station control plane function to use one or more UE contexts for the one or more UEs, wherein the one or more UE contexts comprise one or more UE contexts used by the second base station control plane function. Each context may include for example respective a user plane UE context, and sending the message to the first base station control plane function in step 1104 to cause the first base station control plane function to associate with the one or more UEs may comprise sending the message to the first base station to cause the first base station control plane function to maintain the one or more user plane UE contexts for the one or more UEs. Sending the message to the first base station to cause the first base station control plane function to maintain the one or more user plane UE contexts for the one or more UEs may in some examples comprise sending the message to the first base station to cause the first base station control plane function to maintain the one or more user plane UE contexts for the one or more UEs without performing a RRC reestablishment or RRC setup procedure. For example, this may be performed without interruption or disruption of the user plane contexts or user plane functionality of the one or more UEs. Each UE context may comprise a most recently acknowledged UE context for each UE, for example, as suggested above. Each UE context may comprise for example one or more of a UE RRC context, a UE PDCP context for a control plane, a UE NG-C interface context, a UE control plane context, a UE user plane context and one or more pending RRC procedures.

In some examples, determining that the second base station control plane function is unavailable comprises determining that one or more expected messages or acknowledgements have not been received from the second base station control plane function, in a manner similar to that described above with respect to the first control plane function performing examples of the method 1000.

Figure 12 shows a particular example of communications 1200 within a network according to an embodiment of this disclosure. In this example, there is failure of the second control plane function CU-CP1 1202, and a first control plane function CU-CP2 1204 takes over control plane functions. This is based for example on information stored by CU-CP1 1202 in a UE context database 1206, and a UE Context Failure Detection function 1208 detects the failure of CU-CP1 1202. When the UE Context Failure Detection function 1208 detects the failure of CU-CP1 1202 it can trigger the actions needed on the network side for the CU-CP2 1204 to take over the control plane connection of the UE. The different steps for communications 1200 within the network are described in more detail as follows:

1a) and 1b) UE 1210 control plane connection via CU-CP1 to the Access and Mobility Management Function (AMF) 1212 is present. UE 1210 user plane (for a PDU session) is present via DU 1214 to CU-UP 1216 and further to User Plane Function (UPF) 1218.

2) CU-CP1 1202 writes UE information to the UE context database 1206 in relation to RRC signalling. This update can be performed in some examples over the F1-C association of the UE (e.g. where the UE context database is located within a distributed unit, DU). Step 2a) in Figure 12 shows RRC signaling and step 2b) shows database update communications.

3) Failure of CU-CP1 1202.

4) The UE Context Failure function 1208 detects the failure of CU-CP1 1202.

5) CU-CP2 1204 takes over control plane functions for the UEs/DUs associated with CU-CP1 1202, for example according to the method 1000 described above with reference to Figure 10.

6) CU-CP2 1204 updates the NG-C interface to the AMF 1212, for example by performing a Path Switch procedure indicating only move of NG-AP signaling association. 7) In this example, the same CU-LIP 1216 is still used and the CLI-CP2 1204 takes over the relevant UE contexts in the CU-LIP.

8) CLI-CP2 1204 may trigger an optional resynchronization of the UE RRC state. This step may depend on the UE Context Database context, for example if there are any pending RRC procedures towards the UE.

9a) and 9b) There is no interruption of the user plane for the UE(s) when the CU-CP2 1204 takes over the control plane connection for the UEs previously served by the failed CU- CP1 1202.

In some examples, the UE context database 1206 and UE Context Failure Detection function 1208 are located with the Distributed Unit (DU) 1214. One benefit of the placement of the UE Context Database 1206 in the DU may be that the updates to the database and the outcome of RRC procedures can be synchronized in an efficient manner. For example, both the RRC signaling and the database update can be performed in the same signaling messages. In such examples, CU-CP2 1204 taking over the UE control plane functionality in step 5) above may be implemented in some examples using the following steps: a) The DU 1214 is aware of the UE(s) served by the failed CU-CP1 1202 and holds the needed UE contexts for these UEs in the local UE context database 1206. b) The DU 1214 then selects a new CU-CP, e.g. CU-CP2 1204, to handle the UE(s) previously handled by the failed CU-CP1 1202. c) The DU 1214 forwards the relevant UE context(s), or references to these UE context(s), to the selected CU-CP2 1204. The CU-CP2 1204 can take over the control plane connections for these UE(s). CU-CP2 1204 updates the DU 1214 with new identity allocated for the UE in CU-CP2 1204 (e.g. gNB-CU UE F1AP ID). d) CU-CP2 1204 updates the UE Context Database 1206 in the DU 1214 to indicate that these UE contexts are now served by CU-CP2 1204. This update can alternatively be performed in step b) by the DU 1214.

In some examples, the UE Context Failure Detection function 1208 is located within the DU 1214, whereas the UE context database 1206 is located external to the DU 1214 and also in some examples external to any CU-CP. In such examples two alternatives are presented for step 5) referred to above for CU-CP2 1204 taking over the UE control plane functionality. In a first alternative, as the DU 1214 is aware of the UE(s) served by the failed CU-CP1 1202, it can retrieve the UE context(s) from the UE context database 1206. The DU 1214 then selects a CU-CP, e.g. CU-CP2 1204, and forwards the retrieved UE context(s) to CU-CP2 1204. The CU-CP2 1204 can take over the control plane connections for these UE(s). The CLI-CP2 1204 updates the UE Context Database 1206 to indicate that these UE context(s) are now served by CLI-CP2 1204. CLI-CP2 1204 updates the DU 1214 with new identities allocated for the UE(s) in CU-CP2 1204 (e.g. gNB-CU UE F1AP ID). In addition, the CU- CP2 1204 updates the NG-C interface to the AMF 1212 in the core network. In this example, the same CU-UP 1216 is still used and the CU-CP2 1204 takes over the relevant UE contexts in the CU-UP 1212.

In a second alternative, the DU 1214 is aware of the UE(s) served by the failed CU-CP1 1202. The DU 1214 then selects a CU-CP, e.g. CU-CP2 1204, and forwards database identifiers for the UE(s) previously served by the failed CU-CP1 1202 to the selected CU- CP2 1204. CU-CP2 1204 retrieves the UE context(s) from the UE context database 1206 using the database identifiers for the UE(s). The CU-CP2 1204 can take over the control plane connections for these UE(s). The CU-CP2 1204 updates the UE Context Database 1206 to indicate that these UE context(s) are now served by CU-CP2 1204. CU-CP2 1204 updates the DU 1214 with new identities allocated for the UE(s) in CU-CP2 1204 (e.g. gNB- CU UE F1AP ID). In addition, CU-CP2 1204 updates the NG-C interface to the AMF 1212 in the core network. In this example, the same CU-UP 1216 is still used and the CU-CP2 1204 takes over the relevant UE contexts in the CU-UP.

In some examples, a database (referred to as a UE context database) is employed. In some examples, a control plane function serving one or more UEs may update the continuously, at regular intervals or after some events, to store one or more UE contexts. The control plane function serving the UE(s) may store or update the UE connection state (e.g. RRC connections state/configuration and/or PDCP state, and/or NAS state) in a persistent database, the UE Context Database (e.g. the database 1206 referred to above), which may in some examples include redundancy to survive any failure or restart of nodes implementing the database. This could enable restarting the control plane function (or switching to a new control plane function) based on the stored information in the database while reducing or eliminating the possibility that the UE and network state become desynchronized, which could otherwise lead to connection failure. In some examples, the UE context database contains one or more of the following items of information:

1. UE RRC context including for example UE cell group configurations, radio bearer configuration, measurement configuration, PDCP/RLC/MAC/PHY configuration, security configuration (e.g. algorithms, keys) etc. In addition, the UE RRC context may contain information about pending RRC procedures such as those described above. 2. UE cPDCP (PDCP for control plane) context including PDCP counters, state, sequence number, buffered packets, etc.

3. UE NG-C context including PDU session resource configuration (e.g. QoS parameters, flow information), RAT/frequency selection information, handover restrictions, slice information etc.

4. UE RAN UP context, including information about NG-U as well as which CU-UP is serving the UE. Could also include GTP TEID information for GTP tunnels established between the DU and CU-UP, and between the CU-UP and Core Network (UPF).

Some examples of this disclosure employ a function in the network, referred to as UE Context Failure Detection function (e.g. UE Context Failure Detection function 1208 described above). In these examples, it may be possible for the UE Context Failure Detection function to detect the failure of the control plane function based on, for example, SCTP keep alive signaling on the F1-C interface between the UE Context Failure Detection function and the control plane function. In some examples, the periodicity of such keep alive signaling would be short to be able to detect control plane function failure or unavailability quickly.

Since unavailability or failure of the control plane function (e.g. the second control plane function referred to herein) can happen at any time, it may be possible in some examples that the control plane function updates the database storing UE contexts before it signals any configuration to the UE. (If the opposite is the case, there is a risk that the UE configuration state would be updated, but not the persistent database, which would lead to an unsynchronized connection state when the control plane function is restarted or taken over by another control plane function.) If the database is updated first, it may be possible to retransmit the most recent message(s), including those that may not have yet been sent in some examples, to re-synchronize the state between the UE and the network in case the UE did not receive or acknowledge the most recent message(s).

However, updating the persistent database prior to any transaction may mean that any UE configuration update would be delayed by the update to the persistent database, which may decrease the performance of the radio network (e.g. due to extra delay at handover, which could lead to lost connections). Thus, in some examples the update to the database and the sending of the configuration update to the UE may be performed in parallel. For example, the configuration update may be sent to the UE and the database may be updated without waiting for either of these to be completed or acknowledged. If the database is updated first, the network can keep multiple versions of the UE context and/or configuration until a confirmation is received from the UE acknowledging the receipt and application of a configuration. For example, if the UE is to be configured to configuration x+1, the configuration x+1 may be stored in the database, while configuration x is still retained (e.g. also stored in the database). Once the network receives an acknowledgement from the UE that configuration x+1 has been successfully applied, then the network can remove configuration x.

Some examples of this disclosure take account of pending RRC procedures. For example, the network (e.g. the first control plane function, e.g. CU-CP2 1204) may trigger or re-trigger pending RRC procedures to ensure that the UE RRC state is in synchronization with the network. Therefore, the UE Context Database may in some examples also be updated with information identifying pending RRC procedures. This information may also be associated with a transaction identifier allowing multiple parallel pending RRC procedures. When the serving control plane entity for the UE (e.g. the second control plane function, e.g. CU-CP1 1202) triggers a RRC procedure towards the UE, it also stores information about this pending RRC procedure in the UE Context Database. Once a RRC procedure is completed, the pending flag in the UE Context Database is removed (the record of the procedure may also be removed). The new serving control plane entity for the UE (e.g. the first control plane function, e.g. CU-CP2 1204) can in some examples perform the following actions depending on whether there are any RRC procedures pending when the new control plane entity takes over responsibility for an unavailable control plane entity:

1) If there are no pending RRC procedures in the UE Context Database then no additional actions are needed.

2) If there are pending RRC procedures in the UE Context Database then these RRC procedures are retriggered and sent by the new serving control plane entity for each UE. If the UE has already received the related messages and RRC configuration has been applied then the cPDPC layer on the UE will drop the received RRC messages as duplicates. If the UE has not yet handled the RRC procedure then the UE will action the received messages, e.g. apply a new or updated configuration.

Figure 13 is a schematic of an example of apparatus 1300 in a first base station control plane function for associating with one or more User Equipments (UEs). The apparatus 1300 comprises processing circuitry 1302 (e.g. one or more processors) and a memory 1304 in communication with the processing circuitry 1302. The memory 1304 contains instructions executable by the processing circuitry 1302. The apparatus 1300 also comprises an interface 1306 in communication with the processing circuitry 1302. Although the interface 1306, processing circuitry 1302 and memory 1304 are shown connected in series, these may alternatively be interconnected in any other way, for example via a bus.

In one embodiment, the memory 1304 contains instructions executable by the processing circuitry 1302 such that the apparatus 1300 is operable to determine that a second base station control plane function associated with one or more UEs is unavailable, determine one or more UE contexts for the one or more UEs, and perform control plane functions for the one or more UEs. In some examples, the apparatus 1300 is operable to carry out the method 1000 described above with reference to Figure 10.

Figure 14 is a schematic of an example of apparatus 1400 in a network function in a network for causing a first base station control plane function to associate with one or more User Equipments (UEs). The apparatus 1400 comprises processing circuitry 1402 (e.g. one or more processors) and a memory 1404 in communication with the processing circuitry 1402. The memory 1404 contains instructions executable by the processing circuitry 1402. The apparatus 1400 also comprises an interface 1406 in communication with the processing circuitry 1402. Although the interface 1406, processing circuitry 1402 and memory 1404 are shown connected in series, these may alternatively be interconnected in any other way, for example via a bus.

In one embodiment, the memory 1404 contains instructions executable by the processing circuitry 1402 such that the apparatus 1400 is operable to determine that a second base station control plane function associated with one or more UEs is unavailable, and send a message to a first base station control plane function to cause the first base station control plane function to associate with the one or more UEs. In some examples, the apparatus 1400 is operable to carry out the method 1100 described above with reference to Figure 11.

It should be noted that the above-mentioned examples illustrate rather than limit the invention, and that those skilled in the art will be able to design many alternative examples without departing from the scope of the appended statements. The word “comprising” does not exclude the presence of elements or steps other than those listed in a claim, “a” or “an” does not exclude a plurality, and a single processor or other unit may fulfil the functions of several units recited in the statements below. Where the terms, “first”, “second” etc. are used they are to be understood merely as labels for the convenient identification of a particular feature. In particular, they are not to be interpreted as describing the first or the second feature of a plurality of such features (i.e. the first or second of such features to occur in time or space) unless explicitly stated otherwise. Steps in the methods disclosed herein may be carried out in any order unless expressly otherwise stated. Any reference signs in the statements shall not be construed so as to limit their scope.

Claims

Claims

1. A method in a first base station control plane function of performing control plane functions for one or more User Equipments (UEs), the method comprising: determining that a second base station control plane function associated with one or more UEs is unavailable; determining one or more UE contexts for the one or more UEs; and performing control plane functions for the one or more UEs.

2. The method of claim 1 , wherein performing control plane functions for the one or more UEs comprises generating and sending control plane messages to the one or more UEs, and/or receiving control plane messages from the one or more UEs.

3. The method of claim 1 , comprising sending a message to a base station distributed unit (DU) associated with the one or more UEs that the first base station control plane function is associated with the one or more UEs.

4. The method of any of claims 1 to 3, wherein each context includes respective a user plane UE context, and the method comprises maintaining the one or more user plane UE contexts for the one or more UEs.

5. The method of claim 4, wherein maintaining the one or more user plane UE contexts for the one or more UEs comprises maintaining the one or more user plane UE contexts without performing a RRC reestablishment or RRC setup procedure.

6. The method of any of claims 1 to 5, wherein determining that the second base station control plane function is unavailable comprises receiving an instruction to associate with the one or more UEs.

7. The method of claim 6, comprising receiving the instruction from a base station distributed unit (DU) associated with the one or more UEs or from a network function.

8. The method of any of claims 1 to 5, wherein determining that the second base station control plane function is unavailable comprises determining that one or more expected messages or acknowledgements have not been received from the second base station control plane function.

9. The method of any of claims 1 to 8, wherein determining the one or more UE contexts for the one or more UEs comprises retrieving the one or more UE contexts from a database.

10. The method of claim 9, comprising retrieving the one or more UE contexts from a database via a base station distributed unit (DU) associated with the one or more UEs.

11. The method of claim 9, wherein the first base station control plane function includes the database.

12. The method of claim 11, comprising, before determining unavailability of the second base station control plane function, receiving one or more indications of the one or more contexts and/or one or more updates to the one or more contexts, and storing the contexts or updating the contexts based on the one or more indications.

13. The method of any of claims 9 to 12, comprising updating the database to indicate that the first base station control plane function is associated with the one or more UEs.

14. The method of any of claims 1 to 8, wherein determining the one or more UE contexts for the one or more UEs comprises receiving an indication of the one or more UE contexts from a base station distributed unit (DU) associated with the one or more UEs.

15. The method of any of claims 1 to 14, comprising, if there are any pending or unacknowledged (by the UE) RRC procedures in the one or more UE contexts, performing the RRC procedures.

16. The method of any of claims 1 to 15, wherein each UE context comprises a most recently acknowledged UE context for each UE.

17. The method of any of claims 1 to 16, comprising sending a message to one or more further base station distributed units (DUs) associated with one or more further UEs that are associated with the second base station control plane function that the first base station control plane function is associated with the one or more further UEs.

18. The method of any of claims 1 to 17, wherein determining that the second base station control plane function is unavailable comprises determining failure of the second base station control plane function.

19. The method of any of claims 1 to 18, wherein each UE context comprises one or more of a UE RRC context, a UE PDCP context for a control plane, a UE NG-C interface context, a UE control plane context, a UE user plane context and one or more pending RRC procedures.

20. The method of any of claims 1 to 19, wherein the one or more UE contexts for the one or more UEs comprises one or more UE contexts used by the second base station control plane function.

21. The method of any of claims 1 to 20, wherein the first base station control plane function is or is comprised in a first base station central unit-control plane (CU-CP), and the second base station control plane function is or is comprised in a second base station CU- CP.

22. A method in a network function in a network of causing a first base station control plane function to associate with one or more User Equipments (UEs), the method comprising: determining that a second base station control plane function associated with one or more UEs is unavailable; and sending a message to a first base station control plane function to cause the first base station control plane function to associate with the one or more UEs.

23. The method of claim 22, wherein sending the message to the first base station control plane function to cause the first base station control plane function to associate with the one or more UEs comprises sending the message to the first base station to cause the first base station control plane function to use one or more UE contexts for the one or more UEs, wherein the one or more UE contexts comprise one or more UE contexts used by the second base station control plane function.

24. The method of claim 23, wherein each context includes respective a user plane UE context, and wherein sending the message to the first base station control plane function to cause the first base station control plane function to associate with the one or more UEs comprises sending the message to the first base station to cause the first base station control plane function to maintain the one or more user plane UE contexts for the one or more UEs.

25. The method of claim 24, wherein sending the message to the first base station to cause the first base station control plane function to maintain the one or more user plane UE contexts for the one or more UEs comprises sending the message to the first base station to cause the first base station control plane function to maintain the one or more user plane UE contexts for the one or more UEs without performing a RRC reestablishment or RRC setup procedure.

26. The method of any of claims 23 to 25, wherein each UE context comprises a most recently acknowledged UE context for each UE.

27. The method of any of claims 23 to 26, wherein each UE context comprises one or more of a UE RRC context, a UE PDCP context for a control plane, a UE NG-C interface context, a UE control plane context, a UE user plane context and one or more pending RRC procedures.

28. The method of any of claims 22 to 27, wherein determining that the second base station control plane function is unavailable comprises determining that one or more expected messages or acknowledgements have not been received from the second base station control plane function.

29. The method of any of claims 22 to 28, wherein determining that the second base station control plane function is unavailable comprises determining failure of the second base station control plane function.

30. The method of any of claims 22 to 29, wherein the first base station control plane function is or is comprised in a first base station central unit-control plane (CU-CP), and the second base station control plane function is or is comprised in a second base station CU- CP.

31. A computer program comprising instructions which, when executed on at least one processor, cause the at least one processor to carry out a method according to any of claims 1 to 30.

32. A carrier containing a computer program according to claim 31 , wherein the carrier comprises one of an electronic signal, optical signal, radio signal or computer readable storage medium.

33. A computer program product comprising non transitory computer readable media having stored thereon a computer program according to claim 31.

34. Apparatus in a first base station control plane function for associating with one or more User Equipments (UEs), the apparatus comprising a processor and a memory, the memory containing instructions executable by the processor such that the apparatus is operable to: determine that a second base station control plane function associated with one or more UEs is unavailable; determine one or more UE contexts for the one or more UEs; and perform control plane functions for the one or more UEs.

35. The apparatus of claim 34, wherein the memory contains instructions executable by the processor such that the apparatus is operable to perform the method of any of claims 2 to 21.

36. Apparatus in a network function in a network for causing a first base station control plane function to associate with one or more User Equipments (UEs), the apparatus comprising a processor and a memory, the memory containing instructions executable by the processor such that the apparatus is operable to: determine that a second base station control plane function associated with one or more UEs is unavailable; and send a message to a first base station control plane function to cause the first base station control plane function to associate with the one or more UEs.

37. The apparatus of claim 36, wherein the memory contains instructions executable by the processor such that the apparatus is operable to perform the method of any of claims 23 to 30.

38. Apparatus in a first base station control plane function for associating with one or more User Equipments (UEs), the apparatus configured to: determine that a second base station control plane function associated with one or more UEs is unavailable; determine one or more UE contexts for the one or more UEs; and perform control plane functions for the one or more UEs.

39. Apparatus in a network function in a network for causing a first base station control plane function to associate with one or more User Equipments (UEs), the apparatus configured to: determine that a second base station control plane function associated with one or more UEs is unavailable; and send a message to a first base station control plane function to cause the first base station control plane function to associate with the one or more UEs.