Patent Publication Number: US-2018049024-A1

Title: User Equipment Registration Recovery Upon Core Node Failure

Description:
TECHNICAL FIELD 
     The present invention generally relates to wireless communication networks, and particularly relates to registration of wireless devices with core network entities. 
     BACKGROUND 
     An increasing number of machines or other autonomous devices communicate with each other (or with an application server) without human interaction over cellular networks. A typical scenario is to have sensors sending measurements infrequently, where each of the transmissions consists of only small amounts of data. This type of communication is referred to as machine to machine (M2M) communications or machine-type communication (MTC). Devices in cellular systems (such as Evolved Universal Terrestrial Radio Access or E-UTRA) are often battery driven and the power consumption is therefore an important factor. Sensors and other similar devices may reside in remote locations and the number of deployed devices could be so large that it would be practically infeasible to replace the batteries of such devices. Thus, it is important to reduce the amount of power consumption. 
     An existing means to reduce the battery power consumption is to use discontinuous reception (DRX), a feature in which the device receiver is switched off except during configured intervals. Currently the longest specified DRX cycle length is 2.56 seconds for E-UTRA. However, it would be beneficial to extend the DRX cycles beyond currently specified values to reduce the battery power consumption further especially for such (MTC type of) devices. Extended DRX cycles naturally cause larger delays to reach the user equipment (UE) in the downlink, however this is typically not considered a problem due to the delay insensitive traffic on such devices. 
     MTC devices can sometimes be placed in challenging locations, for which E-UTRA network deployments were not dimensioned for full coverage. For example, smart meters are often placed in building basements and are sometimes even contained in metal enclosures. Similarly, devices may be located in rural and isolated areas to address smart agriculture scenarios. As a consequence, long-range coverage extensions are defined so that the coverage for (low data-rate) MTC devices can be extended. 
     To enhance the radio coverage and reduce the power consumption for these MTC devices, there are ongoing efforts in the wireless community. For the former, it is essential to reduce the signaling overhead since repetition is one of the main techniques for those UEs to access the network, e.g. ensuring that the message sizes are kept small. While for the latter, it is essential to minimize the UE activity periods, e.g. by introducing extended DRX cycles. 
     The Evolved UMTS Terrestrial Radio Access Network (E-UTRAN), standardized by members of the 3rd Generation Partnership Project (3GPP), includes radio base stations called enhanced NodeBs (eNBs or eNodeBs), providing the E-UTRA user plane and control plane protocol terminations towards the user equipment or UE. The eNBs are interconnected with each other using the X2 interface. The eNBs are also connected using the S1 interface to the EPC (Evolved Packet Core), more specifically to the MME (Mobility Management Entity) by means of the S1-MME interface and to the Serving Gateway (S-GW) by means of the S1-U interface. The S1 interface supports a many-to-many relation between MMEs/S-GWs and eNBs. A simplified view of the E-UTRAN architecture is illustrated in  FIG. 1 . 
     The eNB hosts functionalities such as Radio Resource Management (RRM), radio bearer control, admission control, header compression of user plane data towards serving gateway, and/or routing of user plane data towards the serving gateway. The MME is the control node that processes the signaling between the UE and the CN (core network). Significant functions of the MME are related to connection management and bearer management, which are handled via Non Access Stratum (NAS) protocols. The S-GW is the anchor point for UE mobility, and also includes other functionalities such as temporary DL (downlink) data buffering while the UE is being paged, packet routing and forwarding to the right eNB, and/or gathering of information for charging and lawful interception. The PDN Gateway (P-GW, not shown in  FIG. 1 ) is the node responsible for UE IP address allocation, as well as Quality of Service (QoS) enforcement (as further discussed below). The reader is referred to 3GPP TS 36.300 and the references therein for further details of functionalities of the different nodes. 
       FIG. 2  gives a summary of the functionalities of the different nodes, and the reader is referred to the 3GPP document “Evolved Universal Terrestrial Radio Access (E-UTRA) and Evolved Universal Terrestrial Radio Access Network (E-UTRAN); Overall Description; Stage 2,” 3GPP TS 36.300, v. 11.3.0 (September 2012), available at 3gpp.org, and the references therein for the details of the functionalities of the different nodes. In  FIG. 2 , the boxes labeled “eNB,” “MME,” “S-GW,” and “P-GW” depict the logical nodes, the unshaded white boxes within the larger boxes depict the functional entities of the control plane, and the shaded boxes within the box labeled “eNB” depict the radio protocol layers. 
     In E-UTRA, a UE attached to the network has a UE context in the MME, see 3GPP TS 23.401. In this UE context, data essential for the communication is stored. One example is the temporary mobile subscriber identity (S-TMSI), which is used to address the UE over the radio interface. Another example is the tracking area identity (TAI) List, which describes the set of cells, one of which were UE is located. In case of an MME failure, user context information such as the UE context and data like S-TMSI and TAI List is lost. Section 5.6.2.2.2 of 3GPP TS 24.301 defines the following procedure to take place as a recovery measure, to reach and notify the UE of terminating call/data. 
     Paging for Evolved Packet System (EPS) services using International Mobile Subscriber Identity (IMSI) is an abnormal procedure used for error recovery in the network. The network may initiate paging for EPS services using IMSI with core network (CN) domain indicator set to “PS” if the S-TMSI is not available due to a network failure (see example in  FIG. 3 ). 
     In S1 mode, to initiate the procedure, the EPS Mobility Management (EMM) entity in the network requests the lower layer to start paging. If the TAI list is not available due to a network failure, the network may perform the paging within all tracking areas served by the MME (see 3GPP TS 36.331 and 3GPP TS 36.413). When a UE receives a page for EPS services using IMSI from the network before a UE initiated EMM specific procedure has been completed, the UE then aborts the EMM specific procedure and proceeds according to the description in subclause 5.6.2.2.2 of 3GPP TS 24.301. Upon reception of paging for EPS services using IMSI, the UE shall stop timer T3346, if it is running, locally deactivate any EPS bearer context(s) and locally detach from EPS. Additionally, the UE shall delete the following parameters: last visited registered TAI, TAI list, Globally Unique Temporary ID (GUTI) and Key Selection Identifier Access Security Management Entity (KSIASME). The UE shall set the EPS update status to EU2 NOT UPDATED and change the state to EMM-DEREGISTERED. The UE shall stop all timers T3396 that are running. 
     If A/Gb mode or Iu mode is supported by the UE, the UE shall, in addition, handle the General Packet Radio System (GPRS) Mobility Management (GMM) parameters, GMM state, GPRS update status, P-TMSI, P-TMSI signature, RAI, and GPRS ciphering key sequence number as specified in 3GPP TS 24.008 for the case when a paging for GPRS services using IMSI is received. 
     After performing the local detach, the UE shall then perform an attach procedure as described in subclause 5.5.1.2 of 3GPP TS 24.301. If the UE is operating in CS/PS mode 1 or CS/PS mode 2 of operation, then the UE shall perform a combined attach procedure as described in subclause 5.5.1.3. In some cases, user interaction can be required, thus the UE cannot activate the dedicated bearer context(s) automatically. Also, the UE does not respond to the paging except with the attach request, hence timer T3413 in the network is not used when paging with IMSI. 
     It is recognized herein that the procedure described above results in a massive paging load over a large number of cells (“all tracking areas served by the MME”) using the IMSI that requires more encoding bits in the paging message compared to a paging message where S-TMSI is used. The sum of these impacts would consume substantial radio resources used for paging, and even increase the risk of congestion. 
     For UEs that are in a position that requires coverage enhancement techniques, the paging message will be repeated several hundreds of times for successful UE reception. The procedure described above would introduce even higher risk of radio resource congestion. Given that the information that a particular UE may support and need coverage enhancement techniques is lost at the MME failure, the procedure described above will fail as such devices will not be reached. 
     In this case, UE-specific DRX cycle lengths (previously negotiated between the MME and the UE) are lost in the network due to the MME failure. As a result, the network may not be able to reach the UE until next time UE triggers registration procedure. This will disable the recovery procedure described above. 
     SUMMARY 
     Embodiments of the present invention comprise apparatuses and methods for registration recovery of wireless devices upon a loss of user context information by a network node for which the devices are registered. For example, an MME may lose user context information, such as S-TMSIs, TAI lists, DRX cycle length or use of coverage enhancement techniques. Of course, lost user context information may include other data, including but not limited to, coverage level, etc. The loss of user context information may be caused by a failure of the MME. 
     According to some embodiments, the MME determines there is a loss of user context information for wireless devices such as UEs, and signals to radio base stations (e.g., eNBs) connected, or communicatively coupled, to the MME to notify the UEs of the loss of user context information in, for example, broadcast system information (SIB). The MME failure is indicated to radio eNBs connected to the MME using a procedure, message or information element. For example, a new information element may be transmitted in S1 Management procedures, such as in the S1 messages S1 SETUP RESPONSE and/or MME CONFIGURATION UPDATE. 
     As for the eNB, the eNB determines that the MME has lost user context information for UEs registered to the network node. In response to the determination, an information element indicating the loss of user context information is transmitted in broadcast system information. For instance, the connected eNBs will start to transmit a new information element in broadcast system information identifying the affected/failed MMES. The information element may contain: alt1 (List of) Global Unique Mobility Management Entity Identifiers (GUMMEIs) of the Failure MME; alt2 (List of) MME Identifiers (MMEIs) of the Failure MME and/or alt3 (List of) MME codes (MMECs) of the failure MME. 
     UEs in a cell of the connected eNB will be triggered to receive broadcast system information in the cell. This can be achieved by transmitting paging messages in all cells of the connected eNB, indicating that all UEs (i.e. not only UEs addressed in the paging message) shall start to receive broadcast system information. This can also be achieved by UEs regularly (i.e. according to a preconfigured time interval) and autonomously starting to receive broadcast system information in the cell. 
     A UE located in the cell controlled by the connected eNBs receives the new information element in broadcast system information and compares to its locally stored information identifying its registered MME (Globally Unique Temporary ID (GUTI) that contains the MMEC). In the case there is a match (which means the UE is registered to the failed MME), the UE detaches (and releases the bearers locally) and re-attaches to the network ( 3 ), which means a new UE context (including S-TMSI, TAI List, DRX cycle length, use of coverage enhancement techniques, etc.) is created in the MME. 
     In some embodiments, to make sure the UE does not detach and re-attach multiple times as triggered by the same MME failure, a counter (sequence number) can be added to the S1 sequence management procedure ( 1 ) and the broadcast system information ( 2 ). This counter is stepped by one at MME failure. When the UE triggers detach and re-attach ( 4 ), the UE memorizes the counter value, and will trigger yet another detach/re-attach only in case the counter value broadcast in system information is different from the UE-stored count value. 
     The discussions of the solutions are based on E-UTRA; however, these aspects can also be considered valid for other technologies. 
     According to some embodiments, a method, in a radio base station connected to a network node in a wireless communication network, for indicating a loss of user context information stored by the network node, includes determining that the network node has lost user context information for one or more user equipments registered to the network node. The method also includes, in response to said determining, transmitting an information element indicating the loss of user context information in broadcast system information. 
     According to some embodiments, a method, in a user equipment registered to a network node in a wireless communication network, for reattaching to the network node upon loss of user context information stored by the network node, includes receiving an information element in broadcast system information from a radio base station connected to the network node, the information element indicating the network node has lost user context information. The method also includes comparing the network node indicated by the information element to information stored on the user equipment that identifies the network node to which the user equipment is registered. The method further includes, responsive to a determination that the network node indicated by the information element matches the network node in the stored information, detaching and reattaching to the network node. 
     According to some embodiments, a method, in a network node storing user context information for user equipments for which the network node is registered, for recreating new user context information upon a loss of user context information, includes determining there is a loss of user context information for one or more user equipments and signaling to radio base stations connected to the network node to notify the one or more user equipments of the loss of user context information in broadcast system information. 
     According to some embodiments, a radio base station connected to a network node in a wireless communication network and configured to indicate a loss of user context information stored by the network node, includes a transceiver circuit and a processing circuit operatively connected to the transceiver circuit. The processing circuit is configured to determine that the network node has lost user context information for one or more user equipments registered to the network node, and, in response to the determination, transmit, via the transceiver circuit, an information element indicating the loss of user context information in broadcast system information. 
     According to some embodiments, a user equipment registered to a network node in a wireless communication network and configured to reattach to the network node upon loss of user context information stored by the network node, includes a transceiver circuit and a processing circuit operatively connected to the transceiver circuit. The processing circuit is configured to receive an information element in broadcast system information from a radio base station connected to the network node, the information element indicating the network node has lost user context information. The processing circuit is configured to compare the network node indicated by the information element to information stored on the user equipment that identifies the network node to which the user equipment is registered, and, responsive to a determination that the network node indicated by the information element matches the network node in the stored information, initiate detachment and reattachment to the network node. 
     According to some embodiments, a network node storing user context information for user equipments for which the network node is registered and configured to recreate new user context information upon a loss of user context information, includes a communication interface circuit and a processing circuit operatively connected to the communication interface circuit. The processing circuit is configured to determine there is a loss of user context information for one or more user equipments and signal, via the communication interface circuit, to radio base stations connected to the network node to notify the one or more user equipments of the loss of user context information in broadcast system information. 
     Variations of the above-described methods, as well as corresponding apparatuses, computer program products, computer readable medium and functional implementations are described in detail below. 
     Of course, the present invention is not limited to the above features and advantages. Those of ordinary skill in the art will recognize additional features and advantages upon reading the following detailed description, and upon viewing the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram of a simplified view of E-UTRAN architecture. 
         FIG. 2  illustrates functionalities of the different nodes in the E-UTRAN architecture. 
         FIG. 3  illustrates paging for an attachment request. 
         FIG. 4  illustrates a block diagram of a network node configured to recreate new user context information upon a loss of user context information, according to some embodiments. 
         FIG. 5  illustrates a block diagram of a radio base station configured to indicate a loss of user context information stored by the network node, according to some embodiments. 
         FIG. 6  illustrates a block diagram of a user equipment configured to reattach and recover registration to the network node upon loss of user context information stored by the network node, according to some embodiments. 
         FIG. 7  illustrates a method for recreating new user context information upon a loss of user context information, according to some embodiments. 
         FIG. 8  illustrates a method for indicating a loss of user context information stored by the network node, according to some embodiments. 
         FIG. 9  illustrates a method for reattaching to the network node upon loss of user context information stored by the network node, according to some embodiments. 
         FIG. 10  illustrates an overview of a network performing registration recovery of user equipments upon loss of user context information by a network node, according to some embodiments. 
         FIG. 11  illustrates information elements in an example S1 SETUP RESPONSE message, according to some embodiments. 
         FIG. 12  illustrates information elements in an example MME CONFIGURATION UPDATE message, according to some embodiments. 
         FIG. 13  illustrates an example functional implementation of recreating new user context information upon a loss of user context information, according to some embodiments. 
         FIG. 14  illustrates an example functional implementation of indicating a loss of user context information stored by the network node, according to some embodiments. 
         FIG. 15  illustrates an example functional implementation of indicating a loss of user context information stored by the network node, according to some embodiments. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 4  illustrates a diagram of a network node  10 , according to some embodiments. The network node  10  resides in the core network and facilitates communication between access networks and the Internet using communication interface circuit  18 . The communication interface circuit  18  includes circuitry for communicating with other nodes in the core network, radio nodes, and/or other types of nodes in the network for the purposes of providing data and cellular communication services. According to various embodiments, cellular communication services may be operated according to any one or more of the 3GPP cellular standards, GSM, GPRS, WCDMA, HSDPA, LTE and LTE-Advanced. 
     The network node  10  also includes one or more processing circuits  12  that are operatively associated with the communication interface circuit  18 . For ease of discussion, the one or more processing circuits  12  are referred to hereafter as “the processing circuit  12 ”. The processing circuit  12  comprises one or more digital processors  22 , e.g., one or more microprocessors, microcontrollers, Digital Signal Processors (DSPs), Field Programmable Gate Arrays (FPGAs), Complex Programmable Logic Devices (CPLDs), Application Specific Integrated Circuits (ASICs), or any mix thereof. More generally, the processing circuit  12  may comprise fixed circuitry, or programmable circuitry that is specially configured via the execution of program instructions implementing the functionality taught herein, or may comprise some mix of fixed and programmed circuitry. The processor  22  may be multi-core having two or more processor cores utilized for enhanced performance, reduced power consumption, and more efficient simultaneous processing of multiple tasks. 
     The processing circuit  12  also includes a memory  24 . The memory  24 , in some embodiments, stores one or more computer programs  26  and, optionally, configuration data  28 . The memory  24  provides non-transitory storage for the computer program  26  and it may comprise one or more types of computer-readable media, such as disk storage, solid-state memory storage, or any mix thereof. By way of non-limiting example, the memory  24  comprises any one or more of SRAM, DRAM, EEPROM, and FLASH memory, which may be in the processing circuit  12  and/or separate from the processing circuit  12 . 
     In general, the memory  24  comprises one or more types of computer-readable storage media providing non-transitory storage of the computer program  26  and any configuration data  28  used by the network node  10 . Here, “non-transitory” means permanent, semi-permanent, or at least temporarily persistent storage and encompasses both long-term storage in non-volatile memory and storage in working memory, e.g., for program execution. 
     The processor  22  of the processing circuit  12  may execute a computer program  26  stored in the memory  24  that configures the processor  22  to determine there is a loss of user context information for one or more user equipments and signal, via the communication interface  18 , to radio base stations connected to the network node  10  to notify the one or more user equipments of the loss of user context information in broadcast system information. This structure and functionality may be referred to as loss notification circuitry  20  in the processing circuit  12 . 
       FIG. 5  illustrates a diagram of a radio base station  30 , according to some embodiments. The base station  30  provides an air interface to wireless devices, e.g., an LTE air interface for downlink transmission and uplink reception, which is implemented via antennas  34  and a transceiver circuit  36 . The transceiver circuit  36  may include transmitter circuits, receiver circuits, and associated control circuits that are collectively configured to transmit and receive signals according to a radio access technology, for the purposes of providing cellular communication services. According to various embodiments, cellular communication services may be operated according to any one or more of the 3GPP cellular standards, GSM, general packet radio service (GPRS), wideband code division multiple access (WCDMA), high-speed downlink packet access (HSDPA), LTE and LTE-Advanced. The base station  30  may also include a communication interface circuit  38  for communicating with nodes in the core network such as the network node  10 , other peer radio nodes, and/or other types of nodes in the network. The base station  30  may be, for example, an eNodeB. 
     The base station  30  also includes one or more processing circuits  32  that are operatively associated with the communication interface circuit  38  and transceiver circuit  36 . The processing circuit  32  comprises one or more digital processors  42 , e.g., one or more microprocessors, microcontrollers, DSPs, FPGAs, CPLDs, ASICs, or any mix thereof. More generally, the processing circuit  32  may comprise fixed circuitry, or programmable circuitry that is specially configured via the execution of program instructions implementing the functionality taught herein, or may comprise some mix of fixed and programmed circuitry. The processor  32  may be multi-core. 
     The processing circuit  32  also includes a memory  44 . The memory  44 , in some embodiments, stores one or more computer programs  46  and, optionally, configuration data  48 . The memory  44  provides non-transitory storage for the computer program  46  and it may comprise one or more types of computer-readable media, such as disk storage, solid-state memory storage, or any mix thereof. By way of non-limiting example, the memory  44  comprises any one or more of SRAM, DRAM, EEPROM, and FLASH memory, which may be in the processing circuit  32  and/or separate from the processing circuit  32 . In general, the memory  44  comprises one or more types of computer-readable storage media providing non-transitory storage of the computer program  46  and any configuration data  48  used by the base station  30 . 
     The processor  42  may execute a computer program  46  stored in the memory  44  that configures the processor  42  to determine that the network node  10  has lost user context information for one or more user equipments registered to the network node  10 , and, in response to the determination, transmit, via the transceiver circuit  36 , an information element indicating the loss of user context information in broadcast system information. This structure and functionality may be referred to as loss indication circuitry  40  in the processing circuit  52 . 
       FIG. 6  illustrates a diagram of a wireless device, such as a user equipment  50 , according to some embodiments. To ease explanation, the user equipment  50  may also be considered to represent any wireless devices that perform machine to machine (M2M) communications or machine-type communication (MTC). The user equipment  50  communicates with a radio node or base station, such as base station  30 , via antennas  54  and a transceiver circuit  56 . The transceiver circuit  56  may include transmitter circuits, receiver circuits, and associated control circuits that are collectively configured to transmit and receive signals according to a radio access technology, for the purposes of providing cellular communication services. According to various embodiments, cellular communication services may be operated according to any one or more of the 3GPP cellular standards, GSM, GPRS, WCDMA, HSDPA, LTE and LTE-Advanced. 
     The user equipment  50  also includes one or more processing circuits  52  that are operatively associated with the radio transceiver circuit  56 . The processing circuit  52  comprises one or more digital processing circuits, e.g., one or more microprocessors, microcontrollers, DSPs, FPGAs, CPLDs, ASICs, or any mix thereof. More generally, the processing circuit  52  may comprise fixed circuitry, or programmable circuitry that is specially adapted via the execution of program instructions implementing the functionality taught herein, or may comprise some mix of fixed and programmed circuitry. The processing circuit  52  may be multi-core. 
     The processing circuit  52  also includes a memory  64 . The memory  64 , in some embodiments, stores one or more computer programs  66  and, optionally, configuration data  68 . The memory  64  provides non-transitory storage for the computer program  66  and it may comprise one or more types of computer-readable media, such as disk storage, solid-state memory storage, or any mix thereof. By way of non-limiting example, the memory  64  comprises any one or more of SRAM, DRAM, EEPROM, and FLASH memory, which may be in the processing circuit  52  and/or separate from processing circuit  52 . In general, the memory  64  comprises one or more types of computer-readable storage media providing non-transitory storage of the computer program  66  and any configuration data  68  used by the user equipment  50 . 
     The processor  62  of the processor circuit  52  may execute a computer program  66  stored in the memory  64  that configures the processor  62  to receive an information element in broadcast system information from a radio base station, such as base station  30  connected to the network node  10 , the information element indicating the network node  10  has lost user context information. The processing circuit  52  is also configured to compare the network node  10  indicated by the information element to information stored on the user equipment  50  that identifies the network node to which the user equipment  50  is registered. Responsive to a determination that the network node  10  indicated by the information element matches the network node in the stored information, the processing circuit  52  initiates detachment and reattachment of the user equipment  50  to the network node  10 . The user equipment  50  may also perform any other procedure to recover registration upon receiving one or more information elements in the broadcast system information. This functionality may be performed by registration recovery circuitry  60  in processing circuit  52 . 
       FIG. 10  illustrates an example overview of the registration recovery of the user equipment  50  upon a loss of user context information by the network node  10  (block  1010 ), shown as MME  10  in this example. The network may also include a serving gateway  70  and a packet data network gateway  80 .  FIG. 10  will be used as context to explain methods  700 - 900  of  FIGS. 7-9 . 
     Process  1012  of  FIG. 10  shows the MME  10  signaling loss of user context information to the base station  30 . The processing circuit  12  of the MME  10  is configured to perform a method, such as method  700  of  FIG. 7 . The method  700  includes determining there is a loss of user context information for user equipments like the user equipment  50  (block  702 ). The MME  10  then signals to the base station  30  connected to the MME  10  to notify user equipments, including the user equipment  50 , of the loss of user context information in broadcast system information (block  704 ). The user context information lost by the MME  10  may include one or more temporary mobile subscriber identifiers, one or more tracking area identifiers, and/or DRX cycle length information for the user equipments  50 . 
     In some cases, the MME  10  signals an information element indicating the loss of user context information during an S1 management procedure. For example, the information element is transmitted in an S1 SETUP RESPONSE message and/or an MME CONFIGURATION UPDATE message.  FIG. 11  shows a suggested change to the S1 SETUP RESPONSE. This message is sent by the MME  10  to transfer information for a Transport Network Layer (TNL) association. The direction is from the MME  10  to an eNB.  FIG. 12  shows suggested changes to the MME CONFIGURATION UPDATE. This message is sent by the MME  10  to transfer updated information for a TNL association. The direction is from the MME  10  to an eNB. 
     At process  1014 , the base station  30  transmits a page to user equipments to receive system broadcast information (SIB). The page may be sent to all user equipments in all cells of the base station  30 . 
     At process  1016 , an information element indicating the MME  10  lost user context information is transmitted to user equipments in broadcast system information. The processing circuit  32  of the base station  30  is configured to perform a method, such as method  800  of  FIG. 8 . The method  800  includes determining that the MME  10  has lost user context information for one or more user equipments registered to the MME  10  (block  802 ). This determination may be made in response to receiving signaling or an information element from the MME  10  indicating the loss of user context information. 
     In response to the determination, the base station  30  transmits an information element indicating the loss of user context information in broadcast system information (block  804 ). The information element may identify the MME  10 . For example, an information element includes one or more Global Unique Mobility Management Entity Identifiers (GUMMEIs) of the MME, one or more MME Identifiers (MMEIs) of the MME, one or more MME Group Identities (MMEGIs), and/or one or more MME codes (MMECs) of the MME. 
     The user equipment  50  receives the page notifying the user equipment  50  to receive the broadcast system information and initiates reception of the broadcast system information responsive to receiving the page. Alternatively, rather than receiving the page, the user equipment  50  receives the information element in broadcast system information during a periodic monitoring of the broadcast system information. This may involve autonomously starting to receive broadcast system information during periodic intervals. 
     The processing circuit  52  of the user equipment  50  is configured to perform a method, such as method  900  of  FIG. 9 . The method  900  includes receiving an information element in broadcast system information from the base station  30  connected to the MME  10 , the information element indicating the MME  10  has lost user context information (block  902 ). The method  900  also includes comparing the MME  10  indicated by the information element to information stored on the user equipment  50  that identifies the network node or MME to which the user equipment is registered (block  904 ). 
     The method  900  further includes, responsive to a determination that the MME  10  indicated by the information element matches the network node in the stored information, initiating detachment (process  1018 ) and reattachment (attachment request  1020 ) to the network node (block  906 ). The MME  10  may also recreate new user context information for the user equipment  50  or other user equipments that reattach to the MME  10 . 
     To prevent unnecessary detachment and reattachment, the user equipment  50 , according to some embodiments, compares a stored counter value, previously received from the network and representing, directly or indirectly, a number of times the MME  10  has lost user context information, to a loss counter value indicated in the broadcast system information. The processing circuit  52  of the user equipment  50  controls the user equipment to detach and reattach to the MME  10  based on whether the stored counter value matches the loss counter value. For example, when the values match, detachment and reattachment are not performed. If the values are different, then detachment and reattachment are performed. The MME  10  may increment a counter value upon a loss of user context information by the MME  10  and transmit the counter value. While the MME  10  is used as an example network node, the methods are not limited to MMES. 
       FIG. 13  illustrates an example functional module or circuit architecture as may be implemented in the network node  10 , e.g., based on the processing circuitry  20 . The illustrated embodiment at least functionally includes a determining module  1302  for determining there is a loss of user context information for one or more user equipments. The embodiment also includes a signaling module  1304  for signaling to radio base stations connected to the network node to notify the one or more user equipments of the loss of user context information in broadcast system information. 
       FIG. 14  illustrates an example functional module or circuit architecture as may be implemented in the base station  30 , e.g., based on the processing circuitry  40 . The illustrated embodiment at least functionally includes a determining module  1402  for determining that a network node has lost user context information for one or more user equipments registered to the network node. The embodiment also includes a transmitting module  1404  for, in response to said determining, transmitting an information element indicating the loss of user context information in broadcast system information. 
       FIG. 15  illustrates an example functional module or circuit architecture as may be implemented in the user equipment  50 , e.g., based on the processing circuitry  60 . The illustrated embodiment at least functionally includes a receiving module  1502  for receiving an information element in broadcast system information from a radio base station connected to a network node, the information element indicating the network node has lost user context information. The embodiment also includes a comparing module  1504  for comparing the network node indicated by the information element to information stored on the user equipment that identifies the network node to which the user equipment is registered. The embodiment also includes a signaling module  1506  for, responsive to a determination that the network node indicated by the information element matches the network node in the stored information, detaching and reattaching to the network node. 
     Notably, modifications and other embodiments of the disclosed invention(s) will come to mind to one skilled in the art having the benefit of the teachings presented in the foregoing descriptions and the associated drawings. Therefore, it is to be understood that the invention(s) is/are not to be limited to the specific embodiments disclosed and that modifications and other embodiments are intended to be included within the scope of this disclosure. Although specific terms may be employed herein, they are used in a generic and descriptive sense only and not for purposes of limitation.