Patent Publication Number: US-11659389-B1

Title: Architecture for providing cellular roaming support

Description:
BACKGROUND 
     Today, many networks provide roaming support to each other to enhance and increase a serviceable area for their network users. However, as network technologies have advanced some networks have lagged behind or failed to implement the newest network technologies. This growing divide between networks has resulted in some incompatibilities between networks that typically offer roaming support to each other&#39;s users. For example, many networks still rely on circuit switched (CS) systems while the more advanced systems have been upgraded to provided Voice over Long-Term Evolution (VoLTE) systems. Thus, an architecture for the VoLTE networks to provide roaming support for CS network users is desirable. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The detailed description is set forth with reference to the accompanying figures. In the figures, the left-most digit(s) of a reference number identifies the figure in which the reference number first appears. The use of the same reference numbers in different figures indicates similar or identical items or features. 
         FIG.  1    is an example of a block diagram of a network equipped with an International Switched Digital Service configured to host vesting UEs associated with a CS-based home network, in accordance with some examples of the present disclosure. 
         FIG.  2    is an example flow diagram showing an illustrative process associated with hosting a visiting CS-based user equipment on a VoLTE based visited public land mobile network, in accordance with some examples of the present disclosure. 
         FIG.  3    is an example flow diagram showing an illustrative process associated with hosting a visiting CS-based user equipment on a VoLTE based visited public land mobile network, in accordance with some examples of the present disclosure. 
         FIG.  4    is an example of a data flow diagram for registering a VoLTE user equipment to a CS network, in accordance with some examples of the present disclosure. 
         FIG.  5    is an example of a data flow diagram associated with routing a mobile originating call from a visiting user equipment over a visited public land mobile network, in accordance with some examples of the present disclosure. 
         FIG.  6    is an example of a data flow diagram associated with routing a mobile terminating call for a visiting user equipment over a visited public land mobile network, in accordance with some examples of the present disclosure. 
         FIG.  7    is an example of a data flow diagram associated with routing a mobile originating short-message-service from a visiting user equipment over a visited public land mobile network, in accordance with some examples of the present disclosure. 
         FIG.  8    is an example of a data flow diagram associated with routing a mobile terminating short-message-service from a visiting user equipment over a visited public land mobile network, in accordance with some examples of the present disclosure. 
         FIG.  9    is another example of a data flow diagram associated with routing a mobile originating short-message-service from a visiting user equipment over a visited public land mobile network, in accordance with some examples of the present disclosure. 
         FIG.  10    is another example of a data flow diagram associated with routing a mobile terminating short-message-service from a visiting user equipment over a visited public land mobile network, in accordance with some examples of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     Discussed herein are systems, architectures, and methods associated with VoLTE networks to provide registration, voice calling, text-message, and the like for roaming users that are currently associated with a CS network. For instance, many networks support roaming services for each other&#39;s users to enhance and increase a serviceable area associated with the individual networks. In some cases, a roaming partner&#39;s network may still rely on CS (or other legacy standards) based user equipment (UE) that may not be supported on more up-to-date VoLTE UEs. As such, the system, discussed herein, may be configured to register roaming VoLTE UEs to CS networks as a roaming user without maintaining dedicated CS network resources on the UE. 
     In some cases, the system may be configured to route mobile originating (MO) and mobile terminating (MT) calls, short message service (SMS) text-based messages, and the like between the VoLTE UEs roaming on the CS network with other UEs, as discussed below. In some cases, the CS network may act as a visited public land mobile network (VPLMN) to register, process calls and messages, and authenticate the roaming or visiting UE via a home location register (HLR) of a corresponding VoLTE home public land mobile network (HPLMN). A VPLMN is a public land mobile network that a user or UE may access as a visiting or roaming device when leaving the HPLMN associated with the user and/or UE. 
     In one implementation, the VPLMN may be configured with an International Switched Digital Service (ISDS) that is capable of receiving and transmitting requests from components of the VPLMN network on behalf of UEs located within a service area of the VPLMN using a first protocol, such as 4G, 5G, VoLTE and the like. The ISDS may also be configured to transmit and receive requests using various legacy protocols (e.g., CS network based requests) when registering visiting UEs, placing incoming or outgoing calls, and/or placing incoming or outgoing SMS messages. In this manner, the ISDS may be equipped with components to translate or otherwise process requests using multiple standards and/or protocols. 
       FIG.  1    is an example  100  of a block diagram of a network  102  equipped with an ISDS  104  configured to host visiting UEs  106  associated with a CS-based home network  108 , in accordance with some examples of the present disclosure. In some cases, the ISDS  104  may be configured to receive and transmit requests associated with the visiting UE  108  using the VoLTE standard. For example, the VoLTE-based transmissions  110  (e.g., confirmations, data, and requests) may include a registration request, an authentication request, a multimedia authentication request, registration confirmations, authentication confirmations, and the like. In some cases, the VoLTE standard transmissions  110  may be received from and sent to other components of the VPLMN  102 , such as an Integrating-Call Session Control Function, Proxy-Call Session Control Function, a Serving-Call Session Control Function, and the like. 
     The ISDS  104  may also be configured to transmit and receive CS-based transmissions  112  (e.g., confirmations, data, and requests) from the HPLMN  108  associated with the visiting UE  106 . For example, the ISDS  104  may be configured to translate or convert the VoLTE-based transmissions  110  into a CS-based transmissions  112  prior to transmitting to components of the HPLMN  108 . The ISDS  104  may also be configured to translate or convert the CS-based transmissions  112  into a VoLTE-based transmissions  110  prior to transmitting to components of the VPLMN  102 . For instance, as an illustrative example, the ISDS  104  may receive a VoLTE Multimedia Authentication Request and in response send a CS authentication information request to the HPLMN  108  to obtain, for instance, subscriber data associated with the visiting UE  106 . In this example, the ISDS  104  may convert the subscriber data from the CS format into a VoLTE format to complete, for instance, a registration of the visiting UE  106  by other components of the VPLMN  102 . 
       FIGS.  2  and  3    are flow diagrams illustrating example processes associated with hosting visiting CS-based UEs on a VoLTE based network according to some implementations. The processes are illustrated as a collection of blocks in a logical flow diagram, which represent a sequence of operations, some or all of which can be implemented in hardware, software, or a combination thereof. In the context of software, the blocks represent computer-executable instructions stored on one or more computer-readable media that, which when executed by one or more processors, perform the recited operations. Generally, computer-executable instructions include routines, programs, objects, components, encryption, deciphering, compressing, recording, data structures and the like that perform particular functions or implement particular abstract data types. 
     The order in which the operations are described should not be construed as a limitation. Any number of the described blocks can be combined in any order and/or in parallel to implement the process, or alternative processes, and not all of the blocks need be executed. For discussion purposes, the processes herein are described with reference to the frameworks, architectures and environments described in the examples herein, although the processes may be implemented in a wide variety of other frameworks, architectures or environments. 
       FIG.  2    is an example flow diagram showing an illustrative process  200  associated with hosting a visiting CS-based user equipment on a VoLTE based VPLMN network, in accordance with some examples of the present disclosure. In some cases, a CS-based UE may attempt to register with the VoLTE based VPLMN and/or send a mobile originating call or text with respect to another UE. 
     At  202 , an ISDS of the VPLMN may receive a first VoLTE transmission associated with the visiting CS-based UE. For example, the first VoLTE transmission may be a registration request, a Multimedia Authentication Request, an INVITE message, an SMS message, or the like. In these examples, the ISDS may obtain information associated with the CS-based UE from the home network of the CS-based UE (e.g., the CS-based UE&#39;s HPLMN). 
     At  204 , the ISDS may generate a first CS transmission based at least in part on the first VoLTE transmission. For example, the ISDS may convert the CS transmission from a first format associated with the VPLMN to a second format associated with the HPLMN of the CS-based UE. In this manner, the VPLMN may operate using the first format while the ISDS is able to obtain information and data associated with the CS-based UE from the HPLMN using the second format. 
     At  206 , the ISDS may send the first CS transmission to the HPLMN. For example, the first CS transmission may be an authorization information request, a location data request, an INVITE message, a mobile originating-forward short message (MO-FSM), and the like. 
     At  208 , the ISDS may receive from the HPLMN a second CS transmission associated with the visiting CS-based UE. For example, the second CS transmission may be a location response, a mobile subscriber data or information, AKA vectors, INVITE response, MO-FSM response, and the like. 
     At  210 , the ISDS may generate a second VoLTE transmission based at least in part on the second CS transmission. For example, the ISDS may convert or otherwise generate a second VoLTE transmission using data associated with the second CS based transmission. For example, an Inter-Working Function (IWF) associated with the ISDS  122  may build and/or store relevant profile information or data associated with the CS-based UE  102  received as mobile subscriber data from the HPLMN. 
     At  212 , the ISDS may send the second VoLTE transmission to another component of the VPLMN and, at  214 , the other component may perform an operation associated with the CS-based UE. In some cases, the other components may include an Integrating-Call Session Control Function, a Serving-Call Session Control Function, a Proxy-Call Session Control Function, and the like. For example, the Serving-Call Session Control Function may assign network resources (such as a server) to the CS-based UE in response to receiving the second VoLTE transmission. 
       FIG.  3    is an example flow diagram showing an illustrative process associated with hosting a visiting CS-based user equipment on a VoLTE based visited public land mobile network, in accordance with some examples of the present disclosure. In some cases, a CS-based UE may receive a mobile terminating call or text with respect to another UE. 
     At  302 , an ISDS of the VPLMN may receive a first CS transmission associated with the visiting CS-based UE. For example, the first CS transmission may be a send routing information request, an Identity and Access Management Request, or the like. In these examples, the first CS transmission may be associated with a mobile terminating call or text message. 
     At  304 , the ISDS may generate a first VoLTE transmission based at least in part on the first CS transmission. For example, the ISDS may convert the first VoLTE transmission from a first format associated with the HPLMN to a second format associated with the VPLMN. In this manner, the VPLMN may operate using the second format while the ISDS is able to obtain information and data associated with the CS-based UE from the HPLMN using the first format. 
     At  306 , the ISDS may send the first VoLTE transmission to a component of the VPLMN. For example, the first VoLTE transmission may be a Mobile Terminating Forward short message that the ISDS may convert to a VoLTE SIP message for forwarding to the visiting UE via, for example, Serving-Call Session Control Function and/or a Proxy-Call Session Control Function. 
     At  308 , the ISDS may receive from the component of the VPLMN a second VoLTE transmission associated with the visiting CS-based UE. For example, the second VoLTE transmission may be an acknowledgement of the text message or the like. 
     At  210 , the ISDS may generate a second CS transmission based at least in part on the second VoLTE transmission. For example, the ISDS may convert or otherwise generate a second CS transmission using data associated with the second VoLTE transmission. The second VoLTE transmission may then be sent or otherwise provided to the HPLMN associated with the visiting UE. 
       FIG.  4    is an example of a data flow diagram for registering a CS user equipment  402  to a VoLTE network, in accordance with some examples of the present disclosure. For example, the UE  406  may be associated with a CS-based HPLMN  404  but may be attempting to register as a roaming or visiting device on a VoLTE-based VPLMN  406 . In other words, the UE  402  may be at a location that is outside of the network coverage provided by the HPLMN  404  but within the network coverage area of the VPLMN  406 . 
     Initially, at  408 , the UE  402  may discover a Proxy-Call Session Control Function (P-CSCF)  410  associated with the VPLMN  406 . The UE  402  may then transmit or send a registration request  412  to the detected P-CSCF  410 . The P-CSCF  410  may act as a first point of contact between the UE  402  and the VPLMN  406 . The registration request  412  may include a Uniform Resource Identifier (URI) associated with the UE  402  to assist the P-CSCF  410  in processing the registration request. 
     At  414 , the P-CSCF  410  may select an Integrating-Call Session Control Function (I-CSCF)  416  for the UE  402  based on a home domain indicated by the URI in the registration request  412 . For example, the P-CSCF  410  may select a Mobile Network Code (MNC), a Mobile Country Code (MCC), or other identifiers for the HPLMN  404  associated with UE&#39;s  402 . In some cases, the P-CSCF  410  may configure an associated Domain Name Server (DNS) to return an address associated with the I-CSCF  416  to avoid changes to a Unified Access Gateway (UAG) associated with the UE  402 . The I-CSCF  416  may assist with routing requests to an appropriate service function, such as Serving-Call Session Control Function (S-CSCF)  426  discussed below. 
     At  418 , the I-CSCF  416  selected by the P-CSCF  410  may receive the registration request  408  or an indication of the registration request  412 . The I-CSCF  416  may select a Home Subscribers Server (HSS) for the UE  402  based on the MNC and/or MCC associated with the HPLMN  404 . In other examples, the I-CSCF  416  may select the HSS for the UE  402  based on an Internet protocol multimedia private identity (IMPI/IMPU) and/or a domain name indicated by the URI associated with the UE  402 . The I-CSCF  416  may then generate and forward a unified authentication request (UAR)  420  associated with the UE  402  to an International Switched Digital Service (ISDS)  422 . 
     At  424 , the ISDS  422  may check to determine if the UE  402  is an authorized roaming partner device and confirm or determine if the S-CSCF  426  for the UE  402  is assigned. If the UE  402  passes the roaming partner check and the S-CSCF  426  is assigned, the ISDS  422  may return a Unified Authentication Accept (UAA) message  428  to the I-CSCF  416 . The I-CSCF  416  may also send or forward the registration request  412  or an indication of the registration request  412  to the assigned S-CSCF  426 , which in response sends a Multimedia Authentication Request (MAR)  430  to the ISDS  422 . 
     At  432 , the ISDS  422  may initiate an authentication process with the HPLMN  404 . For example, the ISDS  422  may send authentication information  434  to a Home Location Register (HLR)  436  associated with the HPLMN  404 . As discussed above, the ISDS  422  may receive the MAR  430  as a VoLTE based transmission and send the authentication information  434  as a CS-based transmission compatible or readable by the HLR  436  of the HPLMN  404 . The HLR  436  may be a database that contains or stores various data about mobile subscribers of a mobile network, such as the mobile numbers, services and whether a number has been ported to another network, and the like. 
     The HLR  436  may return also known as (AKA) vectors  438  for the UE  402  to the ISDS  422 . Again, the AKA vectors  438  may be transmitted as a CS-based transmission that is received and processed by the ISDS  422 . The ISDS  422  may then send updated location data  440  associated with the UE  402  to the HLR  436  of the HPLMN  404  as another CS-based transmission. In some cases, the ISDS  422  and the HLR  436  may also exchange subscriber data (SD) via a CS-based message  442  and a CS response  444 . The HLR  436  may also return a CS-based confirmation or response  446  associated with or acknowledging the updated location data  440 . In one example, the ISDS  422  may translate a diameter of the CS-based message to a diameter associated with a media access plan or protocol (MAP) readable by the VPLMN  406 . The ISDS  422  may also forward the MAP to the HLR  436  and store the user profile or UE profile data. 
     At  448 , an Inter-Working Function (IWF) associated with the ISDS  422  may build and/or store relevant profile information or data associated with the UE  402  in a format associated with the VPLMN  406  and received from the HLR  436  of the HPLMN  404 . The S-CSCF  426  may then return either an unauthorized message and/or the aka vectors information  438  to the UE  402 , the I-CSCF  416 , and/or the P-CSCF  410 , as illustrated. 
     At  450 , the UE  402  may send a message (such as UAR  452 ) to the ISDS  422 . The ISDS  422  may, in response, provide a server-name  462  to the I-CSCF  416 , with which the ISDS  422  may complete the registration process with the VPLMN  406 , as shown. For example, at  454 , if the authentication is successful, the S-CSCF  426  provides a Server-Assignment-Request (SAR)  456  to the ISDS  422  and a Server-Assignment-Answer (SAA)  458  is returned by the ISDS  122 . Upon receipt of the SAA  458 , the S-CSCF  426  may send a confirmation  460  of the registration back to the UE  402 . 
     At  462 , the S-CSCF  426  may execute an iFC process and the UE  402  may be registered with a Telephone Application Server (TAS)  464  associated with the VPLMN  406 . In some cases, at  466 , the TAS  464  may select a Home Subscriber Server (HSS) for the UE  402  based on the MCC, MNC, the IMPI, the IMPU, the domain name indicated by the URI associated with the UE  402 , a combination thereof, or the like. The TAS  464  may then send a diameter associate with a Sub network router (Sh-SNR)  468  to the ISDS  422 . In response, the ISDS  422  may provide a diameter associated with the Systems Network Architecture (SNA)  470  to the TAS  464 . The TAS  464  may then send an OK or approval message  472  to the S-CSCF  126 . The S-CSCF  426  may then register the UE  402  with an Interface Message Processor (IMP)  474 . 
     At  476 , the IMP  474  may query a contract data requirement list (CRDL) to determine if a Uniform Network Resource Identifier (UNRI) is set or otherwise configured for the UE  402 . In some cases, the TAS  474  may also write or store registering data. Once, the CRDL configuration is confirmed, the TAS  464  may return an OK or approval message  478  to the S-CSCF  426  and the UE  402  is fully registered with the VPLMN  406 . 
       FIG.  5    is an example of a data flow diagram  500  associated with routing a mobile originating call (MOC) from a visiting UE  502  over a VPLMN  504 , in accordance with some examples of the present disclosure. Initially, the VPLMN  504  receives an inbound MOC from a UE  502 . The inbound MOC may be routed in a similar manner as to the VoLTE calls. The MOC may be received at a P-CSCF  506  in the form of an INVITE message  508 . The P-CSCF  506  may forward the INVITE message  508  to a S-CSCF  510  and the S-CSCF  510  may execute the iFC process. The INVITE message  508  may then be received by a TAS  512 . The TAS  512  may execute the originating services and the S-CSCF  510  may route the call to a mobile terminating (MT) UE (not shown) in a traditional manner. 
       FIG.  6    is an example of a data flow diagram  600  associated with routing a mobile terminating call (MTC) for a visiting UE  602  over a VPLMN  604 , in accordance with some examples of the present disclosure. Initially, a Gateway Mobile Switching Center (GMSC)  606  associated with a HPLMN  608  sends a send routing information (SRI) request  608  to a HLR  612  of the HPLMN  608 . In response, the HLR  612  sends a Mobile Subscriber Integrated Services Digital Network Number (MSISDN)  614  to an ISDS  642  associated with the VPLMN  604 . 
     At  616 , the ISDS  642  may allocate, store, and/or check a Mobile Station Roaming Number (MSRN) against the MSISDN  614  received from the HLR  612 . If the ISDS  642  identifies a match, the ISDS  642  may return a MSRN  618  to the HLR  612  associated with the HPLMN  608  and the HLR  612  may send an SRI response  620  to the GMSC  606 . In this example, the MSISDN  614  and the MSRN  618  may be sent and received in a first format (e.g., a CS based transmission) associated with the HPLMN  608 . 
     At  622 , the GMSC  606  may generate an Identity and Access Management (IAM) MSRN request  624  and forward to a Media Gateway Controller Function (MGCF)  626  associated with the VPLMN  604 . The MGCF  626  may translate the IAM MSRN request  624  to an INVITE message  628 . The INVITE message  628  is provided to a I-CSCF  630  associated with the VPLMN  604 . 
     At  632 , the I-CSCF  630  may select an IWF for use with the MTC and send a Local Internet Registry (LIR) request  640  ( 640  is used twice as LIR request and TAS, here and in the PDF) to the ISDS  642 . The LIR request  640  may be in a second format (e.g., a VoLTE based format) associated with the VPLMN  604 . At  634 , the LIR request  640  indicates the MSRN  618 , and the IWF determines the MSISDN  614  associated with the MRSN  618 , returns a custom AVP, and assigns a S-CSCF  636  to the MOC. The Server Name associated with the S-CSCF  636  and the MSISDN  614  are then sent to the I-CSCF  630  which sends an INVITE  638  to the S-CSCF  636 , thereby routing the MTC to the S-CSCF  636 . The INVITE  638  may indicate the MSISDN  614 . The S-CSCF  636  may apply initial filter criteria to route the MTC to a TAS  644 . The TAS  644  may then perform terminating services and route the MTC to the UE  602  via a corresponding P-CSCF  642 , as shown. 
       FIG.  7    is an example of a data flow diagram  700  associated with routing a mobile originating short-message-service (SMS) from a visiting UE  702  over a VPLMN  704 , in accordance with some examples of the present disclosure. In the illustrated example, the UE  702  may send a message  706  to the P-CSCF  708  of the VPLMN  704 . The message  706  may include a SMS-SUBMIT request (in a first format associated with the VPLMN  704 ) and include a R-URI, an indication of a HPLMN  710  associated with the SMS message, a SMS control address, and the like. The P-CSCF  708  may forward the message  706  to a S-CSCF  412 . The S-CSCF  712  may execute an iFC process and send the message to an ISDS  714  in the first format associated with the VPLMN  704 , as illustrated. The ISDS  714  may then convert the message  706  to a second format associated with the HPLMN  710 . For example, the first format may be a VoLTE based transmission format and the second format may be a CS based transmission format. The ISDS  714  may then send a CS based MO-FSM  722  to the SMSC  718  of the HPLMN  710 . 
     The ISDS  714  may receive a CS based MO-FSM response  716  (e.g., in the second format) from the HPLMN  710 . For example, the SMSC  718  may return a CS based MO-FSM response  716  indicating a successful delivery of the SMS message. The ISDS  714  may then convert the CS based MO-FSM response  716  to a VoLTE SMS submit report  720  (e.g., in the first format). The ISDS  714  may then return SMS submit report  720  to the UE  702  via the S-CSCF  712  and/or the P-CSCF  708 , as shown. Finally, the UE  702  may then acknowledge the report with the ISDS  714 . 
       FIG.  8    is an example of a data flow diagram  800  associated with routing a MT SMS from a visiting UE  802  over a VPLMN  804 , in accordance with some examples of the present disclosure. In the currently illustrated example, an SMSC  806  of a HPLMN  808  originating the SMS message may transmit a send-routing information for a short message (SRI-SM)  810  to an HLR  812  associated with the HPLMN  808 . The HLR  812  may then return a SRI-SM response  814  to the SMSC  806 . The SRI-SM response  814  including a visitor location register (VLR) of the UE  802 . 
     The SMSC  806  may then transmit a Mobile Terminating Forward short message (MT-FSM)  816  to an ISDS  818  associated with the VPLMN  804  currently servicing the UE  802 . The MT-FSM  816  may be in a first format, such as a CS based transmission format associated with the HPLMN  810 . The ISDS  818  may convert the MT-FSM  816  to a SIP MESSAGE in a second format then deliver the SIP MESSAGE  818  to the UE  802  via a S-CSCF  820  and a P-CSCF  822 , as shown. The second format may be a VoLTE based transmission format and associated with the VPLMN  804 . In some cases, the UE  802  may acknowledge a receipt of the SIP MESSAGE  818  by sending an OK response to the ISDS  818 . The UE  802  may also send a MESSAGE with a delivery report  824  to the S-CSCF  820 . The S-CSCF  820  may report back to the SMSC  806  via the ISDS  818 , as illustrated. 
     Again, as discussed above, the communication with the components of the HPLMN  810  may be in the first format associated with the HPLMN  810  (e.g., a CS based transmission format) and the communication within the VPLMN  804  may be in the second format (e.g., a VoLTE based transmission format). In this example, the ISDS  818  is responsible for converting or generating transmissions, request, reports, and the like in the appropriate format for each of the components. In this manner, the ISDS  818  acts to translate the transmissions to a diameter or standard associated with the responsible network  804  or  810 . 
       FIG.  9    is another example of a data flow diagram  900  associated with routing a MO SMS from a visiting UE  902  over a VLPMN  904 , in accordance with some examples of the present disclosure. In this example, the UE  902  may initiate an SMS message using a Standard Global Service (SGs). For instance, the UE  902  may send via an uplink (UL) Non-Access Stratum (NAS) transport  906  to a mobility management entity (MME)  908  associated with the VPLMN  904 . The transport  906  may include the SMS data. The MME  908  may translate data associated with the UL NAS transport  906  to UL Unitdata  910 . The UL Unitdata  910  is transmitted to an ISDS  912  associated with the VPLMN  904 . The ISDS  912  may also send an acknowledgment associated with the MO-FSM  914  back to the MME  908  as a downlink Unitdata  922  and the MME  908  may forward to the UE  902  via a DL NAS transport  924 . 
     The ISDS  912  converts the UL Unitdata  910  from a first format associated with the VPLMN  904  to a MO-FSM  914  in a second format associated with the HPLMN  918  and sends to SMSC  916  associated with an HPLMN  618 . For example, the first format may be a VoLTE format and the second format may be a CS format. The SMSC  916  may process the MO-FSM  914  and transmit a MO-FSM response  920  back to the ISDS  912 . The ISDS  912  may convert the MO-FSM response  920  to a DL Unitdata  926  and sends the DL Unitdata  926  to the MME  908  over the SGs. The MME  908  then converts the DL Unitdata  926  to a DL NAS transport  930  and send the DL NAS transport  930  to the UE  902 . In some cases, the UE  902  may provide a UL NAS transport  932  back to the MME  908  acknowledging receipt of the DL NAS transport  930 . 
       FIG.  10    is another example of a data flow diagram  1000  associated with routing a MT SMS from a visiting UE  1002  over a VLPMN  1004 , in accordance with some examples of the present disclosure. In this example, the UE  1002  may receive an SMS message using a SGs. In this example, an SMSC  1006  of a HPLMN  1008  originating the SMS message may transmit a SRI-SM  1010  to an HLR  1012  associated with the HPLMN  1008 . The HLR  1012  may then return a SRI-SM response  1014  to the SMSC  1006  the SRI-SM response  1014  including a visitor location register (VLR) of the UE  1002 . 
     The SMSC  1006  may then transmit a MT-FSM  1016  to an ISDS  1018  associated with the VPLMN  1004  currently servicing the UE  1002 . The ISDS  1018  may convert the MT-FSM  1016  in a format associated with the HPLMN  1008  to a DL Unitdata  1038  in a format associated with the VPLMN  1004 . The DL Unitdata  1038  may then be provided to the UE  1002  via an MME  1020 . The MME  1020  may initiate a service request  1022  to cause the ISDS  1018  to provide DL Unitdata  1024 . The MME  1020  may then convert the DL Unitdata  1024  to a DL NAS transport  1026  which is transmitted to the UE  1002 . 
     The UE  1002  may then initiate a delivery report  1040  to provide back to the SMSC  1006  of the HPLMN  1008 . The MME  1020  may convert the UL NAS transport  1028  to a UL Unitdata  1030  which is provided to the ISDS  1018 . The ISDS  1018  may then cover the UL Unitdata  1030  to a MT-FSM response  1032 . The MS-FSM response  1032  is then sent to the SMSC  1006 . The ISDS  1018  may also respond to the UE  1002  with a final acknowledgement message that may be provided via the MME  1020  as DL Unitdata  1034  and DL NAS transport  1036 , as illustrated. 
     Although the subject matter has been described in language specific to structural features and/or methodological acts, it is to be understood that the subject matter defined in the appended claims is not necessarily limited to the specific features or acts described. Rather, the specific features and acts are disclosed as exemplary forms of implementing the claims.