Abstract:
The subject matter described herein includes methods, systems, and computer readable media for providing a NAS node selection function with CN node bearer circuit availability monitoring and availability-based load sharing. According to one aspect, the subject matter described herein includes a method for providing a NAS node selection function. The method includes performing various steps at a NAS node selection function which may be integrated with or separate from a media gateway. The method includes monitoring bearer circuit availability for each of a plurality of CN nodes. The method further includes storing an indication of bearer circuit availability for each of the CN nodes. The method further includes receiving initial layer  3  messages from radio access nodes in response to mobile station activity. The method further includes, in response to the messages, assigning mobile stations to the MSCs in a load-sharing manner using the stored indications of bearer circuit availabilities.

Description:
TECHNICAL FIELD 
       [0001]    The subject matter described herein relates to assigning mobile stations to core network nodes. More particularly, the subject matter described herein includes methods, systems, and computer readable media for providing a NAS node selection function with CN node bearer circuit availability monitoring and availability-based load sharing. 
       BACKGROUND 
       [0002]    In mobile communications networks, it may be desirable to share core network (CN) node resources among radio access nodes, such as UMTS radio network controllers (RNCs) and GSM base station controllers (BSCs). Conventionally, radio access nodes were restricted to a single core network node. As a result of this strict hierarchy, core node resources were inefficiently used. 
         [0003]    3GPP TS 23.236 defines technical requirements for A/lu-flex, a proposal where radio access nodes, such as RNCs and BSCs, can select any core network node, such as a serving GPRS support node (SGSN) or mobile switching center (MSC) that serves a specific geographic area, referred to as a pool area. The group of CN nodes that serve a pool area is referred to as the MSC pool or the SGSN pool. Allowing access nodes to select from plural CN nodes within a pool area increases the efficiency of utilization of CN node resources. 
         [0004]    One problem with the architecture proposed in 3GPP TS 23.236 is that it requires that the intelligence for selecting the CN node to be in the access node, i.e., in the BSC or RNC. However, legacy BSCs and RNCs may not have the capability to perform such selection. In light of the number of BSCs or RNCs in a network, upgrading all of the BSCs or RNCs in a network may be cost-prohibitive. Another problem with the architecture proposed in TS 23.236 is that it does not specify a load-balancing algorithm for balancing the load between available CN nodes. Instead, TS 23.236 indicates that the load-balancing algorithm is “implementation specific.” 
         [0005]    3GPP TR 23.823 describes an architecture where the NAS node selection function is located above the BSC/RNC. However, like TS 23.236, TS 23.823 indicates that the load-balancing algorithm is implementation specific. In addition, TR 23.823 indicates that the NNSF can be located in a stand-alone intermediary node or co-located with another node, but does not specify the node type with which the NNSF can be co-located. 
         [0006]    Accordingly, in light of these shortcomings, there exists a need for methods, systems, and computer readable media for providing non-access stratum (NAS) node selection function with core network (CN) node bearer-circuit availability monitoring and availability-based load sharing. 
       SUMMARY 
       [0007]    The subject matter described herein includes methods, systems, and computer readable media for providing a NAS node selection function with CN node bearer circuit availability monitoring and availability-based load sharing. According to one aspect, the subject matter described herein includes a method for providing a NAS node selection function. The method includes performing various steps at a NAS node selection function which may be integrated with or separate from a media gateway. The method includes monitoring bearer circuit availability for each of a plurality of CN nodes. The method further includes storing an indication of bearer circuit availability for each of the CN nodes. The method further includes receiving initial layer 3 messages from radio access nodes in response to mobile station activity. The method further includes, in response to the messages, assigning mobile stations to the CN nodes in a load-sharing manner using the stored indications of bearer circuit availabilities. 
         [0008]    The subject matter described herein for providing an NNSF with CN bearer circuit availability monitoring and availability-based load sharing may be implemented on a non-transitory computer readable medium having stored thereon executable instructions that when executed by the processor of a computer control the computer to perform steps. Exemplary computer readable media suitable for implementing the subject matter described herein include disk memory devices, chip memory devices, programmable logic devices, and application specific integrated circuits. In addition, a computer readable medium that implements the subject matter described herein may be located on a single device or computing platform or may be distributed across multiple devices or computing platforms. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]    Preferred embodiments of the subject matter described herein will now be explained with reference to the accompanying drawings of which: 
           [0010]      FIG. 1  is a network diagram illustrating fixed association between BSCs and MSCs; 
           [0011]      FIG. 2  is a network diagram illustrating exemplary MSC pooling where NNSF functionality is co-located with the BSC nodes; 
           [0012]      FIG. 3  is a network diagram illustrating a network where the NNSF functionality is co-located with MG and SG functionality and where the NNSF performs bearer circuit availability-based load sharing according to an embodiment of the subject matter described herein; 
           [0013]      FIG. 4  is a network diagram illustrating an example where SG and NNSF functionality are co-located and where the NNSF performs bearer circuit availability-based load sharing according to an embodiment of the subject matter described herein; 
           [0014]      FIG. 5  is a block diagram illustrating an exemplary NNSF architecture according to an embodiment of the subject matter described herein; 
           [0015]      FIG. 6  is a message flow diagram illustrating exemplary messages exchanged between a BSC, an NNSF, and an MSC in assigning a mobile station to an MSC according to an embodiment of the subject matter described herein; 
           [0016]      FIG. 7  is a network diagram illustrating the bypassing of NNSF functionality at a signaling gateway when NNSF functionality is included in the radio access node according to an embodiment of the subject matter described herein; 
           [0017]      FIG. 8  is a network diagram illustrating transparent mapping of CICs by an NNSF according to an embodiment of the subject matter described herein; and 
           [0018]      FIG. 9  is a flow chart illustrating exemplary steps for CN assignment according to an embodiment of the subject matter described herein. 
       
    
    
     DETAILED DESCRIPTION 
       [0019]      FIG. 1  is a network diagram illustrating a conventional mobile network where each BSC is assigned to a single MSC. In particular, in  FIG. 1 , BSCs  104  and  106  are assigned to MSC  100 , and BSCs  108  and  110  are assigned to MSC  102 . Each BSC  104 ,  106 ,  108 , and  110  has access to only a single MSC without the ability to utilize resources provided by other MSCs. As a result of this fixed relationship between BSCs and MSCs, there is no ability to load-share the assignment of MSCs to mobile stations. 
         [0020]      FIG. 2  illustrates a network as described in 3GPP TS 23.236 where a non-access stratum node selection function (NNSF) is integrated within the BSCs. More particularly, in  FIG. 2 , each BSC  200 ,  202 ,  204 , and  206  includes an NNSF  210 . NNSF  210  allows multiple MSCs  212  and  214  to be assigned to a given single BSC. However, 3GPP TS 23.236 does not specify the method by which NNSF  210  assigns mobile stations to MSCs. Moreover, requiring that the NNSF be located at the BSC node requires that each BSC nodes be upgraded and is therefore unsuitable for networks with large numbers of legacy BSC nodes without NNSF functions.  FIG. 2  also illustrates an MSC pool area, which is the area served by MSCs  212  and  214 . 
         [0021]    According to one aspect of the subject matter described herein, an NNSF may be located within a media gateway that resides between the BSCs and the MSCs to allow load-sharing of MSC assignments among mobile stations. In addition, the load-sharing may be based on bearer circuit availabilities of the MSCs.  FIG. 3  illustrates an example of a network where the NNSF is integrated with media gateways and where MSCs are assigned to mobile stations using bearer circuit availability-based load sharing according to an embodiment of the subject matter described herein. Referring to  FIG. 3 , a plurality of media gateways (MGs)  300  and  302  with integrated signaling gateway (SG) functions resides between BSCs  304 ,  306 , and  308  and MSCs  310  and  312 . Each media gateway  300  and  302  includes an NNSF function  314 . Each NNSF function  314  monitors availability of bearer circuits of each of MSCs  310  and  312  and assigns new mobile stations to MSCs  310  and  312  based on the monitored availabilities and in a load-sharing manner. In the illustrated example, each BSC  304 ,  306 , and  308  is connected to two combined media gateway/NNSF nodes  300  and  302 , which are both active and share the load of the subtending BSCs  304 ,  306 , and  308 . The SG functions of media gateways  300  and  302  handle SCCP signaling between BSCs  304 ,  306 , and  308  and MSCs  310  and  312  and appear as a single node to the BSCs. The SG functions of media gateways  300  and  302  may share the same local point codes and a single BSC linkset connected to both SGs. 
         [0022]    In operation, when a mobile station is first active in a BSC service area, i.e., through initial registration or handover, the serving BSC sends an initial layer 3 message to the MSC assigned to the BSC. For example, BSC  304  may send the initial layer 3 message to MSC  310 . NNSF  312  of MG  300  may intercept the initial layer 3 message, determine whether to assign the mobile station to MSC  310  or MSC  312  based on monitored relative bearer circuit availabilities of the MSCs, and assign the mobile station to the MSC with the highest bearer circuit availability. Bearer circuit availability or relative bearer circuit availability between MSCs of equal processing capacity may be determined by comparing the number or percentage of available bearer circuits of each MSC. If one MSC has a higher number or percentage of available bearer circuits than another MSC of the same processing capacity, then the mobile station may be assigned to the first MSC. Monitoring the bearer circuit availability of an MSC may be performed by keeping track of assignments of bearer circuits by each MSC made through the NNSF  314 . Moreover, each NNSF  314  may periodically audit each MSC  310  and  312  to determine whether the MSC  310  or  312  has blocked or made circuits available. The auditing may be performed using an operations, administration, and maintenance (OA&amp;M) interface of NNSF  314  to communicate with a corresponding OA&amp;M interface of each MSC. Exemplary OA&amp;M protocols that can be used include proprietary protocols and SNMP. Monitoring bearer circuit availability may also include monitoring bearer circuit maintenance messages generated by BSCs  304 ,  306 , and  308  to inform MSCs  310  and  312  of bearer circuit maintenance or equipment failure events. 
         [0023]    In the illustrated example, a pair of NNSFs  314  serves a common subscriber pool, represented by the box surrounding BSCs  304 ,  306 , and  308 . In such an embodiment, CIC assignment messages from MSCs  310  and  312  and maintenance messages from BSCs  304 ,  306 , and  308  may go through either NNSF  314 . As a result, NNSFs  314  may implement communications to track the state of all the bearer circuits. One option for tracking bearer circuit status information where bearer circuit assignment or maintenance messages are not guaranteed to go through the same NNSF is to have the NNSFs communicate with each other regarding bearer circuit status. Such a solution may lead to consistency problems if an NNSF receives bearer circuit status information from both its mated NNSF and from a BSC or MSC. Another option for tracking bearer circuit operational status is to have the NNSF-MSC audit messages request bearer circuit operational state (free or busy) instead of just the administration state (unblocked or blocked). In such an embodiment, it may be unnecessary for the NNSFs to record CIC assignment messages from the MSCs. In yet another example, each MSC may compute its own bearer circuit availability information as an absolute number or a percentage and send that information to each NNSF. Any method for obtaining bearer circuit availability information for core network nodes is intended to be within the scope of the subject matter described herein. 
         [0024]      FIG. 4  illustrates an alternate embodiment of the subject matter described herein where NNSF  314  is separate from MG nodes  300  and  302 . In  FIG. 4 , each NNSF  314  is implemented on a platform that hosts a signaling gateway, but not a media gateway. The operation of the subject matter illustrated in  FIG. 4  with regard to MSC load sharing is similar to that illustrated in  FIG. 3  and a description thereof will not be repeated herein. 
         [0025]      FIG. 5  is a block diagram illustrating an exemplary NNSF architecture according to an embodiment of the subject matter described herein. Referring to  FIG. 5 , each NNSF  314  may reside on a circuit board that is associated with a signaling gateway  502 , or implemented on a separate circuit board. NNSF  314  may communicate with signaling gateway  502  through inter-processor communications  504 . Each NNSF  314  may implement both SCCP and BSSAP signaling layers  506  and  508 . Each NNSF  314  may maintain a list of available bearer channels and corresponding MSCs, illustrated in  FIG. 6  by circuit identifier code (CIC) list  510 . The Network resource identity (NRI) table  511  stores NRIs for all CN nodes accessible by each NNSF  314 . Each NNSF  314  may send and receive BSSAP signaling to and from BSCs and MSCs via a TDM network interface  512  or IP network interface  514 . 
         [0026]    Table 1 shown below illustrates an example of bearer circuit availability data that may be maintained by NNSF  314  according to an embodiment of the subject matter described herein. 
         [0000]    
       
         
               
             
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 MSC CIC Availability Status Information 
               
             
          
           
               
                   
                 CN Node 
                 CICs/BSCs 
                 CICs status 
               
               
                   
                   
               
             
          
           
               
                   
                 MSC1 
                 CIC1 
                 BSC1 
                 available 
               
               
                   
                   
                 CIC2 
                 BSC1 
                 available 
               
               
                   
                   
                 CIC3 
                 BSC2 
                 available 
               
               
                   
                   
                 CIC4 
                 BSC2 
                 available 
               
               
                   
                 MSC2 
                 CIC6 
                 BSC1 
                 blocked 
               
               
                   
                   
                 CIC7 
                 BSC1 
                 available 
               
               
                   
                   
                 CIC8 
                 BSC2 
                 busy 
               
               
                   
                   
                 CIC9 
                 BSC2 
                 available 
               
               
                   
                   
               
             
          
         
       
     
         [0027]    Table 1 shown above illustrates exemplary CIC availability that may be maintained for MSCs or other core network nodes. In the illustrated example, the first column includes MSC identifiers. In practice, each MSC may be identified by any suitable network identifier, such as an NRI, a point code, or an IP address. The next column includes CICs that are assigned to each MSC and the corresponding BSCs with the CICs are associated. In the illustrated example, MSC 1  has CIC 1  and CIC 2  with BSC 1  and CIC 3  and CIC 4  with BSC 2 . Similarly, MSC 2  has CIC 6  and CIC 7  with BSC 1  and CIC 8  and CIC 9  with BSC 2 . If the NNSF with which Table 1 is associated receives an initial layer 3 message from BSC 1 , the NNSF may determine from the stored CIC status information that MSC 1  has two CICs, CIC 1  and CIC 2 , available for BSC 1  and MSC 2  has only one CIC available for BSC 1 . Using this availability status information, the NNSF may assign MSC 1  to the mobile station for which the initial layer 3 message was sent. The NNSF may then forward the initial layer 3 message to MSC 1 , which returns a response to the message to BSC 1 . 
         [0028]      FIG. 7  is a message flow diagram illustrating exemplary MSC assignment by NNSF  314  according to an embodiment of the subject matter described herein. Referring to  FIG. 7 , in line  1 , BSC  700  sends an SCCP connection request (CR) message to NNSF  314 . If the connection request message contains a valid NRI that corresponds to one of the MSCs in the network, then the connection request corresponds to a mobile station that has already been assigned, e.g., due to a previous activation. However, if any of the following conditions are true, the NNSF may perform load sharing:
       The TIMSI does not contain a valid NRI (no match in the static configuration data);   The TIMSI contains the null NRI;   The initial layer three message does not contain a TIMSI (IMSI or IMEI used instead);       
 
         [0032]    For all these cases the NNSF selects the MSC from available MSCs in the pool using the load balancing algorithm described herein, taking into account the MSC&#39;s reachability, service, and load redistribution states. 
         [0033]    Returning to the message flow diagram in  FIG. 6 , in line  2 , NNSF  314  sends the SCCP connection request to MSC  602 . MSC  602  allocates an SCCP local reference number for the connection and embeds its SNRI in the SCCP LRN sent back in the SCCP connection confirm message in line  3 . In line  4 , the SCCP connection confirm message is sent from NNSF  314  to BSC  600 . In line  5 , BSC  600  sends an SCCP DT message to NNSF  314 . NNSF  314  extracts the SNRI from the SCCP destination LRN and distributes the SCCP message to the appropriate MSC, MSC  602 , as illustrated by line  6 . 
         [0034]      FIG. 7  is a network diagram illustrating an example where NNSF  314  is co-located with signaling gateway  700  and interfaces with a first BSC  702  that has A-flex or lu-flex capabilities and a second BSC  704  that does not have A-flex or lu-flex capabilities. As stated above, lu-flex refers to the ability of a radio access node in a UMTS network to select between MSCs in a pool of MSCs. A-flex refers to the same capability in a GSM network where the radio access interface between the BSC and the MSC is referred to as the A or access interface, rather than the lu interface. 
         [0035]    Signaling gateway  800  may determine whether or not a message originates from a node with or without A-flex or lu-flex capabilities by examining a configuration parameter associated with the address of the sending BSC that indicates whether lu-flex or A-flex capabilities exist. If signaling gateway  700  determines that an initial layer 3 message originates from a node that has A-flex or lu-flex capabilities, then NNSF function  314  is bypassed, and the message is forwarded to the appropriate MSC  706  or  708  identified in the message. In the illustrated example, the solid line from BSC  702  to MSC  708  represents the case where NNSF  314  is bypassed. If signaling gateway  700  receives a message from a node that does not have A-flex or lu-flex capabilities, then the message is forwarded to NNSF  314 , which selects the appropriate MSC using load sharing, as described above. The load sharing case is illustrated by the dashed line in  FIG. 7 . 
         [0036]      FIG. 8  is a network diagram illustrating exemplary CIC assignments between BSCs and MSCs and the transparent mapping of CICs performed by each NNSF according to an embodiment of the subject matter described herein. In the illustrated example, each NNSF is a component of a combined MG/SG node  800  or  802 . MG/SG nodes  800  and  802  interface between BSCs  804  and  806  and MSCs  810  and  812 . BSC  804  has CICs  1 - 50  and  101 - 150  that it believes that BSC  804  associates with MSC  810 . However, each NNSF maps CICs  101 - 150  to MSC 2   512  transparently from BSC  804 . Similarly, BSC  806  is assigned CICs  51 - 100  and  151 - 200  that BSC  806  associates with MSC 2   812 . However, the NNSFs of MG/SG nodes  800  and  802  map CICs  51 - 100  to MSC 1   810 . By transparently mapping CICs in the manner shown in  FIG. 8 , each NNSF is able to transparently load share assignment of mobile stations to MSCs without acquiring modification of the BSCs. 
         [0037]      FIG. 9  is a flow chart illustrating the exemplary overall steps that may be performed by an NNSF at an intermediate node according to an embodiment of the subject matter described herein. Referring to  FIG. 9 , in step  900 , the NNSF receives an initial layer 3 message from a radio access node. For example NNSF  314  may receive an initial layer 3 message from a BSC. In step  902 , it is determined whether the radio access node that originated the message includes NNSF functionality. If the NNSF determines that the radio access node has NNSF functionality, control proceeds to step  904  where the NNSF function at the intermediate node is bypassed and then to step  906  where the message is routed to the CN node specified by the message. In step  902 , if it is determined that the sending radio access node does not include NNSF functionality, control proceeds to step  908  where it is determined whether the message includes a valid, assigned NRI. If the message includes a valid, assigned NRI, control proceeds to step  906  where the NNSF routes the message to the CN node specified by the NRI. 
         [0038]    Returning to step  908 , if the message does not include a valid, assigned NRI, control proceeds to step  910  where the core network node is assigned using a load sharing algorithm described herein based on bearer circuit availability. Control then proceeds to step  906  where the message is routed to the assigned CN node. 
         [0039]    It will be understood that various details of the presently disclosed subject matter may be changed without departing from the scope of the presently disclosed subject matter. Furthermore, the foregoing description is for the purpose of illustration only, and not for the purpose of limitation.