Abstract:
A System to perform concurrent GERAN sharing with local switching and traffic shaping, comprising BSC-proxy (proxy base station controller), being connected as a regular BSC (base station controller) to one or multiple PLMNs (public land mobile networks); plurality of BSCs, geographically co-located with BTSs (base transceiver stations) and interfaced to BSC-proxy by A-over-IP and traffic shaping software; and optional signaling probes for GSM MAP (mobile application part). Methods providing concurrent sharing of radio access network and local switching connectivity without involving mobile network cores.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of U.S. application Ser. No. 62/006,423, filed Jun. 2, 2014, the disclosure of which is incorporated by reference herein. 
     
    
     FIELD OF THE INVENTION 
       [0002]    Mobile networking for extending cellular coverage and concurrent sharing between multiple mobile networks and for providing local switching connectivity within radio access network. 
       BACKGROUND OF THE INVENTION 
       [0003]    A segment of a conventional mobile network contains a MSC/VLR (mobile swithing cener/visitor location register) that couples a number of base station controllers (BSC) by means of so-called GSM A-interface, that can be implemented as TDM- or IP-based (time division multiplex- or Internet protocol). Each BSC serves multiple base stations (BTS), connected over an A-bis interface, that can be also carried over TDM or IP communication links. 
         [0004]    When a segment of radio access network (RAN) is connected by a link with expensive or limited resources, and the essential volume of traffic is interconnected within a given area, optimizing amount of data transferred over such a backhaul is a critical challenge. It can be either a link between MSC/VLR and BSC, or in between a BSC and a BTS. An A-interface is exhaustively specified and it can unify equipment by different vendors, while an A-bis interface in between BSC and BTS has vendor-specific flavor. 
         [0005]    There is a challenge to provide GSM EDGE radio access network (GERAN) coverage in low-traffic area. The common practice is either infrastructure sharing between operators, or roaming model (ie coverage is provided by one operator and the remained subscribers are served as roamers, with both visitor and serving networks interconnected by means of signaling system #7 mobile application part—SS7 MAP protocol), or by coverage of a single operator with limited services (emergency calls only) for other subscribers. 
         [0006]    GSM radio access network (GERAN) sharing is unspecified by a standard. It includes a chain consisting of a BTS and a BSC, and it is hosted by a single MSC/VLR belonging to a given mobile network (PLMN—public land mobile network). Each BTS emits a single pair of a mobile country code (MCC) and a mobile network code (MNC) that is a native obstacle for sharing GSM radio access network. A-interface signaling messages also carry fields that identify the given GERAN. 
         [0007]    U.S. Pat. No. 7,561,879 B2 describes usage of pseudo—PLMN at air interface to identify radio access network being shared among core networks, as well as the behavior of a communication device to chose a core network. 
         [0008]    US 20040105429 A1 Patent contains methods of sharing network elements from radio access networks between core networks, where dedicated resources (e.g. frequencies) are exclusively allocated for each network. 
         [0009]    U.S. Pat. No. 7,236,784 B2 Patent describes a system with a shared radio access network, while it assumes that a wireless device chooses a PLMN to be used. 
         [0010]    U.S. Pat. No. 7,280,516 B1 Patent discloses a network architecture for an arrangement in which mobile terminals may have at least two functional modes of operation in which the functional modes are provided by at least two core networks having different functionality. 
         [0011]    As a state of the art, the standard interface between GERAN and a network core is a so-called A-interface that is either a conventional TDM or over IP. In any case, an application level protocol at an A-interface operates with mobile identifications like IMSI (international mobile subscriber identity) or TMSI (temporary mobile subscriber identity), while subscribers are using MSISDN (mobile station integrated services digital network number) numbering. At the BSC side there is no knowledge about MSISDN-IMSI relations, and a legacy BSC is not specified to provide local switching by itself. A conventional TDM-based A-interface functions in such a way, that a call leg passes through an interface, so MSC is involved in a voice path in any case, even when both subscribers are being served by the same BTS or adjacent BTSs. This issue can be eliminated if a network follows a so-called NGN (next generation networks) architecture, so both call legs are represented by corresponding endpoints that can be enforced to use the same codecs, and IP traffic can be switched directly between BTSs. Nevertheless, this approach fails in the case of a conventional MSC. 
         [0012]    An example of a local switching implementation, controlled on core network side, can be found in US 20120178453 A1 Patent, and base station subsystem (BSS)-side procedures to provide aimed local connectivity can be found in the U.S. Pat. No. 8,559,949 B2. 
       SUMMARY OF THE INVENTION 
       [0013]    In one embodiment the invention provides a system that comprises of a BSC-proxy, coupled with one or multiple MSC/VLRs belonging to different PLMNs, and multiple BSCs, connected to a BSC-proxy with traffic-shaping software and co-located with BTSs. Optional signaling probes monitor GSM MAP signaling between MSC/VLRs and HLRs(home location register). 
         [0014]    For each MSC/VLR, the BSC-proxy emulates a conventional BSC which provides a radio access network (RAN) extension for the given PLMN, routes traffic between BSCs and MSC/VLRs and replaces signaling parameters in such a way that the GERAN segment is accepted as being exclusively controlled by each of MSC/VLRs and serves own subset of subscribers. 
         [0015]    Co-locating BSCs and BTSs eliminates the conventional practice of transferring radio resource management procedures over a backhaul that is assumed to be in between a BSC and a BTS. 
         [0016]    From a BSC perspective, the BSC-proxy performs as a single MSC/VLR. The BSC-proxy implements local switching functionality without any support on any MSC/VLR, and enables traffic shaping software to reduce signaling and voice traffic being transferred over a backhaul between BSC-proxy and BSC. The BSC-proxy performs local switching within the complete GERAN extension, regardless which BSCs and MSC/VLRs are involved in controlling call legs being switched. 
         [0017]    Signaling probes provide MSISDN-IMSI pairs to the BSC-proxy to implement local switching. 
         [0018]    In another embodiment, the invention provides a method of transparent GERAN sharing, where a BSC-proxy routes SCCP (signaling Connection Control Part) messages and transactions between BSCs and MSC/VLRs based on IMSIs and TMSIs of subscribers being served. In order to unambiguously correlate a TMSI to a PLMN, the BSC-proxy re-allocates TMSIs locally and keeps TMSI-IMSI knowledge to ensure proper routing. The BSC-proxy replaces signaling parameters like Location Area Identification and Cell Identification, as well as CIC(channel identification code) values. Thus, all BTSs in a given GERAN extension are emitting either a dedicated MCC/MNC or ones from one of the extended network. Functioning that way, the BSC-proxy is accepted by all MSCs involved just as a regular BSC belonging to a corresponding PLMN. 
         [0019]    In another embodiment, the invention provides a method of proving local switching within GERAN extension, served by BSC-proxy, based on IMSI-MSISDN knowledge, where on each mobile-originated (MO) call attempt a calling party number is converted to IMSI, and the mentioned IMSI is charged. For each mobile-terminated (MT) call, the calling party is checked against a list of charged IMSIs, and if found, local switching is implemented either within the location served by BSC or between BSCs. If a MT call to a charged IMSI contains a calling party number, this number is also converted to IMSI and verified to match the MO call leg. 
         [0020]    In another embodiment a method is provided of obtaining IMSI-MSISDN relation by means of GSM mobile application part (MAP) signaling monitoring between MSC/VLRs and HLRs. 
         [0021]    In another embodiment, a method is provided of obtaining IMSI-MSISDN by means of emulating a virtual subscriber, as if being served by BSC-proxy, and making a fake call on behalf of a newly-appeared subscriber after performing Location Update procedure within GERAN extension. Thus, a MT call to a virtual subscriber contains a needed MSISDN as a calling party number. 
         [0022]    In another embodiment, a method is provided of obtaining IMSI-MSISDN by means of getting MSISDN in response of an emulated USSD (unstructured supplementary service data) request on behalf of a newly-appeared subscriber after performing Location Update procedure. 
         [0023]    In another embodiment, a method is provided to start tracking TMSI-IMSI correlation, when BSC-proxy rejects a Location Update attempt, done by an unknown TMSI, to force a subscriber to use IMSI instead. 
         [0024]    In another embodiment, a method Is provided to start tracking TMSI-IMSI correlation by means of initiating a MAP_SEND_IDENTIFICATION procedure to a VLR from a BSC-proxy. 
         [0025]    In another embodiment, a method is provided of canceling a local switching procedure when a lawful interception is applied for any call leg by means of either keeping a list of subscribers being monitored or requesting from a corresponding node a permission for each call to perform local switching. 
         [0026]    In another embodiment, a method is provided of canceling local switching procedure upon request, e.g. when LBS (location-based services) data is requested for any of call legs being involved. 
         [0027]    To provide packet data service, the same radio access network sharing principles as described above for A-interface proxying and multiplexing, can be also be applied for sharing GERAN between several core networks. 
     
    
     
       DESCRIPTION OF DRAWINGS 
         [0028]      FIG. 1  illustrates the block diagram of a system in accordance with the invention. 
           [0029]      FIG. 2  is a flow diagram of a method for processing a Location Update request in a BSC-proxy, when selecting a host PLMN for serving the given subscriber. 
           [0030]      FIG. 3  shows examples of signaling parameters of A-interfaces, that are substituted within BSC-proxy. 
           [0031]      FIG. 4  is a simplified flow diagram of local switching as implemented in BSC-proxy. 
           [0032]      FIG. 5  illustrates local switching within the system of  FIG. 1  for the case when subscribers belong to two different operators-legs of a call being switched are served by two different PLMNs. 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0033]      FIG. 1  is a block diagram of a system in accordance with the invention aimed at extending mobile network coverage for one or several PLMNs  101  in such a way that each PLMN assumes the given GERAN extension to be exclusively under its control and without noticing subscribers belonging to other PLMNs being also extended. 
         [0034]    A BSC-proxy  106  provides a proper option of a GSM A-interface to each PLMN, where A-interface signaling can be carried either over conventional SS7 layers, or SIGTRAN (signaling transport), or as A-over-IP, and media is carried either over conventional TDM or is RTP-packetized (real-time transport protocol) and controlled by means of MGCP (media gateway control protocol) or MEGACO (media gateway control). For each MSC/VLR  102  being coupled , the BSC-proxy is seen as a regular BSC that is exclusively controlled. 
         [0035]    BSC-proxy  106  provides an A-over-IP signaling interface to BSCs  107  with media being shaped by traffic shaping software  109 . BSC-proxy  106  is seen by BSC  107  as a regular MSC/VLR, and the traffic shaping software  109  exposes RTP+MGCP to BSC  107 . 
         [0036]    To perform in such a way, BSC-proxy modifies signaling parameters as shown on  FIG. 3 . During setting up any SCCP connection from BSC  107  to BSC-proxy, the target PLMN is chosen (either by MCC/MNC form IMSI or by a TMSI as described below) as seen on  FIG. 2 . The BSC-proxy keeps knowledge about SCCP Local References at both sides, ie at BSC ( 107 )-BSC-proxy ( 106 ) and BSC-proxy ( 106 )-MSC/VLR ( 102 ) interfaces, as well as relations between them, and substitutes SCCP local references when transferring messages between BSCs  107  and MSC/VLRs  102 . 
         [0037]    As TMSIs are allocated by each MSC/VLR  102  independently, BSC-proxy  106  re-allocates TMSIs towards BSCs  107  itself in such a way, that a newly re-allocated TMSI unambiguously defines PLMN  101 , where the given subscriber is to be served. Knowledge about TMSI relation at both sides of BSC-proxy  106  is stored and updated. BSC-proxy  106  replaces TMSIs when transferring signaling messages between BSCs  107  and MSC/VLRs  102 . 
         [0038]    During initial Location Update procedure, BSC-proxy  106  stores the relation between IMSI and TMSI and updates it. If the initial Location Update is performed by IMSI, IMSI-TMSI relation is obtained by itself. If the Location Update is performed by TMSI, and the given TMSI is not allocated by BSC-proxy  106 , BSC-proxy rejects Location Update and forces a subscriber to initiate Location Update with IMSI. As an alternative, if MSC/VLR is known, TMSI can be translated to IMSI with MAP_SEND_IDENTIFICATION procedure. 
         [0039]    To make GERAN extension  105  native for any PLMN  101 , BSC-proxy  106  substitutes network-specific BSSAP parameters like Location Area Identification and Cell Identification. 
         [0040]    Each BSC  107  maps traffic channels, at BTS  108  being served, to CIC values and media endpoints. BSC-proxy  106  converts media and performs voice transcoding, if needed, towards MSC/VLRs, modifies CIC values within BSSAP messages and routes RTP media. 
         [0041]    BSC-proxy  106  is capable of performing local switching without any impact on A-interfaces towards MSC/VLRs that are controlling call legs, even when a call involves subscribers being served by different PLMNs  101 , as seen on  FIG. 5 . From any of PLMNs&#39;  101  perspective, its subscriber is served in GERAN extension  105 , by a corresponding MSC/VLR  102 , and the second party is connected over public telephone network (PSTN)  120 . In fact, BSC-proxy  106  detects that both call legs are appeared to be within the same GERAN extension, and media endpoints are cross-connected between Traffic Shaping Software  109  of respective sites. Both calls are billed independently in the respective PLMNs: one PLMN  101  accounts MO call from its subscriber within GERAN extension  105  to outside, the second PLMN  101  receives off-net incoming call to a subscriber within GERAN extension  105 . 
         [0042]    The algorithm of detecting call legs to be cross-connected is shown on  FIG. 4 . An actual database with MSISDN-IMSI relations is available for BSC-proxy  106 . When an outgoing call  400  is placed from GERAN extension  105 , the calling party number is converted to IMSI  401 . If local switching capabilities are permitted to both parties  402  (ie if lawful interception procedure is not expected), the mentioned IMSI is charged  403 . Call setup continues in a regular way over a chosen PLMN  101  as described above. 
         [0043]    When a mobile terminated call is delivered  405  to GERAN extension  105 , and the IMSI being called is marked as charged  407 , and a calling party number, converted to IMSI, matches a corresponding MO call, local switching procedure is performed—media endpoints are interconnected at TSS  109 . 
         [0044]    Thus, keeping knowledge about IMSI-MSISDN correlation is a must for the given algorithm of local switching implementation. 
         [0045]    As a preferred non-intrusive method, a Probe  104  passively monitors SS7 MAP signaling traffic, fetches IMSI-MSISDN pairs and supplies them to BSC-proxy  106  to be stored. 
         [0046]    The alternative method of obtaining IMSI-MSISDN knowledge is requesting MSISDN my means of USSD requests: such a technological USSD request is supported by default on most of GSM networks. In the context of the given invention, BSC-proxy  106  emulates USSD request on behalf of a subscriber after Location Update procedure, if MSISDN for the given IMSI is unknown. On USSD response, MSISDN-IMSI pair is stored to be available for further processing. 
         [0047]    Another method of obtaining MSISDN-IMSI correlation is emulating a Virtual subscriber as if it is served within GERAN extension  105 . When a subscriber with unknown MSISDN performs Location Update, BSC-proxy emulates a call on behalf of the given subscriber to a Virtual one, thus BSC-proxy receives MSISDN of the subscriber as a calling party number when MT call to a Virtual subscriber is delivered back to GERAN extension  105 . MSISDN-IMSI pair is stored for further processing. 
         [0048]    When local switching is applied within GERAN extension  105 , the actual voice traffic is not delivered to PLMN  101 , while PLMN assumes that voice traffic to be available for Lawful Interception. To cancel local switching procedure and route traffic in a conventional way, ie through MSC/VLR  102 , one of the methods below can be applied. BSC-proxy can either keep the list of IMSIs being monitored locally, or request a permission from a Lawful Interception module to perform local switching on per-call basis. 
         [0000]    As an alternative method, local switching can be canceled e.g. if data for Location-Based Service (LBS) is being requested for a given subscriber. 
       GLOSSARY OF TERMS USED IN THE SPECIFICATION 
       [0000]    
       
         BSSAP Base Station Subsystem Application Part 
         BSC Base station controller 
         BTS Base transceiver station 
         CIC C Channel Identification Code 
         CFU Call forwarding unconditional 
         CS Circuit switched 
         GERAN Global system for mobile communications/EDGE radio access network 
         GPRS GSM packet radio service 
         GSM Global system for mobile communication (formerly: groupe speciale mobile) 
         HLR Home location register 
         HSS Home subscriber server 
         IMSI International mobile subscriber identity 
         IP Internet Protocol 
         LBS Location Based Services 
         MAP Mobile application part 
         MCC Mobile Country Code 
         MNC Mobile Network Code 
         MSC Mobile Switching Center 
         MO Mobile originated 
         MGCP Media Gateway Control Protocol 
         MEGACO Media Gateway Control 
         MSC Mobile switching center 
         MT Mobile terminated 
         MSISDN Mobile station integrated services digital network number 
         NGN Next generation network 
         PLMN Public Land Mobile Network 
         PRN Provide roaming number 
         PSTN Public Switched Telephone Network 
         RAN Radio access network 
         RTP Real Time Transport Protocol 
         SCCP Signaling Connection Control Part 
         SIGTRAN Signaling Transport 
         SIM Subscriber identification module 
         SS7 Signaling System N27 
         TDM Time division multiplex 
         TMSI Temporary mobile subscriber identity 
         TSS Traffic Shaping Software 
         USSD Unstructured supplementary service data 
         VLR Visitor Location Register