Description of Related Art With the advent and development of mobile telecommunications systems, telecommunications users are no longer physically bound to wireline terminals or fixed locations for telecommunications network communications. Using the added capabilities of roaming and interoffice handoffs, mobile subscribers may travel between multiple Public Land Mobile Networks (PLMNs) utilizing the same telephone number and the same mobile station to originate outgoing calls and to receive incoming calls.

Thus, a mobile subscriber may access telecommunications service from multiple locations utilizing multiple service providers with any incurred fees being charged to a single subscription. Furthermore, calling parties do not necessarily have to know where the mobile subscriber is physically located in order to establish a call connection. Data signals between visitor location registers (VLRs) and a home location register (HLR) automatically update and store subscriber information, in particular subscriber location information, enabling the network to reroute incoming calls to the appropriate mobile switching center (MSC) serving the roaming mobile subscriber. Furthermore, regardless of which MSC is currently serving the mobile station, the VLR associated with the serving MSC communicates with the HLR assigne to the mobile station to retrieve the requisite subscriber data, including subscriber feature data and billing data, and provides uniform mobile service to the mobile station.

Conventionally, if a mobile station associated with a New York (N. Y.) mobile telecommunications network is traveling in Los Angeles (L. A.), an incoming call towards the roaming mobile station is first routed to the gateway mobile switching

center (GMSC) serving the N. Y. public land mobile network (PLMN) associated with the mobile station. Thereinafter, the N. Y. GMSC performs an interrogation with the home location register (HLR) associated with the mobile station to determine the current location of the roaming mobile station. Upon determining the identity of the L. A. mobile switching center (MSC) currently serving the N. Y. mobile station, the received incoming call is rerouted by the N. Y. GMSC to the serving L. A. MSC to enable a speech connection between the calling party terminal and the roaming mobile station.

Accordingly, if the calling party is originating a call setup signal from L. A., a first trunk call connection between the L. A. telecommunications network and the GMSC serving the home PLMN is initially established and then a second trunk call connection between the GMSC back to the L. A. MSC currently serving the mobile station is established. As a result, a"tromboning"of trunk call connections through N. Y. is performed in order to establish a call connection between the two telecommunications subscribers both located in L. A.

Such"tromboning"trunk call connections are disadvantageous for a number of reasons. First, unnecessary trunk connections have to be established and maintained in order to enable two subscribers who are locally located to communicate with each other and result in unnecessary seizure of valuable network resources. Additionally, even though the called party mobile station is receiving an incoming call from a calling party terminal located within the same local access transport area (LATA), such as L. A., the called party mobile station has to incur long distance charges for forwarding the call connection from his or her home PLMN back to the visited PLMN Furthermore, the calling party also has to incur long distance charges to N. Y. for calling another mobile station located within the same area.

Accordingly, there is a need for a mechanism to enable the serving mobile telecommunications network to perform optimal routing of a mobile call towards a roaming mobile station.

It is, therefore an object of the present invention to provide a system, method, and apparatus that eliminates or minimizes tromboning.

It is also an object of the present invention to optimize routing of an incoming call to a mobile phone, particularly a roaming mobile phone.

SUMMARY OF THE INVENTION The present invention is directed to a system, method, and apparatus for optimally routing telephone calls from a calling party telephone terminal to a roaming mobile station. The call from the calling party telephone terminal is routed to a redirecting node. In one embodiment, the redirecting node routes the call to the Home Public Land Mobile Network (H-PLMN) serving the mobile station. At the H-PLMN, the home location register (HLR) is queried to determine the location of the mobile station and the mobile switching center (MSC) serving the mobile station. The foregoing information is transmitted backwards to the redirecting node, which uses the information to determine an alternate route. The redirecting node routes the call to the serving MSC and releases the connection from the redirecting node to the H-PLMN In another embodiment, the redirecting node queries the HLR, determines the location and serving MSC for the mobile station, and routes the call to the serving MSC.

BRIEF DESCRIPTION OF THE DRAWINGS A more complete understanding of the method and apparatus of the present invention may be had by reference to the following detailed description when taken in conjunction with the accompanying drawings wherein: FIGURE 1 is a block diagram of a mobile telecommunications network; FIGURE 2 is a block diagram of a mobile telecommunications network illustrating the routing of an incoming call to a destination mobile station ; FIGURE 3 is a block diagram of a telecommunications network illustrating the tromboning of trunk connections to establish a call connection between a calling party telephone terminal and a roaming called party mobile station; FIGURE 4 is a block diagram of a telecommunications network configured in accordance with the present invention ; and FIGURE 5 is a signal flow diagram describing the operation of the telecommunications network illustrated in FIGURE 4 in accordance with a first embodiment of the present invention ; and

FIGURE 6 is a signal flow diagram describing the operation of the telecommunications network illustrated in FIGURE 4 in accordance with a second embodiment of the present invention.

DETAILED DESCRIPTION OF THE DRAWINGS The numerous innovative teachings of the present application will be described with particular reference to the presently preferred exemplary embodiments. However, it should be understood that this class of embodiments provides only a few examples of the many advantageous uses of the innovative teachings herein. In general, statements made in the specification of the present application do not necessarily delimit any of the various claimed inventions. Moreover, some statements may apply to some inventive features but not to others.

Reference is now made to FIGURE 1 wherein there is shown a plurality of exemplary wireless networks, such as a Public Land Mobile Network (PLMN) 105, e. g., PLMNs 105a, 105b. TheparticularPLMN 105utilizedmayinclude, forexample, an Advanced Mobile Phone Service (AMPS) network, a Digital Advanced Mobile Phone Service (D-AMPS) network, or a Global System for Mobile Communication (GSM). As is well understood in the art, a particular PLMN 105 is composed of a plurality ofMobile Switching Center/Visitor Location Register (MSC/VLR) areas 115, e. g., MSC/VLR areas 115a and 115b, each with a Mobile Switching Center (MSC) 120, i. e., MSC 120a and 120b, respectively, and a Visitor Location Register (VLR) 121, e. g., VLR 121a and VLR 121b. Each MSC 120 is connected to a GatewayMSC (GMSC) 127 which connects the PLMN 105 to a public switched telephone network/integrated services digital network (PSTN/ISDN) 129.

An MSC/VLR area 115 is a defined area in which a mobile station (MS) (terminal) 135 may move freely without having to send location update information to the MSC 120 that controls that MSC/VLR area 115. Mobile Station (MS) 135 is the physical equipment, for example a car phone or other portable phone, used by mobile subscribers to communicate with the cellular network 105, each other, and users outside the subscribed network, both wireline and wireless. Each MSC/VLR Area 115 is divided into a number of cells 138. The MSC 120 is in communication with at least

one base station 150. The base station 150 is the physical equipment, illustrated for simplicity as a radio tower, that provides radio coverage to the geographical part ofthe cell 138 for which it is responsible.

With further reference to FIGURE 1, each PLMN 105 includes a Home Location Register (HLR) 126, e. g., HLR 126a in PLMN 105a and HLR 126b in PLMN 105b, which is a database maintaining and storing subscriber information such as subscriber profiles, current location information, International Mobile Subscriber Identity (IMSI) numbers, and other administrative information. The subscriber services associated with the mobile station 135 are defined in a subscriber profile that is stored in the HLR 126. The HLR 126 may be co-located with a given MSC 120, integrated with the MSC 120, or alternatively can service multiple MSCs 120, the latter of which is illustrated in FIGURE 1.

The VLR 121 is a database containing information about all of the MSs 135 currently located within the MSC/VLR area 115. If an MS 135 moves from MSC/VLR area 115ato anew MSC/VLR area, for example, area 115b, the VLR 121b in MSC/VLR area 115b will request data about that MS 135 from the HLR database 126 (simultaneously informing the HLR 126 about the current location ofthe MS 135), for example, HLR 126b. Accordingly, if the user of the MS 135 then wants to make a call, VLR 121b in MSC/VLR area 115b has access to the requisite service profile information without having to reinterrogate the HLR 126. In the previously described manner, the VLR and HLR databases 121 and 126, respectively, contain various subscriber information associated with a given MS 135.

The key advantage of subscribing to a cellular network, such as a PLMN, is the mobility of the communication service. A subscriber subscribing to a particular PLMN, e. g., PLMN 105a, can make a phone call from anywhere within the geographic area of the PLMN 105a, e. g., areas 115 and area 115a. The geographic area covered by a particular PLMN 105a generally covers the metropolitan area of a large city or even a sizeable portion of a state or province, thereby permitting the subscriber a substantial degree of mobility. However, subscribers traveling to a different country or across the United States are frequently beyond the range of their PLMN 105a. To provide service to the subscriber, the operators of a PLMN 105b serving the

geographic area, i. e., area 115b, where the subscriber is located (now referred to as the Visited PLMN or V-PLMN), permit the subscriber to use the resources and infrastructure of the V-PLMN 105b, as if it were their home PLMN 105a. In this manner, roaming is supported.

FIGURE 2 is a block diagram of a telecommunications network illustrating a mobile telecommunications network routing an incoming call to a destination mobile station 135. Whenever a telecommunications subscriber (mobile or wireline) originates a call connection by dialing a directory number, such as a Mobile Subscriber Integrated Service Digital Network (MSISDN) number, associated with the mobile station 135, a call setup signal 220, such as an Initial Address Message (IAM), is received by the gateway mobile switching center (GMSC) 127 serving the public land mobile network (PLMN) 105 associated with the destination mobile station 135. When the IAM signal 220 is received by a particular telecommunications node, such as the GMSC 127, a corresponding call connection (i. e., trunk call connection) is further established between the originating telecommunications node and the receiving telecommunications node.

Because the mobile station 135 is not restricted to one particular location or PLMN 105, the GMSC 127 receiving the IAM signal needs to interrogate the home location register (HLR) 126 associated with the mobile station 135 to determine the current location of the mobile station 135. Accordingly, a signal 260 requesting routing instruction is transmitted from the GMSC 127 to the HLR 126. The HLR 126, in turn, determines the identity of the mobile switching center (MSC) 120 currently serving the mobile station 135 and further transmits a signal 280 requesting a roaming number to the identified MSC 120. With the help of subscriber data stored at an attached visitor location register (VLR) 121, the serving MSC 120 identifies the called mobile station 135 traveling within its coverage area and, as a result, returns the roaming number representing the called mobile station located within the serving MSC 120 to the HLR 126 (signal 200). The HLR 126 then forwards the received roaming number back to the GMSC 127 (signal 205). By utilizing the received roaming number identifying the serving MSC 120, the GMSC 127 reroutes the received call setup signal 220 to the serving MSC 120. Accordingly, a call connection 210, such as a trunk

connection, is established from the GMSC 127 to the serving MSC 120. Once the rerouted call setup signal is received by the serving MSC 120, the mobile station 135 is paged and a call connection 221 over a radio channel between the mobile station 135 and the serving MSC 120 is established.

Due to such HLR interrogation and rerouting of the incoming call by a serving GMSC, inefficient and wasteful call connections are often times established towards a roaming mobile station. For example, if MS 135 subscribes to PLMN 105a and has roamed to PLMN 105b and is served by MSC 120b, an incoming call to MS 135 is first routed to GMSC 127 of PLMN 105a and is then rerouted to the MSC 120b in PLMN 105b serving the MS 135. The foregoing is known in the art as"tromboning." Reference is now made to FIG. 3 illustrating the tromboning of trunk connections within a telecommunications network to establish a call connection between a calling party terminal 305 and a roaming called party mobile station 135.

The called party mobile station 135 is associated with PLMN 105a (now referred to as H-PLMN) and is currently roaming within PLMN 105b (now referred to as V- PLMN). When telephone terminal (either wireless or wireline, the latter of which is shown) 305 proximately located with the V-PLMN 105b, originates an outgoing call connection towards the called party mobile station 13 5, a call setup signal is originated by a telecommunications switch 310 serving the calling party telephone terminal 305.

The originated call setup signal, such as an IAM signal, is then routed through the connected PSTN/ISDN 129 and delivered to the GMSC 127 serving the H-PLMN 105a. Accordingly, a first trunk call connection, as illustrated by a solid line 320, is established between the telecommunications switch 310 and the GMSC 127. As described above in connection with FIG. 2, the GMSC 127 then performs HLR interrogation by transmitting a connection-less signal 360 towards the HLR 126. In order to retrieve a network number that can be utilized by the GMSC 127 to reroute the incoming call, the HLR 126 requests a roaming number from the MSC serving MS 135, e. g., MSC 120b. Because the MSC 120b serving the called party mobile station 135 is located within the V-PLMN 105b, a Mobile Application Part (MAP) based signal 380 is sent from the HLR 126 to MSC 120b. The MSC 120b then returns the roaming number back to the HLR 126 via another MAP based signal 300. The

received MAP based signal is then transmitted back to the GMSC 127 via signal 305.

Utilizing the received roaming number identifying MSC 120b as the MSC currently serving the mobile station 135, the GMSC 127 reroutes the call setup signal to MSC 120b. Such a rerouting creates a second trunk call connection 311 between the GMSC 127 and MSC 120b.

If the H-PLMN 105a is located in New York, while the V-PLMN 105b is located in Los Angeles, a speech connection between the calling party telephone terminal 305 in Los Angeles and the called party mobile station 135 also in Los Angeles, via the GMSC 127 located in New York is established using the first call connection 320 and the second call connection 311. Such a tromboning of trunk call connections is conventionally unavoidable because the current location of the called party mobile station 135 cannot be determined until the call setup signal reaches the GMSC 127. Unfortunately, tromboning of trunk call connections are wasteful and inefficient. It further forces mobile subscribers to incur additional fees for rerouting incoming calls from their H-PLMN 105a to the V-PLMN 105b. Furthermore, it forces calling parties to incur unnecessary long distance charges.

Referring now to FIGURE 4, there is illustrated a block diagram of an exemplary telecommunications system 400 embodying the present invention. The telecommunications system 400 includes H-PLMN 105a, a V-PLMN 105b, a PSTN/ISDN 129, and a calling party network 460. The calling party network 460 can either comprise a portion of the PSTN/ISDN 129 or a wireless network, such as a PLMN 105.

To better route a phone call from calling party telephone terminal 305 to the called party mobile station 135, the present invention proposes the use of a redirecting node 405. The redirecting node 405 can be integrated with the switch 310 serving the calling party telephone terminal 305, as well as at a tandem switch 410 within the PSTN/ISDN 129, the latter of which is illustrated. During a phone call from calling party telephone terminal 305 to mobile station 135 the redirecting node 405 determines whether an alternate route can be used to connect the call.

In one embodiment, the redirecting node 405 establishes a call to the GMSC 127 in H-PLMN 105a during which an indicator indicating that the node establishing

the call is a redirecting node. When GMSC 127 determines the location of the MSC serving the called party MS 135, e. g. MSC 120b, by querying HLR 126, the information can transmitted backwards to the redirecting node 405. Where the mobile station 135 is roaming, the redirecting node 405 uses the information to determine a more favorable alternate route. After the redirecting node 405 determines a more favorable alternate route, the redirecting node 405 redirects the call to the serving MSC 120b. When the redirecting node 405 redirects the call, the redirecting node 405 can release the call, thereby eliminating an unnecessary connection to H-PLMN 105a.

In a second embodiment, the redirecting node 405 determines the H-PLMN 105a to which MS 135 subscribes, queries the HLR 126 associated with H-PLMN 105a to determine the location of the MSC 120b serving the MS 135, and where the mobile station 135 is roaming, the redirection node 405 uses the information to determine a more favorable alternate route. After the redirecting node 405 determines a more favorable alternate route, the redirecting node 405 redirects the call to the serving MSC 120b. When the redirecting node redirects the call, the GMSC 127 is bypassed, thereby eliminating an unnecessary connection to H-PLMN 105a.

Referring now to FIGURE 5, there is illustrated a signal flow diagram illustrating the operation of telecommunications system described in FIGURE 4 in accordance with a first embodiment of the present invention. A telephone call is originated from calling party telephone terminal 305 to the called party mobile station 135 (signal 505). The telephone call is received at the telecommunications switch 310 serving the calling party telephone terminal 305. The telecommunications switch 310 proceeds to route the call towards GMSC 127 using the PSTN/ISDN 129 (signal 510).

The telephone call is received at a redirecting node 405 which proceeds to direct the call to the GMSC 127 in the H-PLMN 105a (signal 515).

The GMSC 127 interrogates the HLR 126 to determine the geographic location of the called party mobile station 135 (signal 520). Because the called party mobile station 135 has roamed outside the H-PLMN 105a, the reply (signal 525) from the HLR 126 indicates that the called party mobile station 135 has roamed outside the H- PLMN 105a, and is served by MSC 120b. The GMSC 127 transmits the information received from the HLR 126 backwards towards the redirecting node 405 (signal 530).

The redirecting node 405 uses the information to determine an alternate route. After determining an alternate route, the redirecting node 405 directs the call to the serving MSC 120b (signal 535). When the redirecting node 405 directs the call to the serving MSC 120b, the redirecting node 405 also releases the phone call (signal 515), and the pre-existing leg from the redirecting node 405 to the GMSC 127 is thereby disconnected. The serving MSC 120b completes the call to the called party mobile station 135 (signal 545).

Referring now to FIGURE 6, there is illustrated a signal flow diagram illustrating the operation of telecommunications system described in FIGURE 4 in accordance with a second embodiment of the present invention. A telephone call is originated from calling party telephone terminal 305 to the called party mobile station 135 (signal 605). The telephone call is received at the telecommunications switch 310 serving the calling party telephone terminal 305. The telecommunications switch 310 proceeds to route the call towards GMSC 127 using the PSTN/ISDN 129 (signal 610).

The telephone call is received at a redirecting node 405 which proceeds to interrogate the HLR 126 to determine the geographic location of the called party mobile station 135 (signal 615). Because the called party mobile station 135 has roamed outside the H-PLMN 105a, the reply (signal 620) from the HLR 126 indicates that the called party mobile station 135 has roamed outside the H-PLMN 105a, and is served by MSC 120b. The redirecting node 405 uses the information received from the HLR 126 to determine an alternate route. After determining an alternate route, the redirecting node 405 directs the call to the serving MSC 120b (signal 625), which completes the call to MS 135 (signal 630).

Although preferred embodiments of the method and apparatus of the present invention have been illustrated in the accompanying Drawings and described in the foregoing Detailed Description, it will be understood that the invention is not limited to the embodiments disclosed, but is capable of numerous rearrangements, modifications, and substitutions without departing from the spirit of the invention.

Therefore, the invention is limited only by the following claims and their equivalents.