Abstract:
In a medium earth orbit (MEO) satellite system, an embodiment of the present invention employs a system and method for administering mobility management issues for mobile terminals, via a network of base station controllers. The mobility management issues, for example, paging, routing, and handover of communication signals are administered between the base station systems without having to be routed through the serving general packet radio service support node (SGSN).

Description:
[0001]    The present invention claims benefit under 35 U.S.C. section 119(e) of a provisional U.S. Patent Application of Channasandra Ravishankar et. al., entitled ICO Packet Data Mobility Management and Handover, Serial No. ______ , filed Aug. 9, 2000, the entire contents of said provisional application being incorporated herein by reference. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    The present invention relates generally to a system and method for administering mobility management for mobile terminals (e.g. registration, handover, paging and routing), within a satellite network, such as a medium earth orbit (MEO) satellite network. More particularly, the present invention relates to a system and method employing a plurality of MEO satellites, for example, within a Global System for Mobile Communications-General Packet Radio Service (GSM-GPRS) network that enables management of communication signals between multiple base station controllers.  
           [0004]    2. Description of the Related Art  
           [0005]    A satellite-based communications network, such as a telephony network or data transmission network, typically employs at least one base station, one or more satellites, such as a medium earth orbit (MEO) satellite, and a plurality of mobile terminals, such as remote telephones or data terminals. In a typical network, data is transmitted between the mobile terminals and base stations in the form of packets or frames, as can be appreciated by one skilled in the art. The satellite network, as described herein, shares a similar protocol process with the GSM-GPRS standard. Accordingly, this allows compatibility between the satellite, cellular and the Public Switched Telephone Network (PSTN) systems. However, the satellite air-interface poses physical constraints not accounted for in the basic terrestrial GSM-GPRS architecture.  
           [0006]    The GSM-GPRS network permits transmission and reception of data and voice information across a mobile terminal network, wherein the mobile terminals comprise remote telephones and data terminals, as described above. In addition the mobile terminals transmit and receive signaling information corresponding to voice and data communication services. The GSM-GPRS network facilitates instant connections between mobile terminals and a fixed network, for example an internet service provider (ISP), and between two mobile terminals, thereby no longer requiring a dial-up modem or other physical connection. In addition, the GSM-GPRS network employs packet switching, thus ensuring spectrum efficiency. Accordingly, system resources are employed only when sending and receiving information, therefore, allowing users to share network resources, as can be appreciated by one skilled in the art.  
           [0007]    One disadvantage of employing terrestrial GSM-GPRS protocol for a non-geosynchronous satellite system relates to the terrestrial protocol&#39;s static nature. The association between cells corresponding to radio coverage areas remain fixed in a terrestrial network. Accordingly, a mobile terminal (MT) in a terrestrial GSM-GPRS system is associated uniquely with a geographical area (e.g., a cell) on the earth, depending on the radio frequency and the associated base transceiver station (BTS), that the MT employs to communicate with the base station. In a Medium Earth Orbit (MEO) satellite environment, multiple spot beams from different satellites are projected on an MT as the MEO satellites move with respect to the surface of the earth. Multiple base stations controllers (BSC) control the multiple spot beams. Furthermore, the initial BSC may relinquish control of the MT to another BSC as the satellites orbit and the spot beams change location despite the static position of the MT. Therefore, the MT may communicate with multiple BSCs due to the satellites&#39; movement, despite the MT remaining in the same geographic position. In addition, multiple network elements correspond to varying radio coverage areas. Accordingly, a Serving GPRS support node (SGSN) services multiple radio coverage areas. Therefore, a need exists for a management system for the dynamic elements (e.g. BSC, spot beams) within a MEO satellite communication system.  
           [0008]    In the GSM-GPRS network, geographical coverage areas of the radio services spectrum are divided into cells. The cells are further regrouped into routing areas for providing and managing network services. By contrast, within the MEO environment, a network operator divides geographical coverage locations into Service areas. Multiple service areas form service regions. The network operator assembles the above mentioned groupings based on service logic (e.g., a base station associated with a corresponding SGSN (Serving GPRS Support Node) which in turn services a particular area) and international regulations, as well as radio spectrum planning. Accordingly in a terrestrial GPRS system, unless the MT moves, data to and from the MT is routed to and from the SGSN to which the base station (communicating with the MT) is attached, in a conventional manner. By contrast, in a MEO system, even when the MT does not move, it may be necessary to route data to and from the MT as well as to and from an SGSN that is not directly attached to the base station with which the MT is communicating due to the service logic and international regulations. Therefore, a need exists for a method for managing signal routing within a MEO satellite communications network that accounts for service logic, and regulatory requirements.  
           [0009]    A single BSC also administers paging of an MT within a GSM-GPRS network. Paging is performed based on Location Area (LA) for circuit switched services and Routing Area (RA) based on packet switched services, in a conventional manner. However, this type of paging is only adequate if the MT utilizes only one BSC for all of its communication. In such a case, the static relationship between the MT and the BSC is critical since the BSC contains the relevant information in order to page another MT. However, in the MEO satellite communications network, this relationship is not at all static, but on the contrary, is dynamic, as mentioned above. Therefore, a need exists for a paging method within a MEO satellite communications network that accounts for the changing relationship between one MT and a plurality of BSCs.  
           [0010]    [0010]FIG. 1 is an overview of a conventional GSM-GPRS communications network  14 . It is important to note the GSM-GPRS communications network does not employ any type of satellite and is therefore a terrestrial network.  
           [0011]    Accordingly, various cells  15 ,  16  and  17  correspond to unique base trans-receiver sub-systems (BTS)  21 ,  22 , and  23 . These relationships between the cells  15 ,  16  and  17  and the corresponding BTSs  21 ,  22  and  23  are static. Therefore, as the MT  24  changes geographic position from cell  15  to cell  16 , the MT  24  communicates with an associated BTS  21 ,  22  and  23  based upon the MT&#39;s  24  cell location. Thus, the MT  24  is the only dynamic component of the GSM-GPRS communications network  14 , whereas the cells  15 ,  16 , and  17  and the BTSs  21 ,  22  and  23  maintain a static association.  
           [0012]    A BTS, such as BTS  21 , controls transmission for an MT  24  within its cell  15 . In addition, the BTS  21  communicates with a base station controller (BSC)  35 , that is associated with a unique support GPRS serving node (SGSN)  31 . The BTS  21 , and BSC  35  collectively form a Base Station System. Accordingly, an MT  24  within a cell  15 , corresponding to a unique geographic area, communicates via an associated Base station system (BSS)  36 . The BSS  36  comprises a BSC  35  and a BTS  21 . The BSS  36  thus corresponds to a unique SGSN  31 . This implies that there is a one to one mapping between a BSS and a SGSN.  
           [0013]    A conventional terrestrial GPRS network faces various issues that the network is unable to overcome, within a MEO satellite environment, for example. Among the issues are, the fact that regulatory restrictions requiring data originating from a particular region to pass through a specified SGSN, and the radio resources from different satellites are controlled by different base stations physically located in varying regulatory jurisdictions. The issues further comprise the dynamic nature of non-geostationary satellites and their associated spot beams despite the MT remaining static, and the MT&#39;s location is illuminated by spot beams from various satellites, as well as the MT&#39;s ability to communicate with any of the visible satellites within the network. Therefore a need exists for a network between base stations to facilitate routing communication signals between the base stations, implying a one-one mapping between the base stations and the SGSN.  
         SUMMARY OF THE INVENTION  
         [0014]    An aspect of the present invention is to provide a system and method which enables efficient communication, via an inter-base station controller network, between a plurality of mobile terminals within a communications network.  
           [0015]    Another aspect of the present invention is to provide a doublet comprising &lt;TLLI, SGSN-ID&gt; in order to uniquely identify the user and allow the BSC to route data packets from the MT to the registered SGSN, based upon this doublet.  
           [0016]    Another aspect of the present invention is to provide a routing table to facilitate communication between an anchor base station controller (BSC) and a mobile terminal (MT), as well as its associated radio BSC in a communications network. In order to meet certain regulatory requirements in many systems.  
           [0017]    Another aspect of the present invention is for the MT to assist the BSC in routing packets based on the geographical position of the MT or the MT location previously registered with the SGSN.  
           [0018]    Yet another aspect of the present invention is to provide a system and method for the SGSN to transmit to the MT its position as well as the SGSN ID with which the MT is registered.  
           [0019]    A further aspect of the invention is to provide a system and method to deny service based on geographical position of the MT or its International Mobile Subscriber Identity (IMSI).  
           [0020]    An additional aspect of the invention is to provide a system and method that will efficiently page the MT depending on whether the MT is in MM-Ready state or MM-Standby state.  
           [0021]    Another aspect of the invention is to provide a signaling mechanism between base stations that will allow databases with in the BSC to be cleared when an MT performs a Routing Area Update due to the MT&#39;s movement outside its initial routing area.  
           [0022]    Finally an aspect of the invention is to provide a system and method of storing a list of Routing Area Identities (RAI) that will assist the MT in a MEO satellite environment to determine when a Routing Area Update is necessary—such a scheme significantly reduces the satellite spectrum usage.  
           [0023]    These and other objects are substantially achieved by a system and method for communicating between a plurality of base station controllers within a communications network. One of the base station controllers, corresponding to a support node identifier, processes a communication signal from one of a plurality of mobile terminals accessing the communications network. The mobile terminals are located within a geographic area. The base station controller receives the communication signal from a mobile terminal, and frames the communication signal with a support node identifier bit.  
           [0024]    The above objects can further be substantially achieved by a method and apparatus for communicating between a plurality of base station controllers within a communications network. One of the base station controllers corresponding to a support node identifier processes a communication signal from one of a plurality of mobile terminals accessing the communications network. The mobile terminals are located within a geographic area. The method and apparatus employ a plurality of base station controllers corresponding to a plurality of mobile terminals and an interbase station controller network, to enable the network to facilitate high speed communication between the mobile terminals.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0025]    These and other objects, advantages and novel features of the invention will be more readily appreciated from the following detailed description when read in conjunction with the accompanying drawings, in which:  
         [0026]    [0026]FIG. 1 depicts an overview of a conventional GSM/GPRS Cellular Network;  
         [0027]    [0027]FIG. 2 depicts an overview of a MEO satellite communications network arranged in accordance with an embodiment of the present invention;  
         [0028]    [0028]FIG. 3 depicts a more detailed block-diagram illustration of the MEO satellite communications network shown in FIG. 2, having connectivity between multiple Base Station Controllers arranged in accordance with an embodiment of the present invention;  
         [0029]    [0029]FIG. 4 demonstrates multiple Base Station Systems&#39; spot beams projected upon a geographical area controlled by a single SGSN in accordance with an embodiment of the present invention;  
         [0030]    [0030]FIG. 5 illustrates one Base Station Systems&#39; spot beam projected upon multiple geographic areas controlled by multiple SGSNs in accordance with an embodiment of the present invention;  
         [0031]    [0031]FIG. 6 is a ladder diagram depicting an example of the communication between an SGSN, mobile switching center, a mobile terminal, and multiple base station controllers in accordance with an embodiment of the present invention;  
         [0032]    [0032]FIG. 7 depicts a packet data unit with a logical link controller and associated fields, in relation to a ladder diagram illustrating communication between the mobile terminal, the base station controller, and the support GPRS node in accordance with an embodiment of the present invention;  
         [0033]    [0033]FIG. 8 illustrates a flow diagram of the network updating a Mobile terminal&#39;s position in accordance with an embodiment of the present invention;  
         [0034]    [0034]FIG. 9 depicts a scenario in which Routing Area (RA) Update procedures are executed for the MEO satellite communications network and the associated information relating to a particular user employing radio and anchor base stations in accordance with an embodiment of the present invention;  
         [0035]    [0035]FIG. 10 illustrates other elements in the network such as the Mobile Switching Center/Visitor Location (MSC/VLR) which assists in mobility management in accordance with an embodiment of present invention;  
         [0036]    [0036]FIG. 11 depicts the MT switching from one satellite to another satellite, due to local blockage conditions and therefore switching satellite access gateways constructed in accordance with an embodiment of the present invention; and  
         [0037]    [0037]FIG. 12 depicts a satellite beam movement with reference to time during a routing area update in accordance with an embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0038]    [0038]FIG. 2 is an overview of a MEO satellite-based communications network  5  employing an embodiment of the present invention. The network  5  comprises at least two satellites  10  and  20 , multiple base trans-receiver systems (BTS)  50 ,  60  and  70 , multiple satellite access gateways (SAG)  80 ,  90 ,  100 , a public data network (PDN)  110 , and various user terminals, for example a mobile telephone  120  and a laptop  130 . As the MEO satellites  10  and  20  orbit the earth they project spot beams  30  and  40  onto various geographic regions of the earth, as determined by the satellite orbit. Communication between the various elements, for example the BTS, SAG, and the public Data Network (PDN), is accomplished via a packet switched data transmission service that provides for the transmission of data in the form of packets and switches data at the packet level, in a conventional manner.  
         [0039]    When a mobile terminal  120  communicates with another mobile terminal (MT)  130 , or the PDN (e.g. internet), the MT&#39;s  120  and  130  rely on the spot beams  30  and  40  to transmit and receive messages, via antennas  50 ,  60  and  70 , as well as satellite access gateways (SAG)  80 ,  90 , and  100 . Specifically, according to an embodiment of the present invention, a SAG  80  preferably comprises a BSC  350 , an SGSN  360  and a GGSN  370 .  
         [0040]    A more detailed depiction of the SAGs  80 ,  90 , and  100  of FIG. 2 is shown in FIG. 3. The BSC  350  has intelligence to enable it to assist the network in executing handoffs, routing and paging, among other functions. In addition, the SGSN  360  and the GGSN  370  serve as a gateway to route PDUs (packet data units) through to various external networks, such as the PDN  340 .  
         [0041]    It is to be understood that according to another embodiment of the present invention, the SGSN  410  and the GGSN  420  can be considered as one unit. The combined unit interfaces with outside networks, for example the PDN  340 , to facilitate packet data transfers.  
         [0042]    These aspects, in addition to the regulatory requirements, mentioned below, that a user in a particular geographical region communicates with a particular SGSN, requires a fully connected configuration between the BSCs and SGSNs. Therefore, an embodiment of the present invention preferably provides for a high speed inter-BSC network  380  in accordance with an aspect of the present invention as shown in FIG. 3. This network  380  maintains a high speed connection between multiple BSCs  350 ,  360  and  370 . The network  380  allows the MT  120  and  130 , located in separate spot beams  30  and  40  respectively, to communicate via BSCs  350  and  370 , over the network  280 . The details of the communication signaling, for example handover, and paging, are discussed below. In addition, the GSN backbone  400 , and the PDN  340  provide an external gateway (PTSN) to the SGSN  410 , and the GGSN  420 .  
         [0043]    The MEO satellite environment has multiple spot beams, corresponding to multiple satellites, that project onto a mobile terminal, as shown in FIG. 4. A mobile may choose any spotbeam depending on a “goodness” criterion, for example, as determined by the quality of the signal as received by the MT  120  over the broadacast common control channel (BCCH) of a spot beam  40  from the satellite  10 . In addition, multiple base stations control the various spot beams. Furthermore as shown in FIG. 4, the spotbeam  40  is so large that the geographical area covered by a spotbeam  40  is under the control of two different SGSNs  42  and  44 . However, due to various regulatory requirements a specific SGSN may be employed. The regulatory requirements relate to the fact that there are two satellites  10 ,  20  visible to an MT  120  where each satellite is controlled by a different BSC physically located in two different countries. The MT can choose to communicate via any of the two satellites  10 ,  20  depending on a “goodness” criterion, as defined above, and hence any of the two BSCs. However the regulatory requirements provide that regardless of the base station that the MT  120  communicates with, the MT  120  has to communicate with a particular SGSN depending on the geographical location of the MT  120 . For example, if an MT is in the USA and is communicating via a satellite controlled by a BSC in Mexico, the MT employs the BSC in Mexico based upon a goodness criteria, the regulatory requirement mandates the BSC in Mexico to route the packets to and from the MT  120  via the SGSN- 2  in the USA and not via the SGSN- 1  in Mexico of FIG. 5.  
         [0044]    The MT  120  of FIG. 4 is within the range of both satellites  10  and  20 . However, satellites  10  and  20  are associated with two different base station systems (BSS), BSS 1  and BSS 2  although MT  120  is within a geographical area controlled by a single SGSN. The MT  120  communicates with the spot beam that is more readily available for communication. Specifically, if satellite  20  is blocked from the view of the MT  120 , the MT  120  communicates with satellite  10 . Due to the regulatory requirements discussed above, routing is provided to the appropriate SGSN via the concept of a doublet.  
         [0045]    Accordingly, as mentioned above, the BSC is able to uniquely distinguish an MT through the concept of a doublet (e.g., &lt;SGSN-ID, TLLI&gt;) comprising an SGSN identifier (SGSN-ID) and a temporary logical link identifier (TLLI), which is unique within a particular SGSN. These doublets allow the BSC to manage two different users having the same TLLI, yet registered with different SGSNs.  
         [0046]    The SGSN-ID field of the doublet is preferably provided to the MT by the SGSN at the time of initial registration, or contact, and during routing area updates, for example when the MT moves from one geographic location to another. This SGSN-ID is used by the MT as the last registered SGSN-ID (LRSI) in future requests for uplink transmissions to assist the BSC in subsequent routing, handoff, and other mobility management issues. In the GSM-GPRS protocol, uplink access is typically employed with a two phase access, whereby the first phase employs a channel request message that informs the network of its intent to perform an uplink data transfer and a second phase of access employs a packet resource request message that provides additional details such as mobile terminal&#39;s identity (e.g., TLLI), the amount of data to transfer, its priority, and throughput, for example. The packet resource request message from the MT includes LRSI and TLLI information that assists the BSC in routing data from the MT to the appropriate SGSN based on the LRSI. This functionality is therefore useful after a MT has performed a GPRS Attach or Routing Area Update. It can be appreciated that at the time of the Attach or Routing Area Updates, packet routing is based on the geographical location referred to as a Service Area of the MT to meet the regulatory requirements mentioned above. This implies that the BSC has to route certain messages from the MT based on Service Area and others based on LRSI. The MT in accordance with an embodiment of the present invention transmits an associated SA/LRSI bit with the above mentioned doublet to perform appropriate routing. For very short uplink transfers such as Attach Request, the terrestrial GPRS standard employs a single phase access. However, the present invention allows the MT&#39;s identifier bit, the TLLI for example, to be transmitted as part of the Medium Access Control/Radio Link Control (MAC/RLC) block. Therefore, in a preferred embodiment of the present invention, for single phase access, the TLLI, LRSI, and SA/LRSI bits are transmitted in the MAC/RLC block.  
         [0047]    [0047]FIG. 6 illustrates an example of routing packet resource requests between multiple BSC&#39;s depending upon the MT&#39;s location and Service Area (SA). The transmitting MT sends a Channel Request message on the random access channel (RACH) to the BSS which controls the spotbeam that the MT has selected. Based upon reception of a Channel Request  901  message on the RACH channel, BSC 2  determines the Service Area wherein the MT belongs. In addition, the network provides a block of uplink resources to the MT. Employing this block of resources, the MT sends an Attach Request  900  to the BSC 2 . The SA/LRSI bit is also transmitted by the MT in the Medium Access Control/Radio Link Control (MAC/RLC) block. For the case of the Attach Request Message, the MT instructs BSC 2  using the SA/LRSI bit to route the Attach Request Message based on the SA. Employing the SA information, BSC 2  determines that the MT should register with a different SGSN connected to BSC 1 , as opposed to the SGSN coupled to BSC 2 , and therefore routes the Attach Accept to BSC 1  which in turn routes the Attach Request Message to SGSN 1 , or the SGSN directly attached to BSC 1 . BSC 1  is referred to as the anchor BSC as it corresponds to the SGSN to which the user is to be attached. Additionally, the BSC 2  is referred to as the radio BSC corresponding to the spotbeam selected by the MT.  
         [0048]    When anchor BSC 1  receives Attach Request Message from BSC 2 , BSC 1  creates an entry in a routing table for the user based on the TLLI. This context information is used by BSC 1 , anchor BSC, to appropriately route to BSC 2 , radio BSC, whenever BSC 1  receives data for this particular user.  
         [0049]    In FIG. 6, SGSN 1  sends Attach Accept Message to BSC 1 , anchor BSC, as depicted within block  910 . BSC 1 , based on the routing table it had created earlier, routes the Attach Accept to BSC 2 , the radio BSC. It was also described that the MT position, its accuracy and SGSN-ID is provided to the MT at the time of Attach Accept Message. In accordance with an embodiment of the present invention, anchor BSC (BSC 1 ) appends the MT position, its accuracy and SGSN-ID (in this example SGSN 1  ID) to the Attach Accept Message at the end of a logical link controller packet data unit (LLC PDU) containing this message before transmitting it to BSC 2 , radio BSC. It would be spectrally inefficient to do this for all LLC PDUs; therefore it is important that this appending of the MT position, accuracy, and associated SGSN-ID preferably done at the time of Attach Accept and Routing Area Update. Accordingly, this implies the need for the BSC to recognize these messages.  
         [0050]    Turning now to FIG. 7, it is noted that LLC PDUs from the SGSN arrive in ciphered or encrypted form as shown in field  510 . This makes it difficult for the BSC to make a judgement as to whether it is an Attach Accept or not. BSC 1  therefore inspects the LLC header from SGSN (LLC header is not ciphered) and only elongates the LLC PDU for those LLC frames that indicate SAPI=GMM. As shown in FIG. 7, field  505  is not encrypted, whereas field  510  may be encrypted. The elongated LLC PDU is then transmitted to BSC 2 . The elongation of LLC PDU in accordance with the present invention is illustrated in FIG. 7B.  
         [0051]    The method of inspecting the LLC header and deciphering those headers with SAPI=GMM is illustrated by the flowchart in FIG. 8. The BSC  350  appends the LLC PDU with the MT position and accuracy, as depicted in FIG. 7. The BSC  350  receives the LLC PDU from the SGSN  410  as shown in block  600 . The BSC  350  inspects the SAPI field to determine if SAPI=GMM ( 0001 )  610 . If the address field is correct as in block  620  the next step is to attach the MT position and accuracy to the LLC PDU field, as in block  630 . As shown in step  640 , the LLC PDU field is transmitted to the BSC  350  for processing.  
         [0052]    To further protect the information regarding MT position and accuracy from channel errors, a checksum is associated with the appended information in the preferred embodiment. In another embodiment of the present invention, the MT ignores the content of elongation of LLC PDU when a GMM message is not an Attach Accept or Routing Area Update Accept message, for example.  
         [0053]    In order to establish proper routing of the communication signal, the SGSN  410  frames the LLC PDU with an Attach Accept/Routing Area Update message, as shown in FIG. 7. In accordance with an embodiment of the present invention, the base station controller  350  inspects the Service Access Point identifier (SAPI) portion of the address field, as shown in FIG. 7A. The BSC  350  obtains LLC PDU frame  500  from the SGSN  410 . The BSC  350  processes the header portion of the LLC PDU  505 . If the address field contains SAPI=GMM ( 0001 ) the BSC  350  transmits the elongated version of the LLC PDU  530  containing the MT Position and accuracy. The ciphered information field  510  comprises the content portion of the communication signal. If the LLC PDU frames with SAPI=GMM are transmitted in a down-link direction, the BSC  350  adds a Cyclic Redundancy Checksum (CRC) to ensure the MT position and accuracy. If any other frame besides the attach accept or routing area update frame comprises the MT position and accuracy, the MT  120  rejects that information.  
         [0054]    An additional function of the BSC employs appending a registration forbidden flag to a SAPI=GMM LLC PDU frame, thus denying service to an MT for a variety of reasons, including but not limited to, the MT&#39;s location. In accordance with another embodiment of the present invention, the BSC manipulates all GMM messages from MTs belonging to certain areas or having a particular International Mobile Subscriber Identity (IMSI). This manipulation results in an Attach Reject or Routing Area Update Reject. For example, the manipulation method involves replacing the identity of the user by an IMSI in the Attach Request or Routing Area Update Request Message, wherein the user identified by the IMSI is known to be rejected by the SGSN. Therefore, in accordance with another embodiment of the present invention, the BSC manipulates all GMM messages from those MTs in certain service areas provoking the SGSN to send back an Attach Reject or Routing Area Update Reject.  
         [0055]    In the dynamic situation of the MEO satellite arena, associated BSCs and SGSNs are changing based upon the MT location, as highlighted in FIG. 9. Specifically, when an MT  120  moves appreciably from one location  920  to another location  930 , the associated SAG  940  is no longer controlling the MT  120 , rather the SAG  950  controls the MT  120 . Accordingly, when the MT  120  changes location it makes a routing area update as detailed above, with its position and accuracy information. If the new MT position  930  is controlled by a different SAG  950 , then the LLC PDU is routed to the new SAG  950 . According to the to an embodiment of the present invention, when an Routing Area (RA) Update Request is received by BSC in SAG  950 , the BSC in SAG  950  sends a message to the BSC in SAG  940  to purge the contents of its routing table for MT  120 . The old SAG  940  purges its routing table or contexts associated with the MT  120  upon transmission of the LLC PDU to the new SAG  950 . This prevents stale contexts to be retained in the entire system.  
         [0056]    The concept of a MEO satellite communications system managing a paging request from an MT will now be described. An MT  120  is associated with a satellite spot beam  40  that is not directly associated with BSC  350  of FIG. 10. MT  120  may be registered with SGSN  410 . When the MT is in MM-Standby state, SGSN  410  does not know which spotbeam the MT is listening to since the SGSN  410  does not know the cell location of the MT  120  but rather the routing area of the MT  120 . By contrast to MM-Ready mode, where the SGSN  410  knows exactly which cell contains the MT  120 . Accordingly, the BSC  350 , based on the MT position finds all the beams covering the MT position and pages the MT in all those beams. Circuit Switched (CS) paging from the Mobile Switching Center/Visitor Location Register (MSC/VLR)  450  can arrive at the BSC  350  during MM-Standby or MM-Ready mode. When the MT is in MM-Ready, then SGSN  410  indicates the Cell Identifier along with Paging CS PDU. According to another embodiment of the invention, the BSC processes this information to determine whether the user needs to be paged only in a single spotbeam, or multiple spotbeams.  
         [0057]    Another embodiment of the present invention, as shown in FIG. 11, depicts the MT  915  ceasing communication with satellite  10  and beginning communication with satellite  20 , thus switching from Radio SAG  960  to new Radio SAG  965 . During such an event, there are several LLC PDU frames that are pending in the old Radio SAG  960 . Instead of consuming resources by maintaining and attempting to transmit these LLC PDU frames from old SAG  960  to an MT that is not listening, a communication link is maintained between the old Radio SAG  960  and the new Radio SAG  965 , via the anchor SAG  970 . Another embodiment of the present invention comprises the method of the anchor BSC  972  receiving signaling relating to a BSC other than that indicated by the LRSI. In other words, the anchor BSC  972  generates a “Flush LLC Message” to the old radio BSC  962 , causing the BSC  962  to purge all pending LLC PDUs. This new method conserves valuable radio resources. The anchor SAG  970  is a vital link between MT  120  and the various radio SAGs  960  and  965 . The anchor SAG  970  is the original associated SAG however, due to the communication signal or other dynamic elements of the communication system, various other radio SAGs are involved in transmission of the communication signal.  
         [0058]    The above mentioned unplanned handover situation highlights the dynamic nature of the MEO satellite communications network. Despite the fact that an MT is in a stationary position, other components of the communications network, for example the radio BSC, and the anchor BSC, as well as the associated SAGs can change. In addition to these components, the satellites within a MEO satellite network are orbiting the earth at a constant rate, therefore the geographic location of the spot beam is constantly in flux. Accordingly, an MT enters a new spot beam, periodically, even if the MT is stationary. Every time it enters a new spotbeam it is not pageable. Therefore one solution is to continually send Routing Area Update messages to the network, in a conventional manner. However, this consume vital radio resources.  
         [0059]    Therefore, another embodiment of the present invention employs a packet data service within each beam that carries a routing area identity information list comprising a time stamp. The time stamp indicates the time after which the beam will no longer be covering the routing area, as shown in FIG. 12. The network of the present invention maintains the time at which a beam enters a routing area and the time at which the beam leaves the routing area As soon as a beam enters a routing area, the broadcast common control channel (BCCH) broadcasts a routing area identity list comprising a geographic region the beam covers, the list is updated with the time stamp of when the beam will leave the routing area, as mentioned above.  
         [0060]    The MT downloads the routing area information list comprising the routing area identifier from the network via the BCCH. The MT initiates a timer equal to the difference between the time the beam will leave the routing area as indicated by the routing area identity list, and the current time. When the time elapses the MT initiates a routing area update. Therefore, the MT is aware of which spot beam the MT is employing for communication.  
         [0061]    Although only several exemplary embodiments of the present invention have been described in detail above, those skilled in the art will readily appreciate that many modifications are possible in the exemplary embodiments without materially departing from the novel teachings and advantages of this invention. Accordingly, all such modifications are intended to be included within the scope of this invention as defined in the following claims.