Abstract:
The present invention provides a system and method for migrating a control of a wireless communication service provided to a mobile station (MS) in a first network depending on a first technology to a second network depending on a second technology. When receiving a request for the wireless communication service in the first network by a mobile switch center (MSC), at least one instruction is obtained to grant or deny the wireless communication service from a first control device in the second network, the first control device providing the instruction based on its communication to a management subsystem for the control of the wireless communication service. If the wireless communication service is granted, a second control device in the first network controlled by the first control device allows the MS to execute the wireless communication service with a receiver. If the wireless communication service is denied, the second control device in the first network controlled by the first control device prohibits the MS to execute the wireless communication service with the receiver, wherein the first and second control device communicate with each other using a predetermined protocol independent of the network technology used by either the first and the second network, and wherein the first network thus maintains the control of the wireless communication service through the management subsystem connected to the second network without implementing additional management subsystem.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]     The present invention is a continuation of application Ser. No. 09/924,785, filed Aug. 8, 2001, entitled “A Method and Architecture for Providing Network Technology Independent Wireless Services Migration.” 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     The present invention relates generally to voice and data communications, and more particularly, to a wireless system and method for providing communication services to a wireless mobile user of a network through another network using a different network technology.  
         [0003]     With the ever-evolving technology in the wireless industry, new wireless generations are being standardized to provide a new and broader set of wireless services to end users. With this natural evolution, new network architectures and technologies are defined and deployed in the market, which leads to a coexistence of multiple wireless networks built on multiple technologies, with some having the capability to support more services than others. In general, older networks may evolve but seldom are able to provide the same features or support the same capacity as newer networks.  
         [0004]     While operating both an older network and a newer network, one of the concerns for a network operator is how to continue managing some existing services based on the older network technology in the newer network which may use different rules and protocols.  
         [0005]     Taking a wireless Prepaid Service as an example, each subscriber of the Prepaid Service is assigned a record in a network database system which includes a unique account number and an associated prepaid monetary value. The record is maintained and updated based on the user services usage and requests for monetary increases.  
         [0006]      FIG. 1  illustrates a simplified schematic of a current solution for providing the Prepaid Service in two communication networks of two different generations of network technologies. The two independent networks in the example are an IS41 network  10  and a GSM/UMTS network  12 , each having a separate service system for providing the Prepaid Service. For the IS41 network  10 , a full blown intranet based billing subsystem  14  is established to communicate with a Signaling Control Point (SCP)  16  of the network  10  for handling calls made or received by subscribers of the IS41 network  10  (e.g., calls made from the IS41 network to the Public Switch Telephone Network (PSTN)  18 ). For the GSM network  12 , another dedicated SCP  20  is installed to communicate with another billing subsystem  22  for calls made by subscribers of the Prepaid Service using the GSM network  12 . In order to accommodate both technologies (IS41 and GSM in this case), each network has to have an SCP and a billing system, which will cause a wireless service provider higher operational costs and longer service deployment time.  
         [0007]     Another disadvantage of such a solution is that the network architecture shown above does not provide high scalability. That is, for each additional network maintained by the wireless service provider, an additional SCP and billing system must be installed, thereby increasing operational and deployment costs with future network expansions.  
         [0008]     What is needed is a method and system that allows seamless service migration from one network technology to another. The solution must be cost effective due to the large investment that service providers have put into existing networks, and should provide high scalability to the networks so that future capacity increases can easily be supported. In addition, there should be no degradation in the quality and reliability of the services provided to the subscribers and should require very little change to the existing networks and their contained network entities.  
       SUMMARY OF THE INVENTION  
       [0009]     In response to these and other needs, a new and improved system and method provide communication services to a wireless mobile user of a network through another network using a different network technology. Effectively, the system and method interconnect a wireless user from one network type to another. The present disclosure further provides a method and system to provide new services to a mobile user from a different network where a mobile station used by the user may not support such services. The present disclosure also provides a method for controlling the operation of network entities that play a part in the provisioning of different services provided to a mobile station.  
         [0010]     In one example of the present invention, a system and method is provided for migrating a control of a wireless communication service provided to a mobile station (MS) in a first network depending on a first technology, to a second network depending on a second technology. When receiving a request for the wireless communication service in the first network by a mobile switch center (MSC), at least one instruction is obtained to grant or deny the wireless communication service from a first control device in the second network. The first control device provides the instruction based on its communication to a management subsystem for the control of the wireless communication service. If the wireless communication service is granted, a second control device in the first network controlled by the first control device allows the MS to execute the wireless communication service with a receiver. If the wireless communication service is denied, the second control device in the first network controlled by the first control device prohibits the MS to execute the wireless communication service with the receiver. The first and second control device can communicate with each other using a predetermined protocol independent of the network technology used by either the first and the second network, and the first network can maintain the control of the wireless communication service through the management subsystem connected to the second network without implementing additional management subsystem. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]      FIG. 1  illustrates a conventional network architecture for providing a wireless network service such as a Prepaid Service in both the IS41 and GSM/UMTS networks.  
         [0012]      FIG. 2  illustrates a network architecture for providing the wireless network service such as a Prepaid Service in the IS41 network through the GSM/UMTS network according to one example of the present invention.  
         [0013]      FIGS. 3-5  illustrate call flow diagrams for providing the Prepaid Service from the GSM/UMTS network, while the request of the service is received by the IS41 network according to the example of the present invention as illustrated in  FIG. 2 .  
         [0014]      FIG. 6  illustrates another network architecture for providing the wireless network service such as a Prepaid Service in the IS41 network through the GSM/UMTS network according to another example of the present invention.  
         [0015]      FIG. 7  illustrates a network configuration for supporting the wireless network service of multiple IS41 networks from a GSM/UMTS network according to one example of the present invention.  
         [0016]      FIG. 8  illustrates a multi-network communication system for supporting a data service of one network through another network according to one example of the present invention. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0017]     For the purposes of illustrating the present invention, various acronyms are used, and the definitions of which are listed below: 
        ANSI-41 American National Standards Institute—Cellular Radio Telecommunications Intersystem Operations     BSS Base Station System     CAP CAMEL Application Part     GMSC Gateway MSC     GSM Global System for Mobile communications     HLR Home Location Register     IGS Intelligent Gateway Service     IS41 Wireless Network conforming to the IS41 standard     ISDN Integrated Services Digital Network     ISUP ISDN User Part (of SS7)     IVRU Interactive Voice Response Unit     MC Messaging Centre     MEGACO Media Gateway Control protocol     MSC Mobile Switching Centre     PIN Personal Identification Number     PSTN Public Switch Telephone Network     SCP Signalling Control Point     SMSC Short Message Service Centre     SMTP Simple Mail Transfer Protocol     SS7 Signaling System No. 7     T1 Digital communication line that uses time division multiplexing with an overall transmission rate of 1.544 million bits per second.     TCP/IP Transmission Control Protocol/Internet Protocol     UMTS Universal Mobile Telecommunications System     WMG Wireless Media Gateway     WSS Wireless Soft Switch        
 
         [0043]     The present invention is described below with several examples. It is understood, however, that the examples are not necessarily limitations to the present invention, but are used to describe typical embodiments of operation. For instance, although an IS41 and GSM networks are used to describe the present invention, the invention can apply to many different wireless networks. Additionally, although two networks, and a Prepaid Service are used, the present invention can be applied to more than two networks and to any other wireless network services if such services of a network are not otherwise prohibited.  
         [0044]      FIG. 2  illustrates a multi-network communication system  30  for supporting a Prepaid Service of one network through another according to one example of the present invention. Similar to  FIG. 1 , there are two different types of wireless communication networks involved: an IS41 network  10  and a GSM/UMTS network  12 . Assuming a service management subsystem such as a billing subsystem  22  is connected to an SCP  20  of the GSM network  12  through a TCP/IP protocol, the billing subsystem  22  is expected to support any calls for a mobile station (MS)  32  operating in the IS41 network  10  as well as MSs operating in the GSM/UMTS network  12 . For example, a call can be made between the two networks, two subscribers in the same network, or between either network and a receiver  34  in the PSTN  18 . It is understood that the SCP can also be deemed as either as a part of the GSM network  12  or a part of the service management subsystem such as the billing subsystem  22  since the primary function of the SCP is to communicate with a network entity in the network using an appropriate protocol.  
         [0045]     The user of the MS  32  is a subscriber of the Prepaid Service, and her wireless communication service within the IS41 network  10  is provided contingent on the status of a personal account in the billing subsystem  22  in which she has paid an amount of money. Her use of the MS  32  will consume the amount of money in the account, and the billing subsystem  22  should keep her account updated in the fastest and most effective fashion since her wireless communication service will be provided only if there is enough fund in her account. It is understood that the IS41 network  10  is considered to be an older network than the GSM network  12  (or, the GSM network uses a newer network technology), and when the network operator of the IS41 network  10  adopts the new GSM network  12  without abandoning the IS41 network, it would be desirable for the service operator to provide wireless network services such as the Prepaid Service not only to the mobile users of the GSM network, but also to the mobile users of the old IS41 network as well. In order to avoid redundant equipment investment, it is extremely desirable for the network operator to avoid the network design as shown in  FIG. 1  wherein multiple SCPs and multiple billing subsystems have to be used.  
         [0046]     One example of the present invention as illustrated in  FIG. 2  avoids using excessive equipment by implementing a multi-network communication system  30  for supporting the Prepaid Service for mobile users of both the IS41 network and the GSM network. In this example, in order to “migrate” the wireless service from the IS41 network  10  to the GSM network  12 , a WMG  36  and wireless switch device such as a WSS  38  are used to interconnect the IS41 and GSM networks, thereby providing a seamless Prepaid Service to subscribers in both networks.  
         [0047]     The WSS  38  handles calls and exchanges call information with the MSC  40 , the SCP  20 , and the WMG  36 . The WMG  36  is responsible for transferring application data information between the MS  32  and any other receiver end such as a PSTN receiver, another MS in the GSM network, or another MS in the IS41 network. The WSS  38  controls the WMG  36  operation using various control messages. The communications between the WMG  36  and the WSS  38  may conform to a media gateway control protocol such as MEGACO.  
         [0048]     The WMG  36  is connected to one or more MSCs  40  in the IS41 network  10  using communication links such as T1 links. The WMG  36  is also connected to the PSTN network via another communication link such as another T1 link. The MSC  40  connects to an HLR  42  and an MC  44  in the IS41 network  10 , as well as a BSS  46  using SS 7  protocol. The HLR  42  also communicates with the WSS  38  in the GSM network  12  using the ANSI 41 standard protocol. Moreover, the MSC  40  further communicates to the WSS  38  using SS7 ISUP as the signaling protocol. Therefore, from the perspective of the IS41 Network, multiple network entities are in contact with the WSS  38 , and each uses a different protocol.  
         [0049]     In order to communicate with multiple network entities in both the IS41 network and the GSM network, the WSS  38  has multiple interfaces built in that communicate with them using multiple protocols. For example, it uses a dedicated MEGACO interface to control the connection setups of the WMG  36  in the IS41 network. The WSS  38  communicates with the HLR  42  in the IS41 network using an ANSI- 41  interface. The WSS  38  has a connection with the SCP  20  of the GSM/UMTS network  12  which uses CAP as the communication protocol. Additionally, the WSS  38  has an SS 7  ISUP interface to communicate with one or more MSC  48  in the GSM/UMTS network as well as one or more MSCs in the IS41 network  10 . Moreover, the WSS  38  is connected with the PSTN  34  using the SS7 ISUP as the signaling protocol. The MSC  48  in the GSM network  12  further connects to the SCP  20 , an HLR  50 , and an SMSC  52  through GSM connections. In order to process information for the Prepaid Service, the SCP  20  in the GSM/UMTS network  12  is connected to the billing subsystem  22 . There may also be an Email Gateway  54  that links the MC  44  of the IS41 network  10  and the SMSC  52  of the GSM network  12 , and further connected to the billing subsystem  22 . Compared to the network in  FIG. 1 , the same Email Gateway  54  is used for both networks.  
         [0050]     From operational perspective, when the MS  32  located in the IS41 network  10  makes a call to a PSTN number which represents a receiver  34 , its call request is routed from the BSS  46  to the MSC  40  in the IS41 network  10 , and further to the WSS  38  in the GSM/UMTS network. The WSS  38  then passes the control of the call to the SCP  20 , which can check the corresponding subscriber account in the billing subsystem  22  to allow or deny the call to go through. More details are explained below with regard to  FIGS. 3-5 . If it is a GSM subscriber located in the GSM network  12  who makes a call to the PSTN receiver  34 , its call request is first sent to the MSC  48  in the GSM/UMTS network  12 , and the call control is then passed to the SCP  20  directly. Similar to the call initiated from the IS41 network, the SCP  20  then examines the monetary value in the subscriber account of the billing subsystem  22 . With this configuration, the SCP  20  can therefore request the MSC  48  to refuse or disconnect the call if or when the subscriber&#39;s prepaid monetary value becomes insufficient for the requested service. It is understood that the same routes can be reversed for processing calls initiated by a user of the PSTN to a user of either the IS41 network  10  or the GSM network  12 .  
         [0051]      FIG. 3  illustrates a call flow diagram  58  for steps taken to set up a call initiated by an MS according to one example of the present invention. When the MS  32  makes a call request, which reaches the MSC  40 , the MSC  40  sends an ORREQ message to the HLR  42  in step  60  to pass on the mobile user identification information and authenticate the mobile user service request, the HLR  42  sends to the MSC  40  a ORREQ response message in step  62  approving the call origination request and informing the MSC  40  about the mobile user profile. The MSC  42  then communicates with the WSS  38  in an Initial Address Message (IAM) passing on the destination number and calling number of the MS  32  (step  64 ). The WSS  38  then sends a request to the SCP  20  with the mobile user information in step  66 , while the SCP contacts the billing subsystem  22  requesting credit information of the corresponding account for the user of the MS  32  (step  68 ). The billing subsystem  22  then replies with the user credit information (step  70 ). The SCP  20  examines the user&#39;s current credit amount, and if sufficient funds exist in the user&#39;s account, sends a message to the WSS  38  requesting it to continue the service call setup (step  72 ). Once the WSS and WMG have collectively established a context for a connection route between the MSC  40  and the PSTN  18  by sending messages between them (step  73   a  and  73   b ), the WSS  38  immediately sends out an IAM to the PSTN  18  to access the receiver  34  (step  74 ). The PSTN sends an ACM message to the WSS  38  (step  76 ) indicating that the receiver  34  has been successfully accessed and is ringing. The WSS  38  sends a similar ACM message to the MSC  40  indicating that the connection with the receiver has been established. Thereafter, upon receiving an ANswer Message (ANM) from the PSTN indicating that the call made to the receiver  34  has been answered (step  80 ), the WSS  38  sends a similar ANM to the MSC  40  (step  82 ). At this moment, a call originated from a user of the IS41 network subscribing to the Prepaid Service has been set up to go through to reach the receiver  34  in the PSTN  18 .  
         [0052]     It is noted that if in step  70  the billing subsystem  22  indicates that there is a lack of fund to proceed, the SCP  20  would inform the WSS  38  to stop connecting the call to the receiver  34 , the WSS  38  then interfaces with the MSC  40  indicating the same, and the call request of the MS  32  would be turned down.  
         [0053]      FIG. 4  illustrates a call flow diagram  88  for steps taken to tear down the call initiated by an MS as illustrated in  FIG. 3 . When the MS  32  terminates the call, the MSC  40  informs the WSS  38  to release the occupied bandwidth for processing the call (step  90 ). The WSS  38  then sends a call termination indication to the SCP  20  (step  94 ), and the SCP  20  further initiates a Credit Deposit Record message (step  94 ) to the billing subsystem  22  to update the account status of the user of the MS  32 . Once the record is updated in the billing system, the SCP  20  informs the WSS  38  to release the call between the MS  32  and the receiver  34  through the PSTN  18  (step  96 ). The WSS  38  informs the PSTN  18  to release all the bandwidth and circuits allocated for the call between the end receiver  34  and the WMG  36  in step  98 , and the PSTN  18  confirms the completion of the releases in step  100 . The WSS  38  further inform the MSC  40  that the resource release on the PSTN side has been done (step  102 ), and further informs the WMG  36  to release the context of the call and terminate any dealing with the PSTN  18  for the call (step  104 ). Once it is achieve, the WMG  36  confirms it with the WSS  38  with a message in step  106 .  
         [0054]     During the call, the SCP  20  monitors the MS call duration and compares it with the monetary value in the user account that has been extracted from the billing subsystem  22  during the call setup procedure in  FIG. 3  (step  70 ). If at any moment before the end of the call that the prepaid monetary value becomes insufficient, the SCP  20  can request the WSS  38  to refuse or disconnect the call.  FIG. 5  illustrates a detailed call flow diagram  108  for terminating a call when there is a lack of fund in the prepaid account according to one example of the present invention. When the SCP  20  detects that there is a lack of sufficient fund in the billing subsystem  22 , it sends the CDR message to the billing subsystem in step  110  in order to update the billing record information in the billing subsystem  22 , and informs the WSS  38  to terminate the call in step  112 . Steps  114 - 124  execute the termination process in the same way as the steps  90 - 106  of  FIG. 4  except that the release command is initiated by the WSS  38  instead of the MSC  40 .  
         [0055]      FIG. 6  illustrates a multi-network communication system  130  for supporting a Prepaid Service of one network through another according to another example of the present invention. In this configuration, an MSC  132  replaces the WSS  38  in  FIG. 2  to assist with migrating the Prepaid Service to the new GSM network  12 . The MSC  132  connects to the HLR  50 , the SCP  20 , and the SMSC  52  inside the GSM network  12 . On the other side, the MSC  132  has an interface that communicates with the HLR  42  and the MSC  40  of the IS41 network. Similar communications as those described with regard to  FIGS. 3-5  can be made except that the MSC  40  interfaces with the MSC  132  directly without the assistance of the WMG  36  and the WSS  38 .  
         [0056]      FIG. 7  illustrates another multi-network communication system  140  for supporting a Prepaid Service of one network through another according to yet another example of the present invention. In this configuration, the WSS  38  of the GSM network handles multiple WMGs  142   a - 142   d  for supporting the Prepaid Service. It does not matter whether a call comes in from which switch site (e.g., Switch Site A-D), the WMG in that receiving switch site communicates with the WSS  38  and thus receives instructions from the billing subsystem  22  through the SCP  20 . Having one WSS support multiple WMGs in different sites is a very scalable solution since additional WMGs can be easily added if there are more mobile users. Henceforth, the network operator can maximize its benefits with minimum equipment investment when two or more network technologies are supporting two different wireless communication networks.  
         [0057]      FIG. 8  illustrates a multi-network communication system  151  for supporting a data service of one network through another network according to one example of the present invention. The present invention as described above with regard to  FIGS. 2-7  focuses on the wireless network services such as the Prepaid Service working in conjunction with wireless communication services such as the voice service provided by the two different networks. Similar concepts can be applied to the entities in the two different networks to provide data services to users of both networks.  
         [0058]     Referring to  FIG. 8 , the steps shown are for setting up a call initiated by an MS for an email service. Although the two networks  150   a - b  depicted are assumed to be all GSM/UMTS networks, it is understood that any network that provides email service can be similarly supported. For email communications, when an SGSN  152  receives a data connection request from the MS  32  in step  156 , the SGSN  152  communicates with the WSS  38  by sending a connection request message to it in step  158 . At this stage of the call, the application that is used by the MS is unknown. The WSS  38  sends a message to the WMG  36  in step  160  to establish a context connection between the SGSN  152  and the PDN  154 . If the context creation is successful, the WMG  36  sends a confirmation message in step  162  to the WSS  38  which further informs the SGSN  152  that the data connection establishment is complete (step  164 ). The SGSN  152  then informs the MS  32  that its request has been accepted (step  166 ), and that it can start transmitting and receiving packet data. The MS  32  therefore starts sending email data to the WMG  36  through the SGSN  152  (step  168  and  170 ). Before allowing the data to go through, the WMG  36  requests the WSS  38  for the maximum amount of data that the user is allowed to transmit (step  172 ). Upon receiving this request, the WSS  38  sends a request to the SCP  20  in step  174  with the mobile user information, while the SCP  20  contacts the billing subsystem  22  in step  176  requesting credit information of the corresponding account for the user of the MS  32 . The billing subsystem  22  then replies with the MS  32  credit information in step  178 . The SCP  20  examines the user current credit amount and, if sufficient fund exists in the user account, sends a message to the WSS  38  requesting it to continue the call and informs it the amount of data the user is allowed to transmit (step  180 ). Subsequently in step  182 , the WSS  38  informs the WMG  36  of the maximum data allowed. Then in step  184 , the WMG  36  allows the data to go through and monitors the total amount transmitted from the MS, and informs the WSS when the total amount transmitted by the MS exceeds the allowed amount.  
         [0059]     The above disclosure provides many different embodiments, or examples, for implementing the invention. However, specific examples, and processes are described to help clarify the invention. These are, of course, merely examples and are not intended to limit the invention from that described in the claims. For instance, in the example illustrated above, although one SCP is deemed to exist in one of the networks, it is understood that the network can easily contain several SCPs to handle a much larger number of subscribers.  
         [0060]     For simplicity purposes, the example illustrated above provides that the GSM/UMTS network is the network that supports the SCP directly, it is understood that the SCP can also be located in the IS41 network. In that case, subscribers in the GSM/UMTS network access the prepaid services available in the IS41 network using the WSS and WMG inter-network connections. Consistent with this understanding, a wireless communication service does not need to be initiated by a user of the IS41 network, it can be one user of the GSM network. And similarly, the recipient of a call does not need to be a subscriber of the PSTN like the call receiver  34 . The recipient can be a user in the same network where the call is initiated, or can be a user of another network such as the GSM network (while the call is initiated in the IS41 network).  
         [0061]     Referring back to  FIG. 2 , assuming in this scenario a user  33  of the GSM network actually initiates a call, the user  33  of the GSM network communicates with its own MSC  48 , and if the billing subsystem is connected to the GSM network  12  through the SCP  20  and the recipient is one in the PSTN  18  such as the receiver  34 , then the control can be directly asserted by the MSC  48  since it is capable to manage the requested wireless communication service by constantly communicating with the billing subsystem  22 . However, if the recipient is the user in the IS41 network such as the user  32 , then the WSS  38  and WMG  36  participate in the control of the Prepaid Service. Collectively, the MSC  48  for the caller, the SCP  20 , the billing subsystem  22 , the WSS  38 , the WMG  36 , the MSC  40  for the recipient may manage the control of the wireless communication service provided to the user  33  based on his/her subscribed Prepaid Service. In this situation, a typical communication connection route is from the user  33 , to the MSC  48 , the PSTN  18 , the WMG  36 , the MSC  40 , and finally to the user (i.e., his MS)  32  of the IS41 network. If the billing subsystem is connected to the IS41 network, then a “mirror image” of the configuration of  FIG. 2  will be implemented wherein the WSS  38  and SCP  20  will be in the IS41 network, and the WMG  36  will be in the GSM network.  
         [0062]     The present invention as described above thus provides an economical method and system for supporting a wireless network service offered from one wireless network through another network having a different network technology. The present invention does not introduce any change to the existing architectures in the interconnected wireless networks. This is a significant advantage for a network operator or service provider given that there is no need for investing in upgrading existing equipment, and the migration of the services to be supported by the new network can be achieved in much shorter time and at a lower cost. In some cases, to further save the costs for the network operator, the WMG and WSS can be reused as GMSC and Tandem switch. In addition, the present invention provides a maximum leverage of equipment investment since the configured networks are highly scalable and have a high reliability factor.  
         [0063]     The present invention also introduces cost savings by eliminating loop-back trunks existed in each MSC in the IS41 network due to the fact that each MSC now has a direct connection with the WMG and WSS. Moreover, because of the intelligence of the WMG and WSS, the present invention allows the delivery of additional network services such as “in-call warning announcement” in the Prepaid Service to warn the user that his fund has been depleted to a predetermined threshold, and that he better finishes his call as soon as possible.  
         [0064]     The WMG and WSS pair also allow new services to wireless networks of older generations by providing network services such as call forwarding, mobile number portability, home zone services, and location based services, while traditionally, these services cannot be offered in an older IS41 network.  
         [0065]     It will also be understood by those having skill in the art that one or more (including all) of the elements/steps of the present invention may be implemented using software and hardware to develop WSS and WMG nodes, which will then be deployed in a wireless network at appropriate locations with the proper connections.  
         [0066]     Furthermore, while the invention has been particularly shown and described with reference to the preferred embodiment thereof, it will be understood by those skilled in the art that various changes in form and detail may be made therein without departing from the spirit and scope of the invention, as set forth in the following claims.