Patent Publication Number: US-2012026882-A1

Title: Apparatus and method for supporting agps traffic class in mobile communication system

Description:
PRIORITY 
     This application claims the benefit under 35 U.S.C. §119(a) of a Korean patent application filed in the Korean Intellectual Property Office on Jul. 30, 2010 and assigned Serial No. 10-2010-0074152, the entire disclosure of which is hereby incorporated by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an apparatus and method for controlling traffic transmission in a communication system. More particularly, the present invention relates to an apparatus and a method for transmitting traffic of an adaptive Grant and Polling Service (aGPS) service class in a mobile communication system. 
     2. Description of the Related Art 
     A communication system implementing the Institute of Electrical and Electronics Engineers (IEEE) 802.16m standard now supports an aGPS scheduling service as a new Quality of Service (QoS) service class. 
     However, in an initial connection setting process of an aGPS service class defined in the IEEE 802.16m standard, negotiating a plurality of QoS parameter sets to be used by a terminal and a network, and then converting and using a QoS parameter set without transmitting a separate control signal when needed cannot be supported by the processing method of the related art. 
     Therefore, a need exists for an apparatus and method for supporting the aGPS traffic class in a mobile communication system. 
     SUMMARY OF THE INVENTION 
     Aspects of the present invention are to address at least the above-mentioned problems and/or disadvantages and to provide at least the advantages described below. Accordingly, an aspect of the present invention is to provide an apparatus and a method for supporting an adaptive Grant and Polling Service (aGPS) traffic class in a mobile communication system. 
     Another aspect of the present invention is to provide an apparatus and a method for providing a new Quality of Service (QoS) service by defining an efficient network structure and a signal scheme for supporting an aGPS service class in a Worldwide Interoperability for Microwave Access (WiMAX) network system that supports the Institute of Electrical and Electronics Engineers (IEEE) 802.16m standard. 
     In accordance with an aspect of the present invention, a method for changing a QoS of a terminal in a mobile communication system is provided. The method includes, when it is determined to change the QoS, transmitting a packet where a QoS parameter has been changed to a base station, and when a Dynamic Service Change (DSC) performance request is not received from the base station, using the changed QoS parameter. 
     In accordance with another aspect of the present invention, a method for changing a QoS of a base station in a mobile communication system is provided. The method includes, when detecting a QoS parameter change from a packet received from a terminal, determining the changed QoS parameter, transmitting a Generic Route Encapsulation (GRE) packet to which the changed QoS parameter has been applied to an upper node, when performance of a DSC with the terminal is requested by the upper node, performing the DSC with the terminal, and applying the changed QoS parameter. 
     In accordance with still another aspect of the present invention, a method for changing a QoS of a network apparatus in a mobile communication system is provided. The method includes determining whether a GRE packet where a QoS parameter has been changed is received from a base station, when receiving the GRE packet where the QoS parameter has been changed, determining whether to allow a QoS parameter change based on a user QoS policy, and when not allowing the QoS parameter change, requesting the base station to perform DSC. 
     In accordance with yet another aspect of the present invention, an apparatus of a terminal, for changing a QoS in a mobile communication system is provided. The apparatus includes a QoS manager for, when it is determined to change a QoS, determining to transmit a packet where a QoS parameter has been changed to a base station, and for, when a DSC performance request is not received from the base station, using the changed QoS parameter, and a modem for transmitting a packet where the QoS parameter has been changed to the base station, and for receiving the DSC performance request from the base station. 
     In accordance with another aspect of the present invention, an apparatus of a base station, for changing a QoS in a mobile communication system is provided. The apparatus includes a wireless modem for communicating with a terminal, a wired modem for communicating with an upper node, a QoS manager for, when detecting a QoS parameter change from a packet received from the terminal via the wireless modem, determining the changed QoS parameter, for transmitting a GRE packet to which the changed QoS parameter has been applied to the upper node via the wired modem, and for applying the changed QoS parameter, and a DSC processor for, when performance of a DSC with the terminal is requested by the upper node, determining to perform the DSC with the terminal. 
     In accordance with still another aspect of the present invention, a network apparatus for changing a QoS in a mobile communication system is provided. The apparatus includes a modem for communicating with a base station, and a QoS manager for determining whether a GRE packet where a QoS parameter has been changed is received from the base station via the modem, and for, when receiving the GRE packet where the QoS parameter has been changed, determining whether to allow a QoS parameter change based on a user QoS policy, and a DSC processor for, when not allowing the QoS parameter change, requesting the base station to perform DSC. 
     In accordance with still another aspect of the present invention, a system for changing a QoS in a mobile communication system is provided. The system includes a terminal for, when it is determined to change the QoS, transmitting a packet where the QoS parameter has been changed to a base station, and for, when a DSC performance request is not received from the base station, using the changed QoS parameter, the base station for, when detecting a QoS parameter change from a packet received from the terminal, determining the changed QoS parameter, for transmitting a GRE packet to which the changed QoS parameter has been applied to a network apparatus, and for, when performance of a DSC with the terminal is requested by the network apparatus, performing the DSC with the terminal to apply the changed QoS parameter, and the network apparatus for determining whether a GRE packet where the QoS parameter has been changed is received from the base station, for, when receiving the GRE packet where the QoS parameter has been changed, determining whether to allow the QoS parameter change based on a user QoS policy, and for, when not allowing the QoS parameter change, requesting the base station to perform the DSC. 
     Other aspects, advantages, and salient features of the invention will become apparent to those skilled in the art from the following detailed description, which, taken in conjunction with the annexed drawings, discloses exemplary embodiments of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other aspects, features, and advantages of certain exemplary embodiments of the present invention will be more apparent from the following description taken in conjunction with the accompanying drawings, in which: 
         FIG. 1  is a view illustrating a control signal message flow for controlling a data path for transmitting traffic of an adaptive Grant and Polling Service (aGPS) service class according to an exemplary embodiment of the present invention; 
         FIG. 2  is a flowchart illustrating a process for operating a terminal according to an exemplary embodiment of the present invention; 
         FIG. 3  is a flowchart illustrating a process for operating a base station according to an exemplary embodiment of the present invention; 
         FIG. 4  is a flowchart illustrating a process for operating an Access Service Network Gateway (ASN GW) or a Policy Charging Resource Function (PCRF) according to an exemplary embodiment of the present invention; 
         FIG. 5  is a block diagram illustrating a terminal or a base station according to an exemplary embodiment of the present invention; and 
         FIG. 6  is a block diagram illustrating an ASN GW or a PCRF according to an exemplary embodiment of the present invention. 
     
    
    
     Throughout the drawings, like reference numerals will be understood to refer to like parts, components, and structures. 
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     The following description with reference to the accompanying drawings is provided to assist in a comprehensive understanding of exemplary embodiments of the invention as defined by the claims and their equivalents. It includes various specific details to assist in that understanding but these are to be regarded as merely exemplary. Accordingly, those of ordinary skill in the art will recognize that various changes and modifications of the embodiments described herein can be made without departing from the scope and spirit of the invention. Also, descriptions of well-known functions and constructions are omitted for clarity and conciseness. 
     The terms and words used in the following description and claims are not limited to the bibliographical meanings, but, are merely used by the inventor to enable a clear and consistent understanding of the invention. Accordingly, it should be apparent to those skilled in the art that the following description of exemplary embodiments of the present invention is provided for illustration purpose only and not for the purpose of limiting the invention as defined by the appended claims and their equivalents. 
     It is to be understood that the singular forms “a,” “an,” and “the” include plural referents unless the context clearly dictates otherwise. Thus, for example, reference to “a component surface” includes reference to one or more of such surfaces. 
     By the term “substantially” it is meant that the recited characteristic, parameter, or value need not be achieved exactly, but that deviations or variations, including for example, tolerances, measurement error, measurement accuracy limitations and other factors known to skill in the art, may occur in amounts that do not preclude the effect the characteristic was intended to provide. 
     Exemplary embodiments of the present invention provide an apparatus and a method for supporting an adaptive Grant and Polling Service (aGPS) traffic class in a mobile communication system. 
     A system that uses an Institute of Electrical and Electronics Engineers (IEEE) 802.16m standard manages Quality of Service (QoS) of a service provided to a terminal via an Access Service Network Gateway (ASN GW) and Authentication, Authorization and Accounting (AAA), or Policy Charging Resource Function (PCRF) in a QoS and Policy and Charging Control (PCC) structure. 
     For a terminal and a base station to automatically change and use a parameter when needed without transmitting a separate control signal after determining primary/secondary QoS parameters in advance during an initial setting of a relevant connection in an aGPS scheduling service, the following process is provided. 
     Exemplary embodiments of the present invention describe a role of each element of a network for providing an aGPS service and signal operations thereof. 
     First, an exemplary initial network entry procedure is described below. According to a method proposed by an exemplary embodiment of the present invention, during an initial network entry procedure of a terminal or during a process where a terminal generates a new aGPS connection, an ASN GW or a PCRF determines a QoS set ID or a Differentiated Services Code Point (DSCP) value to be used for each QoS parameter set included in each connection and incorporates the same into a Path-Registration-Response (Path-Reg-Rsp) message, and then transfers the same to a base station. 
     The base station stores a QoS set Identifier (ID) or a DSCP value allocated for each QoS parameter set. In addition, the base station informs an ASN GW that a relevant QoS parameter set is used by incorporating a relevant QoS ID or a DSCP value into a header of a Generic Route Encapsulation (GRE) packet and transmitting the same when transferring a data packet corresponding to a currently activated QoS parameter set from the base station via a GRE tunnel to the ASN GW. 
     In the case where a PCRF is used, the ASN GW determines whether a QoS ID or a DSCP value corresponding to the activated QoS parameter set is normally used, and when an error exists, informs the PCRF of the error to perform an additional operation. 
     Next, a procedure for changing an activated QoS parameter set proposed by an exemplary embodiment of the present invention is described. 
       FIG. 1  is a view illustrating a control signal message flow for controlling a data path for transmitting traffic of an aGPS service class according to an exemplary embodiment of the present invention. 
     Referring to  FIG. 1 , when a terminal  110  or a network changes a QoS parameter set depending on necessity, in case of an aGPS service class according to an exemplary embodiment of the present invention, the terminal  110  or a base station  120  transmits a scheduled user data packet to a counterpart node using a changed QoS parameter set without a separate signal procedure. Alternatively, the terminal  110  may transmit a control message for changing a QoS to the base station. 
       FIG. 1  corresponds to a case where the terminal  110  changes a QoS parameter set and transmits a user data packet in step A. Since a case where a network changes a parameter set is described as a similar process, description thereof is omitted for conciseness. An exemplary embodiment of the present invention is described using a case where the terminal  110  starts to change a QoS parameter. 
     The base station  120  that receives a user data packet tries QoS filtering (or regulation) using a currently set (activated) QoS parameter set. At this point, when detecting that the currently set QoS parameter is unavailable due to a QoS parameter change, the base station  120  estimates the changed QoS parameter set by sequentially using different QoS parameter sets additionally in step B. 
     When detecting the change of the QoS parameter set, the base station  120  informs an ASN GW  130  that the activated QoS parameter set of the terminal  110  has changed by replacing a QoS set ID or a DSCP code value included in a GRE packet header transferred via the GRE tunnel with a QoS set ID or a DSCP code value allocated to a relevant changed QoS parameter set, and transferring the same to the ASN GW  130  in step C. 
     In the case where the ASN GW  130  responsible for a Data Plane (DP) and an ASN GW  140  responsible for a Control Plane (CP) are separated, a signaling procedure may be used in which the anchor ASN GW  130  responsible for the DP receives a GRE data packet where a QoS set ID or a DSCP code value has changed from the base station  120 , and transmits an R4 Path-Registration-Request (Path-Reg-Req) message requesting a path change by a QoS parameter change to transfer the same to the Auth ASN GW  140  responsible for the CP in step D. After that, a determination is made of whether to allow the changed QoS parameter set in step  160 . 
     In the case where the ASN GW is responsible for both the DP and CP, the inside of the ASN GW performs the above procedure. 
     When a PCC does not exist, the ASN GW  140  may determine whether to allow the changed QoS parameter set based on a stored user QoS policy in step E, and transfer, in step I, a result thereof to the base station  120  via the ASN GW  130  to which the result was transferred in step H. 
     On the other hand, when the PCC exists, a PCRF  150  determines whether to allow the changed QoS parameter set via relevant signaling in step F based on a stored user QoS policy, and transfers a result thereof to the base station  120  via the ASN GW  140  and  130  in steps G, H, I. 
     During this process, in the case where a QoS parameter set changed by the terminal  110  is not allowable according to a user QoS policy, the ASN GW  140  or the PCRF  150  performs the following process in step  170 . 
     That is, the ASN GW  140  or the PCRF  150  instructs step I the base station  120  to perform DSC procedures of steps K, L, and M for QoS change, thereby allowing the terminal  110  to change to a QoS parameter set allowed to a user. Here, the base station  120  allocates a resource to the terminal  110  in step J. 
     The ASN GW  130  detects that all processes for the above process have been completed from the base station  120  in step N), and informs the base station  120  and the ASN GW  140  of a response for the detection in steps O and P. After that, the activated QoS parameter set is used. 
     In the case where the changed QoS parameter set of the terminal  120  is approved by the ASN GW  140  or the PCRF  150 , a relevant QoS parameter set may be activated and used without a separate process. Alternatively, the ASN GW  140  or the PCRF  150  may approve use of the changed QoS parameter set, and transmit a result thereof to the base station  120  and the terminal  130  via the ASN GW  130 . 
     According to an exemplary embodiment of the present invention, when detecting a change of a QoS parameter set in a DP, a base station may complete a required procedure by only changing a QoS set ID or a DSCP code value of a GRE header. When a changed QoS set ID or DSCP code value is received, the ASN GW may perform a required procedure by triggering a predetermined signal procedure within a CP internally. 
     Instead of using a complicated process of monitoring a QoS change and generating a signal procedure of a CP via a signal inside of a base station when supporting an aGPS service class, according to exemplary embodiments of the present invention, a DP may detect a QoS change and report the same to an ASN GW via a general data packet transferred via a GRE tunnel In that case, an error occurrence probability that may occur during a complicated signal processing procedure may be reduced and a more efficient process may be achieved. 
       FIG. 2  is a flowchart illustrating a process for operating a terminal according to an exemplary embodiment of the present invention. 
     Referring to  FIG. 2 , when a QoS parameter change is required in step  210 , the terminal transmits a user data packet where QoS has changed to a base station in step  220 . A case where a QoS parameter change is required may be generated due to various causes such as a user&#39;s request, a network environment, etc. 
     When it is determined in step  230  that a DSC request is received from a base station, the terminal performs a DSC process for changing a QoS parameter with the base station in step  240 . The terminal uses the changed QoS parameter in step  250 . 
     On the other hand, when it is determined in step  230  that the DSC request is not received from the base station, the terminal may use a QoS parameter changed in a previous step (e.g., step  220 ) in step  250 . 
       FIG. 3  is a flowchart illustrating a process for operating a base station according to an exemplary embodiment of the present invention. 
     Referring to  FIG. 3 , when analyzing a user data packet transmitted by a terminal and detecting a QoS parameter change in step  310 , the base station estimates which is the changed QoS parameter in step  320 . 
     The base station informs an ASN GW of the QoS parameter change in step  330 . At this point, the base station incorporates a data packet including a QoS ID or a DSCP value corresponding to a QoS parameter set that the base station desires to change currently into a header of a GRE packet and transfers the same to the ASN GW via a GRE tunnel. 
     When it is determined in step  340  that a QoS parameter change is allowed by the ASN GW, the base station uses the changed QoS parameter in step  360 . 
     When it is determined that in step  340  that the QoS parameter change is not allowed by the ASN GW, that is, when performance of a DSC process with a terminal is required in order to use the QoS change, the base station performs the DSC process with the terminal in step  350 , and uses the changed QoS parameter in step  360 . 
     The base station determines whether to perform the DSC via an instruction from the ASN GW or the PCRF. When not receiving a DSC performance instruction, the base station may determine to use a changed QoS parameter even without a separate changed QoS parameter use instruction. Alternatively, when receiving a response informing of a QoS parameter change allowance, the base station may determine use of the changed QoS parameter. 
       FIG. 4  is a flowchart illustrating a process for operating an ASN GW or a PCRF according to an exemplary embodiment of the present invention. 
     Referring to  FIG. 4 , the ASN GW is a network entity that can determine a QoS parameter change allowance. 
     When it is determined in step  410  that a QoS parameter change request for a terminal transmitted by a base station is received, the ASN GW or the PCRF determines whether the QoS parameter change is allowable based on stored information. 
     When it is determined in step  420  that the QoS parameter change is not allowable 0 , the PCRF instructs the base station via the ASN GW to use the changed QoS parameter after the terminal and the base station perform a DSC process in step  430 . The ASN GW instructs the base station to use the changed QoS parameter after the terminal and the base station perform the DSC process. 
     On the other hand, when it is determined in step  420  that the QoS parameter change is allowable, the PCRF transmits the changed QoS parameter use instruction to the base station via the ASN GW in step  440 . The ASN GW transmits the changed QoS parameter use instruction to the base station. The QoS parameter use instruction may not be transmitted depending on realization. 
       FIG. 5  is a block diagram illustrating a terminal or a base station according to an exemplary embodiment of the present invention. 
     Referring to  FIG. 5 , as illustrated, the terminal and the base station according to an exemplary embodiment of the present invention include a duplexer  500 , a Radio Frequency (RF) receiver  502 , an Analog to Digital Converter (ADC)  504 , an Orthogonal Frequency Division Multiplexing (OFDM) demodulator  506 , a decoder  508 , a message processor  510 , a QoS manager  511 , a controller  512 , a DSC processor  513 , a message generator  514 , an encoder  516 , an OFDM modulator  518 , a Digital to Analog Converter (DAC)  520 , and an RF transmitter  522 . 
     The duplexer  500  transfers a reception signal from an antenna to the RF receiver  502  and transmits a transmission signal from the RF transmitter  522  via the antenna according to a duplexing scheme. 
     The RF receiver  502  converts an RF signal from the duplexer  500  into a baseband analog signal. The ADC  504  converts an analog signal from the RF receiver  502  into sample data and outputs the same. The OFDM demodulator  506  converts sample data output from the ADC  504  into data in a frequency domain by performing Fast Fourier Transform (FFT). 
     The decoder  508  selects data of subcarriers to be received from data in the frequency domain from the OFDM demodulator  506 , and demodulates and decodes the selected data according to a predetermined Modulation and Coding Scheme (MCS) level. 
     The message processor  510  detects a packet on a predetermined basis from data from the decoder  508 , and performs a header and error test on the detected packet. At this point, when determining a QoS parameter through the header test, the message processor  510  provides a QoS parameter to the controller  512 . That is, the message processor  510  extracts a QoS parameter from a received message and transfers the same to the controller  512 . 
     The controller  512  performs a relevant process based on information from the message processor  510 . In addition, when information transmission is required, the controller  512  generates relevant information and provides the same to the message generator  514 . The message generator  514  generates a message using various information provided from the controller  512  and outputs the same to the encoder  516  of a physical layer. 
     The encoder  516  encodes and modulates data from the message generator  514  according to a predetermined MCS level. The OFDM modulator  518  outputs sample data by performing Inverse Fast Fourier Transform (IFFT) on data from the encoder  516 . The DAC  520  converts the sample data into an analog signal. The RF processor  522  converts an analog signal from the DAC  520  into an RF signal and transmits the same via the antenna. 
     In the above construction, the controller  512  serves as a protocol controller. The controller  512  controls the message generator  514 , the QoS manager  511 , and the DSC processor  513 , and controls an overall operation of the base station and the terminal. That is, the controller  512  may perform the functions of the message processor  510 , the message generator  514 , the QoS manager  511 , and the DSC processor  513 . 
     Separate configuration of the message processor  510 , the message generator  514 , the QoS manager  511 , and the DSC processor  513  in an exemplary embodiment of the present invention is for separately describing each function. However, in actual realization, all or some of the functions of the message processor  510 , the message generator  514 , the QoS manager  511 , and the DSC processor  513  may be processed by the controller  512 . In addition, the functional blocks corresponding to the physical layer (PHY layer) in the drawing may be denoted by a modem. 
     Hereinafter, an operation of the terminal is described with reference to the construction of  FIG. 5 . 
     First, the terminal is described. When a QoS parameter change is required, the QoS manager  511  provides a user data packet where QoS has changed to the message generator  514  via the controller  512 . A case where a QoS parameter change is required may be generated due to various causes such as a user&#39;s request, a network environment, etc. 
     When receiving a DSC request from a base station, the DSC processor  513  controls the modem to perform a DSC process for changing a QoS parameter with the base station. 
     After that, the QoS manager  511  uses the changed QoS parameter. Even when not receiving the DSC request from the base station, the DSC processor  513  controls the modem to use the changed QoS parameter. 
     Hereinafter, an operation of the base station is described with reference to the construction of  FIG. 5 . 
     The operation of the base station is described. When analyzing a user data packet transmitted by a terminal and detecting a QoS parameter change, the QoS manager  511  estimates which is the changed QoS parameter. After that, the QoS manager  511  informs the ASN GW of the QoS parameter change via a modem. At this point, the QoS manager  511  incorporates a data packet including a QoS ID or a DSCP value corresponding to a currently activated QoS parameter set into a header of a GRE packet and transfers the same to the ASN GW via the modem by way of a GRE tunnel When the QoS parameter change is allowed by the ASN GW, the QoS manager  511  controls the modem to use the changed QoS parameter. 
     When the QoS parameter change is not allowed by the ASN GW, that is, when performance of a DSC procedure with a terminal is required in order to use the changed QoS, the DSC processor  513  controls the modem to perform the DSC procedure with the terminal (e.g., step  350  of  FIG. 3 ) and uses the changed QoS parameter. 
     The DSC processor  513  determines whether to perform the DSC via an instruction from the ASN GW or the PCRF. When the DSC processor  513  does not receive a DSC performance instruction, the QoS manager  511  may determine a use of the changed QoS parameter even without a separate instruction of using the changed QoS parameter. 
     The above-described modem is for communication with the terminal and may be called a wireless modem. In addition, the controller  512 , though not shown, includes a wired modem for communicating with an upper node, of course. 
       FIG. 6  is a block diagram illustrating an ASN GW or a PCRF according to an exemplary embodiment of the present invention. 
     Referring to  FIG. 6 , the ASN GW or the PCRF includes a modem  610 , a controller  620 , a storage  630 , a QoS manager  640 , and a DSC processor  645 . 
     The modem  610  serves as a module for communicating with a different apparatus and includes a wired processor and a baseband processor. The wired processor converts a signal received via a wired path into a baseband signal and provides the same to the baseband processor, converts a baseband signal from the baseband processor into a wired signal so that the signal may be transmitted on the wired path, and transmits the signal via the wired path. 
     The controller  620  controls an overall operation of the ASN GW or the PCRF. More particularly, according to an exemplary embodiment of the present invention, the controller  620  controls the QoS manager  640 . 
     The storage  630  stores programs regarding an overall operation of the ASN GW or the PCRF and temporary data occurring during execution of programs. 
     When receiving a QoS parameter change request for a terminal transmitted by a base station, the QoS manager  640  determines whether the QoS parameter change is allowable based on information stored in the storage  630 . 
     When the QoS parameter change is not allowable, after the terminal and the base station perform a DSC procedure, the DSC processor  645  of the PCRF instructs the base station to use the changed QoS parameter via the ASN GW. 
     When the QoS parameter change is not allowable, after the terminal and the base station perform the DSC procedure, the DSC processor  645  of the ASN GW instructs the base station to use the changed QoS parameter. 
     When the QoS parameter change is allowable, the QoS manager  640  of the PCRF transmits an instruction of using the changed QoS parameter to the base station via the ASN GW. 
     When the QoS parameter change is allowable, the QoS manager  640  of the ASN GW transmits an instruction of using the changed QoS parameter to the base station. 
     Here, regardless of the ASN GW or PCRF, the QoS manager  640  may not transmit any instruction when the QoS parameter change is allowable. 
     In the above construction, the controller  620  serves as a protocol controller and controls the QoS manager  640 . That is, the controller  620  may perform the function of the QoS manager  640 . 
     The configuration of the QoS manager  640  is described separately to clarify each function. However, in actual realization, all or some of the functions of the QoS manager  640  may be processed by the controller  620 . 
     According to exemplary embodiments of the present invention, during an initial connection setting procedure of the aGPS service class, a terminal and a network negotiate a plurality of QoS parameter sets to be used, and then may change to a QoS parameter set and use the same without a separate process of transmitting a control signal when needed. 
     While the invention has been shown and described with reference to certain exemplary embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims and their equivalents.