Abstract:
In a base station of an IEEE 802.16 Wireless MAN-based portable Internet system, a forward packet scheduling method and apparatus is provided. The portable Internet system portable Internet service includes a user terminal AT for providing a portable Internet service, a base station AP for supporting an inter-user terminal wireless access and network connection, and a packet access router PAR for performing a mobility control and packet routing function of each user terminal AT. The forward packet scheduler includes a packet-based subsidiary scheduling part for considering a real-time service and a user-based scheduling part for ensuring fairness and increasing system efficiency (throughput). After finishing the 2-step scheduling, the generated frame data are transmitted though a physical layer to a wireless user terminal. Therefore, with such a structure, the forward packet scheduling method may guarantee a maximum of system efficiency and fairness according to each user&#39;s QoS requirements, and may also guarantee a delay performance for a real-time service.

Description:
[0001]     This application claims priority under 35 U.S.C. §119 to a Korean application filed in the Korean Intellectual Property office on Nov. 11, 2005 and allocated Serial No.10-2005-0107961, the contents of which are incorporated herein by reference.  
       BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates to a forward (downlink) packet scheduling method and apparatus in a base station of a portable Internet system, and more particularly, the present invention relates to a forward packet scheduling method and apparatus in a base station of an IEEE 802.16 Wireless MAN-based portable Internet (WiBro) system.  
         [0004]     2. Description of the Related Art  
         [0005]     The IEEE 802.16 Wireless Metropolitan Area Network (MAN)-based (WiBro) system is a 3.5 Generation mobile communication system providing image and high-speed packet data transmission such as various IP-based wireless data services (streaming video, mail, chafting) that the wired Internet provides, using a wireless transmission technology ensuring a spectrum use efficiency in a 2.3 GHz frequency bandwidth.  
         [0006]      FIG. 1  is a schematic diagram of a conventional portable Internet (WiBro) system.  
         [0007]     As shown in  FIG. 1 , a WiBro (Wireless Broadband) system includes a user terminal (also known as Access Terminal or “AT”) for using a portable Internet service, a base station  20  (also known as Access Point or “AP”) supporting a wireless access and network connection between the user terminals  10 , and a packet access router  30  (“PAR”) for performing an each-terminal mobility control and packet routing function.  
         [0008]     The user terminal  10  may receive a wireless data service after the user terminal is recorded at the base station  20 . In addition, the user terminal  10  may configure a plurality of traffic connections having different Quality of Service (QoS) requirements through a negotiation with the base station  20  so as to connect services having different service characteristics such as a web connection and FTP.  
         [0009]     The base station  20  performs access control of the user terminal  10 , packet mapping between the wired and wireless areas, wireless transmitting/receiving control, and wireless band management.  
         [0010]     The packet access router (PAR)  30  performs an each-user-terminal mobility control and packet routing function.  
         [0011]     The WiBro system transmits/receives data every frame so as to support high speed packet data transmission in the wireless area, and is operated based on an OFDM/FDMA/TDD (Orthogonal Frequency Division Multiplexing/Frequency Division Multiple Access/Time Division Duplex) wireless transmission scheme.  
         [0012]      FIG. 2  is a schematic diagram of a wireless frame of a portable Internet (WiBro) system.  
         [0013]     A frame includes a downlink (DL) frame and an uplink (UL) frame. The respective frames transmit a MAP message including frame configuration information at the leading part thereof and then transmit a data burst. Therefore, a scheduler for performing wireless management allocates/manages a sub-channel so as to transmit the respective user data every frame and configures a MAP message based on the sub-channel-related information.  
         [0014]     The MAP message includes a DL-MAP for downstream traffic and a UL-MAP for upstream traffic, and each MAP includes a GMH (Generic MAC Header), IE (Information Element) informing data burst information, and CRC (Cyclic Redundancy Check). The IE defines a data region, stores each data region information (which are needed in each data region), and informs of each data region information.  
         [0015]     Among the respective IEs, Normal DIUC (DL Interval Usage Code) IE, Hybrid Automatic Repeat Request (HARQ) DL MAP IE, Normal UIUC (UL Interval Usage Code) IE, HARQ UL MAP IE, etc. refers to data burst contents and includes information about which user receives the same or similar information.  
         [0016]     The uplink frame includes Channel Quality Indicator Channel (CQICH), Acknowledge Channel (ACKCH), and Code Division Multiple Access (CDMA) ranging in an uplink control symbol (UL control symbol). A CDMA ranging channel includes information for a random access using CDMA code, the CQICH ranging is for adaptive modulation, and the ACK is ACK information for HARQ.  
         [0017]     According to the OFDM/FDMA/TDD scheme, data transmitting/receiving is performed by a sub-channel, wherein the sub-channel is configured by a sub-carrier group. In addition, the respective frames transmit a MAP message including frame configuration information at the leading part thereof and then transmit a data burst (the burst being a set of Packet Data Units (PDUs) to be transmitted for the respective users, and the same radio channel parameter is used so as to transmit a single burst). Therefore, the base station  20  for performing wireless management allocates/manages a sub-channel to transmit/receive the respective user data every frame, configures the MAP message based on the sub-channel concerning information, and the user terminal  10  receives the MAP message, receives the upstream/downstream data burst information allocated to the base station, and transmit/receives the corresponding data burst.  
         [0018]     Generally, in almost all wireless communication systems, the overall efficiency or transmission characteristics thereof largely depends on which scheduling algorithm is used. Therefore, a scheduling algorithm may be chosen according to usage. Representatively, a method for maximizing system transmission efficiency and a method for ensuring fairness between the terminals have been proposed.  
         [0019]     The first method can maximize system throughput because almost all radio resources are allocated to the terminal having the best channel state. However, there is a problem in that the terminal having a poor channel state may have sufficient service because the radio resources are not allocated regardless of a large amount of data to be transmitted.  
         [0020]     The second scheme can fairly allocate a radio resource to all terminals regardless of the channel states because it uniformly allocates radio resources to all the terminals. However, the second scheme has a drawback in that the system throughput may be decreased because a large amount of resources is allocated to terminals having a poor channel state. That is, these two methods each have a fatal drawback. Therefore, recently, wireless systems prefer a proportional-fairness (hereinafter, called “PF”) scheme for enhancing system throughput by means of a system throughput/fairness trade-off. However, the PF scheme has a drawback in that each user&#39;s different QoS requirements and real-time service are not considered.  
         [0021]     The conventional wireless communication system supports a single service. However, the WiBro system supports various multi-media services such as a real-time service, a non-real-time service, and the newest service. Therefore, the WiBro system requires scheduling considering each service&#39;s QoS requirements. In addition, the scheduler of the WiBro system must determine a proper MCS (Modulation and Coding Scheme) level based on the user channel state according to the requirements of the system applying an AMC (Adaptive Modulation and Coding) scheme so as to ensure wireless transmission. In order to easily realize the WiBro system as well as to solve the above-described problems, scheduling must be performed before frame synchronization because frame synchronization is exactly matched to satisfy the frame-based WiBro system operation conditions and system performance, and a minimum process time must be ensured because the MAP message is configured.  
       SUMMARY OF THE INVENTION  
       [0022]     The present invention has been made in an effort to provide a forward packet scheduling method and apparatus in a base station of a portable Internet system having advantages of increasing system efficiency and fairness according to each user&#39;s QoS requirements considering a real-time service by a forward packet scheduler in a base station of the IEEE 802.16 Wireless MAN-based portable Internet (WiBro) system, and reducing a process time.  
         [0023]     A forward packet scheduling apparatus according to the present invention includes a packet classifier for analyzing an IP header of an IP packet received from the wired network and determining the corresponding user, connection ID, and a priority; 
        a storage unit for storing the IP packet received until the wireless period is transmitted;     a scheduler for determining a transmission order according to the priority of the to-be-transmitted data stored at the storage unit, for determining capability of data transmission and a modulation and coding scheme according to the radio channel state, for selecting data to be transmitted every frame selection and capability thereof, for performing wireless period packet transmitting/receiving control, and for performing a wireless frame resource management function; and     a PDU generator for receiving PDU generation information from the scheduler, selecting data from the storage unit, generating a MAC PDU and data burst, and transmitting the generated MAC PDU and data burst through the physical layer to the user terminal.        
 
         [0027]     In addition, a forward packet scheduling method in a base station of a portable Internet system including a user terminal for providing a portable Internet service, a base station for supporting an inter-user terminal wireless access and network connection, and a packet access router for performing a mobility control and packet routing function of each user terminal, according to the present invention, includes analyzing an IP header of an IP packet received from a wired network and determining a corresponding user, a connection ID, and a priority; storing the received IP packet until before the wireless area transmission; determining scheduling of an order of the stored data to be transmitted according to the priority, determining a transmission capability of data to be transmitted and a modulation and coding scheme according to a radio channel state by means of subsidiary scheduling and main scheduling, selecting a to-be-transmitted data capability for the respective frames, controlling wireless packet transmitting/receiving, and managing a wireless frame resource; and receiving PDU generation information according to the scheduling, selecting the stored data, generating a MAC PDU and a data burst, and transmitting the generated MAC PDU and data burst to a physical layer. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0028]     The above and other objects, features and advantages of the present invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings in which:  
         [0029]      FIG. 1  is a schematic diagram of a conventional portable Internet (WiBro) system;  
         [0030]      FIG. 2  is a schematic diagram of a wireless frame of a portable Internet (WiBro) system;  
         [0031]      FIG. 3  is a schematic diagram showing a forward packet process module in a base station;  
         [0032]      FIG. 4  is a schematic diagram showing an operation of a forward packet scheduler according to the present invention;  
         [0033]      FIG. 5  is a flowchart showing a subsidiary scheduling part in a base station scheduler according to the present invention; and  
         [0034]      FIG. 6  is a flowchart showing a main scheduling part in a base station scheduler according to the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0035]     The present invention will hereinafter be described in detail with reference to the accompanying drawings. As those skilled in the art would realize, the described embodiments may be modified in various different ways, all without departing from the spirit or scope of the present invention. Accordingly, the drawings and description are to be regarded as illustrative in nature and not restrictive. Like reference numerals designate like elements throughout the specification.  
         [0036]      FIG. 3  is a schematic diagram showing a forward packet process module in a base station.  
         [0037]     As shown in  FIG. 3 , a forward traffic process module of the base station (AP) includes a packet (for example, Service Data Unit, SDU) classifier  100 , a storage unit  110 , a scheduler  120 , and a PDU generator  130 .  
         [0038]     The packet (SDU) classifier  100  analyzes an IP header of an IP packet (SDU) received from the wired network and determines the corresponding user, connection ID, and a priority.  
         [0039]     The storage unit  110  stores the received IP packet until the wireless period is transmitted, and is realized as a soft storage unit.  
         [0040]     The scheduler  120  (i.e. a forward packet scheduler in the AP) determines a transmission order according to the priority of the transmission data stored in the storage unit  110 , determines capability of data transmission and a modulation and coding scheme (MCS) according to the radio channel state, and performs selection of data (and data capability) that the respective frames can transmit, wireless period packet transmitting/receiving control, and a wireless frame resource management function.  
         [0041]     The PDU generator  130  receives PDU generation information from the scheduler  120 , selects data from the storage unit  110 , generates a MAC PDU and data burst, and transmits the generated MAC PDU and data burst to the physical layer  140 .  
         [0042]     That is, the base station performs scheduling based on the received traffic state information and the respective user radio channel state information, generates frame data by combining the MAC-PDUs, and transmits the frame data to the physical layer  140 .  
         [0043]     Particularly, the scheduler  120  is placed in the forward traffic process module and determines a transmission order according to the priority of the data to be transmitted, configures an MCS according to the radio channel state, and combines transmission data for the respective user terminal (AT) by generating a MAP message and wireless frame data. That is, a wireless packet transmitting/receiving control and wireless frame resource management function is performed. The scheduler  120  receives buffer state information including the priority of the stored data and data capability from the storage unit  110 , receives channel state information from the physical layer  140  by determining the transmission capability of a data block to be transmitted and the MCS and controlling the PDU generator  130  generating the MAC PDU thereby generating a MAC PDU and data burst, and transmits the MAC PDU and data burst through the physical layer  140  to the corresponding user terminal.  
         [0044]      FIG. 4  is a schematic diagram showing an operation of a forward packet scheduler according to the present invention.  
         [0045]     As shown in  FIG. 4 , the forward packet scheduler  120  of the base station performs packet-based subsidiary scheduling for considering a real-time service and user-based main scheduling for ensuring fairness and enhancing system throughput. After the two-step scheduling is finished, the frame data generating the PDU of the stored data are transmitted through the physical layer to the wireless area.  
         [0046]     According to the present invention, the forward packet scheduler  120  of the base station is operated according to the two step scheduling process as shown in  FIG. 4 . That is, the forward packet scheduler  120  performs subsidiary scheduling to calculate a service stand-by time priority value for the respective packets that wait for service for the connections supporting the real-time service, provides service of a MAC layer to the packet having a higher priority, and allocates the radio resource. The forward packet scheduler  120  performs user-based main scheduling to calculate a user link state and a used service data speed priority value, determines a service user, and supports a service capability and MAC service and radio resource allocation for the service stand-by connections of the corresponding user. After the scheduler  120  performs the two-step scheduling, it transmits the generated frame data through the physical layer  140  to the wireless area.  
         [0047]     Performing forward packet scheduling for a base station of a portable Internet will now be described in detail according to the present invention.  
         [0048]     The base station performs scheduling during a predetermined time period every frame. When it enters into the scheduling period, the base station first performs a MAC service and radio resource allocation to a MAC control message between the user terminal and the base station (AT-AP). A service order is determined on an FCFS (First Come First Served) basis. After the control message scheduling is finished, the subsidiary scheduling and main scheduling are performed.  
         [0049]      FIG. 5  is a flowchart showing a subsidiary scheduling part of a base station scheduler according to the present invention.  
         [0050]     A scheduling rule of a subsidiary scheduling part according to the present invention is expressed as Equation 1.  
                 S   =         arg   ⁢           ⁢   max       i   ∈   A       ⁢     {     P   ⁢           ⁢       M   i     ⁡     (   k   )         }         ,     
     ⁢       condition   ⁢           ⁢   …   ⁢           ⁢   P   ⁢           ⁢       M   i     ⁡     (   k   )         ≥     PM   threshold       ,       for   ⁢           ⁢   i     ∈   A       ⁢     
     ⁢     where   ,     
     ⁢         PM   i     ⁡     (   k   )       ⁢     :     ⁢           ⁢   Priority   ⁢           ⁢   Metric   ⁢           ⁢   for   ⁢           ⁢   HOL   ⁢           ⁢   packet   ⁢           ⁢   in   ⁢           ⁢   connection   ⁢           ⁢     (   i   )     ⁢           ⁢   at   ⁢           ⁢   the   ⁢           ⁢     (   k   )     ⁢           ⁢   service   ⁢           ⁢   time       ⁢     
     ⁢       PM   threshold     ⁢     :     ⁢           ⁢   Threshold   ⁢           ⁢   constant   ⁢           ⁢   of   ⁢           ⁢   Priority   ⁢           ⁢   Metirc             (   1   )             
 
         [0051]     Referring to  FIG. 5 , the scheduler of the base station starts downlink (DL) scheduling in step S 101 . When the rest sub-channei number Nrest&gt;0, it starts subsidiary scheduling in step S 102 .  
         [0052]     The scheduler calculates a Priority deterMination (PM) variable PM i  from Head-Of-Line (HOL) packets for buffering packets on a connection (i) of an active real-time service in the present cell in step S 103 . If it is determined that the priority determination variable PM i  is greater than or equal to a threshold value  PM threshold in step S 104 , the corresponding packet information is aligned and stored in a PM queue (PMQ, the PMQ is a list having packet information in a higher PM value order) in step S 105 . When the priority determination variable PM i  is smaller than a threshold value  PM threshold, the scheduler stops a PM value calculation for packets of the selected connection and starts a PM value calculation for packets of other connections. The PM value calculation follows Equation 2.  
                     P   i     ⁡     (   k   )       =       γ   i     ⁢     1     (       W     i   ,   MAX       -       W   i     ⁡     (   k   )         )           ,     
     ⁢       γ   i     ⁢     :     ⁢           ⁢   Connection   ⁢           ⁢     (   i   )     ⁢           ⁢   priority   ⁢           ⁢   factor       ⁢     
     ⁢       W     i   ,   MAX       ⁢     :     ⁢           ⁢   Connection   ⁢           ⁢     (   i   )     ⁢           ⁢   delay   ⁢           ⁢   limit     ⁢     
     ⁢         W   i     ⁡     (   k   )       ⁢     :     ⁢           ⁢   Connection   ⁢           ⁢     (   i   )     ⁢           ⁢   HOL   ⁢           ⁢   packet   ⁢           ⁢   delay             (   2   )             
 
         [0053]     When the PM value calculation is finished for all the active real-time service connections (Active Connections), the MAC service and radio resource allocation are performed for packets in a PMQ order. The selected packets are included in the MAC PDU for the wireless area transmission, and it is checked whether the radio resource for the wireless area transmission is allocated. So as to calculate a necessary radio resource allocation, an MCS level is determined according to a user channel state for the primarily selected packet (i-th PMQ item) in step S 106 . At this time, the MCS level determines a modulation and coding scheme level in an MCS determination table (CQI Threshold-MCS level), which is a cell configuring parameter using channel information (Channel Quality Indicator, “CQI”) received from the terminal every frame. The MCS determination table includes a minimum entry channel quality (Minimum Entry Threshold) for respective MCS levels and a mandatory exist channel quality (Mandatory Exit Threshold). The MCS determination table determines an MCS level in which the CQI value is within these threshold values when CQI values of each user terminal are received. When the MCS level of the corresponding user terminal is determined, the needed sub-channel number Nneeded of the wireless frame for servicing data is calculated according to Equation 3 in step S 107 .  
         [0054]     Needed sub-channel number=allocated data size(byte)/(MODXCR)X6, where MOD: modulation mode factor(QPSK: 2,16QAM:4,64QAM:6) CR: coding rate.  
         [0055]     It is then determined whether the needed sub-channel number (Nneeded) is smaller than the rest sub-channel number Nrest of the present frame in step S 108 . When the needed sub-channel number (Nneeded) is smaller than the rest sub-channel number Nrest of the present frame, the resource allocation for the corresponding packet is finished, and when the needed sub-channel number (Nneeded) is greater than or equal to the rest sub-channel number Nrest of the present frame, capability of service data transmission though the used sub-channel is determined. At this time, when the connection for the corresponding packet may be upper-packet divided, the PDU and burst (Burst) are generated for a part of a transmittable packet, the MAP IE is formed, the rest channel number Nrest is updated, and accordingly the MAC PDU is generated in step S 109 .  
         [0056]     When the MAC service and radio resource allocation for the HOL packet information in the PMQ are finished, the corresponding packet information (PMQ Active items) are deleted in step S 110 , and then the same processes are repeated for the next packet information. However, during the operation, if there are no radio resources needed, the subsidiary scheduling process stops. The QoS of the real-time service may be guaranteed though such subsidiary scheduling.  
         [0057]      FIG. 6  is a flowchart showing a main scheduling part in a base station scheduler according to the present invention.  
         [0058]     The user terminal-based main scheduling part is operated as follows. The scheduler of the base station starts downlink packet scheduling (forward packet scheduling) in step S 201 . When the rest sub-channel number of the present frame Nrest&gt;0 is determined in step S 202 , the priority determination variable PM j  for the user terminal of a presently active state is calculated and a service is preferentially provided for the terminal having the maximum priority determination variable max PMi among the user terminal service request list AtSvcReqList in step S 203 .  
         [0059]     The scheduling rule and PM j  calculation formula is described with reference to Equation 4.  
                 S   =         arg   ⁢           ⁢   max     j     ⁢     {       PM   j     ⁡     (   k   )       }         ,     
     ⁢         PM   j     ⁡     (   k   )       =       γ   j     ⁢         DRC   j     ⁡     (   k   )       /       A   j     ⁡     (   k   )             ,     
     ⁢   where   ,     
     ⁢     k   ⁢     :     ⁢           ⁢   Service   ⁢           ⁢   Time   ⁢           ⁢     (     Frame   ⁢           ⁢   Number     )         ⁢     
     ⁢         γ   j     ⁡     (     =       ρ   j     ×     R   j         )       ⁢     :     ⁢           ⁢   priority   ⁢           ⁢   factor   ⁢           ⁢   for   ⁢           ⁢     AT   ⁡     (   j   )         ⁢     
     ⁢         DRC   j     ⁡     (   k   )       ⁢     :     ⁢           ⁢   Current   ⁢           ⁢   Channel   ⁢           ⁢   State   ⁢           ⁢   for   ⁢           ⁢     AT   ⁡     (   j   )         ⁢     
     ⁢         A   j     ⁡     (   k   )       ⁢     :     ⁢           ⁢   Avarage   ⁢           ⁢   Service   ⁢           ⁢   Rate   ⁢           ⁢   for   ⁢           ⁢     AT   ⁡     (   j   )         ⁢     
     ⁢       R   j     (       =       ∑   i     ⁢     r   i         ,       r   i     ⁢     :     ⁢           ⁢   Target   ⁢           ⁢   Service   ⁢           ⁢   Rate   ⁢           ⁢   for   ⁢           ⁢   Connection   ⁢           ⁢     (   i   )       ,     i   ∈     AT   ⁡     (   j   )           )     ⁢     
     ⁢       :     ⁢           ⁢   Target   ⁢           ⁢   Service   ⁢           ⁢   Rate   ⁢           ⁢   for   ⁢           ⁢     AT   ⁡     (   j   )                 (   4   )             
 
         [0060]      P j is a user priority factor negotiated between the base station (AP) and the user terminal (AT) at the registration of a user, and R j  is the sum of service speeds (a minimum service speed negotiated between the base station and the user terminal on the traffic connection configuration) of the present predetermined traffic connections for the user terminal. DRC j (k) is a variable for indicating a channel state of the present user terminal, and A j (k) is an average data speed serviced for user (j) thus far.  
         [0061]     When the users are aligned in a higher priority determination variable PMj, the needed radio resources are calculated so as to transmit a total capacity of traffic of a service stand-by state with respect to all the predetermined connections to the user terminal having the higher priority determination variable. The needed radio resource calculation for providing a MAC service for the selected user traffic is performed in the same manner as the radio resource calculation method of the subsidiary scheduling part. That is, according to the user&#39;s channel state information (CQI), the modulation and coding scheme (MCS) is determined and the needed sub-channel number Nneeded is calculated according to Equation 3 in step S 204 .  
         [0062]     When it is determined the needed sub-channel number Nneeded is smaller than the rest sub-channel number Nrest of the present frame in step S 205 , the radio resource allocation is finished for all the stand-by packets of the selected user terminal.  
         [0063]     That is, the PDU and Burst are generated and the MAP IE is configured until there is no rest radio resource for all the stand-by packets of the selected user terminal in step S 207 . When there is no rest radio resource, the item is cancelled in the service request list AtSvcReqList of the user terminal in step S 208 , and the rest sub-channel number Nrest is updated in step S 209 .  
         [0064]     When the needed sub-channel number Nneeded is greater than the rest sub-channel number Nrest of the present frame, the service data capability C allow  that is transmittable to the used sub-channel is calculated, the radio resource is allocated to a part of the stand-by traffic, and the MAC PDU is generated in step S 206 , and steps S 207 , S 208 , and S 209  are performed.  
         [0065]     When there is a rest radio resource ((Nrest−Nmargin&lt;=0)), the same steps S 203  to S 209  are repeated for the next user terminal in step S 210 .  
         [0066]     The main scheduling part can increase system efficiency as well as guarantee fairness according to each user QoS negotiation specification because it uses an PF algorithm for supporting the inter-user fairness and the optimum inter-system efficiency trade-off considering QoS of each user.  
         [0067]     According to the present invention, the priority determination variable calculation for the service stand-by packets is performed while being limited in a real-time service and is performed while being limited to packets having a priority determination variable greater than a threshold value rather than for all the stand-by packets, and the main scheduling uses a user-based priority determination variable calculation and radio resource allocation, thereby reducing an operation time. Since the base station forward packet scheduler of a 5 msec frame-based Wibro system must process scheduling and MAC of high speed traffic within an interval of 5 msec, the minimum of processing time according to the present invention has a great merit.  
         [0068]     The recording medium may include all types of recording medium that a computer can read, for example an HDD, a storage unit, a CD-ROM, a magnetic tape, and a floppy disk, and it may also be realized in a carrier wave (e.g., Internet communication) format.  
         [0069]     While this invention has been described in connection with what is presently considered to be practical exemplary embodiments, it is to be understood that the invention is not limited to the disclosed embodiments, but, on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.