Abstract:
In a TCP/IP network, congestion control techniques such as slow start and congestion avoidance are employed. Such networks include wired and wireless links. However, normal operation of the wireless links exhibit different latencies than those exhibited over the wired link. The protocols employed in the wired network do not lend themselves well to efficient communication over wireless connections, and can cause slow start to be triggered. Determining when a sender will timeout due to non-receipt of an ACK, and intervening with a suppression message having an advertised window of zero to pause the user, are employed to prevent congestion control mechanisms such as slow start and congestion avoidance from activation.

Description:
CROSS REFERENCE TO RELATED APPLICATION  
       [0001]     This application is a continuation of U.S. patent application Ser. No. 09/777,557, filed Feb. 5, 2001, which is incorporated by reference as if fully set forth. 
     
    
     BACKGROUND  
       [0002]     Wireless network infrastructure equipment is increasingly being used to allow computing devices to communicate over a wireless medium to a wired network such as the Internet. In a wireless data network, a plurality of local computing devices, such as PCs, are supported via wireless subscriber access units. Each subscriber access unit (SAU) provides a wireless radio link to a base station processor. The base station processor (BSP) is also connected to an Internet gateway that provides a connection to a wired network.  
         [0003]     Wired networks typically employ congestion control techniques to detect the speed with which messages are propagated across the network to a recipient. These techniques reduce congestion through avoiding overburdening a recipient with messages by reducing the rate at which messages are sent, and consequentially reducing throughput. However, normal operation of the wireless network exhibits different latencies than those exhibited during normal operation of the wired network. Accordingly, such techniques can interpret the queuing of messages at the base station processor as congestion, and accordingly, reduce throughput. In general, the protocols employed in the wired network do not lend themselves well to efficient communication over wireless connections.  
         [0004]     In a TCP/IP network, for example, congestion control techniques such as slow start and congestion avoidance are employed. In accordance with the slow start technique, as defined in Internet RFC 2581, an acknowledgment message (ACK) is expected as a response to every second packet (message) sent. A sliding window protocol is employed to regulate the number of unacknowledged messages which can be outstanding at any time. This sliding window, which is indicative of the number of unacknowledged message permitted at any time, is initially set at a low number, typically two messages. The number of messages permitted in the window is gradually increased as the ACKs are received in a timely manner. If, however, an ACK is not received after a timeout threshold, or if duplicate ACKs are received, the window may be reset to the initial value (typically one) and must again be permitted to gradually increase as described above.  
         [0005]     The queuing of messages at the base station processor, however, is not necessarily indicative of congestion in the wireless network. Rather, the queuing is indicative of the propagation delay or the assignment delay of wireless resources. This propagation delay is interpreted, however, as congestion by the wired line protocols such as TCP/IP when the ACK is not received within the timeout expected by the wired network. Accordingly, the wireless connection tends to be throttled back to a sliding window of two by slow start more frequently, thereby reducing throughput.  
         [0006]     Another TCP/IP congestion control parameter employed by congestion avoidance and slow start is an advertised window. The advertised window is contained in the ACK and informs the sender how many more messages the receiver can accept. This prevents a sender from overburdening a receiver with more packets than it can buffer, therefore avoiding sending packets which are likely to be dropped or result in timeout. While technically separate, the slow start and congestion avoidance mechanisms are typically implemented in a complementary manner. The sliding window and slow start are congestion control imposed by the sender, while the advertised window is congestion control imposed by the receiver.  
         [0007]     A further aspect of the advertised window mechanism is a persist mode. When a receiver can accept no more data, it sends a message having an advertised window of zero. This transmission has the additional effect of preventing the sender from sending any more data until the receiver sends another ACK message with a nonzero advertised window. Persist mode, therefore, allows the receiver to “pause” the sender until more messages can be processed at the receiver side. However, an ACK indicating an advertised window of zero does not trigger slow start and reset the sliding window.  
         [0008]     It would be beneficial, therefore, to provide a system and method for determining when a connection including a wireless link is about to experience a timeout, and sending a suppression message to trigger a persist mode to avoid slow start from resetting the window size, while maintaining the same TCP/IP end-to-end connection between the sender and receiver to avoid tearing down and buffering messages to accommodate the wireless latency.  
       SUMMARY OF THE INVENTION  
       [0009]     In a multiplexed system, multiple users share access to physical layer resources, such as radio channels. There are always delays inherent in assignment and reassignment of the physical layer resource, which propagate up to the higher protocol layers. Such additional delays are not insignificant. For example, a Round Trip Transfer (RTT) delay may be on the order of one second in a typical TCP/IP network layer protocol, whereas physical layer resources may require 200 milliseconds or more to reassign. Thus, a time out mechanism which only accommodates the return layer delay will unnecessarily time out prematurely.  
         [0010]     A system and method are disclosed for monitoring and controlling message delivery from a remote node by detecting when an incoming message is received, determining a timeout corresponding to the timing of the acknowledgment message, and sending a suppression message if the acknowledgment message has not been sent to the remote node before the timeout expires. The invention prevents activation of congestion control mechanisms which reduce throughput. By determining when a sender will timeout due to non-receipt of an ACK, and intervening with a suppression message to pause the sender, congestion control mechanisms such as slow start and congestion avoidance are prevented from activation. In this manner, the reduction in message throughput caused by congestion control mechanisms is avoided, thereby allowing the sender to resume message delivery at the same rate at which it was delivering messages when the suppression message was received.  
         [0011]     Sending a suppression message, therefore, prevents the sender from timing out for failure to receive a timely ACK. Since the sliding window is not reset to the initial size, as would have occurred if a timeout occurred, the receiver can send a resume message when the ACK is received. The sender then resumes message transmission with the same window size that was in effect when the suppression message was received. Since a larger window potentially allows more packets to be sent at a time, message traffic throughput is increased.  
         [0012]     The connection and associated parameters between the sender and receiver remain consistent throughout the pause periods. Since the connection remains a single end-to-end connection, no tearing down or setting up of connections and buffering messages received in the interim, is required. Further, no connection parameters at either the sender or receiver in the wired network need be modified to conform to differences in the wired and wireless communication links. In this manner, the users served by the wireless link are less burdened with the wired link congestion control mechanisms which can tend to have a negative result on throughput over a wireless link.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]     The foregoing and other objects, features and advantages of the invention will be apparent from the following more particular description of preferred embodiments of the invention, as illustrated in the accompanying drawings in which like reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the principles of invention.  
         [0014]      FIG. 1  shows a wireless communication system operable to perform message transmission according to the link-aware transmission control protocol as defined herein;  
         [0015]      FIG. 2  shows a wireless gateway connection in the wireless communication system of  FIG. 1 ;  
         [0016]      FIG. 3  shows a protocol stack for performing the link-aware transmission control protocol as defined herein;  
         [0017]      FIG. 4  shows message transmission as defined by the present claims;  
         [0018]      FIG. 5  shows transmission of a suppression message;  
         [0019]      FIG. 6  shows an alternate embodiment modifying the advertised window;  
         [0020]      FIG. 7  shows a flowchart of message transmission; and  
         [0021]      FIGS. 8   a - 8   g  show an example of message transmission as defined herein. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0022]     A description of a preferred embodiment of the invention follows.  FIG. 1  is a block diagram of a communication system  10  operable for link-aware transmissions in a wireless network as defined herein. The communication system includes local computing devices, generally a user PC  12 , a subscriber access unit (SAU)  14 , a base station processor (BSP)  16 , and an internetworking gateway  18 . The user PC  12  is in communication with the subscriber access unit  14  via a wired link  20 . The subscriber access unit  14  is in communication with the base station processor  16  via a wireless link  26 . The base station processor is in communication with an internetworking gateway  18  via a wired link  24 . The internetworking gateway  18  is adapted for communication via a public access network such as the Internet  28  for maintaining a connection with a remote node  30 . Note that a single user PC  12  subscriber access unit (SAU)  14  and base station processor (BSP)  16  are shown for illustrative purposes. Multiple SAUs  14  may be interconnected in an actual implementation.  
         [0023]     The user PC  12  may therefore be provided access to the internetworking gateway  18 , which may include any remote entity located on the Internet  28  or other network, through a combination of the wired  20 ,  24  and wireless links  26  provided. The wired links  20 ,  24  are typically supported by a protocol such as TCP/IP or UP/IP. The wireless link is supported by a wireless link protocol such as IS 95  or another wireless link protocol such as the protocol described in pending U.S. Patent Application entitled “Dynamic Frame Sizing Settings for Multichannel Transmission,” published as PCT application No. WO 99/44341, Sep. 2, 1999.  
         [0024]     Typically, the PC  12  provides a data packet, which may for example be an Internet Protocol (IP) packet, to the subscriber access unit  14  over the wired link  20  which may for example be an Ethernet type connection. The subscriber access unit  14  removes the framing of the data packet and transfers the data in the data packet to the base station processor  16  over the wireless link  26  in accordance with the wireless link protocol. The base station processor  16  extracts the wireless link frames and forwards them, in data packet form, over the wired link  24  to the internetworking gateway  18 .  
         [0025]     Similarly, packets sent from the public access network are sent to the base station processor  16  over the wired link  24 , transmitted to the corresponding subscriber access unit  14  over the wireless link  26 , and sent to the user PC  12  over the wired link  20 . The subscriber access unit  14  and the base station processor  16  therefore denote endpoints of the wireless link  26 , providing for wireless communication from the user PC  12  to the public access network such as the Internet  28 .  
         [0026]     In a network including a wireless link, therefore, a point to point connection is maintained between two entities via the wireless link. Accordingly, since bidirectional communication is provided, the base station processor  16  and the subscriber access unit  14  each provide a wireless gateway  230  supporting the wireless link  26 . Referring to  FIG. 2 , a wireless link  26  is shown between wireless gateways  230 . Since the communication is bidirectional, the system and methods described below are applicable to wireless gateways  230  on either side of the wireless link  26 .  
         [0027]     The wireless gateways  230  each include a timer manager  232 , a link detector  234 , a segment generator  236 , and a packet buffer  238 . The timer manager  232  computes the round trip time corresponding to the time at which an ACK message is expected. When an incoming message is received from a wired link  40 , a timer is set just prior to the time at which the corresponding sender will timeout for failure to receive the ACK. The incoming message is then forwarded over the wireless link  26 . If the timer expires before the corresponding ACK is received over the wireless link  26 , the segment generator  236  generates a suppression message, and sends it to the sender (not shown) over the wired link  40 . The suppression message tells the sender to not send any more messages until the receiver sends a resume message. When the corresponding ACK is received from the wireless network, it is stored in the packet buffer  238 . The segment generator  236  forwards the ACK message to the sender over the wired link  40 . The sender then interprets this as a resume message.  
         [0028]     Previous prior art approaches include MTCP, outlined in Brown, et al., “M-TCP: TCP for Mobile Cellular Networks,” Dept. of Computer Science, University of South Carolina, Jul. 29, 1997. This paper described a method using pico-cell cellular networks which implemented an altered TCP stack. The M-TCP system did not maintain a single point-to-point connection between a wireless subscriber and wired network server, but rather terminated the wired TCP connection and instantiated a separate connection over the wireless link, and employed a slightly modified TCP/IP stack on the user computer.  
         [0029]     Referring to  FIG. 3 , a TCP/IP stack corresponding to the wireless network is shown. The link-aware transmission control protocol (LTCP) defined herein is disclosed. The protocols supporting the point-to-point connection  32  between the remote server  30  and the local user PC  12  are shown, including the transport  33   a , network  33   b , link  33   c , and physical  33   d  layer protocols.  
         [0030]     Referring to  FIGS. 4 and 1 , a diagram of message transmission is shown. The remote server  30  transmits two messages at  34   a  to the BSP  16 , each containing 1460 bytes of data, over the connection on the wired links  24 ,  18 , and  28 . The BSP  16  transmits the messages to the SAU  14  over the wireless link  25  at  34   b . The SAU  14  transmits the messages to the user PC  12  at  34   c  over the wired link  24 . The user PC responds with an ACK advertising a receive window of 8760 back to the SAU  14 , as shown at  34   d.    
         [0031]     It should be noted, for reasons which will become apparent below, that in a TCP/IP connection, the ACK messages indicate the last byte received, and may correspond to more than one received packet. Accordingly, the ACK messages need not complement the received messages on a one to one basis. Further, since the ACK indicates only the last byte successfully received, and not the last packet, an ACK message can indicate successful receipt of a subset of the bytes in the received pack. Additionally, since TCP/IP typically performs other retransmission mechanisms, the methods described herein perform optimally in conjunction with a relatively persistent link layer.  
         [0032]     The SAU  14  transmits the ACK back to the BSP  16  over the wireless link  26  at  34   e . The BSP  16 , however, transmits an ACK message indicating successful receipt of one byte less than the ACK received from the SAU  14  at  34   f . In accordance with the invention as defined by the present claims, the ability to send a suppression message to pause the sender is kept available by retaining one outstanding unacknowledged byte. A TCP/IP connection does not respond well to unsolicited or duplicate ACK messages, and such ACKs can also have the effect of triggering slow start and closing the sliding window. Since an advertised window is typically sent with an ACK of one or more bytes, retention of one unacknowledged byte preserves the ability to send a suppression message, described further below, by generating a TCP/IP message segment including an acknowledgment of the outstanding, unacknowledged byte and an advertised window of zero.  
         [0033]     The remote server  30  receives the ACK of all but the last of the bytes sent over the wired link  24 ,  18 ,  28  at  34   f . Since the advertised window is still 8760, the remote server sends another 2920 (2 * 1460) bytes at  34   g . These message packets are transmitted to the user PC  12  via  34   g ,  34   h , and  34   i  similarly to  34   a ,  34   b , and  34   c  above. The user PC again responds with an ACK of the 2920 bytes in the two message packets at  34   j . The SAU  14  transmits across the wireless link  26  back to the BSP  16  at  34   k . The BSP again sends an ACK of all but the very last outstanding byte back to the remote server  30 . Since an ACK indicates successful receipt of all bytes up to and including the byte indicated in the ACK, this ACK has the effect of also acknowledging receipt of the outstanding byte not ACKed at  34   f.    
         [0034]     In a TCP/IP network, it is preferable to maintain the end-to-end semantics between communicating nodes, rather than implementing a series of connections to maintain communication. In the system as disclosed herein, the end-to-end semantics of the connection between the remote server  30  and the PC  12  are maintained according to the TCP/IP protocol. Accordingly, a single point-to-point TCP/IP connection is maintained.  
         [0035]      FIG. 5  shows transmission of a suppression message. Referring to  FIGS. 5 and 1 , the remote server  30  sends two message packets including 2920 (2 * 1460) bytes of data to the BSP  16  at  36   a . The message packets are transmitted from the BSP  16  to the SAU  14  at  36   b , and from the SAU  14  to the user PC  12  at  36   c . The PC  12  sends the ACK back at  36   d ,  36   e , and  36   f , similar to as in  FIG. 4  above, again reserving the last byte as unacknowledged by the BSP  16  at  36   f . At  36   g , the remote server sends another two packets totaling 2920 bytes (2 * 1460), and the BSP  16  transmits these over the wireless link to the SAU  14  at  36   h , which in turn transmits to the PC  12  at  36   i . At the time shown by dotted line  38 , however, a problem is detected on the wireless link which will delay the transmission of the ACK from the SAU  14  to the BSP  16 . Such a problem includes detection of a loss of the wireless link, as may occur when the SAU  14  travels outside the range of the BSP  16 , or an eminent timeout at the remote server  30  before ACK should have been received, both described further below.  
         [0036]     In response to the delayed ACK, the BSP  16  generates and sends a suppression message at  36   k . The suppression message includes an acknowledgment of the outstanding unacknowledged byte of  36   f , and an advertised window of zero. As described above, in accordance with TCP/IP, an advertised window of zero has the effect of preventing the sender from sending additional message packets until a non-zero advertised window is received, effectively pausing the remote node  30  in a persist mode.  
         [0037]     The actual ACK from the transmission at  36   i  is sent from the PC  12  to the SA  14  at  36   j , and queued at the SAU  14 . At the time shown by dotted line  40 , the wireless link is available and the ACK transmitted to the BSP  16  at  361 . The BSP  16  sends an ACK of all but one outstanding byte to the remote node  30  at  36   m , again reserving the ability to send a suppression message, and indicates a non-zero advertised window of 8760 bytes, permitting the remote node  30  to again send message packets to the PC  12  at  36   n ,  26   o , and  36   p.    
         [0038]      FIG. 6  shows another particular embodiment in which the suppression message reduces the advertised window. In this embodiment, the advertised window is not zero, but is a value reduced from that which was sent by the PC. In this manner, the remote node  30  is not paused in persist mode, but is limited by the amount of data which will be sent as determined by the BSP  16 . Referring to  FIG. 6 , the remote server transmits two message packets to the PC  12  at  38   a ,  38   b , and  38   c . The PC responds with the ACK, sent at  38   d  and  38   e . The BSP  16 , however, reduces the advertised window by a factor of 6 from 8760 to 1460, and again reserves an unacknowledged byte at  38   f . Reducing the advertised window reduces the load on the BSP  16 . This action may occur for various reasons, such as high cell load, low buffer resources, and GoS (Grade of Service) or QoS (Quality of Service) provisions. A similar thread continues from  38   g - 38   m.    
         [0039]     In the above examples, the BSP  16  and SAU  14  are employed as exemplary wireless gateways for illustrative purposes. The system and method described above adaptable to a wireless gateway on either side of a wireless link. Accordingly, the discussion below will employ the term “wireless gateway” to refer to an endpoint on either side of the wireless link, and accordingly, is equally applicable to either a BSP  16  or a SAU  14  or other node operable for wireless communication over an RF (radio frequency) medium.  FIG. 7  shows a flowchart of a particular embodiment of link-aware message transmission at a wireless gateway. Referring to  FIG. 7 , an incoming message packet is detected, as depicted at step  100 . A connection corresponding to the incoming message is examined, as shown at step  102 . A check is performed to determine if this message packet represents a new connection as disclosed at step  104 . If this is a new connection, a corresponding entry is made in the timer table, as disclosed at step  106 . A corresponding entry is also made in the link table, as depicted at step  107 . An expected timeout is computed by first determining the RTT delay. A latency threshold corresponding to the propagation time from the wired link over the wireless gateway is determined. Jacobsen, V. 1990 “Berkley TCP Evaluation 4.3-Tahoe to 4.3-Reno,” Proceedings at the Eighteenth Internet Engineering Task Force, p. 365 (September, 1990), University of British Columbia, Vancouver, B.C., describes a standard technique for determining latency time at periodic given RTT delays. In the preferred embodiment, RTT is determined by noting the time at which a data segment is received from the remote service  30 . The segment is then forwarded over the wireless link, and the BSP  16  then waits for the ACK to be returned from the wireless gateway, as shown at step  108 . The latency threshold is subtracted from the determined RTT to compute an expected timeout entry by which the ACK should be received. The expected timeout is stored in the timer table to correspond to the connection, as disclosed at step  110 . A check is performed to determine if an ACK is received before the expected timeout expires, as shown at step  112 . If the ACK is received, control reverts to step  100  to wait for the next message packet, as shown at step  112 . If the ACK is not received, a check is made to determine if the expected timeout has expired, as depicted at step  114 . If the expected timeout has not expired, a check is performed to determine if the wireless link was dropped, as shown at step  116 . If either the timeout has expired, as shown at step  114 , or the wireless link was lost, as shown at step  116 , a suppression message is sent the remote node  30  to pause the sender in persist mode, as disclosed at step  118 , and control reverts to step  100  to wait for the next message packet. If the wireless link was not lost at step  116 , control reverts to step  112  to again check for the ACK. Although a polling mechanism is shown for illustrative purposes, the corresponding steps could also be performed employing an interrupt driven implementation without departing from the invention as described and claimed.  
         [0040]      FIGS. 8   a - 8   g  disclose an example of link-aware transmission corresponding to the flowchart of  FIG. 7 . Referring to  FIG. 8   a , two connections are established as per the known TCP/IP protocol connection handshake. A first connection C 23  is attempted as the PC  12  sends a SYN message  290 . The remote node ( 30 ,  FIG. 1 ) responds with a SYN  292  over the wireless link  26 . The PC  12  then sends a SYN ACK message  296 , completing connection C 23 . A second connection is similarly established for C 17 , by SYN  294 , SYN  295 , and SYN ACK  298 , establishing connection C 17 .  
         [0041]     Referring to  FIG. 8   b , the user PC  12  sends a bulk data message packet  300  to a remote node via the wireless gateway  230 . Note that in this example, the user PC  12  is transmitting data packets and the remote node  30  is sending ACKs, to illustrate the bidirectional nature of message transmission. The timer manager  232  in the wireless gateway  230  determines that this message corresponds to a new connection C 23 , and creates a corresponding entry  242   a  in the timer table  242  having an expected timeout value of T 1 . Note that the timer table and link table entries are created upon the first transmission of a data message  300 , not during the connection handshake sequence described above with respect to  FIG. 8   a . A new entry  244   a  is also created in the link detector table  244  to correspond to connection C 23  with a link status of U (up). The message  300  is sent over the wireless link  26  at  302 .  
         [0042]     Referring to  FIG. 8   c , another bulk data message packet  304  is sent from the PC  12 , corresponding to connection C 17 . Accordingly, new entries  242   b  and  244   b  are created in the timer manager table for T 2  and the link detector table for U, respectively, and the message  304  is sent over the wireless link  26  at  306 . Also, at a time less than T 1 , an ACK  308  is received for connection C 23 , and advertises a receive window of  1024 . The timer manager  232  cancels the expected timeout T 1  for connection C 23 , and the ACK is modified  310  to leave one unacknowledged byte, and is forwarded through the wireless gateway to the PC  12 .  
         [0043]     Referring to  FIG. 8   d , a message packet containing 512 bytes is sent from the PC  12  to the wireless gateway  230  on connection C 23 . The timer manager computes a new expected timeout T 3  timeout and updates the entry  242   a  for connection C 23 . Alternatively, the entry  242   a  could have been deleted when the ACK  308  ( FIG. 8   c ) was received and a new entry created for the message packet  312 . The message packet  312  is transmitted over the wireless link at  314 . Continuing to refer to  FIG. 8   d , at a time less than T 2 , an ACK  316  is received for connection C 17 . The timer manager updates the timer table entry  242   b  corresponding to connection C 17 , and the ACK is forwarded the PC  12  at  318 .  
         [0044]     Referring to  FIG. 8   e , time T 3  has elapsed and accordingly, expected timeout T 3  of timer table entry  242   a  is triggered. The timer manager  232  directs the segment generator  236  to generate a suppression message for connection C 23  before the PC  12  experiences a timeout for failure to receive an expected ACK. The segment generator  236  generates a suppression message  320  ACKing the last byte of the last acknowledged message packet ( 308 ,  FIG. 8   c ) sent and advertising a receive window of zero, and sends it to the PC  12  at  322 , pausing the PC  12  in persist mode with respect to connection C 23 .  
         [0045]     Referring to  FIG. 8   f , the ACK  324  on connection C 23  corresponding to the message packet  314  is received at the wireless gateway  230  and is queued in the packet buffer  238  at  326 , until it can be sent to the PC  12  at  328 , advertising a window of  512  and removing the user PC  12  from persist, or pause mode. Referring to  FIGS. 8   f  and  8   g , the wireless link corresponding to connection C 17  is dropped, as indicated by link table entry  244   b  having a value of D (down). Accordingly, prior to the expiration of the expected timeout T 2  for connection C 17 , the link detector  234  immediately directs the segment generator to generate a suppression message  328  for connection C 17 , which sent to the PC at  330 . Note that time T 2  need not necessarily precede time T 3  because the RTT determinations for the two connections may be different.  
         [0046]     Those skilled in the art should readily appreciate that the programs defining the operations and methods defined herein are deliverable to a wireless gateway in many forms, including but not limited to a) information permanently stored on non-writeable storage media such as ROM devices, b) information alterably stored on writeable storage media such as floppy disks, magnetic tapes, CDs, RAM devices, and other magnetic and optical media, or c) information conveyed to a computer through communication media, for example using baseband signaling or broadband signaling techniques, as in an electronic network such as the Internet or telephone modem lines. The operations and methods may be implemented in a software executable by a processor or as a set of instructions embedded in a carrier wave. Alternatively, the operations and methods may be embodied in whole or in part using hardware components, such as Application Specific Integrated Circuits (ASICs), state machines, controllers or other hardware components or devices, or a combination of hardware, software, and firmware components.  
         [0047]     While the system and method for link-aware message transmission have been particularly shown and described with references to 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 scope of the invention encompassed by the appended claims. Accordingly, the present invention is not intended to be limited except by the following claims.