Patent Publication Number: US-7715442-B2

Title: Method, apparatus, and system of wireless transmission with frame alignment

Description:
BACKGROUND OF THE INVENTION 
     In some wireless local area networks (WLANs), different stations may transmit frames of different modulations types. For example, the IEEE-Std 802.11, 1999 Edition (ISO/IEC 8802-11: 1999) (“802.11”) set of standards allows coexistence of different formats of physical layer (PHY) protocol data units (PPDUs), or frames, in the same frequency channel. The various formats may differ, for example, in the respective sizes of the transmitted frames. 
     Network stations may use a channel access mechanism and a control mechanism to protect transportation of packets over the network, e.g., to avoid collision of frames. For example, a station may wait for the channel to be clear before transmitting the next frame. One solution may be to utilize a request-to-send/clear-to-send (RTS/CTS) mechanism, including setting a network allocation vector (NAV) to reserve the wireless medium for a predetermined period of time. However, such a protection method may cause significant overhead by taking up part of the available bandwidth and/or power for transmission of management frames. In addition, a network station that is in a power-save mode may not receive the RTS/CTS frames. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The subject matter regarded as the invention is particularly pointed out and distinctly claimed in the concluding portion of the specification. The invention, however, both as to organization and method of operation, together with features and advantages thereof, may best be understood by reference to the following detailed description when read with the accompanied drawings in which: 
         FIG. 1  is a schematic diagram of a wireless communication system in accordance with some demonstrative embodiments of the present invention; 
         FIG. 2  is a schematic diagram of different frame formats that may be helpful in understanding some demonstrative embodiments of the invention; 
         FIG. 3  is a schematic diagram of aligned frames in accordance with one demonstrative embodiment of the invention; and 
         FIG. 4  is a schematic diagram of aligned frames in accordance with another demonstrative embodiment of the invention. 
     
    
    
     It will be appreciated that for simplicity and clarity of illustration, elements shown in the figures have not necessarily been drawn to scale. For example, the dimensions of some of the elements may be exaggerated relative to other elements for clarity. Further, where considered appropriate, reference numerals may be repeated among the figures to indicate corresponding or analogous elements. 
     DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION 
     In the following detailed description, numerous specific details are set forth in order to provide a thorough understanding of the invention. However, it will be understood by those of ordinary skill in the art that the present invention may be practiced without these specific details. In other instances, well-known methods, procedures, components and circuits have not been described in detail so as not to obscure the present invention. 
     Unless specifically stated otherwise, as apparent from the following discussions, it is appreciated that throughout the specification discussions utilizing terms such as “processing,” “computing,” “calculating,” “determining,” or the like, refer to the action and/or processes of a computer or computing system, or similar electronic computing device, that manipulate and/or transform data represented as physical, such as electronic, quantities within the computing system&#39;s registers and/or memories into other data similarly represented as physical quantities within the computing system&#39;s memories, registers or other such information storage, transmission or display devices. 
     Embodiments of the present invention may include an apparatus for performing the operations herein. This apparatus may be specially constructed for the desired purposes, or it may comprise a general-purpose computer selectively activated or reconfigured by a computer program stored in the computer. Such a computer program may be stored in a computer readable storage medium, such as, but not limited to, any type of disk, including floppy disks, optical disks, magnetic-optical disks, read-only memories (ROM), compact disc read-only memories (CD-ROM), random access memories (RAM), electrically programmable read-only memories (EPROM), electrically erasable and programmable read only memories (EEPROM), FLASH memory, magnetic or optical cards, or any other type of media suitable for storing electronic instructions and capable of being coupled to a computer system bus. 
     It should be understood that the present invention may be used in a variety of applications. Although the present invention is not limited in this respect, the circuits and techniques disclosed herein may be used in many apparatuses such as units of a wireless communication system, for example, a wireless local area network (WLAN) communication system and/or in any other unit and/or device. Units of a WLAN communication system intended to be included within the scope of the present invention include, by way of example only, modems, mobile units (MU), access points (AP), wireless transmitters/receivers, and the like. 
     Devices, systems and methods incorporating aspects of embodiments of the invention are also suitable for computer communication network applications, for example, intranet and Internet applications. Embodiments of the invention may be implemented in conjunction with hardware and/or software adapted to interact with a computer communication network, for example, a LAN, wide area network (WAN), a personal area network (PAN), or a global communication network, for example, the Internet. 
     Types of WLAN communication systems intended to be within the scope of the present invention include, although are not limited to, WLAN communication systems as described by “IEEE-Std 802.11, 1999 Edition (ISO/IEC 8802-11: 1999)” standard (“the 802.11 standard”), and more particularly in “International Standard ISO/IEC 8802-11:1999/Amd 1:2000(E) IEEE Std 802.11a-1999 Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) specifications Amendment 1: High-speed Physical Layer in the 5 GHz band” (“the 802.11a standard”), “IEEE-Std 802.11n—High throughput extension to the 802.11” (“the 802.11n standard”), and the like. 
     Although the scope of the present invention is not limited in this respect, the circuits and techniques disclosed herein may also be used in units of wireless communication systems, digital communication systems, satellite communication systems, and the like. Some embodiments of the invention may be used in conjunction with one or more types of wireless communication signals and/or systems, for example, radio frequency (RF), infra red (IR), frequency-division multiplexing (FDM), orthogonal FDM (OFDM), time-division multiplexing (TDM), time-division multiple access (TDMA), extended TDMA (E-TDMA), general packet radio service (GPRS), extended GPRS, code-division multiple access (CDMA), wideband CDMA (WCDMA), CDMA 2000, multi-carrier modulation (MDM), or the like. Embodiments of the invention may be used in various other apparatuses, devices, systems and/or networks. 
     Although the scope of the present invention is not limited in this respect, the system and method disclosed herein may be implemented in many wireless, handheld and portable communication devices. By way of example, wireless, handheld and portable communication devices may include wireless and cellular telephones, smart telephones, personal digital assistants (PDAs), web-tablets and any device that may provide wireless access to a network such, an intranet or the internet. It should be understood that the present invention may be used in a variety of applications. 
     Part of the discussion herein may relate, for demonstrative purposes, to transmitting a frame, e.g., a physical layer (PHY) protocol data unit (PPDU). However, embodiments of the invention are not limited in this regard, and may include, for example, transmitting a signal, a packet, a block, a data portion, a data sequence, a data signal, a data packet, a preamble, a signal field, a content, an item, a message, or the like. 
     Reference is made to  FIG. 1 , which schematically illustrates a wireless communication system  100  in accordance with an embodiment of the present invention. It will be appreciated by those skilled in the art that the simplified components schematically illustrated in  FIG. 1  are intended for demonstration purposes only, and that other components may be required for operation of the wireless devices. Those of skill in the art will further note that the connection between components in a wireless device need not necessarily be exactly as depicted in the schematic diagram. 
     In some demonstrative embodiments of the invention, communication system  100  may for example, a wireless network or a network that may include wireless components. For example, communication system  100  may include or may be a wireless local area network (WLAN) in accordance with the 802.11 family of standards. Although embodiments of the invention are not limited in this respect, communication system  100  may include, for example, a basic service set (BSS) provider such as an access point (AP)  110 , as well as one or more wireless mobile units such as a station (STA), for example stations  120 ,  130 , and  140 . 
     In some embodiments, AP  110  and one or more of STA  120 ,  130 , and  140  may communicate network traffic over a shared access medium using one or more wireless links, e.g., links  128 ,  138 , and  148 , respectively. Links  128 ,  138 , and  148  may each include a downlink and an uplink, as are known in the art. Although embodiments of the invention are not limited in this respect, the traffic that may be carried via links  128 ,  138 , and  148  may include packets, frames, or other collections of signals and/or data, such as, e.g., media access controller (MAC) protocol data units (MPDUs) and/or physical layer (PHY) protocol data units (PPDUs), that may make up a transmission of wireless signals. In accordance with some demonstrative embodiments of the invention, wireless communication system  100  may enable coexistence of different modulation schemes and/or frame formats, as explained in more detail below with reference to  FIG. 2 . 
     Although embodiments of the invention are not limited in this respect, each of AP  110 , STA  120 , STA  130 , and STA  140  may be operatively coupled with at least one radio frequency antenna  119 ,  129 ,  139 , and  149 , respectively, which may include or may be an internal and/or external RF antenna, for example, a dipole antenna, a monopole antenna, an omni-directional antenna, an end-fed antenna, a circularly polarized antenna, a micro-strip antenna, a diversity antenna, or any other type of antenna suitable for transmitting and/or receiving wireless communication signals, blocks, frames, transmission streams, packets, messages and/or data. 
     In some embodiments, AP  110  may be a dedicated device with additional functionality such as, for example, providing a bridge to wired network infrastructure or arbitrating communication of stations in the WLAN. For example, AP  110  may facilitate communication with a wider network such as, for example, the Internet or an intranet, by either wired or wireless communication. A BSS provider such as AP  110  may in some embodiments associate wireless devices such as, for example, STA  130  with other equipment such as, for example, personal computers, workstations, printers, and the like. 
     In some embodiments, AP  110  may include a transmitter  111  and a receiver  112  to transmit and receive network traffic, e.g., over wireless links  128 ,  138 , and  148 . In addition, AP  110  may include a physical layer (PHY)  113  and a media access controller (MAC)  114  to control the operation of the transmitter and receiver. Transmitter  111  and receiver  112  may include any components involved in the process of transmitting and receiving network traffic, respectively, including components of PHY  113  and MAC  114 . Similarly, STA  120  and STA  140  may include, respectively, transmitters  121  and  141 , receivers  122  and  124 , PHYs  123  and  143 , and MACs  124  and  144 . It will be appreciated that AP  110  and STAs  120 ,  130  and  140  may include other suitable software and/or hardware elements, e.g., a memory, a processor, a storage unit, and the like. 
     Although embodiments of the invention are not limited in this respect, AP  110  may be able to transmit and receive frames using several modulation schemes and/or frame formats. For example, AP  110  may use a first modulation scheme, e.g., a high-throughput (HT) modulation scheme such as a multiple-input-multiple-output (MIMO) orthogonal frequency division multiplexing (OFDM) modulation scheme, to communicate traffic with a HT station, e.g., STA  120 . Although embodiments of the invention are not limited in this respect, one or more components of AP  110  and/or STA  120 , e.g., PHYs  113  and  123  and MACs  114  and  124 , respectively, may be adapted to operate in accordance with the 802.11n standard or another wireless communication protocol that allows relatively high throughput, e.g., up to 600 megabytes per second. In accordance with embodiments of the invention, the first modulation scheme may be used with multiple frame formats, e.g., a HT-optimal format and a mixed-mode format, as explained in more detail below with reference to  FIG. 2 . In some embodiments, STA  120  and AP  110  may use different first and second frame formats of the same modulation scheme, e.g., the high-throughput modulation scheme MIMO-OFDM. 
     In some embodiments, one or more stations of WLAN  100 , e.g., STA  140 , may transmit and receive frames using a second modulation scheme, which may be different from the first modulation scheme used by AP  110  and/or the scheme used by other stations of WLAN  100 . For example, STA  140  may use orthogonal frequency division multiplexing (OFDM) in accordance with the 802.11a standard, or any other standard that may have a lower throughput than that of the first modulation scheme. 
     Although embodiments of the invention are not limited in this respect, STA  140  may be a legacy station that may not be able to demodulate and/or decode frames transmitted in the first modulation scheme. It will be appreciated that AP  110  may be able to use the second modulation scheme to communicating traffic to a non-HT station such as legacy STA  140 . Additionally or alternatively, high-throughput STA  120  may not be able to demodulate and/or decode frames transmitted in the first modulation scheme using a different frame format. However, AP  110  may still be able to properly receive frames transmitted by STA  140  and/or STA  120  using the second modulation scheme and/or frame format. 
     In some embodiments, a BSS of communication system  100  may include a high throughput AP, e.g., AP  110  using the first modulation scheme such as MIMO-OFDM, and one or more high throughput stations, e.g., STA  120 , as well as one or more legacy stations, e.g., STA  140  using the second modulation scheme, such as OFDM. Optionally, in some embodiments communication system  100  may include a protection mechanism to prevent collision of frames. For example, when AP  110  transmits traffic including frames of the first modulation scheme, it may be necessary for stations, including stations adapted to use the second modulation scheme, to detect that the wireless medium is busy and not attempt to transmit. Although embodiments of the invention are not limited in this respect, a protection method may depend on an accurate estimate of the frame size and transmittal time, as explained in more detail below with reference to  FIGS. 2-4 . 
     Reference is made to  FIG. 2 , which schematically illustrates different frame formats that may be used in some demonstrative embodiments of the invention, and is helpful in understanding alignment requirements in accordance with embodiments of the invention. Although embodiments of the invention are not limited in this respect, a high-throughput access point, e.g., AP  110 , may be able to transmit and receive frames of multiple formats as described below. 
     Although embodiments of the invention are not limited in this respect, a wireless communication device, e.g., a station of a WLAN such as communication system  100 , may be adapted to transmit and/or receive frames of a specific format, which may correspond to a certain modulation scheme. The frame formats may include a headings and preambles portion  260  and a data portion  270 . The headings and preambles portion  260  may include one or more fields, e.g., a training field and a signal field. The data portion  270  may include one or more data symbols. 
     For example, a “legacy” format  210  may correspond to OFDM modulation in accordance with 802.11a, and a “greenfield” format  220  may correspond to MIMO-OFDM modulation in accordance with 802.11n, as is known in the art. Greenfield format  220  may be optimal for a HT station, e.g., STA  120 . Legacy format  210  may include a legacy signal field  213  in the preamble portion  260 , which may identify the type of modulation scheme used, and may include information such as the data rate and the length of the subsequent data portion  270 . The legacy signal field may be read by a legacy station, e.g., STA  140 . Similarly, greenfield format  220  may include a high-throughput signal field  223 , which may not be readable by a station that is not adapted to use the high-throughput modulation, e.g., a legacy OFDM station. 
     Other frame formats and modulation schemes may be used. For example, a “mixed-mode” format  230  may also correspond to MIMO-OFDM modulation in accordance with 802.11n, but include both a legacy signal field  233  and a high-throughput signal field  234 . Thus, stations using either a first modulation scheme, e.g., MIMO-OFDM, or a second modulation scheme, e.g., legacy OFDM, may be able to obtain information regarding the data rate and length of the subsequent data portion  270 . 
     In some embodiments, the modulation scheme used may include a shortened guard interval (GI) in the data symbols of data portion  270 . For example, a format  240  may be a short GI format corresponding to the regular GI greenfield format  220  and a format  250  may be a short GI format corresponding to the regular GI mixed-mode format  230 . Although embodiments of the invention are not limited in this respect, a data symbol in a regular GI format, e.g., data symbol  239  of mixed-mode format  230 , may have a length measured in 4 units, e.g., 4 microseconds, whereas a data symbol in a short GI format, e.g., data symbol  259  of short GI mixed mode-format  250 , may have a length measured in 3.6 units, e.g., 3.6, microseconds. 
     In accordance with the 802.11 set of standards, an inter frame space (IFS) may begin following the last data symbol in data portion  270  of a transmitted frame. Some demonstrative embodiments of the invention may provide a method to synchronize the IFS between the transmitting station and all receiving stations, so as to start at the same time. It will be appreciated that in order to calculate the IFS start time, it may be necessary for a receiving station to be able to read all relevant parameters of the received frame, e.g., as found in the signal field. The IFS start time may vary according to the modulation scheme used and the number of data symbols in the data portion, which may be of different lengths. For example, the IFS start time may depend on the frame format (e.g., legacy, mixed-mode, or greenfield), the transmission rate (e.g., as indicated in the legacy and HT signal fields), data length (e.g., the byte count of the data, as indicated in the legacy and HT signal fields), and guard interval format (e.g., regular or shortened, as indicated in the HT signal field). Although embodiments of the invention are not limited in this respect, a frame with a regular GI format (e.g., mixed-mode format  230  or greenfield format  220 ) may have an IFS start time  280  at a boundary that is an integer multiple of 4 units, whereas a short GI format (e.g., mixed-mode format  250  or short GI greenfield format  240 ) may have an IFS start time  290  that is not located at an integer multiple of 4 units. Thus, in a network that includes coexistence of different modulation schemes and frame formats, e.g., a WLAN such as communication system  100 , embodiments of the present invention may enable alignment of the IFS start time between transmitted and received frames, thereby to prevent frame collision without resorting to a costly protection mechanism. 
     Reference is again made to  FIG. 1 . In accordance with the 802.11 set of standards, the MAC, e.g., MAC  114  of AP  110 , may perform functionality related to the data link layer of the open systems interconnect (OSI) model, as known in the art, and prepare data for transmission by the PHY, e.g., PHY  113 . For example, the MAC functionality may include delimiting frames, inserting MAC headers, error detection and/or correction functionality, and controlling access to the physical medium. The PHY, e.g., PHY  113 , may include circuitry for encoding, transmission, reception, and decoding of wireless signals, packets, and/or frames, as part of the physical layer of the OSI model. For example, the PHY may receive a PSDU (PHY Service Data Unit) from the MAC, and append physical layer dependent information, e.g., information relating to the modulation scheme used, in the preamble of the PSDU, thereby to form a PPDU (PHY protocol data unit) frame suitable for transmission. 
     An interface  115  between MAC  114  and PHY  113  may use primitives, as known in the art, to communicate information between the PHY and the MAC. For example, a PHY-CCA.indicate primitive, which may hold a value of either busy or idle, may be communicated from the PHY to the MAC. Although embodiments of the invention are not limited in this respect, the PHY may include a carrier sense function to sense the physical (wireless) medium when the station is not actively transmitting or receiving. For example, the PHY may generate a clear channel assessment (CCA) based on a detected energy level. Based on the indication from the PHY, the MAC may decide when to send a frame for transmission. For example, the MAC may delay transmission for a time period corresponding to the IFS, which may begin after receiving a PHY-CCA.indicate(idle) primitive from the PHY. 
     Reference is now made to  FIG. 3 , which schematically illustrates alignment between a transmitted frame  310  and a received frame  320  in accordance with one demonstrative embodiment of the invention. For example, frames  310  and  320  may be of a short GI format, e.g., the mixed-mode short GI format  250  illustrated in  FIG. 2 . 
     During transmission, the PHY of the transmitting station, e.g., PHY  113  of AP  110 , may calculate a transmission time  330  such that the start of the IFS time may be aligned with the IFS start time calculated by the PHY of the receiving station, e.g., PHY  123  of STA  120  and/or PHY  143  of STA  140 . For example, the receiving PHY may calculate the IFS start time according to the information in the signal field of the received frame, e.g., legacy signal field  323  or high-throughput signal field  324  of received frame  320 . 
     It will be appreciated that the receiving PHY may be adapted to align to the legacy GI format and may thus calculate the IFS start time as is known in the art, e.g., at a 4 unit boundary such as time  340 . Thus, in order to align the IFS start time, in some embodiments the transmitting PHY may calculate the transmission time  330  to similarly end at the 4 unit boundary  340 , which may be longer than the actual end of transmission at time  350 . Such an alignment may enable synchronization between stations of different modulation schemes. 
     Although embodiments of the invention are not limited in this respect, transmission time  330  may be calculated using the following equation: 
                   TXTIME   =       T     LEG   ⁢   _   ⁢   PREAMBLE       +     T     LEG   ⁢   _   ⁢   SIGNAL       +     T     HT   ⁢   _   ⁢   PREAMABLE       +     T     HT   ⁢   _   ⁢   SIGNAL       +         T     REG   ⁢   _   ⁢   SYM       ?           ⁢   Ceiling     ⁢           ⁢         ?           ?           ?         ⁢       T     SGI   ⁢   _   ⁢   SYM       /     T     REG   ⁢   _   ⁢   SYM         ⁢         ?           ?           ?         ×     N   SYM                 (     Equation   ⁢           ⁢   1     )               
wherein:
     T LEG     —     PREAMBLE  is the duration of the legacy preamble, e.g., training fields  311  and  312 ;   T LEG     —     SIGNAL  is the duration of the legacy signal field, e.g., signal field  313 ;   T HT     —     PREAMBLE  is the duration of the HT preamble, e.g., training fields  315  and  316 ;   T HT     —     SIGNAL  is the duration of the HT signal field, e.g., signal field  314 ;   T REG     —     SYM  is the time required to transmit a data symbol having a regular guard interval;   T SGI     —     SYM  is the time required to transmit a data symbol having a shortened guard interval; and   N SYM  is the total number of data symbols in the data portion, which may be calculated according to the following formula:   
     
       
         
           
             
               
                 
                   
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     Detailed definitions of the parameters appearing in Formula 1 may be found, for example, in section 4 of “Joint Proposal: High throughput extension to the 802.11 Standard: PHY” which is part of the IEEE 802.11 TGn Joint Proposal Technical Specification, dated Jan. 13, 2006. For example:
     length is the number of octets in the data portion of the PPDU;   m STBC  is equal to 2 when space time block code (STBC) is used, and otherwise 1;   N ES  is the number of encoders used, e.g., 1 or 2;   N DBPS  is the number of data bits per symbol; and   N CBPS  is the number of code bits per symbol.   

     Reference is now made to  FIG. 4 , which schematically illustrates alignment between a transmitted frame  410  and received frames  420  and  430  in accordance with another demonstrative embodiment of the invention. For example, frames  410 ,  420 , and  430  may be of a short GI format, e.g., the short GI greenfield format  240  illustrated in  FIG. 2 . 
     During transmission, the transmitting PHY, e.g., PHY  113  of AP  110 , may calculate a transmission time  440  such that the start of the IFS time may be aligned with the IFS start time calculated by the receiving PHY of the receiving station, e.g., PHY  123  of STA  120  and/or PHY  143  of STA  140 . 
     For example, frame  420  may be received by a high-throughput station, e.g., STA  120 , which may be adapted to use the first modulation scheme that may also be used by the transmitting PHY. In such a case, the receiving PHY, e.g., PHY  123 , may calculate the IFS start time according to the information in the high-throughput signal field  423  of received frame  420  and may, for example, send a PHY-CCA.indicate(idle) primitive at the end of the last received data symbol of the frame. In accordance with embodiments of the invention, the calculated transmission time  440  may be synchronized with the IFS start time  450  after the last received data symbol, which may not be at a 4-unit boundary. 
     In another example, frame  430  may be received by a legacy station, e.g., STA  140 , which may be adapted to use a second modulation scheme that may be different from the scheme used by the transmitting PHY. In such a case, the receiving PHY, e.g., PHY  143 , may not be able to read the high-throughput signal field  433  of received frame  430 . Thus, in some embodiments, the receiving PHY may use a carrier sense function to detect that the wireless medium is busy based on a detected energy level. As indicated in  FIG. 4 , an energy detection period  460  may be aligned with the transmission time  440  and IFS start time  450 . 
     In yet another example, the receiving PHY, e.g., PHY  123  of high-throughput station  120 , may be able to read-the high-throughput signal field  433 , yet not be able to demodulate/decode the received frame  430 . For example, PHY  123  may be adapted to use a regular guard interval greenfield format, while the transmitted frame  410  is of a short GI greenfield format. In such a case, PHY  123  may also use energy detection  460  to align the IFS start time  450  with the calculated transmission time  440 . 
     Although embodiments of the invention are not limited in this respect, transmission time  330  may be calculated using the following equation:
 
 TX TIME= T   HT     —     PREAMBLE   +T   HT     —     SIGNAL   +T   SGI     —     SYM   ×N   SYM   (Equation 2)
 
wherein:
     T HT     —     PREAMBLE  is the duration of the HT preamble, e.g., training fields  411 ,  412 , and  414 ;   T HT     —     SIGNAL  is the duration of the HT signal field, e.g., signal field  413 ;   T REG     —     SYM  is the time required to transmit a data symbol having a regular guard interval;   T SGI     —     SYM  is the time required to transmit a data symbol having a shortened guard interval; and   N SYM  is the total number of data symbols in the data portion, which may be calculated according Formula 1, as detailed above.   

     Embodiments of the present invention may be implemented by software, hardware, or by any combination of software and/or hardware as may be suitable for specific applications or in accordance with specific design requirements. Embodiments of the present invention may include units and sub-units, which may be separate of each other or combined together, in whole or in part, and may be implemented using specific, multi-purpose or general processors, or devices as are known in the art. Some embodiments of the present invention may include buffers, registers, storage units and/or memory units, for temporary or long-term storage of data and/or in order to facilitate the operation of a specific embodiment. 
     While certain features of the invention have been illustrated and described herein, many modifications, substitutions, changes, and equivalents may occur to those of ordinary skill in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the invention.