Abstract:
Apparatus for customer service includes at least one wireless communication terminal, adapted to be deployed at a first location in a facility that is visited by customers, so as to receive digital data over a wireless link from a portable device carried by at least one of the customers in a vicinity of the terminal. A service center, at a second location, which is not in the vicinity of the at least one wireless communication terminal, is in communication with the terminal so as to receive the digital data therefrom and to generate a permanent record of the data for delivery to the at least one of the customers.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application claims the benefit of U.S. Provisional Patent Application No. 60/197,984, filed Apr. 17, 2000, and is a continuation application of non-provisional U.S. patent application Ser. No. 09/835,284, entitled “Wireless Network with Enhanced Data Rate”, filed Apr. 12, 2001, by Ephraim Zehavi, and is incorporated herein by reference. 
     
    
     FIELD OF THE INVENTION  
       [0002]     The present invention relates generally to wireless data communications, and specifically to high-speed wireless local area networks.  
       BACKGROUND  
       [0003]     Wireless local area networks (WLANs) are gaining in popularity, and new applications are being developed. The original WLAN standards, such as “Bluetooth” and IEEE 802.11, were designed to enable communications at 1-2 Mbps in a band around 2.4 GHz. More recently, IEEE working groups have defined the 802.11a and 802.11b extensions to the original standard, in order to enable higher data rates. The 802.11a standard envisions data rates over 20 Mbps over short distances in a 5 GHz band, while 802.11b defines data rates up to 11 Mbps in the 2.4 GHz band. For 11 Mbps operation, the 802.11b standard uses complementary code keying (CCK) or quadrature phase shift keying (QPSK) with packet binary convolutional coding (PBCC). These modulation schemes are further described in the IEEE 802.11b standard, which is incorporated herein by reference.  
         [0004]     Recently the IEEE 802.11 working group has called for proposals to extend the 802.11b standard data rates still further, to 22 Mbps and above. One proposal that has been put forth, by Kodak and Motorola, is based on a wideband binary frequency shift keying (BFSK) scheme, with a bandwidth of up to 28 MHz. While this proposal in itself can be implemented in a straightforward way, using analog signal processing, it is not compatible with existing, lower-rate receivers or with Bluetooth protocols. Furthermore, it will typically enable high data-rate communication only over very short distances, on the order of 2 m.  
         [0005]     Convolutional coding, as mentioned above, is widely used in communication applications to reduce the bit error rate (BER) in signals sent over noisy channels. Pragmatic trellis coded modulation (PTCM) is a particularly useful type of convolutional coding, which has the advantages of reduced computation load and efficient implementation in VLSI devices. PTCM encoders and decoders are capable of handling different modulation techniques, such as binary phase shift keying (BPSK), QPSK, 8-PSK and 16-PSK (referred to collectively as M-PSK techniques). An exemplary trellis encoder and decoder based on PTCM are described in U.S. Pat. No. 5,633,881, whose disclosure is incorporated herein by reference.  
       SUMMARY OF THE INVENTION  
       [0006]     It is an object of some aspects of the present invention to provide improved devices and methods for high-speed wireless data communications, and particularly to provide devices capable of reaching data rates above 11 Mbps while maintaining downward-compatibility with the IEEE 802.11b standard.  
         [0007]     It is a further object of some aspects of the present invention to provide improved methods and systems for conveying electronic images and other large data files over WLANs.  
         [0008]     In preferred embodiments of the present invention, a high-speed wireless modem operates at an enhanced data rate by transmitting and receiving multiple bits of data per symbol, preferably using M-PSK modulation, most preferably together with PTCM. The modem automatically adjusts the number of bits per symbol upon initiation of communication over a wireless link, based on the quality of the link and the capabilities of the device with which it is communicating. The modem thus maintains compatibility with existing equipment that operates at lower bit-per-symbol rates. The increased number of bits per symbol is preferably facilitated by the use of a covering function, which rotates the M-PSK symbols in a pseudo-random fashion in order to reduce the effects of interference in demodulating received signals.  
         [0009]     In some preferred embodiments of the present invention, the modem is compatible with the IEEE 802.11b standard, and operates at 11 Msps (million symbols per second). Preferably, the modem uses 8-PSK to transmit and receive data at 2 or 3 bits of data per symbol, thus achieving a data rate of 22 or 33 Mbps, unlike devices based on the current standard, which can operate at no more than 1 bit per symbol. These enhanced data rates are achieved with minimal modification to the standard, and without restricting the transmission range of the modem relative to existing devices.  
         [0010]     In some preferred embodiments of the present invention, the enhanced data rate (EDR) of the modem is used for transmitting electronic images from a portable digital camera to a storage or processing facility. In one such embodiment, one or more wireless service points, having EDR-capable modems, are deployed in a store or other commercial establishment. A customer, who wishes to have a hard copy, such as photographic prints, or other permanent record made of images stored in his or her digital camera, brings the camera within range of one of the service points. The camera establishes a wireless link with the service point and transmits the image data over the link to the service point, preferably at 22 or 33 Mbps. The service point then conveys the images, preferably via WLAN, to a service center, where the prints (or other media) are produced. The customer stops by the service center later, typically upon exiting the store, to pick up the prints.  
         [0011]     In another, similar embodiment, a WLAN in the store (or other establishment) is configured to operate at the enhanced data rate. In this case, the camera can establish a wireless link at the EDR to transfer the images over the WLAN to the service center from substantially any point in the store.  
         [0012]     Although these preferred embodiments relate specifically to transmission of digital images in a commercial environment, it will be appreciated that the principles of the present invention can similarly be used in other applications and services in which large volumes of data must be transferred over wireless networks at high speed.  
         [0013]     There is therefore provided, in accordance with a preferred embodiment of the present invention, apparatus for customer service, including: at least one wireless communication terminal, adapted to be deployed at a first location in a facility that is visited by customers, so as to receive digital data over a wireless link from a portable device carried by at least one of the customers in a vicinity of the terminal; and a service center, at a second location, which is not in the vicinity of the at least one wireless communication terminal, and in communication with the terminal so as to receive the digital data therefrom and to generate a permanent record of the data for delivery to the at least one of the customers.  
         [0014]     In a preferred embodiment, the digital data include one or more digital images, the portable device includes a digital camera, and the permanent record includes a hard copy of one or more of the images. Typically, the facility includes a commercial establishment, and wherein the first location is in a shopping area of the establishment, while the second location is near an exit from the establishment.  
         [0015]     In a further preferred embodiment, the wireless link belongs to a wireless local area network deployed in the facility. Preferably, the wireless communication terminal is adapted to receive the data at a rate substantially in excess of 11 Mbps. Most preferably, the wireless communication terminal is further adapted to transmit and receive at a rate of 11 Mbps or less substantially in accordance with IEEE standard 802.11, and to receive the data at the rate substantially in excess of 11 Mbps when requested to do so by the portable device. Further preferably, the wireless communication terminal is adapted to receive the data at a symbol rate substantially equal to 11 million symbols per second. Optionally, the communication between the wireless communication terminal and the service center takes place at a rate of 11 Mbps or less.  
         [0016]     There is also provided, in accordance with a preferred embodiment of the present invention, a method for customer service, including: receiving digital data at a first location in a facility over a wireless link from a portable device carried by a customer in a vicinity of the first location; transferring the digital data to a second location in the facility, which is not in the vicinity of the first location; generating a permanent record of the data at the second location; and delivering the permanent record to the customer.  
         [0017]     There is additionally provided, in accordance with a preferred embodiment of the present invention, a high-speed transmitter for digital data having a variable data rate, the transmitter including: a convolutional encoder, adapted to generate, for each group of k input bits in the bitstream, n coded output bits, such that k and n are integers, n equal to or greater than k, and at least one of k and n is variable responsive to the variable data rate of the transmitter; and a modulator, coupled to map the output bits generated by the encoder to a constellation of M symbols for transmission by the transmitter, M an integer, which is variable responsive to the variable data rate of the transmitter.  
         [0018]     Preferably, for a given rate RS of transmission of the symbols by the transmitter, the variable data rate Rb is given by Rb=Rs*log2(M)*Rc, wherein Rc is a code rate equal to kin. Further preferably, the rate of transmission of the symbols is substantially fixed at a standard rate, which most preferably is substantially equal to 11 million symbols per second.  
         [0019]     Preferably, the constellation includes a phase-shift-keyed constellation of order M. Most preferably, after mapping the output bits to the symbols, the modulator is adapted to rotate a phase of the symbols in accordance with a pseudo-random cover function.  
         [0020]     Additionally or alternative, the encoder includes a sequence of delay stages coupled to receive the input bits in a serial stream, and a plurality of adders, which are coupled to receive the input bits from the delay stages and to add the input bits together so as to generate at least two of the coded output bits in parallel. Preferably, the modulator is configured to select the coded output bits from the encoder to be mapped to each of the symbols responsive to the variable data rate.  
         [0021]     There is further provided, in accordance with a preferred embodiment of the present invention, a method for variable-rate, high-speed transmission of digital data, including: specifying a first bit rate at which the data are to be transmitted by a transmitter; applying convolutional encoding to the data so as to generate, for each group of k input bits in the bitstream, n coded output bits, such that k and n are integers, n equal to or greater than k; modulating the output bits to generate a constellation of M symbols, M a variable integer, for transmission of the modulated data at a given symbol rate and at the first bit rate; specifying a second bit rate at which the data are to be transmitted, different from the first bit rate; and changing a value of at least one of k, n and M, so that after applying the convolutional encoding and modulating the output bits using the changed value, the transmitter transmits the modulated data at the given symbol rate and at the second bit rate.  
         [0022]     The present invention will be more fully understood from the following detailed description of the preferred embodiments thereof, taken together with the drawings in which: 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0023]      FIG. 1  is a block diagram that schematically illustrates a system, based on a WLAN, for providing hard copies of digital images, in accordance with a preferred embodiment of the present invention;  
         [0024]      FIG. 2  is a flow chart that schematically illustrates a method for transferring digital images and producing hard copies in the system of  FIG. 1 , inaccordance with a preferred embodiment of the present invention;  
         [0025]      FIG. 3  is a flow chart that schematically illustrates a method for transmitting data at an enhanced data rate over a wireless connection, in accordance with a preferred embodiment of the present invention;  
         [0026]      FIG. 4  is a block diagram that schematically illustrates a system, based on a WLAN, for providing hard copies of digital images, in accordance with another preferred embodiment of the present invention;  
         [0027]      FIG. 5  is a block diagram that schematically illustrates elements of a wireless transmitter, in accordance with a preferred embodiment of the present invention;  
         [0028]      FIG. 6  is a block diagram that schematically illustrates details of a convolutional encoder used in the transmitter of  FIG. 5 , in accordance with a preferred embodiment;  
         [0029]      FIG. 7  is a polar plot illustrating an 8-PSK modulation scheme, in accordance with a preferred embodiment of the present invention; and  
         [0030]      FIG. 8  is a block diagram that schematically illustrates elements of a wireless receiver in accordance with a preferred embodiment of the present invention.  
     
    
     DETAILED DESCRIPTION  
       [0031]      FIG. 1  is a block diagram that schematically illustrates a system  20  for providing hard copies of digital images, in accordance with a preferred embodiment of the present invention. System  20  is typically deployed in a commercial establishment, such as a department store or shopping mall, and enables customers to submit their digital images via a WLAN  22  at multiple points in the establishment, while they are shopping. The customers then pick up their hard copies (or other media that they have requested, such as photo-CDs) when they are ready to check out of the establishment.  
         [0032]     WLAN  22  preferably operates in accordance with the IEEE 802.11b standard, as described hereinabove, at a rate of 11 Mbps. Because of the large volume of data contained in digital images, however, one or more high-speed service points  24  are deployed in the establishment. The service points are equipped with wireless modems, as described herein below, which are capable of operating at an enhanced data rate (EDR).  
         [0033]     Most preferably, the EDR is at least 22 Mbps. Service points  24  are designed to serve cameras  28  (or other user devices) that are similarly equipped with wireless modems, preferably modems that are also capable of EDR operation.  
         [0034]     A customer wishing to submit images for hard copying positions his or her camera  28 , with the images stored in the camera memory, within a prescribed range of one of service points  24 . Preferably, the prescribed range is at least 10 m. When the customer actuates an appropriate control on the camera, the camera makes contact with the service point and negotiates an optimal data rate.  
         [0035]     The data rate depends on the capabilities of the camera and its modem, proximity to the service point, and noise conditions in the area of the service point. When possible, cameras  28  communicate with service points  24  at the EDR, as indicated by broad arrows  30 . In this way, the customer&#39;s images are transferred rapidly to the service point, and the customer is freed to continue with his or her shopping. On the other hand, when EDR is not possible, the camera may communicate with the service point at a normal rate, such as 5.5 or 11 Mbps, as indicated by thin arrows  26 .  
         [0036]      FIG. 2  is a flow chart that schematically illustrates a process by which images are transferred by the customer to a service center  32  in system  20  for hard copying, in accordance with a preferred embodiment of the present invention. At an upload step  40 , a client device, in this case camera  28 , uploads one or more image files from its memory to service point  24 . Details of this step are shown in  FIG. 3  and described with reference thereto. When the upload is complete, the service point signals the customer that the camera can be taken away.  
         [0037]     At a transfer step  42 , the service point passes the image files over WLAN  22  to a service center  32 . Step  42  may overlap in time with step  40 . Since the customer is released at the end of step  40 , however, the amount of time taken to transfer the images to the service center at step  42  is generally not critical. The WLAN may therefore operate at a standard IEEE 802.11b rate, such as 11 Mbps, rather than at the EDR prevailing between the camera and the service point. Alternatively, WLAN  22  may also operate at the EDR. Further alternatively, the WLAN in the embodiment of  FIG. 1  may be replaced by a wired network.  
         [0038]     Service center  32  receives and processes the image files at a hard copy step  44 . Typically, the service center produces photographic prints or, alternatively, other media, such as a photo-CD. The finished hard copies are held at a customer service site  34 , which is preferably at or adjacent to the service center. The customer goes to site  34  at a pickup step  46 , typically when he or she is done shopping and is ready to leave the establishment, and picks up the finished hard copies. The customer is thus able to enjoy the advantage of high-quality hard copies of electronic images, with virtually no time spent waiting for the images to be transferred and only a single stop required at the customer service site to pick them up.  
         [0039]      FIG. 3  is a flow chart that schematically illustrates details of image upload step  40 , in accordance with a preferred embodiment of the present invention. Communications between service point  24  and the client device (camera  28 ) are preferably built on a master-slave model, with the service point acting as the master. To initiate the data upload, the client device passes a request to service point  24  to begin EDR service, at a request step  50 . This step is preferably carried out by communicating at the minimum standard bandwidth of 1 MHz, most preferably in accordance with Bluetooth networking protocols. (The IEEE 802.11 standards specify only the physical layer[PHY] and a media access control [MAC] sublayer for wireless communications.  
         [0040]     Bluetooth specifies higher-level protocols, as well, which can operate over IEEE 802.11.) In response to the client request, service point  24  allocates time slots in which the client device can transmit the image data, at an allocation step  52 . The time slots are preferably allocated based on the same model as is used for Bluetooth time slot allocation, so that EDR transmission can co-exist with low-bandwidth Bluetooth communications. The service point informs the client device of its time slot allocation in a Bluetooth-standard message. This message is preferably sent to the client at a different frequency from the client message at step  50 , in accordance with the Bluetooth protocol.  
         [0041]     At an EDR transmission step  54 , the client begins transmitting the image files or other data at the enhanced data rate. A large bandwidth, preferably up to 28 MHz, is allocated for this transmission. Most preferably, the client device encodes multiple bits per symbol, at the 11 Msps rate provided by 802.11b, so that the transmission at step  54  takes place at 22 or 33 Mbps. The frequency of transmission is preferably held steady at step  54 , as opposed to the above-mentioned frequency hopping that is used at steps  50  and  52 . Upon completion of the transmission, service point  24  informs the client camera  28  that all of the data have been received in good order, at an acknowledgment step  56 . This step preferably returns to using Bluetooth protocols, at the reduced 1 MHz bandwidth.  
         [0042]      FIG. 4  is a block diagram that schematically illustrates an alternative system  60  for providing hard copies of digital images, in accordance with another preferred embodiment of the present invention. In this embodiment, a high-speed WLAN  62  is itself adapted for EDR operation. Cameras  28  and other end devices are thus able to connect to WLAN  62  at substantially any point in an area that the WLAN serves, so as to transfer their digital images or other data directly to service center  32  at data rates of 22 Mbps and above. Alternatively, assuming the WLAN to be downward-compatible to the existing data rates of IEEE 802.11b, transmission at 11 Mbps and below is also possible. Aside from these differences, the methods of  FIGS. 2 and 3  are similarly applicable in system  60 .  
         [0043]     System  60  may also include service points  24 , enabling users to communicate alternatively with the service points or directly with high-speed WLAN  62 .  
         [0044]      FIG. 5  is a block diagram that schematically illustrates elements of a wireless transmitter  70  for use in an EDR modem, in accordance with a preferred embodiment of the present invention. The elements of transmitter  70  shown in the figure are preferably implemented on a single VLSI chip. This transmitter is appropriate for use as part of a modem in cameras  28  and service points  24 , as well as in other equipment that uses EDR service. A receiver suitable for communicating with transmitter  70  is described hereinbelow with reference to  FIG. 8 .  
         [0045]     Transmitter  70  comprises a convolutional encoder  72 , which applies pragmatic trellis coded modulation (PTCM) to encode the data. Details of encoder  72  are shown in  FIG. 6  and described with reference thereto. The encoder outputs a stream of two-, three- or four-bit symbols, depending on the data rate of transmission. These symbols are interleaved by an interleaver  74 . Alternatively, the bits output by the encoder are interleaved, without regard to the symbols. Optionally, an outer code, such as a Reed-Solomon code, is added to the data in order to enhance the reliability of the transmission.  
         [0046]     The interleaved stream of symbols is modulated by a configurable M-PSK modulator  76 . The configuration of the modulator is varied depending on the data rate at which transmitter  70  is to operate, as shown in the following table:  
                             TABLE I                       Data rate (Mbps)   Modulation                                1   Differential BPSK       2   Differential QPSK       5.5   CCK or BPSK/PBCC       11   CCK or QPSK/PBCC       &gt;11   QPSK, 8PSK or 16PSK                  
 
         [0047]     The modulation schemes for 1-11 Mbps are dictated by the 802.11b standard. The M-PSK schemes used above 11 Mbps enable data to be transmitted at these high bit rates, by sending multiple bits/symbol at the standard symbol rate of 11 Msps. Generally speaking, a wide range of different data rates Rb can be achieved at the fixed symbol rate RS by varying the order of modulation M and the code rate Rc, as given by the equation Rb=RS*log2(M)*Rc. Exemplary values are shown in Table II:  
                                                                 TABLE II                                   Rb (Mbps)   M   RS (Ms   Code Rate   Technique                                        11   4   11   ½   Regular Code           14.67   4   11   ⅔   Punctured           16.5   4   11   ¾   Punctured           22   4   11   1   Punctured           16.5   8   11   ½   Regular Code           22   8   11   ⅔   Punctured           24.75   8   11   ¾   Punctured           27.5   8   11   ⅚   Punctured           33   8   11   1   Punctured           22   16   11   ½   Regular code           29.67   16   11   ⅔   Punctured           33   16   11   ¾   Punctured           36.67   16   11   ⅚   Punctured           44   16   11   1   Punctured                      
 
         [0048]      FIG. 6  is a block diagram that schematically illustrates details of convolutional encoder  72 , in accordance with a preferred embodiment of the present invention. The input serial bitstream received by transmitter  70  is converted to two-bit parallel form by a serial/parallel converter  80 . The actual encoding is then performed by a finite impulse response (FIR) filter  82 . In this embodiment, the filter includes six delay stages  84  and two adders  86  and  88 . Filter  82  implements a 64-state, rate 1/2 convolution code giving two output bits Yo and Y 1  for each input bit, with a generator matrix G=[D6+D5+D3+D2+1, D6+D3+D2+D1+1]. Alternatively, other convolution schemes may be used, as are known in the art.  
         [0049]     Together with modulator  76 , encoder  72  is configurable depending on the data rate that is chosen. For example: 
        At 5.5 Mbps with BPSK modulation, each pair of bits Yo and Y 1  is taken serially so as to produce two BPSK symbols, at 1/2 input bit/symbol. Transmitting these symbols at the standard rate of 11 Msps gives the 5.5 Mbps data rate.     At 11 Mbps with QPSK modulation, each pair (Yo, Y 1 ) is used to generate a single symbol.     At 22 Mbps, encoder  72  can operate as a PTCM encoder with a 2/3 rate, i.e., for each two successive input bits, one is fed to filter  82 , to generate outputs Yo and Y 1   r  and the next is passed through directly to Y 2 . The triplet of output bits (Yo, Y 1   r  Y 2 ) is used to generate a single 8PSK symbol, giving two input bits/symbol. Alternatively, certain bits in outputs Yo and Y 1  may be punctured, as is known in the art, after which the symbols are interleaved and mapped to 8PSK.     At 33 Mbps, the input data are mapped directly to modulator  76 , without convolutional encoding, so as to generate 8PSK symbols with three input bits/symbol. Alternative configurations of encoder  72  will be apparent to those skilled in the art. By the same token, 16PSK could be used to transmit 4 bits/symbol (with appropriate modification to the design of encoder  72 ), thus yielding a data rates up to 44 Mbps, as long as the quality of the transmission channel is suitable.        
 
         [0054]      FIG. 7  is a polar plot illustrating an 8PSK constellation that is used in modulator  76 , in accordance with a preferred embodiment of the present invention. For each point in the constellation, the corresponding triplet of output bits (for 22 or 33 Mbps operation) is marked in the format (Y 2   r  Y 1 , Yo). Preferably, the modulator applies a cover function to the output symbols, in the form of a pseudo-random phase rotation that is applied to each symbol in the output sequence. For example, the cover function may be determined based on a pseudo-random bit sequence, running at three times the symbol rate. Each successive group of three bits then determines the phase rotation to be applied to the next output symbol.  
         [0055]     The pattern of phase rotations is known to the receiver, enabling the receiver to apply the appropriate inverse rotation before demodulating the symbols. The variable rotation of the signal from the transmitter means that the phase of any jamming signal affecting the receiver is effectively randomized. Furthermore, when multiple networks are operating simultaneously, as in the environment of system  20  ( FIG. 1 ), for example, mutual interference between the networks is also randomized, since the cover functions are not synchronized with one another. The use of the cover function thus enhances the data rates that can be achieved environments.  
         [0056]      FIG. 8  is a block diagram that schematically illustrates elements of a wireless receiver  100  for use in an EDR modem, in accordance with a preferred embodiment of the present invention. This receiver is designed particularly for use in conjunction with transmitter  70  ( FIG. 5 ), but may alternatively be used with wireless transmitters of other types. Radio signals arriving at receiver  100  are amplified, filtered and digitized by an analog front end (AFE)  102 , as is known in the art. A Bluetooth receiver  104  is provided for standard, low-rate communications, such as initiation of EDR service at step  50  in the method of  FIG. 3 .  
         [0057]     Once high-rate service has been established, the digitized samples generated by AFE  102  are rotated in phase, in a rotator/buffer  106 , and are then passed to a demodulator  108  for processing. For decoding signals received at standard 802.11b rates, the demodulator preferably comprises a conventional rake and demodulator block  110 , as is known in the art. EDR signals, on the other hand, are processed by a M-PSK modem and equalizer block  112 . This block can also be configured to demodulate standard 802.11b QPSK signals. Block  112  is configurable to operate at different orders of modulation, preferably including M=4, 8 and 16. Equalized samples output by block  112  are optionally de-interleaved, as appropriate, by a de-interleaver  114 . A decoder  116  processes the samples to generate a hard decision output, corresponding to the transmitted bitstream. Preferably, decoder  116  comprises a Viterbi decoder, although other types of trellis decoders may also be used, as are known in the art. The bitstream output by demodulator  108  is then passed to a MAC interface  118  for higher-level processing.  
         [0058]     Although preferred embodiments are described herein in relation to transmission of digital images in system  20  or system  60 , it will be appreciated that systems and devices of these types can be adapted for use in other applications and services in which large volumes of data must be transferred over wireless networks at high speed. Furthermore, although these preferred embodiments make specific use of inventive extensions to the IEEE 802.11 standard, those skilled in the art will understand that the principles of the present invention are not tied to one standard or another and may be applied to other standards and methods for high-speed data transmission. It will thus be appreciated that the preferred embodiments described above are cited by way of example, and that the present invention is not limited to what has been particularly shown and described hereinabove. Rather, the scope of the present invention includes both combinations and subcombinations of the various features described hereinabove, as well as variations and modifications thereof which would occur to persons skilled in the art upon reading the foregoing description and which are not disclosed in the prior art.