Abstract:
Disclosed is a integrated wireless local loop (WLL) and wireless local area network (WLAN) transceiver apparatus which connects a wireless local loop (WLL) base station and a plurality of wireless local area network (WLAN) terminals, including a WLL transceiver in communication with a WLL base station, a WLAN transceiver in communication with a plurality of WLAN terminals, and a specialized processing device which translates communications between WLL signaling protocol and WLAN signaling protocol. The apparatus allows a plurality of wireless terminals to simultaneously access the Internet without using a dedicated line or a cable line.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to a wireless network access system, and more particularly, a WLL-WLAN integrated network system which allows Internet access and constructs a network wirelessly by combining a wireless LAN and a wireless local loop (WLL) employing a wideband code division multiple access (W-CDMA) scheme.  
           [0003]    2. Background of the Related Art  
           [0004]    Currently, a W-CDMA system called a third-generation partner projection (3GPP) or a third-generation (3G) mobile wireless technology requires access to a core network such as PSTN, ISDN, IMT-2000, PSDN, the Internet, WAN/LAN, etc., through its own network for a wireless (radio) telecommunication.  
           [0005]    A wireless telecommunication network system is largely classified into a mobile communication network and a wireless local loop (WLL) according to an application field. The mobile communication network can support data communication at transmission rates of up to about  
           [0006]    [0006] 384  Kbps during movement of a mobile and portable wireless device and up to about 2 Mbps during a stop thereof, and the wireless local loop (WLL) can support a data communication at a transmission rate of up to about 2 Mbps.  
           [0007]    Also, the wireless local area network (WLAN) specified by a standard, IEEE 802.11b can support data communication at up to 11 Mbps, but requires a connection to an Ethernet switch using a cable line for the data communication.  
           [0008]    [0008]FIG. 1 is a broad block diagram illustrating the construction of an Internet access system independently employing a wireless local loop (WLL) and a wireless local area network (WLAN) according to the related art.  
           [0009]    As shown in FIG. 1, a general Internet access system  30  of a wireless local loop (WLL) includes a WLL base station  20  connected to the core network (hereinafter, referred to as “Internet”)  10  and a plurality of WLL terminals ( 32  and  34 ) connected to the WLL base station  20  wirelessly. The WLL terminals ( 32  and  34 ) include a mobile WLL terminal  34  connected to a lap top computer and a fixed WLL terminal  32  connected to a desk top computer.  
           [0010]    Basically, the WLL is not much different from a cellular communication system or a personal communication system in the aspect that a base station and a terminal transmit and receive a data to each other wirelessly. It is a telecommunication network constructed using a radio system instead of a wired line which connects the core network and a terminal of a network subscriber. This technique was first developed by the Bell Atlantic Network Services, Inc. (Arlington, Va.) of the United States in the early  1970 &#39;s as a method for cutting down on a facility expense in constructing a telecommunication network in a sparsely populated area such as a farming area or a remote mountain village. Currently, however, with the advanced radio technology and as the WLL draws more attention, its utilization coverage and installation area are being widened. Thanks to its advantages of a low unit cost per line and easy installation compared with a wired line, it is in increasing demand.  
           [0011]    [0011]FIG. 2 is a block diagram of a radio transceiver of a WLL terminal according to the related art. As shown in FIG. 2, the radio transceivers of the WLL terminals  32  and  34  employ a diversity antenna, respectively and a radio-frequency, electromagnetic signal received by each diversity antenna is converted into an electrical signal which is supplied to a band pass filter (BPF) 110  and a duplexer  240  which apply only a necessary frequency band of the electrical signal to low noise amplifiers (LNAs)  120  and  125 , respectively, to amplify the necessary frequency band. The amplified signals generated from the low noise amplifiers (LNAs)  120  and  125  are applied to down mixers  130  and  135  which down-convert in frequency the amplified signals to form intermediate frequency signals, respectively. Then, the down-converted signals generated from the down mixers  130  and  135  are applied to automatic gain controller (AGC) and demodulators  140  and  145  which amplify the down-converted signals to a proper level and demodulate the amplified signals. The signals generated from the automatic gain controller (AGC) and demodulators  140  and  145  are applied to low pass filters (LPFs)  150  and  155  which reject noise components of the signals applied thereto from the automatic gain controller (AGC) and demodulators  140  and  145  for application to A/D converters  160  and  165 . The A/D converters  160  and  165  convert the noise-rejected signals into digital signals for application to a digital baseband processor  170  which, in turn, applies the converted digital signal to a personal computer (PC)  180  so that a user can receive data on the Internet  10 .  
           [0012]    In the meantime, the data transmitted from the personal computer (PC)  180  is transmitted to the digital baseband processor  170 , and then a D/A converter  190  which converts the data transmitted thereto into an analog signal for application to an automatic gain controller (AGC) and modulator  200 . The automatic gain controller (AGC) and modulator  200  controls the-gain of the converted analog signal to a proper level and modulates the converted analog signal for application to a band pass filter (BPF)  210  which, in turn, rejects noise components of the signal applied thereto from the automatic gain controller (AGC) and demodulator  200  for application to an up mixer  220 . The up mixer  220  up-converts in frequency the signal applied thereto from the band pass filter (BPF)  210  for application to a high power amplifier (HPA)  230 . The high power amplifier (HPA)  230  sufficiently amplifies the output of the up-converted signal applied thereto from the up mixer  220  so that it can be sent out by radio for application to the duplexer  240  which, in turn, transmits the amplified signal applied thereto from the high power amplifier (HPA)  230  again to the WLL base station  20 . However, in case of such a WLL Internet access system  30 , there has been a problem in that a plurality of WLL terminals  32  have difficulties in accessing the Internet simultaneously.  
           [0013]    As shown in FIG. 1, a general Internet access system  40  of a wireless local area network (WLAN) includes a HUB  44  connected through wire to the Internet  10 , a wireless access point  46  connected through wire to the HUB  44  for enabling a wireless data transmission between the HUB  44  and a plurality of WLAN terminals  42 , and the plurality of WLAN terminals  42  included in a mobile or fixed computer for connecting to the wireless access point  46 .  
           [0014]    A conventional WLAN Internet access system  40  constructed as described above can enable a data communication with the Internet  10  at a transmission rate of up to 11 Mbps by means of the HUB  44  connected to the Internet  10  through a cable line and the wireless access point  46  for driving the plurality of WLAN terminals  42 , for example, 10 WLAN terminals.  
           [0015]    [0015]FIG. 3 is a block diagram of a WLAN access point according to the related art. As shown in FIG. 3, data transmission from the WLAN Internet access system  40  is performed by a half duplex scheme in which a medium access controller (MAC)  175  controls a dual switch  280  to determine whether to transmit or receive a signal.  
           [0016]    In the case where the dual switch  280  selects a receiving antenna, a radio-frequency, electromagnetic signal received by the selected receiving antenna is converted into an electrical signal supplied to a low noise amplifier (LNA)  128  through the dual switch  280 . The low noise amplifier (LNA)  128  amplifies the electrical signal applied thereto from the selected antenna for application to a band pass filter (BPF)  213  which, in turn, passes only a necessary frequency band of the amplified signal. The filtered signal generated from the band pass filter (BPF)  213  is applied to a down mixer  138  which converts downward in frequency the filtered signal into an intermediate frequency signal. Then, the converted intermediate frequency signal generated from the down mixer  138  is applied to an automatic gain controller (AGC) and demodulator  148  which demodulates the intermediate frequency signal. The demodulated signal generated from the automatic gain controller (AGC) and demodulator  148  is applied to an A/D converter  168  which converts the demodulated signal into a digital signal. The converted digital signal is applied to a computer (PC)  188  through the medium access controller (MAC)  175  so that a user can receive data on the Internet  10 .  
           [0017]    In the meantime, the signal transmitted from the computer (PC)  188  is processed by the medium access controller (MAC)  175  which applies the processed signal to a D/A converter  198 . The D/A converter  198  converts the signal applied thereto from the medium access controller (MAC)  175  into an analog signal for application to an automatic gain controller (AGC) and modulator  208  which modulates the converted analog signal. The modulated analog signal is applied to a up mixer  228  which up-converts in frequency the modulated signal applied thereto from the automatic gain controller (AGC) and modulator  208  for application to a band pass filter (BPF)  218  which, in turn, rejects noise components of the up-converted signal for application to a high power amplifier (HPA)  238 . The high power amplifier (HPA)  230  amplifies the signal applied thereto from the band pass filter (BPF)  218  to transmit the amplified . signal to the wireless access point  46  by way of the dual switch  280 . At this time, the signal received by the wireless access point  46  is transmitted to the HUB  44 , and then the Internet  10  through wire.  
           [0018]    However, in case of such a WLAN Internet access system  40 , there has been a problem in that the HUB  44  must connect to the Internet  10  through a cable line, and the WLL Internet access system  30  and the WLAN Internet access system  40  have been operated as independent network systems.  
           [0019]    The above references are incorporated by reference herein where appropriate for appropriate teachings of additional or alternative details, features and/or technical background. cl SUMMARY OF THE INVENTION  
           [0020]    An object of the invention is to solve at least the above problems and/or disadvantages and to provide at least the advantages described hereinafter.  
           [0021]    Therefore, the present invention has been made in view of the above-mentioned problems, and it is an object of the present invention to provide a plurality of users with a integrated wireless local loop (WLL) and wireless local area network (WLAN) Internet access system which combines a wireless local loop (WLL) and a wireless local area network (WLAN) which have been operated independently of each other at a small-scale office or a home office for small office home office (SOHO).  
           [0022]    According to the present invention, there is provided an integrated wireless local loop (WLL) and wireless local area network (WLAN) transceiver apparatus which connects a wireless local loop (WLL) base station and a plurality of wireless local area network (WLAN) terminals, including a WLL transceiver in communication with a WLL base station, a WLAN transceiver in communication with wireless terminals, and a means for translating signals between the WLL signal protocol and the WLAN signal protocol.  
           [0023]    Additional advantages, objects, and features of the invention will be set forth in part in the description which follows and in part will become apparent to those having ordinary skill in the art upon examination of the following or may be learned from practice of the invention. The objects and advantages of the invention may be realized and attained as particularly pointed out in the appended claims. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0024]    The invention will be described in detail with reference to the following drawings in which like reference numerals refer to like elements wherein:  
         [0025]    [0025]FIG. 1 is a broad block diagram illustrating the construction of an Internet access system independently employing a wireless local loop (WLL) and a wireless local area network (WLAN) according to the related art;  
         [0026]    [0026]FIG. 2 is a block diagram of a radio transceiver of a WLL terminal according to the related art;  
         [0027]    [0027]FIG. 3 is a block diagram of a WLAN access point according to the related art;  
         [0028]    [0028]FIG. 4 is a broad block diagram illustrating the construction of an integrated wireless local loop (WLL) and wireless local area network (WLAN) Internet access system according to the present invention; and  
         [0029]    [0029]FIG. 5 is a block diagram of the construction of an integrated wireless local loop (WLL) and wireless local area network (WLAN) transceiver apparatus according to the present invention.  
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0030]    Reference will now be made in detail to the preferred embodiments of the present invention. FIG. 4 is a broad block diagram illustrating the construction of an integrated wireless local loop (WLL) and wireless local area network (WLAN) Internet access system according to the present invention. As shown in FIG. 4, the integrated wireless local loop (WLL) and wireless local area network (WLAN) Internet access system  50  includes a core network  10 , a wireless local loop (WLL) base station  20  connected to the core network  10 , a computer  52  hereinafter, referred to as “integration terminal”) having an integrated wireless local loop (WLL) and wireless local area network (WLAN) transceiver apparatus, and a plurality of WLAN terminals  54 .  
         [0031]    The core network  10  includes a public switched telephone network (PSTN), an integrated services digital network (ISDN), an IMT-2000 which is the next-generation mobile telecommunication technology for a high-speed and high-capacity transmission, a public switched data network (PSDN), the Internet, a wide area network (WAN) or a local area network (LAN), etc.  
         [0032]    The integrated wireless local loop (WLL) and wireless local area network (WLAN) Internet access system  50  constructed as described above allows a WLL base station  20  connected to the core network (hereinafter, referred to as “the Internet”) through a cable line to be connected to the integration terminal  52  through a wireless (radio) connection. Also, the integrated wireless local loop (WLL) and wireless local area network. (WLAN) Internet access system  50  uses the integration terminal  52  as a server to bundle the plurality of WLAN terminals into a wireless LAN. At this time, the integrated wireless local loop (WLL) and wireless local area network (WLAN) transceiver apparatus included in the integration terminal  52 , which is an apparatus for coupling two independent wireless networks, i.e., a wireless local loop (WLL) and a wireless local area network (WLAN), allows the plurality of WLAN terminals  54  to connect to the Internet  10  through only a wireless (radio) connection. The plurality of WLAN terminals  54  can use a radio transceiver of a conventional WLAN terminal  42  as it is without any addition or replacement of the apparatus.  
         [0033]    [0033]FIG. 5 is a block diagram of the construction of an integrated wireless local loop (WLL) and wireless local area network (WLAN) transceiver apparatus according to the present invention. As shown in FIG. 5, the integrated wireless local loop (WLL) and wireless local area. network (WLAN) transceiver apparatus, which is a combination of the radio transceiver of the WLL terminal and the wireless access point  46  of the wireless local area network (WLAN) Internet access system  40 , includes a WLL transceiver  1 , a WLAN transceiver  2 , and an antenna unit  3 .  
         [0034]    The WLL transceiver  1 , which is adapted to access the WLL base station  20  by a WLL transmission scheme to transmit and receive a radio signal to and from the WLL base station  20 , performs the same function as that of a conventional radio transceiver of the WLL terminal. The WLL transceiver  1  includes a WLL reception processor  4  adapted to receive the radio signal from the WLL base station  20  and allow the received radio signal to undergo a predetermined reception process for application to a digital baseband processor  170 , a WLL transmission processor  5  adapted to receive a signal from the digital baseband processor  170  and allow the received signal to undergo a predetermined transmission process for radio transmission to the WLL base station  20 , and the digital baseband processor  170  adapted to perform a digital signal process (DSP) for the signal applied thereto from the WLL reception processor  4  or the signal applied to the WLL transmission processor  5  therefrom.  
         [0035]    The WLAN transceiver  2 , which is adapted to access the plurality of WLAN terminals  54  by a WLAN transmission scheme to transmit and receive a radio signal to and from the WLL base station  20 , performs the same function as that of a conventional wireless access point  46  of the WLAN Internet access system. 40  of FIG. 1. The WLAN transceiver  2  includes a WLAN reception processor  6  adapted to receive the radio signal from the plurality of WLAN terminals  54  and allow the received radio signal to undergo a predetermined reception process for application to a medium access controller (MAC)  175 , a WLAN transmission processor  7  adapted to receive a signal from the medium access controller (MAC)  175  and allow the received signal to undergo a predetermined transmission process for radio transmission to the plurality of WLAN terminals  54 , and the medium access controller (MAC)  175  adapted to supply the signal applied thereto from the digital baseband processor  170  to the WLAN transmission processor  7  or supply the signal applied thereto from the WLAN reception processor  6  to the digital baseband processor  170 , and perform an access control function to normally carry out the supply of the signal.  
         [0036]    The antenna unit  3  includes a duplexer  245  adapted to apply a radio signal received by a first antenna to the WLL reception processor  4  or the WLAN reception processor  6 , and a triplexer  170  adapted to apply a radio signal received by a second antenna to the WLL reception processor  4  or apply a signal supplied thereto from the WLL transmission processor  5  or the WLAN transmission processor  7  to the second antenna.  
         [0037]    The integrated wireless local loop (WLL) and wireless local area network (WLAN) transceiver apparatus employs a diversity antenna for collecting the radio signal by two antennas in order to prevent a fading phenomenon of a signal received through multiple paths. The two antennas of diversity antenna are spaced apart from each other to maintain an optimum distance between them in consideration of a wavelength. Generally, in case of using two antennas, the first antenna is used as a receive-only antenna and the second antenna is used as a combined transmit/receive antenna.  
         [0038]    [0038]FIG. 5 is a block diagram of the construction of an integrated wireless local loop (WLL) and wireless local area network (WLAN) transceiver apparatus according to the present invention. An operation of corresponding circuits and elements of the WLL and WLAN transceiver apparatus of the present invention when a certain radio signal is transmitted to the plurality of WLAN terminals  54  from the WLL base station  20  will now be described in detail hereinafter with reference to FIG. 5.  
         [0039]    The radio signal transmitted to the plurality of WLAN terminals  54  from the WLL base station  20  is collected by the first and second antennas, respectively. The two identical radio signals each collected by the first and second antennas are applied to the WLL reception processor  4  through the duplexer  245  and the triplexer  270 .  
         [0040]    The two radio signals applied to the WLL reception processor  4  are supplied to low noise amplifiers (LNAs)  120  and  125 , respectively, which amplify the two radio signals. The amplified two signals generated from the low noise amplifiers (LNAs)  120  and  125  are applied to down mixers  130  and  135  which down-convert in frequency the amplified two signals to form intermediate frequency signals, respectively. Then, the down-converted signals generated from the down mixers  130  and  135  are applied to automatic gain controller (AGC) and demodulators  140  and  145 , respectively, which amplify the down-converted signals to a proper level and demodulate the amplified signals. The signals generated from the automatic gain controller (AGC) and demodulators  140  and  145  are applied to low pass filters (LPFs)  150  and  155 , respectively, which reject noise components of the signals applied thereto from the automatic gain controller (AGC) and demodulators  140  and  145  for application to A/D converters  160  and  165 . The A/D converters  160  and  165  convert the noise-rejected signals into digital signals for application to a digital baseband processor  170 . The digital baseband processor  170  detects a signal having an error in the less amount of the two signals applied thereto from the A/D converters  160  and  165 , thereby improving a reliability of the received signals.  
         [0041]    The frequency supplied to the down mixers  130  and  135  is produced by a phase locked loop (PLL)  250  and is divided by 2 by a first distributor  260  which, in turn, outputs the divided two frequencies. One of the two frequencies is applied to a third distributor  267  and the other of them is applied to a second distributor  265 . The second distributor  265  divides the frequency applied thereto from the first distributor  260  by 2 to apply the divided two frequencies to the down mixers  130  and  135 , respectively. The phase locked loop (PLL)  250  is controlled by the digital baseband processor  170 .  
         [0042]    The digital signals processed by the digital baseband processor  170  are applied to the computer (PC)  180  or the medium access controller (MAC)  175  so that a user can receive or download data from the Internet.  
         [0043]    In the meantime, the medium access controller (MAC)  175  processes the signal thereto from the digital baseband processor  170  for application to the WLAN transmission processor  7 . The signal applied to the WLAN transmission processor  7  is converted into an analog signal by a D/A converter  198  which applies the converted analog signal to an automatic gain controller (AGC) and modulator  208 . The automatic gain controller (AGC) and modulator  208  controls a gain of the converted analog signal to a proper level and modulates the converted analog signal for application to a up mixer  228  which up-converts in frequency the modulated signal applied thereto from the automatic gain controller (AGC) and modulator  208  into a high frequency signal conforming to a WLAN transmission scheme for application to a band pass filter (BPF)  218 . The band pass filter (BPF)  218  rejects noise components of the up-converted signal applied thereto from the up mixer  228  for application to a high power amplifier (HPA)  238 . The high power amplifier (HPA)  238  sufficiently amplifies a power of the noise-rejected signal applied thereto from the band pass filter (BPF)  218  so that it can be sent out by radio for application to the triplexer  270  through the dual switch  280 . The triplexer  270  transmits the amplified signal applied thereto from the high power amplifier (HPA)  238  to a corresponding WLAN terminal  54 .  
         [0044]    The frequency supplied to the up mixer  228  is produced by a phase locked loop (PLL)  250  and is divided by 2 by a first distributor  260  which, in turn, outputs the divided two frequencies. One of the two frequencies is applied to a third distributor  267  and the other of them is applied to a second distributor  265 . The third distributor  267  divides the frequency applied thereto from the first distributor  260  by  2  to apply one of the divided two frequencies to the down mixer  220  and the other of them to a fourth distributor  268  respectively. The divided frequency applied to the fourth distributor  268  is again divided by  2 , and one of the two frequencies divided by the fourth distributor  268  is applied to the up mixer  228 .  
         [0045]    The operation of corresponding circuits and elements of the WLL and WLAN transceiver apparatus of the present invention when a certain radio signal is transmitted to the WLL base station  20  from the plurality of WLAN terminals  54  will now be described in detail hereinafter with reference to FIG. 5.  
         [0046]    The radio signal transmitted from the WLAN terminal  54  in the same scheme as a conventional WLAN transmission scheme is collected by the first antenna of the duplexer  245  and is applied to the WLAN reception processor  6  through the dual switch  280 .  
         [0047]    The radio signal applied to the WLAN reception processor  6  is amplified by a low noise amplifier (LNA)  128 . The low noise amplifier (LNA)  128  applies the amplified signal to a band pass filter (BPF)  213  which, in turn, reject noise components of the amplified signal. The filtered signal generated from the band pass filter (BPF)  213  is applied to a down mixer  138  which converts downward in frequency the filtered signal into an intermediate frequency signal. Then, the frequency supplied to the down mixer  138  is produced by the four distributor  268 . The converted intermediate frequency signal generated from the down mixer  138  is applied to an automatic gain controller (AGC) and demodulator  148  which amplifies the converted intermediate frequency signal to a proper level and demodulate the amplified signal. The demodulated signal generated from the automatic gain controller (AGC) and demodulator  148  is applied to an A/D converter  168  which converts the demodulated signal into a digital signal. The converted digital signal is applied to the medium access controller (MAC)  175  which processes the digital signal for application to the digital baseband processor  170 . The digital baseband processor  170  performs a digital signal process (DSP) for the processed signal applied thereto from the medium access controller (MAC)  175  to conform to a WLL transmission scheme for application to the WLL transmission processor  5 .  
         [0048]    The signal applied to the WLL transmission processor  5  from the digital baseband processor  170  is converted in to an analog signal by a D/A converter  190  for application to the automatic gain controller (AGC) and modulator  200 . The automatic gain controller (AGC) and modulator  200  controls a gain of the converted analog signal to a proper level and modulates the converted analog signal for application to a band pass filter (BPF)  210  which, in turn, rejects noise components of the signal applied thereto from the automatic gain controller (AGC) and demodulator  200  for application to the up mixer  220 . The up mixer  220  up-converts in frequency the noise-rejected signal applied thereto from the band pass filter (BPF)  210  into a high frequency signal to conform to the WLL transmission scheme for application to the high power amplifier (HPA)  230 . The high power amplifier (HPA)  230  sufficiently amplifies a power of the up-converted signal applied thereto from the up mixer  220  so that it can be sent out by radio for application to the triplexer  270  which, in turn, transmits the amplified signal applied thereto from the high power amplifier (HPA)  230  again to the WLL base station  20 .  
         [0049]    The frequency supplied to the up mixer  220  is applied from the third distributor  267 . The integrated WLL and WLAN transceiver apparatus allows five mixers to share the phase locked loop (PLL)  250  using a plurality of distributors.  
         [0050]    As can be seen from the foregoing, the integrated WLL and WLAN transceiver apparatus of the present invention allows the integration terminal  52  having the integrated WLL and WLAN transceiver apparatus included therein to access the Internet  10  by way of the WLL base station  20 , so that it enables a data communication with the Internet  10  each other intrinsically and enables a data communication between the plurality of WLAN terminals  54  connected to the integration terminal  52  and the Internet  10  without any wired connection. Moreover, since the integrated WLL and WLAN transceiver apparatus shares such components as the first and second antennas, the phase locked loop (PLL)  250 , etc., overlapped components can be saved, so that the cost price is expected to be reduced.  
         [0051]    The integrated WLL and WLAN transceiver apparatus has an effect of efficiently utilizing limited wireless communication resources in that it enables a configuration of the combined WLL and WLAN Internet access system without any great modification to a conventional Internet access system independently employing WLL and WLAN, and allows a plurality of terminals to simultaneously access the Internet without using a dedicated line or a cable line requiring a payment of an expensive rental fee.  
         [0052]    While this invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not limited to the disclosed embodiment, but, on the contrary, it is intended to cover various modifications within the spirit and scope of the appended claims.  
         [0053]    The foregoing embodiments and advantages are merely exemplary and are not to be construed as limiting the present invention. The present teaching can be readily applied to other types of apparatuses. The description of the present invention is intended to be illustrative, and not to limit the scope of the claims. Many alternatives, modifications, and variations will be apparent to those skilled in the art. In the claims, means-plus-function clauses are intended to cover the structures described herein as performing the recited function and not only structural equivalents but also equivalent structures.