Abstract:
An optical repeater monitoring system, according to the invention, comprises an oscillating source, a reference signal transmitter for transmitting a reference signal of a predetermined frequency generated from an output of the oscillating source to a first optical fiber, and an optical repeater. The optical repeater has a first photodetector for converting light from the first optical fiber into an electrical signal, a reference signal extractor for extracting a component of the reference signal from an output of the first photodetector, a carrier generator for generating a carrier from an output of the reference signal extractor, a monitor signal modulator for modulating the carrier generated by the carrier generator with a monitor signal showing a operating state of the optical repeater, a transmitter for transmitting an output of the monitor signal modulator to a second optical fiber. The system further comprises a demodulating signal generator for generating a demodulating signal from either of the output from the oscillating source and the reference signal, the demodulating signal having a frequency equal to that of the monitor signal carrier, a second photodetector for photodetecting the light propagated on the second fiber, and a monitor signal demodulator for demodulating the monitor signal from outputs of the second photodetector and demodulating signal generator.

Description:
FIELD OF THE INVENTION 
     This invention relates to an optical repeater monitoring system and a method thereof, and more specifically, to a system and a method thereof for transmitting monitored information of a repeater and the like to a terminal station in an optical transmission system. 
     BACKGROUND OF THE INVENTION 
     In an optical transmission system, especially in an optical repeatered transmission system comprising at least one optical repeater for optically amplifying and repeating optical signals, it is necessary to remotely monitor and control an operating state and the like of the optical repeater. In a conventional system, for the purpose of transmitting an operating state of an optical repeater to a terminal station, a local oscillating signal source having a individual or common frequency is disposed in each optical repeater and an output of the signal source is modulated with a repeater monitoring information data and transmitted to the terminal station. 
     The oscillation frequency of the local oscillation signal source, however, fluctuates due to a temperature variation as well as aging and therefore the terminal stations are required to prepare a wider receiving bandwidth in anticipation of the frequency fluctuation. Therefore, in the conventional systems, signal-to-noise ratio (SNR) is deteriorated owing to the inefficiently wider bandwidth. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to solve the aforementioned problems and provide an optical repeater monitoring system and a method thereof for receiving a monitor signal at a high SNR. 
     An optical repeater monitoring system, according to the invention, comprises an oscillating source, a reference signal transmitter for transmitting a reference signal of a predetermined frequency generated from an output of the oscillating source to a first optical fiber, and an optical repeater. The optical repeater has a first photodetector for converting light from the first optical fiber into an electrical signal, a reference signal extractor for extracting a component of the reference signal from an output of the first photodetector, a carrier generator for generating a carrier from an output of the reference signal extractor, a monitor signal modulator for modulating the carrier generated by the carrier generator with a monitor signal showing a operating state of the optical repeater, a transmitter for transmitting an output of the monitor signal modulator to a second optical fiber. The system further comprises a demodulating signal generator for generating a demodulating signal from either of the output from the oscillating source and the reference signal, the demodulating signal having a frequency equal to that of the monitor signal carrier, a second photodetector for photodetecting the light propagated on the second fiber, and a monitor signal demodulator for demodulating the monitor signal from outputs of the second photodetector and demodulating signal generator. 
     With the above-mentioned configuration, it is no longer necessary to dispose local oscillator in an optical repeater since a carrier, which transmits a monitor signal showing an operating state of the optical repeater to a terminal station, can be generated in the optical repeater out of a reference signal from the same or another terminal station. As a result, a receiving side of the monitor signal has no need to consider a frequency fluctuation of a carrier for carrying the monitor signal and therefore it is also not necessary to dispose a receiver having an inefficiently wide bandwidth for receiving the monitor signal. Since synchronous detection can be used for demodulating the monitor-signal-modulated signal, the monitor signal can be demodulated at a high SNR. The monitor signal can be received at either of terminal stations; the one transmits the reference signal or another one. 
     When the reference signal is superimposed on a transmission signal light, an optical fiber transmission line can be effectively utilized. Also, when a dedicated light is used for carrying the monitor signal, a bad influence on the signal light can be reduced. 
     The optical repeater monitoring method, according to the invention, comprises a reference signal transmitting step for transmitting a reference signal having a predetermined frequency from a reference signal transmitter toward an optical repeater through an optical fiber line, a carrier generating step in the optical repeater for generating a carrier from the reference signal; the carrier has a frequency different from that of the reference signal and carries a monitor signal of the optical repeater, a monitor signal modulating step in the optical repeater for modulating the carrier with the monitor signal, a monitor signal transmitting step in the optical repeater for transmitting the modulated wave of the monitor signal by the monitor signal modulating step to a monitor signal receiver, a demodulating signal generating step in the monitor signal receiver for generating a demodulating signal having a frequency equal to that of the carrier, and a monitor signal demodulating step in the monitor signal receiver for demodulating the monitor-signal-modulated signal from the optical repeater is demodulated with the demodulating signal. 
     The above-mentioned configuration produces advantages similarly to the optical repeater monitoring system according to the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 is a schematic block diagram according to an embodiment of the present invention; 
     FIG. 2 is a timing chart of a control signal, a reference signal and a monitor signal according to the embodiment; (A) shows a transmission sequence of the control signal and reference signal and (B) shows a transmission timing of the monitor signal; 
     FIG. 3 is a schematic block diagram of a monitor signal receiving system at a terminal station  12 ; 
     FIG. 4 is a schematic block diagram of an embodiment of a superimposer  30 ; and 
     FIG. 5 is a schematic block diagram of another embodiment of the superimposer  30 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Embodiments of the present invention are explained below in detail with reference to the drawings. 
     FIG. 1 is a schematic block diagram according to an embodiment of the invention. An optical fiber line  14  for transmitting signal light from a terminal station  10  to a terminal station  12  and an optical fiber line  16  for transmitting the signal light from the terminal station  12  to the terminal station  10  are disposed between the terminal stations  10  and  12 . An optical repeater  18  is disposed halfway on the optical fiber lines  14  and  16 . For explanatory convenience, on the optical fiber line  14 , an optical fiber between the terminal station  10  and the optical repeater  18  is expressed as a reference numeral  14   a  and an optical fiber between the optical repeater  18  and the terminal station  12  is expressed as a reference numeral  14   b . Similarly, on the optical fiber line  16 , an optical fiber between the terminal station  12  and the optical repeater  18  is expressed as a reference numeral  16   a  and an optical fiber between the optical repeater  18  and the terminal station  10  are expressed as a reference numeral  16   b.    
     The configuration and operation of the terminal station  10  is explained below. A reference oscillator  20  oscillates a frequency signal (e.g. 6.37 MHz) for use in generating a carrier to be used when the optical repeater  18  transmits a monitor signal including a repeater data to the terminal station  10  (or the terminal station  12 ). The output of the reference oscillator  20  is divided (e.g. into 1/14) by a frequency divider  22  and applied to a control signal modulator  24 . A control/process circuit  26  generates a control signal for controlling the optical repeater  18  and applies it to the control signal modulator  24 . In this embodiment, the terminal station  10  transmits the control signal and reference signal to the optical repeater  18  using the time division. The control signal modulator  24  modulates a frequency signal (a frequency signal to be transmitted to the optical repeater  18 ) from the frequency divider  22  with the control signal from the control/process circuit  26  during the period in which the control signal should be transmitted to the optical repeater  18 , and outputs the output of the frequency divider  22  without processing during the other period; namely the period in which the reference signal should be transmitted to the optical repeater  18 . Preferably, the reference signal should be a tone signal having a single frequency. 
     A laser light source  27  generates a laser light for carrying a signal (e.g. 10 Gbit/s) to be transmitted to the terminal station  12 . An optical modulator  28  modulates the intensity of the output laser light from the laser light source  27  with a transmission signal and outputs an RZ optical pulse train or NRZ optical pulse train. A superimposer  30  superimposes the output of the control signal modulator  24  on the signal light from the optical modulator  28 . As to the methods for superimposing, although details are described later, there are concretely two methods; one is to use a dedicated wavelength light for carrying the control signal (and the reference signal) and the other is to modulate the amplitude of the signal light from the optical modulator  28  with the output of the control signal modulator  24 . The output light of the superimposer  30  inputs and propagates the optical fiber  14   a  of the optical fiber line  14  and then enters the optical repeater  18 . 
     On the other hand, a photodetector  32  converts the input light from the optical fiber line  16  into an electrical signal. The signal light entered the photodetector  32 , as to be described later, also carries a monitor signal including a repeater data of the optical repeater  18 . A receiver  34  converts the input light from the optical fiber line  16  into an electrical signal as well as receives and processes the signal from the terminal station  12 . The output from the photodetector  32  enters a monitor signal demodulator  36 . A frequency divider  38  divides the output of the reference oscillator  20  at a dividing ratio (e.g. 7/148) so as to obtain a frequency (e.g. 43.041 kHz) of a carrier to be used when the optical repeater  18  transmits the monitor signal to the terminal station  10 , and applies the divided signal to a monitor signal demodulator  36 . The monitor signal demodulator  36  demodulates the monitor signal from the output of the photodetector  32  using the output of the frequency divider  38 . The demodulated monitor signal is supplied to a control/process circuit  26 . 
     The configuration and operation of the optical repeater  18  is explained. Optical amplifiers (e.g. optical amplifiers using erbium-doped optical fiber)  40  and  42  are pumped by a pumping light from a pumping circuit  44  and then optically amplify signal lights from optical fibers  14   a  and  16   a  respectively. Optical dividers  46  and  48  output most of the outputs from the optical amplifiers  40  and  42  toward the following optical fibers  14   b  and  16   b  respectively and supply a portion of the outputs to photodetectors  50  and  52  respectively. The photodetectors  50  and  52  convert the light from the optical dividers  46  and  48  into electrical signals respectively. The photodetectors  50  and  52  can be low-speed as far as they can detect the control signal and reference signal from the terminal station  10 . 
     The outputs from the photodetectors  50  and  52  are compounded using wire-OR and applied to a bandpass filter (BPF)  54 . Instead of using the two photodetectors  50  and  52 , it is also applicable that the output lights from the optical dividers  46  and  48  are put together first and then converted into electrical signals by a photodetector. With this configuration, the optical repeater  18  can be monitored from both terminal stations  10  and  12  in the single repeater monitoring circuit. That is, this configuration allows monitoring the optical repeater  18  from both terminal stations  10  and  12  as well as lowering the cost of equipment. 
     The BPF  54  extracts the frequency components of the control and reference signals from the terminal station  10  out of the outputs of the photodetectors  50 ,  52  and supplies them to a reference reproducing circuit  56  and a control signal demodulator  58 . The reference reproducing circuit  56  multiplies the reference signal components contained in the outputs of the BPF  54  using a phase locked loop (PLL), and then a frequency divider  60  produces a carrier for carrying a monitor signal by dividing the outputs of the reference reproducing circuit  56 . That is, the reference reproducing circuit  56  and frequency divider  60  compose a carrier generating circuit. Owing to the aforementioned frequency multiplying and dividing, a carrier with a stable frequency can be obtained. Needless to say, the reference reproducing circuit  56  can be a narrow band pass filter for extracting the frequency component of the reference signal from the terminal station  10 . In this embodiment, for instance, the reference reproducing circuit  56  multiplies the reference frequency (455 kHz) components of the output from the BPF  54  by seven, and the frequency divider  60  divides the output frequency of the reference reproducing circuit  56  by  74 . The output frequency of the frequency divider  60  becomes 43.04 kHz. 
     The control signal demodulator  58  demodulates the control signal from the output of the BPF  54  and supplies it to a control circuit  62 . The control circuit  62  controls or monitors each part according to the control signal from the control signal demodulator  58  and outputs a monitor signal showing a monitored result toward a monitor signal modulator  64 . Applied to the monitor signal modulator  64  is the output of the frequency divider  60 . The monitor signal modulator  64  modulates the output of the frequency divider  60  with the monitor signal from the control circuit  62 . As the modulation method, for instance, amplitude-shift keying (ASK), frequency-shift keying (FSK) or phase-shift keying (PSK) is preferable. 
     The output of the monitor signal modulator  64  is applied to a pumping circuit  44 . The pumping circuit  44  weakly modulates the intensity of the pumping light to be transmitted to the optical amplifier  40  and/or the optical amplifier  42  according to the output from the monitor signal modulator  64 . Consequently, a monitor-signal-modulated signal is superimposed on signal light propagating from the terminal station  12  to the terminal station  10  on the optical fiber line  16  and transmitted to the terminal station  10 . In the embodiment, the gain of the optical amplifier  42  is modulated by the output of the monitor signal modulator  64  in order to transmit the monitor signal to the terminal station  10 . However, it is also applicable to cause Raman amplification on the optical fiber line  16  and change its gain according to the output of the monitor signal modulator  64 . Namely, pumping light for leading the Raman amplification within the wavelength band of the signal light on the optical fiber line  16  is applied to the optical fiber line  16  and the intensity of the pumping light is modulated with the output of the monitor signal modulator  64 . As a result, the gain of the signal light propagating on the optical fiber line  16  fluctuates according to the output of the monitor signal modulator  64  and thus brings the same effect with the case in which the gain of the optical amplifier  42  is fluctuated. 
     FIG.  2 (A) shows transmission timing of the signal from the terminal station  10  to the optical repeater  18 . FIG.  2 (B) shows transmission timing of the monitor signal from the optical repeater  18  to the terminal station  10 . The optical repeater  18  produces the carrier of the monitor signal and transmits the monitor signal to the terminal station  10  while receiving the reference signal from the terminal station  10 . 
     Explained next is the process in which the terminal station  10  makes the optical repeater  18  transmit the monitor signal showing the operating state of the optical repeater  18  toward the terminal station  10 . 
     As shown in FIG.  2 (A), the terminal station  10  first transmits the repeater control signal toward the optical repeater  18 . The repeater control signal, for instance, is such signals for remotely controlling the operating state of the optical repeater  18  and inquiring the operating state of the optical repeater  18 . The control/process circuit  26  outputs a control signal with desired contents (in the embodiment, the signal is for inquiring the operating state of the optical repeater  18 .) toward the control signal modulator  24 . Also applied to the control signal modulator  24  is the reference signal obtained from dividing the output of the reference oscillator  20  by the frequency divider  22 . The control signal modulator  24  modulates the reference signal with the control signal from the control/process circuit  26 . The modulated signal is applied to the superimposer  30 . The superimposer  30  superimposes the output of the control signal modulator  24  on the signal light generated by the laser light source  27  and the optical modulator  28  and outputs it toward the optical fiber  14   a.    
     The light propagating on the optical fiber  14   a  enters the optical amplifier  40  in the optical repeater  18  and is optically amplified there. The light is then divided into two portions by the optical divider  46 ; one portion enters the following optical fiber  16   b  and the other enters the photodetector  50 . The photodetector  50  converts the intensity of the incident light into an electrical signal and applies it to the BPF  54 . The BPF  54  extracts the component of frequency which is equal to the output frequency of the frequency divider  22  from the output of the photodetector  50  and applies it to the reference reproducing circuit  56  and control signal demodulator  58 . At this stage, the control signal demodulator  58  demodulates the output of the BPF  54  by a demodulation method corresponding to the modulation method of the control signal modulator  24  and applies the obtained control signal to the control circuit  62 . The control circuit  62  controls each part according to the input control signal and collects the data showing the operating state of each part. 
     The terminal station  10  transmits the control signal toward the optical repeater  18  for a certain period and then stops supplying the control signal to the control signal modulator  24  for making the control signal modulator  24  in a nonmodulating operating state. By this operation, the output of the frequency divider  22  passes through the control signal modulator  24  without stopping and enters the superimposer  30 . The superimposer  30 , similarly to the case when the control signal is transmitted, superimposes the output of the control signal modulator  24  on the signal light generated by the laser light source  27  and the optical modulator  28  and outputs it toward the optical fiber  14   a . Consequently, the reference signal, which defines the frequency of the carrier used when the optical repeater  18  transmits the monitor signal toward the terminal station  10  (or  12 ), is transmitted from the terminal station  10  to the optical repeater  18 . 
     Similarly to the case of the control signal, in the optical repeater  18 , the BPF  54  extracts the component of frequency equal to the output frequency of the frequency divider  22  from the output of the photodetector  50  and applies it to the reference reproducing circuit  56  and control signal demodulator  58 . The reference reproducing circuit  56  multiplies the frequency of the output (the reference signal) of the BPF  54 , and the frequency divider  60  divides the output frequency of the reference reproducing circuit  56 . Owing to this operation, a carrier for transmitting the monitor signal toward the terminal station  10  (or  12 ) is obtained and applied to the monitor signal modulator  64 . The control circuit  62  applies the monitor signal showing the previously collected repeater data to the monitor signal modulator  64 . The monitor signal modulator  64  modulates the output of the frequency divider  60  with the monitor signal from the control circuit  62  by a digital modulating method such as ASK, FSK or PSK. The output of the monitor signal modulator  64  is applied to the pumping circuit  44 . The pumping circuit  44 , as explained above, weakly modulates the intensity of the pumping light to be transmitted toward the optical amplifier  40  and/or the optical amplifier  42  according to the output of the monitor signal modulator  64 , superimposes the monitor-signal-modulated signal on the signal light propagating from the terminal station  12  to the terminal station  10  on the optical fiber line  16 , and transmits it with the signal light toward the terminal station  10 . 
     In the terminal station  10 , the photodetector  32  converts the input light from the optical fiber line  16  into an electrical signal. The output of the photodetector  32  enters the receiver  34  and the monitor signal demodulator  36 . The frequency divider  38  divides the output frequency of the reference oscillator  20  at a frequency dividing ratio (e.g. 1/148) so as to obtain a carrier frequency (e.g. 43.041 kHz) to be used when the optical repeater  18  transmits the monitor signal toward the terminal station  10  and applies it to the monitor signal demodulator  36 . The monitor signal demodulator  36  demodulates the monitor signal from the output of the photodetector  32  using the output of the frequency divider  38 . The demodulated monitor signal is applied to the control/process circuit  26 . Accordingly, the terminal station  10  can check the detailed operating state of the remote optical repeater  18 . 
     The relation of frequency dividing ratios among the frequency dividers  22 ,  60  and  38  is explained here. As to the carrier of the monitor signal, in a 10,000 km repeatered transmission system of a transpacific length, a 43 kHz band is most suitable in terms of modulating characteristics of optical amplifiers and frequency characteristics due to the multi-stage connection of the optical amplifiers. Considering the simpler process in the optical repeater  18 , the frequency of the reference signal to be transmitted from the terminal station  10  to the optical repeater  18  is set to 455 kHz. The reference reproducing circuit  56  generates 3.185 MHz through multiplying the received reference signal by an odd number (concretely, seven), and the frequency divider  60  generates 43.04 kHz by dividing the output of the reverence reproducing circuit  56  by 74. 
     In the terminal station  10 , when the oscillating frequency of the reference oscillator  20  is 3.185 MHz, the frequency dividing ratio of the frequency divider  22  becomes 1/7 and the duty factor becomes out of 50%. Since the duty factor is preferably 50%, a denominator n of the frequency-dividing factor 1/n of the frequency divider  22  need to be integer. Therefore, the oscillating frequency of the reference oscillator  20  is set to 6.37 MHz which is fourteen times (=2×7) of 455 kHz. The oscillating frequency of the reference oscillator  20  also can be 9.555 MHz which is twenty-one times (=3×7) of 455 kHz. 
     In order to equalize the carrier frequency from the frequency divider  60  in the optical repeater  18  with the output frequency of the frequency divider  38 , the frequency dividing factor of the frequency divider  38  should be 1/148. However, when the oscillating frequency of the reference oscillator  20  is 9.555 MHz, the frequency dividing factor of the frequency divider  38  should be 1/222. By equalizing the output frequency of the frequency divider  38  with the frequency of the carrier of the monitor signal, synchronous detection at the monitor signal demodulator  36  becomes possible and thus the demodulation of the monitor signal becomes much easier. 
     It is also possible to receive the monitor signal at the terminal station  12 . FIG. 3 shows an embodiment for demodulating the monitor signal at the terminal station  12 . In this case, the pumping circuit  44  in the optical repeater  18  modulates the intensity of the pumping light to be transmitted toward the optical amplifier  40  according to the output of the monitor signal modulator  64 . 
     The photodetector  70  converts the input light from the optical fiber  14   b  into an electrical signal. The output of the photodetector  70  is applied to the BPFs  72  and  74 . The BPF  72  extracts the reference signal component to be transmitted from the terminal station  10  to the optical fiber line  14 . The BPF  74  extracts the frequency component of the monitor-signal-modulated signal to be transmitted from the optical repeater  18  to the optical fiber line  14  using the gain modulation of the optical amplifier  40 . The output of the BPF  72  is applied to the reference reproducing circuit  76 . The reference reproducing circuit  76 , which has similar structure to the reference reproducing circuit  56  in the optical repeater  18 , multiplies the output frequency of the BPF  72  using PLL and the frequency divider  78  generates the frequency signal for synchronously detecting the monitor-signal-modulated signal by dividing the output frequency of the reference reproducing circuit  76 . The reference reproducing circuit  76  and frequency divider  78  are respectively equal to the reference reproducing circuit  56  and frequency divider  60  in the optical repeater  18  and similarly function. That is, the output frequency of the frequency divider  78  is equal to that of the frequency divider  60 . The monitor signal demodulator  80  synchronously detects the monitor signal by multiplying the output of the frequency divider  78  with the output of the BPF  74 . 
     In the embodiment, since the carrier frequency (455 kHz) of the reference signal to be transmitted from the terminal station  10  to the optical fiber line  14  and the carrier frequency (43.041 kHz) used for transmitting the monitor signal from the optical repeater  18  to the terminal station  12  are different, the terminal station  12  can easily distinguish the reference signal from the monitor signal carrier. 
     The superimposing method of the signal light and reference signal (or the control signal modulated signal) in the superimposer  30  is explained below. FIG. 4 shows an example of configuration in which the amplitude of the transmission signal light is modulated with the reference signal (or the control signal modulated signal) and transmitted. FIG. 5 shows an example of configuration to provide a dedicated wavelength for transmitting the reference signal (or the control signal modulated signal). 
     FIG. 4 is explained below. Laser light sources  82 - 1 ˜ 82 -n are laser-oscillated at respectively different wavelengths λ 1 ˜λn, and optical modulators  84 - 1 ˜ 84 -n modulate the intensities of output light from the respective laser light sources  82 - 1 ˜ 82 -n with transmission signals # 1 ˜#n. A multiplexer  86  multiplexes the output lights from the optical modulators  84 - 1 ˜ 84 -n. An optical modulator  88  weakly modulates the intensity of the output from the multiplexer  86  with the output (namely, the reference signal or the control signal modulated signal) from the control signal modulator  24 . The output of the optical modulator  88  is output toward the optical fiber line  14 . In this example, the optical modulator  88  acts a role of the superimposer  30 . The configuration of the optical repeater  18  in the embodiment shown in FIG. 1 corresponds to the superimposing method shown in FIG.  5 . 
     FIG. 5 is explained below. Laser light sources  90 - 1 ˜ 90 -n laser-oscillate at respectively different wavelengths λ 1 ˜λn, and optical modulators  92 - 1 ˜ 92 -n modulate the intensities of output light from the respective laser light sources  90 - 1 ˜ 90 -n with transmission signals # 1 ˜#n. A laser light source  94  is laser-oscillated at a wavelength λa different from the wavelengths λ 1 ˜λn of the signal light, and an optical modulator  96  modulates the intensity of output light from the laser light source  94  with the output (namely, reference signal or control signal modulated signal) from the control signal modulator  24 . A multiplexer  98  multiplexes the output lights from the optical modulators  92 - 1 ˜ 92 -n and the optical modulator  96  and outputs toward the optical fiber line  14 . In this configuration, the laser light source  94 , the optical modulator  96  and the multiplexer  98  compose the superimposer  30 . In the optical repeater  18 , an optical filter for removing the wavelength λa should be disposed at the optical stage. 
     As readily understandable from the aforementioned description, according to the invention, since a carrier, which carries a monitor signal showing a operating state of an optical repeater toward a terminal station, can be generated inside the optical repeater based on a reference signal from the same terminal station or another terminal station, it is no need to dispose a local oscillator in the optical repeater. As a result, it is no longer necessary for a monitor signal receiving side to consider a frequency fluctuation of a carrier for carrying a monitor signal and thus it is unnecessary for a receiver to have an inefficiently wide bandwidth for receiving the monitor signal. As the synchronous detection can be used for demodulating a monitor-signal-modulated signal at a terminal station, a monitor signal can be demodulated at a high signal-to-noise ratio. 
     While the invention has been described with reference to the specific embodiment, it will be apparent to those skilled in the art that various changes and modifications can be made to the specific embodiment without departing from the spirit and scope of the invention as defined in the claims.