Abstract:
A plurality of radio networks for transmitting data include a head end unit and a plurality of work stations. The head end unit receives a radio signal output from a given work station, converts the reception signal into a radio signal of a different frequency, and re-radiates the signal to all the work stations. Each work station includes a data processor, a receiving section for receiving and demodulating the radio signal from the head end unit to obtain data supplied from another work station, and supplying the obtained data to the data processor, and a unit for receiving transmission data generated by the data processor, and converting the transmission data into a radio signal and radiating the radio signal, thereby transmitting the transmission data to another work station through the head end unit.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a network system in which a plurality of work stations transmit/receive data by radio signals through head end units. 
     2. Description of the Related Art 
     Recently, so-called work stations have been made compact and therefore used on desks of individual operators. Cables are used as a medium for mutually connecting such work stations. 
     In a network system adopting cable connection, however, a wiring arrangement must be changed in order to move work stations, resulting in very troublesome operation. In addition, the appearance of the cable-connected network system is not desirable. 
     SUMMARY OF THE INVENTION 
     It is, therefore, an object of the present invention to provide a network system which can be easily operated. 
     In order to achieve the above object of the present invention, there is provided a network system comprising: 
     a head end unit (1) for receiving a radio signal of a first frequency (fU), and converting the reception signal into a radio signal of a second frequency (fD) different from the first frequency (fU) and radiating the converted signal; and 
     a plurality of work stations (WA) for performing data processing, each of the work stations (WA) comprising: 
     a data processing circuit (3) for performing data processing, receiving data supplied from another work station, and generating data to be transmitted to another work station; 
     a receiving circuit (2), connected to the data processing circuit, for receiving the radio signal of the second frequency from the head end unit (1), demodulating the radio signal to obtain data supplied from other the work station, and supplying the obtained data to the data processing circuit; and 
     a transmitting circuit (2), connected to the data processing circuit, for receiving the transmission data generated by the data processing circuit (3), and converting the transmission data into the radio signal of the first frequency and radiating the radio signal, thereby transmitting the transmission data to the another work station through the head end unit. 
     According to the above arrangement, since the work stations can mutually transmit/receive data by radio signals and therefore need not be connected through wires, the network system can be easily operated. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram showing an arrangement of a network system according to a first embodiment of the present invention; 
     FIG. 2 is a block diagram showing an arrangement of a work station shown in FIG. 1; 
     FIG. 3 is a block diagram showing an arrangement of a head end unit shown in FIG. 1; 
     FIG. 4 is a flow chart for explaining the operation of the network system shown in FIG. 1; 
     FIG. 5 is a schematic view showing a data format transmitted in the network system shown in FIG. 1; 
     FIG. 6 is a block diagram showing an arrangement of a network system according to a second embodiment of the present invention; 
     FIG. 7 is a schematic view showing a positional relationship of frequencies used in the network system shown in FIG. 6; 
     FIG. 8 is a block diagram showing an arrangement of a transmit/receive circuit used in a network system according to a third embodiment of the present invention; 
     FIG. 9 is a block diagram showing an arrangement of a network system according to a fourth embodiment of the present invention; and 
     FIG. 10 is a block diagram showing an arrangement of a bridge unit shown in FIG. 9. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Embodiments of the present invention will be described in detail below with reference to the accompanying drawings. 
     Referring to FIG. 1, a first embodiment of the present invention will be described. In the first embodiment, work stations mutually transmit/receive data by radio signals. 
     This network system using radio signals comprises head end unit 1 and a plurality of work stations WA l  to WA n . Unit 1 receives a radio signal (carrier wave fU) transmitted from certain work station WA. Unit 1 converts the reception signal into a radio signal (carrier wave fD) having a frequency different from that of the reception signal. Unit 1 transmits the converted radio signal to all work stations WA l  to WA n . Each station WA checks a destination address of a received data frame. Each station WA receives a data frame having a destination address coincident with its own address and neglects the other data frames. 
     Work stations WA respectively comprise transmit/receive circuits 2 l  to 2 n  ; and terminal units 3 l  to 3 n  such as computer terminals and word processors. 
     An arrangement of each work station will be described below with reference to FIG. 2. In FIG. 2, CPU 11, main memory 12, keyboard controller 13, external memory controller 14, VRAM (video memory, image memory) 15, display circuit 16, control memory 17, and transmit/receive circuit 18 are connected to system bus 22. Antenna ANT is connected to circuit 18. Keyboard unit 19 is connected to keyboard controller 13. Plasma display panel (PDP) 2 l  is connected to display circuit 16. 
     CPU 11 is a main unit of a control section of this system. Application software or the like is loaded in main memory 12. Keyboard controller 13 controls keyboard unit 19. External memory controller 14 controls external memory unit 20. VRAM 15 stores display data. Display circuit 16 controls PDP 21 and displays an image corresponding to the image data stored in VRAM 15. Control memory 17 stores various control data and the like. Transmit/receive circuit 18 converts transmission data supplied from bus 22 into a radio signal and outputs the signal through antenna ANT. Circuit 18 receives and demodulates a radio signal and supplies reception data to CPU 11 and the like. Keyboard unit 19 has keys so that various information can be keyed in from unit 19 to the system. Bus 22 comprises address lines, data lines, control lines and the like and transmits addresses, data and control signals between CPU 11 and the respective units. External memory unit 20 stores application programs and the like under the control of controller 14. PDP 21 displays an image with four gradation levels. 
     Head end unit 1 has, for example, an arrangement as shown in FIG. 3. Signals received by antenna ANT are supplied to band-pass filter 31, and only a signal of frequency fU is extracted therein. The extracted signal is amplified by amplifier 32 and supplied to mixer 33. Mixer 33 mixes the reception signal with a signal (frequency fU minus fD) from local oscillator 34. Amplifier 35 amplifies the mixed signal. Signals of unnecessary frequencies are cut by band-pass filter 36, and a radio signal of frequency fD is output through the antenna. 
     An operation of the network system according to the first embodiment will be described below. Assume that certain work station WA requests transmission of data. CPU 11 of this work station reads out transmission data stored in main memory 12, a register in CPU 11 or the like (step S1). The transmission data has, for example, a format as shown in FIG. 5 in which a sync flag, a destination address, a source address, text data, CRC data and an end flag are arranged in the order named. CPU 11 designates an I/O address of transmit/receive circuit 18 and writes the readout data therein. In this manner, the transmission data is transferred to circuit 18 through bus 22 (step S2). Circuit 18 converts the transmission data into serial data (step S3), converts the serial data into a radio signal of frequency fU and transmits the radio signal through antenna ANT (step S4). 
     Head end unit 1 receives the signal of frequency fU (step S5), converts the signal into a signal of frequency fD (step S6) and retransmits the converted signal (step S7). 
     All work stations WA l  to WA n  receive the radio signals from unit 1 (step S8). Each station WA takes synchronization of the reception data in accordance with the sync flag (step S9) and checks the destination address (step S10). Transmit/receive circuit 18 checks whether the destination address coincides with an address assigned to a work station to which circuit 18 belongs (step S11). If the addresses do not coincide with each other, a reading operation is interrupted thereafter (step S12). Meanwhile, if the addresses coincide with each other, circuit 18 converts the reception serial data into parallel data, stores the converted data into a buffer or the like and informs CPU 11 of this reception (step S13). Circuit 18 outputs the reception data to bus 22 under the control of CPU 11 (step S14). In this manner, data can be transmitted/received between the work stations through the head end unit. 
     If the network system using radio signals is enlarged in scale, a plurality of head end units must be used to constitute network systems in units of specific areas. FIG. 6 shows the second embodiment in which three network systems are formed in three areas A, B and C, respectively. In FIG. 6, head end units 1A to 1C and work stations WA Al  -WA An  to WA Cl  -WA Cp  constitute local area networks 4A to 4C, respectively. 
     If areas A, B and C are arranged adjacent to each other and radio signals of the same frequency are used in the respective areas, interference of the radio signals or the like occurs between the areas. For this reason, data must be transmitted/received by different channels (transmission/reception frequencies) in the respective areas. Therefore, in this embodiment, a frequency band is divided into a transmission band and a reception band and different frequencies are used in areas A to C. 
     In an arrangement of the network systems according to the second embodiment (FIG. 6), different channels (frequencies) are used in the respective areas, therefore a work station used in a certain area cannot be directly used in another, so a transmit/receive circuit of the work station must be replaced or readjusted. However, replacement or readjustment of a transmit/receive circuit performed each time a work station is moved is very troublesome. 
     Therefore, in a third embodiment to be described later, each work station automatically detects a channel to be used in an area to which it belongs and automatically changes reception and transmission frequencies. 
     An overall arrangement of a network system according to the third embodiment is substantially the same as that shown in FIG. 6, and a basic arrangement of a work station is substantially the same as that shown in FIG. 2. In order to automatically change a frequency, however, head end units 1A to 1C output corresponding radio signals normally (continuously) or at predetermined periods. Transmit/receive circuit 18 (2 in FIG. 6) is arranged as shown in FIG. 8. In FIG. 8, a reception RF signal is supplied to wideband filter 41, which limits the reception signal to a reception frequency area as shown in FIG. 3. An output signal from filter 41 is supplied to receiving amplifier 42. An output signal from amplifier 42 is mixed with a signal of frequency fLR to be described later by mixer 43, which supplies an output signal to band-pass filter (BPF) 44. BPF 44 has center frequency fm and a comparatively narrow passband width (i.e., passband width required for data transmission). BPF 44 then outputs a signal to demodulator 45, which supplies output data to transmission/reception controller 46 and controller 46 then checks the destination address assigned to the reception data. If controller 46 determines that the data is addressed to its own work station, controller 46 converts the data into parallel data and outputs the converted data. The reception data supplied to CPU 11 is used in a variety of data processing. If controller 46 determines that the data is addressed to another work station, controller 46 neglects it. 
     The output signal from BPF 44 is also supplied to reception frequency detector 47. Detector 47 checks whether a signal level of the input signal exceeds a predetermined level. If detector 47 determines that the signal level of the input signal exceeds the predetermined level, detector 47 outputs frequency fixing control signal L to local oscillator 48. If detector 47 determines that the signal level of the input signal is at the predetermined signal level or less, detector 47 outputs oscillation frequency variable control signal M to oscillator 48. Oscillator 48 is e.g., a VFO which oscillates at a fixed frequency in response to fixing control signal L or continuously changes its oscillation frequency in response to variable control signal M. Oscillator 48 supplies a signal of frequency fLR to mixer 43 in response to the output signal from detector 47. 
     Controller 46 converts transmission data into serial data and supplies the serial data to modulator 49, which modulates the transmission data into a signal of frequency fs and supplies an output signal to mixer 50. Mixer 50 then mixes the signal of frequency fs from modulator 49 with the signal of frequency fLT from oscillator 48 so that the signal of frequency fs is converted into a signal of frequency fU. An output signal from mixer 50 is supplied to band-pass filter (BPF) 51 for passing an RF signal of a transmission frequency band. A signal output from filter 51 is amplified by output amplifier 52 and sent as a transmission signal (carrier wave fU) from antenna ANT. 
     An operation of the circuit shown in FIG. 8 will be described below. 
     For the sake of better understanding, assume that a work station having the circuit shown in FIG. 8 is currently used in area A. 
     In this embodiment, head end units 1A to 1C normally output corresponding radio signals. In a reception operation, the work station receives at antenna ANT an RF signal of frequency (frequency of a carrier wave) fD transmitted from unit 1A, and BPF 41 of the work station limits the signal to a reception frequency band. The reception signal is amplified by amplifier 42 and supplied to mixer 43. Mixer 43 also receives a local signal of frequency fLR A  for area A generated by local oscillator 48, mixes the reception signal of frequency fD A  with the local signal of frequency fLR A , and outputs an intermediate frequency signal of frequency fm (fm=fD A  - fLR A ). The intermediate frequency signal is supplied to BPF 44 and passed therethrough. The signal output from BPR 44 is supplied to demodulator 45 and reception frequency detector 47. Demodulator 45 demodulates the reception signal passed through BPF 44 and generates serial data. Controller 46 checks a destination address of the serial data. If controller 46 determines that the data is addressed to its own work station, controller 46 converts the data into parallel data and supplies the parallel data to CPU 11. 
     In the above state, since the output signal from mixer 43 passes through BPF 44, detector 47 detects that a signal level of the input signal exceeds a reference value. For this reason, detector 47 outputs oscillation frequency fixing control signal L, and therefore oscillator 48 continuously outputs oscillation frequency fLR A . As a result, a reception state is maintained. 
     In a transmission operation, transmission data output from CPU 11 is supplied to transmission/reception controller 46. Controller 46 converts the transmission data into serial data and supplies the serial data to modulator 49, which modulates the transmission data into a signal of frequency fs and supplies the signal to mixer 50. Mixer 50 mixes the signal of frequency fLT A  for area A supplied from oscillator 48 with the signal of frequency fs. BPF 51 extracts only a signal of frequency fUA (=fLT A  -fs) for area A. An output signal from BPF 51 is radiated from antenna ANT. In this manner, the work station transmits/receives the data in area A. 
     Assume that the work station is moved to area B. 
     As described above, head end unit 1B in area B normally outputs a signal of frequency fD B , and the work station receives the signal at antenna ANT. Immediately after the work station is moved to area B, however, the frequency of an output signal from oscillator 48 is fLR A , so that a frequency (=fD B  -fLR A ) of an output signal from mixer 43 is not fm. Therefore, the signal level of an output signal from BPF 44 becomes substantially zero or near. For this reason, reception frequency detector 47 detects that the input signal level is a reference value or less and outputs frequency variable control signal M. In response to signal M, oscillator 48 gradually changes an oscillation frequency to increase/decrease the frequency of an output signal. When the frequency of the output signal becomes a predetermined level (frequency fL RB  (=fD B  +fm) for area B), the frequency of the output signal from mixer 43 becomes fm. As a result, the output signal from mixer 43 passes through BPF 44. Since the signal passes through BPF 44, detector 47 detects that the input signal level exceeds the reference value and outputs frequency fixing control signal L. In response to signal L, oscillator 48 continuously outputs a signal of frequency fL RB  different from frequency fLR A  in area A, thereby maintaining a reception state of the work station in area B. 
     If a frequency relationship as shown in FIG. 7 is adopted in areas A, B and C, oscillator 48 changes frequency fLT by an amount identical to a change amount (=fLR A  -fLR B ) of the frequency. That is, oscillator 48 outputs a signal of a frequency with which the frequency of an output signal from mixer 50 becomes fU B   to mixer 50. For this reason, the frequency of the output signal from mixer 50 becomes fU B  to enable data transmission from the work station to head end unit 1B in area B. 
     In this embodiment, reception frequency detector 47 detects the signal level of the output signal from BPF 44 and thereby checks whether a signal of a frequency corresponding to a channel of an area to which detector 44 belongs is correctly received. If detector 47 determines that the signal is not correctly received, frequency fLR is changed to select a channel. After the channel is selected, the reception and transmission states are fixed to the channel. 
     In this manner, in the third embodiment, each work station automatically selects a channel of an area to which it belongs. For this reason, if a certain work station is moved to another area, the work station can be directly used without performing any channel switching operation and the like. 
     In the third embodiment, head end units 1A to 1C normally output radio signals. Such a head end unit can be easily obtained by adding, to the arrangement shown in FIG. 3, an oscillator for normally oscillating at frequency fD and a circuit for adding an output from the oscillator to an output signal from BPF 36. 
     The present invention is not limited to the circuit arrangements of the above embodiments but can be modified. For example, instead of the circuit shown in FIG. 8, a frequency of a signal having the highest signal level of reception signals may be detected to calculate frequencies fLR and fLT, so that oscillator 48 outputs a signal of this frequency. If a difference between frequencies fm and fs is substantially the same as the width of the transmission or reception band, frequencies fLR and fLT may be identical. In addition, the frequencies used in the respective areas have predetermined intervals in the arrangement in FIG. 7, but they may be arbitrarily arranged. Moreover, in the arrangement in FIG. 8, an oscillation frequency of oscillator 48 is continuously changed. However, a table storing oscillation frequencies for all the areas may be prepared in advance to change the oscillation frequencies step-by-step in accordance with the contents of the table. 
     In the network system of the third embodiment, data cannot be transmitted/received between work stations belonging to different areas. For this reason, various types of software or hardware sources cannot be commonly used in a plurality of areas. For example, a single high-speed printer or a data base cannot be commonly used in areas A to C. In order to solve the above problem, in a fourth embodiment to be described below, data transmission is enabled between work stations belonging to different areas. 
     A network system according to the fourth embodiment will be described below with reference to FIG. 9. In a system shown in FIG. 9, data transmission can be performed between LANs 4 A  and 4 B  located in areas A and B, respectively, in FIG. 6. In FIG. 9, therefore, the same reference numerals as in FIG. 6 denote the same parts. The system shown in FIG. 9 comprises, e.g., taken-controlled type radio front end LANs 4 A  and 4 B&#39;  located in areas A and B, respectively, CSMA/CD (Carrier Sense Multiple Access/Collision Detection)-controlled type back end LAN 64 for mutual data transmission between LANs 4 A  and 4 B , and bridge units 62 A  and 62 B  having a relay function for connecting LANs 4 A  and 4 B  with LAN 64, respectively. 
     As in the arrangement shown in FIG. 6, front end LANs 4 A  and 4 B  respectively comprise work stations WA Al  to WA An  and WA Bl  to WA Bm , and head end units 1 A  and 1 B . Bridge units 62 A  and 62 B  and back end LAN 64 are connected by cables 63 A  and 63 B . Bridge units 62 A  and 62 B  and head end units 1 A  and 1 B  are connected by cables 61 A  and 61 B . 
     In this embodiment, each of head end units 1 A  and 1 B  comprises circuit 65 shown in FIG. 3, and circuit 66, having an arrangement similar to that of the circuit shown in FIG. 8, for demodulating and supplying reception data to a corresponding one of bridge units 62 A  and 62 B  and converting data supplied from a corresponding one of units 62 A  and 62 B  into a radio signal and outputting the signal. (If the reception and transmission frequencies are fixed, reception frequency detector 47 and the like need not be used.) 
     Referring to FIG. 10, an arrangement of each of bridge units 62 A  and 62 B  will be described. Unit 62 A  (62 B ) comprises: transmit/receive LSI (transmission/reception interface LSI) 71, connected to cable 63 A  (63 B ), for CSMA/CD control; token control transmit/receive LSI 72 connected to cable 61 A  (61 B ); data buffer memory 73; microprocessor 74 for controlling overall unit 62 A  (62 B ); memory 75 for storing operation programs and the like of microprocessor 74; and internal bus 76 for mutually connecting LSIs 71 and 72, buffer memory 73, microprocessor 74 and memory 75. 
     A predetermined area of memory 75 has a management table representing a relationship between sources (addresses inherent in the sources) such as the respective work stations in the system and the LANs to which the respective sources belong. 
     An operation of the fourth embodiment will be described below. 
     Assume that data is to be transmitted from work station WA Al  to WA An  in a single LAN. In this case, station WA Al  transmits data having a destination address representing station WA An  at its header portion to head end unit 1 A . Unit 1 A  receives the data and transmits a radio signal of frequency fD A  to work stations WAAl to WA An , and at the same time supplies the reception data to bridge unit 62 A  through cable 61 A . Each of stations WA Al  to WA An  receives the radio signal from unit 1 A  and fetches the data therein only when the destination address coincides with its own address. In this case, only station WA An  fetches the reception data. 
     The data transmitted from unit 1 A  to unit 62 A  is received by LSI 72 and stored in buffer memory 73 by a DMA through internal bus 76. When the data is stored in memory 73, microprocessor 74 retrieves the management table in memory 75 using the destination address assigned to the data. If microprocessor 74 determines by this retrieval that the data (reception data from unit 1 A ) is not destined to a work station in front end LAN 4A , microprocessor 74 sends the data to back end LAN 64. Alternatively, if microprocessor 74 determines that the data is destined to a work station in LAN 4A , microprocessor 74 disposes the data and interrupts data transmission (relay) to LAN 64. Therefore, since the data is transmitted from station WA Al  to WA An  in this case, the data flows through only LAN 4A . 
     The above operation is similarly performed in mutual communication between arbitrary work stations in LAN 4A  or in LAN 4B  installed in area B. 
     Mutual communication between front end LANs 4 A  and 4 B  will be described below assuming that data is transmitted from work station WA An  to WA Bm . First, data is transmitted by a radio signal of frequency fU A  from station WA An  to head end unit 1 A . Unit 1 A  receives the transmission data. Unit 1 A  converts the reception data into a radio signal of frequency fD A  and transmits the signal to stations WA Al  to WA An . In this case, stations WA Al  to WA An  check a destination address of the data and neglects the data. Unit 1 A  also supplies the reception data to bridge unit 62 A . The data (transmission data from station WA An ) supplied from unit 1 A  to unit 62 A  is received by LSI 72 in unit 62&#34; and stored in buffer memory 73. Microprocessor 74 in unit 62 A  retrieves a management table in memory 75 using the destination address assigned to the data stored in memory 73. If microprocessor 74 determines that the data is not destined for any of stations WA Al  to WA An , it sends the data to back end LAN 64 through internal bus 76, LSI 71, and cable 63 A . 
     Data (transmission data from station WA Al ) on back end LAN 64 is supplied to bridge unit 62 B  through cable 63 B . LSI 71 in unit 62 B  receives data on LAN 64 through cable 63 B  and stores it in buffer memory 73. Microprocessor 74 in unit 62 B  retrieves a management table in memory 75 using the destination address assigned to the data stored in memory 73. If microprocessor 74 determines that the data is destined to a work station (in this case, station WA Bm ) in LAN 4B , microprocessor 74 supplies the data to head end unit 1 B  through internal bus 76, LSI 71 and cable 61 B . Unit 1 B  transmits the data supplied from bridge unit 62 B  to stations WA Bl  to WA Bm  by radio signals of frequency fD B . Each of stations WA Bl  to WA Bm  receives the radio signal of frequency fD B  and fetches the data therein only when the destination address of the data coincides with its own address. Otherwise, each station disposes the data. In this case, only station WA Bm  fetches the reception data (transmission data from station WA An ) therein. 
     In the above embodiment, two front end LANs are connected to back end LAN. The present invention, however, can be applied to a system in which three or more front end LANs are connected to a back end LAN. 
     In this embodiment, units 1A and 1B output a signal of, frequency fD A  and a signal of frequency fD B  when they receive a signal of frequency fU A  and a signal of frequency fU B . Alternatively, they can output signal of frequencies fD A  and fD B  only when bridge circuits 62 A  and 62 B  determine that the received data is destined to any work station incorporated in local area network LANs 4 A  and 4 B  to which units 62 A  and 62 B  belong, respectively. 
     According to the fourth embodiment, the various sources can be shared in the system, and transmission data significant in only a certain front end LAN is not sent to a back end LAN. As a result, the back end LAN can be efficiently used. 
     As has been described above, according to the present invention, (1) a network system can be arranged using radio signals, (2) radio network systems can be arranged in units of areas, (3) when a plurality of radio network systems are arranged, each network system can be rearranged without any trouble if a work station is moved therein, and (4) data transmission can be performed between work stations located in different areas so that various sources can be shared in the system. In addition, since data significant to only a certain front end LAN is not transmitted to a back end LAN, the back end LAN can be efficiently used.