BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a data communications apparatus and, more particularly, a data communications apparatus capable of performing bidirectional data communications through a cable network. This application for a communications apparatus is based on Korean patent application No. 2004-70119, which is incorporated by reference herein for all purposes. 
   2. Description of the Related Arts 
   The most common methods for implementing high-speed Internet subscriber network include the use of a DSL modem or a cable modem, which transmits and receives digital data through an existing twisted wire pair or a cable TV network, respectively. While the DSL modem is cost-effective in case that cable TV lines are broadly installed to a large majority of residences, the cable modem may be more effective alternative if cable TV lines are available in most of the residences. In a data communications network based on the cable TV network, each subscriber premise is connected to or equipped with a cable modem, which communicates with a counterpart, i.e., a headend to help the subscriber premise with Internet access. 
     FIG. 1  illustrates an example of a data communications system based on the cable TV network. In the system shown in  FIG. 1 , a headend or a cable modem termination system (CMTS)  10  is connected to a plurality of cable modems  34 A,  34 B through a tap  20  and/or a splitter  30 . The CMTS  10  is connected to an Internet service provider (ISP) server  12 , and the cable modems  34 A,  34 B are connected to client PCS  36 A,  36 B, respectively. The CMTS  10  modulates data to be transmitted from the ISP server  12  to a client PC  34 A or  34 B to transmit through the cable network, so that a desired cable modem  34 A, for example, demodulates such data to provide to the client PC  34 A. Similarly, the cable modem  34 A modulates data generated by the client PC  34 A to transmit to the CMTS  10 , so that the CMTS  10  demodulates such data to provide to the ISP server  12 . 
   Generally, a cable network equipment is manufactured to be compliant with Data Over Cable Service Interface Specification (DOCSIS) proposed by Multimedia Cable Network System (MCNS) and IEEE802.14 standard proposed by Institute of Electronic and Electrical Engineering (IEEE). According to the DOCSIS standard, the CMTS modulates the downstream data by 64 Quadrature Amplitude Modulation (QAM) or 256 QAM scheme in a frequency band of 5 MHZ to 45 MHZ to provide a data rate of 20 Mbps to 30 Mbps. The cable modem modulates the upstream data by Quadrature Phase Shift Keying (QPSK) or 16 QAM scheme in a frequency band of 65 MHZ to 750 MHZ to provide a data rate of 200 Kbps to 10 Mbps. Meanwhile, regarding medium access control (MAC), frequency domain multiple access (FDMA)/time division multiplexing (TDM) is used to facilitate communication from the CMTS to each cable modem and FDMA/time domain multiple access (TDMA) is used to facilitate communication from each cable modem to the CMTS. 
   However, the conventional CMTS and cable modem adopting FDMA or TDMA along with TDM are disadvantageous in that it is difficult for them to provide high data rates, for example, above 50 Mbps. Besides, the conventional CMTS and cable modem has drawbacks that they are much complex in their hardware configuration and show less reliability because different frequency bands and modulation schemes are used for the transmission of upstream and downstream data. 
   SUMMARY OF THE INVENTION 
   To solve the above problem, one object of the present invention is to provide a data communications apparatus which uses the same frequency band and modulation scheme with those used in the counterpart apparatus, and has a simple configuration and shows high reliability in transmitting and receiving high-rate data through a cable network. 
   The data communications apparatus for achieving the above object is connected to a data processing device through a first port and connected to a counterpart apparatus through a second port and cable network. The apparatus receives first data from the data processing device and modulates the first data to transmit first modulated signal to the counterpart apparatus, and receives second modulated signal from the counterpart apparatus and demodulate the second modulated signal to recover second data and provide to the data processing device. 
   The data communications apparatus includes a main processing unit, a RF unit, and medium access control performing means. The main processing unit receives the first data from the data processing device to perform orthogonal frequency division multiplexing (OFDM) on the first data to generate first OFDM signal, and demultiplexes second OFDM signal from the counterpart apparatus to recover the second data and provide to the data processing device. The RF unit transforms the first OFDM signal into a radio frequency band to generate the first modulated signal and transmit to the counterpart apparatus through the second port and the cable network. Also, the RF unit receives the second modulated signal from the counterpart apparatus to recover a second OFDM signal and provide to the main processing unit. The medium access control performing means detects collision in the cable network and selectively activates the main processing unit so that the RF unit transmits and receives signal to and from the counterpart apparatus through a common cable line. 
   Preferably, the medium access control performing means includes a MAC unit electrically connected to the first port and including a buffer for buffering the first data provided to the main processing unit; and a half-duplex controller for determining availability of the cable network and controlling the MAC unit. It is preferable that the main processing unit includes a quadrature amplitude modulator/demodulator and an OFDM unit. The quadrature amplitude modulator/demodulator performs quadrature amplitude modulation on the first data from the MAC unit to generate first QAM signal and demodulating second QAM signal to recover the second data. The OFDM unit performs OFDM on the first QAM signal to generate the first OFDM signal, and demultiplexes second OFDM signal to recover the second QAM signal. The main processing unit preferably further includes an error-correction encoder and an error-correction decoder. The error-correction encoder error-correction encodes the first data from the MAC unit to provide first error-correction encoded data to the quadrature amplitude modulator/demodulator. The error-correction decoder receives second error-correction encoded data from the first data from the quadrature amplitude modulator/demodulator to recover the second data and provide to the MAC unit. 
   Preferably, the half-duplex controller controls the MAC unit by a Distribution Coordination Function of CSMA/CA mode and centrally-controlled Point Coordination Function. Also, the MAC unit communicates with the data processing device in data frame compliant with IEEE 802.11 standard. 
   The RF unit may include a modulating and detecting unit and a RF transformer. The modulating and detecting unit modulates the first OFDM signal to the first modulated signal and detects the second OFDM signal from the second modulated signal. The RF transformer amplifies and filters the first modulated signal to transmit to the counterpart apparatus, and filters and amplifies signal received through the second port and the cable network to provide the second modulated signal to the modulating and detecting unit. 
   It is preferable, but not limited to, that the apparatus operates in frequency band below 2.5 GHZ. More preferably, the apparatus operates in frequency band below 1 GHz. 
   By employing the OFDM scheme having been used typically in wireless communications system along with half-duplex communication mode (for example, IEEE802.11 compliant medium access control mechanism), the inventor have found that the communications system of the present invention accomplish data rate higher than 50 MHz. Also, the apparatus of the present invention enhances the efficiency of frequency bandwidth because data transmission is performed in the same frequency as that used in the reception. Further, the simple architecture of the apparatus can reduce the system purchasing or deploying cost. 


   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above objects and advantages of the present invention will become more apparent by describing in detail preferred embodiments thereof with reference to the attached drawings in which: 
       FIG. 1  illustrates an example of a data communications network based on a cable TV network; 
       FIG. 2  illustrates an embodiment of a data communications network of the present invention; 
       FIG. 3  is a schematic diagram showing a preferred embodiment of the OFDM termination system and one of the OFDM modems shown in  FIG. 3 ; 
       FIG. 4  shows the frame format specified in IEEE 802.11 standard; 
       FIG. 5  is a detailed block diagram of the QAM unit, the OFDM unit, and the RF unit of the OFDM termination system shown in  FIG. 3 ; 
       FIG. 6  illustrates an example of implementation of the OFDM termination system; 
       FIG. 7  illustrates an example of implementation of the OFDM modem; and 
       FIGS. 8A through 8C  illustrates another embodiments of the data communications apparatus of the present invention. 
   


   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Referring to  FIG. 2 , a data communications system of the present invention may be implemented based on a cable network. The data communications system includes an OFDM termination system  100  and OFDM modems  140 A,  140 B each for respective subscriber and connected to the OFDM termination system  100  through a cable network. The OFDM termination system  100  functionally corresponds to the conventional CMTS  10  shown in  FIG. 1 , and OFDM modems  140 A,  140 B correspond to the cable modems  34 A,  34 B, respectively. 
   The OFDM termination system  100  is connected to the OFDM modems  140 A,  140 B by coaxial cable lines  112 ,  114 ,  116  of the cable network through a tap  110  and/or a splitter  120 . The coaxial cable network represented by the coaxial cable lines  112 ,  114 ,  116  may partially be implemented by a hybrid fiber coax (HFC) network. The OFDM termination system  100  is connected to an Internet service provider (ISP) server  12  through optical cable or unshielded twisted pair (UTP) cable, and the OFDM modems  140 A,  140 B are connected to client PCS  142 A,  142 B, respectively, through UTP cable or USB cable. The tap  110  divides and combines signals to and from several residences so as to accommodate a large number of subscribers, and the splitter  120  enables a television receiver and the client PC to share a single cable TV line. 
   In a preferred embodiment, the OFDM termination system  100  modulates data to be transmitted from the ISP server  102  to one of the client PCS  34 A,  34 B by QAM and OFDM schemes to transmit through the cable network, so that a desired OFDM modem  140 A, for example, demodulates such data to provide to the client PC  142 A. Similarly, the OFDM modem  140 A modulates data generated by the client PC  142 A by QAM and OFDM schemes to transmit to the OFDM termination system  100  through the cable network, so that the OFDM termination system  100  demodulates such data to provide to the ISP server  102 . Thus, the system of  FIG. 2  has a point-to-multipoint topology and facilitates the bidirectional communications between the ISP server  10  and the client PCS  142 A,  142 B. 
     FIG. 3  is a schematic diagram of an embodiment of the OFDM termination system  100  and the OFDM modem  140 A. 
   The OFDM termination system  100  includes a MAC  200 , a main processing unit  210 , and a RF unit  270 . The MAC  200  decapsulates Ethernet packets received from the ISP server  102  and encapsulates data to be transmitted to the ISP server  102  into Ethernet packets. Also, the MAC  200  includes a first in first out (FIFO) buffer, and temporarily stores data to be modulated by the main processing unit  210 , so that the modulation process of the main processing unit  210  is mediated while avoiding data collision in the coaxial line. The main processing unit  210  receives, from the MAC  210 , data to be transmitted to the OFDM modem  140 A to modulate by QAM and OFDM schemes, demodulates signal received from the OFDM modem  140 A, and supports half-duplex communication by detecting and avoiding data collision in the coaxial line based on the received signal. The RF unit  270  upconverts the frequency band of the modulated signal into the RF band to transmit to the OFDM modem  140 A, and downconverts the frequency band of signal received from the OFDM modem  140 A to provide to the main processing unit  210 . 
   The main processing unit  210  includes a QAM unit  220 , an OFDM unit  240 , and a half-duplex controller  265 . The QAM unit  220  performs QAM process for the data received from the MAC  200 , and the OFDM unit spreads the spectrum of the QAM signal by performing inverse Fast Fourier Transform (IFFT) with respect to the signal. The half-duplex controller  265  determines the busy status of the transmission medium, i.e., the coaxial line based on the signal received through the RF unit  270 , and controls the input and output of the MAC  200  depending on the determination result. In case that the medium is not being used, data is allowed to transfer from the MAC  200  to the main processing unit  220 . the medium is being used, however, the data transfer from the MAC  200  to the main processing unit  220  is inhibited. By repetitively controlling the operation of the MAC  200 , the half-duplex controller  265  enables the half-duplex communications. 
   Similarly, the OFDM modem  140 A includes a MAC  300 , a main processing unit  310 , and a RF unit  380 . The MAC  300  decapsulates Ethernet packets received from the client PC  142 A and encapsulates data to be transmitted to the client PC  142 A into Ethernet packets. Also, the MAC  300  includes a first in first out (FIFO) buffer, and temporarily stores data to be modulated by the main processing unit  310 , so that the modulation process of the main processing unit  310  is mediated while avoiding data collision in the coaxial line. The main processing unit  310  receives, from the MAC  310 , data to be transmitted to the OFDM termination system  100  to modulate by QAM and OFDM schemes, demodulates signal received from the OFDM termination system  100 , and supports half-duplex communication by detecting and avoiding data collision in the coaxial line based on the received signal. The RF unit  380  upconverts the frequency band of the modulated signal into the RF band to transmit to the OFDM termination system  100 , and downconverts the frequency band of signal received from the OFDM termination system  100  to provide to the main processing unit  310 . Since the configuration and operation of the main processing unit  310  are similar to those of unit  210 , detailed description thereof is omitted. 
   Preferably, the OFDM termination system  100  and the OFDM modem  140 A of the present invention use frame format specified in IEEE 802.11.  FIG. 4  shows the frame format of IEEE 802.11, which format is applied to all the frames transmitted between the OFDM termination system  100  and the OFDM modem  140 A regardless of the type of frames. The frame shown in the drawing is comprised of a frame header  402 , a frame body  404 , and a frame check sequence (FCS)  406 . 
   IEEE 802.3 compliant Ethernet packets transferred from the OFDM termination system  100  or the OFDM modem  140 A are mapped into the variable-length frame body  404  as payloads. The frame body  404  is preceded by the frame header  402 , which consists of following components: FRAME CONTROL field, DURATION/ID field indicating back off time which depends on the transferred frame, ADDRESS1 to ADDRESS4 fields depending on the type of the transferred frame, and SEQUENCE CONTROL field. The FRAME CONTROL field consists of PROTOCOL VERSION field, DATA TYPE field, SUBTYPE field, TO DS field, FROM DS field, MORE FRAG field, RETRY field, POWER MANAGEMENT field, MORE DATA field, WEB field, and ORDER field. Meanwhile, a 32-bit wide frame check sequence is calculated based on cyclic redundancy code (CRC) and appended behind the frame body  404  as the frame check sequence (FCS)  406 . 
   In addition to the frame format of IEEE 802.11, the OFDM termination system  100  and the OFDM modem  140 A preferably use the medium access control (MAC) mechanisms specified in IEEE 802.11: (1) the basic access mechanism referred to as the Distributed Coordination Function (DCF) based on Carrier-Sense Multiple Access with Collision Avoidance (CSMA/CA), and (2) a centrally-controlled access mechanism referred to as the Point Coordination Function (PCF) based on polling. 
   In DCF mechanism, a station, i.e., the OFDM termination system  100  or the OFDM modem, willing to transmit data packet first transmits a short control packet in a contention period before sending the data packet in order to determine whether another station is transmitting data over the desired channel. The station receives the signal in the channel just after sending the control packet and determines whether a collision has occurred in the channel by finding a signal other than that sent by itself. In case that the medium is free for a specified time, then the station is allowed to transmit the data packet. If a collision occurred, however, the station defers the transmission of the data packet and retries the transmission after a random backoff time. 
   In PCF mechanism which is an optional mode for allowing the transmission of time-bounded and contention-free frames, the OFDM termination system  100  plays the role of a point coordinator. At the beginning of a contention-free period, the OFDM termination system  100  has a chance of obtaining coordination authority. In this operation mode, medium access of all the OFDM modems connected to the cable network is controlled by the OFDM termination system  100 . 
     FIG. 5  illustrates, in detail, the QAM unit  220 , the OFDM unit  240 , and the RF unit  270  of the OFDM termination system  100  shown in  FIG. 3 . 
   The QAM unit  220  includes a forward-error-correction (FEC) encoder  222 , an interleaver  224 , a QAM mapping unit  226 , a QAM demapping unit  228 , a deinterleaver  232 , and a FEC decoder  234 . The FEC encoder  222  error-correction-codes transmit data to enhance the error robustness, and the interleaver  224  interleaves the error-correction-coded data so as to change an burst error which may be introduced in the channel into random errors. The QAM mapping unit  226  performs 16 QAM operation, for example, on the interleaved data to generate 16 QAM I and Q signals and provide to the OFDM unit  240 . The QAM demapping unit  228 , the deinterleaver  232 , and the FEC decoder  234  perform inverse processes of the processes performed by the counterparts equipped with the OFDM modem  140 A similarly to the FEC encoder  222 , the interleaver  224 , and the QAM mapping unit  226 , thereby restoring the data sent by the client PC  142 A. 
   In the OFDM unit  240 , a pilot insertion unit  242  inserts pilot symbols into the QAM symbol sequence from the QAM unit  320 . A serial-to-parallel converter  242  converts the symbol sequence inserted with the pilot symbols into parallel format. An IFFT block  246  performs IFFT operation on parallel symbol sequences to spreads the frequency band and form OFDM symbols. A parallel-to-serial converter  248  converts the parallel OFDM symbols back to serial format. A cyclic extension adder  250  adds a cyclic prefix to the OFDM symbols. Meanwhile, a cyclic extent remover  252  removes, from the received signal, the cyclic prefix added by the OFDM modem  104 A. A serial-to-parallel converter  254  converts prefix removed data into parallel format. A FFT block  256  performs FFT operation on parallel data from the serial-to-parallel converter  254  to despread the received OFDM symbols. A parallel-to-serial converter  258  converts the despreaded data back to serial format. A channel correction unit  260  removes pilot symbols from the despreaded QAM symbol sequence. 
   In the RF unit  270 , IQ modulator  272  modulates the carrier of 40 MHZ from an oscillator  274  to 16 states with the OFDM IQ symbols. A upconverter  276  upconverts the frequency band of the modulated signal using the signal of 22 MHZ from an oscillator  278 . A high power amplifier (HPA)  280  amplifies the upconverted signal, and a bandpass filter (BPF)  282  filters the amplified signal by selectively passing desired frequency bands to transmit through the cable network. 
   A BPF  284 , having the same pass band as the BPF  282  bandpass-filters the signal received through the cable network. A low noise amplifier  286  amplifies the filtered signal, and a downconverter  288  downverts the frequency band of the amplified signal using the signal of 22 MHZ from the oscillator  278 . An automatic gain control (AGC) amplifier  290  amplifies the downconverted signal depending on the intensity of the downconverted signal. An IQ detector  292  demodulates the amplified signal using the signal of 40 MHZ from an oscillator  294  to recover the OFDM I and Q symbols. The output signal of the oscillator  274  is controlled by an clock recovery unit  296 . A timing and frequency synchronizer  298  controls timing and frequency of the demodulated signal. 
   On the other hand, the QAM unit  320 , the OFDM unit  340 , and the RF unit  370  of the OFDM modem  140 A have the same configuration as those of the OFDM termination system  100 , and thus the OFDM termination system  100  and the OFDM modem  140 A operates symmetrically. 
     FIG. 6  illustrates an example of implementation of the OFDM termination system  100 . The OFDM termination system  100  may be implemented using a microcontroller  300  having an embedded CPU, and a radio chip  410 . The MAC unit  200  and the main processing unit is implemented by program executed by the microcontroller  400 . In this example, the microcontroller  400  is connected to an Ethernet transceiver  402  and/or an optical transceiver  404 . The Ethernet transceiver  402  and the optical transceiver  404  are connected to the ISP server  102  by twisted pair  403  and optical fiber  405 , respectively. Also, the OFDM termination system  100  is connected to the cable network through a F-connector  416 . 
     FIG. 7  illustrates an example of implementation of the OFDM modem  140 A. In this example, the OFDM modem  140  has the same configuration as that of the OFDM termination system  100  of  FIG. 6  except that a USB transceiver  440  may be used instead of the optical transceiver. 
   The frequency band for the operation of the communication devices is limited typically by the characteristics of the transmission medium and the taps. The OFDM termination system  100  and the OFDM modem  140 A may be used in the frequency band below 2.5 GHZ preferably in consideration of the signal transferring capability of the taps, and below 1 GHZ more preferably. 
   While the OFDM termination system or the OFDM modem can be manufactured and used in stand-alone type, such an apparatus can be employed in an application system which requires an network connection capability.  FIGS. 8A through 8C  shows such alternative embodiments of the present invention. 
   In  FIG. 8A , the OFDM modem  500  is employed in a voice over Internet Protocol (VoIP) system  510  to enable the system  510  to access Internet through the cable network. With the assistance of the OFDM modem  500 , a VoIP service module  512  which includes protocol stacks and user interfaces provides VoIP service to the user. The configuration of the OFDM modem  500  is similar to that shown in  FIG. 3  except that the modem  500  may be directly coupled to the VoIP sercive module  512 . 
   In  FIG. 8B , the OFDM modem  500  is employed in a settop box system  520  for providing video on demand (VOD) service through the Internet. In the system of  FIG. 8B , a settop box service module  522  which includes protocol stacks and user interfaces communicates an appropriate Internet server to provide VOD service to the user. 
   In  FIG. 8C , the OFDM modem  500  is employed in a wireless LAN access point (AP)  530  for facilitating wireless terminals to access Internet. In the system of  FIG. 8C , a wireless LAN service module  522  which includes RF circuits and protocol stacks communicates with the OFDM modem communicates in data frames compliant with IEEE 802.11 standard. 
   Although the present invention has been described in detail above, it should be understood that the foregoing description is illustrative and not restrictive. Those of ordinary skill in the art will appreciate that many obvious modifications can be made to the invention without departing from its spirit or essential characteristics. We claim all modifications and variation coming within the spirit and scope of the following claims.
A data communications apparatus which uses the same frequency band and modulation scheme with those used in the counterpart apparatus, and has a simple configuration and shows high reliability in transmitting and receiving high-rate data through a cable network. The main processing unit receives the first data from the data processing device to perform orthogonal frequency division multiplexing (OFDM) on the first data to generate first OFDM signal, and demultiplexes second OFDM signal from the counterpart apparatus to recover the second data and provide to the data processing device. The RF unit transforms the first OFDM signal into a radio frequency band to generate the first modulated signal and transmit to the counterpart apparatus through the second port and the cable network. Also, the RF unit receives the second modulated signal from the counterpart apparatus to recover a second OFDM signal and provide to the main processing unit. The medium access control performing means detects collision in the cable network and selectively activates the main processing unit so that the RF unit transmits and receives signal to and from the counterpart apparatus through a common cable line.