Abstract:
A PON (Passive Optical Network) and method employing CDMA for upstream communications to avoid complicated systems such as Machine Access Control. The downstream communications are broadcast in a non-CDMA protocol such as TDM. The PON includes: A PON (Passive Optical Network) employing CDMA (Code Division Multiple Access) comprising: a plurality of ONTs (Optical Network Terminals) corresponding to subscribers; WDM filters for dividing upstream and downstream wavelengths; an OLT (Optical Line Terminal) for receiving optical signals transmitted from the ONTs and transmitting the received optical signals to a higher network, the OLT transmitting signals transmitted from the higher network into ONTs; and an optical coupler, wherein: each ONT includes a first switching unit connected to at least one lower interface, such as computers, a level transformer for converting Ethernet signals having levels of ‘0’ and ‘1’ into data signals of levels of ‘−1’ and ‘+1’, a first code generator for generating CDMA codes as specific codes so as to discriminate each ONT, and a first multiplier for performing spread spectrum function by multiplying the data signals by the CDMA codes, thereby transmitting Ethernet signals transmitted from the lower interfaces into the higher network; and the OLT includes an optical receiver for receiving optical signals transmitted from the ONTs, a branching filter for branching upstream CDMA signals received through the optical receiver, a plurality of second code generators for generating codes for despread, a plurality of second multipliers for multiplying received signals by the codes generated from the second code generators, and a plurality of data decider for extracting data through correlation calculation, thereby transmitting Ethernet signals transmitted from higher interfaces into the higher network.

Description:
CLAIM OF PRIORITY  
         [0001]    This application claims priority to an application entitled “Passive optical network employing code division multiple access,” filed in the Korean Industrial Property Office on Mar. 12, 2003 and assigned Serial No. 2003-15397, the contents of which are hereby incorporated by reference.  
         BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    The present invention relates to a passive optical network (PON) for providing a large scale of data at high speed to subscribers More particularly, the present invention relates to a passive optical network that comprises an Optical Line Terminal (OLT) for providing large scale communication service of 100 Mbps or more, high speed communication service and broadcasting service to subscribers, a plurality of Optical Network Terminals (ONTs) and a passive optical branching/coupling device, and employs CDMA (Code Division Multiple Access).  
           [0004]    2. Description of the Related Art  
           [0005]    Recently, many services such as internet service providers (ISPs), are accessed by most internet service subscribers using one of ADSL (Asymmetric Digital Subscriber Line), cable modems, dial-up modems, Metro-Ethernet, etc., at speeds of 56 kbps to several Mbps. Also, with the increase of bandwidth required by subscribers downloading larger and larger files, it has been possible to provide data service to subscribers at about 10 Mbps by using VDSL (Very High Bit-rate Digital Subscriber Line) and so forth. However, in order to provide various services—such as a large quantity of visual information service, VoD (Video on Demand) service, high quality broadcasting service and so forth—to subscribers, transmission of data at about 100 Mbps is required, and thus it is impossible to provide the various services with only some of the technologies described above. Therefore, the necessity of construction of optical subscriber networks using optical communication is rapidly increasing, so that a PON (Passive Optical Network) has been suggested and is being developed as a method capable of most economically forming an optical subscriber network.  
           [0006]    The PON comprises at least one OLT (Optical Line Terminal), a plurality of ONUs (Optical Network Units) or ONTs (Optical Network Terminals) (hereinafter, designated as “ONTs” for the purpose of simplicity), and a passive optical coupler. The PONs are largely classified into three kinds according to their implementation methods.  
           [0007]    [0007]FIG. 1A shows a schematic view of an ATM-PON employing an ATM (Asynchronous Transfer Mode) according to the prior art, FIG. 1B shows a schematic view of an Ethernet PON employing an Ethernet mode according to the prior art, and FIG. 1C shows a schematic view of a WDM-PON employing a WDM (Wavelength Division Multiplex) according to the prior art. Also, FIG. 1D shows a schematic view of an optical subscriber network employing CDMA (Code Division Multiple Access) technology.  
           [0008]    One of the three kinds of PONs is the ATM-PON shown in FIG. 1A, in which ATM cells are transmitted at 155 Mbps and with a wavelength of 1310 nm for upstream communication and data are transmitted at 155/622 Mbps and with a wavelength of 1550 nm in cell unit for downstream communication. A second of the three kinds of PONs, which is an Ethernet PON shown in FIG. 1B, has the upstream and downstream wavelength same as the ATM-PON, while using Gigabit Ethernet signals at 1.25 Gbps for both upstream and downstream signals. The ATM PON uses cells of fixed length, while the Ethernet PON uses Ethernet frames of variable length. A third of the three kinds of PONs is a WDM-PON which assigns transmitting and receiving wavelengths to each ONT individually. Therefore, the WDM-PON, as shown in FIG. 1C, uses wavelength multiplexer/demultiplexers not a passive optical coupler, unlike the ATM-PON and the Ethernet PON.  
           [0009]    In addition, another method is shown in FIG. 1D in which an optical subscriber network employs CDMA (Code Division Multiple Access) technology. This method employs CDMA (Code Division Multiple Access) technology for both upstream and downstream communications. Herein also, upstream and downstream data according to this method are transmitted at about 10 Mbps.  
           [0010]    The ATM-PON and the Ethernet PON of the prior art use TDM (Time Division Multiplexing) technology for downstream communication and TDMA (Time Division Multiple Access) technology for upstream communication in order to transmit data. Then, in the case of the downstream signals, data are transmitted in a broadcasting method, resulting in a problem of signal collision. However, in the case of the upstream signals, the same wavelength is used when two or more ONTs simultaneously transmits their signals to an OLT, so that signal collision may be caused in the passive optical coupler. Therefore, the ATM-PON and the Ethernet PON have to use a very complicated Media Access Control (MAC) protocol in order to solve this problem. Also, since distances between the OLT and each ONT are different from each other, various optical signals of different strengths are inputted into an optical receiver in the OLT, so that a Burst Mode IC (BMIC) is necessarily required so as to receive the various optical signals in stabilization. An optical transmitter in the ONT needs a BMIC to operate the transmitter only in a case in which signals to transmit exist, and it is largely restricted for the ATM PON and the Ethernet PON to receive a guaranteed bandwidth because the PONs use MAC (Media Access Control) and so forth.  
           [0011]    In the case of the WDM-PON, since the MAC is not used, operation of the PON system is simple and a broad bandwidth can be efficiently guaranteed, however, it is difficult to product optical transmitter/receiver modules in a low cost, so that continuous studies and development have been made on a low cost of optical transmitter/receiver modules.  
           [0012]    Meanwhile, in the case of an optical subscriber network employing CDMA, because CDMA is applied to upstream communication, it has an advantage in that the use of MAC is not required. However, the CDMA technology is also applied to downstream communication in the optical subscriber network in spite of the fact that the downstream communication adopting broadcasting method doesn&#39;t need MAC, so that the construction of the ONT and the OLT is complicated, thereby increasing the cost. Also, in the conventional optical network employing CDMA, data must be divided according to each subscriber by an switch in the OLT before transmission of the data, thereby complicating the operation of the OLT.  
         SUMMARY OF THE INVENTION  
         [0013]    Accordingly, the present invention has been made to overcome the above-mentioned problems and provides additional advantages, by providing a PON (Passive Optical Network) using TDM technology like the Ethernet PON for downstream signals so as to provide a large quantity of data at a high speed to subscribers, while using CDMA technology, not TDMA technology like the prior art, for upstream signals being transmitted from ONTs to an OLT, thereby not requiring the use of the complicated MAC.  
           [0014]    The present invention also provides a PON in which the optical transmitter/receiver commercially used in the prior art can be used without a Burst Mode IC (BMIC) for an optical receiver of an OLT and optical transmitters of ONTs.  
           [0015]    Additionally, the present invention provides a PON capable of guaranteeing bandwidth at all times by enabling data to be transmitted to an OLT whenever each ONT has data to be transmitted.  
           [0016]    The present invention also provides a PON capable of solving a security problem identified as a problem in the PON by enabling upstream transmission signals to be easily encoded by the use of the CDMA.  
           [0017]    The present invention also provides a PON having simpler constructions of OLTs and ONTs than the conventional optical subscriber networks employing CDMA according to the prior art.  
           [0018]    In order to accomplish these objects, there is provided a PON (Passive Optical Network) employing CDMA (Code Division Multiple Access) comprising: a plurality of ONTs (Optical Network Terminals) corresponding to subscribers; WDM filters for dividing upstream and downstream wavelengths; an OLT (Optical Line Terminal) for receiving optical signals transmitted from the ONTs and transmitting the received optical signals to a higher network, the OLT transmitting signals transmitted from the higher network into ONTs; and an optical coupler, wherein: each ONT includes a first switching means connected to at least one lower interface, such as computers, a level transformer for converting Ethernet signals having levels of ‘0’ and ‘1’ into data signals of levels of ‘−1’ and ‘+1’, a first code generator for generating CDMA codes as specific codes so as to discriminate each ONT, and a first multiplier for performing spread spectrum function by multiplying the data signals by the CDMA codes, thereby transmitting Ethernet signals transmitted from the lower interfaces into the higher network; and the OLT includes an optical receiver for receiving optical signals transmitted from the ONTs, a branching filter for branching upstream CDMA signals received through the optical receiver, a plurality of second code generators for generating codes for despread, a plurality of second multipliers for multiplying received signals by the codes generated from the second code generators, and a plurality of data decider for extracting data through correlation calculation, thereby transmitting Ethernet signals transmitted from higher interfaces into the higher network. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0019]    The above and other objects, features and advantages of the present invention will be more apparent from the following detailed description taken in conjunction with the accompanying drawings, in which:  
         [0020]    [0020]FIG. 1A is a schematic view of an ATM-PON employing an ATM according to the prior art;  
         [0021]    [0021]FIG. 1 b  is a schematic view of an Ethernet PON employing an Ethernet mode according to the prior art;  
         [0022]    [0022]FIG. 1 c  is a schematic view of a WDM-PON employing a WDM according to the prior art;  
         [0023]    [0023]FIG. 1 d  is a schematic view of an optical subscriber network employing CDMA technology;  
         [0024]    [0024]FIG. 2 is a schematic view illustrating a PON employing CDMA according to the present invention;  
         [0025]    [0025]FIG. 3 is a schematic view illustrating a CDMA-employing PON for accommodating 32 number of ONTs according to a first aspect of the present invention;  
         [0026]    [0026]FIG. 4 is a schematic view illustrating a construction of an ONT in the CDMA-employing PON according to the first aspect shown in FIG. 3;  
         [0027]    [0027]FIG. 5 is a schematic view illustrating a construction of an OLT in the CDMA-employing PON according to the first aspect shown in FIG. 3;  
         [0028]    [0028]FIG. 6 is a schematic view illustrating a CDMA-employing PON for accommodating 32 number of ONTs according to a second aspect of the present invention;  
         [0029]    [0029]FIG. 7 is a schematic view illustrating a CDMA-employing PON for accommodating 32 number of ONTs according to a third aspect of the present invention;  
         [0030]    [0030]FIG. 8 is a schematic view illustrating a construction of an ONT in a first group of ONTs of the CDMA-employing PON for accommodating 32 number of ONTs according to the third aspect of the present invention;  
         [0031]    [0031]FIG. 9 is a schematic view illustrating a construction of an ONT in a second group of ONTs of the CDMA-employing PON for accommodating 32 number of ONTs according to the third aspect of the present invention;  
         [0032]    [0032]FIG. 10 is a schematic view illustrating a construction of an OLT in the CDMA-employing PON for accommodating 32 number of ONTs according to the third aspect of the present invention;  
         [0033]    [0033]FIG. 11 is a schematic view illustrating yet another aspect of the invention in which the CDMA-employing PON method according to the present invention is applied to a WDM-PON;  
         [0034]    [0034]FIG. 12 is a schematic view illustrating a construction of an ONT in the WDM-PON shown in FIG. 11;  
         [0035]    [0035]FIG. 13 is a schematic view illustrating a construction of an OLT in the WDM-PON shown in FIG. 11;  
         [0036]    [0036]FIG. 14 is a view illustrating waveform of input signals in a simulation for verifying the operation of a CDMA-employing PON according to the present invention;  
         [0037]    [0037]FIG. 15 is a view illustrating waveform of upstream signals outputted from an optical coupler in a simulation for verifying the operation of a CDMA-employing PON according to the present invention;  
         [0038]    [0038]FIG. 16 is a view illustrating a waveform resulting from correlation calculation of a CDMA receiver in an OLT in a simulation for verifying the operation of the CDMA-employing PON according to the present invention; and  
         [0039]    [0039]FIG. 17 is a view illustrating waveform of decoded output data in a simulation for verifying the operation of the CDMA-employing PON according to the present invention.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0040]    Hereinafter, a PON (Passive Optical Network) employing CDMA (Code Division Multiple Access) according to preferred aspects of the present invention will be described with reference to the accompanying drawings. For the purposes of clarity and simplicity, a detailed description of known functions and configurations incorporated herein will be omitted as it may make the subject matter of the present invention unclear.  
         [0041]    [0041]FIG. 2 is a schematic view illustrating a PON employing CDMA according to the present invention.  
         [0042]    Referring to FIG. 2, a PON employing CDMA according to the present invention comprises sixteen ONTs (Optical Network Terminals)  101  to  116  corresponding to the number of subscribers, a plurality of OLTs (Optical Line Terminals)  301  to  30 N which transmit optical signals received from the ONTs  101  to  116  into a higher network and transmit signals received from the higher network into the ONTs  101  to  116 , and an optical coupler  200 .  
         [0043]    Each of the ONTs  101  to  116  includes a first switching unit  10 , a level transformer  12 , a code generator  24 , a multiplier  14 , a laser driver  16 , a laser diode  18 , an optical receiver (photo diode)  22 , and a WDM (Wavelength Division Multiplexing) filter  20 . The first switching unit  10  may comprise a hub or any other switching device, is connected to lower interface  2 , such as computers. The level transformer  12  transforms 100 Mbps Ethernet signals having levels of ‘0’ and ‘1’ into those having levels of ‘−1’ and ‘+1’. The code generator  24  generates CDMA codes of 1.6 Gcps (chips per second) assigned to each of the ONTs  101  to  116 . The multiplier  14  multiplies data signals by the CDMA codes for spread spectrum. The laser driver  16  is a device for controlling laser drive current. The laser diode  18  modulates electric signals into optical signals. The optical receiver  22  receives 1.25 Gbps Ethernet signals transmitted from the OLTs. The WDM filter  20  is a device for dividing upstream wavelengths and downstream wavelengths.  
         [0044]    Each of the OLTs  301  to  30 N comprises a WDM filter  26 , an optical receiver  28 , a 1×16 branching filter  29 , at least one code generator  34 , at least one multiplier  32 , at least one data decider  36 , a second switching unit  40 , and an optical transmitter  38 . The WDM filter  26  divides upstream wavelengths and downstream wavelengths. The optical receiver  28  receives optical signals transmitted from the ONTs  101  to  116 . The 1×16 branching filter  29  branches received upstream CDMA signals. Each code generator  34  is a device for despreading. Each multiplier  32  multiplies received signals by code. The data decider  36  extracts data through correlation calculation. The second switching unit  40  may comprise that of an aggregator or any other switch device, and converts 100 Mbps Ethernet signals into 1.25 Gbps Ethernet signals. The optical transmitter  38  is a device for transmitting 1.25 Gbps Ethernet signals to downstream. Also, each of the OLTs  301  to  30 N is connected to an upper network through a third switching unit  400  having 1.25 Gbps Ethernet interface.  
         [0045]    The operation of the PON employing CDMA of FIG. 2 will be explained. Data transmitted from the lower interface  2 , such as computers, are subjected to a switching process or an aggregation process in the first switching unit  10 , and then are inputted to the level transformer  12  in the form of 100 Mbps Ethernet signals. The level transformer  12  transforms data signals having levels of ‘0’ and ‘1’ into those having levels of ‘−1’ and ‘+1’. The code generator  24 , which generates codes of 1.6 Gcps for pertinent ONT  101  to  116 , generates specific codes for discriminating pertinent subscriber among the whole subscribers. For example, in the case that 16-chip Walsh Hadamard codes are used, ‘1111111111111111’ of code-1 is assigned to a first ONT  101  and ‘1-1-11-111-1-111-11-1-11’ of code-16 is assigned to a sixteenth ONT  116 . The codes represented above are repetitively assigned to each data bit, so that the code generator  24  for 1.6 Gcps Ethernet must be used so as to endow 100 Mbps Ethernet signals with a 16-chip sequence. The Walsh Hadamard codes, which are used as CDMA codes in the present invention, may be replaced by code.  
         [0046]    Data signals outputted from the level transformer  12  are multiplied by the code generated from the code generator  24  in the multiplier  14 , thereby being spread spectrum. The spread-spectrum signals are converted to laser-driving levels in the laser driver  16 , are optical-modulated in the laser diode  18  having an upstream wavelength λ UP , and then are transmitted to the OLT  301  through the WDM filter  20 . Also, 1.25 Gbps Ethernet signals of a downstream wavelength λ DOWN  transmitted from the OLT  301  are converted to electric signals at the optical receiver  22  in each of the ONTs  101  to  116  after passing the WDM filter  20 , and then are transmitted to the lower interface  2 , such as computers, through the first switching means  10 .  
         [0047]    Optical signals of λ UP  transmitted from each of the ONTs  101  to  116  are connected with each other in the 1×16 optical coupler  200 , and then are transmitted to the OLT  301 . The optical signals transmitted through the 1×16 optical coupler  200  are divided by the WDM filter  26  so as to be transferred to the optical receiver  28 , and then are converted into electric signals in the optical receiver  28 . The electric signals are branched into sixteen signals through the 1×16 branching filter  29 , and then each of the branched electric signals is respectively inputted to each corresponded CDMA receiver  30  in which each CDMA receiver  30  comprises a code generator  34 , a multiplier  32  and a data decider  36 . In each CDMA receiver  30 , the code generator  34  synchronized with the ONTs  101  to  116  generates codes—for example, CDMA codes having a first code to a sixteenth code—so as to decode data transmitted from each ONT, and the codes are multiplied with received signals in the multiplier  32 .  
         [0048]    The multiplied signals are subjected to a process, such as correlation calculation, in the data decider  36 , thereby decoding 100 Mbps Ethernet signals transmitted from each of the ONTs  101  to  116 . The decoded 100 Mbps Ethernet signals are converted into 1.25 Gbps Ethernet signals in the second switching unit  40 , and then are transmitted to other OLTs  302  to  30 N or a upper network through the switch  400  which are connected to the OLTs  301  to  30 N.  
         [0049]    In accordance with the process of the present invention described above, it can be seen that the complicated MAC (Media Access Control) isn&#39;t required when each of the ONTs  101  to  116  transmits upstream data to the OLT  301  to  30 N and furthermore a BMIC (Burst Mode IC) isn&#39;t required in receiving optical signals in each of the OLTs  301  to  30 N. Also, a BMIC for the optical receiver in each of the ONTs  101  to  116  isn&#39;t needed since an optical transmitter in each of the ONTs  101  to  116  is always kept in a ‘switched-on’ state so that the CDMA receiver decodes received data. The 1.25 Gbps Ethernet signals, which are transmitted from other OLTs  302  to  30 N or a upper network through the switch  400 , are optical-modulated at the optical transmitter  38  in the OLT  301 , pass through the WDM filter  26 , and then are branched through the 1×16 optical coupler  200  so as to be transmitted to the ONTs  101  to  116 .  
         [0050]    [0050]FIG. 3 is a schematic view illustrating a CDMA-employing PON for accommodating  32  number of ONTs according to a first aspect of the present invention, FIG. 4 is a schematic view illustrating a construction of an ONT in the CDMA-employing PON according to the first embodiment shown in FIG. 3, and FIG. 5 is a schematic view illustrating a construction of an OLT in the CDMA-employing PON according to the first aspect shown in FIG. 3.  
         [0051]    Referring to FIG. 3 to FIG. 5, a CDMA-employing PON, which accommodates 32 number of ONTs according to a first aspect of the present invention, comprises 32 number of ONTs  121  to  152 , an OLT  230  and a 1×32 optical coupler  202 . The ONTs are classified into two groups  210  and  220  so that each group includes sixteen ONTs. In this method, each of the ONTs  121  to  152  has the same construction as the ONT shown in FIG. 2 except that a PN (Pseudo-random Noise) code generator  25  and a multiplier  15  is included additionally. The OLT  230  additionally includes a 1×32 branching filter  55 , and a PN code generator  53  and a multiplier  51  for despreading PN codes.  
         [0052]    The operation principle of the PON for accommodating  32  number of subscribers according to the first aspect of the present invention will be explained with reference to FIG. 3 to FIG. 5. In this method, 32 number of ONTs  121  to  152  are classified into two groups  210  and  220  so that each group includes sixteen ONTs. Each of the ONTs  121  to  152  is discriminated by using 16-chip-sequence CDMA codes as shown in FIG. 3. Also, each group is discriminated by PN codes without regard to the discrimination for each ONT. That is, a first group  210  is discriminated with a first PN code, and a second group  220  is discriminated with a second PN code. Therefore, each of the ONTs  121  to  152  comprises a PN code generator  25  for discriminating its group besides the 16-chip-sequence code generator, and performs a spread spectrum function through a multiplier  15 . Signals generated from each of the ONTs  121  to  152  are connected in the 1×32 optical coupler  202 , and then are transmitted to the OLT  230 . The signals received in the OLT  230  are divided into 32 number of signals through the 1×32 branching filter  55 . First to sixteenth signals of the divided signals are multiplied in the multiplier  51  with a first PN code generated from a first PN code generator  53  so as to encode signals transmitted from the ONTs  121  to  136  of the first group  210 . Seventeenth to thirty-second signals of the divided signals are multiplied in the multiplier  61  with a second PN code generated from a second PN code generator  63  so as to encode signals transmitted from the ONTs  137  to  152  of the second group  220 . The signals, which are divided according to groups through the processes described above, are subjected to the process with codes generated from the 16-chip-sequence code generator as shown in FIG. 2, and thus 100 Mbps Ethernet data transmitted from each of the ONTs  121  to  152  are decoded. When the method described above, that is, the method of additionally multiplying the PN codes, is applied to the construction shown in FIG. 2, it is possible to encode signals to be transmitted, so that a problem of security can be solved.  
         [0053]    [0053]FIG. 6 is a schematic view illustrating a CDMA-employing PON for accommodating  32  number of ONTs according to a second aspect of the present invention.  
         [0054]    [0054]FIG. 6, which illustrates a PON for accommodating  32  number of ONTs according to the second aspect, comprises 32 number of ONTs  161  to  192 , a 1×32 optical coupler  204  and a plurality of OLTs  501  to  50 N. Each of the ONTs  161  to  193 . Each of the OLTs  501  to  50 N have the same construction as the ONT  101  to  116  and the OLT  301  to  30 N shown in FIG. 2 respectively except that a 3.2-Gcps code generator  72  or  74  is included. Also, a 1×32 branching filter  76  is required in each of the OLTs  501  to  50 N.  
         [0055]    The operation principle of the CDMA-employing PON for accommodating  32  number of subscribers according to the second aspect of the present invention is almost similar to the CDMA-employing PON shown in FIG. 2, and has only a little difference. The construction of FIG. 2 includes CDMA code generators having 16-chip sequence so as to accommodate sixteen subscribers, while the construction of FIG. 4 includes 3.2-Gcps CDMA code generators  72  for generating a 32-chip sequence so as to accommodate  32  number of subscribers. Signals transmitted from each ONT are coupled in the 1×32 optical coupler  204 , and then are converted into electric signals by an optical receiver  28  in each of the OLTs  501  to  50 N. The electric signals are divided into 32 number of signals by the 1×32 branching filter  76 , and then are encoded to original 100 Mbps Ethernet data in 32 number of CDMA receivers in which each CDMA receiver has a code generator so as to generate each of the CDMA codes.  
         [0056]    [0056]FIG. 7 is a schematic view illustrating a CDMA-employing PON for accommodating  32  number of ONTs according to a third aspect of the present invention.  
         [0057]    [0057]FIG. 8 is a schematic view illustrating a construction of an ONT in a first group of ONTs of the CDMA-employing PON for accommodating  32  number of ONTs according to the third aspect of the present invention.  
         [0058]    [0058]FIG. 9 is a schematic view illustrating a construction of an ONT in a second group of ONTs of the CDMA-employing PON for accommodating  32  number of ONTs according to the third aspect of the present invention.  
         [0059]    Also, FIG. 10 is a schematic view illustrating a construction of an OLT in the CDMA-employing PON for accommodating  32  number of ONTs according to the third aspect of the present invention.  
         [0060]    Referring to FIGS.  7  to  10 , a CDMA-employing PON, which accommodates 32 number of ONTs according to a third aspect of the present invention, comprises 32 number of ONTs  601  to  632 , an OLT  260  and a 1×32 optical coupler  206 . The ONTs are classified into two groups  240  and  250  including sixteen ONTs respectively, and use different upstream-transmission wavelengths from each other, according to groups. That is, each ONT of a first group  240  uses a laser diode  82  having an upstream wavelength λ UP1 , and each ONT of a second group  250  uses a laser diode  84  having an upstream wavelength λ UP2 . Also, the OLT  260  includes a wavelength-demultiplexing filter  86  for dividing wavelengths transmitted from each group, two optical receivers  87  and  88 .  
         [0061]    As shown in FIGS.  7  to  10 , in the third aspect of the present invention, sixteen ONTs  601  to  632 , like those of the first aspect, is divided into two groups  240  and  250  each including sixteen ONTs. The ONTs  601  to  632  in each group use 16-chip-sequence CDMA codes like those of the first aspect. The ONTs  601  to  632  in this aspect use different upstream-transmission wavelengths according to groups so as to distinguish each group. That is, each ONT of a first group  240  uses an upstream wavelength λ UP1 , and each ONT of a second group  250  uses an upstream wavelength λ UP2 . Therefore, each of the ONTs  601  to  616  of a first group  240  uses a laser diode  82  having an upstream wavelength λ UP1  and each of the ONTs  617  to  632  of a second group  250  uses a laser diode  84  having an upstream wavelength λ UP2 . Signals transmitted from each of the ONTs  601  to  632  are coupled in the 1×32 optical coupler  206 , and then are divided in the wavelength-demultiplexing filter  86  in the OLT  260  according to wavelengths. The optical signals divided according to wavelengths λ UP1  and λ UP2  are converted into electric signals by the optical receivers  87  and  88 , and then are encoded to original 100 Mbps Ethernet data in the same CDMA receivers as those shown in FIG. 2.  
         [0062]    [0062]FIG. 11 is a schematic view illustrating an aspect in which the CDMA-employing PON method according to the present invention is applied to a WDM-PON.  
         [0063]    [0063]FIG. 12 is a schematic view illustrating a construction of an ONT of the WDM-PON shown in FIG. 11, and FIG. 13 is a schematic view illustrating a construction of an OLT of the WDM-PON shown in FIG. 11.  
         [0064]    Referring to FIGS.  11  to  13 , a WDM-PON comprises a plurality of ONT groups  270  to  280 , an OLT  292 , a wavelength multiplexing/demultiplexing device  290  and a plurality of optical couplers  207  and  208 , while each ONT group consists of sixteen ONTs  651  to  666  or  667  to  682 . In this aspect, each ONT includes an optical transmitter  90  and an optical receiver  92  for transmitting and receiving data at an assigned wavelength, and also includes an optical circulator  91  in the case of using the same wavelength for transmission and receipt. The OLT  292  includes a optical circulator  93 , a wavelength multiplexer  97 , a demultiplexer  94 , optical receivers  95  and  96  for receiving each wavelength and optical transmitters  98  and  99  for transmitting each wavelength.  
         [0065]    In the third aspect, the ONTs forms n number of groups  270  to  280  comprising sixteen ONTs respectively. Then, each group uses different wavelengths form each other according to groups. That is, a first group uses a first wavelength λ 1 , a second group uses a second wavelength λ 2 , and a n th  group uses a n th  wavelength λ. Therefore, each ONT includes the optical transmitter  90  and the optical receiver  92  for transmitting and receiving data at an assigned wavelength, and also includes an optical circulator  91  in the case that transmitting and receiving wavelengths is the same. In a case that the transmitting and receiving wavelengths are different from each other as an other aspect, the optical circulator  91  may be replaced by a WDM(Wave Division Multiplex) filter or an optical coupler. The sixteen ONTs  651  to  666  and  667  to  682  in each group are distinguished by means of 16-chip-sequence CDMA codes, likewise to the previous aspects described above. Signals, which are optical-modulated in each of sixteen ONTs  651  to  666  or  667  to  682  in each group, are coupled in the 1×16 optical couplers  207  and  208 , are multiplexed through the wavelength multiplexing/demultiplexing device  290 , and then are transmitted into the OLT  292 . Optical signals, which have transmitted upstream from the optical circulator  93  in an input section of the OLT  292 , are transmitted to the demultiplexer  94  so as to be divided according to wavelengths, are converted into electric signals by the optical receivers  95  and  96 , and then are encoded to original data by the same CDMA receivers as those shown in FIG. 2. 1.25 Gbps Ethernet downstream signals are optical-modulated in the optical transmitters  98  and  99  having wavelengths assigned differentially according to groups, are multiplexed in the wavelength multiplexer  97 , and then are transmitted downstream through the optical circulator  93 . The optical circulator  93 , like that in the ONTs  651  to  682 , may be replaced by an optical coupler or an WDM filter. These signals, which are transmitted downstream through the optical circulator  93 , are divided according to wavelengths in the wavelength multiplexing/demultiplexing device  290 , and then are transmitted to ONTs in each group through the optical couplers  207  and  208 .  
         [0066]    Table 1 is a Walsh code table having sixteen chips assigned to sixteen subscribers as a sequence when the Walsh Hadamard codes are used as CDMA codes. Each code has a perfectly orthogonal characteristic.  
                                                                                                                                                                   TABLE 1                                       Chip order            Walsh code   c0   c1   c2   c3   c4   c5   c6   c7   c8   c9   c10   c11   c12   c13   c14   c15                    Code 1   1   1   1   1   1   1   1   1   1   1   1   1   1   1   1   1       Code 2   1   −1   1   −1   1   −1   1   −1   1   −1   1   −1   1   −1   1   −1       Code 3   1   1   −1   −1   1   1   −1   −1   1   1   −1   −1   1   1   −1   −1       Code 4   1   −1   −1   1   1   −1   −1   1   1   −1   −1   1   1   −1   −1   1       Code 5   1   1   1   1   −1   −1   −1   −1   1   1   1   1   −1   −1   −1   −1       Code 6   1   −1   1   −1   −1   1   −1   1   1   −1   1   −1   −1   1   −1   1       Code 7   1   1   −1   −1   −1   −1   1   1   1   1   −1   −1   −1   −1   1   1       Code 8   1   −1   −1   1   −1   1   1   −1   1   −1   −1   1   −1   1   1   −1       Code 9   1   1   1   1   1   1   1   1   −1   −1   −1   −1   −1   −1   −1   −1       Code 10   1   −1   1   −1   1   −1   1   −1   −1   1   −1   1   −1   1   −1   1       Code 11   1   1   −1   −1   1   1   −1   −1   −1   −1   1   1   −1   −1   1   1       Code 12   1   −1   −1   1   1   −1   −1   1   −1   1   1   −1   −1   1   1   −1       Code 13   1   1   1   1   −1   −1   −1   −1   −1   −1   −1   −1   1   1   1   1       Code 14   1   −1   1   −1   −1   1   −1   1   −1   1   −1   1   1   −1   1   −1       Code 15   1   1   −1   −1   −1   −1   1   1   −1   −1   1   1   1   1   −1   −1       Code 16   1   −1   −1   1   −1   1   1   −1   −1   1   1   −1   1   −1   −1   1                  
 
         [0067]    [0067]FIG. 14 shows the waveform of output signals (input data) of a level transformer in an ONT in a simulation for verifying the operation of a CDMA-employing PON, and  
         [0068]    [0068]FIG. 15 shows waveform of output signals of a 1×16 optical coupler connecting upstream signals transmitted from sixteen ONTs in a simulation for verifying the operation of a CDMA-employing PON.  
         [0069]    [0069]FIG. 16 shows waveform of correlation signals of a first CDMA receiver in an OLT in a simulation for verifying the operation of a CDMA-employing PON, the correlation signals representing correlation-output signals of received signals and CDMA code-1.  
         [0070]    [0070]FIG. 17 shows waveform of output signals encoded, that is, output data encoded, by a first CDMA receiver in an OLT in a simulation for verifying the operation of the CDMA-employing PON.  
         [0071]    As described above, the PON for providing a large quantity of data at a high speed to subscribers according to the present invention, unlike the ATM-PON and Ethernet PON according to the prior art, doesn&#39;t need the use of the complicated MAC protocol by adopting the CDMA method instead of the TDMA method as an upstream data transmission method, while the complicated MAC protocol has been necessarily used in the prior art. Therefore, the PON according to the present invention enables the ONTs to maintain a state capable of transmitting data at all times, thereby guaranteeing a wide bandwidth of 100 Mbps at all times, unlike the prior art. Also, the PON according to the present invention can use the optical transmitter and receiver utilized commercially without a BMIC (Burst Mode IC), though the BMIC has been necessarily required for optical receivers of OLTs and optical transmitters of ONTs in the ATM-PON and the Ethernet PON according to the prior art. Also, the PON according to the present invention adopts CDMA method for the upstream signals, and thus can easily encode the signals.  
         [0072]    Accordingly, the construction of the CDMA-employing PON according to the present invention can henceforth be efficiently applied to large-scale optical subscriber networks, also can be excellently applied to WDM-PONs generally recognized as a ultimate structure for optical subscriber networks. Therefore, the construction of the CDMA-employing PON according to the present invention solves the problems of the ATM-PON and the Ethernet PON according to the prior art, and thus henceforth can be applied to large-scale optical subscriber networks in forms of FTTC/B or FTTH.  
         [0073]    While the invention has been shown and described with reference to certain preferred aspects thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims. For example, the upstream protocol and the downstream protocol could be some other form of transmission (although CDMA and TDMA are preferred) so long as complicated protocols such as MAC are not needed because of the reduction of the collision problem.