Abstract:
A signal processing apparatus for use in an optical line termination or optical network unit in a gigabit passive optical network encapsulates Ethernet signals, time-division multiplexed signals, and asynchronous transfer mode signals in the same way in a novel type of frame. The same input and output circuits can accordingly be used to support all three types of communication. A low-cost chip set including at least the input and output circuits of the apparatus can be combined with conversion circuits as necessary to provide a flexible answer to the needs of specific gigabit passive optical network systems.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to a signal processing apparatus, a signal processing method, and a signal frame structure for a gigabit passive optical network (GPON), more particularly to its transmission convergence structure. 
         [0003]    2. Description of the Related Art 
         [0004]    Known systems that provide access to networks such as the Internet through optical fibers include fiber-to-the-home, fiber-to-the-curb, fiber-to-the-node, fiber-to-the-premises, and other such systems, all of which may be conveniently denoted FTTx. A passive optical network (PON) is one type of network that can be used to implement these FTTx systems. 
         [0005]      FIG. 1  shows the general structure of a PON. An optical line termination (OLT) unit is connected through a single optical fiber to a passive optical coupler called a splitter  702 , which branches the optical signal from the OLT  701  onto a plurality of optical fibers  703 , enabling the OLT to connect with a plurality of optical network units (ONU)  704 - 1  to  704 - n . The OLT  701  is connected to an Internet protocol (IP) network  705  such as a local area IP network or the Internet, and the ONUs  704  are connected to respective communication terminals  706 - 1  to  706 - n  such as personal computers. 
         [0006]    The wavelength of the downstream optical signals used on the PON in transmission to the ONUs differs from the wavelength of the upstream optical signals used on the PON in transmission to the OLT. Bidirectional communication can accordingly be performed on a single strand of optical fiber. 
         [0007]    Downstream transmission from the OLT to the ONUs is point-to-multipoint. The OLT  701  sends downstream signal frames Fd addressed to individual ONUs, as indicated by the characters  2 ,  3 ,  1 , . . . , n in the downstream frames Fd in the drawing, to all of the ONUs  704 - 1  to  704 - n . Each of the ONUs  704 - 1  to  704 - n  extracts the frames addressed to it from the received data stream by a method such as decryption, and discards the other frames. 
         [0008]    Upstream transmission from the ONUs  704 - 1  to  704 - n  to the OLT is point-to-point. Upstream frames Fu, numbered  1  to n in the drawing, are transmitted to the OLT  701  from the ONUs at timings assigned by the OLT. The timings are assigned so that the frames from different ONUs do not collide in the splitter  702 . The timing assignments take into consideration the different round-trip (upstream and downstream) transmission delays between the OLT and the ONUs  704 - 1  to  704 - n , which are due to different distances between the splitter  702  and the ONUs. 
         [0009]    GPON, standardized as Recommendation G.984 of the Telecommunication Standardization Sector of the International Telecommunications Union (ITU-T), is one of several known varieties of PON. GPON is an optical access network system capable of carrying Ethernet, time-division multiplexing (TDM), and asynchronous transfer mode (ATM) communication. The Ethernet communication system is used in the Internet and in local area networks, TDM is used in existing telephone networks, and ATM is usable in all sorts of voice, data, and video communication media. Known documents that disclose GPONs include Japanese Patent Application Publication Nos. 2004-320745 and 2004-320746, and ‘Series G: Transmission System And Media, Digital System And Networks’, February 2004, International Telecommunications Union, compiled by ITU-T Study Group 15. 
         [0010]    Ethernet, incidentally, is a registered trademark. 
         [0011]    The methods by which the three types of communication systems (Ethernet, TDM, and ATM) are accommodated in a GPON will be described below, first for downstream communication, then for upstream communication. 
         [0012]      FIG. 2  shows the conceptual structure of the conventional GPON communication frame standardized in ITU-T Recommendation G.984. The frame used in GPON downstream communication fits into a 125-microsecond time slot and is referred to as a GPON transmission convergence (GTC) frame. A GTC frame includes overhead and a payload. 
         [0013]    The overhead section, which is necessary for communication control, maintenance, and operation, includes a frame header known as a downstream physical control block (PCBd) that gives a variety of information about the GTC frame. Part of the PCBd is an upstream bandwidth map that gives information for controlling upstream transmission by the ONUs  704 - 1  to  704 - n . In the example in  FIG. 2 , a time slot consisting of bytes  100  to  300  is allotted to ONU  704 - 1 , which has allocation identifier (ALLOC ID) ‘1’, a time slot consisting of bytes  400  to  500  is allotted to ONU  704 - 2  (ALLOC ID ‘2’), and a time slot consisting of bytes  520  to  600  is allotted to the ONU  704 - 3  (ALLOC ID ‘3’). 
         [0014]    The payload section, which carries user&#39;s signals, includes an ATM partition and a GPON encapsulation mode (GEM) partition. The ATM partition carries unaltered ATM cells. The GEM partition accommodates a GEM frame. The GEM frame may include Ethernet or TDM signals as described below. The PCBd in the overhead gives information indicating the boundary between the ATM and GEM partitions. 
         [0015]    In upstream transmission, each ONU sends a frame including overhead and a payload. If the upstream signal is an ATM signal, one or more ATM cells are directly mapped onto the payload as in the ATM partition in a downstream signal. If the upstream signal is an Ethernet signal or a TDM signal, the signal is mapped onto a GEM frame, and the GEM frame is mapped onto the payload. 
         [0016]    The transmit and receive processing in the ONU is carried out in a series of layers referred to as a protocol stack.  FIG. 3  shows the conceptual structure of the conventional GTC frame layer in the protocol stack. There is a similar protocol stack in the OLT, but only the ONU protocol stack will be described here. 
         [0017]    A downstream GTC frame is received by a GTC framing sublayer  910  of the GTC frame layer in each of the ONUs  704 - 1  to  704 - n . In the GTC framing sublayer  910 , an ATM signal is read from the ATM partition in the GTC frame or a GEM frame is read from the GEM partition, according to the ONU&#39;s allocation identifier, and the ATM signal or GEM frame is passed to a transmission convergence (TC) adaptation sublayer  920 . When the GTC framing sublayer  910  reads an ATM signal, it is received by an ATM TC adapter  922  in the TC adaptation sublayer  920 , and a VPI/VCI filter  925  identifies the logical path of the signal from the virtual path identifier (VPI) and virtual channel identifier (VCI) given as connection information in each cell, before outputting the signal to an ATM client that provides ATM service to the subscriber. When the GTC framing sublayer  910  reads a GEM signal, it is received by a GEM TC adapter  921  in the TC adaptation sublayer  920 , and a port-ID and PTI filter  923  identifies its logical path from the port identification (ID) value and payload type indicator (PTI) code given as connection information in the signal, before outputting the signal to a GEM client, which may provide either Ethernet service or TDM service. 
         [0018]    In upstream transmission, the GTC framing sublayer  910  in each of the ONUs  704 - 1  to  704 - n  generates a container corresponding to the ONU&#39;s allocation identifier, maps the ATM signal or the GEM frame onto the payload of the container, and transmits the container (see  FIG. 2 ). The containers sent from the ONUs  704 - 1  to  704 - n  are passively multiplexed in the splitter  702  and sent to the OLT  701  (see  FIG. 1 ). 
         [0019]      FIG. 4  shows the general structure of a conventional GPON signal processing apparatus. This is one example of an ONU signal processing apparatus that realizes the GTC frame layer of the protocol stack described above. The apparatus is depicted as a collection of functions and interfaces, which are implemented by a combination of hardware and software in one or more integrated circuits. 
         [0020]    In downstream transmission, the GTC deframing function  1050 , GEM extraction function  1052 , and ATM extraction function  1053  in  FIG. 4  correspond to the multiplexer  911 , GEM partition  913 , and ATM partition  914 , respectively, in the GTC framing sublayer  910  in  FIG. 3 . 
         [0021]    The distribution function  1054 , GEM-to-Ethernet conversion function  1062 , and GEM-to-TDM conversion function  1064  correspond to the port-ID and PTI filter  923  in the TC adaptation sublayer  920 , the conversion functions forming a GEM interface (IF). The ATM interface  1076  corresponds to the VPI/VCI filter  925 . Although no blocks are shown N corresponding to the GEM TC adapter  921  and ATM TC adapter  922  in  FIG. 3 , the functions of these adapters are realized when GEM frames are passed from the GEM extraction function  1052  to the distribution function  1054 , and ATM signals are passed from ATM extraction function  1053  to the ATM interface  1076  in  FIG. 4 . 
         [0022]    In upstream transmission, the GTC framing function  1034 , GEM mapping function  1032 , and ATM mapping function  1033  in  FIG. 4  correspond to the multiplexer  911 , GEM partition  913 , and ATM partition  914 , respectively, in the GTC framing sublayer  910  in  FIG. 3 . The Ethernet-to-GEM conversion function  1012  and TDM-to-GEM conversion function  1014  correspond to the port-ID and PTI filter  923  in the TC adaptation sublayer  920 . Although no blocks corresponding to the ATM TC adapter  922  and VPI/VCI filter  925  in  FIG. 3  are shown, the corresponding functions are realized when ATM signals are passed from the ATM interface  1006  to the bandwidth management buffer function (for ATM)  1031 . Similarly, although no block corresponding to the GEM TC adapter  921  is shown, the corresponding function is realized when GEM frames are passed from the Ethernet-to-GEM conversion function  1012  and TDM-to-GEM conversion function  1014  to the bandwidth management buffer function (for GEM)  1030 . 
         [0023]    The conventional apparatus in  FIG. 4  also includes a pair of Ethernet interfaces  1002 ,  1072 , a pair of TDM interfaces  1004 ,  1074 , a port-ID manager  1020 , and a mapping information extraction function  1040 . The mapping information extraction function  1040  passes bandwidth allocation information from the GTC deframing function  1050  to the GEM mapping function  1032 , the ATM mapping function  1033 , and the GTC framing function  1034 , to enable the upstream GTC frames to be transmitted at the proper timings. 
         [0024]      FIG. 5  shows the conceptual structure of a GEM frame. The GEM frame includes five bytes (40 bits) of overhead and a payload consisting of an arbitrary number of bytes. The overhead includes a 12-bit payload length indicator (PLI), a 12-bit port identifier (ID), a 3-bit PTI code, and a 13-bit header error control (HEC) section. The meaning of the PTI code is defined in ITU-T Recommendation G.984 as shown in  FIG. 6 . 
         [0025]    In ITU-T Recommendation G.984, a GEM frame accommodates Ethernet and TDM signals as described above. A single Ethernet or TDM signaling unit (e.g., an Ethernet packet) may be mapped onto a single GEM frame, or may be divided among a plurality of GEM frames. A single Ethernet or TDM signaling unit is mapped onto a single GEM frame in the example shown in  FIG. 7 , onto two GEM frames in the example shown in  FIG. 8 , and onto three GEM frames in the example shown in  FIG. 9 . In the overhead of the last GEM frame (or a single GEM frame), ‘001’ is set in the PTI field; in the overhead of the other GEM frames, ‘000’ is set in the PTI field. Furthermore, when a single Ethernet signal or TDM signal is mapped onto a plurality of GEM frames, the GEM frames may be mapped onto a plurality of GTC frames. The payload of a GTC frame is thereby used efficiently, and transmission efficiency can be increased by inserting urgent frames into spaces between fragments of non-urgent frames. In  FIG. 10 , for example, one fragment (FRAG) of a first GEM frame (GEM 1   a ) and an entire second GEM frame (GEM 2 ) are mapped onto a first GTC frame (GTC 1 ), and another fragment of the first GEM frame (GEM 1   b ) and an entire third GEM frame (GEM 3 ) are mapped onto a second GTC frame (GTC 2 ). In  FIG. 10 , upstream physical layer overhead (PLOu) is placed in an overhead section that includes the preamble of the GTC frame. 
         [0026]      FIG. 11  shows how Ethernet signals are mapped onto GEM frames. Ethernet signals are made up of packets. An Ethernet packet includes an inter-packet gap (IPG), which is a signal equivalent to the delay between the preceding packet transmission and the present packet transmission, a preamble and a start frame delimiter (SFD), which are signals for indicating the start of frame transmission, a destination address (DA), a source address (SA), length/type information indicating the length of the data field or the type of upper layer protocol, the data field, a frame check sequence (FCS) for detecting errors, and an end of frame (EOF) code that indicates the end of the Ethernet packet. The data field is labeled MAC client in the drawing because the data are processed in the MAC (media access control) client layer of the protocol stack. In GPON, the DA, SA, length/type, MAC client, and FCS fields of the Ethernet packet are mapped onto the payload of a GEM frame. 
         [0027]      FIG. 12  shows how TDM signals are mapped onto a GEM frame. The unaltered TDM signal is placed in the payload of the GEM frame. 
         [0028]    As described above, ITU-T Recommendation G.984 maps the unaltered ATM cells of an ATM signal onto an ATM partition of a GTC frame, and maps Ethernet and TDM signals onto a GEM partition (see  FIG. 2 ). A VPI and VCI are used for processing ATM signals, while a port identifier (ID) value and PTI code are used for processing Ethernet and TDM signals (see  FIG. 3 ). An OLT or ONU conforming to ITU-T Recommendation G.984 accordingly needs separate functions for processing ATM signals and GEM frames (Ethernet signals and TDM signals), increasing the cost of the OLT and ONU components of the GPON. 
         [0029]    ATM communication service is currently provided in fewer countries and territories than Ethernet and TDM communication service. Furthermore, since ATM communication service is not heavily used, when GPONs are constructed, many of them may not support ATM communication. Accordingly, there is a need for a method of processing ATM signals at a low cost. 
       SUMMARY OF THE INVENTION 
       [0030]    An object of the present invention is to provide a simple, low cost ATM-capable signal processing apparatus for use in GPON equipment such as an OLT or ONU. 
         [0031]    Another object of the invention is to provide a signal processing method and a GTC frame for use in the invented signal processing apparatus. 
         [0032]    The invented signal processing apparatus comprises an Ethernet-to-GEM conversion function, a TDM-to-GEM conversion function, a non-GEM-to-GEM conversion function, a GEM-to-Ethernet conversion function, a GEM-to-TDM conversion function, a GEM-to-non-GEM conversion function, a GTC input section, a GTC output section, and a mapping information management function. 
         [0033]    The Ethernet-to-GEM conversion function converts an input Ethernet signal to a GEM frame. The TDM-to-GEM conversion function converts an input TDM signal to a GEM frame. The non-GEM-to-GEM conversion function converts a non-GEM signal to a GEM frame. 
         [0034]    The GTC output section assigns GEM frames generated by the Ethernet-to-GEM conversion function, the TDM-to-GEM conversion function, and the non-GEM-to-GEM conversion function to output time slots, based on mapping information generated by the mapping information management function, adds overhead to the assigned GEM frames to create GTC frames, and outputs the GTC frames. 
         [0035]    The GTC input section extracts GEM frames from received GTC frames, determines which one of an Ethernet signal, a TDM signal, and a non-GEM signal is included in each GEM frame, and sends GEM frames including Ethernet signals to the GEM-to-Ethernet conversion function, GEM frames including TDM signals to the GEM-to-TDM conversion function, and GEM frames including non-GEM signals to the GEM-to-non-GEM conversion function. 
         [0036]    The GEM-to-Ethernet conversion function converts GEM frames to Ethernet signals. The GEM-to-TDM conversion function converts GEM frames to TDM signals. The GEM-to-non-GEM conversion function converts GEM frames to non-GEM signals. 
         [0037]    A non-GEM signal is a signal, such as an ATM signal, that is not placed in a GEM frame under the conventional GPON practice. A TDM signal with a different bandwidth from the TDM signals input to the TDM-to-GEM conversion function and output from the GEM-to-TDM conversion function may also be treated as a non-GEM signal. 
         [0038]    The mapping information management function generates mapping information from the overhead of the GTC frame input to the GTC input section and sends the mapping information to the GTC output section. 
         [0039]    In a preferred embodiment of the above signal processing apparatus, the GTC input section includes a GTC deframing function, a GEM extraction function, and a distribution function, and the GTC output section includes a bandwidth management buffer function, a GEM mapping function, and a GTC framing function. 
         [0040]    The GTC deframing function disassembles input GTC frame into overhead and a payload, and sends the overhead to the mapping information management function and the payload to the GEM extraction function. The GEM extraction function extracts a GEM frame from the payload and sends the GEM frame to the distribution function. 
         [0041]    The distribution function determines which one of an Ethernet signal, a TDM signal, and a non-GEM signal is included in the GEM frame, and sends the GEM frame to the GEM-to-Ethernet conversion function if it includes an Ethernet signal, to the GEM-to-TDM conversion function if it includes a TDM signal, and to the GEM-to-non-GEM conversion function if it includes a non-GEM signal. 
         [0042]    The bandwidth management buffer function stores GEM frames received from the Ethernet-to-GEM conversion function, the TDM-to-GEM conversion function, and the non-GEM-to-GEM conversion function temporarily in a buffer, awaiting output, and sends the GEM frames to the GEM mapping function responsive to commands from the GEM mapping function. 
         [0043]    The GEM mapping function assigns the GEM frames received from the bandwidth management buffer function to the output time slots of GTC frames according to the mapping information received from the mapping information management function. 
         [0044]    The GTC framing function generates a frame header, attaches the frame header as overhead to one or more GEM frames or fragments thereof to generate a GTC frame, and outputs the GTC frame in the time slot to which the GEM frame or frames in its payload were assigned. 
         [0045]    In a preferred embodiment, the GTC input section, GTC output section, mapping information management function, Ethernet-to-GEM conversion function, GEM-to-Ethernet conversion function, TDM-to-GEM conversion function, and GEM-to-TDM conversion function are implemented in a chip set, and the non-GEM-to-GEM conversion function and GEM-to-non-GEM conversion function are implemented outside the chip set. 
         [0046]    Alternatively, the GTC input section, GTC output section, and mapping information management function may be implemented in a chip set, and the Ethernet-to-GEM, GEM-to-Ethernet, TDM-to-GEM, GEM-to-TDM, non-GEM-to-GEM, and GEM-to-non-GEM conversion functions may be implemented outside the chip set. 
         [0047]    The invention provides a signal processing method for use in generating GTC frames. First, in a conversion step, an input Ethernet signal TDM signal, or non-GEM signal is converted to a GEM frame. Next, in a mapping step, the GEM frame is assigned to a time slot, or is divided into fragments which are assigned to different time slots. Next, in a GTC framing step, overhead is generated for the GEM frames and/or fragments assigned to each time slot, and these GEM frames and/or fragments are output together with the overhead as a GTC frame in the assigned time slot. 
         [0048]    The invention also provides a signal processing method for use in receiving GTC frames. First, in a GTC deframing step, a GTC frame is input and disassembled into overhead and a payload. A GEM frame is then extracted from the payload. Whether the GEM frame includes an Ethernet signal, a TDM signal, or a non-GEM signal is determined, and the GEM frame is converted to an Ethernet signal, a TDM signal, or a non-GEM signal, accordingly. 
         [0049]    The GTC frame provided by the present invention for use in a gigabit passive optical network comprises an overhead section accommodating information necessary for control, maintenance, and operation, and a payload accommodating user signals. The payload one or more GEM frames, or fragments thereof. A GEM frame may encapsulate an Ethernet signal, a TDM signal, or a non-GEM signal. 
         [0050]    In the novel signal processing apparatus and method and GTC frame, Ethernet, TDM, and ATM signals or other non-GEM signals are all converted to GEM frames, thereby providing a more unified form of signal processing than in conventional GPON systems, which convert only Ethernet and TDM signals to GEM frames and do not convert ATM signals to GEM frames. 
         [0051]    Consequently, the invented signal processing apparatus does not require a separate bandwidth management buffer function for ATM, a separate ATM mapping function, and a separate ATM extraction function, and has a correspondingly simpler circuit configuration. 
         [0052]    If the GTC input section, GTC output section, mapping information management function, Ethernet-to-GEM conversion function, GEM-to-Ethernet conversion function, TDM-to-GEM conversion function, and GEM-to-TDM conversion function are implemented in a chip set, and the non-GEM-to-GEM conversion function and GEM-to-non-GEM conversion function are implemented outside the chip set, then the chip set can be used in signal processing apparatus that does not support ATM service without burdening the apparatus with unnecessary ATM signal-processing circuitry. In this case, the non-GEM-to-GEM and GEM-to-non-GEM conversion functions can be used to process time-division multiplexed signals having a different bandwidth (bit rate) from the TDM signals processed by the TDM interfaces in the chip set. 
         [0053]    Alternatively, the chip set may include only the GTC input and output sections and the mapping information management function, forming a core that is independent of the types of communication service being supported. The low-cost core chip set can then be used in all GPON systems, and can be supplemented with only the necessary additional chips in each system in which it is used, where the additional chips provide the Ethernet-to-GEM conversion function, GEM-to-Ethernet conversion function, TDM-to-GEM conversion function, GEM-to-TDM conversion function, non-GEM-to-GEM conversion function, and GEM-to-non-GEM conversion function as necessary. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0054]    In the attached drawings: 
           [0055]      FIG. 1  is a schematic drawing of a PON; 
           [0056]      FIG. 2  is a conceptual diagram illustrating the configuration of a conventional GPON communication frame; 
           [0057]      FIG. 3  is a schematic drawing of a conventional GTC frame and the corresponding layer in the protocol stack; 
           [0058]      FIG. 4  is a schematic block diagram of an exemplary conventional GPON signal processing apparatus; 
           [0059]      FIG. 5  illustrates a GEM frame; 
           [0060]      FIG. 6  explains the PTI code in  FIG. 5 ; 
           [0061]      FIG. 7 to 10  are conceptual diagrams showing how GEM frames are mapped onto GTE frames; 
           [0062]      FIG. 11  is a conceptual diagram showing how Ethernet signals are mapped onto a GEM frame; 
           [0063]      FIG. 12  is a conceptual diagram showing how TDM signals are mapped onto a GEM frame; 
           [0064]      FIG. 13  is a conceptual block diagram illustrating the configuration of a novel ONU signal processing apparatus in a GPON system; 
           [0065]      FIG. 14  is a conceptual diagram showing how the novel apparatus encapsulates an ATM cell in a GEM frame; 
           [0066]      FIG. 15  is a conceptual diagram illustrating a novel configuration of the GTC frame layer configuration in the GPON protocol stack; 
           [0067]      FIG. 16  is a schematic diagram illustrating the configuration of a novel OLT signal processing apparatus in a GPON system; 
           [0068]      FIG. 17  is a schematic diagram illustrating the configuration of another novel signal processing apparatus; and 
           [0069]      FIG. 18  is a schematic diagram illustrating the configuration of another novel signal processing apparatus. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0070]    An embodiment of the invention will now be described with reference to the attached drawings, in which like elements are indicated by analogous reference characters. When the same function appears in different apparatus, reference characters with three numeric digits will be used, the last two numeric digits identifying the function, the first numeric digit identifying the apparatus. For example, the bandwidth management buffer function  130  in the signal processing apparatus  100  in  FIG. 13  performs the same operations as the bandwidth management buffer function  430  in the signal processing apparatus  400  in  FIG. 16 . 
         [0071]    The description will refer back to  FIG. 1 , using the notation ONU  704  to refer to a general one of the ONUs  704 - 1  to  704 - 2 , . . . and communication terminal  706  to refer to the one of the communication terminals  706 - 1  to  706 - n  to which the ONU  704  is connected. 
         [0072]      FIG. 13  shows the general structure of a novel signal processing apparatus for an ONU in a GPON. An ATM signal will be used as an example of a non-GEM signal which was not mapped onto a GEM frame in the conventional GTC frame described with reference to  FIG. 2 . 
         [0073]    The signal processing apparatus  100  in  FIG. 13  processes electrical signals. The ONU also includes a PON interface (PON IF, not shown) that converts the electrical signals to optical signals for transmission on the PON, and converts optical signals received from the PON to electrical signals. 
         [0074]    The signal processing apparatus  100  in  FIG. 13  comprises a core section  101   a  having a structure independent of the types of communication service supported and a service section  101   b  having a structure that depends on these types. The service section  101   b  includes an Ethernet-to-GEM conversion function  112 , a TDM-to-GEM conversion function  114 , an ATM-to-GEM conversion function  116 , a GEM-to-Ethernet conversion function  162 , a GEM-to-TDM conversion function  164 , a GEM-to-ATM conversion function  166 , and a port-ID manager  120 . The core section  101   a  includes a GTC output section  101   c  having a bandwidth management buffer function  130 , a GEM mapping function  132 , and a GTC framing function  134 , a GTC input section  101   d  having a GTC deframing function  150 , a GEM extraction function  152 , and a distribution function  154 , and a mapping information extraction function  140 . 
         [0075]    The signal processing apparatus  100  also comprises an Ethernet interface  102  for output of Ethernet signals, a TDM interface  104  for output of TDM signals, an ATM interface  106  for output of ATM signals, an Ethernet interface  172  for input of Ethernet signals, a TDM interface  174  for input of TDM signals, and an ATM interface  176  for input of ATM signals. 
         [0076]    The signal processing apparatus  100  may be used in any of the ONUs  704  in  FIG. 1 , and may receive Ethernet, TDM, and/or ATM signals from the corresponding subscriber&#39;s communication terminal  706 . 
         [0077]    Ethernet interface  102  converts a received Ethernet signal to a format internal to the ONU, and sends the converted Ethernet signal to the Ethernet-to-GEM conversion function  112 . The Ethernet-to-GEM conversion function  112  converts the converted Ethernet signal to a GEM frame. In this process, the Ethernet-to-GEM conversion function  112  receives the port identifier (ID) necessary for generation of the GEM frame from the port-ID manager  120 , and uses the port ID to generate the GEM frame. 
         [0078]    TDM interface  104  converts a received TDM signal to a format internal to the ONU, and sends the converted TDM signal to the TDM-to-GEM conversion function  114 . The TDM-to-GEM conversion function  114  converts the converted TDM signal to a GEM frame. In this process, the Ethernet-to-GEM conversion function  112  receives the port identifier (ID) necessary for generation of the GEM frame from the port-ID manager  120 , and uses the port ID to generate the GEM frame. 
         [0079]    The ATM interface  106  converts a received ATM signal to a format internal to the ONU, and sends the converted ATM signal to the ATM-to-GEM conversion function  116 . The ATM-to-GEM conversion function  116  converts the converted ATM signal to a GEM frame. In this process, the Ethernet-to-GEM conversion function  112  receives the port identifier (ID) necessary for generation of the GEM frame from the port-ID manager  120 , and uses the port ID to generate the GEM frame. 
         [0080]    In each case, the generated GEM frame is sent to the bandwidth management buffer function  130 , where it waits in a predetermined buffer being until to the OLT  701 . 
         [0081]    In signal processing apparatus according to the present invention, ATM signals as well as Ethernet and TDM signals are mapped onto GEM frames. An ATM signal may be mapped onto a GEM frame, that is, encapsulated in a GEM frame, by any preferred method.  FIG. 14  shows an example in which a single ATM cell is mapped onto a GEM frame by encapsulating the ATM cell without alteration in the payload of the GEM frame. A plurality of entire ATM cells may be encapsulated in this way the payload of a single GEM frame. 
         [0082]    The bandwidth management buffer function  130  sends each GEM frame awaiting output in the predetermined buffer to the GEM mapping function  132  responsive to a command from the GEM mapping function  132 . 
         [0083]    The GEM mapping function  132  sends the command to the bandwidth management buffer function  130  according to mapping information received from the mapping information extraction function  140 , which operates as the mapping information management function, and assigns the GEM frame to an appropriate output time slot for output in a GTC frame. The upstream transmission timings of GTC frames are determined according to the mapping information so as to multiplex the transmissions of different ONUs. The functions of the mapping information extraction function  140  will be described in more detail below. 
         [0084]    The GTC framing function  134  generates GTC frames. More specifically, the GTC framing function  134  maps each GEM frame assigned to an output time slot onto the payload of a GTC frame, generates a frame header for the GTC frame, and places the header in the overhead part of the frame. 
         [0085]    In upstream transmission, a GTC frame generated in the GTC framing function  134  is converted to an optical signal in the PON interface (not shown) of the ONU, and sent to the OLT. 
         [0086]    In downstream transmission, the ONU receives a GTC frame from the OLT. The GTC frame is converted from an optical signal to an electrical signal in the PON interface (not shown) and sent to the GTC deframing function  150 . 
         [0087]    The GTC deframing function  150  disassembles the GTC frame into overhead and a payload. The GTC deframing function  150  sends the payload of the GTC frame to the GEM extraction function  152 , and the overhead to the mapping information extraction function  140 . 
         [0088]    The mapping information extraction function  140  (the mapping information management function) generates GEM mapping information by extracting an upstream bandwidth map, added by the OLT  701  ( FIG. 1 ), from the overhead of the GTC frame. The GEM mapping information is sent to the GEM mapping function  132 , where it is used to determine the upstream transmission timing, so that upstream signals from different ONUs can be multiplexed in the splitter  702  without colliding. 
         [0089]    The GEM extraction function  152  extracts a GEM frame from the payload of the GTC frame. The extracted GEM frame is sent to the distribution function  154 . 
         [0090]    The distribution function  154  determines which one of an Ethernet signal, a TDM signal, and an ATM signal is included in the GEM frame, according to the port ID information received from the port-ID manager  120 . The distribution function  154  sends a GEM frame including an Ethernet signal to the GEM-to-Ethernet conversion function  162 , a GEM frame including a TDM signal to the GEM-to-TDM conversion function  164 , and a GEM frame including an ATM signal to the GEM-to-ATM conversion function  166 . 
         [0091]    Upon receiving a GEM frame, the GEM-to-Ethernet conversion function  162  converts the GEM frame to an Ethernet signal, and sends the Ethernet signal to the Ethernet interface  172 . The Ethernet interface  172  converts the Ethernet signal, which is formatted in the internal ONU format, to an appropriate Ethernet signal format, and outputs the converted Ethernet signal to the subscriber&#39;s communication terminal  706 . 
         [0092]    Similarly, upon receiving a GEM frame, the GEM-to-TDM conversion function  164  converts the GEM frame to a TDM signal, and sends the TDM signal to the TDM interface  174 . The TDM interface  174  converts the TDM signal, which is in the internal ONU format, to an appropriate TDM signal format, and outputs the converted TDM signal to the communication terminal  706 . 
         [0093]    The GEM-to-ATM conversion function  166 , when it receives a GEM frame, converts the received GEM frame to an ATM signal, and sends the ATM signal to the ATM interface  176 . The ATM interface  176  converts the ATM signal, which is also in the internal ONU format, to an appropriate ATM signal format, and outputs the converted ATM signal to the communication terminal  706 . 
         [0094]      FIG. 15  shows the conceptual structure of the novel GTC frame and the novel GTC frame layer in the protocol stack. 
         [0095]    The novel GTC frame includes an overhead section (not shown), which includes information necessary for communication control, maintenance, and operation, and a payload section, which accommodates user signals. A detailed description of the overhead section of the novel GTC frame will be omitted, since it is the same as in the conventional GTC frame described with reference to  FIG. 2 . 
         [0096]    The payload section includes only a GEM partition, which accommodates one or more GEM frames or fragments thereof. Each GEM frame includes only one type of signal: an Ethernet signal, a TDM signal, or a non-GEM signal. 
         [0097]    The difference between the GTC frame used in the present invention and the conventional GTC frame is that the payload section is not divided into an ATM partition and a GEM partition. The entire payload section is treated as a GEM partition; there is no ATM partition. The ATM cells that were mapped onto the ATM partition in a conventional GTC frame are mapped onto the GEM frame partition in the novel GTC frame. More precisely, ATM signals, like TDM and Ethernet signals, are encapsulated in GEM frames, which are mapped onto the GEM partition of the GTC frame (see  FIG. 15 ). 
         [0098]    In the downstream direction, the novel GTC frame has the conventional physical control block, specifying the start and end of each ONU&#39;s bandwidth allocation. The overhead section of the frame complies with ITU-T Recommendation G.984, so the frame can be transported on a GPON complying with ITU-T Recommendation G.984. 
         [0099]    Downstream GTC frames are received by a GTC framing sublayer  310  in the ONU, and GEM frames read from the payloads of according to the ONU&#39;s bandwidth allocation, which is identified in the frame overhead. The GTC deframing function  150  and GEM extraction function  152  in  FIG. 13  correspond to the multiplexer  311  and GEM partition  313 , respectively, in the GTC framing sublayer  310  in  FIG. 15 . 
         [0100]    When the GTC framing sublayer  310  reads a GEM frame, it is received by a GEM TC adapter  321  in the TC adaptation sublayer  320 , and a port-ID and PTI filter  323  identifies its logical path from the port ID value and PTI code. If the GEM frame includes an Ethernet or TDM signal and is destined to a GEM client, the port-ID and PTI filter  323  sends the frame signal to the GEM client. 
         [0101]    When the GEM frame includes an ATM signal, the port-ID and PTI filter  323  sends the signal to a VPI/VCI filter  325 . The VPI/VCI filter  325  identifies the logical path of the signal from the VPI and VCI in the ATM header information encapsulated in the frame, and sends the signal to an ATM client. 
         [0102]    The distribution function  154 , GEM-to-Ethernet conversion function  162 , GEM-to-TDM conversion function  164 , and GEM-to-ATM conversion function  166  in  FIG. 13  correspond to the port-ID and PTI filter  323  in the TC adaptation sublayer  320  in  FIG. 15 . The GEM-to-ATM conversion function  166  corresponds to the VPI/VCI filter  325 . 
         [0103]    Although no block in  FIG. 13  corresponds directly to the GEM TC adapter  321  in  FIG. 15 , the adapter function is carried out when GEM frames are passed from the GEM extraction function  152  to the distribution function  154 . Other processing is also performed, such as identifying the logical path of an Ethernet signal from its medium access control (MAC) address, for example, but a description will be omitted as this processing is well known. 
         [0104]    In upstream transmission, the GTC framing function  134 , GEM mapping function  132 , and bandwidth management buffer function  130  in  FIG. 13  correspond to the multiplexer  311 , GEM partition  313 , and allocation ID filter  315 , respectively, in the GTC framing sublayer  310  in  FIG. 15 . The Ethernet-to-GEM conversion function  112 , TDM-to-GEM conversion function  114 , and ATM-to-GEM conversion function  116  correspond to the port-ID and PTI filter  323  in the TC adaptation sublayer  320 . 
         [0105]    Although there is no block in  FIG. 13  corresponding directly to the VPI/VCI filter  325  in  FIG. 15 , the corresponding filter function is carried out when ATM signals are passed from the ATM interface  106  to the ATM-to-GEM conversion function  116 . Similarly, although no block in  FIG. 13  corresponds directly to the GEM TC adapter  321  in  FIG. 15 , the adapter function is carried out when GEM frames are passed from the GEM extraction function  152  to the distribution function  154 . 
         [0106]      FIG. 16  shows the general structure of a novel signal processing apparatus for use in an OLT in a GPON. The OLT also includes a PON interface (PON IF, not shown) for conversion between electrical and optical signals. 
         [0107]    The OLT receives Ethernet, TDM, and ATM signals from, for example, an IP based network. 
         [0108]    The OLT comprises an Ethernet interface  402 , a TDM interface  404 , and an ATM interface  406 . The Ethernet interface  402  converts a received Ethernet signal to a format internal to the OLT, and sends the converted Ethernet signal to an Ethernet-to-GEM conversion function  412 . The TDM interface  404  converts a received TDM signal to the internal OLT format, and sends the converted TDM signal to a TDM-to-GEM conversion function  414 . The ATM interface  406  converts a received ATM signal to the internal OLT format, and sends the converted ATM signal to an ATM-to-GEM conversion function  416 . 
         [0109]    The Ethernet-to-GEM conversion function  412 , TDM-to-GEM conversion function  414 , ATM-to-GEM conversion function  416 , a port-ID manager  420 , bandwidth management buffer function  430 , GEM mapping function  432 , and GTC framing function  434  cooperate to generate GTC frames from the Ethernet, TDM, and ATM signals in the same way as the corresponding ONU elements in  FIG. 13 . A detailed description will be omitted. 
         [0110]    In downstream transmission, GTC frames generated in the GTC framing function  434  are output from the PON interface in the OLT to the ONUs. 
         [0111]    In upstream transmission, the OLT receives GTC frames as optical signals from the connected ONUs. The PON interface in the OLT converts the GTC frames to electrical signals and sends them to the GTC deframing function  450 . 
         [0112]    The GTC deframing function  450 , GEM extraction function  452 , distribution function  454 , GEM-to-Ethernet conversion function  462 , GEM-to-TDM conversion function  464 , GEM-to-ATM conversion function  466 , Ethernet interface  472 , TDM interface  474 , and ATM interface  476  in the OLT in  FIG. 16  generate Ethernet, TDM, and ATM signals from GTC frames in the same way as the corresponding elements in the ONU in  FIG. 13 . A detailed description will be omitted. 
         [0113]    A mapping information generation function  441 , which controls mapping and multiplexing, generates upstream GEM mapping information by calculating bandwidth allocations from the bandwidth control information in the overhead of the GTC frames received from the ONUs. The mapping information generation function  441  sends the generated GEM mapping information to the GEM mapping function  432 , thereby operating as the mapping information management function. 
         [0114]    The ONU and OLT signal processing apparatus described above converts Ethernet signals, TDM signals, and ATM signals to GEM frames for use in GPON systems, thereby providing a more unified form of signal processing than in conventional GPON systems. As the signal processing apparatus does not require a separate bandwidth management buffer function for ATM, a separate ATM mapping function, and a separate ATM extraction function, it has a simpler configuration than the conventional apparatus in  FIG. 4 . 
         [0115]    The novel signal processing apparatus may be implemented as a chip set, that is, a set of two or more monolithic integrated circuits designed to operate together. Such a chip set may include all of the functional elements shown in  FIG. 13  or  16 , but this is not necessary. 
         [0116]    Referring to  FIG. 17 , for example, the chip set  580  may include a core section  501   a  identical to the core section  101   a  in  FIG. 13 , and a service section  501   b  that includes a pair of GEM interfaces  508 ,  578  instead of the ATM interfaces  106 ,  176 , ATM-to-GEM conversion function  116 , and the GEM-to-ATM conversion function  166  of the service section  101   b  in  FIG. 13 . This chip set  580  may be used either in an ONU that supports ATM communication or an ONU that does not support ATM communication. 
         [0117]    The signal processing apparatus  500  in  FIG. 17  is used in an ONU that supports ATM communication, so it includes an ATM-to-GEM conversion function  516 , a GEM-to-ATM conversion function  566 , and a pair of ATM interfaces  506 ,  576  in addition to the chip set  580 . The ATM-to-GEM conversion function  516  converts ATM signals to GEM frames and inputs them to GEM interface  508  in the chip set  580 . This GEM interface  508  sends the GEM frames directly to the bandwidth management buffer function  530  in the GTC output section  501   c . Similarly, GEM frames including ATM cells are routed from the distribution function  554  in the GTC input section  501   d  through GEM interface  578  to the GEM-to-ATM conversion function  566 , which converts them to ATM signals and sends the ATM signals to ATM interface  576 . 
         [0118]    If the signal processing apparatus  500  does not need to support ATM communication, then the GEM interfaces  508 ,  578  can be used for other purposes. For example, the ATM-to-GEM conversion function and GTM-to-ATM conversion function can be replaced with a TDM-to-GEM conversion function and a GEM-to-TDM conversion function. The TDM interfaces  504 ,  574 , the TDM-to-GEM conversion function  514 , and the GEM-to-TDM conversion function  564  in the chip set  580  can then be used to process TDM signals having one bandwidth, and the GEM interfaces, the external TDM-to-GEM conversion function, and the external GEM-to-TDM conversion function can be used to process TDM signals having a different bandwidth. 
         [0119]    Alternatively, if only Ethernet signals and one type of TDM signals need to be processed, the GEM interfaces  508 ,  578  in the chip set  580  can be left unused. 
         [0120]    The invention can also be practiced as shown in  FIG. 18 , by implementing the core section  601   a  in the chip set  680 , and implementing the entire service section  601   b  outside the chip set.  FIG. 18  shows an example in which the chip set  680  is used in an ONU that supports ATM communication, so the service section  601   b  has the same structure as the service section  101   b  in  FIG. 13 . One advantage of this chip set  680  is that if the ONU only needs to process one type of signal, e.g., Ethernet signals, then only one pair of interfaces and conversion units, e.g. the Ethernet interfaces  602 ,  672 , the Ethernet-to-GEM conversion function  612 , and the GEM-to-Ethernet conversion function  662 , have to be implemented in the service section  601   b . Another advantage is that by providing the appropriate interface and conversion circuits, the chip set  680  can be used in a communication system that does not carry Ethernet, TDM, or ATM signals but carries some other type of signal instead, without the incurring the cost of unnecessary Ethernet, TDM, and ATM signal processing circuitry. 
         [0121]    The present invention can accordingly provide a low-cost core chip set can then be used in all GPON systems, and can be supplemented with only the necessary additional chips in each system in which it is used. 
         [0122]    In addition to these variations of the embodiment shown in  FIGS. 13 to 16 , those skilled in the art will recognize that further variations are possible within the scope of the invention, which is defined in the appended claims.