Abstract:
A method and apparatus for receiving burst data in an OLT of an EPON are provided. The method includes the steps of: a) receiving burst data from the plurality of ONUs; b) generating a multipoint control protocol (MPCP) LOS signal for reducing a synch time of the burst data; c) reducing the synch time in response to generation of the MPCP LOS signal; and d) recovering the received burst data by performing a code-group sort in a period where the MPCP LOS signal is not generated.

Description:
RELATED APPLICATION  
       [0001]     The present application is based on, and claims priorities from, Korean Application Number 2005-120234, filed Dec. 8, 2005 and Korean Application Number 2006-62705, filed Jul. 4, 2006, the disclosure of which is hereby incorporated by reference herein in its entirety.  
       BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates to a method and apparatus for receiving and processing burst data in an optical line terminal (OLT) of an Ethernet passive optical access network, and more particularly, to a method and apparatus for receiving and processing burst data for improving a burst mode receiving performance in an optical line terminal (OLT).  
         [0004]     2. Description of the Related Art  
         [0005]     As the number of Internet users using wireless communication has abruptly increased, high-speed Internet technologies have been evolved to provide faster Internet services to users. A wireless local area network (LAN) is one of representative technologies for providing high speed Interne services to users. The wireless LAN has shortcomings of a large gap between a wide area network and an end user and a bottleneck problem of a bandwidth at the end users. In order to overcome the bottleneck program of the bandwidth, a passive optical network (PON) was introduced recently. Such a PON technology may be classified into an ATM PON (APON) and an Ethernet PON (EPON). The EPON technology is a control chip technology that allows users to access a Fiber to the Home (FTTH) network, which is capable of providing various communication services including Internet, Internet TV, digital TV and telephone with affordable cost, through one optical fiber.  
         [0006]      FIG. 1  is a diagram illustrating a structure of Ethernet passive optical network (EPON) according the related art. The EPON is a system that transfers a signal to an end user through an optical cable network. The EPON is classified into FTTC, FTTB and FTTH according to a location of termination.  
         [0007]     Referring-to  FIG. 1 , the EPON includes an optical line terminal (OLT)  10  cooperated with a communication service provider side such as an IP network, a broadcasting network, or a TDM network, and an optical network unit (PNU)  20  connected to a user terminal  30  placed at a user end of an optical access network, such as STB or PC. The EPON uses an optical fiber and generally has a point-to-multipoint tree structure. An optical star coupler/splitter  40  may split an optical signal transferred to an Ethernet passive optical network system according to whether the light thereof is upstream or downstream. The split signals are transferred through a plurality of optical fibers, or the split signals are coupled and transferred through one optical fiber. As an example, a method of transmitting data in EPON will be described with reference to  FIG. 2 .  
         [0008]     The EPON transmits data using an Ethernet frame as a transmission unit. In an upstream path, each of ONUs  20  is allocated to a time slot dynamically or fixedly, and transmits data to a common OLT  10 . An optical star coupler  40  multiplexes the data outputted from each of the ONUs  20  and transmits them to the OLT  10 . ON the contrary, in the downstream path, when the OLT  10  transmits a downstream, the optical star coupler  40  inverse multiplexes the downstream and transmits them to each of the ONUs  20 .  
         [0009]     The OLT  10  recovers burst data transmitted in a burst mode from an ONU  20  through an upstream. Since the EPON is a technology for high speed Internet service, the Internet speed would be significantly influenced if a time of restoring burst data delays. Such a delay also wastes the bandwidth of the upstream channel. The delay is mainly caused by a time required for restoring transmitting data from an ONU at an OLT, and a laser on/off time of an ONU. A sum of such times except the time of transmitting data is a guard time.  
         [0010]     Therefore, it is required to reduce a clock and a data restoring time when the OLT  10  receives burst data. However, a conventional EPON only considers the allocation of synch times.  
         [0011]     Generally, a receiver of an OLT in an EPON system uses an Ethernet SerDes chip. According to the type of the Ethernet SerDes chip and a method of using the same, a CDR time of about 200 nano seconds to about several nano seconds is required. The CDR time becomes a synch time by being added to an OLT optical receiving level recovery time. If the CDR time is significantly lengthened, the synch time becomes lengthened, accordingly. Also, if the synch time becomes lengthened, a upstream band would be wasted as much as the lengthened synch time, and if the number of branches increases, the overhead becomes also bigger as much as the increased number of the branches.  
       SUMMARY OF THE INVENTION  
       [0012]     Accordingly, the present invention is directed to a method and apparatus for receiving and processing burst data in an OLT that substantially obviates one or more problems due to limitations and disadvantages of the related art.  
         [0013]     An object of the present invention is to provide an apparatus and method for receiving and processing burst data for improving upstream band efficiency by shortening a receiving recovery time (synch time) at an optical line terminal of an Ethernet passive optical network (EPON).  
         [0014]     Another object of the present invention is to provide an apparatus and method for processing burst data for shortening a receiving recovery time (synch time) by disposing a SerDes chip where a LOS signal generated from a MPCP at an OLT is applied.  
         [0015]     Additional advantages, objects, and features of the invention will be set forth in part in the description which follows and in part will become apparent to those having ordinary skill in the art upon examination of the following or may be learned from practice of the invention. The objectives and other advantages of the invention may be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings.  
         [0016]     To achieve these objects and other advantages and in accordance with the purpose of the invention, as embodied and broadly described herein, a method and apparatus for receiving and processing burst data in an optical line terminal (OLT) of an Ethernet passive optical access network.  
         [0017]     According to an aspect of the present invention, there is provided a method for processing burst data received from a plurality of optical network units (ONU) at a optical line terminal (OLT) in an Ethernet passive optical access network (EPON) formed of the OLT and the plurality of ONUs, including: receiving burst data from the plurality of ONUs; generating a multipoint control protocol (MPCP) LOS signal for reducing a synch time of the burst data; reducing the synch time in response to generation of the MPCP LOS signal; and recovering the received burst data by performing a code-group sort in a period where the MPCP LOS signal is not generated.  
         [0018]     According to another aspect of the present invention, there is provided an apparatus for processing a burst data in an Ethernet passive optical network, including: an optical module for receiving burst data of different optical power levels from a plurality of optical network units (ONU) and converting an optical signal of the burst data to an electric signal; an access controller for generating a multipoint control protocol (MPCP) LOS signal for reducing a synch time of the burst data; and a SerDes unit for reducing a synch time when the MPCP LO signal is generated, and recovering the received burst data by performing a code-group sort in a period where the MPCP LOS signal is not generated.  
         [0019]     It is to be understood that both the foregoing general description and the following detailed description of the present invention are exemplary and explanatory and are intended to provide further explanation of the invention as claimed. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0020]     The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of this application, illustrate embodiment(s) of the invention and together with the description serve to explain the principle of the invention. In the drawings:  
         [0021]      FIG. 1  is a block diagram illustrating an Ethernet passive optical network according to the related art;  
         [0022]      FIG. 2  is a block diagram illustrating a method for transmitting data in an Ethernet passive optical network according to the related art;  
         [0023]      FIG. 3  is a block diagram illustrating a conventional optical line terminal (OLT) shown in  FIG. 1 ;  
         [0024]      FIG. 4  is a block diagram illustrating an OLT according to an embodiment of the present invention;  
         [0025]      FIG. 5  is a timing diagram illustrating signals generated from an optical line terminal according to an embodiment of the present invention; and  
         [0026]      FIG. 6  is a flowchart illustrating a discovery procedure for managing a synch time in an Ethernet passive optical access network according to an embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0027]     Reference will now be made in detail to the preferred embodiments of the present invention, examples of which are illustrated in the accompanying drawings.  
         [0028]     An apparatus and method for receiving burst data according to an embodiment of the present invention will be described with an Ethernet passive optical network (EPON) applied. As shown in  FIG. 2 , an optical line terminal (OLT) receiving burst data transmitted from optical network units (ONU) through an upstream channel will be described. Operations of an OLT for reducing a receipt and recovery time by disposing a SerDes chip at an OLT will be described. The time for recovering stable data from a burst optical signal at an OLT is a level recovery time of an optical module and a clock and data recovery (CDR) time in a SerDes chip. The sum of them is a synch time. Since the CDR time is about several hundreds nano-seconds to about several nano-seconds in generally, the CDR time must be reduced to improve the upstream band efficiency.  
         [0029]     At first, the upstream and downstream data transmission of an EPON system will be briefly described again for convenience to describe the embodiment of the present invention.  
         [0030]     Referring to  FIG. 2 , the EPON system has a point to multipoint structure formed of an OLT  10 , and a plurality of ONUs  20  through a passive splitter  30 .  
         [0031]     The OLT  10  broadcasts downstream-frames transmitted to the plurality of ONUs  20  as a variable length packet of IEEE 802.3 format. Herein, each frame can be processed by a predetermined ONU  20  or all of ONUs  20 , and the processed frames are transferred to the user of ONU  20 .  
         [0032]     The plurality of ONUs  20  transmit upstream-frames only for a predetermined time assigned by the OLT  10  not to make collision with that transmitted from adjacent ONUs  20  based on a time division multiple access (TDMA) scheme. Therefore, the plurality of ONUs  20  transmit data in a burst mode. Accordingly, the OLT  10  should have a capability to receive the burst data transmitted from the plurality of ONUs  20 .  
         [0033]     To transmit the upstream-data, a laser on/off time in an ONU  20 , and a level recovery time, a CDR time, and a code group sorting time of an OLT  10  are also required beside the time of transmitting real data. In order to effective use a bandwidth, that is, to improve the usability of the upstream transmission band, these times are required to be reduced.  
         [0034]     Hereinafter, an apparatus and method for receiving burst data at an OLT in consideration of these times according to an embodiment of the present invention will be described with reference to accompanying drawings. For convenience, an apparatus for receiving burst data according to the related standard will be described at first, and then an apparatus for receiving burst data according to an embodiment of the present invention will be described.  
         [0035]      FIG. 3  is a block diagram illustrating a conventional optical line terminal (OLT) shown in  FIG. 1 .  
         [0036]     Referring to  FIG. 3 , the conventional OLT  10  includes a receiver (OLT OTRx)  11  which is an optical module, a SerDes unit (PMA)  12 , and an access controller (OLT EPON MAC)  13 , which are embodied as a chip.  
         [0037]     The receiver  11  generates a LOS signal to the SerDes unit  11  when receiving burst data from each of the ONUs  20 . Herein, a gap is present between burst signals when receiving the burst data, thereby generating a period without an optical signal.  
         [0038]     Generally, the SerDes unit  12  tries to lock a PLL for abnormal signal even in the no-optical signal period. Therefore, a clock may seriously change. Since a time of locking the PLL becomes significantly lengthened in receiving a next ONU burst data after releasing the locked PLL, the CDR time becomes lengthened as much as the lengthened time of locking the PLL. Therefore, the SerDes unit  12  shortens theses times by receiving the LOS signal from the optical module. That is, when the SerDes unit  12  receives the LOS signal from the optical module, the SerDes unit  12  locks a TBC, which is a transmission clock, in the LOS period in order to generate a stable receipt clock instead of locking a receiving signal from the receiver  11 . Afterward, the SerDes unit  12  has a fast CDR lock characteristic.  
         [0039]     The SerDes unit  12  receives the LOS signal from the optical module receiver  11  through a LCK_REF pin or a SYNC_EN pin. When the LCK_REF pin is in an inactive state as an active low signal, a receiving PLL is normally driven, thereby locking serial data from the receiver  11 . When the LCK_REF pin is in an active state, a transmitting clock (TBC) from the access controller  13  is locked. On the contrary, if the SYNC_EN(EN_CDET) pin receives an Enable Comma Detect signal that sorts code-group in an no-LOS signal period.  
         [0040]     As described above, the conventional OLT is not suitable for receiving burst data in an EPON because the LOS signal of the optical module does not have a fast operating characteristic as like the real optical signal and have a slow response characteristics.  
         [0041]     In a MPCP of an OLT, a burst data receiving time can be obtained from an ONU. Therefore, in the present embodiment, a LOS signal is generated to be identical to the real receiving optical signal using the obtained burst data receiving time. In consideration of the laser on/off time of an OLT optical module and a level recovery time of an OLT optical module, the MPCP LOS signal must be generated to extend a delay or a LOS period. When a reset signal is required at an EPON OLT optical module, an OLT MPCP reset signal is generated and transmitted to an OLT optical module, thereby reducing the level recovery time.  
         [0042]      FIG. 4  is a block diagram illustrating an OLT according to an embodiment of the present invention.  
         [0043]     The OLT  100  according to the present embodiment is described with reference numerals different from the conventional OLT  10  in order to clearly distinguish the OLT according to the present embodiment from the conventional OLT  10 .  
         [0044]     Referring to  FIG. 4 , the OLT  100  of EPON includes a receiver (OLT OTRx)  110  that is an optical module, a serialization and de-serialization (SerDes)  120  and an access controller (OLT EPON MAC)  130 , which are formed as a chip.  
         [0045]     The EPON provides a TDMA scheme for an upstream channel. A traffic collision may be arisen because a plurality of ONUs  20  may transmit Ethernet frames at the same time when an ONU  20  transmit Ethernet frames to the OLT  10 . In order to avoid the traffic collision, the EPON provides a scheme of sharing the upstream channel through a time division multiple access (TDMA) using a MPCP. Therefore, the OLT  100  of the EPON applies the OLT MPCP to the access controller  130 .  
         [0046]     The optical module receiver  110  receives burst data of different optical power levels from each of the ONUs  20 , recovers the level and converts the optical signal to an electric signal. When the optical module receiver  110  receives the burst data, the optical module receiver  110  generates a receiving bit (rx_bit) signal to the SerDes unit  120 , and receives a transmitting bit (tx_bit) signal from the SerDes unit  120 .  
         [0047]     The SerDes unit  120  is a chip of a PMA layer, and recovers data and a clock. Also, the SerDes unit  120  performs a code-group sort. The SerDes unit  120  receives a receiving bit (rx_bit) signal from the receiver  110 , and transmits a transmitting bit (tx_bit) signal to the receiver  110  in response to a transmitting clock (tx_clk, TBC) generated from the access controller  130 . The SerDes unit  120  receives a MPCP LOS signal from the access controller  130 . The MPCP LOS signal inputs to a LOCK_REF pin or a SYNC_EN pin as like the LOS signal of the optical module in  FIG. 3 .  
         [0048]     When the MPCP LOS signal is generated, the SerDes unit  120  inputs a signal to the LCK_REF pin to drive a receiving PLL normally in case of the inactive state, thereby locking serial data from the receiver  11 , and to lock a transmitting-clock (TBC) from the access controller  13  in case of the active state.  
         [0049]     On the contrary, if the MPCP LOS signal is not generated, the SerDes unit  120  inputs a signal to the SYNC_EN(EN_CDET) to perform a code-group sort in an no-LOS signal period.  
         [0050]     The access controller  130  generates the MPCP LOS signal to shorten the CDR lock time and transmits the MPCP LOS signal to the SerDes unit  120 .  
         [0051]     The access controller  130  uses an OLT MPCP that is a protocol for control in a media access control (MAC) layer. Using the OLT MPCP, not only a gap between burst data but also a starting point of the next burst can be obtained when receiving the burst data from each of the ONUs  20 . It is because the OLT MPCP sets the ONUs  20  to transmit burst data to an OLT only at a time assigned by the OLT MPCP. Therefore, the access controller  130  can generate the OLT MPCP LOS signal suing the OLT MPCP.  
         [0052]     The access controller  130  generates a MPCP RESET Signal when the optical module receiver  110  requires a reset signal, and transmits the MPCP RESET signal to the receiver  110 . Accordingly, the receiver  110  can uses the MPCP RESET signal to reduce the time of recovering a burst level.  
         [0053]     As described above, the CDR lock time can be reduced and the receiving clock is prevented from being seriously tilted by generating the MPCP LOS signal at the OLT  100  instead of the LOS signal of the optical module. Therefore, the burst data receiving performance of the OLT can be improved. Such a performance improvement is clearly shown in a timing diagram of  FIG. 5 .  
         [0054]     As shown in  FIG. 5 , a receiving clock (OLT Optical Rx) signal is generated by receiving burst data at the optical module receiver  110 . Herein, the conventional OLT  10  generates an optical module LOS (OTRx LOS) signal. However, a delay is made by a gap generated after generating a first clock (ONU i) of the receiving clock signal. Therefore, the conventional OLT  10  generates the LOS signal after the second clock (ONU i+1) is generated, which was generated after the delay.  
         [0055]     On the contrary, as shown in  FIG. 5 , the access controller  130  according to the present embodiment generates the MPCP LOS signal at a time of starting the gap after the first clock (ONU i) of the receiving clock signal is generated. The OLT  100  can extend a delay or a LOS period in consideration of the laser on/off time of the ONT optical module and the level recovery time of the OLT optical module by including an extension in the MPCP LOS signal. Also, the MPCP RESET signal is generated at the same time of generating the MPCP LOS signal.  
         [0056]     According to the standard of EPON, the sync time information of the OLT  10  exchanges with ONUs when a new ONT  20  is registered, and the synch time is used with same value after the synch time is set in a registration procedure, which is a discovery procedure. Such a synch time can be reduced by applying the above described scheme to the OLT  100 . However, the new ONT  20  may not be appropriately registered because of errors generated when receiving burst data from the ONU  20 , which are caused by the lack of the synch time.  
         [0057]     As described above, an accurate burst data transmission starting time must be obtained from ONUs to apply a MPCP LOS signal or a MPCP RESET signal according to the present embodiment. It is possible if the ONUs are previously registered. However, the synch time cannot be set shortly by applying the MPCP LOS signal or the MPCP RESET signal when a new ONU requests to be registered. Therefore, in the present embodiment, the synch time is set differently when an OLT and ONUs are registered in an EPON system.  
         [0058]     Hereinafter, a registration procedure according to an embodiment of the present invention, which manages a synch time shortly by applying the MPCP LOS signal or the MPCP RESET Signal when the new ONU is registered, will be described with accompanying drawings.  
         [0059]      FIG. 6  is a flowchart illustrating a recovery procedure for managing a synch time in an Ethernet passive optical access network according to an embodiment of the present invention.  
         [0060]     Referring to  FIG. 6 , at step S 210 , an OLT  100  transmits a GATE message to an ONU  20 . The GATE message includes information about DA, SA, Grant, Sync_Time 1 . Then, the ONU  20  generates a random delay time from a Grant starting point, and responses to the GATE message after the random delay time. Accordingly, the access controller  130  of the OLT  100  cannot be aware of a time of receiving burst data from the ONU  20 . Therefore, the OLT  100  sets the first synch time (Sync_Time 1 ) included in the GATE message to be different from the second synch time (Sync_Time 2 ) included in a register message. That is, at the beginning, the first synch time is set to be longer for the registering procedure, and then the second synch time is set to shorter for transmitting data after completing the registration.  
         [0061]     After the random delay time, the OLT  100  receives a register request message (REGISTER_REQ) from the ONU  20  at step S 220 . The register request message includes DA information, SA information, Grant information, and Pending grants information.  
         [0062]     At step S 230 , the OLT  100  transmits the REGISTER message to the ONU  20 . The REGISTER message includes DA information, LLID information, Sync_Time  2  information, and echo of pending grants information. Then, the ONU  20  sets and stores information included in the received REGISTER message.  
         [0063]     At step S 240 , the OLT  100  transmits a GATE message to the ONU  20 . The GATE message includes DA information, SA information, LLID information and Grant information. Then, the OLT  20  a REGISTER_ACK message as a response for the GATE message. At step S 250 , the OLT  100  receives the register response message transmitted from the ONU  20 . The register response message includes DA information, SA information, echo of LLID information, and echo of Sync_Time information.  
         [0064]     As described above, when a new OLU requests the registration, the chip controller cannot be accurately aware of a time of starting burst data transmission using the MPCP at the registration initial period because the new OLU transmits burst data within a time set by the chip controller (OLT MPCP). Therefore, a synch time for receiving burst data must be set longer. In the present embodiment, the first synch time for registration procedure is set to be sufficiently long, and then the synch time is reset to be shorter after it can obtain the time for starting the burst data transmission from the ONU. That is, the registering procedure is performed by setting the first synch time differently from the second synch time.  
         [0065]     As described above, in the present invention, the access controller of the OLT generates the MPCP LOS signal and the MPCP RESET signal using the OLT MPCP instead of the LOS signal of an optical module, which has a slow response characteristic. Since the OLT MPCP is identical to the real receiving optical signal, the burst data receiving time can be obtained from the ONU. By applying the MPCP LOS signal and the MPCP RESET signal into the SerDes chip, the receiving recovery time can be shortened through the fast operating characteristic as like the real optical signal.  
         [0066]     Also, the synch time information exchanges between the OLT and the ONU by setting the firstly synch time for the registration procedure and the second synch time for burst data transmission, differently in the present invention. Therefore, the synch time can be reduced, and, at the same time, the lack of the synch time problem generated from the OLT using the MPCP LOS can be solved.  
         [0067]     As described above, in the present invention, the synch time, that is, the receiving recovery time, can be reduced through the fast operating characteristics as like the optical signal by generating the MPCP LOS signal and the MPCP RESET signal using the OLT MPCP at the OLT and applying them into the SerDes chip. Therefore, the upstream band efficiency and the burst data receiving performance can be improved.  
         [0068]     It will be apparent to those skilled in the art that various modifications and variations can be made in the present invention. Thus, it is intended that the present invention covers the modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents.