Abstract:
An apparatus automatically maintains bi-directional communication between an optical network unit (ONU) and a central office (CO) in a passive optical network (PON) when the CO changes from a first PON mode to a second PON mode. The apparatus senses a PON mode relating to downstream data flow from the CO to the ONU; and upon detecting a change in the PON mode from the first PON mode to the second PON mode, synchronizes the ONU to operate according to the downstream and upstream configurations of the second PON mode.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of U.S. Provisional Patent Application No. 60/687,442 filed on Jun. 6, 2005, the contents of which are wholly incorporated herein by reference. 
    
    
     REFERENCES 
     Patents and Published Applications 
     
       
         
               
               
               
             
           
               
                   
               
             
             
               
                 6,229,788 
                 May 2001 
                 Graves, et al. 
               
               
                 6,385,366 
                 May 2002 
                 Lin 
               
               
                 20030058505 
                 March 2003 
                 Arol; et al. 
               
               
                 20040202470 
                 October 2004 
                 Se-Youn; et al. 
               
               
                 20040208631 
                 October 2004 
                 Jae-Yeon; et al. 
               
               
                 20040218534 
                 November 2004 
                 Jae-Yeon; et al. 
               
               
                 20040264961 
                 December 2004 
                 Hong Soon; et al. 
               
               
                   
               
             
          
         
       
     
     Other References 
     
         
         1) IEEE 802.3ah 
         2) ITU-T G.984 
         3) ITU G.983 
       
    
     FIELD OF THE INVENTION 
     The present invention relates generally to passive optical networks (PONs), and more particularly to a method for automatically upgrading PONs. 
     BACKGROUND OF THE INVENTION 
     As the demand of users for bandwidth is rapidly increasing, optical transmission systems, where subscriber traffic is transmitted using optical networks, is installed to serve this demand. These networks are typically referred to as fiber-to-the-curb (FTTC), fiber-to-the-building (FTTB), fiber-to-the-premise (FTTP), or fiber-to-the-home (FTTH). Each such network provides an access from a central office (CO) to a building, or a home, via optical fibers installed near or up to the subscribers&#39; locations. As the transmission quantity of such an optical cable is much greater than the bandwidth actually required by each subscriber, a passive optical network (PON) shared between a plurality of subscribers through a splitter was developed. 
     An exemplary diagram of a typical PON  100  is schematically shown in  FIG. 1 . The PON  100  includes M optical network units (ONUs)  120 - 1 ,  120 - 2 , through  120 -M, coupled to an optical line terminal (OLT)  130  via a passive optical splitter  140 . To the extent that reference is made to the ONUs without regard to a specific one thereof, such ONUs will be referenced as  120 . Traffic data transmission may be achieved, for example, by using asynchronous transfer mode (ATM) cells over two optical wavelengths, one for the downstream direction and another for the upstream direction. Other multiplexing methods are available such as Ethernet frames in EPON or data fragments in GPON. Downstream transmission from OLT  130  is broadcast to all ONUs  120 . Each ONU  120  filters its respective data according to, for example, pre-assigned ATM VPI/VCI values. The ONUs  120  transmit respective data to the OLT  130  during different time slots allocated by the OLT  130  for each ONU  120 . The splitter  140  splits a single line into multiple lines, for example 1 to 32. 
     In the related art, PONs are classified into one of the following: an ATM PON (APON), a broadband PON (BPON), an Ethernet PON (EPON or GE-PON), and a Gigabit PON (GPON). The APON uses the ATM protocol; the BPON is designed to provide broadband services over an ATM protocol; the EPON accommodates an Ethernet protocol; and the GPON is an evolution of BPON with higher speeds as well as the ability to accommodate various types of information (e.g., ATM, Ethernet, TDM, and so on) natively. Each type of PON is a standard technology. 
     The OLT  130  and ONUs  120  provide the interface between the optical network and homes or businesses, and thus define the type of the PON. For example, in order to establish a BPON the OLT  130  and ONUs  120  should be compatible with the BPON standards, ITU-T-G.983 series. ONU&#39;s components adapted to operate in a dual mode (e.g., BPON and GPON or EPON and GPON) or components that can function in a triple mode (e.g., BPON, GPON and EPON) are currently being developed. In a case where a mixed-mode ONU is utilized, the network type is determined by the OLT installed at the CO and the ONUs detect the mode of the PON supported by the OLT. 
     To save costs on infrastructure, service providers have installed mixed-mode ONUs in homes or businesses, and thus can upgrade, or provide additional services with minimal or without additional investments. Specifically, to upgrade the network from a first type of PON to a second type of PON, the OLT in the CO is replaced with an OLT that supports the second PON and mixed-mode ONUs are configured to operate in accordance with the second OLT. Presently, each ONU  120  has to be physically replaced, by a technician, when the mode and/or standard implemented on the PON changes. This is of course a time consuming and labor-intensive task that extends the time required for upgrading the network and involves significant costs for service providers. Furthermore, the labor intensive replacement of the ONUs  120  can result in prolonged interruption of services to the subscribers. 
     It would be, therefore, advantageous to provide a method for automatically upgrading ONUs to operate a new type of PON, and thus allowing the smooth migration between the different types of PON. 
     SUMMARY OF THE INVENTION 
     It is an object of the invention to provide a method for automatically upgrading a passive optical network (PON) from a first PON mode to a second PON mode. 
     This object is realized in accordance with a first aspect of the invention by a method for automatically maintaining bi-directional communication between an optical network unit (ONU) and a central office (CO) in a passive optical network (PON) when the CO upgrades from a first optical line terminal (OLT) having a first PON mode to a second OLT having a second PON mode, the method comprising: 
     obtaining a PON mode relating to downstream data flow from the OLT to the ONU; and 
     upon detecting a change in said PON mode from the first PON mode to the second PON mode:
         synchronizing said ONU to operate according to a downstream configuration of the second PON mode; and   synchronizing said ONU to operate according to an upstream configuration of the second PON mode.       

     In accordance with a second aspect of the invention there is provided a method for automatically upgrading a passive optical network (PON) having an optical network unit (ONU) coupled via a first splitter to a first optical line terminal (OLT) in a central office (CO), so as to maintain bi-directional communication between the ONU and the CO when the CO changes from a first PON mode to a second PON mode, the method comprising:
         installing in the CO a second OLT capable of operating in said second PON mode;   connecting a second splitter between the second OLT and the ONU;   connecting the second OLT to said second splitter;   connecting the ONU to the second splitter; and   automatically activating said ONU in said second PON mode.       

     In accordance with a third aspect of the invention there is provided an apparatus for automatically maintaining bi-directional communication between an optical network unit (ONU) and a central office (CO) in a passive optical network (PON) when the CO upgrades from a first optical line terminal (OLT) having a first PON mode to a second OLT having a second PON mode, said apparatus comprising:
         a sensor couplable to the ONU for sensing a PON mode relating to downstream data flow from the CO to the ONU;   a downstream configuration synchronizer coupled to the sensor and responsive to a change in said PON mode from the first PON mode to the second PON mode for synchronizing said ONU to a downstream configuration of the second PON mode; and   an upstream configuration synchronizer coupled to the sensor and responsive to a change in said PON mode from the first PON mode to the second PON mode for synchronizing said ONU to an upstream configuration of the second PON mode.       

    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In order to understand the invention and to see how it may be carried out in practice, some embodiments will now be described, by way of non-limiting example only, with reference to the accompanying drawings, in which: 
         FIG. 1  is an schematically diagram of a typical prior art PON; 
         FIG. 2  is a diagram of a PON illustrating the process for upgrading a PON; 
         FIG. 3  is a flowchart describing the method for automatically activating a mixed-mode ONU to operate in a new PON mode in accordance with one embodiment of the present invention; 
         FIG. 4  is a flowchart describing the step for retrieving a PON configuration in accordance with one embodiment of the present invention; 
         FIG. 5  is a table that lists the prior art line-rates and line-codes defined in the EPON, GPON and BPON standards; and 
         FIG. 6  is a block diagram showing functionality of an apparatus designed for implementing the process for automatically activating an ONU to operate in a new PON in accordance with one embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     In accordance with the disclosed invention a method for automatically upgrading passive optical networks (PONs) from a first PON mode to a second PON mode is used. Each of the first and second PON modes include, but are not limited to, an Ethernet PON (EPON), a Gigabit PON (GPON), and a broadband PON (BPON). The method enables the changing of an operating PON by automatically configuring each optical network unit (ONU), with no service impact on other subscribers. The ONU utilized for this purpose is a mixed-mode ONU which is capable of processing more than one, but not limited to the following EPON, BPON and GPON traffic. By installing mixed-mode ONUs at end-user sites and invoking the method disclosed herein, service providers can significantly reduce the network shutdown time and costs involved with the upgrading of their networks. 
       FIG. 2   a  shows a non-limiting diagram of a PON  200 , illustrating the process for upgrading PONs. For the purpose of this example the current operation mode of the PON  200  is BPON and it is upgraded to GPON mode. The PON  200  includes a BPON OLT  210 - 1  and a GPON OLT  210 - 2  both installed at a central office (CO)  220 , a splitter hub  230 , and M ONUs  240 - 1  through  240 -M located in a service area  270 . The ONUs  240  are mixed-mode ONUs which are capable of processing traffic of at least the two different PON modes. In one configuration, each ONU  220  may include a PON processor adapted to serve a plurality of PON applications. An example for such PON processor may be found in U.S. patent application Ser. No. 11/238,022 filed Sep. 29, 2005 and commonly assigned to the same assignee as the present application, and which is hereby incorporated for all that it contains. 
     In order to upgrade PON  200  to provide GPON services, a GPON OLT  210 - 2  is installed and tested in the CO  220 . A new splitter  250 - 2  is allocated in the splitter hub  230  and a fiber strand  262  is wired between the splitter  250 - 2  and the GPON OLT  210 - 2 . Once a subscriber orders GPON services, a fiber strand  263  from an ONU  240 - 2  (i.e., from the subscriber home) is patched from the splitter  250 - 1  to the splitter  250 - 2 . It should be noted that only the ONU  240 - 2  is switched from splitter  250 - 1  to the new splitter  250 - 2 , and thus the service interruption affects only the subscriber(s) serviced by the ONU  240 - 2 . Subsequently, a process for re-activating the ONU  240 - 2  in the GPON mode is applied. As will be described in greater detail below, this process automatically configures the ONUs to operate in the new mode. 
       FIG. 2   b  shows the PON  200  once all ONUs  240  have been transferred to the GPON mode. As can be seen, the BPON OLT  210 - 1 , the splitter  250 - 1  and the fiber  261  are no longer necessary, and therefore removed from the PON  200 . 
     Referring to  FIG. 3 , there is shown a non-limiting flowchart  300 , describing the method for automatically activating a mixed-mode ONU to operate in a new PON mode, in accordance with one embodiment of the present invention. The method will be described herein with reference to a specific embodiment of the invention where the ONU is a triple-mode ONU equipped to handle GPON, BPON, and EPON traffic. However, other embodiments will be apparent to those of ordinary skill in the art. For example, the mixed-mode ONU may be adapted to double-mode ONU utilizing the standards of EPON and GPON or BPON and GPON, or any other combination, or to any multi-mode ONU accommodating multiple PON standards, existing or future. 
     The triple-mode ONU automatically recognizes the line-code (i.e., a code chosen for use within a communications system for transmission purposes) and the line-rate (i.e., the transmission rate) that are active on the PON. The EPON, BPON, and GPON modes are defined in different standards, each of which determines its own line-codes as well as downstream and upstream line-rates. The line-rates and line codes of the various PON standards are tabulated in  FIG. 5 . 
     At S 310 , it is checked if a light signal is received at the ONU, and if so execution proceeds to S 320 ; otherwise, execution waits until such a signal is detected. At S 320 , a configuration flag (hereinafter the “config_flag”), which indicates the tested configuration (i.e., a line-code and a line-rate) of the optical network is initialized to a string value pointing to “last configuration”. At S 330 , the PON downstream configuration is retrieved according to the value of the config_flag. Each time execution reaches this step a different set of configuration values is retrieved. 
       FIG. 4  shows the execution of step S 330  in greater detail. The PON downstream configuration is obtained from the configuration register to allow the synchronization of an ONU to operate in the line-code and the downstream line-rate of the PON. Specifically, the method first tries to synchronize an ONU according to the PON&#39;s latest operating line-code and downstream line rates. If the ONU is unsynchronized, then there is an attempt to try to connect again with the PON configuration used prior to the latest setting. After the second attempt, if the ONU is still unsynchronized, an attempt is made to synchronize the ONU according to the line rates and line-codes that are pre-defined in the internal registers and memory of the ONU. 
     At S 331 , it is checked if config_flag equals to “last configuration” and if so, at S 332 , the last known working configuration of the PON is retrieved. Then, at S 333 , config_flag is set to a string value “prior to last”. If S 331  results with an affirmative answer, execution continues with S 334  where it is checked if config_flag equals to “prior to last”. If so, at S 335 , the working configuration of the PON prior to the last known configuration is retrieved, and at S 336  config_flag is set to “pre-defined”. If S 334  results in a negative answer, then at S 337  the various combinations of downstream line-rates and line codes as defined in the ONT configuration for the new mode(s) are retrieved. As an example, the GPON standard defines the different line rates: 2.4xx Gbit/s and 1.244 Gbit/s, each operative with a scrambled non return to zero (NRZ) line-code. 
     Referring back to  FIG. 3 , execution continues with S 340  where an attempt is made to synchronize the ONU to the downstream configuration retrieved at S 330 . The ONU is considered successfully synchronized if it reaches its SYNC state. The definition of a SYNC state varies from one PON standard to another. For example, the SYNC state of an EPON is reached when a correct multi-point control protocol (MPCP) frame is received, the SYNC state of a GPON is accomplished when a received value of a physical synchronize field (Psync) is correct, and the SYNC state of BPON is reached once a correct synchronization frame is received. A detail description of the SYNC state can be found in the respective standards IEEE 802.3ah, ITU-T G.984, and ITU G.983, each of which is incorporated herein by reference for all they contain. At S 350 , it is checked if the ONU reaches the SYNC state, and if so execution continues with S 370 ; otherwise, execution continues with S 360  where another check is made to determine if config_flag equals a string value “pre-defined”. If the value of config_flag is different than “pre-defined”, execution returns to S 330  to retrieve a different set of configuration values; otherwise, execution continues with S 380 , where it is checked whether the whole set of pre-defined configurations were tested, and if so execution ends; otherwise, execution returns to S 330  where a new set of a predefined configuration values is retrieved. 
     At S 370 , the ONU is synchronized to the upstream line-rate and line-code of the OLT. Generally, all types of PONs are point-to-multipoint networks, where the upstream traffic is controlled by the OLT, and thus the ONUs are governed to transmit at line-rate compliant with the OLT. In most PON architectures, the upstream line-rate is determined according to downstream rate, i.e., a PON is configured with a pair of upstream and downstream line-rates. Specifically, for the EPON a single upstream line-rate is defined (as shown in  FIG. 5 ), and thus this rate is always chosen. In PONs for which such a pair of line-rates does not exist, the method automatically detects the upstream line-rate. The BPON standard allows two different upstream rates (622.08 Mbit/s and 155.52 Mbit/s) and the GPON standard defines three line rates. Accordingly, an ONU upstream line-rate is determined by utilizing a ranging process. The ranging process is used to adjust the timing of each ONU to compensate for the differential distance from the OLT to the ONUs. The ranging process is further discussed in the GPON and BPON standards. The method of the present invention selects a first upstream line-rate (e.g., 155.52 Mbit/s for BPON) and applies the ranging process. If the process fails after a pre-determined number of connection attempts, another upstream line-rate is selected (e.g., 622.08 Mbit/s for BPON) and the ranging process is applied once more. If the ranging process does not fail, the ONU is synchronized to the selected upstream line-rate. It should be noted that the upstream line-rate cannot be higher than the downstream line-rate. The upstream line-code is the same as the downstream line-code. The ONU upstream transceiver is configured to the upstream line-code which changes from a first PON to a second PON mode. 
     At S 390 , it is checked as to whether the ONU is successfully synchronized to the upstream line-rate, and if so the configuration of the ONU ends and it is ready to operate in the new mode; otherwise, execution continues with S 395  where an internal error message is generated, indicating that the ranging of the ONU should go again through the possible values of the upstream line-rate. In one embodiment of the present invention, the activation process is restarted if an internal error message is generated. 
     The methods described herein can be executed in two scenarios: 1) warm ONU, and 2) cold ONU. In the warm ONU, the activation is performed if a fiber (e.g., fiber  261 ) were disconnected or the OLT were restarted. In the cold-ONU activation is performed if ONU were restarted. It is further noted that the method may be implemented in software, firmware, hardware, or any combination thereof. Thus, hardware or firmware may be embedded within the ONU for sensing a change in PON mode and synchronizing the downstream and upstream PON configurations. Alternatively, a standalone apparatus may be coupled to the ONU for sensing a change in the downstream PON configuration and synchronizing the downstream and upstream PON configurations to the new PON mode. 
       FIG. 6  shows a non-limiting block diagram of such an apparatus  600  operative in an ONU and designed for implementing the process for automatically activating an ONU to detect and operate in one of multiple PON modes supported by the ONU. The apparatus  600  includes a sensor  610  connected to fiber originating from a splitter (e.g., splitter  250 ), a downstream configuration synchronizer  620 , an upstream configuration synchronizer  630 , and a memory unit  640 , which stores the upstream configurations and the downstream configurations of the PON. The memory unit  640  may optionally be an external unit coupled to the downstream and upstream configuration synchronizers. The sensor  610  senses the optic fiber to check if a light signal is received. Once the signal is received, the sensor  610  detects the mode of the PON as supported by the OLT. The downstream configuration synchronizer  620  retrieves the downstream configuration of the detected PON mode and tries to reach a SYNC state. Specifically, the synchronizer  620  first tries to synchronize to an ONU according to the PON&#39;s latest operating line-code and downstream line rates. If the ONU is unsynchronized, the synchronizer  620  again tries to synchronize again with the PON configuration used prior to the latest setting. After the second attempt, if the ONU is still unsynchronized, the synchronizer  620  attempts to synchronize the ONU according to the line rates and line-codes that are pre-defined in the memory unit  640 . The upstream configuration synchronizer  630  synchronizes to the upstream line-rate and line code of the OLT. 
     The apparatus may be a suitably programmed computer, in which case the functionality of the downstream configuration synchronizer and the upstream configuration synchronizer may be implemented by the CPU of the computer. Likewise, the invention contemplates a computer program being readable by a computer for executing the method of the invention. The invention further contemplates a machine-readable memory tangibly embodying a program of instructions executable by the machine for executing the method of the invention. 
     It will be appreciated that modifications can be made to the manner of carrying out the invention without departing from the scope thereof as defined in the appended claims. For example, in the embodiments described, a change in PON mode is sensed by retrieving the PON mode from a data packet that is communicated downstream from the CO to the ONU. This assumes that the PON mode appears in a header that is pre-pended to a downstream data packet and is readable by a mixed-mode ONU regardless of to which PON mode it is configured to operate. Alternatively, a change in PON mode may be inferred by an inability to read a downstream data packet and the new PON mode of the CO may then be determined by trial and error, i.e. by setting the PON mode of the ONU to different modes until the downstream data packet can be read.