Abstract:
A method for performing a protection in passive optical networks. The method comprises forming a protection maintenance link between an active optical line terminal (OLT) and a standby OLT; forming a synchronization link between the active OLT and the standby OLT; computing a base differential distance value; continuously measuring round trip time (RTT) values by the active OLT using the protection maintenance link; periodically sending at least RTT values calculated by the active OLT to the standby OLT over the synchronization link; and computing, by the standby OLT, a new RTT value based on at least a RTT value measured by the active OLT and a standby differential distance value, when a switch-over action is triggered, thereby allowing the standby OLT to serve optical network units (ONUs) in the PON without performing a ranging process.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of U.S. Provisional Application No. 61/141,489 filed on Dec. 30, 2008, the contents of which are herein incorporated by reference. 
     
    
     TECHNICAL FIELD 
       [0002]    The present invention relates generally to passive optical networks (PONs), and more particularly to protection of such networks. 
       BACKGROUND OF THE INVENTION 
       [0003]    A passive optical network (PON) comprises an optical line terminal (OLT) connected to multiple optical network units (ONUs) in a point-to-multi-point network. New standards have been developed to define different types of PONs, each of which serves a different purpose. For example, the various PON types known in the related art include a Broadband PON (BPON), an Ethernet PON (EPON), a Gigabit PON (GPON), and others. 
         [0004]    An exemplary diagram of a typical PON  100  is schematically shown in  FIG. 1 . The PON  100  includes N ONUs  120 - 1  through  120 -N (collectively known as ONUs  120 ) coupled to an OLT  130  via a passive optical splitter  140 . In a GPON, for example, traffic data transmission is achieved using GPON encapsulation method (GEM) encapsulation over two optical wavelengths, one for the downstream direction and another for the upstream direction. Thus, downstream transmission from the OLT  130  is broadcast to all the ONUs  120 . Each ONU  120  filters its respective data according to pre-assigned labels (e.g., GEM port-IDs in a GPON). The splitter  140  is 1 to N splitter, i.e., capable of distributing traffic between a single OLT  130  and N ONUs  120 . 
         [0005]    In most PON architectures, the upstream transmission is shared between the ONUs  120  in a TDMA based access, controlled by the OLT  130 . TDMA requires that the OLT first discovers the ONUs and measures their round-trip-time (RTT), before enabling coordinated access to the upstream link. For example, in a GPON network, during initial set-up an ONU  120  and the OLT  130  may be in one of the following operational states: serial number acquisition or ranging. In the serial-number acquisition state, the OLT  130  tries to detect the serial number of an ONU  120 . If the OLT  130  and an ONU  120  have not completed the serial number acquisition stage, due to a low power signal, the ONU  120  independently changes its optical power output until a successful detection of the serial number. In the ranging state, the OLT  130  tries to determine the range between the terminal units (i.e., ONUs  120 ) to find out at least the round trip time (RTT) between OTL  130  and each of the ONUs  120 . The RTT of each ONU  120  is necessary in order to coordinate a TDMA based access of all ONUs  120  to the shared upstream link. 
         [0006]    During a normal operation mode the range between the OLT  130  to the ONUs  120  may change over time due to temperature changes on the fiber links (which results with varying signal propagation time on the fiber). The OLT  130  continuously measures the RTT and adjusts the TDMA scheme for each ONU accordingly. 
         [0007]    In order to enable protection in PONs a redundant optical link and OLT are connected to a splitter. This type of a configuration is usually referred to as a duplex PON system. An example for such a system is a protection type B, defined in ITU-T standard G.984.1. An illustration of a duplex PON  200  is provided in  FIG. 2 . As can be noticed two OLTs  210 - 1  and  210 - 2  are respectively connected via optical fibers  220 - 1  and  220 - 2  to a splitter  230 . The splitter  230  splits incoming traffic to N OUNs  240 - 1  and  240 -N. That is, in this example, the splitter  230  is a 2 to N splitter. 
         [0008]    In the duplex PON  200  one of the OLTs is set as active while the other as a standby. When there is a failure in the active OLT or its respective fiber a fail-over is performed and the operation is switched to the standby OLT and traffic is sent through its respective fiber. Typically, a synchronization link is established between the OLTs  210  to transfer database updates, PON status messages and switch-over trigger signals. 
         [0009]    However, in order to ensure minimal service interruption due to a switch-over action, the standby OLT should perform at least the ranging process when turning into an active OLT. The lengths of the standby and active optical links are not the same. Performing such a process when establishing the network is an error prone approach as the “range” between the OLTs  210  and the terminal units (i.e., ONUs  240 ) may change over time and therefore there may be different RTT times and optical power levels for signals transmission power. Performing the ranging process for each ONU when switching-over is a time-consuming task and typically results with a long service interruption time, as the PON remains idle for duration that the ranging process takes place. 
         [0010]    In addition, the topology of the network may change over time. For example, an ONU may be added or removed from the network. Thus, the standby OLT should maintain updated information relating to the status of the PON, as acquiring such information when switching-over is a time consuming process through which the PON remains idle. 
         [0011]    A protection mechanism should maintain a fast and reliable communication channel between the two OLTs, whether the active and standby OLTs  210  are collocated on the same shelf, rack, or reside in geographically remote sites. In addition, a logic unit controlling the protection mechanism should be continuously updated with the status of both the fiber links connecting the standby and active OLTs  210  to the splitter  230 . Since the standby OLT (e.g., OLT  210 - 1 ) cannot transmit data on its link (e.g.,  220 - 1 ) while is in standby mode, thus, it is necessary to monitor the standby OLT link before a switch-over operation. 
         [0012]    It would be therefore advantageous to provide an efficient protection mechanism for PONs. 
       SUMMARY OF THE INVENTION 
       [0013]    Certain embodiments of the invention include a method for performing a protection in a passive optical network (PON). The method comprises forming a protection maintenance link between an active optical line terminal (OLT) and a standby OLT; forming a synchronization link between the active OLT and the standby OLT; computing a base differential distance value; continuously measuring round trip time (RTT) values by the active OLT using the protection maintenance link; periodically sending at least RTT values calculated by the active OLT to the standby OLT over the synchronization link; and computing, by the standby OLT, a new RTT value based on at least a RTT value measured by the active OLT and a standby differential distance value, when a switch-over action is triggered, thereby allowing the standby OLT to serve optical network units (ONUs) in the PON without performing a ranging process. 
         [0014]    Certain embodiments of the invention also include a duplex passive optical network (PON). The duplex PON comprises a first optical line terminal (OLT) serving as an active OLT; a first optical network unit (ONU) collocated to the first OLT; a second OLT serving as a standby OLT; a second ONU collocated to the second OLT; and an optical splitter for connecting the first OLT and the second OLT and their collocated first ONU and second ONU to a plurality of ONUs. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]    The subject matter that is regarded as the invention is particularly pointed out and distinctly claimed in the claims at the conclusion of the specification. The foregoing and other features and advantages of the invention will be apparent from the following detailed description taken in conjunction with the accompanying drawings. 
           [0016]      FIG. 1  is a schematic diagram of a PON. 
           [0017]      FIG. 2  is a schematic diagram of a conventional duplex PON. 
           [0018]      FIG. 3  is a schematic diagram of a duplex PON constructed in accordance with an embodiment of the present invention. 
           [0019]      FIG. 4  is a diagram illustrating a process for measuring the differential distance. 
           [0020]      FIG. 5  is a flowchart describing a protection process implemented in accordance with an embodiment of the present invention. 
           [0021]      FIG. 6  is a diagram of an optical splitter constructed in accordance with an embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0022]    It is important to note that the embodiments disclosed are only examples of the many advantageous uses of the innovative teachings herein. In general, statements made in the specification of the present disclosure do not necessarily limit any of the various claimed inventions. Moreover, some statements may apply to some inventive features but not to others. In general, unless otherwise indicated, singular elements may be in plural and vice versa with no loss of generality. In the drawings, like numerals refer to like parts through several views. 
         [0023]      FIG. 3  shows an exemplary block diagram of a duplex PON  300  constructed in accordance with an embodiment of the invention. The PON  300  includes two OLTs  310 - 1  and  310 - 2  respectively coupled to a splitter  321 - 1  or  321 - 2  via optical links (e.g., optical fibers)  330 - 1  and  330 - 2 . The splitters  321 - 1  and  321 - 2  are coupled to a number of N ONUs  340  where a single optical link connects the splitter and each of the ONUs  340 . In the configuration of the PON  300  the optical links  330  as well as OLTs  310  are protected. That is, one OLT and its respective optical link (e.g., OLT  310 - 1  and optical link  330 - 1 ) are active while the other pair (e.g., OLT  310 - 2  an optical link  330 - 2 ) are standby. 
         [0024]    In accordance with principles of the invention, two output optical links  350  and  360  respectively couple two output ports of the splitters  321 - 1  and  321 - 2  to the OLT links  330 - 1  and  330 - 2 , and then these ONU signals are coupled by splitters  370 - 1  and  370 - 2  to ONUs  380 - 1  and  380 - 2 , which are respectively collocated to OLTs  310 - 1  and  310 - 2 . This arrangement enables each OLT to control and manage all ONUs  340  and the ONU collocated to its peer OLT via the fibers  330  connecting the OLTs  310  to the splitter  320 . For example, OLT  310 - 1  controls ONUs  340  as well as ONU  380 - 2 . 
         [0025]    The optical links  350  and  360 , the splitters  370 - 1 ,  370 - 2 ,  321 - 1  and  321 - 2 , and the fiber links  330  create a link (hereinafter “the protection maintenance link”) between the OLTs  310  and the ONUs  380 . The protection maintenance link is used for continuous calculation of the differential distance as will be described in greater detail value. 
         [0026]    OLT  310  manages a collocated ONU function  380 , which acts as any ONUs  340 . That is, an OLT  310 - 1  (e.g., active OLT) manages the ONU  380 - 2  collocated with the OLT  310 - 2  (e.g., standby OLT) and an active OLT  310 - 2  manages the ONU  380 - 2  collocated with the OLT  310 - 1 . The OLT  310 - 1  and collocated ONU  380 - 1  are connected to the splitter  320  through a 2:1 splitter  370 - 1  and the OLT  310 - 2  and collocated ONU  380 - 2  are connected to a 2:1 splitter  370 - 2 . In an exemplary embodiment the splitter  370 - 1  or  370 - 2  is used to multiplex both OLT signals and ONU signals on the same fiber. A non-limiting diagram of the splitters  370 - 1 ,  370 - 2 ,  321 - 1  and  321 - 2  constructed in accordance with an embodiment of the invention is provided in  FIG. 6 . 
         [0027]    In accordance with one embodiment of the invention, both OLTs  310 - 1  and  310 - 2  include a database having the same information, synchronized between the OLTs. The databases maintain the updated round-trip time and PON related information. It should be appreciated by one of ordinary skill in the art that sharing such information between the OLTs  310 - 1  and  310 - 2  allows fast switch-over from an active link to a standby link, as the standby OLT does not require to acquire this information when switching-over. 
         [0028]    In another embodiment of the invention, a synchronization link between the two OLTs  310 - 1  and  310 - 2  can be implemented over the protection maintenance link. The OLTs share updated round-trip time and PON related information. For example, this information includes progress reports of an ONU activation process, ONU alarms indications, learned passwords, RTT (or equalization delay) measurements, and switch-over trigger signals. In one embodiment of the invention the protection maintenance link between the OLTs can be exploited as a synchronization link. This is especially useful when, for example, the two OLTs reside in geographically remote sites. 
         [0029]    As mentioned above in order to ensure fast protection switch-over, the ranging process should not be performed by the standby OLT when switching-over. With this aim, in accordance with an embodiment of the invention a “base differential distance” value is computed. The base differential distance is the differential distance of the OLTs  310  from a designated ONU  340  as measured during installation. 
         [0030]    As illustrated in  FIG. 4  to measure a base differential distance, an active OLT  410  measures the RTT to a designated ONU  420  and then deactivates the ONU  420 . Thereafter, a standby OLT  430  activates the ONU  420  and measures the RTT to the ONU  420  and then deactivates it. Finally, the OLTs  410  and  430  exchange the RTT measurements. The base differential distance is the difference between the RTT measurements. This process is performed at the installation of the PON. It should be noted that the measurement of the differential distance can be performed with any of the ONUs on the PON, including, as illustrated in  FIG. 3 , the ONU  380  collocated to the OLT  310 . 
         [0031]    The standby OLT continuously measures and maintains an updated differential distance value as this value may change over time. In accordance with an embodiment of the invention, the differential distance DiffRTT s (t) from a standby OLT to an ONU at a time ‘t’ is computed as follows: 
         [0000]      DiffRTT S ( t )=(RTT_ONU S ( t )−BaseRTT_ONU S )+BaseDiffRTT S    
         [0032]    wherein a RTT_ONU S (t) is a RTT value measured from the active OLT (OLT A ) to the ONU collocated to the standby OLT (ONU S ) at time T, and the BaseRTT ONU S  is the RTT measured from an OLT A  to an ONU S  at installation time, when BaseDiffRTT S  is computed. The RTT_ONU S (t) and BaseRTT_ONU S  are calculated using the protection maintenance link. 
         [0033]    During normal operation of the PON, the active OLT (OLT A ) continuously monitors the RTT of the ONUs in the network, and measures the RTT(i,t) of an ONU T at time ‘t’. These measures are periodically sent to the standby OLT over the synchronization link. When a switch-over occurs, the standby OLT becomes active, and calculates a new RTT value of ONU ‘i’, as follows: 
         [0000]      New-RTT( i,t )=RTT( i,t )+DiffRTT S ( t ) 
         [0034]    This way, when the standby OLT becomes active, it can immediately provide services to the ONUs, using the New RTT values, and does not need to perform a ranging process. 
         [0035]      FIG. 5  shows an exemplary and non-limiting flowchart  500  describing the process for performing a protection in a PON implemented in accordance with an embodiment of the invention. The method is performed after the protection maintenance link and a synchronization link between the OLTs are established. As mentioned above the synchronization link can be established over the protection maintenance link. 
         [0036]    At S 510 , the active and standby links are determined by setting each OLT  310  and each optical link  320  to their respective states. At S 520 , each link is activated, on its turn, to compute the base differential distance values (BaseDiffRTT S  and BaseRTT_ONU S ) as described in detail above. These values are used by the standby OLT when switching-over. In addition, the active OLT continuously calculates the RTT_ONU S (t) values using the protection maintenance link. 
         [0037]    At S 530  during a normal operation of the PON, the active OLT sends over the synchronization link updated RTT and PON related information. This information includes, for example, progress reports of ONU activation process, ONU alarms indications, learned passwords, RTT(i, t) and RTT_ONU S (t) measurements, and so on. At S 540 , upon receiving this information, the standby OLT saves the updates in its database. In one embodiment of the invention, a new RTT value is computed once the RTT(i, t) and RTT_ONU S (t) measurements are received. The new RTT value is saved in the database. 
         [0038]    At S 550 , as a switch-over is triggered, the standby OLT received a switch-over signal, and thereafter computes the new RTT values for each ONUs as described in detail above. These values are computed using the RTT(i, t) and RTT —  ONU S (t) measurements. In the case where the new RTT value is already saved in the database, then upon triggering a switch-over, this value is retrieved from the database and the computation step (S 550 ) is not performed. Subsequently, at S 560 , the standby OLT starts to transmit data to the ONUs over its respective link. That is, the active link and standby link initially set at S 510  are switched. 
         [0039]    There are various faults that can trigger a switch-over of the standby link to become active. These faults include, but are not limited to, loss of signal (LOS)/loss of frame (LOF) of an active link, a faulty OLT (e.g., OLT&#39;s transceiver), loss of a communication channel between the OLT, and so on. The switch-over may be also triggered manually by a user, e.g., a network administrator. 
         [0040]    In certain exemplary embodiments of the invention, the control of the protection mechanism can be either centralized or distributed. A switch-over decision is based on the various faults and link-status information. The failover mechanism is continuously updated with regard to the status of each of the OLT links using the collocated ONU connected to the standby link. It should be noted that in certain embodiments such configuration is required as a standby OLT cannot communicate over its fiber. 
         [0041]      FIG. 6  shows an exemplary diagram of an optical splitter  600  constructed in accordance with an embodiment of the invention. The optical splitter  600  is based on two optical circulators  611  and  612 . An optical circulator is a three-port device that allows light to travel in only one direction. That is, from port  1  to port  2 , then from port  2  to port  3 . The optical circulator  611  is coupled to splitters  614  and  615 . A signal from the splitter  614  is received at port  1  and output at port  2  of the circulator  611 . A signal received at port  2  of the circulator (from the splitter  615 ) is transferred to the splitter  614  via port  3  of the circulator  614 . In a similar manner, the optical circulator  612  is coupled to splitters  613  and  615 . A signal from the splitter  613  is received at port  1  and output at port  2  of the circulator  612 . A signal received at port  2  of the circulator  612  is transferred to the splitter  613  via port  3  of the circulator  612 . In accordance with an embodiment of the invention the 2:N splitters  370 - 1 ,  370 - 2 ,  321 - 1 , and  321 - 2  are implemented using the 2:N optical splitter  600 . 
         [0042]    It should be appreciated that the invention described herein can be adapted to implement efficient protection mechanisms in any type of a PON including, but not limited to, a BPON, an EPON, and a GPON. Furthermore, the teachings of the invention can be adapted to implement a protection of more than two links. 
         [0043]    The principles of the invention are implemented as hardware, firmware, software or any combination thereof. Moreover, the software is preferably implemented as an application program tangibly embodied on a program storage unit or computer readable medium. The application program may be uploaded to, and executed by, a machine comprising any suitable architecture. Preferably, the machine is implemented on a computer platform having hardware such as one or more central processing units (“CPUs”), a memory, and input/output interfaces. The computer platform may also include an operating system and microinstruction code. The various processes and functions described herein may be either part of the microinstruction code or part of the application program, or any combination thereof, which may be executed by a CPU, whether or not such computer or processor is explicitly shown. In addition, various other peripheral units may be connected to the computer platform such as an additional data storage unit and a printing unit. 
         [0044]    All examples and conditional language recited herein are intended for pedagogical purposes to aid the reader in understanding the principles of the invention and the concepts contributed by the inventor to furthering the art, and are to be construed as being without limitation to such specifically recited examples and conditions. Moreover, all statements herein reciting principles, aspects, and embodiments of the invention, as well as specific examples thereof, are intended to encompass both structural and functional equivalents thereof. Additionally, it is intended that such equivalents include both currently known equivalents as well as equivalents developed in the future, i.e., any elements developed that perform the same function, regardless of structure.