Abstract:
A method and apparatus for protecting faults in an optical network. Protection is based on 1:n protection at an Optical Line Terminator (“OLT”). Each working interface module in the OLT is coupled via a fiber to a 2:N splitter which provides communication with N Optical Network Units (“ONU”). A protection interface module is coupled via a fiber to a 1:n switch whose output is coupled to each of the 2:N splitters. In the event of a fiber break, protection switching is performed by forming a backup link to the 2:N splitter associated with the failed fiber through the protection interface module. The 1:n protection arrangement may be replicated and extended to a g*(1:n) protection arrangement. A uni-ranging process speeds up protection switching by ranging only one ONU associated with a failed fiber, rather than all ONUs associated with a failed fiber.

Description:
[0001]    This application claims the benefit of U.S. Provisional Application No. 60/254,913, filed Dec. 12, 2000. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    This invention is directed toward automatic protection switching in an asynchronous transfer mode passive optical network (APON). More specifically, the present invention is directed toward a protection method and APON architecture that efficiently protects against system faults, such as a fiber cut, in the network. The present invention also includes a method for rapidly acquiring phase equalization data needed to complete a protection switching operation.  
           [0003]    Delivering broadband data through fiber directly to the home is becoming a commercial reality. However, the cost of fiber-to-the-home (FTTH) deployment has been one of the major obstacles to hinder the fast pace of FTTH deployment. Designing reliable and cost effective APON access network equipment has been a challenge for telecommunication engineers. In order to eliminate a single point of failure, protection mechanisms must be incorporated. Protection methods for an APON interface have recommended previously. However, previously proposed methods may not provide suitable tradeoffs between reliability and cost.  
           [0004]    As shown in FIG. 1, a general APON architecture  1  consists of four components, an Optical Line Terminator (OLT)  101 , an Optical Distribution Network (ODN)  102 , an Optical Network Unit (ONU)  103 , and an Element Management System (EMS)  104 . The OLT  101  functions as an asynchronous transfer mode (ATM) concentrator or edge switch with a passive optical network (PON) access interface. On the network side, the OLT  101  can connect to the ATM backbone  105  or digital cross-connect network  106  using their respective interfaces, such as OC-3c  107  or DS3  108 . Each APON interface module (not shown) in the OLT  101  can support N ONUs  103 , where N is defined as the splitter ratio. The ODN  102  has a tree configuration with the roots connecting to the OLT  101  and the branches distributing to the ONUs  103 . The ODN  102  is completely constructed from passive optical components such as fibers  109 , splitters  201 , and connectors (not shown). It may typically span up to 20 km distance with certain power budget restriction. The optical signals are carried on one single fiber to the connected ONUs  103 . The ONU  103  terminates the optical signal at a PON interface and converts the signal to the proper interfaces with customer premises equipment (CPE). Depending on the applications, the ONU  103  can be placed in different locations to support all FTTx configurations, such as FTTH, fiber-to-the-business (FTTB), fiber-to-the-curb (FTTC), or fiber-to-the-cabinet (FTTCab). The EMS  104  is used to manage the whole system for provisioning, performance monitoring, operation, administration, and maintenance.  
           [0005]    APON uses several unique techniques to provide robust duplex data communication through a single fiber. To support bi-directional transmission, Wavelength Division Multiplexing (WDM) employing two lasers at different wavelengths is used in the interface. In the downstream direction, data is broadcast at a wavelength of 1550 nm for point-to-multipoint transmission. In the upstream direction, a 1310 nm wavelength is used along with a Time Division Multiple Access (TDMA) protocol to support the multipoint-to-point communication. Since all ONUs  103  on an APON interface receive the entire data stream broadcast from the OLT  101 , a security measure called churning may be deployed.  
           [0006]    The APON protocol uses standard ATM cell structures in which the most important two types of ATM cells are data and Physical Layer Operation Administration Management (PLOAM) cells. The PLOAM cells carry command, control, and status information whereas the data cells carry the payload for data communication. Since the ONUs  103  can be a significant distance from each other and the OLT  101 , a process called ranging is performed prior to the start of data transmission to avoid collision. Ranging allows an OLT  101  to compensate for the time delay caused by the vast distances separating the OLT  101  and ONUs  103  with phase equalization. The ranging process is carried out through PLOAM cells sent from the OLT  101 . A ranging process standard has been defined by International Telecommunication Union-Telecommunication in the publication ITU-T G.983.1 standard, the entire text of which is hereby incorporated by reference. The ranging process creates the unique requirements for automatic protection design.  
           [0007]    Because a single APON interface module at the OLT  101  will accommodate multiple ONUs  103  in the field, the protection of APON interface modules is very important. In ITU-T G.983.1, four protection schemes have been defined as type A, B, C, and D. In the following paragraphs, each protection type is briefly discussed.  
           [0008]    In protection type A, a spare fiber is equipped between the OLT and the splitter. The APON interface can detect a fiber cut in a primary fiber and switch to the spare fiber. During switching, signal loss or even cell loss may be inevitable. However, all the connections between the service node and the terminal equipment should be held during the fiber switching. Re-ranging of all ONUs connected may be necessary because the total fiber length may be changed. There is no redundant equipment in the OLT and ONUs. A 1:2 optical switch along with 2:N splitter is required to implement this feature. The protection switch message is reported back to the EMS. In order to reduce the reflection caused by the open end of the fiber at the optical switch, a special optical switch, an attenuator between the splitters, or APON scaling may be needed.  
           [0009]    In protection type B, the APON network is partially protected. This configuration uses a working APON interface module and a cold stand-by protection APON interface module in the OLT side and no redundant parts in the ONUs. An APON interface module failure or a fiber cut between an OLT and a splitter will cause “tree” protection switching whereas the individual ONU PON interface failure will not cause “branch” protection switching. The signal loss or even cell loss is, in general, inevitable in the switching period. However, all the connections supported between the service node and the terminal equipment should be held after this switching. A 2:N splitter is used for this protection type. A selector at the OLT is used to switch between working and protection APON interface modules.  
           [0010]    In protection type C, both the OLT and ONUs are equipped with redundant modules. In this case, the hot stand-by protection PON circuits in both OLT and ONU sides makes hitless switching possible. Constant synchronization between the working and protection modules is required for hitless switching. PON interface module failures at the OLT side and a fiber cut between OLT and splitter will cause a “tree” switching. Individual PON interface failures at the ONU side can be recovered by single branch switching so that other ONUs will not be disturbed. In this protection scheme, single point failure scenarios in PON interface are all covered.  
           [0011]    In protection type D, a redundant ODN is implemented besides using a protection PON interface modules at both the OLT and ONU. In such case, Multiple Point Failure (MPF) can be protected against in the ODN. It is the most reliable PON interface. However, it carries a higher cost and the management of such a PON interface is complicated.  
           [0012]    Protection types B and C have been recommended for APON systems for FTTB deployment. For FTTH, it is more cost effective to use types of A or B. However, type A only protects the fiber failure. On the other hand, type B with 1:1 protection at OLT side is cost prohibitive for a majority of FTTH applications because multiple redundant APON interface modules are required.  
           [0013]    Thus, there is a need for a fault protection method and architecture for APON that is cost effective for FTTH applications.  
         SUMMARY OF THE INVENTION  
         [0014]    The present invention provides an alternative to the prior art fault protection schemes. An alternative type H protection is based on 1:n protection at the OLT. Each of n working APON interface modules in the OLT is coupled via a fiber to a 2:N splitter which provides communication with N ONUs. Also at the OLT is a protection APON interface module coupled via a fiber to a 1:n switch whose output is coupled to each of the 2:N splitters. In the event of a failure of one of the n working APONs or of one of the n fibers emanating from the working APONs (such as a fiber break), a backup link to the 2:N splitter associated with the failed working APON is established through the protection APON.  
           [0015]    The present invention also provides a pre-ranging and uni-ranging method which speeds up automatic protection switching. Pre-ranging is performed at the system setup period or period when the least traffic is running in the system. The working APON interface modules are switched to a standby mode one by one until all n modules are switched. The equalization data obtained for the standby module will be stored in a memory. The advantage of this method is that all equalization data for the ONUs associated with each working APON interface module are readily available in the memory.  
           [0016]    The present invention also provides a uni-ranging process for ranging during protection switching. Instead of ranging every ONU, only one ONU associated with a failed APON interface module is ranged after protection switching. Since the 1:n protection at an OLT is tree switching, the distance differences between to the various ONUs remains intact. Therefore, after protection switching, one ONU chosen from the group can be ranged first. By comparing the previous equalization data stored in the memory to the newly obtained range, the equalization data for other ONUs associated with the failed APON interface module can be calculated. Uni-ranging speeds up the automatic switching dramatically by reducing the multiple ranging processes to one.  
           [0017]    This 1:n protection type is more economical than previously defined types. At the same time, it will supply adequate protection during Single Point of Failure (SPF). With the developed pre-ranging and uni-ranging methods discussed herein, the fast protection switching can be achieved. Such protection is suitable for an FTTH system in which the cost is the vital factor for successful and massive deployment. Therefore, it is more suitable for an FTTH application in which cost reduction is essential. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0018]    [0018]FIG. 1 is a plan view of an asynchronous transfer mode passive optical network;  
         [0019]    [0019]FIG. 2 is a plan view of one embodiment an APON network using type H protection;  
         [0020]    [0020]FIG. 3A is a plan view of an APON network using type H protection;  
         [0021]    [0021]FIG. 3B is a plan view of an APON network using type H protection;  
         [0022]    [0022]FIG. 4 is a plan view of an alternative embodiment of an APON network using type H protection;  
         [0023]    [0023]FIG. 5 is a flow diagram of the protection management procedure;  
         [0024]    [0024]FIG. 6 is a flow diagram of the uni-ranging procedure;  
         [0025]    [0025]FIG. 7 is a plan view of an alternative embodiment of an APON network using type H protection; and  
         [0026]    [0026]FIG. 8 is a plan view of an alternative embodiment of an APON network using type H protection. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0027]    Referring now to FIG. 2, an embodiment of the present invention is shown. The OLT  101  has n working APON interface modules  301   a , and one protection APON interface module  301   b . The protection APON interface module  301   b  can be installed in a fixed slot or any slot (not shown). Each of the n working APON interface modules  301   a  is connected by fiber  109  to a 2:N splitter  303 . Each of the 2:N splitters  303  is connected by fiber  109  to the PON interface  603  of a plurality (N x ) of ONUs  103 . The various splitter ratios N 1  through N n  of the 2:N splitters  303  need not be the same. The protection APON interface module  301   b  is connected by fiber  109  to a 1:n optical switch  602 . The 1:n optical switch  602  is connected by fiber  109  to each of the 2:N splitters  303 .  
         [0028]    Referring now to FIG. 3 a , the 1:n optical switch  602  may be installed at a Central Office (CO) (FIG. 3 a ) or in the field (in the ODN  102 ) (FIG. 3 b ). For the 1:n optical switch  602  that is installed at a CO, multiple fibers  109  instead of one need to be installed for the ODN  102 . The 1:n optical switch  602  can be controlled locally from the OLT  601 . Therefore, no active component is deployed in the ODN  102 . Short root fiber connected to the 1:n optical switch  602  in an indoor environment reduces the possibilities of fiber cut. This scheme will lead to high reliability networks.  
         [0029]    Referring now to FIG. 3 b , the 1:n optical switch  602  may be alternatively installed in the field (in the ODN  102 ). Most optical switches based on the technologies such as Optomechanical, Mirco-Optoelectromechanical, Planer Wave Guides, Semiconductor Optical Amplification, or Liquid-Crystal, contain active components. In this case, operating the 1:n optical switch  602  in the ODN  102  will involve metallic wiring and a power supply (not shown). Hybrid or composite cables will be needed for this implementation. The benefit of this deployment is reducing the long multiple optical fibers necessary when the 1:n optical switch  602  is installed in the CO.  
         [0030]    Referring now to FIGS. 7 and 8, an alternative embodiment of the arrangement shown in FIGS. 3 a  and  3   b  is shown. The OLT  901  has g groups of working APON interface modules, each group having an associated protection APON interface module. The embodiment shown in FIGS. 7 and 8 may be called g*(1:n) protection, in which the 1:n scheme show in FIGS. 3 a  and  3   b  is replicated in each of the g groups of APON interface modules.  
         [0031]    A Common Control Card (CCC)  701  is connected to the 1:n optical switch  602  via control bus  702 . The CCC  701  is also connected to the APON interface modules  301   a ,  301   b  via bus  703 . Each APON interface module  301   a ,  301   b  has information associated with it which includes, but is not limited to, identification codes (PON IDs) of the PON interfaces  603  (shown in FIG. 2) with which it communicates, ONU serial numbers, ONU passwords, ranging intervals, bandwidth information, and a current alarm status. This information associated with the APON interface modules  301   a ,  301   b  will be stored in a memory in the CCC and used to supervise protection switching in the event of a system fault, such as a cut in the fiber connecting a working APON interface module  301   a  to a 2:N splitter  303 , or an internal failure in the APON interface module  301   a  itself.  
         [0032]    Referring now to FIG. 4, the 1:n optical switch  602  described in connection with FIGS. 2, 3 a , and  3   b  may be replaced with a 1:n splitter  801 , and a 2:1 optical switch  802  located at each of the n 1:N splitter  803 , which will replace the n 2:N splitters  303  described in connection with FIGS. 2, 3 a , and  3   b . The 1:n optical switch solution is relative expensive and may contain active components. The 1:n splitter solution will be cost effective because one 1:n splitter plus n 2:1 optical switches are relatively cheaper than one 1:n optical switch. However, a larger power loss associated with a 1:n splitter will limit its application to small protection ratios. For example, 1:32 splitter will result in more than 15 dB power loss. For a smaller protection ratio and a short distance application, the splitter solution is more economical. Whereas for a larger protection ratio and a long distance application, using an optical switch installed at a CO will be a suitable choice.  
         [0033]    Referring again to FIGS. 3 a  and  3   b , the protection switching procedure controlled by the CCC  701  will be described. The procedure consists of four major portions: synchronization, failure detection, switching, and fast ranging. The synchronization functions running both at the CCC  701  and APON interface cards will keep the ONUs  103  and APON status updated. Therefore, whenever an APON interface module  301   a  fails, the information associated with the failed APON interface module  301   a , which is stored in the CCC  701 , can be recovered and copied to the protection APON interface module  301   b.    
         [0034]    As soon as a working APON interface module  301   a  detects a loss of signal (LOSi), it will report an alarm code to the CCC  701 . Then the CCC  701  sends a control signal to the 1:n optical switch  602  creating a connection with the 1:N splitter  303  associated with the working APON interface module  301   a  that reported a LOSi. Therefore, the failed APON traffic can detour to the ODN via the protection APON interface module  301   b . In the case of more than one working APON interface module  301   a  failing at the same time, the one with more traffic flow or with higher priority assigned by the operators will be switched to the protection APON interface module  301   b  to reduce revenue loss. The switching time is restricted so that connections on the failed working APON interface module  301   a  will not be dropped. For POTS service, the switching time should be less than 120 ms.  
         [0035]    The time consumed for protection switching is very critical for quality of signal in the APON interface. To switch APON interface modules at the OLT  601  will involve performing a ranging process for multiple ONUs  103  connected. Conventional ranging of ONUs is an inherently slow process, as described in Meredith Schelp, Xudong Wang, Wei Yen, and Elton Ho, “The Ranging Protocol for ATM Passive Optical Networks: Analysis and Improvements,”  Annual Multiplexes Telephony Conference  (AMTC) 2000 Proceedings, July 2000.  
         [0036]    An alternative procedure called pre-ranging, will be described. An EMS  104  orchestrates the whole process of pre-ranging. Pre-ranging is performed at the system setup period or period when the least traffic is running in the system. During system setup time, no live data traffic is running in the system. The protection APON interface module  301   b  can be operated as a working module for the pre-ranging purpose. The working APON interface modules  301   a  are switched to a standby mode one by one until all n modules are switched. The equalization data obtained for the standby module will be stored in a memory in the CCC. The advantage of this method is that all equalization data for the ONUs  103  associated with each working APON interface module  301   a  are readily available in the CCC memory. This will lead to fast protection switching. However, it will not always be the case that a complete new system set up can be performed. System upgrading and adding ONUs to an existing APON interface will complicate the pre-ranging process.  
         [0037]    A second process for ranging during protection switching, called uni-ranging, will be described. Instead of ranging every ONU  103 , only one ONU  103  associated with a failed APON interface module  301   a  will be ranged after protection switching. Since the 1:n protection at an OLT  101  is tree switching, the distance differences between to the various ONUs  103  remains intact. That is, although the total distance from the protection APON interface module  301   b  to a particular ONU number i may differ from the distance from the failed APON interface module to that same ONU, the differences between the distances to any two ONUs  103  is the same for both the failed APON interface module and the protection APON interface module. Therefore, after protection switching, one ONU chosen from the group can be ranged first. By comparing the previous equalization data stored at the CCC to the newly obtained range, one can calculate the equalization data for other ONUs  103  associated with the failed APON interface module. Uni-ranging speeds up the automatic switching dramatically by reducing the multiple ranging processes to one. Fine adjustment for equalization data will be performed periodically as specified by ITU G.983.1.  
         [0038]    Referring now to FIG. 5, a protection management procedure will be described. The procedure is initiated by the CCC at step  901 . At step  902 , data associated with the working APON interface modules  301   a ,  301   b  are copied to a memory in the CCC.  
         [0039]    At step  903 , a determination is made as to whether any of the working APON interface modules  301   a  is in an alarm state indicating a failure. If the determination at step  903  is negative, the process returns to step  902 . If the determination at step  903  is positive, the process proceeds to step  904 .  
         [0040]    At step  904 , a determination is made as to whether more than one working APON interface module is in an alarm state indicating a failure. If the determination at step  904  is negative, the procedure proceeds directly to step  906 . If the determination at step  904  is positive, the procedure proceeds to step  905 .  
         [0041]    At step  905 , a determination is made about which of the multiple failed APON interface modules to protect. This determination may be made, for example, by determining which failed APON interface module was handling the greatest amount of traffic. Alternatively, this determination may be made by determining which failed APON interface module was handling the traffic with the highest priority. After the determination at step  905 , the procedure proceeds to step  906 .  
         [0042]    At step  906 , the protection APON interface module  301   b  receives from the CCC a copy of the data for ONUs  103  connected to the failed APON interface module. The data has been previously stored at the CCC and updated periodically through synchronization functions running at the CCC and APON interface modules.  
         [0043]    At step  907 , control is switched from the failed APON interface module to the protection APON interface module  301   b . In the embodiment shown in FIGS. 3 a  and  3   b , a signal is sent to the 1:n optical switch  602  to make a connection between the protection APON interface module  301   b  and the 2:N splitter  303  associated with the failed APON interface module. In the embodiment shown in FIG. 4, a signal is sent to one of the 2:1 optical switches to make a connection between the protection APON interface module  301   b  and the 1:N splitter  803  associated with the failed APON interface module.  
         [0044]    At step  908 , the uni-ranging process begins.  
         [0045]    Referring now to FIG. 6, the details of the uni-ranging process will be described. At step  10 , an ONU  103  from the group associated with the failed APON interface module is chosen for uni-ranging. For example, the selected ONU can be the one with the shortest distance or the smallest serial number. The chosen ONU will be referred as the uni-ONU.  
         [0046]    At step  20 , the distance difference represented by phase equalization data will be calculated based on the chosen uni-ONU. The distance may be calculated as follows:  
         Δ Td   ji   =Td   j   −Td   i ;  
         [0047]    where j ∈ [1, N], and i represents the uni-ONU number and N is the splitter ratio of the 2:N splitter  303 , for example.  
         [0048]    At step  30 , the ranging of the uni-ONU is performed. If the uni-ONU&#39;s serial number is known and stored in the CCC and transferred to the protection APON interface module, the ranging mask may be sent to the uni-ONU only, thus avoiding a time-consuming binary tree search. After ranging, the new phase equalization data, Td i ′, for the uni-ONU should be obtained.  
         [0049]    At step  40 , new distances of the rest of the ONUs  103  associated with the failed APON interface module are calculated. Based on the new phase equalization data of the uni-ONU, all distances for the other ONUs  103  which are associated with the failed APON interface module can be calculated. The new distances may be calculated as follows:  
           Td   j   ′=Td   i   ′+ΔTd   ji ;  
         [0050]    where j ∈ [1, N], and i represents the uni-ONU number, N is the splitter ratio of the 2:N splitter  303 , for example, and Td j ′ is the new phase equalization data for ONU number j.  
         [0051]    At step  50 , the new equalization data, Td j ′, is sent to the ONUs  103  with triple redundancy from the protection APON interface module  301   b  at the OLT  601 . The ONUs  103  will use the new value Td j ′ for distance compensation.  
         [0052]    At step  60 , the ONUs  103  are set into operational status.  
         [0053]    While the invention has been described in its preferred embodiments, it is understood that the words which have been used are words of description, rather than limitation, and that changes may be made without departing from the true scope and spirit of the invention in its broader aspects. Thus, the scope of the present invention is defined by the claims that follow.