Abstract:
An Ethernet passive optical network (EPON) ring for providing protection against fiber failures. The optical network unit (ONU) is coupled to the ring fiber by a three-port passive optical splitting module that has three two-way optical passages. By the three two-way optical passages, the OUN receives/transmits data from/to the two ends of the optical line termination (OLT) to provide protection while the fiber failure. Moreover, it provides better authorization of users and simpler collision detection by the two-way transmission of the three-port passive optical splitting module to prevent hackers from invading and to reduce collisions.

Description:
This Non-provisional application claims priority under 35 U.S.C. §119(a) on Patent Application No(s). 092129937 filed in Taiwan on Oct. 28, 2003, the entire contents of which are hereby incorporated by reference. 
   BACKGROUND OF THE INVENTION 
   1. Field of Invention 
   The invention relates to an Ethernet passive optical network ring used in a local optical network system. In particular, the invention relates to an Ethernet passive optical network ring that can prevent the whole system from breaking when the network ring fails without the need of additional active or passive optical devices in any form. 
   2. Related Art 
   The structure of a normal Ethernet passive optical network (EPON) ring is shown in  FIG. 1 . It uses an optical line termination (OLT)  11  to manage several optical network units (ONU)  12 . The two ends of an optical ring  10  are connected to the OLT  11 . Each of the ONU  12  uses an optical splitter  13  to connect to the optical ring  10 . Therefore, controlled by the OLT  11 , they can receive/transmit data. Such an optical network structure is usually suitable for systems with low-density users. 
   Due to intrinsic defects, the system breaks as the optical ring  10  fails. With reference to  FIGS. 2A and 2B , the optical splitter  13  only provides one-way transmissions for the ONU  12 . In other words, the OLT  11  can transmit data only in the clockwise direction along the optical ring  10  (see the arrows shown in  FIGS. 1 and 2A ), and receive data in the counterclockwise direction along the optical ring  10  (see the arrows shown in  FIGS. 1 and 2B ). Therefore, once the optical ring  10  breaks, as shown in  FIG. 3 , the downstream ONU also breaks. The drawing shows in order the first ONU  121 , the second ONU  122 , the third ONU  123 , the fourth ONU  124 , and the fifth ONU  125 . When the optical ring  10  breaks between the third ONU  123  and the fourth ONU  124 , both the fourth ONU  124  and fifth ONU  125  break. 
   To solve this problem, the U.S. Pat. No. 6,327,400 proposed a switching method to let the optical splitter couple to one end. When the network breaks, it provides a temporary solution. However, the switching method increases device costs and involves higher complexity in controls. 
   On the other hand, since the optical ring  10  only provides a single ONU  12  to transmit data at a time, collisions will happen when other ONU  12  are using the optical ring  10  to transmit data or perform authorization at the same time. Consequently, one needs to make collision detection beforehand to reduce collisions. With reference to  FIG. 4 , a 3×N optical splitter  142  connects two of the three ports on the left-hand side (LHS) using an isolator  141 . When light sends data from the first ONU  121  at the port on the right-hand side (RHS) to the optical splitter  142 , the connected ports on the LHS reflect the signal because of the isolator  141 . The reflected signal passes through the optical splitter  142  and reaches the second ONU  122 . The second ONU  122  has a wavelength division multiplexing (WDM) system  144 , an optical receiving unit  142 , a coupler  145 , an optical transmitting unit  147 , and a carrier sensor  146 . The carrier sensor  146  receives the returned signal. The advantage of this method is to avoid collisions as a result of sending several signals at the same time the first ONU  121  transmits data. However, one has to include in addition a 3×N optical splitter  142  and an isolator  141 . This does not only increase the cost but also makes the system structure more complicated. 
   SUMMARY OF THE INVENTION 
   In view of the foregoing, the invention provides an Ethernet passive optical network ring and its method of authorization and collision detection. It can prevent the whole system from breaking when the network ring fails without the need of any new type of active or passive optical devices. With this structure, the invention can more effectively verify log-in users and prevent hackers from invading the system. With any additional devices, the invention can provide collision detection to reduce chances of network collisions. 
   According to the disclosed network ring and its authorization and collision detection method, the structure contains an optical ring, an optical line termination (OLT), several optical network units (ONU), and corresponding three-port passive optical splitting modules. The three-port passive optical splitting modules are installed at the intersections of the ONU and the optical ring. Each of the three-port passive optical splitting modules contains three optical ports, which are connected via three two-way passages so that the ONU can transmit/receive data via the two ends of the optical ring to/from the OLT. Therefore, even if the network ring breaks, the downstream ONU can still transmit/receive data using the other end. 
   Using the disclosed structure, one can determine the location of a user from the difference between the two times a signal is transmitted from an ONU to the two ends of the OLT, thereby verifying the user&#39;s identity. Since the invention uses the three-port passive optical splitting modules, any ONU can simultaneously transmit/receive data while another one is using the network. This can be used to detect collisions and to reduce collisions. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will become more fully understood from the detailed description given hereinbelow illustration only, and thus are not limitative of the present invention, and wherein: 
       FIG. 1  is a schematic view of a conventional Ethernet passive optical network ring; 
       FIGS. 2A and 2B  are schematic views of the passages of conventional optical splitters; 
       FIG. 3  is a schematic view of a broken conventional Ethernet passive optical network ring; 
       FIG. 4  is a schematic view of a conventional structure for detecting collisions; 
       FIG. 5  is a schematic view of the disclosed optical splitter; 
       FIGS. 6A and 6B  show an embodiment of the disclosed optical splitter; 
       FIG. 7  is a schematic view of the protecting state of the disclosed Ethernet passive optical network ring; 
       FIG. 8  is a schematic view of the breaking point detection state of the disclosed Ethernet passive optical network ring; 
       FIG. 9  is a schematic view of the disclosed OLT; 
       FIG. 10  is a schematic view of user authorization according to the invention; 
       FIG. 11  is a schematic view showing how the invention prevent hackers from invading the system; and 
       FIG. 12  is a schematic view of the collision detection state according to the invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   In view of the drawbacks in the conventional Ethernet passive optical network ring, the invention uses a two-way transmission structure to guarantee that the downstream network still has a connection with the optical line termination (OLT)  11  even if the network has a failure. Moreover, it avoids the use of the switching method introduced in the prior art. The invention can directly perform two-way transmissions without the introduction of any new type of active or passive optical devices. 
   As shown in  FIG. 5 , the invention mainly utilizes a three-port passive optical splitting module  24  at the intersection of an optical network unit (ONU)  23  and the optical ring  22 . The three-port passive optical splitting module  24  has three ports (the first port  221 , the second port  222 , and the third port  223  shown in the drawing) to connect the ONU  23  and the optical ring  22 . The three-port passive optical splitting module  24  uses three passages (the first passage  241 , the second passage  242 , and the third passage  243  in the drawing) to connect the first port  221 , the second port  222 , and the third port  223 , respectively. Therefore, the three ports can pass data back and forth simultaneously. If the network ring breaks, the downstream ONU  23  can still transmit data via the other end (to be described later). 
   An embodiment of the three-port passive optical splitting module  24  is shown in  FIG. 6A . Three optical splitters  247  along with three sub-fibers  248  are employed to achieve three two-way passages. On the other hand, one can also directly use a plane-wave waveguide  249  to achieve the same goal, as shown in  FIG. 6B . Of course, these are only examples of the invention; people skilled in the art can make any other variations without departing from the spirit of the invention. 
   We use an example to explain the invention. With reference to  FIG. 7 , the Ethernet passive optical network ring contains an OLT  21 , an optical ring  22 , several ONU (the first ONU  231 , the second ONU  232 , the third ONU  233 , the fourth ONU  234 , and the fifth ONU  235  shown in the drawing), and corresponding three-port passive optical splitting modules  24 . The optical ring has a first end  244  and a second end  245  connecting to both ends of the OLT  21  to form a closed ring. The optical ring  22  is also coupled with the first ONU  231 , the second ONU  232 , the third ONU  233 , the fourth ONU  234 , and the fifth ONU  235 , thus defining five intersections. The associated three-port passive optical splitting modules are installed at these intersections, providing two-way transmissions. In other words, the system can transmit/receive data via the first end  244  and the second end  245  (in the clockwise and counterclockwise directions following the arrows shown in the drawing). 
   Suppose the optical ring  22  breaks (i.e. when there is a breaking point  246  on the ring) between the third ONU  233  and the fourth ONU  234  as shown in the drawing. The first ONU  231 , the second ONU  232 , and the third ONU  233  can still use the first end  244  of the optical ring  22  to receive data in the clockwise direction and to transmit data in the counterclockwise direction. The fourth ONU  234  and the fifth ONU  235  use the second end  245  of the optical ring  22  to receive data in the counterclockwise direction and to transmit data in the clockwise direction. 
   As shown in  FIG. 8 , one can also determine the location of the breaking point  246  according to the data transmission directions. In the current example, the signals received by the OLT  21  via the first end  244  are only from the first, second, and third ONU  231 ,  232 ,  233 , while those received by the OLT  21  via the second end  245  are only from the fourth and fifth ONU  234 ,  235 . Therefore, the breaking point  246  is seen to be between the third and fourth ONU  233 ,  234 . One can also use the design of lights as shown in Table 1 to identify the breaking point  246  more easily. 
   
     
       
             
             
             
             
             
             
             
           
             
             
             
           
         
             
                 
               TABLE 1 
             
             
                 
                 
             
           
           
             
                 
                 
               231 
               232 
               233 
               234 
               235 
             
             
                 
               244 
               □ 
               □ 
               □ 
               ▪ 
               ▪ 
             
             
                 
               245 
               ▪ 
               ▪ 
               ▪ 
               □ 
               □ 
             
           
        
         
             
                 
               ↑ 
                 
             
             
                 
               246 
             
             
                 
                 
             
           
        
       
     
   
   To facilitate controls, the OLT  21  can be designed to contain a main server  211  and a backup server  212  connecting to the first end  244  and the second end  245  of the optical ring  22 , respectively (see  FIG. 9 ). Normally, the backup server  212  also receives signals and data but does nothing with them. All the processes are done by the main server  211 . The backup server  212  starts processing only when there is a breaking point  246 . 
   Since the disclosed structure supports two-way transmissions, the system can be used to develop an authorization method for different users. As shown in  FIG. 10 , when the user at the first ONU  231  logs into the system, the OLT  21  receives the signal from both the first end  244  and the second end  245 . One can use the receiving times (t 1  and t 2 ) to compute the difference td=|t 2 −t 1 |. The difference td is used to verify the user&#39;s identity. t 1  and t 2  have another relation that their sum is the time it takes a signal to travel around the optical ring  22 . One thus gets the round-trip signal traveling time is twice t 1  plus td (assuming t 1  is smaller). This relation thus can be used to help verifying the user, preventing hackers from forging signal transmission time. Of course, the difference td can be used to first locate the user before authorization. For convenience, the difference td of each ONU  23  can be recorded so that, in the future, the system only needs to compare the user data with the corresponding difference td. 
   In the following, we use an example to explain the invention. With reference to  FIG. 11 , suppose the user logs in via the first ONU  231 , gets authorized, and temporarily leaves the position. If a hacker uses the user&#39;s identity to enter the network from the second ONU  232 , the difference th is obviously different from the difference td of the first ONU  231 . Therefore, the OLT  21  determines it as an illegal invasion. Even if the identity data are entered correctly, the network can still catch the hacker. 
   As shown in  FIG. 12 , the invention utilizes the three-port passive optical splitting module  142  to perform simultaneous two-way transmissions. Thus, the information sent from the first ONU  231  enters the second ONU  232  too. The second ONU  232  has its WDM system  251  to transmit the information to its coupler  255 , but not the optical receiving unit  252  (this is because the wavelength of the transmitted and received signal is different). The coupler  255  passes the signal to the carrier sensor  254 , which uses a threshold for determination. If it is over the threshold, it means that other users are transmitting data too and therefore the second ONU  232  is not allowed to transmit data. Moreover, the carrier sensor  254  can contain a low-pass filter and a threshold sensor for a more precise determination. The low-pass filter first checks the received signal, and the threshold sensor makes the decision. Since collisions mostly happen when various users log in at the same time, therefore, the disclosed mechanism can effectively prevent collisions from occurring. 
   Certain variations would be apparent to those skilled in the art, which variations are considered within the spirit and scope of the claimed invention.