Abstract:
A test apparatus and method for testing passive optical networks is provided. The test apparatus includes an optical circuit having an optical coupler for splitting off a portion of optical traffic. During testing of a passive optical network, the optical circuit is coupled into an optical path of the passive optical network. A bit stream corresponding to an activating procedure is captured and analyzed to extract identification information of the module that sent the bit stream.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present invention claims priority from U.S. Patent Application No. 61/863,129 filed Aug. 7, 2013, which is incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The present invention relates to passive optical networks, and in particular to passive optical networks test equipment and methods. 
     BACKGROUND 
     Passive optical networks (PONs) are point-to-multipoint networks, in which a central optical line terminal (OLT) is connected with a plurality of optical network units (ONUs) by means of an optical distribution network (ODN) including spans of optical fiber connected by optical splitters and couplers (OSCs). Typically, a single OLT controls the communication with all ONUs installed in a PON, broadcasting downstream signals to the ONUs, and organizing upstream communication from the ONUs to the OLT using time-domain multiple access (TDMA). In TDMA, each ONU is assigned a time slot, within which the ONU may transmit an upstream signal. The OLT is configured to ensure that upstream transmissions from different ONUs do not collide, that is, do not occupy a same time slot. Due to the TDMA organization of a PON, upstream transmissions from ONUs to the OLT are usually bursty in nature, while the downstream transmissions from the OLT to ONUs tend to be more continuous. A technician servicing a PON travels to various locations of the ODN, and checks optical power levels at those locations, to ensure that optical signals propagate to appropriate ODN destinations with acceptable optical loss. Both average optical power levels and peak optical power levels may be measured and recorded. Peak optical power levels may be useful for characterization of bursty upstream signals. 
     Referring to  FIG. 1 , a prior-art PON test device  100  is shown. The PON test device  100  was disclosed by Ruchet in US Patent Application Publication 2006/0171711 A1. The PON test device  100  includes a 2×2 coupler  32 , a wavelength division multiplexor (WDM)  68 , and first  38 , second  42 , and third  44  photodetectors for detecting optical signals at wavelengths of 1310 nm, 1490 nm, and 1550 nm, respectively. 
     In operation, the test device  100  is coupled in an optical path between an OLT  110  and an ONU  120 . The OLT  110  generates downstream optical signals S 2  at 1490 nm and S 3  at 1550 nm, which are coupled to a first through port  28  of the 2×2 coupler  32 . The 2×2 coupler  32  power-splits the downstream optical signals S 2  and S 3 . Eighty percent (80%) of optical power of the downstream optical signals S 2  and S 3  are coupled to a second through port  30  of the 2×2 coupler  32 . Then, the signals S 2  and S 3  (attenuated by 20% by the 2×2 coupler  32 ) propagate to the ONU  120 . Twenty percent (20%) of the optical power of the downstream optical signals S 2  and S 3 , denoted in  FIG. 1  as S 2 ′ and S 3 ′, are coupled to a first drop port  36  of the 2×2 coupler  32 . The first drop port  36  is coupled to the WDM  68 . The WDM  68  separates the signals S 2 ′ and S 3 ′, directing the resulting signals S 2 ″, S 3 ″ to the second  42  and third  44  photodetectors, respectively. 
     The ONU  120  generates an upstream optical signal S 1  at 1310 nm, which is coupled to the second through port  30  of the 2×2 coupler  32 . The 2×2 coupler  32  power-splits the upstream optical signal S 1 . Eighty percent (80%) of the optical power of the upstream optical signal S 1  is coupled to the first through port  28  of the 2×2 coupler  32 , and the attenuated signal S 1  propagates to the OLT  110 . Twenty percent (20%) of the optical power of the upstream optical signal S 1 , denoted in  FIG. 1  as S 1 ′, is coupled to a second drop port  34  of the 2×2 coupler  32 . The split signal S 1 ′ is coupled to the first photodetector  38 . As a result, the optical power of the signals S 1 , S 2 , and S 3  propagating between the OLT  110  and the ONU  120 , may be measured without breaking an optical link between the OLT  110  and the ONU  120 . 
     In a PON, a large number of ONUs may be connected to a single OLT. When the PON is expanded or reconfigured, some ONUs remain connected to the ODN, and some ONUs are transferred to be connected to different optical fibers within the ODN. Due to a high reconfiguration rate of the PON, and due to a large number of optical connections within the PON, network operators are increasingly facing a problem that network documentation is not synchronized with a current configuration of the PON, making network servicing difficult. It is not uncommon for a service technician to call the central office for a network update, and/or disconnect individual subscribers successively one by one, in an attempt to find a correct optical fiber to take an optical power measurement. Besides being tedious and prone to misconnection errors, this procedure is disruptive to subscribers. 
     SUMMARY 
     A testing apparatus according to the invention may capture activation bit streams between ONU and OLT, and analyze the activation bit streams to identify network equipment participating in the communication. 
     In accordance with an aspect of the invention, there is provided an apparatus for testing a passive optical network comprising an optical line terminal, a plurality of optical network units including a first optical network unit, and an optical distribution network for carrying bidirectional optical traffic between the optical line terminal and the first optical network unit, the bidirectional optical traffic comprising downstream traffic from the optical line terminal to the first optical network unit, and upstream traffic from the first optical network unit to the optical line terminal, wherein the optical line terminal is configured to use an activation procedure for establishing communication with the first optical network unit, the apparatus comprising: 
     an optical circuit comprising an optical coupler and first and second input optical ports for optically coupling the optical circuit into the optical distribution network between the optical line terminal and the first optical network unit, the optical coupler comprising first and second through ports for carrying the bidirectional optical traffic therebetween, and a first drop port for dropping a portion of the upstream traffic, wherein the first through port is optically coupled to the first input optical port; 
     an upstream receiver optically coupled to the first drop port for receiving the upstream traffic portion; and 
     a controller operationally coupled to the upstream receiver and the optical breaker switch and configured to: 
     capture the upstream traffic portion received by the upstream receiver during the activation procedure; and 
     extract identification information of the first optical network unit from the captured upstream traffic portion. 
     In one exemplary embodiment, the optical circuit further comprises an optical breaker switch serially coupled to the optical coupler, the optical breaker switch comprising a first switch port optically coupled to the second through port of the optical coupler, and a second switch ports optically coupled to the second input optical port; 
     wherein the controller is further configured to cause the optical breaker switch to uncouple and then re-couple the first and second switch ports to cause a temporary interruption of the bidirectional optical traffic therebetween, thereby initiating the activation procedure between the optical line terminal and the first optical network unit upon re-coupling of the first and second switch ports. 
     In one exemplary embodiment, the optical coupler further comprises a second drop port for dropping a portion of the downstream traffic, and the apparatus further includes a downstream receiver optically coupled to the second drop port of the optical coupler for receiving the downstream traffic portion. The controller may be operationally coupled to the downstream receiver and configured to capture the downstream traffic portion received by the downstream receiver after the interruption, during the activation procedure, and to extract identification information of the optical line terminal from the captured downstream traffic portion. 
     In accordance with an embodiment of the invention, there is further provided a method for testing a passive optical network comprising an optical line terminal, a plurality of optical network units including a first optical network unit, and an optical distribution network for carrying bidirectional optical traffic between the optical line terminal and the first optical network unit, the bidirectional optical traffic comprising downstream traffic from the optical line terminal to the first optical network unit, and upstream traffic from the first optical network unit to the optical line terminal, wherein the optical line terminal is configured to use an activation procedure for establishing communication with the first optical network unit, the method comprising: 
     (a) coupling an optical circuit into the optical distribution network between the optical line terminal and the first optical network unit, the optical circuit including an optical coupler comprising first and second through ports for carrying the bidirectional optical traffic therebetween, and a first drop port for dropping a portion of the upstream traffic; 
     (b) initiating the activation procedure between the optical line terminal and the first optical network; 
     (c) capturing the portion of the upstream traffic after the interruption of step (b), during the activation procedure; and 
     (d) extracting identification information of the first optical network unit from the upstream traffic portion captured in step (c). 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Exemplary embodiments will now be described in conjunction with the drawings, in which: 
         FIG. 1  illustrates a schematic view of a prior-art PON optical tester; 
         FIG. 2  illustrates an exemplary schematic view of a PON being tested in accordance with the invention; 
         FIG. 3  illustrates an exemplary schematic view of a test apparatus according to one embodiment of the invention; 
         FIGS. 4A to 4D  illustrate exemplary schematic views of a test apparatus of the invention, including a single photodiode for each of the upstream and downstream paths ( FIGS. 4A and 4B ); a power splitter and two photodiodes for each of the upstream and downstream paths ( FIG. 4C ); and a common optical filter, power splitter, and two photodiodes for each of the upstream and downstream paths ( FIG. 4D ); 
         FIG. 5  illustrates an exemplary flow chart of a method for testing a PON according to the invention; 
         FIG. 6A  is a schematic data structure diagram indicating a location of an ONU identification number (ONU-ID) and an ONU serial number (ONU-SN) in a gigabit-PON (G-PON) upstream data burst; 
         FIG. 6B  is a schematic data structure diagram indicating a location of an ONU identification number (ONU-ID) and an ONU serial number (ONU-SN) in a 10 gigabit-capable PON (XG-PON) upstream data burst; 
         FIG. 6C  is a schematic data structure diagram indicating a location of an ONU network registration number (REG-ID) in a 10 gigabit-capable PON (XG-PON) upstream data burst; 
         FIG. 6D  is a schematic data structure diagram indicating a location of a PON identification number (PON-ID) in a 10 gigabit-capable PON (XG-PON) downstream data frame; 
         FIG. 6E  is a schematic data structure diagram indicating a location of a PON identification number (PON-ID) in a gigabit PON (G-PON) downstream data frame; 
         FIG. 6F  is a schematic data structure diagram showing a logical link ID (LLID) and a source MAC address (SA) in an upstream burst of a 10 GB/s Ethernet PON (10G-EPON); 
         FIG. 6G  is a schematic data structure diagram showing a logical link ID (LLID) and a source MAC address in a downstream frame of a 10 GB/s Ethernet PON (10G-EPON); 
         FIG. 7A  illustrates an exemplary flow chart of a method for extracting an ONU-ID of  FIG. 6A  from an upstream burst in a G-PON; 
         FIG. 7B  illustrates an exemplary flow chart of a method for extracting an ONU-ID of  FIG. 6B  and a REG-ID of  FIG. 6C  from an upstream burst in an XG-PON; 
         FIG. 7C  illustrates an exemplary flow chart of a method for extracting a PON-ID of  FIG. 6E  from a downstream frame in a G-PON; 
         FIG. 7D  illustrates an exemplary flow chart of a method for extracting a PON-ID of  FIG. 6D  from a downstream frame in an XG-PON; 
         FIG. 7E  illustrates an exemplary flow chart of a method for extracting an LLID and a MAC address of  FIG. 6F  from an upstream burst of an EPON and 10G-EPON; 
         FIG. 7F  illustrates an exemplary flow chart of a method for extracting a MAC address of  FIG. 6G  from a downstream frame of an EPON and 10G-EPON; and 
         FIG. 8  illustrates a PON summary screen shown on a display of the testing device of  FIG. 3 . 
     
    
    
     DETAILED DESCRIPTION 
     While the present teachings are described in conjunction with various embodiments and examples, it is not intended that the present teachings be limited to such embodiments. On the contrary, the present teachings encompass various alternatives and equivalents, as will be appreciated by those of skill in the art. 
     Referring to  FIG. 2 , an exemplary network system  200  includes a PON under test  211 . The PON  211  may include a plurality of ONUs  206 . 1  . . .  206 . m  and  208 . 1 ,  208 . 2 , . . .  208 . n  optically coupled to an OLT  202  via an ODN  210 . The ODN  210  typically includes only passive components, for instance optical fibers  203 , optical power splitters/combiners (OSC)  204  and  205 , and the like. The OLT  202  is typically connected to another network  201 , e.g. an Ethernet™ network, which serves as an external source and recipient of communications. The ONUs  206 . 1  . . .  206 . m ,  208 . 1 ,  208 . 2 , . . .  208 . n  may be connected with respective subscriber networks or subscriber devices  207 . 1  . . .  207 . m ,  209 . 1 ,  209 . 2 , . . .  209 . n , which may be a source and recipient of payload data transmitted in the PON from the subscriber&#39;s side. The terms “optical network unit” or “ONU” and “optical network terminal” or “ONT” are used interchangeably herein, because they behave similarly with respect to embodiments of this invention. ONUs are usually deployed near a group of customer premises or “at a curb”, whereas ONTs are usually deployed at individual customer premises. For simplicity, the term “ONU” is selected to denote both ONU and ONT, unless specified otherwise. It is to be noted that the ONU(s)  206 . 1  to  206 . m  and  208 . 1  to  208 . n  may receive and transmit data from and to the PON  211 , and may be connected to other networks, such as Ethernet, digital subscriber lines, standard telephones (PSTN), or network devices, such as computer terminals, video devices, Ethernet units, and the like. 
     In operation, the ODN  210  carries bidirectional optical traffic between the OLT  202  and the ONUs  206 . 1  to  206 . m ,  208 . 1  to  208 . n  using standard communication protocols, e.g. those described in ITU-T G.984/7 or IEEE 802.3ah/av communication standards. The bidirectional optical traffic includes downstream traffic  212  and upstream traffic  213 . In a typical network configuration, the downstream traffic carries data broadcast by the OLT  202  to the ONUs  206 . 1  to  206 . m ,  208 . 1  to  208 . n , such as, for example, television data, as well as data intended to individual ONUs  206 . 1  to  206 . m ,  208 . 1  to  208 . n , tagged with identification data to identify intended recipients. The downstream traffic  212  is power divided by the OSC  204 ,  205 . In general, a hierarchy of OSCs  204 ,  205  may exist in the ODN  210 . Each of the OSCs  204 ,  205  power-divides the downstream traffic  212 , typically equally between its output legs. 
     The upstream traffic  213  is commonly organized using TDMA. The OLT  202  operates as a “master”, which assigns a time slot to each of the ONU(s)  206 . 1  to  206 . m ,  208 . 1  to  208 . n , during which an upstream transmission may be performed. The OSCs  204 ,  205  combine all signals arriving at downstream facing ports into the combined upstream traffic  213 , in which all of the upstream signal bursts e.g.  214 ,  215 ,  216 , arrive properly separated. For example, the signal burst  214  is sent from the “first” ONU  206 . 1  to the OLT  202 . Herein, the terms “first”, “second”, and the like are not intended to denote the order of appearance, but are merely used as identifiers. 
     The PON  211  may be tested by disconnecting optical fibers at a test point  217 , and inserting a test apparatus  300  by coupling its first  301  and second  302  input optical ports to the disconnected optical fibers. In this manner, the downstream traffic  212  and the upstream traffic  214  between the OLT  202  and the first ONU  206 . 1  can be made to flow through the test apparatus  300 . From here on in, the term “upstream traffic” will be used to identify the upstream burst  214 , which carries upstream information sent by the “first” ONU  206 . 1  to the OLT  202 . It is to be understood that tests can be performed by inserting the test instrument  300  at another test point, not shown, of the ODN  210 . In other words, the test point  217  and the first ONU  206 . 1  are considered only by way of a non-limiting example, which is given to illustrate operation of the test apparatus  300 . 
     Referring to  FIG. 3 , the test apparatus  300  has an optical circuit  310 , which may include an optical coupler  304  and an optical breaker switch  303  serially coupled together between the first  301  and second  302  input optical ports. The optical circuit  310  is shown coupled into the ODN  210  at the test point  217  between the OLT  202  and the first ONU  206 . 1  ( FIG. 2 ). The optical coupler  304  ( FIG. 3 ) includes first  331  and second  332  through ports for carrying the bidirectional optical traffic between the first  331  and second  332  through ports. The optical coupler  304  further includes a first drop port  341  for dropping a portion  214 A of the upstream traffic  214 . The first through port  331  is coupled to the first input optical port  301 . 
     The optical breaker switch  303  includes first  361  and second  362  optical ports, termed herein “switch ports”. The optical breaker switch  303  is configured to optically couple and uncouple the first  361  and second  362  switch ports in a controllable fashion. For example, the coupling and uncoupling may be performed in response to a command e.g. an electrical signal on a control line  363 . The first switch port  361  is optically coupled to the second through port  332  of the optical coupler  304 , and the second switch port  362  is optically coupled to the second input optical port  302 . The optical breaker switch  303  may be also coupled between the first through port  331  and the first optical port  301 . 
     An upstream receiver  306  is optically coupled to the first drop port  341  of the optical coupler  304  for receiving the upstream traffic portion  214 A. A controller  320  is operationally coupled to the upstream receiver  306 . The controller  320  may be coupled to the optical breaker switch  303  via the control line  363 . The controller  320  may be configured to cause the optical breaker switch  304  to uncouple and then re-couple the first  361  and second  362  switch ports, so as to cause a temporary interruption of the bidirectional optical traffic between the first  361  and second  362  switch ports. The purpose of this brief interruption, for example no longer than 1 second and more preferably between 200 ms and 500 ms, is to initiate an activation, or “discovery” procedure between the OLT  202  and the first ONU  206 . 1  upon re-coupling of the first  361  and second  362  switch ports. The activation procedure is known and corresponds to a communication standard used in a particular network. The controller  320  may be further configured to capture the upstream traffic portion  214 A received by the upstream receiver  306  after the interruption and during the activation procedure. The captured upstream traffic portion includes identification information of the first ONU  206 . 1 . The controller  320  may be configured to extract this information, thereby identifying the first ONU  206 . 1 . The upstream receiver  306  may include an optical power meter (PM)  370  for determining an optical power level of the upstream traffic  214  from the dropped upstream traffic portion  214 A. A display  321  may be further provided for displaying the optical power level of the upstream traffic  214  together with the identification information of the first ONU  206 . 1 . In one embodiment, an external display may be used. 
     The test apparatus  300  may be implemented without the optical breaker switch  303 . For this embodiment, the controller  320  may be configured to capture the upstream traffic portion  214 A received by the upstream receiver  306  upon coupling of the first  301  and second  302  optical connectors into the ODN  210  at the test point  217 , during the activation procedure, which automatically follows the coupling. It is preferred to include the optical breaker switch  303  into the optical circuit  310 , because the optical breaker switch  303  provides a controllable, repeatable interruption of the optical communication between the OLT  202  and the first ONU  206 . 1 . 
     In the embodiment shown in  FIG. 3 , the optical coupler  304  includes an optional second drop port  342  for dropping a portion  212 A of the downstream traffic  212 . In this embodiment, the apparatus  300  further includes a downstream receiver  307  optically coupled to the second drop port  342  of the optical coupler  304  for receiving the downstream traffic portion  212 A. The controller  320  may be operationally coupled to the downstream receiver  307  and configured to capture the downstream traffic portion  212 A received by the downstream receiver  307  after the interruption, preferably during the activation procedure, and to extract identification information of the OLT  202  and/or the PON  211  from the captured downstream traffic portion  212 A. The capturing and the identification information extraction from the downstream traffic portion  212 A does not have to take place during the activation procedure, and may be performed at any time during downstream transmission. 
     The downstream receiver  307  may further include its own optical power meter  370 . In this configuration, an optical power level of both the upstream  214  and downstream  212  traffic may be determined from the dropped respective upstream  214 A and downstream  212 A traffic portions. The optical power level of the upstream traffic  214  may be displayed on the display  321  together with the identification information of the first ONU  206 . 1 , and the optical power level of the downstream traffic  212  may be displayed on the display  321  together with the identification information of the OLT  202 . 
     Referring now to  FIG. 4A , a test apparatus  400 A is an embodiment of the test apparatus  300  of  FIG. 3 . A non-transitional memory  422  has stored computer instructions for the processor  320 . Captured bit streams and/or test results may also be stored in the non-transitional memory  422 . A user interface  421  may include the display  321  (not shown in  FIG. 4A ), along with a user input device such as a set of buttons, for example. In the embodiment shown in  FIG. 4A , the upstream receiver  306  of the test apparatus  400 A includes an upstream optical filter  408  coupled to the first drop port  341  of the optical coupler  304 , for selecting a wavelength band corresponding to the upstream traffic  214 . Similarly, the optional downstream receiver  307  may include a downstream optical filter  414  coupled to the second drop port  342  of the optical coupler  304 , for selecting a wavelength band corresponding to the downstream traffic  214 . By way of a non-limiting example, the downstream optical filter  414  may transmit wavelengths of 1490 nm±10 nm and/or 1578 nm±10 nm, and the upstream optical filter  408  may use wavelengths of 1270 nm±10 nm and/or 1310 nm±10 nm. The optical filters  408  and  414  may be based on thin film filters, metal-dielectric filters, color glass filters, and the like. 
     The upstream receiver  306  of the test apparatus  400 A may further include a photodetector  471  optically coupled to the upstream optical filter  408 , for providing an electrical signal in response to the upstream traffic portion  214 A. A burst mode amplifier  418  may be electrically coupled to the first photodetector  471  for broadband amplification of the electrical signal to provide an upstream traffic electrical waveform. A burst mode clock data recovery circuit  412  may be electrically coupled to the burst mode amplifier  418  and the processor  320 , for recovering clock data from the upstream traffic electrical waveform. In the embodiment shown, the upstream receiver  306  of the test apparatus  400 A also includes a pre-amplifier  410  coupled to a peak/average signal detector  411 , for determining a peak and/or average optical power of the upstream traffic  214 . Other types of amplifiers may be used. 
     The downstream receiver  307  of the test apparatus  400 A may also include its own photodetector  471  optically coupled to the downstream optical filter  414 , for providing an electrical signal in response to the downstream traffic portion  212 A. A broadband amplifier  419  may be electrically coupled to the photodetector  471  for broadband amplification of the electrical signal to provide an upstream traffic electrical waveform. A downstream clock data recovery circuit  413  may be electrically coupled to the broadband amplifier  419  and the processor  320 , for recovering clock data from the downstream traffic electrical waveform. The downstream receiver  307  of the test apparatus  400 A may also include its own pre-amplifier  410  coupled to the peak/average signal detector  411 , for determining a peak and/or an average optical power of the downstream traffic  212 . Many types of amplifiers may be used. Furthermore, the downstream clock data recovery circuit  413  may be operationally coupled to the burst mode clock data recovery circuit  412 , to facilitate the clock recovery of a bursty upstream signal. 
     Referring to  FIG. 4B , a test apparatus  400 B is a variant of the test apparatus  400 A of  FIG. 4A . In the apparatus  400 B of  FIG. 4B , the optical breaker switch  303  and the 2×2 optical coupler  304  are swapped with each other: the second through port  332  of the 2×2 optical coupler  304  is coupled to the second input optical port  302 , and the first through port  331  of the 2×2 optical coupler  304  is coupled to the optical breaker switch  303 , which is coupled to the first input optical port  301 . 
     Turning to  FIG. 4C , a test apparatus  400 C is a variant of the test apparatus  400 A of  FIG. 4A . In the test apparatus  400 C of  FIG. 4C , the upstream receiver  306  further includes an optical power splitter  401  coupled to the upstream optical filter  408 . A second upstream optical filter  409  may be optically coupled to the optical power splitter  401 , with a second photodetector  472  coupled to the second upstream optical filter  409 . The second photodetector  472  provides an electrical signal in response to the upstream traffic portion  214 A. The pre-amplifier  410  may be electrically coupled to the second photodetector  472 , for amplifying the electrical signal. The peak or average signal detector  411  may be coupled to the pre-amplifier  410  and the processor  320 , for detecting a peak or average value of the electrical signal. Together, the second photodetector  472 , the pre-amplifier  410 , and the peak or average signal detector  411  make up the optical power meter  370  of the upstream receiver  306  of  FIG. 3 . In  FIG. 4C , the upstream optical power meter  370  is coupled to the optical power splitter  401 , and is calibrated for determining the optical power level of the upstream traffic  214  from the detected portion  214 A. Using two photodetectors  408  and  409  allows one to individually optimize performance of light detection for power measurement and electrical signal waveform generation purposes. 
     In the embodiment of  FIG. 4C , the downstream receiver  307  also includes the optical power splitter  401 , which is coupled to the downstream optical filter  414 . A second downstream optical filter  415  may be optically coupled to the optical power splitter  401 , with the second photodetector  472  coupled to the second downstream optical filter  415 . The second photodetector  472  provides an electrical signal in response to the downstream traffic portion  212 A. Another pre-amplifier  410  may be electrically coupled to the second photodetector  472 , for amplifying the electrical signal. Finally, another peak or average signal detector  411  may be coupled to the pre-amplifier  410  and the processor  320 , for detecting a peak or average value of the electrical signal. Together, the second photodetector  472 , the pre-amplifier  410 , and the peak or average signal detector  411  make up the downstream optical power meter  370  of the downstream receiver  307  of  FIG. 3 . In the test apparatus  400 C of  FIG. 4C , the downstream optical power meter  370  is coupled to the optical power splitter  401 , and is calibrated for determining an optical power level of the downstream traffic  212  from the detected portion  212 A. 
     Referring now to  FIG. 4D , a test apparatus  400 D is a variant of the test apparatus  400 C of  FIG. 4C . In the apparatus  400 D of  FIG. 4D , the upstream optical filter  408  and the downstream optical filter  414  are placed before the power splitters  401 , alleviating the need for second upstream  409  and downstream  415  respective optical filters. 
     Turning to  FIG. 5  with further reference to  FIGS. 2 and 3 , a method  500  ( FIG. 5 ) for testing the PON  211  ( FIG. 2 ) includes a step  501  of coupling the optical circuit  310  of the test apparatus  300  into the ODN  210  between the OLT  202  and the first ONU  206 . 1 . In a step  502 , the optical breaker switch  303  is operated to open and then close the optical circuit  310 , so as to cause a temporary interruption of the bidirectional optical traffic in the optical circuit  310 . As a result of the interruption, an activation procedure is initiated between the OLT  202  and the first ONU  206 . 1  (and other ONUs  206 . 2  to  206 . m ,  208 . 1  to  208 . n ) upon closing the optical circuit  310 . In a step  503 , the portion  214 A of the upstream traffic  214  is captured after the interruption  502 , during the activation procedure. The portion  212 A of the downstream traffic  212  may also be captured in this step. The capturing may include storing the received activation bit stream(s) in the memory  422  of the test apparatuses  400 A to  400 D of  FIGS. 4A to 4D , respectively. In a step  504 , identification information of the first ONU  206 . 1  is extracted from the upstream traffic portion  214 A, captured in step  503 . Identification information of the OLT  202  and/or the PON  211  may also be extracted in this step. 
     In an optional step  505 , optical power levels of the upstream  214  and/or downstream  212  traffic may be determined. By way of an example, optical power meters  370  shown in  FIGS. 3, 4C, and 4D  may be used for this purpose. Finally, in an optional step  506 , the determined optical power levels may be displayed on the display  321  ( FIG. 3 ) along with the identification information of the first ONU  206 . 1  and the OLT  202 . In this manner, a user of the test apparatus  300  and  400 A to  400 D may associate the measured optical power levels with a particular ONU or OLT of the PON  211 . 
     Referring back to  FIG. 3  and  FIGS. 4A to 4D , the processor  320  may be configured to extract the ID information from the upstream and/or downstream traffic in the step  504  of the method  500 , to identify the ONU  206 . 1  to  206 . m ,  208 . 1  to  208 . n , and/or the OLT  202 . Specific implementation of the extracting step  504  of the method  500  of  FIG. 5  depends on a specific PON type. Data structures and extraction methods for most widely deployed PON types, i.e. GPON, XG-PON, EPON, 10G-EPON, are considered below with reference to  FIGS. 6A to 6G  and  FIGS. 7A to 7F . Only relevant data components will be described. 
     Referring first to  FIG. 6A , a structure of a typical GPON upstream burst  600 A is shown. The upstream burst  600 A includes a preamble portion  601 , which may have a dynamically defined length. To the right of the preamble portion  601 , there is a three byte long delimiter portion  602 , which denotes the start of a burst header portion  603 . The burst header portion  603  contains an ONU-ID field  605 , for identification of an ONU that generated the burst  600 A. Next to the burst header  603  there is a physical layer operation and maintenance upstream (PLOAMu) message  604 . In  FIG. 6A , the PLOAMu message  604  is a Serial_Number_ONU physical layer operation and maintenance (PLOAM) message as specified in the ITU-T G984.3 communications protocol. The Serial_Number_ONU PLOAM message  604  contains a one byte long ONU-ID field  606  and an eight bytes long ONU-SN field  607 , which holds a vendor-specific serial number of the ONU which has sent the upstream burst  600 A, e.g. the ONU  206 . 1  of  FIG. 2 . 
     Turning to  FIG. 6B , a structure of a typical XG-PON upstream burst  600 B is shown. The upstream burst  600 B includes an upstream physical synchronization block (PSBu)  610 , which may have a dynamically defined length. Next to the PSBu  610  a XG-PON transmission convergence (XGTC) header portion  611  is disposed. The XGTC header portion  611  contains a ten bit long ONU-ID field  612  and optionally a PLOAMu message  613 . In  FIG. 6B , the PLOAMu message  613  is a Serial_Number_ONU PLOAM message as specified in the standard ITU-T G987.3. The Serial_Number_ONU PLOAM message  613  contains a ten bit long ONU-ID field  614  and an eight byte long ONU-SN field  615  holding a vendor-specific serial number of the ONU which has sent the upstream burst  600 B, e.g. the ONU  206 . 1  of  FIG. 2 . 
     Referring to  FIG. 6C , a structure of a typical XG-PON upstream burst  600 C is similar to the PON upstream burst  600 B of  FIG. 6B , but instead of the PLOAM message  613  including the Serial_Number_ONU field  615  in  FIG. 6B , an upstream burst with a Registration PLOAM message  622  is shown in  FIG. 6C , pursuant to the ITU-T G987.3 communications protocol. The Registration PLOAM message  622  contains a thirty six bytes long REG-ID field  623 , which holds a registration identifier usable for identifying the ONU which has generated the upstream burst  600 C, e.g. the ONU  206 . 1  in the PON  211  of  FIG. 2 . 
     Turning to  FIG. 6D , a structure of a typical XG-PON downstream frame  600 D according to ITU-T G987.3 communications standard is illustrated. The frame  600 D includes a physical control block downstream (PSBd) structure  631 . The PSBd structure  631  contains a PON-ID structure  632 . The PON-ID structure  632  contains a fifty one bit long PON-ID field  633  holding a PON-ID of the OLT which has sent the downstream frame  600 D, e.g. the OLT  202  of  FIG. 2 . 
     Referring to  FIG. 6E , a structure of a typical G-PON downstream frame  600 E according to ITU-T G984.3 communications standard is presented. The G-PON downstream frame  600 E includes a PCBd structure  641 . The PCBd structure  641  contains a physical layer operation and management downstream (PLOAMd) message  642 . In  FIG. 6E , the PLOAMd message  642  includes a PON-ID PLOAM message  644  as specified in the ITU-T G984.3 communications protocol, Amendment 3. The PON-ID PLOAM message  644  contains a seven bytes long PON-ID field  643  holding a PON-ID of the OLT which has sent the downstream frame  600 E, e.g. the OLT  202  of  FIG. 2 . 
     Turning to  FIG. 6F , a structure of a typical EPON or 10G-EPON upstream burst  600 F is shown. The upstream burst  600 F comprises a delimiter bit pattern  651 , used for determining the start of an Ethernet frame  654 . The EPON or 10G-EPON upstream burst  600 F further includes a logical link identifier (LLID) field  652  and a fifteen bit long LLID value and a source address (SA) field  653 , which contains a media access control (MAC) address of the source transmitting the upstream frame. In EPON or 10G-EPON systems this source address may be used for identifying the ONU which has generated the upstream burst  600 F, e.g. the ONU  206 . 1  in the PON  211  of  FIG. 2 . 
     Referring now to  FIG. 6G , a structure of a typical EPON or 10G-EPON downstream frame  600 G is shown. The EPON or 10G-EPON downstream frame  600 G comprises an LLID field  661  and a SA field  662 , which contains a MAC address of the source of the downstream frame  600 G. In EPON or 10G-EPON systems, this source address may be used as an identification means for the OLT which has sent the downstream frame  600 G, e.g. the OLT  202  of  FIG. 2 . 
     Exemplary processes of capturing relevant bit streams and extracting the ONU and OLT identifiers will now be considered in detail with reference to  FIGS. 7A to 7F . These exemplary processes represent possible variants of the capturing 503 and extracting  504  steps of the method  500  ( FIG. 5 ). 
     Referring first to  FIG. 7A , an exemplary process  700 A may be used to obtain the ONU-SN  607  and the ONU-ID  605 ,  606  ( FIG. 6A ) from an upstream burst in a GPON system. The process  700 A ( FIG. 7A ) starts at  701 . In a step  702 , a first upstream burst sent from the first ONU  206 . 1  ( FIG. 3 ) is received and stored in the non-transitional memory  422  ( FIGS. 4A to 4D ) as “bit sequence A”. The bit sequence A may contain Serial_Number_ONU PLOAMu message  604  ( FIG. 6A ). In a step  703 , a second upstream burst sent from the first ONU  206 . 1  ( FIG. 3 ) is received and stored in the non-transitional memory  422  ( FIG. 4A ) as “bit sequence B”. Bit sequence B may include the ONU-SN  607 . In both bit sequences A and B, the delimiter bit pattern  602  may be used to determine the start of the burst header section  603  ( FIG. 6A ). In a step  704 , the ONU-ID  605 ,  606  and the ONU-SN  607  may be extracted from the bit sequence A and stored for further data processing. The ONU-ID  605 ,  606  of the bit sequence A may contain an “unassigned” ONU-ID, which is a default value for ONUs in the serial number state. In a step  705 , the ONU-ID  605 ,  606  field and the ONU-SN  607  may be extracted from the bit sequence B and stored for further data processing. Since the ONU-ID  605 ,  606  and the ONU-SN  607  may be obtained only from bit sequence B without processing the bit sequence A, the step  704  of processing the bit sequence A is optional. The process  700 A ends at  706 . 
     Turning to  FIG. 7B , a process  700 B may be used to obtain the ONU-SN  615 , ONU-ID  612 ,  614  ( FIG. 6B ), and the REG-ID  623  ( FIG. 6C ) of an ONU from an upstream burst in an XG-PON system. The process  700 B of  FIG. 7B  includes steps  711 ,  712 ,  713 ,  714 ,  715 , and  716  similar to the respective steps  701 ,  702 ,  703 ,  704 ,  705 , and  706  of the process  700 A of  FIG. 7A . One difference of the process  700 B of  FIG. 7B  is that, if the bit sequence B comprises a Registration PLOAM message, the REG-ID  623  ( FIG. 6C ) may be extracted and stored for further data processing in the step  714 . 
     Referring to  FIG. 7C , a process  700 C may be used to obtain the PON-ID  633  ( FIG. 6E ) from a downstream data frame in a GPON system. The process  700 C ( FIG. 7C ) starts at  721 . In a step  722 , a bit sequence is extracted from a downstream frame, which may be sent e.g. by the OLT  202  ( FIG. 3 ). The bit sequence may have the structure shown in  FIG. 6E . In a step  723 , a check is performed whether the bit sequence includes the PON-ID PLOAM message  632 , as specified in ITU-T G984.3 Amendment 3. If yes, the PON-ID  633  ( FIG. 6E ), that is, a network ID number, is extracted and stored for further processing in a step  724 . If the bit sequence does not comprise the PON-ID PLOAM message  632 , the bit sequence received is discarded and another downstream bit sequence is received. The process  700 C ends at  725 . 
     Turning to  FIG. 7D , a process  700 D may be used to obtain the PON-ID  643  ( FIG. 6D ) from a downstream data frame in a XG-PON system. The process  700 D ( FIG. 7D ) starts at  731 . In a step  732 , a received bit sequence is converted from a downstream frame, which may have been sent e.g. by the OLT  202  ( FIG. 3 ). The bit sequence may have the structure shown in  FIG. 6D . In a step  733 , the PON-ID  643  ( FIG. 6D ) is extracted from the bit sequence. The process  700 D ends at  734 . 
     Referring to  FIG. 7E , a process  700 E may be used to obtain the LLID  652  and the MAC source address (SA)  653  ( FIG. 6F ) from an upstream burst in a EPON or 10G-EPON system. The process  700 E ( FIG. 7E ) starts at  741 . In a step  742 , a bit sequence is extracted from an upstream burst, which may be generated by the first ONU  206 . 1  ( FIG. 3 ). The received bit sequence may be stored in the non-transitional memory  422  ( FIGS. 4A to 4D ). The received bit sequence may have the structure shown in  FIG. 6F . The delimiter pattern  651  may be used to determine the start of the Ethernet frame  654  ( FIG. 6F ). In a step  743  ( FIG. 7E ), the LLID field  652  is extracted from the bit sequence. In a step  744 , a check is performed whether the extracted LLID is within a range of valid LLIDs for registered ONUs. If yes, then in a step  745 , the source MAC address  653 , which is the MAC address of the ONU sending the upstream signal e.g. the first ONU  206 . 1  ( FIG. 2 ), is extracted and stored for further data processing. If the extracted LLID  652  is not within a range of valid LLIDs for registered ONUs, the bit sequence received in the receiving step  742  is discarded and another bit sequence is captured. The process  700 E ends at  746 . 
     Turning to  FIG. 7F , a process  700 F may be used to obtain the LLID  661  and the MAC SA  662  ( FIG. 6G ) from a downstream frame in a EPON or 10G-EPON system. The process  700 F ( FIG. 7F ) starts at  751 . In a step  752 , a bit sequence is extracted from a downstream frame e.g. a downstream frame sent by the OLT  202  ( FIG. 3 ). In a step  753  ( FIG. 7F ), the LLID field  661  is extracted from the captured bit sequence. In a step  754 , a check is performed whether the extracted LLID  661  is within a range of valid LLIDs. If yes, then in a step  755 , the MAC SA  662  ( FIG. 6G ), which is the MAC address of the OLT sending the downstream frame, is extracted and stored for further data processing. If the extracted LLID  661  is not within a range of valid LLIDs, the bit sequence is discarded and another bit sequence is captured. The process  700 F ends at  756 . 
     Once the identification information is collected as explained above, and optical power levels measured by the test apparatus  300  of  FIG. 3 or 400A to 400D  of  FIGS. 4A to 4D , the identification information of ONUs and/or OLT may be displayed together with corresponding optical power levels. Referring to  FIG. 8 , an example summary screen  800  may be shown e.g. on the display  321  of the test apparatus  300  of  FIG. 3 . The summary screen  800  ( FIG. 8 ) may include wavelengths  802 ,  806  and optical power levels  803 ,  807  of either the upstream optical signal or the downstream optical signal or both. Information for identifying the OLT and/or ONU may be displayed. In GPON and XG-PON systems, the ONU(s) are typically identified via their vendor specific serial number (ONU-SN)  804 , or their ONU-ID  805 . The ONU-ID  805  is a unique number within a given PON. In GPON and XG-PON systems OLTs are typically identified via their PON-ID  808 . In EPON and 10G-EPON systems ONUs and OLTs are typically identified via their MAC address (not shown in  FIG. 8 ). Furthermore, ONUs may be identified via their LLID (not shown in  FIG. 8 ), which is a unique number within a given PON. The PON type is shown at  801 , which typically is a device setting selected prior to starting the test. 
     The hardware used to implement the various illustrative logics, logical blocks, modules, and circuits described in connection with the aspects disclosed herein may be implemented or performed with a general purpose processor, a digital signal processor (DSP), an application specific integrated circuit (ASIC), a field programmable gate array (FPGA) or other programmable logic device, discrete gate or transistor logic, discrete hardware components, or any combination thereof designed to perform the functions described herein. A general-purpose processor may be a microprocessor, but, in the alternative, the processor may be any conventional processor, controller, microcontroller, or state machine. A processor may also be implemented as a combination of computing devices, e.g., a combination of a DSP and a microprocessor, a plurality of microprocessors, one or more microprocessors in conjunction with a DSP core, or any other such configuration. Alternatively, some steps or methods may be performed by circuitry that is specific to a given function. 
     The foregoing description of one or more embodiments of the invention has been presented for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed. Many modifications and variations are possible in light of the above teaching. It is intended that the scope of the invention be limited not by this detailed description, but rather by the claims appended hereto.