Patent Publication Number: US-6907196-B2

Title: Arrangement and method for testing fiber optic telecommunications circuits over dense wavelength division multiplexing (DWDM)

Description:
RELATED CASES 
   This patent application is a continuation of U.S. patent application Ser. No. 09/758,496; filed on Jan. 11, 2001 now U.S. Pat. No. 6,778,778; entitled “Arrangement And Method For Testing Fiber Optic Telecommunications Circuits Over Dense Wavelength Division Multiplexing (DWDM); and hereby incorporated by reference into this patent application. 

   BACKGROUND OF THE INVENTION 
   The present invention generally relates to an arrangement and method for testing telecommunications circuits over Dense Wavelength Division Multiplexing (DWDM). Yet more particularly, the present invention relates to an arrangement and method that enable testing of cross country circuits over DWDM by a single person. 
   As is known, telecommunication service providers have created cross country or ultra-long haul networks. These networks are driven, in part, by the nature of internet and data traffic and by ever expansive enterprise networks. Voice traffic, on the other hand, is characterized as more regional and served principally by metropolitan and regional networks. Network planners visually depict these two developing traffic patterns as network overlays, served by different transmission equipment. Internet and data traffic over these networks has been increasing at a significant rate and currently dominates voice traffic for many long distance service providers. 
   Telecommunication service providers continually look toward new technologies leading to greater network carrying capacity, or bandwidth, and increasing transmission distances, which are the length separations between system transmitting and receiving terminals. DWDM is the favored optical technology for increasing bandwidth on an optical fiber. DWDM operates by multiplexing and transmitting a number of signals, i.e. OC-48 or OC-192, simultaneously at different wavelengths on the same optical fiber. As a result, a single optical fiber provides a number of virtual optical fibers by carrying a number of simultaneous signals. This permits greater network traffic through increased bandwidth. 
   The most aggressive service providers have deployed 40 channel DWDM systems with transmission distances limited to around 500 km before requiring Optical-Electrical-Optical (O-E-O) regeneration of the optical bit streams. Thus, DWDM systems are connected back-to-back for cross country connectivity. In order to overcome these limitations, DWDM system manufacturers are presently offering to at least double both channel count and transmission distance. More elaborate technologies will lead to greater length/bandwidth products and true ultra-long haul systems. 
   Along with the interests aimed at increasing both transmission length and bandwidth a greater importance is being placed on developing more efficient network management tools and test equipment. Service providers have been instrumental in driving this development and embedding performance measures and diagnostic tools into their system elements. Circuit tests that are performed by field operators include, for example, tests for optical power levels, loss of signal modulation, and code violations. 
   Currently, field technicians cannot trouble-shoot an entire cross country DWDM circuit without tremendous group coordination. Each cross country DWDM circuit comprises a number of sub-circuits that must be administered by a local field technician during a test. For example, a circuit from State  1 , which is located in one region of a country, to State  4 , which is located in a distant region of the country, might comprise multiple, sequential sub-circuits from State  1  to State  2 , State  2  to State  3 , and State  3  to State  4 . During a test, a local field technician for a sub-circuit can only monitor and trouble-shoot their individual sub-circuit. Therefore, to obtain information relating to another sub-circuit, a field technician must either communicate with another field technician who is monitoring the other sub-circuit or rely on personnel in a Network Operation Center (NOC) that can survey the entire circuit. 
   Less reliance on the NOC is desired. It is impractical for a NOC to be heavily involved in turn-up testing or prolonged maintenance tasks. A NOC should rather be focused on in-service traffic management. 
   It is therefore desirable to empower field operations by providing the tools and test equipment needed for a network to efficiently manage turn-up and maintenance requirements. 
   SUMMARY OF THE INVENTION 
   It is therefore an object of the present invention to provide an arrangement and a method for testing that 1) reduces the cost of test equipment and 2) consumes less time and manpower than known arrangements and methods. 
   The present disclosure provides one or more embodiments directed to improvements in testing fiber optic telecommunications circuits over DWDM. These improvements can be provided in a single all-encompassing unit or practiced separately. 
   To this end, in one embodiment, there is provided a method for testing a circuit over Dense Wavelength Division Multiplexing (DWDM). A test-drive signal is transmitted on the circuit and a performance of the circuit is monitored at points along the circuit based on the transmitted test-drive signal. 
   In an embodiment, the field technician accesses downstream Network Elements (NE) over an Operations Support System (OSS) Network. The OSS Net is accessed via a LAN connection or via a dial-up connection and providing login information. For example, a field technician may Telnet to a downstream NE and provide the necessary login information to bring up that NE&#39;s management system or Craft Interface as if being on site and physically connected to the NE. Performance measures are visible at the Craft Interface display screen. As a result, the present invention enables testing of cross country circuits over DWDM by a single person. 
   In another embodiment, there is provided an arrangement for testing a telecommunications circuit over Dense Wavelength Division Multiplexing (DWDM). The arrangement comprises a transmitter for transmitting a test-drive signal on the circuit, which is optically coupled to a near end of the circuit. A network monitors performance of the circuit at points along the circuit based on the transmitted test-drive signal. As a result, the present invention provides an arrangement for testing an entire cross country circuit that requires only a single transmitter. A single field operator, who operates the transmitter, also monitors performance of any point on the circuit as described above. Other field operators can also monitor the circuit from their locations. 
   A DWDM circuit can comprise a number of sub-circuits, each sub-circuit having terminals at each end and possibly having at least one optical add-drop multiplexer and/or optical line amplifier. The performance of the circuit is monitored via the network at least one of a number of network elements along the circuit. Network elements comprise, for example, optical add-drop multiplexers and optical line amplifiers. 
   In another embodiment, there is provided a display, such as a laptop computer or Craft Interface Device (CID), which is connected to the network for displaying the monitored performance of the circuit. The display device can be connected to the wide area network via one of a local area network or dial-up connection. 
   In another embodiment, the transmitter is similarly controlled via the OSS Net or by direct physical connection using the same display device or CID or by a separate display device or CID. 
   The present invention is applicable to any transport framing structure with system support for “in-band” signaling and detection of key performance measures. For example, in an embodiment, SONET and SDH signals are transported over a DWDM system with performance monitoring at system client side ingress and egress points for optical power levels, loss of signal modulation, and code violations. 
   In the present discussion, IP or ATM circuits merely describe example cross country DWDM circuits; IP and ATM are traffic descriptors of the services transported within the payload area of a concatenated SONET or SDH signal. SONET and SDH provide various other channelized mixtures and payload mappings for transport of, for example, DS3, E1, or DS1 services. 
   These and other features of the invention will become clearer with reference to the following detailed description of the presently preferred embodiments and accompanying drawings. 

   
     DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is schematic block diagram of an arrangement for testing an IP or ATM circuit constructed and operated in accordance with the present invention. 
       FIG. 2  is a plan view of a transmitter constructed and operated in accordance with the present invention. 
   

   DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS 
   As discussed above, there is provided an arrangement and a method for testing fiber optic telecommunications circuits over Dense Wavelength Division Multiplexing (DWDM) that, among other things, enable testing of cross country circuits over DWDM by a single person. 
   In  FIG. 1 , there is illustrated an arrangement  10  for testing a circuit over DWDM that embodies principles of the present invention. As illustrated, a transmitter  12  is coupled to a near end  14  of a DWDM circuit  16  at a fiber access panel  20  using a fiber optic transmit cable or patch cord  38 , as described below. The circuit  16  can comprise a single circuit between, for example, two closely located cities. Alternatively, the circuit  16  can comprise, for example, a multi-point cross country circuit between distant cities with a number of contiguous sub-circuits spanning intermediate cities. 
   The circuit  16  typically comprises a number of optical fibers, or channels, for transmission, including a number of critical spare channels. The channels terminate at the near end  14  and far end  18  of the circuit  16  at fiber access panels  20 . The channels of any intermediate sub-circuits may also terminate at fiber access panels  20 . In the present discussion, fiber access panels  20  cross-connect network elements, such as add-drop multiplexers, IP routers, or ATM switches, to individual channels on a DWDM system. 
   Currently, circuits are tested with a device that has both a transmitter for transmitting a test-drive signal on a circuit and a receiver for receiving the test-drive signal after it is returned from a far end of the circuit. Thus, there is a disadvantageously high cost associated with the current testing devices because they must incorporate both a transmitter and a receiver. The present arrangement  10  inventively overcomes this known disadvantage by providing a “bare bones” transmitter  12  to meet routine transmission demands. Thus, the cost of the present transmitter  12  is significantly lower than the cost of known transmitters because it does not require a receiver. 
   Referring to  FIG. 2 , the transmitter  12  has a rack mounted chassis  26 . The chassis  26  is portable and can be provided with a carrying handle  28  and/or mounted in a cross-connect cabinet or equipment rack. 
   One or more modular transmitter cards  30 ,  32  are mounted in slots  34  of the chassis  26 . Each transmitter card  30 ,  32  produces a test-drive signal. In one embodiment, at least one of the transmitter cards  30  is an Optical Carrier-48 (OC-48), such as Synchronous Transport Signal-48c (STS-48c), transmitter. Additional transmitter cards can also be provided, such as an OC-192 (STS-192c) transmitter card  32  and/or a 40 Gigabit or higher transmitter card. Each transmitter card  30 ,  32  includes a laser transmit output port (not shown) for outputting the test drive signal. Timing and control of the transmitter cards  30 ,  32  is performed by a CPU/clock card  36 , which is mounted in one of the slots  34 . Additional slots  34  can be provided in the chassis  26  to accommodate additional cards, such as transmitter cards, receiver cards, or 4:1 passive splitter cards. 
   The transmitter cards  30 ,  32  are optically coupled to the fiber access panel  20  via a fiber optic transmit cable  38 . The transmitter card  30 ,  32  optical interface to the transmit cable  38  complies with GR-253-CORE short reach specifications. 
   The transmitter  12  is designed to meet daily field requirements. A field technician controls the transmitter  12  via actuators (not shown) on the various transmitter cards  30 ,  32 . A laser power actuator (not shown) toggles power to the laser (not shown) of the selected transmitter card  30 ,  32  and starts/stops the test-drive signal transmission. The transmitter cards  30 ,  32  also have an error insert actuator (not shown), which allows a field technician to inject a single error, such as a B1 byte error, into the test drive signal during BER testing. 
   Alternatively, a field technician can control the transmitter  12  by connecting a craft interface device either directly to the unit utilizing, for example, an RS-232 controlled connection or Ethernet port on the unit or over a LAN or Internet connection from afar, utilizing the Ethernet port on the unit. In an embodiment, where a field technician controls the transmitter  12  over an Internet connection, the unit&#39;s IP address is provided on a secure web site. 
   The test-drive signal need only conform to Short-Reach (SR) SONET specifications, amounting to a savings over Long Reach (LR) specifications, and preferably has a −3 dBm maximum output power at a wavelength of 1310 nm. Alternatively, the test drive signal can conform to other specifications. For example, the test drive signal can conform to LR SONET specifications or have a wavelength of, for example, 1280 nm to 1625 nm. In an embodiment, the test drive signal further comprises a test pattern of “1010 . . . ,” for full bandwidth exercise, and a 2 23 -1 Pseudo Random Bit Sequence (PRBS) at OC-48 (STS-48c), which is a common stress pattern that simulates live traffic on the circuit  16 . A 2 31 -1 PRBS is preferred at OC-192 (STS-192c). Test patterns may be selected by software control of the transmitter. 
   Preventive maintenance and trouble-shooting are ongoing activities for a field technician. They require testing of the entire circuit  16  to ensure that no problem areas exist. Thus, it is critical that a field technician has the ability to monitor the performance of the circuit  16  at all sub-circuit ingress and egress points. As stated above, testing of a cross country circuit currently involves the coordination of resources along the circuit. Each field operator can only monitor their individual sub-circuit. Therefore, to obtain information relating to another sub-circuit, a field operator must either communicate with another field operator who is monitoring the other sub-circuit or communicate with a NOC that can survey the entire circuit. During circuit testing, a variety of tests are routinely performed that include tests for optical power levels, loss of signal modulation, and code violations. Much of the time of circuit testing is spent, based on actual field experience, in performing continuity tests on turn-up testing before final Bit Error Ratio (BER) testing. This may be easily handled by keying in a number of B1 byte errors and confirming counters correctly incremented downstream; some technicians may use simple on/off laser control and look for Loss of Signal (LOS) and Loss of Frame (LOF) at downstream system ingress and egress points. Lacking continuity may require an installations technician to be dispatched to trouble-shoot and complete a cross-connect. 
   The present invention overcomes this known disadvantage by providing a network  24 , such as an OSS network, for monitoring the performance of the entire circuit  16  and which can be viewed by any field technician who has access to the network  24 . Thus, a single field technician can simultaneously control the test-drive signal transmitter  12  and monitor, through the network  24 , the performance of the entire circuit  16 . 
   The network  24  comprises a communications network operating under, for example, an Ethernet communications protocol. Circuit  16  performance data is gathered from NEs, such as Optical Add-Drop Multiplexers (OADMs)  58 , Optical Line Amplifiers (OLAs)  56 , and terminals  60  for monitoring on the network  24 . An intermediate DWDM management system (not shown) can be provided to interface directly to the NEs  56 ,  58 ,  60  for embedded performance monitoring and to communicate the performance data to the network  24 . 
   A field technician can monitor the performance of the circuit  16  simply by accessing the network  24  and telneting to the NE&#39;s IP address, made available on a secure web site. This access is preferably done through a display device  62 , such as a laptop computer or CID, that is coupled to the network  24  via, for example, an Ethernet or dial-up connection. 
   Thus, the present invention eliminates the geographical constraints and manpower problems associated with current circuit testing methods and devices by providing an arrangement and method for monitoring performance of a circuit  16  from any location. A field technician can remotely access the network  24 , telnet to a distant NE  56 ,  58 ,  60 , and monitor the performance at any point on the cross country circuit  16 . Further, the transmitter  12 , which can be operated via a remote connection, can also be connected to the network  24  for remote control by the field technician. Therefore, a single field technician can both operate the transmitter  12  and monitor the entire circuit  16  from a display device  62  at a remote location. 
   It is preferable that field access to the DWDM management system via the network  24  be restricted to “read only” user privileges except for allowing B 1  counters, or the like, to be reset to zero. Full access to the DWDM management system provides on/off control of all system vitals. Only the local field technician or NOC personnel have full administrative privileges. NE IP addresses should be posted on a secure web site. 
   During a test of the circuit  16 , a field technician manually couples the transmitter  12  to the fiber access panel  20  if the transmitter  12  is not already coupled thereto. The field operator then activates the test-drive signal by pressing the laser power actuator (not shown). Accordingly, the test-drive signal, which preferably comprises the OC-48 or OC-192 signal with a desired default test pattern and a 2 23 -1 PRBS, is transmitted on the circuit  16 . Other test patterns may be selected by software control of the transmitter settings. 
   Through the display device  62  connected to the network  24 , the field technician will monitor data collected from the various network elements  56 ,  58 ,  60  along the circuit  16 . Both a transmission path  66  and a return path  68  of the test-data signal can be monitored at one time by providing a loop-back  70  at the far end  18  of the circuit  12 . In order to monitor a final section  72  of the return path  68  from the near end system egress point to the fiber access panel  20 , an output channel of the return path  68  can be patched to an input of another channel, such as a spare channel  76 , through the use of a coupling  74 . The loop-back  70  can either be a hard loop-back or a soft loop-back. The performance of the entire circuit, i.e. BER measurement, can be monitored at the SPARE channel system ingress point. 
   Loop-backs and manual patching at fiber access panels may one day be supplanted by soft or facility loop-backs at  20 . There is a demand for remote provisional x-connects or wavelength routers, as they may be referred to. System ingress and egress points may also feature a facility loop-back. 
   During BER testing, a field technician monitors the BER performance of the test drive signal at sub-circuit ingress and egress points  60  and  58 . In order to simulate a B 1  byte error, the field operator presses the error insert actuator (not shown) on the transmitter card  30 ,  32 . This injects a single B 1  byte error into the test-drive signal, which error can be monitored downstream at the network device  58  and  60 . The field operator may zero a B 1  byte counter through the display device  62 . This is as an exception to the network  24  accounts being read-only accounts. Further, other types of errors can be injected into the test-drive signal. 
   The foregoing provides a cost effective apparatus and method for testing cross country circuits over DWDM by a single person. 
   Further, the present invention is applicable to any transport framing structure with system support for “in-band” signaling and detection of key performance measures. For example, in an embodiment, SONET and SDH signals are transported over a DWDM system with performance monitoring at system client side ingress and egress points for optical power levels, loss of signal modulation, and code violations. In the present discussion, IP or ATM circuits merely describe example cross country DWDM circuits; IP and ATM are traffic descriptors of the services transported within the payload area of a concatenated SONET or SDH signal. SONET and SDH provide various other channelized mixtures and payload mappings for transport of, for example, DS3, E1, or DS1 services 
   As is apparent from the foregoing specification, the invention is susceptible to being embodied with various alterations and modifications which may differ particularly from those that have been described in the preceding specification and description. It should be understood that it is desired to embody within the scope of the patent warranted herein all such modifications as reasonably and properly come within the scope of the presently defined contribution to the art.