Abstract:
A system and method of selecting and viewing communication traffic transmitted over an optical channel selected from among a plurality of possible channels without disrupting the communication traffic occurring over the selected channel or other channels is presented. The system and method includes an optical channel analyzing switch which taps each of the possible plurality of channels and selects a specific channel for routing to a network analyzer. The signal on the selected channel, prior to being analyzed by the network analyzer, undergoes clock and data recovery and retiming/recombination to mitigate contamination from routing and switching the selected original signal between the signal source and the network analyzer. The retimed and recombined channel signal results in a signal, as presented to the network analyzer, which is representative of the original channel signal.

Description:
BACKGROUND OF THE INVENTION 
     1. The Field of the Invention 
     The present invention relates to a circuit and system for selecting and evaluating an optical communication channel. In particular, the invention relates to an optical integrated in-line switching device for selecting a specific communication channel for evaluation. 
     2. Background and Related Art 
     Communication system channels have largely been composed of metallic conductors such as copper or other low resistance metals. Systems using such conductors have generally been relatively easy to monitor and evaluate without great disruption or intrusion into the communication channel since current flows throughout the entire conductor and portions of the conductor can be externally “tapped” with another conductor attached to the test equipment that bleeds-off a negligible amount of test current. 
     Additionally, conductive fibers that transmit light have also been used as communication channel medium and have proven to be advantageous for the transmission of large amounts of information, both in digital and analog form. Fiber conductors, unlike metallic conductors, propagate the information signal in a very longitudinally directional path. Furthermore, the information signal propagates down a very narrow internal portion of the conductor making the non-intrusive external “tapping” of the fiber impractical. 
     Therefore, in order to monitor a fiber channel, a splitter also known as a coupler, must be placed “in-line” with the fiber channel to reflect a portion of the light from the main conductive fiber channel to another conductive fiber channel that can be coupled to a network analyzer or other test equipment. In  FIG. 1 , a system  100  for monitoring a plurality of fiber channels  102 – 108  is depicted with a corresponding plurality of dedicated couplers  110 – 116  connecting with a corresponding plurality of dedicated test equipment  118 – 124 . While such an arrangement makes in-line testing possible, the installation of such couplers into the individual channels has been complex and tedious. 
     Additionally, even when the couplers are inserted into the various individual fiber channels, the logistics and expense of connecting dedicated test equipment to each channel soon becomes prohibitively expensive. Also, even if a single piece of test equipment is reused on multiple channels, the logistics of disconnecting and reconnecting to each of the various couplers becomes expensive, tedious, and, especially when remote monitoring is desired, impractical or impossible to timely access and physically re-couple with each of the channels. 
     There is a need to provide a non-intrusive solution that efficiently uses network analysis resources while allowing the channel to remain intact without interrupting the flow of traffic on the channel. Furthermore, a need exists for providing convenient selection of channels for monitoring without impacting the flow of communications traffic on the channel under analysis. There further exists a need to efficiently utilize test equipment without requiring deployment of a full suite of test equipment dedicated to each communication channel. 
     BRIEF SUMMARY OF THE INVENTION 
     A system and method of selecting and viewing communication traffic transmitted over an optical channel selected from among a plurality of channels without intruding upon the normal traffic of that selected channel or requiring a separate channel dedicated to monitoring and analyzing is presented. The system and method for selecting and analyzing the channel from among a plurality of channels includes an optical channel analyzing switch for selecting the channel to be monitored from among several channels and test equipment such as a network analyzer for evaluating the selected channel. 
     The optical channel analyzing switch includes an optical coupler for each of the plurality of possibly analyzed channels. The optical coupler receives an input optical signal and splits the signal into two paths, a first pass-through path that provides continuous normal routing of optical channel traffic and a second analyzable output optical path that “taps” the channel and routes the input optical signal for that channel for routing to analyzer equipment when selected. 
     Because the analyzable output signal will be routed through various switching and control elements before arriving at the test equipment or network analyzer, the analyzable output optical signal is converted from an optical or light signal into an analyzable electrical signal. This conversion is performed by a receiver having an optical input and an electrical signal output. 
     The optical channel analyzing switch further includes a multiplexor coupled on the inputs to the analyzable electrical signals of the severally available optical channels. The multiplexor is preferably computer controlled to select an output from among one of the potentially several inputs. While the optical channel analyzing switch may operate as an individual unit, in another embodiment, the optical channel analyzing switch may be cascaded to a second or more optical channel analyzing switch for selecting a channel for output to the test equipment or network analyzer from among the first plurality of channels connected to the first optical channel analyzing switch or a second or more plurality of channels connected to the second optical channel analyzing switch. This cascading of switches together is preferably accomplished by interconnecting the multiplexors of each of the switches as discussed in detail below. 
     Because of the extended signal path associated with routing the tapped optical input signal to the analyzing test equipment and further in view of the noise introduced into the signal through the multiplexor and other related electronics, it is desirable to retime the signal after the multiplexor output to recover the clock and data signal and to realign those signals in relation to each other. Retiming is desirable in order to restore the signal integrity after passing through various electronic paths that distort the signal and prior to presenting the signal to the test equipment or network analyzer. Without retiming the signal, false errors generated by the jitter, distortion and noise not present in the original optical signal but introduced by the electronic components could trigger errors in the test equipment that were not present in the original input optical signal. 
     The optical channel analyzing switch also converts the retimed electrical output signal into a retimed optical signal through the use of a transmitter which outputs an optical output signal compatible with optical front-end test equipment such as an optical network analyzer. The optical signal is then ready and available for analysis and monitoring by a single dedicated test equipment such as a network analyzer. 
     These and other objects and features of the present invention will become more fully apparent from the following description and appended claims, or may be learned by the practice of the invention as set forth hereinafter. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       To further clarify the above and other advantages and features of the present invention, a more particular description of the invention will be rendered by reference to specific embodiments thereof which are illustrated in the appended drawings. It is appreciated that these drawings depict only typical embodiments of the invention and are therefore not to be considered limiting of its scope. The invention will be described and explained with additional specificity and detail through the use of the accompanying drawings in which: 
         FIG. 1  illustrates monitoring of multiple channels on an optical network, in accordance with the prior art; 
         FIG. 2  illustrates a block diagram of a plurality of optical channels having a switching mechanism capable of selecting one of the optical channels for routing to a common or shared analyzer; 
         FIG. 3  illustrates a block diagram of a cascaded array of optical analyzing switches forming an extended network evaluation system; 
         FIG. 4  illustrates a detailed block diagram of a network evaluation system, in accordance with a preferred embodiment of the present invention; and 
         FIG. 5  is a schematic diagram of an optical channel analyzing switch for selecting from among a plurality of optical channels, in accordance with the preferred embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIGS. 2 and 3  represent different embodiments of optical network evaluation systems and are presented herein to facilitate the understanding of the manner in which the novel optical channel analyzing switches described below in reference to FIGS.  4  and  5 A–D can be used. The network evaluation system  200  of  FIG. 2  provides a system and method of viewing traffic over an optical channel without impacting the performance of the individual channel under observation or requiring disconnection and recoupling of the test equipment with each successively observed channel. 
     Referring to  FIG. 2 , network evaluation system  200  includes an optical channel analyzing switch  202  and a channel-shared test equipment such as a network analyzer  204 . The optical channel analyzing switch  202  selects a particular channel for monitoring and/or analyzing from among a plurality of channels, for example, channels  206 – 212 . The architecture of  FIG. 2  enables a single or shared test equipment  204  to monitor a plurality of channels. 
     The network evaluation system  200  may operate within a network configuration which, by way of example, may include a full-duplex or half-duplex Gigabit Ethernet or Fibre Channel configuration. Those of skill in the art appreciate that Gigabit Ethernet may operate on either single-mode fiber or multi-mode fiber at data rates that require optical connections. Similarly, Fibre Channel details computer channel communications over fiber optics at transmission speeds from 132 Mbps to 1062.5 Mbps at distances of up to 10 kilometers. 
     As illustrated in  FIG. 2 , optical channel analyzing switch  202  receives optical channels  206 – 212  and “taps” each of the channels using optical couplers  214 – 220  to provide a sample of each of the channels to a switching array, depicted in  FIG. 2  as multiplexor  222 . Multiplexor  222  selects a specific channel from among a possible plurality of channels under direction from a control signal  224  which may be discretely controlled by a network administrator from a remote location or manually controlled through local means. 
     It should be appreciated that the routing of the “tapped” sample signal from optical channels  206 – 212  to the input of test equipment  204  introduces jitter and noise to the signal and reduces the signal to noise ratio of the signal carried on the channel under evaluation. In order to mitigate such signal contamination, additional functionality, illustrated in subsequent FIGS.  4  and  5 A–D, restores or retimes the data and clock relationship. Additionally, prior to being input to the test equipment, the signal is processed according to the invention to comply with the input signal requirements of the test equipment or network analyzers that evaluate optical channels. For example, if the network analyzer is optical, the signal is converted from an electrical signal to an optical signal. If the network analyzer requires an electrical signal, the signal is transduced according to the input signal requirements of the network analyzer. 
       FIG. 3  illustrates a block diagram of a cascading array of optical analyzing switches forming a network evaluation system for analyzing an additional quantity of optical channels, and represents another way in which the optical channel analyzing switches of the invention can be used. According to  FIG. 3 , network evaluation system  300  includes a first optical channel analyzing switch  302  cascaded with a second optical channel analyzing switch  304  for selecting a channel for analysis from among a first plurality of input channels  306  and a second plurality of input channels  308 . 
     The selection of the channel for analysis or monitoring by test equipment  310  is directed by control signal  312  received at first optical channel analyzing switch  302  which is also coupled to second optical channel analyzing switch  304  via a control signal  314 . Physical selection and routing of the specific channel to the test equipment  310  is performed by respective multiplexors  316  and  318 . If a channel from first optical channel analyzing switch  302  is selected, it is routed into a cascade multiplexor input  320  for facilitating a single routing connection to test equipment  310 . 
       FIG. 4  is a functional block diagram of an optical channel analyzing switch in accordance with a preferred embodiment of the present invention. An analyzing system  400  is depicted as including an optical channel analyzing switch  402  for selecting a specific channel from among a plurality of channels  404  for coupling with test equipment  406 . Plurality of channels  404  is comprised of optical channels which may be implemented as single-mode or multi-mode fibers and operated at various channel standards and capacities such as Gigabit Ethernet or Fibre Channel. The present invention facilitates the monitoring and evaluation of a specific channel without interruption to that specific channel&#39;s traffic. Such an implementation is facilitated by coupling channels  404  to individual optical couplers  408  which split or “tap” each of the individual channels and provide two groups of outputs, one being a group of pass-through outputs  410  and a second group of outputs depicted as analyzable output optical signals  412 . 
     One benefit of the cascading capabilities of this embodiment is that two or more units and associated switches can be combined to tap and analyze more channels than could be handled by a single unit. The combined, or cascaded, switches can be controlled together as a single combined system. These features are in contrast with the system configuration that would otherwise be required, in which multiple independent switches would be used to tap different channels, with each switch being controlled separately one from another. 
     Analyzable output optical signals  412  directly couple with receivers  414  which perform optical-to-electrical conversion thereby facilitating the signal timing and manipulation in electrical form as opposed to the more complex optical signal manipulation. Receivers  414  convert analyzable output optical signals  414  into analyzable electrical signals  416  which are coupled to a multiplexor  418 , which in  FIG. 4  is depicted for illustrative purposes only as being an 8-to-1 multiplexor. Multiplexor  418  selects, according to control signal  420 , one of the input signals from among analyzable electrical signals  416  as the output signal depicted as multiplexor output signal  422 . Signal  422  then undergoes various signal modifications in order to restore the timing relationship of the signal which has been contaminated by the extended propagation path through optical channel analyzing switch  402  as well as the noise contamination inherent in electrical devices and components within optical channel analyzing switch  402 . 
     In optical channel analyzing switch  402 , a retimer  424  receives a multiplexor output signal  422  in electrical form and performs a clock recovery function which extracts the clock from the serial data and generates retimed data signal. This retiming operation reduces the jitter that would otherwise be introduced into the signal provided to the test equipment  406 . In this manner, the optical channel analyzing switches of the invention provide significant advantages over switches of the prior art. 
     In order to prepare retimed electrical analyzable output signal  434  to be evaluated by optical test equipment  406 , the output signal is converted into an optical format. A transmitter  436  receives retimed electrical analyzable output signals  434  in electrical form and transforms those electrical signals into a retimed optical analyzable output signal  438  which is an approximation in optical form of the selected input signal from among the plurality of channels  404  selected by multiplexor  418 . 
     If the test equipment analyzes electrical signals rather than optical signals, no conversion of the output signal  438  to optical form is needed. Instead, the transmitter  436  included in optical channel analyzing switch  402  performs transducing operations to process the output signal  438  such that it complies with the input signal requirements of the test equipment. Accordingly, optical channel analyzing switch  402  includes a transmitter  436  that is selected to process the output signal  438  in an appropriate manner such that the output signal complies with the input signal requirements of the test equipment. The type of transmitter  436  is typically determined by the type of test equipment (optical or electrical) with which the optical channel analyzing switch is to be used. 
     As described above in  FIG. 3 , the present invention also includes an embodiment capable of cascading or coupling a plurality of optical channel analyzer switches, such as  402 , for selecting from among an even greater plurality of inputs  404 . Transmitter  436  includes electrical outputs  440 , which may be further coupled with a multiplexor of another optical channel analyzer switch as depicted in  FIG. 3 . 
       FIGS. 5A–5D  represent a schematic diagram of a single channel of the optical channel analyzing switch, in accordance with one implementation of the preferred embodiment of the present invention. In  FIGS. 5A–5D , input optical signal  502  is coupled to a multi-mode wide-band fiber coupler  504 . In addition to an input, coupler  504  is further comprised of two output signals, a pass-through output signal  506  and an analyzable output optical signal  508  in optical form. It is desirable that coupler  504  exhibit low insertion loss, high directivity, high stability and reliability and low excess loss. By way of example and not limitation, coupler  504  may be comprised of a multi-mode coupler such as an MMC-Multimode Wideband Fiber Coupler, manufactured by Transwave Fiber, Inc., of Fremont, Calif. 
     Optical signal  508  is coupled to a receiver portion which exhibits acceptable operational characteristics in converting from optical to electrical transmissions. It would be desirable for a receiver  510  to exhibit high-speed data rates up to and in excess of 2.125 Gbit/sec which is compatible with Fibre Channel and Gigabit Ethernet data rates. Additionally, receiver  510  would desirably exhibit very low jitter, low power dissipation, and for ease of integration exhibit a small form-factor. By way of example and not limitation, receiver  510  may be comprised of transceiver implemented in a receiver mode only such as a 2 gigabit/2×5 transceiver FTRJ-8519-1-25 available from Finisar Systems of Sunnyvale, Calif. 
     A receiver  510  generates analyzable electrical signals  512 , now in electronic rather than optical form which are coupled to a multiplexor  514 . It is desirable that multiplexor  514  exhibit sufficient addressability for individually selecting from among the plurality of possible channels presented to the optical channel analyzing switch  500 . Also, multiplexor  514  desirably operates at propagation delays and frequencies consistent with the frequencies of the communication standards being evaluated.  FIGS. 5A–5D  illustrate multiplexor  514  implemented using a plurality of discrete 4-to-1 multiplexors arranged to implement an 8-to-1 multiplexor configuration. By way of example, multiplexor  514  is implemented using a plurality of multiplexor devices such as the MC10EP57 and MC10EL57 available from ON Semiconductor, Phoenix, Ariz. 
     The selected output signals  516  including reference clock signals  520  are coupled to a retimer circuit  518  to generate output data signals  522  and output clock signals  524 . Retimer  518  extracts the clock from the serial data and generates retimed clock signal  524  and retimed data signal  522 . Retimer circuit  518  desirably performs continuous-rate clock and data recovery, at the desirable data rate standards of at least Fibre Channel and Gigabit Ethernet. It is also desirable for retimer circuit  518  to exhibit low jitter and sufficient input sensitivity. By way of example and not limitation, retimer circuit  518  may be comprised of an S3056 clock recovery device that performs the clock recovery function for various optical standards including SONET, Fibre Channel, and Gigabit, Ethernet. The S3056 is capable of operating at 30 Mbps to 2.7 Gbps continuous-rate clock and data recovery. The exemplary device is available from Applied Micro Circuits Corporation of San Diego, Calif. 
     Output signals  522  and  524  are further coupled to a latch or flip-flop configuration  526 . The purpose of latch  526  is to recombine the timing-realigned separated clock signal  524  and data signal  522  into combined retimed electrical analyzable output signal  528 . By way of example and not limitation, an exemplary latch configuration  526  may be comprised of a “D” flip-flop such as an MC100EP52 available from various sources including ON Semiconductor, Phoenix, Ariz. 
     Outputs  528  are further coupled to a transmitter for converting from an electrical signal to an optical signal by way of a transmitter  530 . Transmitter  530  generates an optical output  532  for coupling with the test equipment. Transmitter  530  also alternatively generates cascading signals  534  for coupling with additional switches in an alternate embodiment, as discussed above. By way of example and not limitation, an exemplary transmitter  530  may be comprised of a FTRJ-8519-1-25 available from Finisar Systems of San Jose, Calif. 
     The present invention may be embodied in other specific forms without departing from its spirit or essential characteristics. The described embodiments are to be considered in all respects only as illustrative and not restrictive. The scope of the invention is, therefore, indicated by the appended claims rather than by the foregoing description. All changes which come within the meaning and range of equivalency of the claims are to be embraced within their scope.