Abstract:
A method and system for optimizing a response time of a monitoring loop with forward error correction. Characteristics of a fiber optic communications channel are adjusted based on the number of errors corrected in the FEC decoder. An adaptive BER is calculated much faster by using a signal from an FEC decoder, than by comparing input and output transmission. Thereby, the lag time in adjusting the transmission characteristics of the fiber optic channel is minimized and the overall performance of the system is improved.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application is based upon and claims the benefit of priority from the prior U.S. Provisional Application No. 60/717,194 filed on Sep. 16, 2005, the entire contents of which is incorporated herein by reference.  
         [0002]     This application is related to and incorporates in its entirety, nonprovisional U.S. Patent Application entitled “Apparatus and Method for Adaptive Adjustment and Performance Monitoring of Avalanche Photo-Diode Optical Receiver and Laser Transmitter for Fiber Link Long Haul Applications,” filed on Sep. 18, 2006. 
     
    
     BACKGROUND OF THE INVENTION  
       [0003]     1. Field of the Invention  
         [0004]     This invention relates to methods and systems for control loop response time optimization. In particular, this invention relates to optimizing the response time of a control loop in a 10 Gigabyte-per-second (Gbps) Fiber Communication Channel with Forward Error Correction.  
         [0005]     2. Background of the Technology  
         [0006]     The advantages of network computing are increasingly evident, as the convenience and efficiency of providing information, communication, or computational power to individuals at their personal computers or other end user devices has led to rapid growth of such network computing, including Internet and intranet systems and applications.  
         [0007]     Today&#39;s networks carry vast amounts of information. High bandwidth applications supported by these networks include streaming video, audio, and large aggregations of voice traffic. In the future, these bandwidth demands are certain to increase.  
         [0008]     Recently, fiber optic communications has emerged as a viable means for transmitting data information over a network. The demand for quick reliable data transmission means continues to increase. Fiber optic communication channels provide means for reliable and efficient transmission of large volumes of data.  
         [0009]     As bandwidth requirements increase, correcting errors in data transmission becomes increasingly important. Early methods of error correction, such as handshaking, required prior communication between the transmitting system and the receiving system. This method has many shortcomings, however, especially for systems which are transmitting information from one transmitter to multiple receivers at a time.  
         [0010]     Another known method implements a monitoring loop which continuously calculates the Bit Error Rate (BER) and adjusts various system parameters in the attempt to decrease BER. One drawback to the use of a monitoring loop is that if the monitoring loop is based on the number of errors detected in the communication channel, then the response on an increase of the error rate cannot be faster than the measurement time, usually in the hundredths of seconds. For example, if a change occurs in one of the characteristics of the transmitter, the parameter cannot be adjusted faster than the measurement time. During the period while new measurements are taking place, the traffic across the media is subject to an increased BER for this extended period of time.  
       SUMMARY OF THE INVENTION  
       [0011]     There is a need in the art for methods and systems optimizing the response time of a monitoring loop, without the disadvantage of exposing network traffic to an increased BER for extended periods of time. The present invention solves these needs, as well as others, by providing a method and system for optimizing the response time of a control loop in communications channels with forward error correction. Specifically, in one embodiment of the present invention, the characteristics of a fiber optic communications channel which are adjusted based on the number of errors corrected in the FEC decoder. By determining the BER using the FEC decoder, rather than by comparing input transmission with output transmission, the system can determine the adaptive BER much faster. This reduces the lag time in making adjustments to the transmission characteristics of the fiber optic channel and improves the overall performance of the system.  
         [0012]     Other objects, features, and advantages will be apparent to persons of ordinary skill in the art from the following detailed description of the invention and the accompanying drawings. 
     
    
     BRIEF DESCRIPTION OF THE FIGURES  
       [0013]     The above and other features and advantages of the present invention will become more apparent to those of ordinary skill in the art by describing in detail exemplary embodiments thereof with reference to the attached drawings in which:  
         [0014]      FIG. 1  presents a computer system implementation capable of carrying out the functionality of one embodiment of the current invention.  
         [0015]      FIG. 2  is a generalized scheme of a communication channel utilizing Forward Error Correction (FEC).  
         [0016]      FIG. 3  is a high-level diagram of one embodiment of the performance monitoring system of the present invention.  
         [0017]      FIG. 4  is a flowchart showing operation of one embodiment of the present invention.  
         [0018]      FIG. 5  is a diagram of the architecture of an embodiment of the performance monitoring system of  FIG. 3 .  
         [0019]      FIG. 6  is a graph showing receiver sensitivity and the relationship between the BER with and without FEC coding. 
     
    
     DETAILED DESCRIPTION  
       [0020]     The present invention will now be described more fully hereinafter with reference to the accompanying drawings, in which exemplary embodiments of the invention are shown. The invention may, however, be embodied in different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. In the figures, the dimensions of elements may be exaggerated for clarity of illustration. Like reference characters refer to like elements throughout.  
         [0021]     The present invention may be implemented using hardware, software or a combination thereof and may be implemented in one or more computer systems or other processing systems. In one embodiment, the invention is directed toward one or more computer systems capable of carrying out the functionality described herein. An example of such a computer system  200  is shown in  FIG. 1 .  
         [0022]     Computer system  200  includes one or more processors, such as processor  204 . The processor  204  is connected to a communication infrastructure  206  (e.g., a communications bus, cross-over bar, or network). Various software embodiments are described in terms of this exemplary computer system. After reading this description, it will become apparent to a person skilled in the relevant art(s) how to implement the invention using other computer systems and/or architectures.  
         [0023]     Computer system  200  can include a display interface  202  that forwards graphics, text, and other data from the communication infrastructure  206  (or from a frame buffer not shown) for display on the display unit  230 . Computer system  200  also includes a main memory  208 , preferably random access memory (RAM), and may also include a secondary memory  210 . The secondary memory  210  may include, for example, a hard disk drive  212  and/or a removable storage drive  214 , representing a floppy disk drive, a magnetic tape drive, an optical disk drive, etc. The removable storage drive  214  reads from and/or writes to a removable storage unit  218  in a well known manner. Removable storage unit  218 , represents a floppy disk, magnetic tape, optical disk, etc., which is read by and written to removable storage drive  214 . As will be appreciated, the removable storage unit  218  includes a computer usable storage medium having stored therein computer software and/or data.  
         [0024]     In alternative embodiments, secondary memory  210  may include other similar devices for allowing computer programs or other instructions to be loaded into computer system  200 . Such devices may include, for example, a removable storage unit  222  and an interface  220 . Examples of such may include a program cartridge and cartridge interface (such as that found in video game devices), a removable memory chip (such as an erasable programmable read only memory (EPROM), or programmable read only memory (PROM)) and associated socket, and other removable storage units  222  and interfaces  220 , which allow software and data to be transferred from the removable storage unit  222  to computer system  200 .  
         [0025]     Computer system  200  may also include a communications interface  224 . Communications interface  224  allows software and data to be transferred between computer system  200  and external devices. Examples of communications interface  224  may include a modem, a network interface (such as an Ethernet card), a communications port, a Personal Computer Memory Card International Association (PCMCIA) slot and card, etc. Software and data transferred via communications interface  224  are in the form of signals  228 , which may be electronic, electromagnetic, optical or other signals capable of being received by communications interface  224 . These signals  228  are provided to communications interface  224  via a communications path (e.g., channel)  226 . This path  226  carries signals  228  and may be implemented using wire or cable, fiber optics, a telephone line, a cellular link, a radio frequency (RF) link and/or other communications channels. In this document, the terms “computer program medium” and “computer usable medium” are used to refer generally to media such as a removable storage drive  214 , a hard disk installed in hard disk drive  212 , and signals  228 . These computer program products provide software to the computer system  200 . The invention is directed to such computer program products.  
         [0026]     Computer programs (also referred to as computer control logic) are stored in main memory  208  and/or secondary memory  210 . Computer programs may also be received via communications interface  224 . Such computer programs, when executed, enable the computer system  200  to perform the features of the present invention, as discussed herein. In particular, the computer programs, when executed, enable the processor  204  to perform the features of the present invention. Accordingly, such computer programs represent controllers of the computer system  200 .  
         [0027]     In an embodiment where the invention is implemented using software, the software may be stored in a computer program product and loaded into computer system  200  using removable storage drive  214 , hard drive  212 , or communications interface  224 . The control logic (software), when executed by the processor  204 , causes the processor  204  to perform the functions of the invention as described herein. In another embodiment, the invention is implemented primarily in hardware using, for example, hardware components, such as application specific integrated circuits (ASICs). Implementation of the hardware state machine so as to perform the functions described herein will be apparent to persons skilled in the relevant art(s).  
         [0028]     In yet another embodiment, the invention is implemented using a combination of both hardware and software.  
         [0029]     Under International Telecommunication Union Telecommunication Standardization Sector Standards G.709 (ITU-T G.709) and G.975 (ITU-T G.975), which are incorporated by reference herein in their entirety, certain fiber optic communication channels, for example, a 10GE/OC-192 fiber communication channel, as featured in one embodiment of the present invention, is equipped with FEC, and a system for monitoring the performance of the data transmission.  FIG. 2  depicts a communications channel utilizing FEC. In  FIG. 2 , data is fed into FEC coder  110 . The encoded data is then sent to a modulator  120 , where the data is transmitted across a media  130 , for example, a fiber optic cable. The signal is received at a demodulator  140 , and the BER is calculated at the demodulator and is designated by BERDM. The demodulated signal is then sent to the FEC decoder  150 , which completes the error correction and corrects the signal. The BER is then calculated at the FEC decoder and is designated by BERFEC. BERFEC is ideally multiple orders of magnitude smaller than BERDM. The error-corrected signal is then sent as the data output.  
         [0030]     Referring now to  FIG. 3 , therein shown is a data transmission system  300  according to one embodiment of the present invention. FEC encoder  310  receives a data stream as input and outputs an encoded data stream. In one embodiment of the present invention, the FEC encoder is a Reed-Solomon encoder, for example, but any suitable FEC encoding device may be used. The encoded signal is then sent to transmission unit  320 . Transmission unit  320 , which is described in more detail in reference to  FIG. 5 , receives signals P adj  and M adj  from the power and modulation controller  370 . Based on signals P adj  and M adj , transmission unit  320  adjusts the optical signal λ 1 , which is transmitted through a medium  330 , such as a fiber or a cable. The optical signal λ 1  is received by the receiving unit  340 , which is described in more detail in reference to  FIG. 5 . The received signal is then sent to the decoder, which decodes the signal using FEC. The decoder outputs the decoded and error-corrected data stream Data Out, and also outputs the number of errors corrected by the FEC decoder N err  to the control unit  360 . In one embodiment of the method of the present invention, shown in  FIG. 4 , at step  420 , the control unit  360  outputs two electrical signals, HV adj  and T adj , which control the APD receiver. At step  425 , control unit  360  outputs an optical signal λ 2 , which is sent back across the medium  330  for controlling the power and modulation control unit  370 . At step  435 , power and modulation control unit  370  outputs two signals, P adj  and M adj , which control the laser output power (L) and modulation amplitude of the laser.