Abstract:
A system and method for transmitting data streams subject to crosstalk, in which an adjustable parameter, preferably data rate, is adjusted using feedback of performance characteristics, preferably signal-to-noise ratio (SNR) and line attenuation, maximizing total throughput of the data streams. The invention is particularly applicable to inverse multiplex (IMUX) systems, where total throughput is more important than the throughput of any individual data stream.

Description:
FIELD AND BACKGROUND OF THE INVENTION  
         [0001]    The present invention relates to a system for transmitting data streams via multiple links in the presence of crosstalk, and more particularly to a system that uses feedback to optimize the transmission rates of a group of communication links that are subject to crosstalk so as to maximize the overall data transmission rate, subject to transmission quality constraints.  
           [0002]    As used herein, the term “crosstalk” means the coupling of a portion of a signal transmitted via a communication link to another communication link.  
           [0003]    As used herein, the term “unacceptable crosstalk” means crosstalk that causes performance of a communication link to be poorer than the performance specified for that communication link.  
           [0004]    A system for optimizing the transmission rates of a group of communication links which are subject to crosstalk, as provided by the present invention, is of particular value in inverse multiplex data transmission systems, although such a system for optimizing transmission rates is also applicable to other types of communication systems that are subject to crosstalk. The application of the system of the present invention for optimizing transmission rates to inverse multiplex systems is included in the scope of the present invention. The application of the system of the present invention for optimizing transmission rates to other communication systems that are subject to crosstalk is included in the scope of the present invention.  
           [0005]    In an inverse multiplexing system, multiple data links are joined together in parallel to form a single aggregate link whose total data transmission capacity is close to or equal to the sum of the data transmission capacities of the individual links.  
           [0006]    At the transmitter, an inverse multiplexing system must divide the data stream into separate data streams, one for each of the links being used, while the receiver must recombine the several streams to recover the original data stream. See co-pending U.S. patent application Ser. No. 10/335872, which is incorporated by reference for all purposes as if fully set forth herein.  
           [0007]    As used herein, the term “symbol”, as applied to a digital communication link, means the basic unit of information transmitted by that communication link.  
           [0008]    In some links, for example, RS-232, a symbol is a single bit, while in others, for example, QAM, a symbol includes more than one bit. The information may be carried by amplitude, frequency, phase, other signal phenomena, or combinations thereof.  
           [0009]    When communication links, such as those including twisted wire pairs, are in close physical proximity to each other, they are subject to the coupling of signals from one link to another, a phenomenon known as “crosstalk”. By adding an unwanted signal to a link, crosstalk degrades the performance of that link. For example, in an analog telephone system, crosstalk can make a conversation on another line be audible, interfering with conversation. In a digital system, crosstalk can change signals received by a receiver sufficiently to cause the receiver to mistake one symbol for another, increasing the bit error rate (BER) of the link.  
           [0010]    As larger numbers of links are concentrated in a limited space, the problem of crosstalk worsens.  
           [0011]    As an example, consider a cable that includes several twisted pairs. If only a single one of the twisted pairs is in use, there is no problem of crosstalk among the twisted pairs. This allows the link to operate using modulation techniques that transmit many bits per symbol, but are very sensitive to interference. If another of the twisted pairs is activated, crosstalk between these two pairs may reduce the number of bits per symbol that either link can carry for any particular BER.  
           [0012]    If several communication links that are subject to crosstalk are started at the same time, it is possible that some will suffer unacceptable levels of crosstalk. If the links are started one after the other, the links started early in the process will begin to suffer more and more crosstalk as other links are started. A mechanism for adjusting the data rates of individual communication links to allow a combination of data rates that maximizes total throughput of such a system would be highly desirable, especially in inverse multiplex data transmission systems, because the total data rate for the communication links is more important than the data rate for any individual communication link in inverse multiplex data transmission systems.  
           [0013]    There is thus a widely recognized need for, and it would be highly advantageous to have, a system for transmitting data streams via multiple links in the presence of crosstalk that optimizes the transmission rates of the communication links so as to maximize the overall data transmission rate, subject to transmission quality constraints.  
         SUMMARY OF THE INVENTION  
         [0014]    According to the present invention there is provided a system for transmitting data streams that are subject to crosstalk, including: (a) a plurality of communication links, each communication link having a respective performance characteristic, at least one of the communication links having an adjustable parameter; (b) a mechanism for measuring the performance characteristics; and, (c) a mechanism for adjusting the one or more adjustable parameters in response to the mechanism for measuring the performance characteristics.  
           [0015]    According to the present invention there is provided a method for transmitting data streams via a plurality of communication links that are subject to crosstalk, the method including the steps of: (a) providing: (i) a mechanism for measuring a respective performance characteristic of each of the communication links; and, (ii) a mechanism for adjusting a respective adjustable parameter of at least one of the communication links; (b) transmitting a signal via at least one of the communication links in accordance with the adjustable parameter thereof; (c) measuring the respective performance characteristic of at least one of the communication links; and, (d) if one of the one or more performance characteristic is outside of a respective preselected range, adjusting the one or more adjustable parameters until every one of the one or more performance characteristics is inside the preselected range thereof.  
           [0016]    Preferably, the communication links include digital communication links.  
           [0017]    Preferably, the at least one adjustable parameter includes a data transmission rate.  
           [0018]    Preferably, the performance characteristics include signal-to-noise ratios.  
           [0019]    Preferably, the performance characteristics include line attenuations.  
           [0020]    Alternatively, the performance characteristics include bit error rates.  
           [0021]    Preferably, the communication links include communication links of an inverse multiplex data transmission system.  
           [0022]    Preferably, each communication link has an adjustable parameter, and the system is operative to adjust all the adjustable parameters.  
           [0023]    Preferably, the system further includes: (d) a controller operative to set an initial value of an adjustable parameter to a value unlikely to induce unacceptable crosstalk.  
           [0024]    Alternatively, the system further includes: (d) a memory operative to store a value of at least one of the one or more adjustable parameters; and (e) a controller operative to set an initial value of an adjustable parameter based on a value thereof stored in the memory.  
           [0025]    Turning now to the method of the present invention, preferably, in the transmitting step, two or more respective signals are transmitted via corresponding communication links, substantially simultaneously.  
           [0026]    Preferably, each of the one or more adjustable parameters includes a data transmission rate.  
           [0027]    Preferably, the performance characteristics include signal-to-noise ratios.  
           [0028]    Preferably, the performance characteristics include line attenuations.  
           [0029]    Alternatively, the performance characteristics include bit error rates.  
           [0030]    Preferably, each communication link has an adjustable parameter, and all the adjustable parameters are adjusted to bring every one of the one or more performance characteristics within the preselected range thereof.  
           [0031]    Preferably, the method further includes the step of: (e) selecting an initial value, of each adjustable parameter, that is unlikely to induce unacceptable crosstalk.  
           [0032]    Alternatively, the method further includes the step of: (e) storing a respective value of one of the one or more adjustable parameters.  
           [0033]    Preferably, the method further includes the step of: (f) upon starting the system, initializing an adjustable parameter based on a corresponding stored value thereof.  
           [0034]    If several communication links that are subject to crosstalk are started at the same time, it is possible that some will suffer unacceptable levels of crosstalk. If the links are started one after the other, the links started early in the process will begin to suffer more and more crosstalk as other links are started. A feedback mechanism, as provided by the present invention, can be used to adjust the data rates of the individual communication links to allow a combination of data rates that maximizes total throughput of such a system. This mechanism is especially applicable to inverse multiplex data transmission systems, because the total data rate for communication links is more important than the data rate for any individual communication link in such a system.  
           [0035]    The present invention successfully addresses the shortcomings of the presently known systems for transmitting data streams via multiple links in the presence of crosstalk by providing a system that uses feedback of information regarding the quality of data transmission via individual links in the presence of crosstalk to guide the selection of transmission speeds for the individual links so as to optimize total data throughput for the collection of links. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0036]    The invention is herein described, by way of example only, with reference to the accompanying drawings, wherein:  
         [0037]    [0037]FIG. 1 is a schematic illustration of a system for transmitting data streams via multiple links according to the present invention;  
         [0038]    [0038]FIG. 2 is a flowchart illustrating the process of establishing link data rates for a preferred embodiment of the present invention. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0039]    The present invention is of a communication system having a plurality of communication links that addresses the problem of crosstalk between the communication links by adjusting transmission parameters of the communication links according to feedback of performance parameters of the communication links. This invention is particularly applicable to an inverse multiplexing system that can be used to transmit a single data stream via multiple links, making optimal use of the available transmission capacity.  
         [0040]    The principles and operation of a system for transmitting data streams via multiple links according to the present invention may be better understood with reference to the drawings and the accompanying description.  
         [0041]    Referring now to the drawings, FIG. 1 is a schematic illustration of a preferred embodiment of a system for transmitting data streams via multiple links according to the present invention.  
         [0042]    The system in FIG. 1 includes a collection of independent communication subsystems, each independent communication subsystem including a respective transmitter  20 , a respective link  16 , and a respective receiver  22 . Links  16  may be subject to crosstalk. Each transmitter  20  includes a respective data rate adjuster  10 . Each receiver  22  includes a respective monitor  12 , operative to measure at least one performance characteristic. In this preferred embodiment, the performance characteristics measured include signal-to-noise ratio (SNR) and line attenuation. In a variation of this preferred embodiment, the performance characteristics measured include the bit error rate (BER). The monitor  12  may be implemented in hardware or software or a combination thereof. Respective feedback paths  18  are operative to transmit information about performance characteristics from monitors  12  to a controller  14 . Controller  14  is operative to transmit commands, including commands for the data rate adjusters  10 , to transmitters  20 , via control paths  24 . A variation of this preferred embodiment includes a memory  26  that is operative to store values of data rates at which links  16 , or links of other, comparable communication systems, have operated with acceptable levels of crosstalk in the past, and controller  14  selects initial values for setting data rate adjusters  10  from values stored in memory  26 . Although memory  26  is shown in FIG. 1 as being internal to controller  14 , part or all of memory  26  may, alternatively, be external to controller  14 . Such use of a memory  26  is included in the scope of the present invention.  
         [0043]    Although, for simplicity, the discussion here centers mostly on transmission of data in a single direction, it will be clear to those skilled in the art that similar considerations apply to bidirectional transmission of data. Bidirectional transmission of data is included in the scope of the present invention.  
         [0044]    [0044]FIG. 2 is a flowchart illustrating the process of establishing link data rates for a preferred embodiment of the present invention.  
         [0045]    This preferred embodiment may be better understood by following the process illustrated in the flowchart of FIG. 2. Beginning at the start box  30 , control passes to a process  32  labeled “Determine rates for links”. In process  32 , an adjustable parameter that has an influence on the level of crosstalk between links, such as data rate or signal power, is chosen for each link  16 . The preferred adjustable parameter is the data rate for each link  16 . There are many different ways to select these data rates, all of which are within the scope of the present invention. One way is to select a respective low data rate for each link  16 , so that crosstalk is minimal during the initial phase of establishment of data rates for links  16 . Another way is to select respective data rates based upon historical data collected during past operation of the system or comparable systems initializing each link  16  with a respective data rate at which that link  16 , or comparable links of this or other systems, have transmitted data successfully. Memory  26  is used to store these historical data.  
         [0046]    Control next passes to a process  34  labeled “Transmit signals on all links”. In process  34 , each transmitter  20  transmits a respective signal, which may include test data or other test signals, such as sinewaves, via its link  16 , to be received by a respective receiver  22 .  
         [0047]    Although, in this preferred embodiment of the present invention, only test data are used during the startup procedure, it may be desirable in some situations, such as file transfers, to include payload data during the startup procedure. Payload data that are successfully transmitted, as determined by well-known methods such as cyclic redundancy checks, during the startup procedure may be used as payload data by the end-user of the system, while payload data that are not successfully received may be retransmitted, as is commonly done in systems that transmit data via imperfect links. Such use of payload data for testing quality of transmission via links  16  allows the transmission system to be used by the end-user of the system for the transmission of payload data sooner than would be possible if payload data are sent only after the transmission system is fully operational at optimum data rates, although data transmission during startup may be significantly slower than the optimum rate during this phase of operation. The use of payload data for testing quality of transmission via links  16  is included in the scope of the present invention.  
         [0048]    Control next passes to a process  36  labeled “Measure quality for all links”. In this process  36  one or more performance characteristics, such as SNR and line attenuation, are measured for each link  16 .  
         [0049]    Control next passes to a decision block  38  labeled “Are all links of acceptable, but not superfluous, quality?”. If the performance characteristics for all links are at least at a lower threshold, but not above an upper threshold, the transmission rates of the communication links are optimal, control passes to finish box  40 , the initialization process is completed, and the system is ready to transmit payload data.  
         [0050]    If the condition of decision block  38  is not met, control passes to a process  42  labeled “Adjust rates for all links”.  
         [0051]    If the performance characteristic for any link  16  is below a lower threshold, new data rates for the several links  16  are selected, preferably lower data rates.  
         [0052]    Note that, if the performance characteristics of all links  16  are at least at the lower threshold, then if the performance characteristic of any link  16  is above the upper threshold, it probably is possible to transmit data at an even higher rate on that link  16 . Because, for some data transmission protocols, including Single-pair High bit-rate Digital Subscriber Line (SHDSL), transmitting data at a higher rate mostly introduces energy at higher frequencies than those found in the transmission of data at a lower data rate, without substantially increasing the energy content of lower frequencies, and the respective receiver  22  corresponding to each respective link  16  filters out energy content at frequencies higher than those necessary for operation at the data rate of the respective link  16 , increasing the data rate of a link  16  does not substantially increase crosstalk caused by that link  16  impinging upon other links  16  operating at lower data rates.  
         [0053]    Therefore, for those links  16  whose performance characteristic is above the upper threshold, the system of the present invention selects a new data rate increased by an amount that preferably depends on how much beyond the upper threshold the performance characteristic is. Preferably, the amount of the data rate increase for these links  16  is limited to the increase that would be chosen for the poorest-performing of the above-threshold links  16 . As an example, assume that a link  16  performing 3 dB above threshold can probably have its data rate increased by 1 megabit per second (1 Mbps), and that a link  16  performing 6 dB above threshold can have probably have its data rate increased by 2 Mbps. If one link  16  is found to be performing 3 dB above threshold, and a second link  16  is found to be performing 6 dB above threshold, then, in this preferred embodiment, it is preferred that, in this step, the new data rate selected for both the first link  16  and the second link  16  represent an increase of 1 Mbps.  
         [0054]    Control then passes back to the process  34  labeled “Transmit data on all links”.  
         [0055]    The measurement of performance characteristics of all links  16  and adjustment of data rates are repeated, cyclically, until an optimal combination of data rates for all communication links  16  is found.  
         [0056]    While the invention has been described with respect to a limited number of embodiments, it will be appreciated that many variations, modifications and other applications of the invention may be made.