Abstract:
A system and method for improving communications, wherein Frequency Division Multiplexed (FDM) modems are communicatively coupled over a bi-directional channel. Bandwidth is apportioned to satisfy a desired downstream-to-upstream data rate ratio by measuring upstream and downstream channel parameters and determining a data rate ratio. The determined ratio is compared to the desired ratio, and if the determined ratio is less than the desired ratio, the bandwidth is adjusted, and the process repeats until the desired ratio is obtained.

Description:
FIELD OF THE INVENTION  
         [0001]    The invention relates to improvements in communications technology and in particular the improvements in Asymmetric Digital Subscriber Line (ADSL) systems using Frequency Division Multiplexed (FDM) modems to maintain a desired downstream to upstream data rate ratio in the face of long and severely impaired loops.  
         BACKGROUND OF THE INVENTION  
         [0002]    New technologies have been applied to solve problems created by a bandwidth hungry communications applications. One of the new technologies is the Asymmetric Digital Subscriber Line (ADSL). ADSL has promised high downstream rates to support services such as high speed Internet data access, video and audio streaming. While ADSL can be implemented in several different forms, in one form Frequency Division Multiplexed (FDM) systems provide bandwidth for both upstream and downstream flow. Desirably, the upstream rates are much lower as compared to the downstream rates. Typically the upstream spectrum for ADSL is placed lower in the frequency band as compared to the downstream spectrum.  
           [0003]    For normal loops, loops which are considered short to medium in length, it is possible to achieve the high downstream rates. However using FDM systems implementing ADSL for long and severely impaired loops, that is loops impaired with cross talk and reasonably high noise floors, the modems may either fail to connect or if they synchronize, the downstream data rates are much lower than the upstream rates. This of course is not what ADSL is designed to provide. Particularly, the goal is to have downstream data rates larger, and perhaps much larger, than upstream data rates.  
         SUMMARY OF THE INVENTION  
         [0004]    The foregoing problem can be overcome in accordance with the invention in an Asymmetric Digital Subscriber Line (ADSL) system that includes a pair of separately-placed Frequency Division Multiplex (FDM) modems connected to a bi-directional channel. Typical FDM implementations of ASDL place the upstream spectrum at the low end of the frequency range and the downstream spectrum at the high end. However, under the current invention, bandwidth is reapportioned, e.g., the dividing line between the upstream and downstream spectrum is moved down in frequency so as to increase the bandwidth allocated to the downstream spectrum (and correspondingly reduce the bandwidth allocated to the upstream spectrum) so as to maintain or attempt to maintain the desired downstream to upstream data rate ratio.  
           [0005]    The first modem is configured to transmit downstream and the second modem is configured to transmit upstream. A method is further employed to apportion bandwidth between the upstream and downstream portions of the channel (i.e., between two separate modems) so as to satisfy a desired downstream to upstream data rate ratio. The method is capable of execution from the standpoint of either modem. The method includes the following features:  
           [0006]    a. measuring at least one upstream and downstream channel parameter for a prevailing bandwidth apportionment,  
           [0007]    b. determining, from the measured parameters, a downstream to upstream data rate ratio,  
           [0008]    c. comparing the determined data ratio to a desired ratio, and if the determined ratio is less than the desired ratio,  
           [0009]    d. adjusting the bandwidth apportionment and repeating steps a) through c).  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]    The invention will be further described in the following portion of the specification when taken in conjunction with the attached drawings in which;  
         [0011]    [0011]FIG. 1 is a block diagram showing the context in which the invention is employed;  
         [0012]    [0012]FIG. 2 discloses an embodiment of the invention via an Asymmetric Digital Subscriber Line;  
         [0013]    [0013]FIG. 3 a  is a flow diagram of the method steps implemented at the central location which cooperate with other method steps implemented at the remote location;  
         [0014]    [0014]FIG. 3 b  illustrates the method steps implemented at the remote location to cooperate with the method steps implemented at the central location in order to implement the invention; and  
         [0015]    [0015]FIG. 4 graphically illustrates the division between upstream and downstream spectrum which may be adjusted in accordance with the present invention  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0016]    [0016]FIG. 1 discloses a block diagram of the invention, where one or more modems ( 101 - 104 ) are configured within a Central Office end (ATU-C). The Central Office modems each communicate with a respective remote modem ( 105 - 108 ) that is configured at a remote or Subscriber end (ATU-R). In accordance with an embodiment of the invention, signal to noise ratio is measured at the first (central) modem. Once measured, the ratio is transmitted to the second (remote) modem, e.g. the modem transmitting upstream. Thus the second modem employs the measured signal to noise ratios to determine a downstream to upstream data rate ratio and then compare the determined ratio to a ratio desired by the user or subscriber. Furthermore, the second modem is configured to determine the adjusted levels for bandwidth apportionment. For example, the remote modem(s) (ATU-R) can move a portion of upstream bandwidth to the downstream side of the dividing line and subsequently transmit that information to the first modem so the reapportionment can be implemented.  
         [0017]    The invention is also applicable in an Asymmetric Digital Subscriber Line (ADSL) system, illustrated in FIG. 2. A pair of Frequency Division Multiplex (FDM) modems ( 201 ,  202 ) are connected to a bi-directional channel. In this configuration, the first modem  201  transmits downstream and a second modem  202  transmits upstream, where the system includes a processing device  203  that establishes a desired downstream to upstream data rate ratio. The available spectrum is broken into fixed bandwidth bins, where frequency Δf of each bin is preferably fixed at 4.3125 kHz. As per standards, the upstream bandwidth is typically from 25.875 kHz to 138 kHz (see FIG. 4), thus with Δf=4.3125 kHz the upstream bins could be configured to have 26 upstream bins, with each carrying 2 to 15 bits/Hz. The processing device  203  is embedded or programmed to measure at least one upstream and downstream channel parameter or ratio for a prevailing bandwidth apportionment. Once the parameter is measured, a desired downstream-to-upstream rate ratio is targeted. A comparator (not shown) in the processing device then compares the determined ratio to the desired ratio. If the determined ratio is determined to be less than the desired ratio, the processing device executes a control mechanism or algorithm in order to adjust the prevailing bandwidth apportionment. While FIG. 2 exemplifies a single processing apparatus, it is understood that the above may be identically executed with multiple processing apparatuses of varying configurations.  
         [0018]    The illustration of FIGS. 3A and 3B shows the steps carried out to control the apportionment of bandwidth. From the central office, the process is initiated  301 , where the processing apparatus establishes the size of the upstream (N U ) and Downstream (N D ) spectrums in step  302 . An illustration of the bandwidths N U  and N D  can bee seen in FIG. 4. The processing apparatus then measures the signal-to-noise ratio (SNR) at the central office modem (ATU-C), step  303 . Once established, the SNR is sent to the remote site modem (ATU-R) in step  304 . If the bandwidth apportionment is completed  305 , the process ends  307 . However, if a new or further reapportionment is required, the processing apparatus receives the parameters N U  and N D  for the next iteration, and the process reverts back to  303  as shown in FIG. 3A.  
         [0019]    [0019]FIG. 3B further illustrates the steps carried out for bandwidth reapportioning at the remote modem side. After the remote modem begins the process  311 , the size of the upstream and downstream spectrum is established  312 . Once the size is determined, the SNR is then measured  313 . Once the SNR is received  314  (sent in step  304  of FIG. 3A), the remote site computes the downstream (DS) and upstream (US) data rates  315  and then makes a determination in  316  whether the DS/US ratio is greater than a desired ratio K T . If the DS/US ratio is determined to be at an acceptable level, the algorithm completes  317  and then establishes and sends the ratio to the remote modem (ATU-R)  318 .  
         [0020]    If the ratio is not acceptable, the algorithm proceeds to  319 , where N U  and N D  is redefined and then sent back to the central modem (ATU-C)  320 , where the process in FIG. 3A is executed.  
         [0021]    For example, once the upstream and downstream SNR&#39;s are measured, the algorithm may compute and determine that the downstream data rate is less than the upstream data rate. In this example, it is assumed that 8 bins were assigned to the downstream and 4 bins were assigned to the upstream. If the last five bins in the downstream channel have a low SNR, those bins may not be used since they cannot carry bits that are dependent on the line characteristic and can vary from line to line. Thus, if a fixed number of bits are assigned to each bin (8 bits/Hz/bin):  
         upstream data rate=4 kHz×8 bits/Hz×4=128 k bits  
         downstream data rate=4 kHz×8 bits/Hz×3=9.6 k bits.  
         [0022]    From this case it is evident that the downstream rate will be less than the upstream rate (thus the DS/US will be less than the desired ratio [DS/US&lt;1]). Under the present invention, the downstream rates are modified in order to keep them greater than the upstream rates at any given time during the transmission. Under one embodiment, the protocol increases the downstream data rates in relation to the upstream rates by reassigning the bins according to the predetermined ratio. In other words the number of bins assigned to the downstream transmission (N D ) is increased, and as a consequence, N U  is decreased. The desired ratio is typically greater than one, and is determined at the time of startup. The protocol under the present invention can either re-define the upstream and the downstream spectral bands, or may assign the initially assigned upstream bins to the downstream for increasing the downstream data rates. It is understood that the channel bandwidth apportionment may also be configured dynamically, so that channel characteristics that change over time may be taken into configuration. It is also understood that the defined DS/US ratio may be fixed at various values in accordance with the designer&#39;s needs.