Abstract:
A method and apparatus for transmit power control in a subscriber loop for high frequency data services are described. Transmit signal power levels are maintained at minimum levels for achieving acceptable communications between a transmitter and a receiver. Transmit signal power levels are adjusted dynamically on a link-by-link basis by employing command controllable transmit power attenuators at the transmitters. The transmit power attenuators are controlled by transmit signal power controllers which issue commands to the transmit power attenuators. Excess transmit power is avoided and parasitic signal coupling between communication links is reduced. The advantages are cross-talk reduction, longer loop length and higher data rates.

Description:
FIELD OF THE INVENTION 
     The present invention relates to data delivery on telecommunications links and, in particular, to an improved apparatus and method for power control data delivery for digital subscriber access to data services through the Public Switched Telephone Network (PSTN) using wire loops. 
     BACKGROUND OF THE INVENTION 
     An emerging trend in the telecommunications industry is to provide data services deployed on existing telephone twisted pair copper wires (local loops) utilizing a frequency spectrum above the voice frequency band. These services are susceptible to interference created by frequency compatible and frequency incompatible services carried in the same and/or adjacent binder groups. A binder group is a bundle of twisted pairs of copper wires bound together in a cable consisting of 12, 25, 50 or 100 such twisted pairs. Each twisted pair provides voice and/or data services to a subscriber. As high frequency services are added to a binder group, the high frequency traffic causes interference called “cross-talk” that reduces the effective loop reach and transmit capacity of data services in the cable. Consequently, loop reach and data capacity of data services using frequencies above the voice frequency band suffer from a loss in link reliability as more data service subscribers are added to a binder group. 
     Access to the data services is generally provided by a collection of provider transceivers at a central site connected through twisted pairs to subscriber transceivers. The twisted pairs are of various lengths due to the different location of each subscriber and the physical routing of wires. Some subscribers are near the central site while others are much farther away. Current practice has subscriber transceivers transmitting at close to maximum regulation power which is more than is generally required to maintain reliable communications with the central site at an optimum data rate, and contributes to cross-talk. 
     Attempts have been made at minimizing the cross-talk between the communications links described above. One proposal is for a better allocation of the frequency spectrum. Another is for implementation of power spectral density masks. Frequency coordination has been suggested as a way to control the use of the spectrum. The objective is to have different allocations for transmit and receive frequencies which prevent transmitters from occupying the receive spectrum. So far, however, there is no general agreement within the industry or regulatory bodies on frequency spectrum allocation. The power spectral density masks have been proposed but these limit the maximum transmit power in any frequency spectrum. So far there is no agreement in the industry as to the maximum transmit power permitted for many of the high frequency services currently being deployed. 
     There is a practical limit on the transmit signal power levels output by transceivers referred to as the “maximum transmit power”. There is also a practical minimum signal-to-noise ratio for enabling clear signal detection, referred to as a “link margin”. It is well known that signal attenuation occurs as a signal is carried by a twisted pair between transceivers and that attenuation increases with loop length. The quality of the loop and other components in the signal path between the subscriber transceiver and the provider transceiver also contribute to signal attenuation. Careful management of the physical facilities increases the loop reach and the reliability of communications, but it has been recognized that transmit signal power level control is required to enable a mass deployment of digital subscriber data services using a telephone network. 
     The current state of the art in subscriber line transmit signal power level control is described in Applicant&#39;s co-pending U.S. patent application Ser. No. 09/031,647 to Darveau filed on Feb. 27, 1998, the entire specification of which is incorporated herein by reference. Darveau teaches that the amount of cross-talk within a data transmission system in which digital data is transmitted at high speeds over a telephone network can be reduced. Darveau also teaches the use of intelligence at the remote subscriber units to reduce differences in received data signal strength at a central site, thus reducing the potential for cross-talk corruption of the data. Cross-talk is reduced by analyzing a service provider&#39;s signal received at a subscriber transceiver. All of the service provider&#39;s tranceivers transmit at a known power level. By analyzing the strength of the signal received at the subscriber tranceiver, the subscriber transceiver is enabled to determine an appropriate transmit signal power level for its transmitted signal to achieve a target signal strength at the provider tranceiver. The subscriber&#39;s transceiver transmit signal power is adjusted in coarse increments so that a data signal transmitted from the subscriber transceiver at the adjusted transmit signal power arrives at the central location at approximately the target signal strength. 
     Although this method has merit, it does not provide an optimal solution for reducing cross-talk between wire loops used for the delivery of high frequency data services. 
     As the volume and number of data services provided over twisted pair wire loops increases, transmission power management which results in more predictable and consistent data delivery rates and increased local loop reach is required. 
     OBJECTS OF THE INVENTION 
     It is therefore an object of the present invention to provide a method of transmission power management which accommodates a plurality of simultaneous communications sessions at a central site. 
     It is another object of the invention to provide a method of transmission power management which provides a more reliable data delivery. 
     It is another object of the invention to provide a method of transmission power management which enables a longer subscriber loop reach. 
     It is another object of the invention to provide a method of transmission power management which reduces the use of transmission power. 
     It is another object of the invention to provide a method of transmission power management which continuously optimizes transmission power usage. 
     It is yet another object of the invention to provide a method of transmission power management which optimizes transmit signal power levels based on existing connection conditions. 
     It is a further object of the invention to provide an apparatus for providing a data service over a twisted copper wire loop that enables a more reliable link, longer loop reach and higher transmit rates with lower bit error rates. 
     SUMMARY OF THE INVENTION 
     In accordance with the invention, there is provided a system for providing power control data delivery consistency between a central site equipped with provider transceivers and a plurality of remote subscriber transceivers located varying distances from the central site, each subscriber transceiver being connected to a provider transceiver by a wire loop, comprising: 
     a transmitter at each end of the wire loop, the transmitter including a transmit signal amplification stage having a command controllable transmit power attenuator; 
     a receiver at each end of the wire loop, the receiver including means for analyzing properties of a received signal; and, 
     at least one transmit signal power controller for each transceiver pair connected by a wire loop, the transmit signal power controller being adapted to receive the properties of the received signal and to control transmit power attenuation by the command controllable transmit power attenuator of a transmitter sending the received signal. 
     In accordance with a further aspect of the invention, there is provided a method of providing power control data delivery consistency between a central site equipped with provider transceivers and a plurality of remote subscriber transceivers, the provider transceivers and the subscriber transceivers being interconnected in pairs by a plurality of wire loops having various respective lengths, comprising the steps of: 
     receiving a communication signal at one of the transceivers transmitted by the other of the transceivers; 
     determining at least one predefined characteristic of the communication signal received; 
     determining from the at least one predefined characteristic whether the power level used to transmit the communication signal should be adjusted; and 
     issuing control commands to a command controllable transit power attenuator in the other of the transceivers to adjust the power level of the communication signal transmitted, if it is determined that the power level of the communication signal should be adjusted. 
     The invention therefore provides a method and apparatus for controlling transmit signal power levels in subscriber loops used for high frequency data services. Transmit signal power controllers in at least one of the provider and subscriber transceivers analyze communications signals transmitted by the other of the transceivers. The transmit signal power controllers examine a plurality of predetermined characteristics of the communication signal to ascertain whether the transmit signal power level should be adjusted. The goal is to regulate transmit signal power levels so that a minimum power for acceptable communications is dynamically maintained. Command controllable transit power attenuators are used to regulate transmit signal power levels. The command controllable transit power attenuators are preferably enabled to control transmit power outputs in small increments of 1-2 db. 
     Preferably, a transmit signal power controller and a command controllable transmit power attenuator are provided on each of the subscriber and provider transceivers. Proper control of transmit signal power levels reduces cross-talk within and between twisted pair binder groups, enables subscriber loop lengths to be extended and increases data transfer rates. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will now be described by way of example only, and with reference to the accompanying drawings, in which: 
     FIG. 1 is a connection diagram showing subscribers&#39; transceivers connected to a central service site; 
     FIG. 2 is a flow diagram showing a process by which the subscriber and provider transceivers synchronize to each other&#39;s transmit signals; 
     FIG. 3 is a flow diagram showing a process by which the subscriber and provider transceivers check the validity of an established connection; 
     FIG. 4 is a functional block diagram showing a subscriber&#39;s transceiver connected to a provider&#39;s transceiver, in which transmit power control in accordance with the invention is enabled on each of the subscriber and provider transceivers; 
     FIG. 5 is a flow diagram showing a process by which the provider transceiver optimizes the transmit signal power level of the subscriber transceiver according to an embodiment of the invention in which transmit power control is enabled on each of the subscriber and provider transceivers; 
     FIG. 6 is a flow diagram showing a process by which the subscriber transceiver optimizes the transmit signal power level of the provider transceiver according to an embodiment of the invention in which transmit power control is enabled on each of the subscriber and provider transceivers; 
     FIG. 7 is a functional block diagram showing a subscriber&#39;s transceiver connected to a provider&#39;s transceiver, in which transmit power control in accordance with the invention is enabled on the provider&#39;s transceiver; 
     FIG. 8 is a flow diagram showing a process by which the provider transceiver optimizes the transmit signal power level of the subscriber transceiver according to an embodiment in which transmit power control is enabled on the provider transceiver; and 
     FIG. 9 is a flow diagram showing a process by which the provider transceiver optimizes its own transmit signal power level according to an embodiment in which transmit power control is enabled on the provider transceiver. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     As shown in FIG. 1, a subscriber transceiver  10  is used by a subscriber computing system  12  located at a distance from a central site  30  to gain access to data services through a communications link, such as a twisted pair copper wire local loop  16 . Similarly, another subscriber transceiver  20  is used by another subscriber computing system  22  located closer to the central site  30  to gain access to data services through another wire loop  26 . Although wire loop  16  is longer than wire loop  26  both twisted pairs form part of a binder group  40 . At the central site  30  the wire loops  16  and  26  are connected to provider transceivers  31  and  32 , respectively. 
     If the subscriber transceivers  10  and  20  transmit at maximum power, then a signal power level received at the central site  30  from the subscriber transceiver  20  located close to the central site  30  will be much greater than the signal power level from the subscriber transceiver  10  located far from the central site  30 . 
     Due to the nature of cable construction and wiring at the central site  30 , signals from adjacent loops, and especially from loops in the same binder group, parasitically couple into one another. This parasitic coupling will likely have no effect on the communications link  26 , because the received signal strength at the central site in the communications link  16  is relatively weak and therefore its coupling into the communication link  26  is proportionally weak. However, the strength of the signal received on the communication link  26  generally causes strong coupling into the communication link  16 , which produces noise that may completely mask the transmitted signal from tranceiver  10 . Such coupling can thus have an undesirable affect on the performance of the communications link  16  manifested in a reduced data transfer rate capacity and a shorter local loop reach. 
     Excessive signal coupling onto communications link  16  can lead to situations in which transceiver  31  synchronizes to the coupled signal from communications link  26 . A process is therefore required to mitigate this situation. 
     Establishing a Communication Link 
     Communications link setup and link validation is done on a link-by-link basis. FIG. 2 is a flow diagram showing the steps taken to setup a communications link between a Subscriber Transceiver (ST) and a Provider Transceiver (PT). Data transmitted by the ST to the PT is carried by a Subscriber Transmit Signal (STS). Data transmitted by the PT to the ST is carried by a Provider Transmit Signal (PTS). 
     The default state of the PT is to continuously poll the twisted pair for the ST. On power up the ST performs a power-on reset  52 . A first step in the power-on reset  52  is to disable communications  54  with the subscriber&#39;s computing system. Once the ST has reset, it turns on its carrier signal providing an STS burst over the twisted pair at maximum power ( 56 ). 
     Detection of the STS carrier signal for 100 ms at the PT serves as an interrupt for the polling process of the PT ( 58 ). The interrupt causes the PT polling process to chirp ( 60 ) its carrier signal providing a PTS burst over the twisted pair. If the PTS carrier signal is not detected at the ST ( 62 ), the ST chirps ( 64 ) the STS providing a maximum power burst for a short period of time. Detection of the PTS carrier signal at the ST ( 62 ) triggers an attempt ( 66 ) by the ST to synchronize to the PTS carrier signal. Failure to synchronize puts the ST back into its polling state  64 . 
     Once synchronization on PTS carrier signal is achieved, the ST assesses the received power level of the PTS carrier signal and compares it with a reference value representing the maximum transmission power level of the PTS carrier signal at the PT. Based on this comparison, the ST calculates (step  68 ) the attenuation over the PTS link and computes (step  70 ) an appropriate STS carrier transmit signal power level. On detecting the STS carrier signal at the PT (step  72 ), synchronization on the STS carrier is attempted in step  76 . There are preferably a total of two attempts, detailed by steps  74 ,  76 ,  78 ,  80  and  82 . 
     Failure to synchronize on the STS carrier signal results in turning off the PTS carrier ( 84 ) and resuming the PT polling process (step  60 ) after the expiry of a predetermined time interval. Synchronization on the STS carrier ends the PT polling process  60  and enables the PTS carrier signal ( 86 ). To ensure correct synchronization the PT proceeds to validate the link ( 100 ). 
     Connection Link Validation 
     As detailed in the flow diagram shown is FIG. 3, connection validation (step  100 ) is necessary to ensure that the PT has synchronized to the STS signal from the intended ST and not to a coupled signal propagated from an STS of another ST. To begin the validation process, the PT issues (step  102 ) a “get key” command over the PTS link. The receiving ST generates (step  104 ) a key and sends it (step  106 ) to the PT. On receiving the key (step  108 ), a PT sends the key back (step  110 ) over the PTS. On receiving the key (step  112 ), the ST validates (step  114 ) that the key corresponds to the key sent. 
     A receipt of a non-valid key turns off the STS and PTS carriers (steps  118  and  120 ) and initiates resumption of the polling processes (steps  60  and  64 ) on both the ST and PT ends. A valid key enables a valid communication link (step  122 ) and an ACKnowledge message (step  124 ) is sent to the PT. At the same time communications with the subscriber computing system is enabled at the ST (step  128 ). 
     After validating the full duplex communications link, both PTS and STS transmission power level optimization is begun. 
     Transmit Power Optimization Hardware—Symmetric Implementation 
     In order to maintain an acceptable link margin, both the provider and subscriber transceivers are preferably enabled to transmit at variable power levels that may be controlled in small increments. Transmit power expended beyond what is necessary to maintain the link margin, is not only wasted, it may cause undesirable levels of coupling into adjacent communications links. 
     Preferably, transmit signal power levels are dynamically maintained at a minimum required to achieve an acceptable link margin between a subscriber transceiver and a provider transceiver. Consequently, excess transmit power is avoided and the parasitic coupling between communications links is reduced. 
     In a preferred implementation shown in FIG. 4, a full duplex communication link is set up between an ST  132  and a PT  131 . Each transceiver includes a transmitter and a receiver, as is well known in the art. To establish the full duplex communication link between the transceivers  132  and  131 , the provider transmitter  140  is linked to the subscriber receiver  150 , and the subscriber transmitter  142  is linked to the provider receiver  152 . The full duplex communications link between the transceivers  132  and  131  is enabled by a unidirectional PTS  144  and STS  146 . 
     In describing the end-to-end PTS  144  it should be understood that similar components make up the end-to-end STS  146  because the implementation is symmetric. 
     When a subscriber launches an information request, a data stream is provided by the data service provider system. The data stream is buffered in an input data buffer  161 . Data packets from the input data buffer are passed to an error check generator  163  which appends error checking bits to the data packets, forming augmented data packets. The augmented data packets are processed by a modulator  165  which outputs a transmit signal. This transmit signal is passed to a transmit signal amplification stage  167 . Under higher control  194 , the amplitude of the transmit signal is controlled by a command controllable transmit power attenuator  169 . An attenuated transmit signal output by the command controllable transmit power attenuator  169  is amplified by a signal driver  171  which outputs the communications signal corresponding to the PTS  144 . 
     The communications signal carried by the PTS  144  is received at a received signal conditioning stage  173 . Besides signal manipulation operations well known to a person skilled in the art, received signal information  192  about the received signal, such as background noise level and signal level, is extracted from the received PTS. The received signal is then demodulated by demodulator  175  into received augmented data packets and passed to an error checking stage  177 . Error checking information  190  about the signal is extracted from the received augmented data packets and in the process, error bearing received data packets are discarded. Error free data packets are passed to an output data buffer  179  which provides the subscriber system with a data stream. 
     The higher level control required to enable the present invention is provided by a transmit signal power controller  191 . The transmit signal power controller  191  accepts as input the received signal information  192  and the error checking information  190 . The information may include, but is not limited to: the number of dropped data packets or bit error rate, background noise level, received signal level, signal-to-noise ratio etc. Using this information, the transmit signal power controller  191  computes a transmit signal power level and may optionally compute a received signal detection threshold level. A received signal detection threshold adjustment, if required, is communicated to the received signal conditioning stage  173  in order to keep the signal detection threshold above the background noise level. If required, a transmit signal power level adjustment is communicated to the command controllable transmit power attenuator  169  over the STS in order to keep the link margin of the PTS within acceptable limits. Preferably, the command controllable transmit power attenuator may be controlled in increments of 1-2 db. 
     In the preferred implementation there is one transmit signal power controller  191  per transceiver, and each controller is closely related to the receiver circuitry from which signal information  190  and  192  is received. Transmit signal power level adjustment commands are issued to the respective command controllable transmit power attenuators  169  of the PTS  144  and STS  146  via dedicated communications channels. FIG. 4 shows the communications channel  194  between the transmit signal power controller  191  of the subscriber receiver  150  and the command controllable transmit power attenuator  169  of the provider transmitter  140 . The communications channels are themselves unidirectional and are multiplexed with the transmitted signals in a manner well known to persons skilled in the art. Specifically, in the preferred embodiment the communications channel  194  is multiplexed on the STS  146 . 
     Transmit Power Optimization Process—Symmetric Implementation 
     Transmit power optimization of the Subscriber Transmit Signal is performed repetitively in a program loop which dynamically computes transmit signal power level adjustments. The algorithms described below deal exclusively with transmit signal power level control. Persons skilled in the art will realize that it may be necessary to control data transfer speeds as well as transmit signal power levels. In order to enable a concise description of the invention, the control of data transfer speed is not described, although it may be incorporated into any of the algorithms described below. 
     FIG. 5 shows a flow diagram of a first optimization loop  126 . After a predetermined time delay  200 , the provider transceiver takes a reading of the subscriber transmitted signal bit error rate  202 . The time delay is preferably variable and may be influenced by several factors including transmission speed and bit error rate, the implementation being largely a matter of design choice. If the bit error rate exceeds a predetermined threshold, step  204 , the provider transceiver sends a transmit signal power level increment message, step  206 , over the provider transmit signal to the subscriber transceiver. The subscriber transceiver  132  adjusts the subscriber transmit signal power level accordingly, step  208 . Although not illustrated, it is to be understood that before the transmit signal power level is adjusted, a check is performed to ascertain if the transmit signal power level is already at maximum, in which case, the request is ignored. If the bit error rate is determined to be below the threshold in step  204 , then the provider transceiver reads the received signal information (signal-to-noise ratio of the subscriber transmit signal, for example) in step  210 . If the signal-to-noise ratio does not exceed a predetermined lower threshold, step  212 , the provider transceiver sends a transmit signal power level increment message request, step  214 , over the provider transmitter signal to the subscriber transceiver, and the subscriber transceiver adjusts the transmit signal power level of the subscriber transmit signal accordingly, step  216 . If the signal-to-noise ratio does exceed the lower threshold, another comparison is made to determine if the signal-to-noise ratio exceeds an upper threshold, step  218 . If so, the provider transceiver sends a transmit signal power level decrease message request, step  220 , over the provider transmit signal to the subscriber transceiver. The subscriber transceiver adjusts the transmit signal power level of the subscriber transmit signal accordingly, step  222 . On the provider transceiver side, the transmit signal power level decrease message request, step  220 , triggers synchronization monitoring, step  224 . If synchronization on the subscriber transmit signal is lost, the provider transceiver sends a power level increase request message over the provider transmit signal, step  226 , to the subscriber transceiver, which reacts accordingly by increasing the transmit signal power level by the requested increment, step  228 . If the synchronization is maintained, then the power optimization loop of the subscriber transmit signal returns to step  126 . 
     FIG. 6 shows a flow diagram of a process performed by the subscriber transceiver  132  to optimize the transmit signal power level of the provider transmit signal  144 . After a predetermined time delay ( 300 ) determined in the same way as described above, the subscriber transceiver takes a reading of the bit error rate of the provider transmit signal, step  302 . A determination is made (step  304 ) by the subscriber transceiver of whether the bit error rate exceeds a predetermined threshold. If the bit error rate exceeds the threshold, a transmit signal power level increase message request is sent in step  306  over the subscriber transmit signal to the provider transceiver  131 . The provider transceiver responds by adjusting the transmit signal power level of the provider transmit signal accordingly, step  308 . If the bit error rate is below the threshold, then the subscriber transceiver examines the transmit signal information (signal-to-noise ratio, for example), of the received provider transmit signal, step  310 . If the signal-to-noise ratio is below a lower threshold, step  312 , the subscriber transceiver sends a transmit signal power level increase message request, step  314 , over the subscriber transmit signal to the provider transceiver. The provider transceiver responds by adjusting the transmit signal power level of the provider transmit signal in step  316 . If the signal-to-noise ratio is above the lower threshold, in step  312  the subscriber transceiver further determines whether the signal-to-noise ratio is below an upper threshold, step  318 . If the upper threshold is exceeded, the subscriber transceiver sends a transmit signal power level decrease message request, step  320 , over the subscriber transmit signal. The provider transceiver responds by adjusting the transmit signal power level of the provider transmit signal accordingly, step  322 . On the subscriber transceiver side, the transmit signal power level decrease message request also invokes a synchronization check, step  324 . If synchronization on the provider transmit signal is lost, then the subscriber transceiver sends a transmit signal power level increase message request over the subscriber transmit signal, step  326 . The provider transceiver adjusts the transmit signal power level of the provider transmit signal accordingly, step  328 . If synchronization is maintained, power reduction of the provider transmit signal has been achieved and the transmit power optimization loop of the provider transmit signal returns to step  130 . 
     Alternate Implementation 
     An alternate implementation employs a centralized transmit signal power controller which is part of the provider transceiver  431  or the subscriber transceiver  416 . As seen in the embodiment shown in FIG. 7, signal information  490 ,  492 ,  498  and  499  about both PTS  444  and STS  446  is fed into transmit signal power controller  491  and computed transmit signal power levels are communicated through communications channel  496  to the subscriber transmitter  442  and through communications channel  494  to the provider transmitter  440 . 
     If only the provider transceiver is enabled to perform transmit power optimization, a transmit power optimization loop optimizes the transmit signal power levels of both the subscriber transmit signal and the provider transmit signal. 
     Remote transmit power optimization of the subscriber transmit signal is shown in FIG. 8, the steps of which are similar to those shown in FIG.  5 . The difference is that after remote transmit power optimization of the subscriber transmit signal is performed, local transmit power optimization of the provider transmit signal is also performed, step  600 . 
     FIG. 9 shows the local transmit power optimization of the provider transmit signal by the provider transceiver. At regular intervals, the subscriber transceiver sends the bit error rate of the provider transmit signal to the transmit signal power controller  491  (FIG.  7 ), step  602 . Upon receiving the bit error rate in step  604 , the transmit signal power controller  491  determines whether the bit error rate exceeds a predetermined threshold. If so, the transmit signal power controller  491  increments the transmit signal power level of the provider transmit signal, step  608 . The subscriber transceiver periodically sends received signal information, such as the signal-to-noise ratio of the provider transmit signal, step  610  to the transmit signal level controller  491 . On receiving the signal-to-noise ratio in step  612 , the transmit signal power controller determines whether the signal-to-noise ratio exceeds a lower threshold, step  614 . If not, the transmit signal power controller  491  increments the transmit signal power level of the provider transmit signal in step  616 . Further, if the signal-to-noise ratio exceeds the upper threshold, step  618 , the transmit signal power controller  491  decreases the transmit signal power level of the provider transmit signal, step  620 . If the power level of the provider transmit signal is decreased, that action triggers a synchronization check, step  626 . If the subscriber transceiver determines that synchronization was lost on the provider transmit signal in step  622 , it sends a synchronization lost message in step  624 , to the transmit signal power controller  491 . On receiving a synchronization lost message, the transmit signal power controller sends an appropriate command to the command controllable transmit power attenuator to increase the transmit signal power level of the provider transmit signal, step  628 . Having thus optimized the transmit signal power level of the provider transmit signal, the loop restarts at step  500  (FIG.  8 ). 
     The Bit Error Rate 
     The bit error rate of the received transmit signals described above is accumulated on a continuous basis using a sliding window type analysis that is well known in the art. This type of bit error accumulation provides for a historical trend in the variation of the transmission quality. Preferably, after each power level adjustment of the transmit signal power level, the accumulated bit error rate is re-initialized, as is the sliding window. The bit error rate threshold discussed above is preferably a ratio of about 10 −6 . To facilitate processing, a table may be used to store a maximum number of bit errors for each given transmission speed. The bit error rate threshold test therefore becomes a simple matter of comparing the accumulated bit errors with a value obtained in a lookup table indexed by data transmission speed. 
     Signal-to-Noise Ratio Thresholds 
     As described above, the power attenuation range of about 40 db is preferably enabled in the provider and subscriber transmitters. The transmit power attenuators are preferably controllable in 1-2 db increments, and under normal operating conditions, the transmit signal power level is not adjusted by more that ±1-2 bd in any power level adjustment. A suitable link margin is about 6 db over noise level on the link. The low threshold and the high threshold for the signal-to-noise threshold tests described above are therefore preferably offset about equally on respective sides of the preferred 6 db link margin. By reducing the transmit power of all transmitters at both the central site and subscriber sites, so that only as much transmit power is utilized as is required to meet the link margin, parasitic signal coupling is reduced. This enables longer loop reach, faster data transfer speeds, and data delivery consistency. Constant monitoring of the transmit signal power levels enables dynamic adaptive response to unpredictable environmental electromagnetic noise events that could otherwise interrupt a communications link. 
     The preferred embodiments of the invention described above are intended to be only exemplary of the invention and are not intended in a limiting sense. The scope of the invention is therefore intended to be limited solely by the scope of the appended claims.