Abstract:
In a particular embodiment, a system for communicating data includes a data switch coupled to one or more CPE devices. The data switch may communicate with one or more CPE devices using a first predetermined PSD and using a second predetermined PSD. The operability of the switch is provided by software embodied in a computer-readable medium. In a more particular embodiment, the data switch may also communicate substantially simultaneously with two or more CPE devices using at least two different PSDs.

Description:
TECHNICAL FIELD OF THE INVENTION 
   This invention relates in general to the field of communications and in particular to a switching system supporting data communications supported by multiple power spectra. 
   BACKGROUND OF THE INVENTION 
   High-frequency signals transmitted on separate lines within a bundle of twisted pair copper cables may interfere with each other. The nature and degree of interference may vary depending on the nature of the cabling and the characteristics of the communication. For this reason, national and international standards and regulations may govern transmissions across trunk lines within communication networks. For example, transmissions across trunk lines within a proprietary network interfacing directly with a public switched telephone network (PSTN) may be regulated by standards adopted by the American National Standards Institute (ANSI) or the European Telecommunications Standards Institute (ETSI), depending on the location of the PSTN. It may therefore be necessary for a switch using such a network to communicate with customer premises equipment (CPE) devices within the network using a power spectral density (PSD) that complies with ANSI or ETSI standards. In contrast, transmissions across lines within a proprietary network coupled to a PSTN via a private branch exchange (PBX) may be unconstrained by such standards. Therefore, a switch supporting a network coupled to a PSTN via a PBX may be free to communicate with CPE devices within the network using a PSD that provides a higher bit rate or greater reliability than PSDs that comply with ANSI or ETSI standards. 
   Traditionally, field-replaceable line cards have been used to provide switches that may support communication in different spectra. However, the use of line cards has several drawbacks. For example, the use of line cards adds to costs associated with setting up and operating switches and increases the size of switch “footprints.” Moreover, a single line card may support communication in only one power spectrum. Network administrators may therefore have to replace line cards supporting communication between the switch and CPE devices to effect a change in the PSD used for data communication within the network. 
   SUMMARY OF THE INVENTION 
   According to the present invention, disadvantages and problems associated with previous switching systems supporting communication in multiple power spectra are substantially reduced or eliminated. 
   In a particular embodiment, a system for communicating data includes a data switch coupled to one or more CPE devices. The data switch may communicate with one or more CPE devices using a first predetermined PSD and using a second predetermined PSD. The operability of the switch is provided by software embodied in a computer-readable medium. In a more particular embodiment, the data switch may also communicate substantially simultaneously with two or more CPE devices using at least two different PSDs. 
   The present invention provides a number of important technical advantages over previous switching systems supporting communication in multiple power spectra. A switch may support communication in multiple power spectra without the use of field-replaceable line cards, which may reduce or eliminate costs associated with line cards, decrease switch footprints, and make it easier for network administrators to change switch port configurations. In one embodiment, a switch may communicate with different CPE devices using different PSDs. This may benefit networks where the optimal PSD for communication between the switch and CPE devices varies from link to link. Moreover, the present invention may allow network administrators to define their own PSDs for communication between a switch and CPE devices, providing for even greater flexibility in communication between the switch and CPE devices within the network. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     To provide a more complete understanding of the present invention and the features and advantages thereof, reference is made to the following description, taken in conjunction with the accompanying drawings, in which: 
       FIG. 1  illustrates an exemplary system for providing voice and data services; 
       FIG. 2  illustrates an exemplary switch; 
       FIG. 3  illustrates an exemplary physical media controller (PMC); 
       FIG. 4  illustrates an exemplary CPE device; 
       FIGS. 5A through 5D  illustrate various PSDs for communication between switches and CPE devices; and 
       FIGS. 6A through 6D  illustrate various methods for establishing and maintaining links in desired modes. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1  illustrates an exemplary system  10  for providing voice and data services. System  10  may provide voice and data services for a subscriber community, which may include one or more businesses, apartment complexes, or other communities in which subscribers share communications infrastructure or resources, such as switch  12  and PBX  14 . Each subscriber in the served community may access voice and data services using one or more associated telephones  16 , personal computers  18 , or other suitable devices for interfacing with a telephone network, such as PSTN  20 , or a data network, such as wide area network (WAN)  22 . 
   CPE devices  24  and downstream splitters  26  couple PCs  18  and telephones  16  with cable trunks  28 . Downstream splitters  26  separate downstream data traffic from downstream voice traffic for communication to PCs  18  and telephones  16 , respectively, and combine upstream data traffic with upstream voice traffic for communication across cable trunks  28  to switch  12  and PBX  14 , respectively. Reference to voice traffic is meant to encompass any appropriate signals that may be communicated between telephones  16  and PSTN  20 , and reference to data traffic is meant to encompass any appropriate voice, video, multimedia, data, or other wideband or broadband calls, traffic, or other signals that may be communicated between WAN  22  and PCs  18 . Upstream splitters  30  separate upstream data traffic from upstream voice traffic for communication to switch  10  and PBX  14 , respectively, and combine downstream data traffic with downstream voice traffic for communication across trunks  28  to PCs  18  and telephones  16 , respectively. 
   Trunks  28  provide links between downstream splitters  26  and upstream splitters  30  and may carry both voice and data traffic. Transmissions across trunks  28  may be affected by the physical characteristics of trunks  28 . For example, attenuation across trunk  28  may be affected by trunk length. Typically, attenuation is greater for longer trunks than on shorter trunks. As a result, the optimal PSD for communicating data across trunk  28  may vary according to the length of trunk  28 . Herein, a particular PSD used for communications between switch  12  and CPE devices  24  may be referred to as a mode, and vice versa. In an “unregulated spectra” environment, as shown in  FIG. 1 , transmissions across trunks  28  are not constrained by public standards, regulations, or other constraints. The interface between system  10  and PSTN  20  provided by PBX  14  may eliminate the need for compliance with standards or public regulations within system  10 . Accordingly, the PSDs for transmissions across trunks  28  may vary trunk-to-trunk and may be set by a system administrator to optimize the transmission of data across trunks  28 . In a “regulated spectra” environment, transmissions across trunks  28  may be subject to public standards or regulations. For example, ANSI standards may govern the PSD used for communications across trunks  28 . Typically, public standards and regulations define PSDs that are, in effect, a “lowest common denominator,” supporting communications across a variety of networks under many different circumstances. As a result, PSDs defined by public standards and regulations may be substantially inefficient, providing, for example, substantially low bit rates in comparison with that which may be achieved in an optimized configuration. Public standards or regulations may govern transmissions across trunks  28  for any valid reason. For example, system  10  may exclude PBX  14 , resulting in a direct connection between upstream splitters  30  and PSTN  20  and transmissions across trunks  28  may be required to comply with standards governing communications with PSTNs. 
   Switch  12  may provide an interface between system  10  and WAN  22  or other suitable data network. Switch  12  may be a DSL access multiplexer (DSLAM) unit or other device for routing or aggregating data traffic communicated between WAN  22  and PCs  18 . In one embodiment, switch  12  is a very high speed DSL (VDSL) switch. Switch  12  may include one or more ports  32  coupling switch  12  with switch links  34 . For example, port  32   a  may couple switch  12  with switch link  34   a , which provides a link between switch  12  and PC  18   a , along with trunk  28   a . Switch  12  may operate in regulated spectra environments and in unregulated spectra environments. In a regulated spectra environment, switch  12  may communicate with CPE devices  24  using a PSD that complies with applicable standards or regulations. In an unregulated spectra environment, switch  12  may communicate with CPE devices  24  using non-compliant PSDs, which may vary from trunk-to-trunk and may be individually optimized for each line coupling CPE devices  24  with switch  12 . Reference to line is meant to encompass the physical link between switch  12  and a CPE device  24 , which may include a switch link  34 , an upstream splitter  30 , a trunk  28 , and a downstream splitter  26 . In one embodiment, the parameters of different modes may be stored by switch  12 , such that a network administrator may optimize a line by picking from a set of modes for communication between switch  12  and CPEs  24 . 
     FIG. 2  illustrates an exemplary switch  12 . Switch  12  may include memory  36 , switch controller  38 , switch fabric  40 , and physical media controllers (PMCs)  42 . Memory  36 , described more fully below in connection with switch controller  38 , may contain information used by switch controller  38  to establish communication links between CPE devices  24  and switch  12 . Switch fabric  40  may provide an internal switching infrastructure linking WAN  22 , switch controller  38 , and PMCs  42 . PMCs  42 , also described more fully below, generally provide a physical-layer interface between switch fabric  40  and switch links  34 . 
   In general, switch controller  38  controls the operation of switch  12 . When a CPE device  24  is coupled to switch  12 , switch controller  38  may receive control packets from CPE device  24  notifying switch controller  38  of the network presence of CPE device  24 . Such control packets may include, for example, framed “sync” messages communicated using reduced quadrature amplitude modulation (QAM). In this way, the VDSL chipsets may actively try to achieve link with a suitable device that may be able to receive messages from CPE device  24 . Switch controller  28  may set up links between CPE devices  24  and switch  12 . Reference to link in this context is meant to include any suitable system enabling communication defined by a particular PSD. In a regulated spectra environment, all links between CPE devices  24  and switch  12  must comply with the applicable standards or regulations. In an unregulated spectra environment, switch controller  38  may set up different links between CPE devices  24  and switch  12  according to particular needs. For example, switch controller  38  may set up a “long line” mode link between switch  12  and CPE  24   a  and a “short line” mode link between switch  12  and CPE  24   b . A long line mode link may be suitable for communication over a line that is approximately 4,000 feet long, and a short line mode link may be suitable for communication over a line that is approximately 2,000 feet long. Switch controller  38  may set up links according to one or more selections made by a network administrator. For example, switch controller  38  may set up a long line mode link between switch  12  and CPE device  24   n  in response to a network administrator selecting long line mode for communication between switch  12  and CPE device  24   n . Switch controller  38  may also set up a link between switch  12  and each CPE device  24  in system  10  that complies with a particular standard or set of standards in response to a network administrator selecting such a mode. The mode selected by a network administrator for communication over a particular line may be referred to as the desired mode for communication over the line. In one embodiment, network administrators may select from a set of modes provided by the vendor or manufacturer of the switch or create their own modes for communication between switch  12  and CPE devices  24 . Switch controller  38  may determine the desired mode for communication over a line by accessing the profile for the port coupling switch  12  with the line. Port profiles may be stored in memory  36  coupled to switch controller  38  and modified by network administrators as needed. A particular port profile may contain information reflecting the desired mode for a line that may be supported by the corresponding port. Memory  36  may also contain information reflecting a “high probability” mode, described more fully below, which may be used for communication between switch  12  and CPE devices  24  for setting up desired mode links. 
   As described above, transmissions across trunks  28  are not constrained by public standards in an unregulated spectra environment, and the PSDs for transmissions across trunks  28  may vary from trunk  28  to trunk  28  to optimize the transmission of data across trunks  28 . In contrast, transmissions across each trunk  28  in a regulated spectra environment may be required to comply with one or more public standards or regulations, which may restrict transmissions across all trunks  28  to a single PSD complying with the applicable public standards or regulations. Accordingly, when switch  12  is set for operation in a regulated spectra environment (by a network administrator or otherwise) switch controller  38  may automatically configure each port  32  of switch  12  to use a particular PSD complying with the applicable public standards or regulations for communications with CPE devices  24 . In this way, switch  12  may communicate with all CPE devices  24  using a particular standards-compliant PSD in a regulated spectra environment, but may communicate with different CPE devices  24  using different PSDs in an unregulated environment. 
   To establish a desired mode link between a switch  12  and a CPE device  24 , switch controller  38  may modify some of control registers  44 ,  46 , and  48  within PMC  42  supporting the line between switch  12  and CPE Device  24  to reflect the parameters of the desired mode. A mode may be defined by six or more parameters, including the following: the upper and lower frequency limits for downstream traffic; the upper and lower frequency limits for upstream traffic; and the power levels for transmissions within those frequency ranges. As an example, switch controller  38  may modify registers  44 ,  46 , and  48  within PMC  42   a  to establish a desired mode link between switch  12  and CPE device  24   a . Switch controller  38  may also modify registers  50 ,  52 , and  54  within CPE devices  24  to establish desired modes links by communicating control packets to CPE devices  24 . For example, switch controller  38  may communicate to CPE device  24   a  control packets containing information reflecting the desired mode parameters which CPE device  24   a  may use to modify registers  50 ,  52 , and  54  within CPE device  24   a  accordingly. 
   When a link goes down, control processor  38  may modify registers  44 ,  46 , and  48  within PMC  42  supporting the link to reflect high-probability mode, described more fully below. A link may go down when, for example, there is a discontinuity in the line supporting the link, which may be caused by a physical break in the line. In one embodiment, switch controller  38  may wait a predetermined amount of time, such as thirty seconds, after a link has gone down before modifying registers  44 ,  46 , and  48  to reflect high-probability mode. In this way, a desired mode for communication across a link does not have to be re-established if the link goes down for less than the predetermined amount of time. Switch controller  38  may access an internal or external timer to determine the amount of time that has passed since a link has gone down. 
     FIG. 3  illustrates an exemplary PMC  42 . PMC  42  may include modulator  56 , demodulator  58 , frequency generator  60 , and PMC controller  62 . Modulator  56  receives data from switch fabric  40 , encodes that data for transmission across the line coupling switch  12  with CPE device  24  supported by PMC  24 , and communicates that data downstream across switch link  34  to CPE device  24 . The signal power output of modulator  56  may be determined by mode parameter information stored in register  44 . For example, modulator  56  may transmit downstream data at power levels between minus one hundred decibels per hertz (dBm/Hz) and minus fifty dBm/Hz if register  44  contains parameter information reflecting that range. The signal frequency output of modulator  56  may be set by frequency generator  60 , which communicates a frequency signal to modulator  56 . Demodulator  58  may receive data from CPE devices  24 , decode that data for communication to switch fabric  40 , and communicate that data to switch fabric  40  for communication to switch controller  38  or WAN  22 . Demodulator  58  may decode data using information reflecting the current mode for communication between switch  12  and CPE device  24  stored in register  46  and the frequency signal communicated to demodulator  58  by frequency generator  60 . Frequency generator  60  may generate frequency signals that may be used by modulator  56  and demodulator  58  to encode and decode transmissions across lines between switch  12  and CPE devices  12 . PMC controller  62  may receive control messages from switch controller  38  and modify registers  44 ,  46 , and  48  in modulator  56 , demodulator  58 , and frequency generator  60 , respectively, to reflect the desired mode for communication between switch  12  and CPE device  24 . Registers  44 ,  46 , and  48 , may also be modified to store parameters defining a high-probability mode. 
     FIG. 4  illustrates an exemplary CPE device  24 . Exemplary CPE device  24  may include modulator  64 , demodulator  66 , frequency generator  68 , CPE controller  70 , and memory  72 . Modulator  64  receives data from PC  18 , encodes that data for transmission across the line coupling CPE device  24  with switch  12 , and communicates that data upstream to switch  12 . Similar to PMC modulator  56 , the signal power output of modulator  64  may be determined by mode parameter information stored in register  50 , and the signal frequency output of modulator  64  may be set by frequency generator  68 , which communicates a frequency signal to modulator  64 . Demodulator  66  receives data from switch  12 , decodes that data for communication to PC  18 , and communicates that data downstream to PC  18 . Demodulator  66  may decode data using information about the current mode for communication between switch  12  and CPE device  24  stored in register  52  and a frequency signal communicated to demodulator  66  by frequency generator  68 . Frequency generator  68  generates frequency signals that may be used by modulator  64  and demodulator  66  to encode and decode transmissions across the line between switch  12  and CPE device  24 . Frequency generator  68  contains register  54  for generating the appropriate frequency signals according to the current mode for communication between switch  12  and CPE device  24 . CPE controller  70  may receive control messages from switch controller  38  and modify registers  50 ,  52 , and  54  to reflect the desired mode for communication between switch  12  and CPE device  24 . Registers  50 ,  52 , and  54  may also be modified to reflect high-probability mode parameters, which may be stored in CPE memory  72 . CPE memory  72  may include a read-only memory (ROM) device, such as an electrically erasable programmable ROM (EEPROM), containing information reflecting high-probability mode parameters. 
   When a desired mode link goes down, CPE controller  70  may reset registers  50 ,  52 , and  54  to high-probability mode. CPE controller  70  may wait a predetermined amount of time, such as thirty seconds, after a desired mode link has gone down before resetting registers  50 ,  52 , and  54 . Similar to switch controller  62 , CPE controller may access an internal or external timer to determine the amount of time that has passed since a link has gone down. 
     FIGS. 5A through 5D  illustrate various PSDs for communication between switch  12  and CPE devices  24 .  FIG. 5A  illustrates a PSD that may be used for communication between switch  12  and CPE devices  24  in a regulated spectra environment where communications across trunks  28  are governed by ANSI T1E1.4 standards. In this example, downstream data is communicated at frequencies between nine hundred kilohertz and three thousand four hundred kilohertz and at power levels between minus fifty dBm/Hz and minus one hundred dBm/Hz. Upstream data is communicated at frequencies between four megahertz and five thousand two hundred fifty kilohertz, also at power levels between minus fifty and minus one hundred dBm/Hz. Downstream and upstream data may be received at a power level low than that transmitted due to attenuation over the line, which may vary according to line length and cable characteristics. Data received at power levels below minus one hundred forty dBm/Hz may be unretrievable due to noise. Accordingly, a “noise floor” exists at minus one hundred forty dBm/Hz, and the PSD illustrated in  FIG. 5A  excludes area  74 . The area between the received PSD and the noise floor may govern the rate at which data may be transported across the link. 
     FIG. 5B  illustrates a PSD that may be used for communications between switch  12  and CPE devices  24  in an unregulated spectra environment across substantially short lines. A short line may, for example, be approximately two thousand feet long or shorter. Downstream traffic may be transmitted at frequencies between nine hundred kilohertz and three thousand two hundred kilohertz. Upstream traffic may be transmitted at frequencies between three thousand two hundred kilohertz and eight megahertz. Both upstream and downstream traffic may be transmitted at power levels between minus fifty dBm/Hz and minus one hundred dBm/Hz. For a short line, the received PSD is substantially the same as the transmitted PSD.  FIG. 5C  illustrates a PSD as received at the end of substantially long lines, corresponding to transmissions depicted in  FIG. 5B . The received power in this case may be minus one hundred dBm/Hz. Thus, the area shown above the noise floor, which is the area available for noise communication, is much smaller than that for short lines. In order to optimize transmissions across long lines, CPE devices  24  may use a modulation requiring a lower signal to noise ratio. Also, CPE devices  24  may lower the maximum frequency for transmissions as the high frequency signals suffer greater attenuation. 
     FIG. 5D  illustrates a “high-probability” PSD that may be used for initial communications between switch  12  and CPE devices  24 . High-probability mode may be a default mode used for communications between switch  12  and CPE devices  24  for establishing links in desired modes. In one embodiment, high-probability mode may be used for communications in any environment and complies with all standards that may govern transmissions across trunks  28 . The chances of establishing a link in high-probability mode may be greater than in other modes, but the rate of transmission across a high-probability mode link may be lower. 
     FIGS. 6A through 6D  illustrate exemplary methods for establishing and maintaining links in desired modes between switch  12  and CPE devices  24 . 
     FIG. 6A  illustrates an exemplary method for establishing a link between switch  12  and a CPE device  24  in a desired mode. The method begins at step  100  where CPE device  24  is physically coupled to switch  12 . This may occur when, for example, CPE device  24  is first connected to trunk  28  supporting communication between switch  12  and CPE device  24 . At step  102 , switch  12  communicates control packets with CPE device  24  in high-probability mode requesting a high-probability mode link with CPE device  24 . CPE device  24  communicates, at step  104 , control packets to switch  12  in high-probability mode to make its presence on the line known to switch  12 . As described above, these control packets may include, for example, framed “sync” messages communicated using reduced quadrature amplitude modulation (QAM). In this way, the VDSL chipsets may actively try to achieve link with a suitable device that may be able to receive messages from CPE device  24 . In one embodiment, switch  12  continuously “listens” for CPE devices  24  and may detect CPE devices  24  when they are coupled to switch  12 . Switch  12  receives the control packets from CPE device  24  at step  106  and determines, at step  108 , whether a high-probability mode link has been established between switch  12  and CPE device  24 . If a high-probability mode link has not been established, switch  12  waits, at step  110 , to receive from CPE device  24  control packets communicated in high-probability mode. If a link has been established, the method proceeds to step  112 , where switch controller  38  accesses the port profile for port  32  coupling switch  12  with CPE device  24  to determine the desired mode for communication between switch  12  and CPE device  24 . After determining the desired mode for communication, switch controller  38  communicates to CPE controller  70  control packets containing the desired mode parameters for communication between switch  12  and CPE device  24  at step  114 . CPE controller  70  then receives the control packets at step  116  and modifies registers  50 ,  52 , and  54  to reflect the desired mode parameters for communication between switch  12  and CPE device  24  at step  116 . At step  118 , switch controller  38  modifies PMC registers  44 ,  46 , and  48  to reflect the desired mode parameters for communication between switch  12  and CPE device  24 . Switch  12  then attempts, at step  120 , to establish a link with CPE device  24  in the desired mode. Switch  12  may accomplish this by communicating packets with CPE device  24  in the desired mode. If a desired mode link is established at step  122 , switch  12  and CPE device  24  start communicating data in the desired mode at step  124 , and the method ends. If a desired mode link is not established at step  122 , CPE device  24 , at step  126 , waits thirty seconds from the time registers  50 ,  52 , and  54  were modified for the desired mode link to be established. In one particular embodiment, CPE device  24  may wait thirty seconds from the time PMC registers  44 ,  46 , and  48  are modified to reflect the desired mode parameters. If the desired mode link is not established within that time, CPE controller  70  resets registers  50 ,  52 , and  54  to reflect high-probability mode parameters. In this way, CPE device  24  resets itself to high-probability mode if a desired mode link is not established within a set amount of time. Similarly, switch controller  38  waits, at step  130 , thirty seconds from the time registers  44 ,  46 , and  48  in PMC  42  coupling CPE device  24  with switch fabric  40  are modified for the desired mode link to be established. If thirty seconds pass without a desired mode link being established, the method proceeds to step  132  where switch controller  38  modifies registers  44 ,  46 , and  48  to reflect high-probability mode parameters. In this way, switch  12  and CPE device  24 , after having been unable to establish a desired mode link, reset themselves to communicate in high-probability mode to try again to establish a desired mode link. In a particular embodiment, CPE device  24  and switch  12  may use the same thirty-second timer, which may begin to run substantially immediately after step  118 , to determine when to reset their respective registers. Accordingly, the method proceeds to step  134 , where CPE device  24  communicates control packets to switch  12  to request switch  12  to establish a high-probability link between switch  12  and CPE device  24 . The method then proceeds to step  104 , where switch  12  and CPE device  24  attempt to establish the desired mode link for communication. 
     FIG. 6B  illustrates an exemplary method for reacquiring a desired mode link that has gone down. As described above, a link may go down when there is a discontinuity in the line coupling switch  12  with CPE device  24 . The method begins at step  136 , where the desired mode link between switch  12  and CPE device  24  goes down. The system administrator is notified, at step  138 , by switch  12  that the desired mode link has gone down, and switch  12  attempts, at step  140 , to reacquire the desired mode link. Switch  12  may attempt to reacquire a desired mode link by communicating with CPE device packets in desired mode. At step  142 , if the desired mode link is reacquired, switch  12  and CPE device  24  resume communication in the desired mode at step  144 , and the method ends. If, however, the desired mode link is not reacquired, the method proceeds to step  146 , where CPE device  24  and switch  12  wait thirty seconds from the time the desire mode link went down. As described above, switch controller  38  and CPE controller  70  may each contain timers for determining the amount of time that has passed since a link has gone down. After 30 seconds have passed, switch controller  38  modifies registers  44 ,  46 , and  48  in PMC  42  coupling switch  12  with CPE device  24  to reflect high-probability mode parameters. Similarly, at step  150 , CPE controller  70  modifies registers  50 ,  52 , and  54  to reflect high-probability mode parameters. Once CPE device  24  and switch  12  have reset to high-probability mode, the method proceeds to step  102  in  FIG. 6A , where CPE device  24  and switch  12  attempt to establish a link in the desired mode. 
     FIGS. 6C and 6D  illustrates an exemplary method for changing a link from one desired mode to another. The method begins at step  152 , where a system administrator changes the desired mode for communication between switch  12  and a CPE device  24 . Switch controller then communicates, at step  154 , to CPE controller  70  in the old desired mode control packets containing the desired mode parameters for communication between switch  12  and CPE device  24 . CPE controller  70  then receives the control packets at step  156  and modifies registers  50 ,  52 , and  54  to reflect the new desired mode parameters. At step  158 , switch controller  38  also modifies registers  44 ,  46 , and  48  and PMC  42  coupling CPE device  24  with switch fabric  40  to reflect the new desired mode parameters. After these registers have been modified, switch  12  attempts to establish a link with CPE device  24  in the new desired mode at step  160 . If a desired mode link is established at step  162 , the method proceeds to step  164 , where switch  12  and CPE device  24  begin communicating in the new desired mode, and the method ends. If, however, the desired mode link is not established at step  162 , the method proceeds to step  166 , where CPE controller  70  waits thirty seconds from the time registers  50 ,  52 , and  54  are modified for the new desired mode link to be established. If thirty seconds pass without the desired mode link being established, CPE controller  70  modifies, at step  168 , registers  50 ,  52 , and  54  to reflect high-probability mode parameters. In this way, when a predetermined amount of time elapses without a new desired mode link being established, CPE device  24   a  resets itself to high-probability mode. At step  170 , switch controller  38  also waits thirty seconds from the time registers  44 ,  46 , and  48  in PMC  42  coupling CPE device  24  with switch fabric  40  are modified for the new desired mode link to be established. Likewise, if thirty seconds pass without the new desired mode link being established, switch controller  38  notifies the system administrator that the new desired mode link has not been established and modifies registers  44 ,  46 , and  48  in PMC  42  coupling CPE device  24  with switch fabric  40  to reflect high-probability mode parameters at step  172 . Once PMC  42  and CPE device  24  have been reset to communicate in high-probability mode, CPE device  24  communicates, at step  174 , control packets to switch  12  to request switch  12  to establish a high-probability mode link between switch  12  and CPE device  24 . As described above, these control packets may include, for example, framed “sync” messages communicated using reduced quadrature amplitude modulation (QAM). In this way, the VDSL chipsets may actively try to achieve link with a suitable device that may be able to receive messages from CPE device  24 . At step  176 , switch  12  receives the control packets from CPE device  24  and, at step  178 , communicates control packets with CPE device  24  in high-probability mode to establish a high-probability mode link with CPE device  24 . 
   At step  180 , if a high-probability mode link is not established between switch  12  and CPE device  24  the method proceeds to step  182 , where switch  12  waits to receive from CPE device  24  control packets communicated in high-probability mode. If a high-probability mode link is established at step  180  between switch  12  and CPE device  24 , the method proceeds to step  184 , where switch controller  38  communicates to CPE controller  70  control packets containing the old desired mode parameters for communication between switch  12  and CPE device  24 . (Alternatively, switch controller  38  may re-try to establish the new desired mode link one or more times.) CPE controller  70  then receives, at step  186 , the control packets and subsequently modifies registers  50 ,  52 , and  54  to reflect the old desired mode parameters for communication between switch  12  and CPE device  24 . Switch controller  38  then modifies registers  44 ,  46 , and  48  in PMC  42  coupling CPE device  24  with switch fabric  40  to reflect the old desired mode parameters at step  188 . After switch  12  and CPE device  24  have been reset to communicate in the old desired mode, switch  12  attempts, at step  190 , to establish a link with CPE device  24  in the old desired mode. At step  192 , if the old desired mode link is established, the method proceeds to step  194 , where switch  12  and CPE device  24  begin communicating in the old desired mode, and the method ends. If, however, the old desired mode link is not established at step  192 , the method proceeds to step  196 , where CPE device  24  waits thirty seconds from the time registers  50 ,  52 , and  54  were modified for the old desired mode link to be established. If thirty seconds pass without the old desired mode link being established, CPE controller  70  resets registers  50 ,  52 , and  54  to reflect high-probability mode parameters at steps  196  and  198 . Switch controller  38  also waits thirty seconds from the time PMC registers  44 ,  46 , and  48  were modified for the old desired mode link to be established at step  200 . If thirty seconds pass from the time registers  44 ,  46 , and  48  are modified to reflect the old desired mode without the old desired mode link being established, the method proceeds to step  202 , where switch controller  38  modifies registers  44 ,  46 , and  48  in PMC  42  coupling CPE device  24  with switch fabric  40  to reflect high-probability mode parameters. After switch  12  and CPE device  24  have been reset to communicate in high-probability mode, the method proceeds to step  174 , where switch  12  and CPE device  24  attempt to establish a high-probability mode link before establishing a link in the old desired mode. 
   The invention described herein provides numerous advantages over the prior art. For example, the present invention provides for greater flexibility in configuring links between switch  12  and CPE devices  24 . Switch  12  may support data communication in multiple power spectra, may support communication in both regulated spectra and unregulated spectra environments, may communicate with different CPE devices  24  using different PSDs, and may communicate using “custom” PSDs defined by network administrators. Additionally, links may be individually optimized in an unregulated spectra environment to maximize bit rates and minimize data loss across communication lines. The present invention also provides compatibility with existing data networks and provides for robust links between switch  12  and CPE devices  24  in that switch  12  may reacquire links that have gone down without intervention by network administrators. 
   Although the present invention has been described with one embodiment, divers changes, variations, alterations, transformations, and modifications may be suggested to one skilled in the art, and it is intended that the present invention encompass such changes, variations, alterations, transformations, and modifications as fall within the scope and spirit of the appended claims.