Patent Document

TECHNICAL FIELD 
     The present invention relates to a system and a method for distributing power to telecommunication subscriber lines. 
     BACKGROUND 
     The Plain Old Telephony Service (POTS) has since its origin been based on remote power feeding also called battery feed. The phone set at the subscriber premises is powered by using the current flowing in the subscriber loop provided by a DC power supply in the central office. Using remote power feeding the phone also works in case of mains outage, a feature sometimes called “life-line”. 
     The power needed to feed a subscriber line depends on the loop length. Longer subscriber lines need a higher voltage level than shorter lines and can be up to 60 Volt DC. This level has been selected to be high enough to get through kilometers of subscriber lines but still low enough to be safe. 
     Excess power that is not transmitted along the subscriber line (due to impedance mismatch) is dissipated as heat at the subscriber line interface unit (line card) and is referred to as power dissipation. 
     Different solutions to provide power feeding to subscriber lines are known. One solution is to use a DC power supply (today often a DC/DC converter) for each subscriber line. By using one DC power supply per subscriber line, each subscriber line can be fed by a voltage level just above what it needs to operate and the power dissipation is minimized. 
     One disadvantage with this solution is that it is expensive. Another disadvantage is that the design becomes very complex for line cards comprising a large number of subscriber line interface circuits (SLIC). All DC/DC converters also consume a large area on the line card. 
     Yet another disadvantage is that when using a large number of DC/DC converters on the same line card, the converters add electromagnetic interference when they independently regulate the different subscriber lines. 
     Addressing some of these disadvantages, a number of other solutions have been proposed. 
     One solution is disclosed in U.S. Pat. No. 6,760,430. In this patent, a common voltage regulator feeds a number of subscriber line interface circuits (SLIC) or line drivers. The common voltage regulator has to provide a voltage level sufficiently high so that all subscriber lines including the longest subscriber line can operate. This means that for each subscriber line that is shorter than the longest one there is excess power that dissipates as heat from the line card. 
     Another solution is disclosed in U.S. Pat. No. 6,351,534. In this patent, subscriber lines are grouped according to their loop lengths. In one embodiment a group with the loop length 0 to 300 meters is connected to one line card and another group with the loop length 300 to 1000 meters is connected to another line card and so on. The line cards are mounted in a rack. Each group is fed by a power supply mounted on each line card with a sufficient voltage level sufficient to operate all subscriber lines in that group. 
     This solution reduces power dissipation compared to a solution where all subscriber lines (independent of loop lengths) are fed from the same voltage regulator as in U.S. Pat. No. 6,760,430. 
     One disadvantage with the solution in U.S. Pat. No. 6,351,534 is that it uses fixed range groups. This will result in a number of limitations when it comes to reconfiguring the rack comprising the line cards. 
     For example, when connecting a new 200 meters long subscriber line and if the line card configured for the range 0 to 300 m is full, it is necessary to add a new card for that range instead of using an unused subscriber line interface circuit on any of the other line cards. Another example, if one subscriber in the range 300 m to 1 km ends his/her subscription, the free subscriber line interface circuit is left unused until a new subscriber within the same range is identified. 
     It has been observed that the lengths in a set of subscriber lines connected to a central office seldom are evenly distributed. The normal case is that the line lengths are concentrated around certain ranges which in turn can vary depending on where the central office is located (such as rural or highly populated areas). Moreover, the ranges can also differ very much from country to country. Therefore, using a solution with fixed range groups there may also be a need to make additional market configurations and/or adaptations. 
     SUMMARY 
     The present invention is a power distribution system comprising at least two power supply units (such as DC/DC converters) that are feeding a set of subscriber line interface circuits (SLIC). Between the power supply units and the subscriber line interface circuits there is a switch unit that is adapted to connect (switch) current from the power supply units to the subscriber line interface circuits. The power distribution system also comprises a control unit. This control unit is adapted to set different power supply voltage levels for each power supply unit. The control unit is also adapted to determine the loop voltage of the subscriber lines when they are engaged and to control the switch unit so that each one of the subscriber line interface circuits is fed from a power supply unit having the lowest power supply voltage level but a level sufficiently high to operate the corresponding subscriber line. 
     The invention further includes a method to reduce power dissipation in the subscriber line interface circuits where the method comprises the step of setting different power supply voltage levels for the power supply units. At least one power supply unit is set to a power supply voltage level sufficiently high to operate any subscriber line. The method further comprises the step of determining the loop voltage of each corresponding subscriber line when engaged. When the loop voltage has been determined the method further comprises the step of connecting each line interface circuit to the power supply unit having the lowest power supply voltage level but a level sufficiently high to operate the corresponding subscriber line which in other words means the combination that is having the least power dissipation. 
     In one embodiment, the method further includes an algorithm to calculate the voltage level for each power supply unit so that the sum of the differences between the power supply voltage levels and voltage levels needed to operate each subscriber line is minimized. 
     The invention has the advantages of allowing for great flexibility and scalability when configuring a central office with POTS subscriber lines. It is not necessary to manually connect the subscriber line to a particular POTS line card (or group of line cards) that matches the loop length of the subscriber line. On the contrary, the subscriber line can be connected to any POTS line card with a free subscriber line interface circuit. 
     When adding new subscriber lines and if no more free subscriber line interface circuits are available, an additional POTS line card can simply be inserted and the new subscriber lines can be connected to this line card without considering the different loop lengths of each subscriber line. 
     When the subscriber line is connected, the power distribution system automatically determines the needed voltage level for the subscriber line and selects the power supply unit giving the lowest power dissipation. 
     Another advantage is that there is no need to define and configure a specific set of voltage levels according the country of deployment or the location of the central office as this is automatically set by the present invention. 
     Yet another advantage is that a single version of the power distribution system can be designed meeting all market requirements which makes it cheaper. 
     The objective with the present invention is therefore to provide a flexible power distribution system where the power dissipation in the line cards is reduced. 
     The invention will now be described in more detail and with preferred embodiments and referring to accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram illustrating remote power feeding according to prior art. 
         FIG. 2  is a block diagram illustrating remote power feeding according to an embodiment of the present invention. 
         FIG. 3  is a block diagram illustrating a switching system with a number of line interface units each with a power distribution system. 
         FIG. 4  is a block diagram illustrating a switching system with a number of line interface units fed by a common power distribution system. 
         FIG. 5  is a flow chart illustrating a method for reducing power dissipation according to the present invention. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  illustrates a rack  100  with a power distribution system known from prior art (e.g. U.S. Pat. No. 6,351,534). In this system, subscriber lines are grouped together depending on its loop length. For example, subscriber lines  114 , 115  having a loop length within a range ‘A’ (for example 0-300 meters) are connected to a first line card  110 . Subscriber lines  124 , 125  having a loop length within a range ‘B’ (for example 300-1000 meters) are connected to a second line card  120 . Finally, subscriber lines  134 , 135  having a loop length within a range ‘C’ (for example &gt;1000 meters) are connected to a third line card  130 . Each line card  110 , 120 , 130  has a compensating power supply  111 , 121 , 131 . 
     Each power supply  111 , 121 , 131  monitors the loop voltage for the subscriber lines within each range A, B, C respectively and adapts each output voltage to a sufficient value where all subscriber lines within that range can operate. The power supply units  111 , 112 , 113  are themselves fed by a common power source P. 
       FIG. 2  illustrates a preferred embodiment of a power distribution system  210  according to the present invention. The power distribution system  210  is adapted to serve a plurality of subscriber line interface circuits (SLIC)  221 - 226 . To each subscriber line interface circuit  221 - 226  a subscriber line  231 - 236  can be connected. 
     The power distribution system  210  is mounted on a POTS line card  310 , 320 , 330 , 340  in a switching system  300  as illustrated by  FIG. 3 . In another embodiment it can be a common resource  210  in a switching system  400  serving a plurality of POTS line cards  410 , 420 , 430 , 440  as illustrated by  FIG. 4 . 
     The power distribution system  210  in  FIG. 2  comprises three power supply units  211 , 212 , 213 . The purpose of each power supply unit  211 , 212 , 213  is to provide different power supply voltage levels to the subscriber line interfaces  221 - 226  and the subscriber lines  231 - 236 . The inventive concept is not limited to exactly three power supply units  211 , 212 , 213 . At least two power supply units can be used, but normally only a few compared to the number of subscriber lines. The embodiment with three power supply units  211 , 212 , 213  is preferred because it provides a significant reduction of power dissipation but on the same time achieves the desired effect of reducing costs and complexity in the implementation. The power supply units  211 , 212 , 213  are themselves normally fed by a common power source P. 
     Each power supply unit  211 , 212 , 213  is connected to a common switch unit  219 . This switch unit  219  is adapted to switch/connect current from the power supply units  211 , 212 , 213  to the subscriber line interface circuits  221 - 226 . 
     The power distribution system  210  further comprises a control unit  218  that is connected to all power supply units  211 , 212 , 213  and to the switch unit  219 . In the preferred embodiment the control unit  218  is connectable to all subscriber line interface circuits  221 - 226  on the same line card  310  as the power distribution system  210 . 
     The control unit  218  is adapted to monitor the subscriber line interface circuits  221 - 226  in order to determine the loop voltage for each connected subscriber line  231 - 236 . The loop voltage is determined when the subscriber line is in the engaged mode (off-hook). The control unit  218  is further adapted to control the voltage levels in each power supply unit  211 , 212 , 213  and to control the switch unit  219  so that the current from a specific power supply unit  211 , 212 , 213  is fed to a specific group of subscriber line interface circuits. The control unit  218  is preferable implemented as a processor P  2181  with a memory area M  2182  comprising executable code to perform the functions described above and further below. 
     With the three power supply units  211 , 212 , 213  and the switch unit  219 , the need to connect subscriber lines having loop lengths within a certain range to a specific line card is eliminated. Subscriber lines with an arbitrary loop length (within accepted telecom standards of course) can be connected to the same line card and fed by the power supply unit  211 , 212 , 213  giving the least power dissipation for that particular subscriber line. 
     When determining which subscriber line interface  221 - 226  is to be connected to which power supply unit  211 , 212 , 213  giving the least power dissipation, the method comprises inter alia the step of determining the loop voltages for the subscriber lines  231 - 236 . This is described more in detail further below. 
     As discussed above, the power distribution system  210  is preferably implemented on the same line card  310  as the served subscriber line interface circuits  221 - 226 .  FIG. 3  illustrates a switching system  300  with a plurality of line cards  310 , 320 , 330 , 340  each having a power distribution system  210  according the present invention. In another embodiment as illustrated by  FIG. 4 , the power distribution system  210  is a common resource in a switching system  400 . In this case the power distribution system  210  is serving subscriber line interface circuits  411 , 421 , 431 , 441  on several line cards  410 , 420 , 430 , 440  respectively. 
     The switch system  400  could very well be equipped with a plurality of common power distribution systems  210 ,  490  for example for capacity reasons or for redundancy and back-up at failure. 
     When reducing the power dissipation in the subscriber line interfaces  221 - 226 , it is important that each subscriber line interface  221 - 226  is connected to the power supply unit  211 , 212 , 213  having the lowest power supply voltage level but still having a level high enough so that the subscriber line  231 - 236  can operate. Optionally it is an advantage that also the sum of the power dissipations for all connected subscriber lines is minimized. In the preferred embodiment this means that the sum of the power dissipations for the subscriber lines  231 - 236  connected to the line card  310  is minimized. 
     The minimum voltage level Vmin needed for a subscriber line  231 - 236  to be operable is Vmin=Vab+Vas where Vab is the determined loop voltage and Vas is a predefined value for the anti saturation gap. Vas is the overhead voltage applied to front end amplifiers in the subscriber line interface circuits in order to allocate the requested dynamics for the voice signal in order to avoid signal clipping. The value of Vas is usually around 10V. 
     The value of the loop voltage level Vab for each subscriber line  231 - 236  is determined by the control unit  218  when the subscriber line is engaged. 
     The principal method of reducing the power dissipation in the subscriber line interfaces  221 - 226  is illustrated by  FIG. 5  and described below. It is assumed that the subscriber line interfaces  221 - 226  are initially connected to a power supply unit  211 , 212 , 213 . In order to determine the loop voltage Vab of a subscriber line  231 - 236 , the subscriber line  231 - 236  must be fed with a voltage level high enough so it can operate. To achieve this the power supply units  211 , 212 , 213  are in step  501  set with initial power supply voltage levels where at least one voltage level is set sufficiently high to operate any of the subscriber lines  231 - 236 . The loop voltages Vab for the subscriber lines  231 - 236  when they are engaged (off-hook) are determined in step  502 . When the loop voltage Vab has been determined, each subscriber line interface circuit  221 - 226  is connected to a power supply unit  211 , 212 , 213  having the lowest power supply voltage level but at least a level Vmin=Vab+Vas as described above. When the difference between the power supply voltage level and Vmin is small, the power dissipation is also small. It is assumed that the line current is the same for all subscriber lines  231 - 236  which means that finding the optimal voltage levels also results in finding the minimal power dissipation. The setting of the initial power supply voltage levels could optionally involve additional calculations as described in detail further below. 
     As a further option, the total power dissipation for a set of subscriber lines  231 - 236  can be further minimized in step  504  by minimizing the sum of the differences between the power supply voltage levels and Vmin for each corresponding subscriber lines  231 - 236 . To minimize the sum of the differences means basically to find the optimum power supply voltage levels so that the sum of power dissipations for all connected subscriber lines  231 - 236  is minimized. Embodiments of an algorithm to calculate these power supply voltage levels are described in detail further below. 
     If the calculation results in new power supply voltage levels then they are set in step  505  for the power supply units  211 , 212 , 213 . Also, if needed, some subscriber line interface circuits  221 - 226  may be connected in step  506  to another power supply unit  211 , 212 , 213 . The calculation can optionally be repeated at regular intervals by starting a timer T 1  in step  507 . When the timer T 1  times out in step  508 , the loop voltages for the engaged subscriber lines are again determined in step  509 . If changes are determined in step  510  a new calculation is made in step  504  and so on. If no changes are determined the timer T 1  is started again in step  511 . 
     An embodiment of the method is described below where the power distribution system  210  is mounted on a line card  310  with  64  line interface circuits and where the power distribution system  210  comprises three power supply units  211 , 212 , 213 . The power supply voltage levels in the power supply units  211 , 212 , 213  are set to initial values at power up of the line card  310 . One power supply voltage level Vh_init is set to a value sufficiently high so that any subscriber line  231 - 236  connected to the line card  310  can operate. These levels could be set by the power units  211 , 212 , 213  themselves or after a command from the control unit  218 . Initially all subscriber line interface circuits  221 - 226  are connected to the power supply unit  211 , 212 , 213  with the highest voltage level. 
     In order to determine which power supply unit  211 , 212 , 213  is generating the least power dissipation for a particular subscriber line interface circuit  221 - 226  it is necessary to determining the loop voltage for each subscriber line  231 - 236  when engaged. 
     In this embodiment where the power distribution system  210  is connected to 64 subscriber lines, the algorithm could be exemplified by the following pseudo code: 
     
       
         
               
               
             
               
               
               
             
               
               
             
               
               
               
             
               
               
             
           
               
                   
               
             
             
               
                   
                 maxVab = 0 
               
             
          
           
               
                   
                 for m = 1 to 64  
                 (read Vab value for each connected 
               
               
                   
                   
                 line up to 64) 
               
             
          
           
               
                   
                 Vab = measured loop voltage on line ‘m’ 
               
             
          
           
               
                   
                 if Vab &gt; maxVab then  
                 (take the max Vab value amongst 
               
               
                   
                   
                 all the lines) 
               
             
          
           
               
                   
                 maxVab = Vab 
               
               
                   
                 end If 
               
               
                   
                 next m 
               
               
                   
               
             
          
         
       
     
     When maxVab has been determined, the highest value Vh for the power supply units  211 , 212 , 213  is set to Vh=maxVab+Vas where Vas is the predefined value for the anti saturation gap. 
     A safe operational limit value minVh is normally preset in advance. If the determined value maxVab is less than minVh, then Vh is set to Vh=minVh+Vas. Otherwise Vh is set Vh=maxVab+Vas as above. 
     The remaining power supply voltage levels for the other power supply units are calculated to different initial values below Vh. In this embodiment the power distribution system  210  has three power supply units  211 , 212 , 213 . This means that three voltage levels have to be calculated, the highest voltage level Vh (calculated as described above) and a medium voltage level Vm and a low voltage level Vl. 
     The value of the medium voltage level Vm can be set by an algorithm illustrated by the following pseudo code: 
     
       
         
               
               
               
             
               
               
             
               
               
               
             
               
               
             
               
               
               
             
               
               
             
               
               
               
             
           
               
                   
               
             
             
               
                   
                 VM_max = Vh − deltaVhm 
                 ‘upper acceptable limit for 
               
               
                   
                   
                 VM according to selected Vh 
               
               
                   
                   
                 value’ 
               
             
          
           
               
                   
                 maxVab = 0 
               
             
          
           
               
                   
                 Ntot = 0 
                 ‘Ntot being the total number of 
               
               
                   
                   
                 line connected to level Vm or Vl’ 
               
               
                   
                 for m = 1 to 64 
                 ‘read Vab value for each 
               
               
                   
                   
                 connected lines up to 64’ 
               
             
          
           
               
                   
                 Vab = ′measured loop voltage on line ‘m’ 
               
             
          
           
               
                   
                  If Vab &lt; (VM_max − Vas) then 
                 ‘for each Vab value, which is 
               
               
                   
                   
                 less than (VM_max − Vas)’ 
               
               
                   
                  Ntot = Ntot + 1 
                 ‘add this line to the 
               
               
                   
                   
                 total number of lines fed by 
               
               
                   
                   
                 Vm or Vl’ 
               
               
                   
                   if Vab &gt; maxVab then 
                 ‘and search for the maximum 
               
               
                   
                   
                 of those Vab values 
               
               
                   
                   
                 which are less than (Vm_max − 
               
               
                   
                   
                 Vas)’ 
               
             
          
           
               
                   
                   maxVab = Vab 
               
               
                   
                   End If 
               
               
                   
                  End If 
               
               
                   
                 Next m 
               
             
          
           
               
                   
                 Vm = maxVab + Vas 
                 ‘set the initial estimated 
               
               
                   
                   
                 value for Vm’ 
               
               
                   
               
             
          
         
       
     
     The parameter deltaVhm is a predefined value for the minimum difference between Vh and Vm. 
     In short, this algorithm sets value Vm to the maximum value maxVab in the set of Vab values less than (Vm max-Vas) plus the anti saturation gap Vas, that is Vm=maxVab Vas. 
     The value of the low voltage level Vl is set to a value between Vl max=Vm−deltaVml and the minimum acceptable voltage level Vl min=Vphone 4 Vas. The parameter deltaVml is a predefined value for the minimum difference between Vm and Vl. The constant value ‘4’ represents the voltage drop due to the shortest subscriber loop length connecting the user phone to the subscriber line interface circuit  221 - 226 . It is considered to be 4 V corresponding to 700 meters of smallest wire gauge at 20 mA. This means that a safe minimum value Vl is set. The parameter Vphone indicates the voltage drop present at that phone depending on its impedance and the loop current. Voltage drop value is usually in the range of 7 to 12 volt with loop current of 20 up to 40 mA. 
     The initial value of Vl could be set by using the following algorithm: 
     
       
         
               
               
             
               
               
               
             
               
               
             
               
               
               
             
               
               
             
               
               
               
             
               
               
             
               
               
               
             
               
               
             
               
               
               
             
               
               
             
               
               
               
             
               
               
             
               
               
               
             
               
               
             
               
               
               
             
               
               
             
               
               
               
             
               
               
             
               
               
               
             
               
               
             
               
               
               
             
               
               
             
               
               
               
             
               
               
             
               
               
               
             
               
               
             
               
               
               
             
               
               
             
           
               
                   
               
             
             
               
                   
                 for n = 1 to 120 
               
             
          
           
               
                   
                  p(n) = 0 
                 ′P(n) is the array used to 
               
               
                   
                   
                 estimated power dissipation 
               
             
          
           
               
                   
                 next n 
               
               
                   
                 Lmax = Int(Vl_max − Vas) * 2 ′ let Lmax be the number in ½ 
               
             
          
           
               
                   
                   
                 volt steps up to Vl_max 
               
             
          
           
               
                   
                 S = 0 
               
               
                   
                 for n = 1 to Lmax ′ assuming Vl being any values between 
               
             
          
           
               
                   
                   
                 0,5V up to (Vl_max−Vas) 
               
               
                   
                   
                 in steps of 0,5V 
               
             
          
           
               
                   
                  S = S + ′number of lines requiring at most n·0,5V 
               
             
          
           
               
                   
                   
                 ′ S represents the total number of 
               
               
                   
                   
                 lines fed by Vl, when it is supposed 
               
               
                   
                   
                 to be equal to n·0,5V plus the anti 
               
               
                   
                   
                 saturation gap Vas 
               
             
          
           
               
                   
                  Plow = S * (n / 2 + Vas) 
               
             
          
           
               
                   
                   
                 ′ power consumption for all the lines 
               
               
                   
                   
                 fed by Vl is accumulated here. 
               
               
                   
                   
                 Actually the multiplication by loop 
               
               
                   
                   
                 current is omitted considering it as 
               
               
                   
                   
                 a common constant value. 
               
             
          
           
               
                   
                  Pmid = (Ntot − S) * Vm 
               
             
          
           
               
                   
                   
                 ′ power consumption for all the lines 
               
               
                   
                   
                 fed by Vm is accumulated here. 
               
               
                   
                   
                 Since Ntot were previously evaluated 
               
               
                   
                   
                 as the total number of lines fed 
               
               
                   
                   
                 by Vm  or  Vl, (Ntot−S) are the lines 
               
               
                   
                   
                 fed by Vm 
               
               
                   
                   
                 Actually the multiplication by loop 
               
               
                   
                   
                 current is omitted considering it as 
               
               
                   
                   
                 a common constant value. 
               
             
          
           
               
                   
                  p(n) = Plow + Pmid 
               
             
          
           
               
                   
                   
                 ′ total power consumption for all the 
               
               
                   
                   
                 lines fed by both Vm and Vl 
               
               
                   
                   
                 is accumulated here. Phigh, due to 
               
               
                   
                   
                 lines fed by Vh, remains constant in 
               
               
                   
                   
                 this phase. 
               
             
          
           
               
                   
                   next n 
               
               
                   
                 ′ given the initial Vm value, the minimum in power 
               
               
                   
                 dissipation is found using the above estimated value for 
               
               
                   
                 both Vm and Vl. 
               
               
                   
                 ′ finding the minimum of power consumption versus Vl value 
               
               
                   
                 Pmin = p(1) 
               
               
                   
                 h = 1 
               
               
                   
                 for n = 2 to Lmax 
               
               
                   
                  If p(n) &lt; Pmin then 
               
               
                   
                   Pmin = p(n) 
               
               
                   
                   h = n 
               
               
                   
                  end if 
               
               
                   
                 next n 
               
             
          
           
               
                   
                 Vl = h / 2 + Vas 
                 ′ set the initial estimated value 
               
               
                   
                   
                 for Vl 
               
             
          
           
               
                   
                 ′ check if the initial estimated VBL value is acceptable 
               
               
                   
                 If Vl &lt;V_min then 
               
             
          
           
               
                   
                   
                 ′ if the initial estimated value 
               
               
                   
                   
                 for Vl is not acceptable 
               
               
                   
                  minVab = Vl_max 
                 ′ starting from maximum acceptable 
               
               
                   
                   
                 Vl value 
               
             
          
           
               
                   
                  for m = 1 to 64 
               
             
          
           
               
                   
                   
                 ′ read Vab value for each 
               
               
                   
                   
                 connected lines up to 64 
               
             
          
           
               
                   
                  Vab = ′measured loop voltage on line ‘m’ ′ 
               
               
                   
                  if Vab &gt;= (Vl_min − Vas) then 
               
             
          
           
               
                   
                   
                 ′ for each Vab not less then the 
               
               
                   
                   
                 minimum stated for Vl 
               
             
          
           
               
                   
                   if Vab &lt; minVab then 
               
             
          
           
               
                   
                   
                 ′ save Vab as minimum if less then 
               
               
                   
                   
                 the other values checked so far 
               
             
          
           
               
                   
                    minVab = Vab 
               
               
                   
                   end if 
               
               
                   
                  end if 
               
               
                   
                  next m 
               
               
                   
                  Vl = minVab + Vas 
               
             
          
           
               
                   
                   
                 ′ set the initial estimated value 
               
               
                   
                   
                 for Vl 
               
             
          
           
               
                   
                  h = Int(Vl − Vas) * 2 
               
             
          
           
               
                   
                   
                 ′ h represents Vl in steps of ½ 
               
               
                   
                   
                 volt 
               
             
          
           
               
                   
                 end if 
               
               
                   
               
             
          
         
       
     
     The subscriber line interfaces  221 - 226  can now be connected to the power supply unit  211 , 212 , 213  having the lowest voltage level but sufficiently high to operate the corresponding subscriber line  231 - 236 . Again, the subscriber line interface  221 - 226  is connected to the power supply unit  211 , 212 , 213  having the lowest power supply voltage level but having at least a voltage level Vmin=Vab+Vas. 
     The procedure above describes some procedures for setting the initial values of the power supply voltage levels to the power supply units  211 , 212 , 213  and connecting the subscriber line interfaces  221 - 226  to the power supply unit  211 , 212 , 213  generating the least power dissipation. 
     In order to minimize the sum of power dissipations for all the subscriber lines  231 - 236  connected to the line card  310 , further trimming may be needed. This trimming can be initiated at regular intervals (for example each 15 minutes) in order to cope with changes in the configuration. The initial highest voltage level Vh, remains the same until it has been determined that the maximum loop voltage for all the subscriber lines has decreased. This could for example be the case when the subscriber line with the highest loop voltage has been uninstalled and/or removed from the line card  310 . In this case the highest power supply voltage level Vh can be decreased, but never below the value Vh=minVh+Vas. 
     The optimal power supply voltage levels for the remaining power supply units are calculated based on the distribution of the determined loop voltage levels Vab for the subscriber lines. An algorithm to determine the optimal values is to try out combinations of all power supply voltage levels below Vh in steps (for example in steps of 0.5 Volt) until the sum of the power dissipations for all connected subscriber lines has reached a minimum. This algorithm can be used for any number of power supply units  211 , 212 , 213 . When the power supply voltage levels have been calculated the power supply units  211 , 212 , 213  are set with the calculated values and the subscriber lines are connected to the power supply units  211 , 212 , 213  having the lowest power supply voltage level but sufficiently high to operate the corresponding subscriber line. 
     In the embodiment with three power supply units  211 , 212 , 213  the algorithm is to try out each combination of power supply voltage levels Vm, Vl in steps. An example is shown below: 
     
       
         
               
               
             
               
               
               
             
               
               
             
               
               
               
             
               
               
             
               
               
               
             
               
               
             
               
               
               
             
               
               
             
               
               
               
             
               
               
             
               
               
               
             
               
               
             
               
               
               
             
               
               
             
               
               
               
             
               
               
             
               
               
               
             
               
               
             
               
               
               
             
               
               
             
               
               
               
             
           
               
                   
               
             
             
               
                   
                 ′ optimizing the estimated VM value 
               
             
          
           
               
                   
                 Vm_min = Vl + deltaVml 
                 ′ lower acceptable limit for Vm 
               
               
                   
                 Nlow = 0 
                 ′ number of lines fed by Vl 
               
             
          
           
               
                   
                 for n = 1 to h 
               
             
          
           
               
                   
                  Nlow = Nlow +  
                 ′ number of lines requiring at most 
               
               
                   
                   
                 n·0,5V′ 
               
               
                   
                 next n 
                 ′ Nlow represents the total number 
               
               
                   
                   
                 of lines fed by Vl, assuming Vl 
               
               
                   
                   
                 equal to h·0,5V plus the anti 
               
               
                   
                   
                 saturation gap Vas 
               
               
                   
                 for n = 1 to 120 
                 ′ reset the P array 
               
             
          
           
               
                   
                  p(n) = 0 
               
               
                   
                 next n 
               
             
          
           
               
                   
                 Lmin = h + 1 
                 ′minimum value for which a line is 
               
               
                   
                   
                 switched to Vm 
               
             
          
           
               
                   
                 Lmax = Int(Vm − Vas) * 2 
               
             
          
           
               
                   
                   
                 ′maximum value for which a line is 
               
               
                   
                   
                 switched to Vm 
               
             
          
           
               
                   
                 S = 0 
               
               
                   
                 for n = Lmin to Lmax 
               
               
                   
                   S = S + ′number of lines requiring n·0,5V′ 
               
             
          
           
               
                   
                   
                 ′ S represents the total number of 
               
               
                   
                   
                 lines fed by Vm, assuming Vm equal 
               
               
                   
                   
                 to n·0,5V plus the anti saturation 
               
               
                   
                   
                 gap Vas 
               
             
          
           
               
                   
                   Pmid = S * (n / 2 + Vas) 
               
               
                   
                   Phigh = ((64 − Nlow) − S) * Vh 
               
               
                   
                   p(n) = Pmid + Phigh 
               
               
                   
                   next n 
               
               
                   
                  ′finding the minimum of power consumption versus Vm value 
               
               
                   
                 Pmin = p(Lmin) 
               
               
                   
                 k = Lmin 
               
               
                   
                 for n = Lmin + 1 to Lmax 
               
               
                   
                   if p(n) &lt; Pmin then 
               
               
                   
                    Pmin = p (n) 
               
               
                   
                    k = n 
               
               
                   
                   end if 
               
               
                   
                 next n 
               
             
          
           
               
                   
                  Vm = k / 2 + Vas 
                 ′set the initial optimal estimated 
               
               
                   
                   
                 value for Vm 
               
             
          
           
               
                   
                 ′check if the optimal estimated Vm value is acceptable with 
               
               
                   
                 respect to Vh 
               
               
                   
                 if Vh − Vm &gt; maxDeltaVhm then 
               
             
          
           
               
                   
                   
                 ′maximum voltage difference 
               
               
                   
                   
                 between Vh and Vm 
               
             
          
           
               
                   
                  maxVab = 0 
               
               
                   
                  for m = 1 to 64 
               
             
          
           
               
                   
                   
                 ′read Vab value for each connected 
               
               
                   
                   
                 lines up to 64 
               
             
          
           
               
                   
                   Vab = ′measured loop voltage on line ‘m’ ′ 
               
               
                   
                   if Vab &lt; (Vh − maxDeltaVhm − Vas) then 
               
               
                   
                    if Vab &gt; maxVab then 
               
               
                   
                     maxVab = Vab 
               
               
                   
                    end If 
               
               
                   
                   end If 
               
               
                   
                  next m 
               
             
          
           
               
                   
                  Vm = maxVab + Vas  
                 ′set the optimal estimated value 
               
               
                   
                   
                 for Vm. It will correspond to the 
               
               
                   
                   
                 closest Vab for which : Vh − Vm &gt; 
               
               
                   
                   
                 maxDeltaVhm is verified 
               
             
          
           
               
                   
                 end If 
               
               
                   
                 ′check if the initial estimated Vl value is acceptable with 
               
               
                   
                 respect to Vm 
               
               
                   
                 If Vm − Vl &gt; maxDeltaVml then 
               
               
                   
                  maxVab = 0 
               
               
                   
                  for m = 1 to 64 
               
             
          
           
               
                   
                   
                 ′read Vab value for each connected 
               
               
                   
                   
                 lines up to 64 
               
             
          
           
               
                   
                   Vab = ′measured loop voltage on line ‘m ’ ′ 
               
               
                   
                   if Vab &lt; (VM − maxDeltaVml − Vas) then 
               
               
                   
                    if Vab &gt; maxVab then 
               
               
                   
                     maxVab = Vab 
               
               
                   
                    end if 
               
               
                   
                   end if 
               
               
                   
                  next m 
               
               
                   
                 Vl = maxVab + Vas 
               
             
          
           
               
                   
                   
                 ′set the optimal estimated value 
               
               
                   
                   
                 for Vl It will correspond to the 
               
               
                   
                   
                 closest Vab for which : Vm − Vl &gt; 
               
               
                   
                   
                 maxDeltaVMl is verified 
               
               
                   
                 end If

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