Method and apparatus for synchronizing a data communications system to a periodic digital impairment

Abstract

A pulse code modulation modem system is configured to transmit data from a first modem to a second modem over the public switched telephone network (PSTN). The PSTN employs robbed bit signaling (RBS) such that symbols affected by RBS arrive at the second modem in a periodic manner based on a period of six symbols. The modem system is configured such that signal segments are formatted and transmitted with six symbols per segment. After obtaining symbol synchronization, the second modem initializes a modulo-6 symbol counter such that the zero count corresponds to the first symbol of each received signal segment. Those symbols affected by RBS are identified and analyzed to determine optimized signal point constellations that may be used to compensate for the RBS on a symbol-by-symbol basis during subsequent encoding and decoding. Upon a loss of synchronization, the second modem resets its modulo-6 counter in response to the detection of the first symbol in a subsequent signal segment. In this manner, the modem system can again use the optimized signal point constellations determined previously.

What is claimed is:  
     
       1. A synchronization method for use in a data communication system having a first device configured to communicate with a second device over a communication network, said communication network employing robbed bit signaling (RBS) such that symbols affected by RBS arrive at said second device in a periodic manner based on a period of N symbols, said method comprising the steps of: 
       transmitting a plurality of signal segments from said first device to said second device, each of said signal segments being represented by an integer multiple of N symbols;  
       receiving said signal segments at said second device;  
       synchronizing said second device to symbols transmitted by said first device;  
       initializing a modulo-N symbol counter at said second device in response to receipt of an initial signal segment by said second device, said modulo-N symbol counter being synchronous to a symbol count associated with said signal segments;  
       detecting the effect of RBS in a received signal segment, said detecting step being performed by said second device;  
       determining at least one symbol position, relative to said received signal segment, where RBS was employed by said communication network;  
       identifying a reference-position symbol of one of said signal segments after loss of an initial symbol synchronization between said second device and said first device; and  
       resetting said modulo-N symbol counter in response to said identifying step.  
     
     
       2. A method according to  claim 1 , further comprising the step of deriving, for each of said at least one symbol position, a compensating signal point constellation for use during subsequent encoding and decoding of symbols affected by RBS. 
     
     
       3. A method according to  claim 1 , wherein N=6. 
     
     
       4. A method according to  claim 1 , wherein said transmitting step transmits signal segments having N symbols. 
     
     
       5. A method according to  claim 1 , wherein said initializing step initializes said modulo-N symbol counter in response to receipt of the first symbol of a subsequent signal segment that follows said initial signal segment. 
     
     
       6. A method according to  claim 1 , wherein said initializing step initializes said modulo-N symbol counter after receipt of a reference position symbol of said initial signal segment. 
     
     
       7. In a data communication system having a first device configured to transmit data to a second device, a method for compensating for digital impairments in a transmit signal, where said digital impairments arrive at said second device in a periodic manner based on a period of N symbols, said method comprising the steps of: 
       formatting a plurality of signal segments such that each of said signal segments is represented by an integer multiple of N symbols;  
       starting a first symbol counter at said first device upon transmission of a first symbol associated with one of said signal segments;  
       transmitting said signal segments from said first device to said second device;  
       initializing a second symbol counter at said second device after receipt of said first symbol by said second device;  
       utilizing compensating signal point constellations, in response to said second symbol counter, for symbols affected by digital impairments;  
       detecting the first symbol of one of said signal segments after loss of an initial symbol synchronization between said second device and said first device; and  
       resetting said second symbol counter in response to said detecting step.  
     
     
       8. A method according to  claim 7 , further comprising the step of determining at least one symbol position, relative to each of said signal segments, where a digital impairment was introduced, said determining step being responsive to said initializing step. 
     
     
       9. A method according to  claim 7 , wherein said digital impairment comprises robbed bit signaling (RBS). 
     
     
       10. A method according to  claim 7 , further comprising the step of selecting a plurality of optimized signal point constellations associated with different states of said second symbol counter, said selecting step being performed by said second device. 
     
     
       11. A method according to  claim 10 , further comprising the step of sending information indicative of said optimized signal point constellations from said second device to said first device. 
     
     
       12. A method according to  claim 7 , wherein said second symbol counter comprises a modulo-N symbol counter. 
     
     
       13. A method according to  claim 7 , wherein: 
       said first symbol is associated with an initial signal segment; and  
       said initializing step initializes said second symbol counter in response to receipt of a first symbol of a subsequent signal segment that follows said initial signal segment.  
     
     
       14. A synchronization method for a communication device connected to a communication network employing robbed bit signaling ( RBS ) , such that symbols affected by RBS arrive at the communication device in a periodic manner based on a period of N symbols, the method comprising the steps of:    
       
         receiving a plurality of signal segments at the communication device with each of the signal segments being represented by an integer multiple of N symbols;  
       
       
         synchronizing the communication device to the integer multiple of N symbols;  
       
         initializing a modulo - N symbol counter in the communication device in response to receipt of an initial signal segment by the communication device, the modulo - N symbol counter being synchronous to a symbol count associated with a signal segment of the plurality of signal segments;    
       
         detecting the effect of RBS in a received signal segment of the plurality of signal segments;  
       
       
         determining an at least one symbol position, relative to the received signal segment, where RBS was employed by the communication network;  
       
         identifying a reference - position symbol of one of the signal segments after loss of an initial symbol synchronization with the communication network; and    
         resetting the modulo - N symbol counter in response to the identifying step.   
     
     
       15. The method of  claim 14 , further comprising the step of deriving, for each of the at least one symbol position, a compensating signal point constellation for use during subsequent encoding and decoding of symbols affected by RBS. 
     
     
       16. The method of  claim 14 , wherein N= 6 . 
     
     
       17. The method of  claim 14 , wherein the initializing step initializes the modulo- N symbol counter in response to receipt of the first symbol of a subsequent signal segment that follows the initial signal segment.   
     
     
       18. The method of  claim 14 , wherein the initializing step initializes the modulo- N symbol counter after receipt of a reference - position symbol of the initial signal segment.