Error monitoring in digital transmission systems

Abstract

Errors in digital transmission are monitored by employing a cyclical-redundancy-check (CRC). A CRC code word having a predetermined number of bits is generated (via 310) from a block of bits (ESF) of a presently transmitted time division multiplexed (XTDM) signal. The code word bits are then inserted (via 304, 305, 306) into predetermined bit positions of the next subsequent block of bits (ESF) of the XTDM signal. In a receiver (FIG. 7), bits of a presently received time division multiplexed (RTDM) signal are compared (801, 802) to bits of a CRC code word generated from the last previously received block of bits to indicate errors in transmission. In a specific example, a 6-bit CRC code word is employed and the code word bits are inserted into predetermined framing bit positions.

What is claimed is: 
     
       1. Apparatus for formatting a time division multiplexed signal including a series of blocks of bits, each block including a plurality of data bits and a plurality of additional bit positions, characterized by   means (310) responsive to bits in a block of bits of the time division multiplexed signal (XTDM) for generating in accordance with a prescribed criterion a code word (CRC BITS) having a predetermined number of bits less in number than the plurality of additional bit positions; and   means (301, 304-306) for inserting said code word bits in predetermined ones of the additional bit positions of a subsequent block of bits of the time division multiplexed signal being transmitted to monitor errors in transmission.   
     
     
       2. Apparatus .Iadd.including means .Iaddend.for monitoring a received time division multiplexed signal including a series of blocks of bits, each block including a plurality of data bits and a plurality of additional bit positions, characterized by   means (702) responsive to all the bits of a block of bits of the received time division multiplexed signal (RTDM) for generating .Iadd.in .Iaddend.accordance with a prescribed criterion a code word (CRC BITS) having a predetermined number of bits less in number than the plurality of additionsal bit position; and   means (703) for comparing said code word bits with bits of a subsequent received block of bits of the time division multiplexed signal to generate error indications.   
     
     
       3. Apparatus for transmitting and receiving time division multiplexed signals including a series of blocks of bits, each block including a plurality of data bits and a plurality of additional bit positions, characterized by   means (300) for formatting the time division multiplexed signal (XTDM) including means (310) responsive to bits in a block of bits of the time division multiplexed signal (XTDM) for generating in accordance with a prescribed criterion a code word (CRC BITS) having a predetermined number of bits less in number than the plurality of additional bit positions, and means (301, 304-306) for inserting said code word bits in predetermined ones of the additional bit positions of a subsequent block of bits of the time division multiplexed signal being transmitted to monitor errors in transmission; and   means (700) for monitoring a received time division multiplexed signal including means (702) responsive to bits of a block of bits of the received time division multiplexed signal (RTDM) for generating in accordance with a prescribed criterion a code word (CRC BITS) having a predetermined number of bits less in number than the plurality of additional bit positions, and means (703) for comparing said code word bits with bits of a subsequent received block of bits of the time division multiplexed signal to generate error indications.   
     
     
       4. Apparatus as defined in claims 1 or 2, wherein said code word generating means (310; 702) includes means (501) for generating said code word bits (CRC BITS) from a block having a predetermined number of bits of the time division multiplexed signal (XTDM; RTDM) and means for serially supplying said code word bits (502) during a next subsequent block of bits of the time division multiplexed signal. 
     
     
       5. Apparatus as defined in claim 1 or 2, wherein said prescribed criterion includes a predetermined cyclic code generator polynomial. 
     
     
       6. Apparatus as defined in claim 5 wherein said predetermined number of code word bits is six. 
     
     
       7. Apparatus as defined in claim 5 wherein said means for generating said code word from said block of bits includes a feedback shift register (501). 
     
     
       8. Apparatus as defined in claims 1 or 3 wherein said means for inserting includes means (306) for generating a timing signal (CRC BIT SYNC) including pulses concurrent with said code word bits (CRC BITS). 
     
     
       9. Apparatus as defined in claim 8 wherein said code word generating means (301) includes means (501) for generating said code word bits from a block (ESF) having a predetermined number of bits of the time division multiplexed signal (XTDM) and means (502) for serially supplying said code word bits to said means for inserting during a next subsequent block of bits of the time division multiplexed signal. 
     
     
       10. Apparatus as defined in claim 9 wherein said predetermined additional bit positions are predetermined framing bit positions. 
     
     
       11. Apparatus as defined in claim 10 wherein said means for inserting includes means (301, 304) responsive to said timing signal pulses (CRC BIT SYNC) and said code word bits (CRC BITS) for inserting said code word bits in said predetermined framing bit positions. 
     
     
       12. Apparatus as defined in claims 2 or 3 wherein said means for monitoring further includes means (705) for generating a timing signal (CRC BIT SYNC) having pulses concurrent with said code word bits (CRC BITS), wherein said code word generating means (702) includes means for generating said code word bits from a block having a predetermined number of bits of the time division multiplexed signal and means for serially supplying said code word bits during a next subsequent block of bits of the time division multiplexed signal, and wherein said comparing means (703) is responsive to said bits of the received time division multiplexed signal, to said code word bits, and to said timing signal pulses for generating said error indications (CRC BLOCK ERRORS). 
     
     
       13. A method for formatting a time division multiplexed signal including a series of blocks of bits each block including a plurality of data bits and a plurality of additional bit positions, the method comprising the steps of: generating a code word having a predetermined number of bits less in number than the plurality of additional bit positions from bits in a block of bits of the time division multiplexed signal in accordance with a prescribed criterion; and   inserting the code word bits in predetermined ones of the additional bit positions of a subsequent block of bits of the time division multiplexed signal.   
     
     
       14. A method for monitoring errors in a received time division multiplexed signal including a series of blocks of bits each block including a plurality of data bits and a plurality of additional bit positions, the method comprising the steps of: generating a code word having a predetermined number of bits less in number than the plurality of additional bit positions from bits of a block of bits of the time division multiplexed signal in accordance with a prescribed criterion; and   comparing the code word bits with bits of a subsequent received block of bits of the time division multiplexed signal to generate error indications. .Iadd.   
     
     
       15.  Apparatus as defined in claim 2 wherein said means for monitoring further includes means for utilizing said error indications to monitor transmission performance. .Iaddend. .Iadd.16. Apparatus as defined in claim 10 wherein said means for monitoring further includes means for utilizing said error indications to monitor transmission performance. .Iaddend. .Iadd.17. Apparatus as defined in claim 12 wherein said means for monitoring further includes means for utilizing said error indications 
     
     
        to monitor transmission performance. .Iaddend. .Iadd.18.  Apparatus for transmitting and receiving time division multiplexed signals including a series of blocks of bits, each block including a plurality of data bits and a plurality of additional bit positions, characterized by   means (300) for formatting the time division multiplexed signal (XTDM) including means (310) responsive to bits in a block of bits of the time division multiplexed signal (XTDM) for generating in accordance with a prescribed criterion a code word (CRC BITS) having a predetermined number of bits less in number than the plurality of additional bit positions, and means (301, 304-306) for inserting said code word bits in predetermined ones of the additional bit positions of a subsequent block of bits of the time division multiplexed signal being transmitted to monitor errors in transmission; and   means (700) for monitoring a received time division multiplexed signal including means (702) responsive to bits of a block of bits of the received time division multiplexed signal (RTDM) for generating in accordance with a prescribed criterion a code word (CRC BITS) having a predetermined number of bits less in number than the plurality of additional bit positions, means (703) for comparing said code word bits with bits of a subsequent received block of bits of the time division multiplexed signal to generate error indications, and means (704) for utilizing said error indications to monitor transmission performance. .Iaddend. .Iadd.19. A method for monitoring errors in a received time division multiplexed signal including a series of blocks of bits each block including a plurality of data bits and a plurality of additional bit positions, the method comprising the steps of:   generating a code word having a predetermined number of bits less in number than the plurality of additional bit positions from bits of a block of bits of the time division multiplexed signal in accordance with a prescribed criterion,   comparing the code word bits with bits of a subsequent received block of bits of the time division multiplexed signal to generate errors indications; and   utilizing said error indications to monitor transmission performance. .Iaddend.