Zero byte substitution method and apparatus for telecommunications equipment

An interface circuit for coupling a digital subscriber line to a digital transmission facility, comprising:

• a first latch having an input and an output, for receiving and storing a current data byte from said digital transmission facility;
• a second latch having an input and an output, for receiving and storing a previous data byte from said digital transmission facility;
• a first comparator having first and second inputs coupled to the outputs of said first and second latches, respectively, and an output;
• and
• a control circuit having a first input coupled to the output of said comparator, a second input coupled to the output of said first latch, and a data output, said control circuit being responsive to an output from said first comparator indicating that said current and previous data bytes are different by passing said current data byte to said data output, and being responsive to an output from said comparator indicating that said current and previous data bytes are identical by producing an all-zero data byte at said data output.

A method for coupling a digital subscriber line to a digital transmission facility, comprising:

• receiving a current data byte from said digital subscriber line;
• detecting whether said current data byte is an all-zero data byte;
• when said current data byte is not an all-zero data byte, coupling said current data byte to said digital transmission facility;
• when said current data byte is an all-zero data byte, replacing said current data byte with a previous data byte received from said digital subscriber line that is not an all-zero data byte, and coupling said previous data byte to said digital transmission facility;
• and
• producing a control bit indicating whether said current data byte has been replaced with said previous data byte and coupling said control bit to said digital transmission facility.

An interface circuit for coupling a digital subscriber line to a digital transmission facility, comprising:

• a first shift register having an input and an output, for receiving and storing a current data byte from said digital subscriber line;
• a first latch for receiving from said digital subscriber line a previous data byte which is not an all-zero data byte and for storing said previous data byte, said first latch having an input coupled to the output of said first shift register, an output, and a control input for controllably coupling data at the input of said first latch to the output thereof;
• and
• a zero byte detector having an input coupled to the input of said first shift register for detecting a current data byte which is an all-zero byte, and an output coupled to the control input of said first latch, said zero byte detector causing said first latch to load the current data byte from said first shift register and couple said current data byte to the output of said first latch when said current data byte is not an all-zero data byte, and to couple the previous data byte to the output of said first latch without loading the current data byte from said first shift register when said current data byte is an all-zero data byte.

An interface circuit for coupling a digital subscriber line to a digital transmission facility, comprising:

• a first latch having an input and an output, for receiving and storing a current data byte from said digital transmission facility;
• a second latch for receiving and storing a previous data byte from said digital transmission facility, said second latch having an input coupled to the output of said first latch and an output;
• a first comparator having first and second inputs coupled to the outputs of said first and second latches, respectively, and an output;
• and
• a control circuit having a first input coupled to the output of said comparator, a second input coupled to the output of one of said first and second latches, and a data output, said control circuit being responsive to an output from said first comparator indicating that said current and previous data bytes are different by passing said current data byte to said data output, and being responsive to an output from said comparator indicating that said current and previous data bytes are identical by producing an all-zero data byte at said data output.