| 摘要 |
A channel unit is disclosed for interfacing a digital subscriber line (DSL) to a PCM digital transmission facility, for example, T carrier. The basic rate interface (BRI) 2B+D format is converted to a unique three time slot format for transmission in the DS1 PCM signal format. The two communication B channels are interrogated (via 402, 403) to determine if they have a logical 0 in each bit position, i.e., a zero byte. If so, a non-all-logical 0 bit pattern is substituted (via 405 and 407 or 406 and 408) and a logical 1 indication bit is inserted (via 404) in an associated bit position in a control channel. In a receiver, the associated indicator bit positions in the control channel are interrogated to determine if an all logical 0 bit pattern, i.e., a zero byte, had been replaced. If so, an all logical 0 bit pattern is inserted in the associated communication channel. A logical 1 is inserted (via 404) in bit position 2 of the control channel to negate the possibility of emulating a T carrier yellow alarm condition. A logical 1 is also inserted (via 404) in bit position 3 of the control channel to satisfy the logical 1's density requirement for digital transmission facilities.
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