This invention generally relates to a desynchronizer for telecommunication signals. The invention more particularly relates to an apparatus for generating an ungapped DS-3 signal from the data component of a gapped STS-1 payload signal.
The telecommunications network servicing the Unites States and the rest of the world is presently evolving from analog transmission to digital transmission with ever-increasing bandwidth requirements. Fiber optic cable has proved to be a valuable tool of such evolution, replacing copper cable in nearly every application from large trunks to subscriber distribution plants. Fiber optic cable is capable of carrying much more information than copper with lower attenuation.
While fiber optic cable represents the future in telecommunications, presently there remains an entire telecommunications network comprised of various cable types, served by equipment of different vintages, and run according to various coexisting transmission standards. While older standards, cables, and equipment will be eventually phased out, for the time being it is necessary that all the old and new standards, equipment, and transmission lines be as compatible as possible. For example, in a wire plant, every signal should be connectable to every other signal. To achieve this, it is not enough to simply multiplex signals from lower to higher orders and vice-versa. In addition to a mux/demux function, signal format conversion operations must be performed before connectibility can be achieved. For instance, a DS-3 signal cannot simply be connected to an STS-1 signal as these signals are at different rates (51.84 MHz.+-.20 ppm for the STS-1 signal, and 44.736 MHz.+-.20 ppm for the DS-3 signal) and use different multiplexing formats. Thus, a conversion from a DS-3 signal to an STS-1 signal requires the addition of overhead bytes, stuff, control information, etc. which are accommodated in an increased data rate. Likewise, in recovering the DS-3 signal from the STS-1 signal in which it is carried, the overhead bytes, stuff, control information, etc. must be stripped out of the STS-1 signal as seen in the prior art FIG. 1, thereby producing gaps in the clock of the extracted DS-3 signal from which an ungapped slower DS-3 signal must be regenerated.
As seen in FIG. 1, for each row of ninety bytes of an STS-1 signal, three bytes of transport overhead and one byte of path overhead must be removed. Of the remaining eighty-six bytes, six bytes of fixed stuff (R) must be removed, as well as three bits of information (RCC) containing fixed stuff and stuff control, one byte of information (CCRRRRRR) containing stuff control and fixed stuff, and either seven or eight bits (CCRROORS) containing stuff control, fixed stuff and overhead communication bits. Whether seven or eight bits are removed from byte CCRROORS depends on whether the stuff opportunity bit S of the particular signal contains data or stuff. Knowledge of whether bit S is a stuff or a data signal is obtained from the stuff control signals C. Details of the STS-1 frame format and the means used to remove the overhead, stuff, and control information from the STS-1 signal are not particularly relevant to the instant invention, but may be seen with reference to prior art documents: Bellcore TR-TSY-000253; ANSI - T1.105-1988; and ANSI - Draft Proposed Technical Report T1X1/90-029. What is relevant, is that the data signal received from whatever is removing the overhead, stuff and control information of the STS-1 signal is a severely gapped data signal with a six hundred twenty-one or twenty-two data bits per row at a clock rate of 51.84 MHz.+-.20 ppm, and an average rate of 44.736 MHz.+-.20 ppm. This gapped STS-1 data payload signal is then preferably transformed into an ungapped DS-3 signal at the 44.736 MHz.+-.20 ppm rate.