1. Field of the Invention
The present invention relates to a method and apparatus for correcting the phase of transmitted data frames, and more particularly to a method and apparatus for correcting the phase of frames/multiframes transmitted over incoming lines in subscriber loop carrier systems.
2. Description of the Related Art
Subscriber loop carrier systems have been employed to accommodate subscribers, at geographically remote locations from switching offices, economically and with high transmission quality, and also to alleviate the problem of the ever increasing number of telephone cables in urban areas. In conventional subscriber loop carrier systems, a plurality of subscriber lines are multiplexed as PCM data with a frame/multiframe structure at a remote terminal (RT), and the thus multiplexed PCM data are then transmitted over a fiber optic or other transmission medium to a central office terminal (COT), where the data are demultiplexed into a plurality of subscriber lines for connection to switching equipment. Furthermore, in recent years, high-order multiplexing has come to be adopted wherein a plurality of multiplexed data, instead of undergoing demultiplexing at a COT, are further multiplexed into a high-level multiplexed signal, for direct connection to a switching network such as SONET. To achieve this, a multiplexer for remultiplexing a plurality of multiplexed data is provided with a time slot interchanger (TSI) for interchanging time slots to accomplish dynamic rearrangement on a user-by-user basis. The TSI is provided to accommodate a larger number of subscriber lines than the number of channels in a multiplexed transmission path. To facilitate the interchanging of time slots in the TSI, the phases of the frames/multiframes of the PCM data transmitted over different transmission paths from different RTs must be corrected to achieve synchronization. In the prior art, this phase correction is done by storing the data in memory at the receiving end. This requires the provision of a memory for storing the data, and furthermore, transmission delays are caused because of temporarily storing the data.
If the differences between transmission delay times from the different RTs are within one-frame period, the phase correction is only necessary for the frame, but no phase correction is needed for the multiframe. In this case, a memory that can store one frame of data will suffice for the purpose, and the transmission delay can be kept within one frame period. On the other hand, if the differences between transmission delay times from the different RTs are larger than one frame period, the phase correction must be done not only for the frame but for the multiframe that typically consists of 12 frames. In such a case, the memory capacity required will be much larger, and significant transmission delays will result. Furthermore, in order to use a single fiber optic cable for carrying both the incoming and outgoing lines, the adoption of a time compression multiplexing (TCM) system is considered. If the TCM system is adopted, phase correction for the multiframe becomes essential.