for proper deletion of the bits on the receiving end. The downside to the PDH system is the added overhead and complexity created by performing the redundant ADD and DROP operations required by the bit stuffing. In addition, PDH has difficulty recovering and rerouting signals following a network failure.SDH, on the other hand, is more suited for the ATM to satellite interface since it was originally designed to take advantage of a completely synchronized network. The fiber optic transmission signal typically used for an ATM network transfers a very accurate clock rate throughout the network. The key ingredient for the SDH protocol is the inclusion of pointer bytes that indicate the beginning of the cell payload. This helps avoid any data loss due to bit slippage caused by slight phase and/or frequency variations. SDH has other advantages over PDH since it can handle higher data rates, support easier and less expensive multiplexing and demultiplexing, and has increased provisions for network management. The PLCP transport method was originally designed to carry ATM cells over existing DS3 facilities. The PLCP format consists of 12 ATM cells in a sequential group, with each cell being preceded by 4 overhead bytes. A frame trailer of either 13 or 14 nibbles is appended to the end of the group of 12 cells to facilitate nibble stuffing. Each individual 12 cell and overhead combinations requires a 125 microsecond interval for transmission. Unfortunately, PLCP is susceptible to corruption caused by burst errors that can effect the perceived number of nibbles required for stuffing, resulting in frame misalignment.SDH appears to be the logical choice for cell transport in this type of system. However, an important point to consider when using SDH is the possibility of an incorrect payload pointer. This situation may produce faulty payload extraction, causing previously received cells to be corrupted and necessitate their dismissal. ...
