5, the ASIU is placed between the last leg of the ATM network and the front of the satellite system equipment. Therefore, the ASIU can perform all of its necessary functions such as data segmentation and reassembly, and the features described above. As mentioned above, the ASIU performs many functions, all contributing to the proper operation of the ATM to satellite interface. Five of the most interesting and important functions of this component are the Cell Transport Method, the Satellite Link Layer, error control, traffic management, and bandwidth management. These five issues will be explored in the following discussions. Figure 5. Cell Transport MethodsCell transport across an ATM over satellite network can make use of an existing digital cell transport format. The three existing cell transport protocols that have been considered for potential use with this type of system are Plesiochronous Digital Hierarchy (PDH), Synchronous Digital Hierarchy (SDH), and Physical Layer Convergence Protocol (PLCP). The most feasible and promising protocol for cell transport within this system turns out to be SDH for the reasons delineated below.The PDH transmission system was originally developed to carry digitized voice efficiently in major urban areas. (PDH however, is being replaced by other transport methods such as SONET and SDH.) In this scheme, the multiplexer at the sending end has access to multiple tributaries for transport that can have varying clock speeds. The multiplexer reads each tributary at the highest allowed clock speed but will also check to see if there are no bits in the input buffer. If this is the case, the multiplexer will use bit-stuffing to move the signal up to a higher clock speed for better performance. The multiplexer subsequently notifies the receiving demultiplexer that the data contains stuffed bits...
