e below for a discussion on traffic management.) The HEC is used for transmission error checking incorporating the CRC-8 polynomial. (CRC-8 is one of several commonly used polynomials for Cyclic Redundancy Checking within network protocols.)ATM can be run over several different physical media and physical-layer protocols including SONET and FDDI. To adapt ATM to these various media layers and accomplish the necessary segmentation and reassembly of the ATM cells, a protocol layer called AAL (ATM Adaptation Layer) is inserted into the network protocol stack between the ATM layers and other protocol layers desiring ATM use. (See Figure 3 for a typical ATM protocol stack.) ATM OVER SATELLITE To take full advantage of the benefits of both satellite and ATM networks, a network architecture and protocol stack must be implemented that allows communication between these very different technologies. The solution to this problem comes in the form of the key component of this system known as the ATM Satellite Internetworking Unit (ASIU). (This component is also referred to by some as the ATM Adaptation Unit.) The ASIU is the essential piece of equipment for the properly-functioning interface between the satellite and ATM system. It is responsible for handling many complex issues of the system interface such as management and control of system resources, real-time bandwidth allocation, network access control, and call monitoring. In addition, the ASIU must take care of system timing and synchronization control, error control, traffic control, and overall system administrative functions. (See Figure 4.) Figure 4. The ASIU is the single interface that acts as a bridge between the ATM and satellite systems, allowing the desired data to be exchanged back and forth across these two distinctly different types of transmission mediums. As can be seen on both sides of the typical ATM to satellite protocol stack of Figure...
