rst in the case of overload. 6.8.2.5 Header Error Control (HEC) HEC is a checksum for the first 4 bytes. It makes possible the detection of multiple errors and the correction of a single error. ATM cells with more than one error will be discarded by the network. It is up to the end systems to detect and recover from such losses. The end systems also have to detect Introduction to Telecommunications Network Engineering errors in the user data. When an ATM switch updates the virtual circuit and path identifications of a cell, it calculates a new HEC for the following hop to the next switch. 6.8.3 Physical Layer of ATM ATM cells can, potentially, be carried over most physical layer media but the ITU-T has defined SDH to carry ATM cells with speeds of 155.52 and 622.08 Mbps. Figure 6.61 illustrates how ATM cells are inserted into the payload of an SDH frame STM-1. The frame includes 9 X 270 bytes and it is transmitted 8,000 times per second. We review here only the STM-1 frame of SDH as an example, although this principle is valid for SONET as well but the detailed structure of the frame is different. The SDH frame includes section overhead (SOH) that contains framing information, network management data, and other overhead information needed by optical SDH transmission systems. ATM cells are merged together with path overhead (POH) to the payload of an SDH frame. POH and data (ATM cells) together make up the virtual container (VC), which can start at any point inside the payload of the frame. The administrative unit (AU) pointer tells where the frame (VC) containing path overhead and ATM cells starts inside the payload [1]. In Figure 6.61 this starting point is assumed to be at the beginning of the payload. At an SDH interface the concept of a continuous stream is used and dummy cells that are called idle cells are inserted if there is no traffic. In ATM cell mapping onto an STM-1 frame: 270 columns (octets) Payload 9 rows AU pointers Cell header ATM cell Continuous stream of cells; dummy cells are inserted if there