is found in the header of a cell which cannot be corrected, the cell is discarded. The error ASYNCHRONOUS TRANSFER MODE Multi-bit error detected (cell discarded) No error detected Correction Detection Error detected (no action) (cell discarded) No error detected (no action) Single bit error detected (correction) Figure 20.6 HEC: receiver modes of operation. (Based on ITU-T Rec. 1.432.) Table 20.3 Header Pattern for Idle Cell Identification Octet 1 Octet Octet 3 Octet 4 Octet 5 Header pattern 00000000 00000000 00000000 00000001 HEC = Valid code 01010010 Source: ITU-T Rec. 1.432, Table 4/1.432, p. 19 (Ref. 4). protection function provided by the HEC allows for both recovery of single bit errors and a low probability of delivery of cells with errored headers, under bursty error conditions. 20.4.2 Idle Cells Idle cells cause no action at a receiving node except for cell delineation including HEC verification. They are inserted and extracted by the physical layer in order to adapt the cell flow rate7 at the boundary between the ATM layer and the physical layer to the available payload capacity of the transmission media. This is called cell rate decoupling. Idle cells are identified by the standardized pattern for the cell header illustrated in Table 20.3. The content of the information field is 01101010 repeated 48 times for an idle cell. 20.5 CELL DELINEATION AND SCRAMBLING Cell delineation allows identification of the cell boundaries. The cell HEC field achieves cell delineation. Keep in mind that the ATM signal must be self-supporting, in that it has to be transparently transported on every network interface without any constraints from the transmission systems used. Scrambling is used to improve security and robustness of the HEC cell delineation mechanism discussed in the next paragraph. In addition, it helps the randomizing of data in the information field for possible improvement in transmission performance. Cell delineation is performed by using correlation between the header bits to be pro- tected (the first 