Header error control

From Hill2dot0

The header error control (HEC) field is carried in the final octet of the header and provides an eight-bit cyclic redundancy check (CRC) remainder covering the entire header. The HEC value can be used to correct single-bit errors or detect multiple-bit errors. Correction of single-bit errors makes sense because optical-fiber-based transmission systems tend to have a mix of single-bit errors (i.e., more than 99 percent of fiber errors are single bits) and large burst errors; therefore, the HEC provides broad coverage for the bit errors expected in the fiber-based B-ISDN environment. The ATM Forum (ATMF), however, does not require error correction with the HEC field because the ATMF has defined interfaces using nonfiber media. Copper-based media, in fact, show a very different pattern of bit errors, and an error correction scheme assuming single-bit errors over copper would demonstrate very poor performance.

Even when it is not used for error correction in ATMF configurations, the HEC field is used for cell delineation. The visual shows the various states of the receiver for cell delineation. A receiver will be in the sync state as long as the incoming HEC values indicate the absence of bit errors. If too many HEC errors occur in a short period, cell synchronization is considered lost, and the receiver enters a hunt state. In this state, the receiver attempts to reacquire cell delineation by calculating the CRC on a sliding, bit-by-bit basis until the HEC value indicates a valid header. At this point, the receiver enters a pre-sync state, a short interval during which the receiver can verify whether it has, in fact, reacquired cell framing. If one HEC error is detected while in the pre-sync state, the receiver returns to the hunt state; after a number of valid HECs are found, the receiver returns to the sync state. The number of HEC errors that cause a transition from the sync to hunt state, and the number of correct HEC values that cause a transition from the pre-sync to sync state, are configurable parameters called ALPHA and DELTA, respectively.

Using the HEC field for cell delineation is the preferred method under normal operation. It eliminates the ATM Layer’s reliance on the Physical Layer’s (PHY) ability to delineate cells. While SONET/SDH can provide pointers to the beginning of a cell, current strategies offer no such aids for cell delineation. Therefore, cell delineation must be an ATM Layer function now.