CRC-based Framing - Overview of CRC-based Framing

Overview of CRC-based Framing

The method of CRC-Based framing re-uses the header cyclic redundancy check (CRC), which is present in ATM and other similar protocols, to provide framing on the link with no additional overhead. In ATM, this field is known as the Header Error Control/Check (HEC) field. The header CRC/HEC is needed for another purpose within an ATM system, to improve the robustness in cell delivery. Using this same CRC/HEC field for the second purpose of link framing provided a significant improvement in link efficiency over what other methods of framing, because no additional bits were required for this second purpose.

A receiver utilizing CRC-based framing bit-shifts along the received bit stream until it finds a bit position where the header CRC is correct for a number of times. The receiver then declares that it has found the frame. A hysteresis function is applied to keep the receiver in lock in the presence of a moderate error rate.

In links where there is already a byte lock mechanism present such as within an E-carrier or SDH frame, the receiver need only byte-shift (rather than of bit-shifting) along the receive data stream to find lock.