The asynchronous transfer mode (ATM) is a promising technique for broadband switching that is capable of supporting high-bit-rate multimedia services. Progress in all-optical parallel processing shows that photonics may be used in the future in full-functionality ATM switching nodes. All-optical switching fabrics and buffers have already been demonstrated. Fewer studies have been dedicated to ATM header-processing functions. As an example of photonic header processing, the implementation of a header-error control (HEC) subsystem by means of an optical circuit is investigated. Although traditional electronic decoders perform HEC in a serial mode, the architecture chosen for the optical HEC subsystem is based on parallelism, and an appropriate parallel-decoding algorithm was used in designing it.