Abstract: Exceptions to valid protocol categories are filtered from a bit stream. Bit sequences between flags are processed to detect bit sequences that do not qualify as valid packet data but do qualify as exceptions to other nonpacket protocol categories. The exceptions or nonconforming bits are filtered from the bit stream before being further processed. Because the nonconforming bits are filtered from the bit stream, extraneous packet errors are prevented from being generated by the packet decoder. The bit stream is generated in a packet protocol, such as HDLC. The flags and bits sequences are detected according to bit patterns used in the transmitted packet protocol.