Complex sentence: At the beginning of the course, the numeric "header" has unknown meaning; the meanings of various components of this header (such as a serial number assigned by the transmitting organization's message center) can be infered through traffic analysis.
Split into simpler sentences: At the beginning of the course, the meaning of the numeric "header" is not known. The meanings of various components of this header, such as a serial number assigned by the transmitting organization's message center, can be infered through traffic analysis.