As previously described, a single byte of preamble can be lost across the
1000BASE-X system (the actual loss occurs in the 1000BASE-X PCS transmitter state
machine).
- There is no specific statement for this preamble loss in the IEEE 802.3-2008 specification; the preamble loss falls out as a consequence of the state machines (see figures 36-5 and 36-6 in the IEEE 802.3-2008 specification).
- IEEE 802.3ah-2004 does, however, specifically state in clause 65.1.3.2.1:
Note: The 1000BASE-X PCS transmit function replaces
the first octet of preamble with the /S/ code-group or it discards the first octet
and replaces the second octet of preamble with the /S/ code-group. This decision is
based upon the even or odd alignment of the PCS transmit state diagram (see Figure
36-5)."