Spartan 6, PCS/PMA Ethernet 1000BASE-X

Hi

I am using IP core Ethernet 1000BASE-X PCS/PMA or SGMII v.11.5,  and i 
have a problem with transmitting frame from PCS/PMA to GTP transciever 
while using gmii user interface - according datasheet I turn gmii_tx_en 
on first octet of preambule and turn it down at the end of frame, so i 
thought that out signal from PCS/PMA will be like <interframe gap><my 
frame><interframe gap>. But chip scope shows that instead of the first 
octet of my frame placed FB(hex) byte, and at the end of frame added 5 
bytes. So as I understand, I have broken frame as result. Could anybody 
help?

Sorry for my english)