The start frame delimiter field marks the start of the frame and must contain the pattern 10101011. If a collision is detected during the transmission of the preamble or start-of-frame delimiter fields, the transmission of both fields is completed.
The receive data valid signal from the PHY ( phy_dv ) can go active during the preamble but is active prior to the start frame delimiter field. For transmission, this field is always automatically inserted by the AXI Ethernet Lite MAC core and should never appear in the packet data provided to the AXI Ethernet Lite MAC core. For reception, this field is always stripped from the packet data.