The Gigabit Media Independent Interface (GMII), a parallel interface connecting a MAC to the physical sublayers (PCS, PMA, and PMD), is defined in IEEE 802.3-2008, clause 35. For a MAC operating at a speed of 1 or 2.5 Gb/s, the full GMII is used; for a MAC operating at a speed of 10 Mb/s or 100 Mb/s, the GMII is replaced with a Media Independent Interface (MII) that uses a subset of the GMII signals.
The Serial-GMII (SGMII) is an alternative interface to the GMII/MII that converts the parallel interface of the GMII/MII into a serial format capable of carrying traffic at speeds of 10 Mb/s, 100 Mb/s, and 1 Gb/s or 2.5 Gb/s. This radically reduces the I/O count and for this reason is often preferred by Printed Circuit Board (PCB) designers. The SGMII specification is closely related to the 1000BASE-X or 2500BASE-X PCS and PMA sublayers, which enables it to be offered in this core.