Ethernet Architecture Overview at Xilinx

 

Ethernet Architecture Overview

 

 

Figure 1. displays the Ethernet MAC architecture from the MAC to the right, as defined in the IEEE
802.3 specification, and also illustrates where the supported physical interfaces fit into the architecture.

 

MAC
The Ethernet MAC is defined in the IEEE 802.3 specification clauses 2, 3, and 4. A MAC is responsible
for the Ethernet framing protocols and error detection of these frames. The MAC is independent of, and
can connect to, any type of physical sublayer.

 

GMII/MII
The Media Independent Interface (MII), defined in IEEE 802.3 clause 22, is a parallel interface that connects
a 10-Mbps and/or 100-Mbps capable MAC to the physical sublayers. The Gigabit Media Independent
Interface (GMII), defined in IEEE 802.3 clause 35, is an extension of the MII used to connect a
1-Gbps capable MAC to the physical sublayers. MII can be considered a subset of GMII, and as a result,
GMII/MII can carry Ethernet traffic at 10 Mbps, 100 Mbps, and 1 Gbps.

 

RGMII
The Reduced-GMII (RGMII) is an alternative to GMII/MII. RGMII achieves a 50-percent reduction in
the pin count, achieved by the use of double-data-rate (DDR) flip-flops. For this reason, RGMII is preferred
over GMII by PCB designers. RGMII can carry Ethernet traffic at 10 Mbps, 100 Mbps, and 1Gbps.

 

SGMII
The Serial-GMII (SGMII) interface is an alternative to GMII/MII. SGMII converts the parallel interface
of the GMII/MII into a serial format using a RocketIO GTP or GTX transceiver, radically reducing the
I/O count. For this reason, it is often the preferred interface of PCB designers. SGMII can carry Ethernet
traffic at 10 Mbps, 100 Mbps, and 1 Gbps.

 

PCS, PMA, PMD
The combination of the Physical Coding Sublayer (PCS), the Physical Medium Attachment (PMA), and
the Physical Medium Dependent (PMD) sublayer comprise the physical layers of the Ethernet protocol.

Two main physical standards are specified for Ethernet:

   • BASE-T, a copper standard using twisted pair cabling systems
   • BASE-X, usually a fibre optical physical standard using short and long wavelength laser

BASE-T devices, supporting 10 Mbps, 100 Mbps, and 1 Gbps Ethernet speeds, are readily available as
off-the-shelf parts. As illustrated in Figures 1 and 2, these can be connected using GMII/MII, RGMII, or
SGMII to provide a tri-speed Ethernet port.
The Ethernet MAC has built-in 1000BASE-X PCS/PMA functionality and can be connected to a RocketIO
GTP or GTX transceiver to provide a 1 Gbps fibre optic port, as illustrated in Figure 3.