What is MDI, the communication standard connecting PHYs? | NEXTY Electronics, an electronic components and semiconductor trading company

When a PHY communicates with an opposing PHY, it converts the signal before sending or receiving it. Digital signals are received from OSI reference model layer 2 devices such as SWITCH using an interface called MII (Media Independent Interface), and then converted to analog signals and sent to the opposing PHY using an interface called MDI (Medium Independent Interface). On the other hand, when receiving from the opposing PHY, the analog signal is converted to digital and then sent to the layer 2 device. This article explains the signals sent and received between MDI and automotive Ethernet PHY.

MDI (Medium Independent Interface)

MDI is an interface for wired communication between two devices. Information is transmitted using analog signals such as voltage changes or flashing lights via coaxial CABLES, twisted pair CABLES, or optical fiber.
For example, 100BASE-T1 and 1000BASE-T1 are used for automotive Ethernet PHYs, which use twisted pair CABLES as a medium and transmit data to the opposing PHY using analog voltage signals.

Signal conversion (100BASE-T1)

100BASE-T1 performs data conversion to transmit data via MDI. Data separated into 4-bit blocks is converted into data separated into 3-bit blocks, then converted into an analog signal and sent to the other device. When receiving a signal, the reverse conversion is performed.

4B/3B conversion

To convert to an analog signal, the data that was divided into 4-bit units is divided into 3-bit units again. After 4B/3B conversion, if the number of bits in a 3-bit packet is not a multiple of 3, one or two stuff bits are added to the end of the packet.

The timing diagram for 4B/3B is as follows: In 100BASE-T1, the frequencies of TX_CLK and pcs_txclk are 25 MHz and 33.333 MHz respectively to maintain the same bit-wise throughput in 4B/3B conversion.

3B2T

Data separated into 3 bits is converted into two ternary signal levels. This allows the communication frequency to be reduced. In the case of 100BASE-T1, it can be reduced from 100MHz to 66.67MHz. The first octet (SSD) and the last octet (ESD) of the transmitted data have AT and BT values of 0.

PAM3

PAM3 (Pulse Amplitude Modulation 3-level) is a ternary encoding method that transmits information using three values: -1, 0, +1 or 0, 1, 2. 100BASE-T1 transmits TA and TB alternately using three voltage values: -1, 0, +1.

Signal conversion (1000BASE-T1)

100BASE-T1 performs data conversion to transmit data via MDI. Data separated into 4-bit blocks is converted into data separated into 3-bit blocks, then converted into an analog signal and sent to the other device. When receiving a signal, the reverse conversion is performed.