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MDIO Managment Data Input/Output or I2C Interface?

MDIO Managment Data Input/Output Interface

MDIO - A short history

Managment Data Input/Output Interface or MDIO is a serial interface to read and write the control and status registers of the PHY. MDIOs configure each PHY before operation and monitor link status during operation. For most pluggable optical transceivers the interface used for monitor and control is the I2C interface.

Defined as part of MII in IEEE802.3 clause45, MDIO can also be used in high speed optical transceivers like CFP, CFP2 or CFP4 instead of 100 Megabit. A MDIO (Clause45) Interface consists of 8 wires, 2 wires for power supply, 5 wires of Port Address, and one wire for Global Alarm. MDIO works with 1.2V supply with a clock rate of 4MHz (downward compatibility to 100 kHz). MDIO supports full internal operation for DDM / Digital Diagnostic Monitoring data and uses a 16 bit adress space.

I2C - A quick overview

The I2C interface was developed in 1982 by Philips Smiconductors (today NXP) to enable communication between different parts of a devices. For optical transceivers the I2C Interface is mostly known for DDM or Digital Diagnostic Monitoring defined in SFF-8472. It is widly used in different formfactors like SFP, SFP+, XFP and QSFP. Also all information for the identification of the transceiver inside the EEPROM that is accessable via I2C.

A I2C Interface consists of just of 2 wires (Clock (SCL) and DATA (SDA)). It works (for optical transceivers) with a clock rate of 1KHz (Standard Mode) or 4KHz (Fast Mode). I2C supports full internal operation for DDM / Digital Diagnostic Monitoring data and uses a 7 or 10 bit adress space.

I2c Interface

DDM Digital Diagnostic Monitoring

DDM / Digital Diagnostic Monitoring - What is this?

With DDM you are able to read in real time the supply voltage, laser bias current, laser average output power, laser received input power and temperature. Digital Diagnostic Monitoring has become one of the most important tools to maintain and do diagnstics on optical networks. Todays most hardware and optical transceivers support DDM / Digital Diagnostic Monitoring.

So - whats the difference?

In fact, for the end customer, its not a big difference which technology is used. MDIO and I2C technology can provide Identification and DDM / Digital Diagnostic Monitoring information. From the technical view, there are some key difference like the different clock rate, different power supply and informations that can be provided internally.

Form factors using MDIO: CFP, CFP2, CFP4 and XENPAK

Form factors using I2C: SFP, SFP+, XFP, X2, QSFP, QSFP28 and SFP28

MDIO Managment Data Input/Output vs I2C Interface