Gigabit Ethernet Specification

High-Speed Digital Design Online Newsletter: Vol. 2 Issue 6

Sy Wong writes:

In your article on Gigabit Ethernet in Nov. 6 EDN, you said:

"GMII is a point-to-point, dual unidirectional interface, meaning that the specification defines one set of wires for use in the transmit direction and another separate set of wires for use in the receive direction. In each direction, there are eight parallel data bits, two control bits, and a clock, plus a few other signals. The interface is clocked at 125mhz."

Am I correct that there are 8+2+1+a_few wires in each direction? Old Ethernet can go many tens of feet, say 100". Is GMII designed to go that far or farther.

You mentioned Quackenbush's proposal "was to connect the driver to the near end of a 1-nsec, 50-ohm transmission line." I am curious what are the specifications of the physical wiring and connector GMII use and how far it will go. Is there a specification for worst case differential delay, crosstalk and ground wire noise among so many wires, particularly between the clock and signal lines?

Also, is point-to-point now an accepted mode architecturally for federated computer systems?

Thank you very much

Dr. Johnson replies:

Thanks for your interest in High-Speed Digital Design.

The GMII is designed as a chip-to-chip interface. The expected link distance is therefore about 3 to 12 inches. Architecturally, the system looks like this: 

     CPU
      |
      |  bus of your choice
      |
     MAC chip (implements packet format, etc.)
      |
      | GMII interface (3 to 12 inches, PCB trace, no connector)
      |
     PHY chip (drives the wire or fiber)
      |
      | wire of your choice (hundreds of meters, with connectors)
      |
    goes to next PHY

The wires coming out of the PHY are all star-connected into a central hub. The choices for PHY-to-PHY wiring supported in the current version of the Gigabit Ethernet standard (draft D3.3) include:

  • 100BASE-LX long wave 1300nm fiber-optics for use over single mode (3 km) or multimode (460m) fiber
  • 100BASE-SX low-cost shortwave 850nm fiber-optics for use over multimode (460m) fiber
  • 100BASE-CX lowest cost solution using two pairs of 150 balanced shielded (25m) cable

Unlike the old Ethernet AUI interface, which would support a cable up to 15m long, the new GMII is not specified as a connectorized interface. It's just a chip-to-chip interconnect standard.

Best Regards,
Dr. Howard Johnson