Newcomer to Networking. I would like to connect an embedded device to an existing Fibre Gigabit LAN. The device would source and sink very little data so I am considering my choices.

  1. A microcontroller that supports R/GMII with an appropriate PHY would mean Gigabit traffic throughout the network but drives me to a more powerful micro
  2. A 100Mb device design, meaning its fiber tranceiver, PHY and MCU, need only support 100Mb Ethernet. Connecting 100Mb fibre ethernet (at switch on the GbE network) would work (as I understand it) so long as jumbo frames are not sent with the intention that they would get to the embedded device AND as long as the traffic intended for the device is minimal. Have I got this right?
  3. Not sure if this exists but is there such a thing as a 10/100/1000 PHY chip to which GbE could be connected, but where the MII interface can be used interfacing only to 100Mb MAC? I can't see how that's better than option 2, but just wondering if this is supportedby the standards & therefore implemented in a product.

Apologies for the seemingly very simple question. I'm always happy to hear what I've got wrong and what is best practice.

Many thanks.

2 Answers 2


The MII is the interface between MAC and PHY - so you do need a PHY (PCS, PMA & PDM, with appropriate setup) to interface with anything.

The Ethernet variants supported (100BASE-FX, 100BASE-TX, 1000BASE-T, 1000BASE-X + SFP, ...) by any PHY at hand depends entirely on its specs and the configuration.

Generally, a gigabit MII variant also supports 100 and 10 Mbit/s - but do check the documentation.

  • Thanks - so if I go MCU <---MII---> 100MbPHY <-----> FiberTXVR then fiber to the fiber port on a Gigabit switch, what else do I have to look out for in the configuration of the gigabit network, apart from the frame size (MTU) for packets destined for the device and the sheer volume of traffic to and from the device?
    – BlueTwin
    Commented Mar 1, 2023 at 13:10
  • 1
    The frame size for Ethernet is standardized (to 1518/1522 bytes) for a payload of 1500 bytes. Jumbos are non standard. In any case you need to make sure that Auto Negotiation is implemented, even if minimally, only for a single speed. Also when using 100BASE-FX(?) for linking to a "fiber gigabit switch" you need to check whether the switch's SFP is compatible with that - a matching 100BASE-FX SFP module is advised. Alternatively, a 100BASE-SX PHY should link with a 100/1000-compatible 1000BASE-SX SFP (you always need matching wavelengths and backward compatibility when mixing).
    – Zac67
    Commented Mar 1, 2023 at 13:17
  • Also, single-fiber 100BASE-BX requires pairing -D and -U SFPs - there's no single "100 Mbit fiber Ethernet".
    – Zac67
    Commented Mar 1, 2023 at 13:20
  • Perfect, thanks, really useful info. I'll make sure I check all of those points!
    – BlueTwin
    Commented Mar 1, 2023 at 13:26
  • Please don't forget to eventually accept an answer. (And you may vote for any useful ones, too.)
    – Zac67
    Commented Mar 1, 2023 at 14:06

A phy converts the electrical signals from the internal medium independent form to an external medium dependent form. However it does not have any buffer, it cannot change speeds. Going from a slower speed to a faster one requires a buffer than can hold a complete packet, going from a faster speed to a slower one without too much packet loss requires the ability to hold multiple complete packets.

It would be possible to build a buffer device that translated speeds, but it would be a highly niche product, and it would probably end up with a silicon cost at least broadly comparable to a switch IC.

In the copper world multi-speed PHYs are the norm, but in the fiber world that is not the case. Most PHYs work at only a single speed. Fortunately many switches support modular transceivers, so it is likely possible to install 100 megabit fiber options in a gigabit switch. I would want to get confirmation before commiting to a soloution though.

Another option to research is incorporating a switch in your product. Unfortunately, while it's easy to find switch ICs with two base-T ports and one port for a MII variant it seems much harder to find switch ICs that support multiple media-independent ports.

  • Regarding the buffering - that's essentially a switch. ;-)
    – Zac67
    Commented Mar 3, 2023 at 20:13
  • In addition to buffering a switch also has to keep track of MAC addresses and decide what packets to send out of which interface. So a switch is more complex than a speed changing buffer would be, but I don't think it's so much more complex as to push it into a different price ballpark. Commented Mar 3, 2023 at 20:19

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