Adding fiber optic ethernet to a circuit

Question

I'm working on a project that involves transmitting data over very long distances (10-25km) where radio communication is unavailable, and I have been looking into the possibility of adding a fiber optic ethernet connection to the board.

The first issue I have run into is trying to figure out what specification I can or should be using. My first thought was to use 100BASE-FX sfp transceivers, and connect the other end to a TRENDnet TFC-110S60i which claims a maximum distance of 60km, however further investigation seems to suggest there is a 2km limit for 100BASE-FX connections. If my objective is to connect over these long distances, what specification should I follow?

The second issue I've run into is actually connecting a fiber optic transceiver, be it SFP or otherwise, to the controller. At this time no decision has been made for what controller will be used on the board, so I'm trying to select as generic an interface as possible. Unfortunately most chips I've found such as the Microchip KSZ8041 are only PHYs, or the Microchip KSZ8441 which uses a host bus that I am unfamiliar with how to implement. I've also found the Microchip LAN7500 which handles MAC and provides an SPI interface, but the documentation doesn't specifically say if 100BASE-FX/BX/LX/etc. is supported. Would I be able to connect this chip to an SPI socket on a circuit board, or do I need something that specifically says it supports a fiber ethernet format?

Answer 1

The 2km limit is imposed by the speed of light. Ethernet works by listening for "quiet ether" and then sending its signal out. It reads back what signal is on the Ethernet. If someone else also decided to send, their transmission will garble your transmission. And your transmission garbles theirs. So they stop, back off, and wait a certain amount of time, and try again. To avoid repeating conflicts, each sender backs off a random amount based on a negative-exponential distribution.

There is a minimum size of an Ethernet packet. If the transmitter and receiver are too far apart, the sender will see a perfect transmission and not bother to retry, because the short packet will have been fully transmitted before the other packet arrives to interfere with it. You can avoid this by using half-duplex trasnsmission, and using a different protocol with timeouts, but then it wouldn't be Ethernet, it would be something else. You are free to implement a "something else" protocol on your fiber network, but you will have to write your own protocol stack. Or ignore the Ethernet entirely and just implement the "something else" protocol directly. Half-duplex will significantly degrade your bandwidth, and you need a way to hand-off transmittership; a two-node "token ring" might work. But if you want Ethernet, it is not going to work, because the low-level protocols critically depend upon the speed of light.