Ethernet + Serial port in Linux

Question

I want to implement a driver in Linux, that has a Ethernet stack but the data going out on hardware will be a serial port. Basically, I want to register my serial port as a Ethernet driver. Does anyone have any idea if this is possible? I want to be able to push IPv6 and/or UDP packets out of the serial port and in a similar way receive the packets via a serial port and pass it up the Ethernet stack.

I do not want to use the solution of serial-to-ethernet convertors(external hardware that convert a serial port to a ethernet port) but have that in my PC itself.

I tried PPP over the serial port and it works well. I am also told that I can do FTP, HTTP etc using the PPP. Reference to this - http://www.faqs.org/docs/Linux-HOWTO/Serial-Laplink-HOWTO.html

I have tried to hack the code from a RealTek Ethernet driver with a serial driver but not able to gain much success. Rather I do not know the stack of either to actually do anything meaningful. Any advice, guidance or tutorials would be helpful.

Thanks Aditya

Is there any reason the PPP over serial port solution does not suit you? — d-_-b
You have a fundamental problem in that ethernet is a packet-oriented protocol, but serial has no such thing as packets. You would have to invent your own packet framing to be able to reconstruct packet boundaries on the receiving side. You should use PPP. — TJD
The reason is I need to have it registered as a "eth0" ethernet driver. Basically, the project needs to not have anything "serial port" about it. To the outside world it should not look like I am talking to a serial port but to a Ethernet port.:-/ — Aditya Tantry
Reply to TJD, I am not worried about the framing part. I have a module at the other end.(let's say its a blackbox that understands the frames coming in). So data being pushed in IPv6 or UDP is understood. So it will push back the same formatted IPv6 or UDP packet back to the serial port if need be. — Aditya Tantry
@AlanCurry - Nice idea :), but I would still want it to be a ethernet driver :-| — Aditya Tantry

Mr_GQ Mr_GQ · Accepted Answer · 2012-08-24T06:25:17

You need to get back to de basics on networking, the way I understand you question is: "I have a serial port and I want to use is an Ethernet link". Sorry to crush your dreams but you don't have the real hardware to do so, I'll elaborate on it.

A serial connection is a physical connection that requires 3 wires (at least) tx, rx and ground. On the logical side you have an IC that coverts binary data into signals that are represented by discrete voltage ranges.

Ethernet is a layer 2 protocol, the layer 1 is provided by the technology used to transmit the signals (coax, up, fiber etc.) As you might see by now, you need a different set of hardware to convert the logical Ethernet frames into a stream of digital numbers, in fact this is call framing.

Since Ethernet has been an easy to use protocol it has been implemented as e preferred protocol for many network operators, of course one of the biggest is PPPoE where you have a PPP session over an Ethernet link. Of course this won't work with your example neither since you're trying the opposite.

If you're just learning and have all the time in the world you can attempt to write your own Ethernet framer over serial lines. This means you need to implement IEEE802.3 into the driver and then you need to serialize the data to push it as a stream of bits over the serial line. Of course note the following drawbacks:

Your driver won't be able to fully support Ethernet, you need some support at hardware level to implement some signaling (example, auto negotiation, CSMA/CD, etc)
You driver will be pretty much useless unless you back in time where 115.2kbps is top speed in data transfers

IMHO there are more exciting projects that you can pick up in the networking field for device drivers. You can for example attempt to buy a NIC and develop the device driver for it from scratch and you can optimize certain areas. Finally, remember that most of the Ethernet implementations are now done in hardware so you don't have to do anything but filling a few registers on the MAC and voila!

Ethernet + Serial port in Linux

2 Answers