Ethernet to Ethernet ovr slow serial link?

Dear Community - I'm thinking of having a Ethernet communication "bridge" over a much slower serial line. My questions are:

- Does anybody built it already? Hopefully with schematics etc. Lniks on the Net?

- Exists time-out values in the Ethernet spec? If I have a slow link there maybe several hundred milliseconds before the answer comes back. Overflow possible?

- I think of using a W3100A or Realtek Ethernet Controller along with a AVR. Is it possible to use a generic Ethernet Controller?

- I don't know if the circuit would be a Router, Bridge, Rub, Switch, Gateway in the Ethernet sense. What is the simplest answer?

- Would it be possible to make the circuit entirely protocol "transparent" (other than the long delay)?

- Anyone interested to share infos/project? BTW: I'm a electronics design engineer and don't want infos about commercial products. Regards - Henry

Reply to
Henry
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This is not a new idea. An ethernet-to-serial port at each end.

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Henry wrote:

Reply to
hamilton

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Did you _read_ any of the hits? I read the first 10 or so, and none of them had anything to do with what the OP wants to do.

I even designed the software for one of the products sited on one of the hits you posted. My product doesn't do what the OP wants, and I'm not aware of any competing products that do either.

Putting any of the cited serial-to-ethernet devices at each end of a cable will not create an Ethernet bridge. The most you could hope to do with the cited products is to tunnel a single UDP or TCP port through the serial line.

Probably -- protocols designed to run on top of Ethernet may going to tolerate the latencies.

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Grant Edwards                   grante             Yow!  Like I always
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Reply to
Grant Edwards
2nd: I should clearly state that I want to make a "packet link". Each Ethernet packet will be send across a link (serial like V24, wireless, etc) in duplex mode. I found nothing on the Net. That's why it is impossible or to difficult? Surely there exists applications for it. Think of having your PC in one room with ordinary Ethernet cable to another PC and you want a little embedded PC somewhere connected with a cheap ordinary telephone cable (That can be your freezer :-)))

- Henry

Reply to
Henry

It's difficult.

Possibly. More likely people will want to route IP over a serial link. That's what PPP is for.

In theory, you can use PPP to encapsulate anything, but IP is the most common.

FWIW, "cheap ordinay telephone cable" is what most of us run

10baseT and 100baseT Ethernet on. Cat-3 or even Cat-5 is pretty cheap and common (at least in the US, it is what phone wiring is done with).
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Grant Edwards                   grante             Yow!  World War III? No
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Reply to
Grant Edwards

Seems odd to route ethernet over a serial link but anyway.

You have a couple of options.

Use it as a router, (assuming IP) and have it connect subnets, then packets are sent to the mac of the Ethernet card at each end when they need to cross the link. Then you can just use something simple like slip to transport the packet to the other end and send it out the other card. This would be my preference in almost every situation

Use it as a bridge, passing all packets. You stick the Ethernet chip in promiscuous mode (where it received all packets on the lan) and transport them all. I assume the serial link is many times slower than the Ethernet link so this could be a problem.

for either of these tasks you could use a couple of ethernut boards to experiment quite quickly and easily.

Have fun.

Ralph

Reply to
Ralph Mason

Use the PPP encapsulation. If you do not need negotiated options, you can forget about PPP link control protocols and the initial negotiation. If needed, you can also transfer the Ethernet frame header with MAC addresses and type/size word.

Been there - done that. (It's sloooow).

HTH

Tauno Voipio tauno voipio @ iki fi

Reply to
Tauno Voipio

Unless I'm misunderstanding your question, you just want a remote bridge, as implemented by every (non-low-end) bridge/router vendor in the world. Bridges, as opposed to routers, work below the network (eg. IP) layer and forward frames based on the MAC address. If you want to build such a bridge that's interoperable with other bridges (and switches, as switches are just plain old multi-port bridges with better marketing), see the 802.1 (spanning tree bridge) spec. The fact that one of the link is over a slow serial line is immaterial, except, perhaps, to some overly timeout sensitive higher level protocols.

If you don't care to be 802.1 interoperable - and I don't recommend this since you'll break with even the most basic switch (as opposed to pure hub) on either side of the bridge - the basic procedure is as follows. Open the network cards in promiscuous mode. Capture all the packets and inspect the destination MAC address. If that address is on the "other" side of the link, forward the packet. The trick is coming up with a good scheme to figure out which MAC address is on which segment, which is exactly what 802.1 addresses.

There is bridging software available on various Linux distributions as well, if you want access to source code.

Reply to
Robert Wessel

Check-out the IEEE 802.1d MAC bridge specification. No ... its *not* "free". We built one of these during the eighties. To reduce the amount of traffic over the slow Wide Area Network (WAN) link, you need to do some filtering at each ethernet port.

To do this, there is an algorithm that looks at the MAC source address of each received packet, and saves it as a known local MAC address. Incomming destination MAC addresses are compared to this list of local addresses, and if the destination is not found in the local list, then the frame is forwarded over the WAN link to the other end of the bridge.

Yes, thats an oversimplification of the algorithm. It gets even more ... interesting ... when there is more than a simple point-to-point connection.

The nice thing about MAC bridges is that they are independent of the higher level protocols (e.g. IP).

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Reply to
Michael N. Moran

It sounds like what any ISP with dial-up lines does with PPP. The appliance-side should just need PPP code. The other end could be a Linux/xBSD box configured as a gateway to route to the serial link.

The "grasshopper" DNS and BIND O'Reilly book might be a good place to start. (Why a grasshopper when one of the authors is _Cricket_ Liu, I dunno. :^)

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Ron Sharp.
Reply to
Android Cat

On second thought, perhaps the PPP solution seems too easy. Did I miss something? Is there a reason that you want to route packets at the Ethernet level or do you just need to route IP packets from an Ethernet LAN across a serial link?

The packet level makes a big difference. The former seems like a lot of work; the latter is off-the-shelf gateway and router config. (And with TCP/IP, you get handshake and retry for free.)

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Ron Sharp.
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Reply to
Android Cat

We tend to do that over many wide area network (WAN) connections with speeds of 56K up to T3. I'd guess that what the OP is requesting is more like what some people do with a dialup except instead of routing using bridging. In the late '80s I had heard that refered to as half bridges (2 makes a complete bridge). So it is possible (examine the Open Source code for tcp/ip, bridging, & PPP). At one time I would have guessed that it wasn't possible to it with 8/16 bit processors but I've seen too much imaginative work to doubt it anymore.

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Linux Home Automation         Neil Cherry        ncherry@comcast.net
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Reply to
Neil Cherry

IEEE has convinced some folks to "sponsor" free (personal use) access to most of the current 802.x specs.

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and click on "Get IEEE 802".

Reply to
Robert Wessel

Thanky you all folks for the really informative answers. Maybe the project will come true...

- Henry

Reply to
Henry

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