modem to modem without dialing. . . possible?

[popcan]

i was wondering if there was a way i could set up a serial link between two linux computers using just modems, since these computers happen to be more than 300 feet apart it's no go for ethernet, but there is phone line between the two i could isolate and use.

i've read various sources online about how to use ppp to "become your own isp" and whatnot, but all of these are with dialing and seem rather dated. i'm not too good with modems, but would it be as simple as sending the AT commands to pick up the phone to both of them so the client could hear the carrier? or since modems are seen as serial ports, would it be possible to use something like hdlc instead?

i'm just brainstorming at this point, but any input before i go ahead with the project will be appreciated

[papal_authority]

If you just want file transfers, mail and remote execution, would uucp work for you? I still use it for some tasks. If you want a login, it shouldn't be to hard with getty.
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The free market gave me gonorrhea.

[tuxmin]

It is not possible using the phone line only. You need at least a power supply and some more stuff (I remember an article about this very problem a few years ago -- I don't remeber the details, but it wasn't trivial).
If you have enough wires you could try soldering a Null Modem cable?


Alex!!
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[papal_authority]

The above poster is correct, I thought you'd be using an actual phone line. So yeah you'd need power in the line to emulate the phone company. Why not just get a repeating hub and put it in between the two computers so you can use Ethernet?
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The free market gave me gonorrhea.

[popcan]

i guess that would be why i couldn't find anything on how to do it!

there's no place to plug in a bridge or hub along the way otherwise i'd have done that. i'll look into making the null modem cable though, shouldn't be too hard. thanks for the replies!

[NaN]

You could just use a hardwired serial connection using RS422, but you'd need two cable pairs (one each for Rx and Tx). The distance is too far for RS232 (standard comm port) - at least if you want something reliable.

RS422 adapters are freelay available to convert RS232 into RS422, or alternatively to connect to a USB port. RS422 also has the advantage that you could run it much faster - 1 Mbps should be no problem over that distance. The other advantage is you do not need to rely on a good ground (earth) connection between your two computers.

[jklmnop]

try the ethernet solution first. you'll need a crossover ethernet cable to plug
directly between the machines. the 300ft limit is due to timing issues on a shared
connection. roughly it goes like this...

the smallest ethernet frame is N octets.
the distance is calculated so that in time it takes for a machine at
one end to send 1/2 the frame, the machine at the far end is just
recieving the first bit of the sent frame. so, if there were a collision
on the network, the far machine would have time to send the jam
signal back to the sending machine before the frame was completely
sent (so it could be retried later). if the distance were greater than
300 feet then it would be possible for the sending machine to finish
transmiting the frame before it got a jam signal back from the far
end.

so, the 300 foot limit was originally for collision avoidance. with only two
machines on a null ethernet... there is no possibility of collision and with
a good cable you can stretch it quite a bit further than 300 feet and
still get atleast a good 10Mb connection.

[NaN]

jklmnop is correct that there is a limitation due to timing issues, however I thought the total length of all segments due to timing was quite a bit more than 100m (300 ft) and that the ~100m (300ft) length limit on a single segment is due to the attenuation of the electrical signal in the cat5 cable. For single mode fibre optic ethernet the length limitation is around 3000m if my memory serves me correctly, and at these distances the timing skew that arises is becoming the dominant consideration. To put it in context 100m times 2 (round trip) is approximately 1 microsecond for the signal propagating in copper wire.

I have successfully run single segments well over 100m (up to ~150m) using good quality cat5. It's certainly worth a try if you're prepared to run a cable. If you were willing to go to single-mode fibre you could get a few km.

You'll get a similar distance for 100Base-T. It uses more advanced signal processing to achieve similar ranges to 10Base-T, at 10 times the speed.