Author Topic: Connecting a pair of Dialup Modems  (Read 1039 times)

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Offline @rt

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Connecting a pair of Dialup Modems
« on: July 13, 2018, 11:58:11 pm »
Hi Guys :)

I’m trying to connect two terminals together, each with their own 56k dialup modem, and an ordinary phone cable connected between them.
One is an old XP laptop running Win XP using the Hyperterminal program, and it’s internal modem, and the other is a 56k portable travel modem.

Both modems can echo back to their respective terminal programs, and both also respond to AT commands with “OK” etc.

I’ve had no luck getting them to connect though. There are a few small articles about, and a YouTube video demo of the same,
and they all use ATA for the terminal doing the answering. The command used to dial varies from a simple “ATD” to something that might cause
the modem to ignore the fact that there is no dial tone without a real phone exchange. Some examples are “ATX3D” or “ATX1D”.
I hear that the function of the “X” command can vary between modems. I can get both modems to make noises by answering,
and one modem to make noises by dialling with some variations of the “ATD” command. It doesn’t matter if I have to dial with one of them all the time.

Ultimately, they both quit with a “NO CARRIER” message, and I have never heard anything that suggests to me they are responding to each other.
Is there a physical (electrical) reason for this (assuming everything is working), or a possible incompatibility that would prevent them even connecting to each other?
Cheers, Brek.

« Last Edit: July 13, 2018, 11:59:42 pm by @rt »
 

Offline CJay

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Re: Connecting a pair of Dialup Modems
« Reply #1 on: July 14, 2018, 12:15:46 am »
Seem to remember 56K modems won't work back to back, at least not at 56K, as they rely on some trickery with the digital telephone exchanges so you might need to persuade them to connect at a slower speed.

But, first, are you expecting them to connect over an actual phone line or just a piece of phone wire?

If they've not got the relevant voltages on the line then they may just not work at all.

M0UAW
 

Offline @rt

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Re: Connecting a pair of Dialup Modems
« Reply #2 on: July 14, 2018, 12:30:07 am »
With a straight phone wire. I have seen this done on YouTube, and know the fellow who did it.

56k should connect at 33.6 or so if that’s the uplink speed of each I assume.
 

Offline bd139

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Re: Connecting a pair of Dialup Modems
« Reply #3 on: July 14, 2018, 12:33:46 am »
You can do this with older Hayes modems. I had a 28.8k building to building ppp link running for several years on some Hayes Optima modems over one spare pair. ATX1D (blind dial no busy detect) Needs to be configured or it’ll wait for a dial tone. That had Pegasus mail running over it (yuck) as part of a supposed loose air gap.

It should negotiate 33.6k max but no guarantees with 56k/v90 cack. Depending on the chipset you might be able to turn that off and negotiate 33.6k max.
 

Offline Berni

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Re: Connecting a pair of Dialup Modems
« Reply #4 on: July 14, 2018, 12:36:38 am »
The phone line injects power into the wires. That's where phones get there power from.

You can get two phones to talk to each other by connecting a 9V battery in series with one of the phones terminals before connecting them with together with a phone cable. The battery pushes current around the loop to power both of them, munch like the telephone exchange would. Audio is transmitted by varying how much current is drawn from the line.

Perhaps the battery trick works with modems too in order to trick them into thinking they are on a real line. Hopefully they don,t also expect to hear a dial tone too, but perhaps that can also be bypassed by simulating the line ringing by injecting 100V AC on the line for a short moment. When you pick up a ringing line you don't get any dial tone, it's straight to audio(apart from the caller id signal nowadays)
 

Offline CJay

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Re: Connecting a pair of Dialup Modems
« Reply #5 on: July 14, 2018, 12:42:29 am »
They should be able to connect at 33.6K if they support V34, I'm pretty sure V.90 and V.92 rely on the exchange trickery and won't connect without seeing an exchange between them, maybe see if you can rate/standard limit them?

Which models do you  have and how are they connected to the hosts?

Some modems relied on having line voltage to power the DAA, most often laptop modems with an external 'lump' to connect to the telephone line which the 'travel modem' sounds like so it might never work if you just sling wire between them but it's been a *long* time...
M0UAW
 

Offline @rt

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Re: Connecting a pair of Dialup Modems
« Reply #6 on: July 14, 2018, 12:56:38 am »
When I first tried the laptop on it’s own it complained it had no dial tone and did nothing,
but I assume one of the commands to turn that off worked, because now it will just go ahead.

The travel modem on the other side might still be waiting for a dial tone, but it doesn’t complain that there isn’t one.
As I said though, it would be fine with me if the modem that will dial is always used to dial.

I have seen a trick to put a 9V battery, or a pair of them in series with one of the phone wires.
It supposedly doesn’t matter which wire, or the polarity of the battery.
I’ve heard of this, but always assumed it was just to get a phone to ring. Thinking about it though, DC shouldn’t get a phone to ring.


 

Offline CJay

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Re: Connecting a pair of Dialup Modems
« Reply #7 on: July 14, 2018, 01:01:10 am »
There are AT commands to persuade modems to dial without dialtone and answer without hearing a ring so I don't think it's that, you've got round the dial problem and ATA should make the other modem answer, the reference manual for the modems should detail the 'advanced' AT commands to limit protocol, allow you to monitor the line via the speaker etc.
M0UAW
 

Offline @rt

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Re: Connecting a pair of Dialup Modems
« Reply #8 on: July 14, 2018, 01:03:48 am »
Does it sound correct that it wouldn’t matter what polarity the battery was connected?
Looking at an open PCMCIA modem I have here, it has tantalum caps on the isolated phone side. This isn’t the one I’m using, but might be similar to one of them.
 

Offline CJay

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Re: Connecting a pair of Dialup Modems
« Reply #9 on: July 14, 2018, 01:12:35 am »
Tip should be positive, Ring should be negative but it probably doesn't matter.


Sorry, that's would be better said as Tip is usually positive, Ring is usually negative.
M0UAW
 

Offline @rt

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Re: Connecting a pair of Dialup Modems
« Reply #10 on: July 14, 2018, 01:15:54 am »
It ends up that one modem will get the wrong polarity anyway.
If you look at the diagram here: https://electronics.howstuffworks.com/telephone4.htm
There’s obviously no way to make the circuit where both red wires are positive for both phones.
 

Offline Berni

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Re: Connecting a pair of Dialup Modems
« Reply #11 on: July 14, 2018, 01:16:20 am »
There is nothing special about the 9V. It just happens to push about the right amount of current around the loop.

A real phone line actually has 48V DC on it, but with a high impedance so when you pick up the phone the speaker in the earpiece loads it down with its internal resistance to bring it down in to a few volts range with a few miliamps of DC bias current flowing trough the line. Phone lines don't have a defined polarity so phones are designed to work when connected backwards.

The ring is a >100V AC signal applied on the line and is designed to push enough current trough the high impedance electromagnet of the ringer bell mechanism to make it ring. This signal can be pretty low impedance so it has plenty of grunt to give you a nasty shock if you happened to hold the line when it rings. You could recreate this signal by simply connecting a mains transformer with a 100V winding right across the phone line, it won't be spot on but it would make phones ring and likely fool a modem too. Tho don't attempt this is you are not experienced with handling high voltage as you can really shock yourself.

It also might be possible to trick a modem into thinking the line is ringing by modding the circuit that detects the ring and wiring a switch in to make it "ring" by a push of a button.
 

Offline CJay

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Re: Connecting a pair of Dialup Modems
« Reply #12 on: July 14, 2018, 01:20:28 am »
It does, you're right, but the 'standard' is as I said above, in practice it rarely ever matters and if the DAA needs power they usually have a bridge rectifier which sorts out the polarity for you anyway.
M0UAW
 

Offline @rt

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Re: Connecting a pair of Dialup Modems
« Reply #13 on: July 14, 2018, 01:24:40 am »
The ringing I’m not so interested in since the “ATA” command works with both modems to answer. Easier than modding the modem and pushing a button.
I’ve used two 9V in series with 330R resistor. Here goes nothing.
 

Offline @rt

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Re: Connecting a pair of Dialup Modems
« Reply #14 on: July 14, 2018, 01:33:59 am »
That totally worked!!! :)

Since photo bucket started asking for money to host images, I can’t easily explain why that is so exciting.

The laptop modem is ordinary, but the travel modem takes 2xAA batteries, plugs into the phone line on one side, and it’s only other interface is IRDA.
So using that as a dongle hanging out of a phone socket on my laptop is very neat. One small thing hanging off one short wire to a phone socket flush with the side of the laptop,
for an IRDA serial port that I needed.

Thanks for all the replies, and of course, regarding my setup, I’d appreciate any suggestions. I assume 330R resistor is closest real value I’ll get to 300R/ does 300 Ohm even exist?
and maybe I could do the 48V DC supply, but if there’s no issues with errors is that really a problem?

BTW it was just “ATD” & “ATA”, but previous setup has obviously removed the need for a dial tone. Probably one of the ATXn commands.


« Last Edit: July 14, 2018, 01:37:55 am by @rt »
 

Offline bd139

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Re: Connecting a pair of Dialup Modems
« Reply #15 on: July 14, 2018, 02:02:55 am »
 :-+

Use imgur.com for image hosting.

 

Offline SeanB

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Re: Connecting a pair of Dialup Modems
« Reply #16 on: July 14, 2018, 02:20:31 am »
Typical telephone system is designed for a 48V central office battery, a 600R coil of the off hook relay doing triple duty as both transformer, off hook detection and a line choke. The phone itself is designed to emulate a 600R load ( which is why you have 600R transformers in audio use) and the line itself will allow over 1k before it degrades too far. Long lines fix the capacitance between the 2 wires with 88mH coils every few miles, which turn it into a rough 600R transmission line.

Thus with 9V and a 330R resistor you probably are emulating the line as if it is a long line, but there is enough headroom on most phone interfaces to handle it fine, and it will not do any harm, as the opposite can fry the line interfaces. You probably are drawing around 5mA from the battery, which is fine, though you probably should place a 100uF-470uF 16V capacitor across the battery terminals to lower the audio impedance, which will slightly increase the operating life of the battery before it's rising ESR causes the modem to drop out.
 

Offline @rt

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Re: Connecting a pair of Dialup Modems
« Reply #17 on: July 14, 2018, 02:25:56 am »
Hi Thanks for the replies.

Please check back Sean, I’ll have another question for you about your explanation, but it’s 2:20am here and I’m fighting to stay awake.
Goodnight! :D
 

Offline CJay

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Re: Connecting a pair of Dialup Modems
« Reply #18 on: July 14, 2018, 03:25:49 am »
What speeds do you get?
M0UAW
 

Offline @rt

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Re: Connecting a pair of Dialup Modems
« Reply #19 on: July 14, 2018, 04:39:19 am »
I’m in bed but the sleep thing isn’t working.
They connected at the 33 something, and I could type,
but I haven’t tried a modem protocol to move a file yet.

There’s enough grey matter here to put the last few bits
together to finish something off if I ask the right questions.
Now is the time I’ll screw it up.

Numbers will be wrong here, I’m making it up.
I made a ringer from scratch some years ago starting with a
555 timed for the 20Hz to ring a phone, modulated with some
gated outputs of a decade counter clocked with another 555
timed to pulse at a derivative of the Australian ring pattern.
Select outputs of the decade counter were ORed such that they
handled the ring timing, and the counter’s own reset.

From there, the twice modulated ring was used to drive some
current from another supply into the primary of a 1:10 transformer
(with some heavy transistors),
and it’s output is the 90-100V standard Australian phone ring.

But that only was intended to make an old Bakelite phone ring.

To superimpose the ringer straight over the DC circuit, you’d need to
either switch between one or the other, or protect the battery
pack with diodes or something, is that right?
There’s also the mater of what causes the phone to stop ringing, and
connect the DC circuit when it’s taken off the hook.

For this modem situation, I have one of the old hand cranked dynamos
which would be funny to try getting a modem to pick up on its own.
But at that moment, when something takes the modem off hook,
 surely you must immediately prevent the 100V AC
ringer circuit from working don’t you?




« Last Edit: July 14, 2018, 04:46:17 am by @rt »
 

Offline Berni

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Re: Connecting a pair of Dialup Modems
« Reply #20 on: July 14, 2018, 05:00:36 am »
The battery will be fine since you should connect the ringing voltage in parallel acros one of the two phones. And yes the moment you pick up the phone the ringing waveform stops. Telephones are electrically pretty tough things tho(especially the old rotary ones as there are no semiconductors or even active devices)

It's amazing what a layering of legacy technologies the phone network is. And yet the same 90V AC signal is being used today by modern voip enabeled router boxes at home to tell a cordless phone that it's time to ring.(While the said router box gets to the Internet via ADSL on the old telephone wires that no longer carry the analog signal)
« Last Edit: July 14, 2018, 05:04:24 am by Berni »
 

Offline @rt

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Re: Connecting a pair of Dialup Modems
« Reply #21 on: July 14, 2018, 09:56:05 am »
Technology rotating one bit to the right :D

For a phone line the ringing might stop when you pick up the phone,
but if I were to connect the ringing signal across mine,
what makes that stop when a modem picks up?
and even if that doesn’t damage a modem,
The event of it going off hook would need to be used to turn it
off anyway, or it will still make the first modem to hang up start ringing again.
 

Offline Berni

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Re: Connecting a pair of Dialup Modems
« Reply #22 on: July 14, 2018, 04:20:20 pm »
Yeah you don't have detection for that. But if you only apply it for a few 100s of a millisecond then even if the on hook resistor can't handle the full waveform it not have time to get hot. The modem will proabobly still see it as a ring tho as sometimes the waveform from the exchange starts half way trough the ring
 

Offline 2N3055

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Re: Connecting a pair of Dialup Modems
« Reply #23 on: July 14, 2018, 06:14:13 pm »
« Last Edit: July 14, 2018, 07:55:51 pm by 2N3055 »
 

Offline SeanB

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Re: Connecting a pair of Dialup Modems
« Reply #24 on: July 14, 2018, 06:39:23 pm »
Trying to emulate a complete phone line is going to be more effort. first you will need a 48VDC supply, but it only has to supply 100mA at most. Each phone will need a 48V 600R coil relay to provide it with power, and you will need a 600R 1:1 audio transformer to couple the audio between them. Ringing is simply 100VAC 25Hz applied via a 2uF capacitor to the phone line, and turning off the ring voltage is when the phone is off hook and the relay contacts close.

However just to connect 2 modems this is a little overkill, though you can make a nice intercom using regular phones with this. There are circuits online (IIRC Don Lancaster published a few) top do this as well.
 

Offline @rt

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Re: Connecting a pair of Dialup Modems
« Reply #25 on: July 14, 2018, 07:48:29 pm »
That links comes up “Forbidden” for me.
I have been told the 48V is present when the line is idle,
and drops to 8-12V for the voice circuit. Is this correct?
It does seem to explain why other diagrams use a single 9V battery.
 

Offline 2N3055

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Re: Connecting a pair of Dialup Modems
« Reply #26 on: July 14, 2018, 07:56:54 pm »
That links comes up “Forbidden” for me.
I have been told the 48V is present when the line is idle,
and drops to 8-12V for the voice circuit. Is this correct?
It does seem to explain why other diagrams use a single 9V battery.

Sorry it is broken to me now too.. I uploaded image up there.
 

Offline 2N3055

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Re: Connecting a pair of Dialup Modems
« Reply #27 on: July 14, 2018, 08:08:42 pm »
Originally it was like this:

1. 48V power
2. 48V high sensitivity relay. It serves as a current source, an AC block (high inductance), and also as an Off-hook detector
3. Voltage was 48-60V open 10-12V with phone off hook. Current draw was 10-20mA.

Good results can be had with 24V (many small inexpensive Panasonic in house switches had 24 V, better ones had 48V).
Also 12V/220Ohm combination should work quite well, it is lower impedance that in should be for the voice signal, but that is not a problem when locally connected.
You can create dial tone with arduino pin, lowpass filter it and attenuate to get 100mV ish level, couple it with a capacitor to the line.

It can be done, but it deceptively complicated to do it right..
 

Offline pamperchu

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Re: Connecting a pair of Dialup Modems
« Reply #28 on: July 15, 2018, 05:04:51 am »
Set Up A Dial-In Server ,

h++ps://www.howtoforge.com/linux_dialin_server
 

Offline Berni

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Re: Connecting a pair of Dialup Modems
« Reply #29 on: July 15, 2018, 05:42:32 am »
Oh and there is also the easy way out of simply buying one of those household telephone exchanges that connects 2 to 10 phones to one POTS line. Connect all your modems to it as phones and simply dial the internal number of the other one. You get the dial tone, ring signal and perhaps even a caller id signal to both of the modems.

By the way 2N3055 I'm in your country on holiday. Good luck with the big soccer match.
 
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Offline 2N3055

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Re: Connecting a pair of Dialup Modems
« Reply #30 on: July 15, 2018, 08:01:32 am »
Oh and there is also the easy way out of simply buying one of those household telephone exchanges that connects 2 to 10 phones to one POTS line. Connect all your modems to it as phones and simply dial the internal number of the other one. You get the dial tone, ring signal and perhaps even a caller id signal to both of the modems.

By the way 2N3055 I'm in your country on holiday. Good luck with the big soccer match.

Thanks a lot! I hope you are having a good time, enjoy the holidays... Lijep pozdrav!!
 

Offline kakabouras

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Re: Connecting a pair of Dialup Modems
« Reply #31 on: July 15, 2018, 08:22:36 am »
Back in 1989 I was assigned the task to make two dial-up modems to talk to each other over a leased line.

After a few experiments the following solution was implemented :

- I used one modem as a call initiator by using a command like ATX0DT5555555.

- X0 means do not detect dial or busy tone.

- DT5555555 means send DTMF tones corresponding to number five ie 1336Hz and 770Hz.

- At the other end I implemented a dual tone detector using two LM567 with their outputs tied in a logic AND configuration. I could have used a special DTMF detection IC like  CMD CM8870CSI  but those were not easily available in Greece then.

- When the DTMF signal was detected a hardware interrupt signal was used by the receiving end computer to send an ATA command to the receiving end modem.

It worked like charm for this telemetry project for many years until it was replaced by internet DSL lines in 2007.


When I described the idea to my boss he said  : "That's too simple to work".
Afterwards when we delivered the project he called the whole idea "the Columbus'  egg" .
« Last Edit: July 15, 2018, 08:30:39 am by kakabouras »
 

Offline 2N3055

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Re: Connecting a pair of Dialup Modems
« Reply #32 on: July 15, 2018, 08:32:11 am »
You don't have to detect dialed number, just play dial-tone after off hook, insert few second pause, ring other side, and after it goes off hook connect them..
Or if you put ignore dial-tone in dial string for modem, no need for dial-tone at all.
 

Offline @rt

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Re: Connecting a pair of Dialup Modems
« Reply #33 on: July 15, 2018, 02:40:07 pm »
Those telephone line simulators are neat,
but still surprisingly expensive on eBay for technology on the very edge of obsolescence.
 

Offline CJay

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Re: Connecting a pair of Dialup Modems
« Reply #34 on: July 16, 2018, 07:45:26 pm »
Those telephone line simulators are neat,
but still surprisingly expensive on eBay for technology on the very edge of obsolescence.

Because that stuff is on the verge of being obsolete it's possible to buy office PABX boxes for cheap, they'd do just fine but they don't tend to be particularly small.
M0UAW
 

Offline Peabody

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Re: Connecting a pair of Dialup Modems
« Reply #35 on: July 17, 2018, 12:12:44 am »
This is going back a long way, but my memory is that the DAA, which is the part of the modem circuit that connects to the phone line, usually runs the lines through a FWBR because often Tip and Ring are wired backwards.  The FWBR makes the modem work with either polarity.  But with a direct connection with no voltage on the line, nothing gets through the two diode drops of the FWBR.  So you need to put some DC voltage on the line so the audio will get through.  But I'm confused that this was described earlier as putting the 9V battery "in series".  The battery would be *across* tip and ring.  Right?

And I also thought there were AT commands to get around the absence of dial tone and ringing.
 

Offline @rt

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Re: Connecting a pair of Dialup Modems
« Reply #36 on: August 08, 2018, 08:08:33 pm »
The ring signal goes across the ring and tip, which puts the phone’s ringer solenoid in series I presume.
The circuit for speech/data is a series circuit (in series with either the ring or tip).

Just in case this thread is searched, the current arrangement worked well until I tried a PCMCIA card modem.
To get this working I use a pair of 12V SLA batteries in series with a 510 Ohm resistor.

Funny enough though, this is also a Fax modem, and when I tried it with Telstra’s fax test number,
it couldn’t interpret the fax to return it. The report was as if my modem was silent after connecting.
I had to go to a friend’s house for this, as he still has a POT line which is getting pretty rare in Australia, as they are being phased out.
 

Offline NiHaoMike

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Re: Connecting a pair of Dialup Modems
« Reply #37 on: August 09, 2018, 12:55:46 pm »
I had a 28.8k building to building ppp link running for several years on some Hayes Optima modems over one spare pair.
Nowadays, it's pretty easy to hack some Homeplug adapters and send data at a few hundred Mbps through a few hundred feet of telephone wire, basically a cheap way to get a pair of really fast "short haul modems". Isn't technology amazing? :)
Cryptocurrency has taught me to love math and at the same time be baffled by it.

Cryptocurrency lesson 0: Altcoins and Bitcoin are not the same thing.
 


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