Author Topic: Connecting a pair of Dialup Modems  (Read 2950 times)

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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.
 

Online 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.
 

Online 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..
 

Online 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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Online 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 »
 

Online 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.
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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? :)
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Offline Rasz

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Re: Connecting a pair of Dialup Modems
« Reply #38 on: September 23, 2018, 07:02:08 am »
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? :)

dont you run into the same problem of needing to pump AC voltage into the line first?
do those Homeplug devices work over dead wire?
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Online SeanB

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Re: Connecting a pair of Dialup Modems
« Reply #39 on: September 24, 2018, 10:54:23 pm »
dont you run into the same problem of needing to pump AC voltage into the line first?
do those Homeplug devices work over dead wire?

You separate the power pins from the RF injection side, and run the RF over phone cable or CAT3 cable to the other side, where you have done the same. The units do not check for AC power, as they are powered by the AC line, so if there is AC power they work, sending out a query RF pulse every so often to see if there is another unit on the line, and listening for a query back. Then they use Rf to negotiate the link, they do not sense line voltage at all, and will after modification work well over a dry cable link.
 
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Offline Rasz

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Re: Connecting a pair of Dialup Modems
« Reply #40 on: September 25, 2018, 10:00:06 am »
dont you run into the same problem of needing to pump AC voltage into the line first?
do those Homeplug devices work over dead wire?

You separate the power pins from the RF injection side, and run the RF over phone cable or CAT3 cable to the other side, where you have done the same. The units do not check for AC power, as they are powered by the AC line, so if there is AC power they work, sending out a query RF pulse every so often to see if there is another unit on the line, and listening for a query back. Then they use Rf to negotiate the link, they do not sense line voltage at all, and will after modification work well over a dry cable link.

interesting, found this https://forum.openrov.com/t/teardown-of-a-homeplug-adapter/305/32
I remember from reading up on X10 20 years ago that it detected power phase and only transmitted inside zero crossing window, somehow expected something similar here.
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Online tooki

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Re: Connecting a pair of Dialup Modems
« Reply #41 on: October 08, 2018, 09:48:08 pm »
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.
Yep. Pre-56K, your ISP had banks of modems, and when you called, you got a switched circuit (in theory; read on) to the ISP's incoming phone lines, where a free modem would then take the call and connect. It was very much like how a pre-VoIP call center worked, with banks of operators waiting to take your call.

But of course, we haven't actually used true circuit-switched lines for phones in most places for decades. Instead, at the local exchange (where your phone line physically terminates), an A/D converter would digitize your audio, compress it down to 64 or 56Kbps, and then send it digitally through the phone network, and then at the local exchange at the receiving end, it would be converted back to analog.

Of course, the A/D conversion and compression algorithms were designed for human voice, not for modem noises (or music, which is why hold music sounds so awful). Somebody realized this is silly for a modem; you were taking digital, converting it to analog, converting it to digital, compressing it, decompressing it, converting it back to analog, and then converting that back to digital!

Enter the 56K modem.

In essence, the 56K speeds worked by not actually connecting the client modem through to a modem at the ISP. Instead, the ISP modem is actually at the local phone exchange, and from there it went digitally to the ISP itself. By avoiding the voice-centric ADCs and compression algorithms and instead using the 56Kbps channel bandwidth for the digital data directly, it increased bandwidth and slightly reduced latency. (56K was chosen because not all phone networks supported 64Kbps channels — some used 8Kbps for signaling, leaving 56Kbps payload.)

The other advantage is that only the "last mile" (aka the local loop: the wire between your phone jack and the local exchange) were analog, compared to potentially much, much longer on a true analog circuit-switched line, where it might be many miles of analog signaling between you and the ISP.

As an aside, ISDN essentially connected you directly to the 64Kbps digital channel. The total absence of A/D conversion is why ISDN was a lot faster for Internet use, despite having similar bandwidth: far less latency.


ADSL (ADSL, VDSL, etc) works the same way as 56K modems, in that the ISP equipment (the DSLAM) is actually in the phone exchange. The only-the-last-mile-is-analog advantage applies here, too. In fact, more so, since ADSL depends on using only frequencies beyond what POTS lines were designed for (since ADSL leaves the voice frequencies free so that it can piggyback on an active voice line). POTS lines' frequency bandwidth plummets with distance, taking data rate "bandwidth" with it. Because of this design, the uplink channels are in one block of frequencies, and the downlink channels are in a different, much larger block of frequencies, so the modem can only send one block of frequencies and receive the other, and the opposite for the DSLAM.

In contrast, the much rarer SDSL (SDSL, SHDSL, etc) (which doesn't piggyback on an active voice line, and uses voice frequencies) can theoretically have the endpoint in the local exchange, but was more commonly used with a leased pair (a true pair of wires permanently wired through from you to the other end, onto which you can place whatever signal you want, within reason). As such, SDSL modems must be able to both send and receive all the same frequencies. Consequently, two ADSL modems are categorically incapable of directly communicating with each other, while two SDSL modems will happily connect over a copper pair.
 
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Offline Beamin

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Re: Connecting a pair of Dialup Modems
« Reply #42 on: October 12, 2018, 12:27:19 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)

Why don't cell phones have the text caller ID like land lines did? Its like we went a step backwards. Also phones should sound crystal clear but they don't you really notice this when you face time someone then call them right away. Im sure it cuts into profit but a voice would be such little data compared to everything else.

Doesn't the phone run at 50 or 80 volts? with the ringer at 110 ac? I remember getting shocked from the red wire but not the green.
What happened to ODE TO SPOT by Data? Seriously I don't remember changing it.
 
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Online SeanB

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Re: Connecting a pair of Dialup Modems
« Reply #43 on: October 12, 2018, 04:19:05 am »
Caller ID is a network operator setting, often they turn off the number reception unless you pay for it, though here that is only on contract SIM services, where there is a $1 per month charge for the "privilege" of getting it, but the prepaid market has it by default turned on. The voice quality is simply because they compress the heck out of the data, and add a little noise so that they can send it at a very low data rate in the individual packets. The original spec had a roughly 4kb/s data rate, and only roughly 6 bit resolution, so the data could be packed densely to save transmit power in the phones. Just had to pass the phone bandwidth of roughly 300Hz to 6kHz for bandwidth, and as the compression was logarithmic it was very compressed amplitude wise, with a 2 bit dither added so that silence still gave a data stream of some minimum rate, so the cell network would not think the phone was out of range of the tower and drop the call thinking it was ended, instead of simply having a quiet background and thus almost no output from the CODEC in the phone.
 

Online tooki

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Re: Connecting a pair of Dialup Modems
« Reply #44 on: October 14, 2018, 05:19:48 am »
Why don't cell phones have the text caller ID like land lines did?
I have wondered this for years, too!!!!

I have a theory on this. Land-line caller ID was a kluge, built onto the analog POTS network. It worked by using a (IIRC) 2400 baud modem signal between rings to send the Caller ID information. This was introduced in 1987-88, but didn’t catch on for a few more years. Meanwhile, also in 1988, ISDN (native digital phone lines; exceedingly rare in USA for domestic use, somewhat more common in Europe, for example) was born, and to the best of my knowledge, only included numerical Caller ID. Now, if you look at the feature sets of GSM and ISDN, they are practically identical. One could easily assume that when they were designing GSM, someone said “GSM should just be wireless ISDN”. This is absolute speculation on my part, but there’s some plausibilty to it. Letting ISDN and GSM use the same functions would certainly simplify the phone network to some extent. And with ISDN having only numerical caller ID, so did GSM.

With that said, at least modern mobile networks do support alphanumeric caller ID — at least to some extent, namely in SMS text messaging. SMS seems to allow alpha caller ID, as witnessed by commercial SMS messages, like the specials I get from IKEA occasionally, whose caller ID is “IKEAFAMILY”. (And no, you can’t reply to them, in case you were wondering!)

But I’ve never seen a mobile network enable alphanumeric caller ID for calls.

Oh yeah. It may be worth explaining to European (and perhaps other non-North American) readers what we even mean, since it was only ever available in North America as far as I know: Caller ID on land-lines used a little box with an LCD, and it would show not only the number, but also the name of the caller (and the name was indeed being transmitted from the phone company, it wasn’t just looking it up in the customer terminal). In contrast, cellphones only receive the number, so if you see the name, it’s because the phone has a matching name in your saved contacts.


Also phones should sound crystal clear but they don't you really notice this when you face time someone then call them right away. Im sure it cuts into profit but a voice would be such little data compared to everything else.
Actually some mobile phones and networks do have improved-quality mode. (The official name is “wideband audio, but most carriers market it as “HD Voice”.) But both handset and network support is so spotty that almost no calls seem to actually use it.

Doesn't the phone run at 50 or 80 volts? with the ringer at 110 ac? I remember getting shocked from the red wire but not the green.
No, a POTS line runs at 48V DC, but when the phone picks up and loads the line, it drops to 3-9V. The ring signal is 90V AC.

Caller ID is a network operator setting, often they turn off the number reception unless you pay for it, though here that is only on contract SIM services, where there is a $1 per month charge for the "privilege" of getting it, but the prepaid market has it by default turned on.
Really? Caller ID was always a premium option for POTS lines, but I’ve never ever heard of a mobile network charging for it!

The voice quality is simply because they compress the heck out of the data, and add a little noise so that they can send it at a very low data rate in the individual packets. The original spec had a roughly 4kb/s data rate, and only roughly 6 bit resolution, so the data could be packed densely to save transmit power in the phones. Just had to pass the phone bandwidth of roughly 300Hz to 6kHz for bandwidth, and as the compression was logarithmic it was very compressed amplitude wise, with a 2 bit dither added so that silence still gave a data stream of some minimum rate, so the cell network would not think the phone was out of range of the tower and drop the call thinking it was ended, instead of simply having a quiet background and thus almost no output from the CODEC in the phone.
POTS bandwidth is specified as 300-3400Hz. Analog local loop circuits often did much better (hence why DSL is even possible, but also why old phones with large speakers and microphones could sound so clear), but the amplifiers, filters, and multiplexers (in the purely analog days) and the ADCs and compressed codecs (once we went digital) adhered to those limits to maintain efficiency on the trunk lines. As such, the mobile codecs had no reason to exceed that — and often do worse, since they have to adapt to poor link conditions.


The modern HD voice standards extend the analog bandwidth to 50-7000Hz, with some taking the upper bounds all the way to the edges of human hearing, at 20KHz. They also sometimes use higher bit depths to reduce noise.
 

Offline janoc

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Re: Connecting a pair of Dialup Modems
« Reply #45 on: October 14, 2018, 05:39:25 am »
Really? Caller ID was always a premium option for POTS lines, but I’ve never ever heard of a mobile network charging for it!

I seem to recall that back in the early 90s when GSM was coming to Slovakia we had CLIP (caller ID) and CLIR (blocking of display of your number when calling someone) as a paid service. It was stupid and the telcos quickly abandoned it. I believe only CLIR was kept as some sort of a premium service for a while.
 
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Online tooki

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Re: Connecting a pair of Dialup Modems
« Reply #46 on: October 14, 2018, 07:56:03 am »
Weird. I’ve had SIM cards (or mobile accounts for networks that don’t use SIMs) from many countries (Switzerland, USA, UK, Guatemala, Thailand) and I’ve never had one that charged for any of those services! Not saying that I don’t believe you, of course... the price models for mobile calls have varied wildly over the years (e.g. how in USA, the mobile user pays for incoming minutes, too, or how they charge extra to enable tethering)!!
 

Offline Beamin

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Re: Connecting a pair of Dialup Modems
« Reply #47 on: October 14, 2018, 11:01:29 am »
Really? Caller ID was always a premium option for POTS lines, but I’ve never ever heard of a mobile network charging for it!

I seem to recall that back in the early 90s when GSM was coming to Slovakia we had CLIP (caller ID) and CLIR (blocking of display of your number when calling someone) as a paid service. It was stupid and the telcos quickly abandoned it. I believe only CLIR was kept as some sort of a premium service for a while.
. Why was it stupid? I loved it. Plus our 900 MHz digital cordless phone supported it . Best phone ever long range too. People thought it was a cell phone.
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Online tooki

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Re: Connecting a pair of Dialup Modems
« Reply #48 on: October 14, 2018, 12:42:51 pm »
I’m pretty sure it’s the charging extra part that he’s calling stupid.
 
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Offline janoc

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Re: Connecting a pair of Dialup Modems
« Reply #49 on: October 14, 2018, 08:25:22 pm »
I’m pretty sure it’s the charging extra part that he’s calling stupid.

Yes, sorry for not being clear. It was abandoned because it was actually more costly for the telcos to not provide the service (the caller's number is transmitted by default and they had to block it for everyone not paying for it).
 
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