Hardware for telephone exchange

I am learning hardware for telephone exchanges. I have seen a Digitum card, (that I saw in the free market), that adapts to the PCI inputs of a PC. Model 410, 3fxo, 1fxs My question to the seller was: And if I wanted to increase the number of extensions or lines, I can join 2 cards or place an adapter. The desired software to implement is FreePBX. ·
And the seller answers me: if you want to increase the number of extensions you can add another card, even in that case it may be better for you to start with an 8-port digium card to which you can initially install only 1, 2, etc. 3 or 4 extensions or lines as required and later you can install one or two 4-port modules to expand it, another option could be to start with a 4-port sangoma card which can be expanded up to 24 ports using only one PCI slot for the first card, additional cards are connected to the first by a connector.. But how would this adaptation be, that is, how would these connections be? If possible in an image that shows a real connection. I hope you understand me.

I would stick to the Digium cards, the Sangoma cards tend to be more finicky.

How many PCI slots do you actually have available? How many lines do you want to have, FXO/FXS? These are 2 important questions you have not answered.

I would avoid adapters/connectors. Those can cause issues on the data bus that result in catastrophic issues with higher numbers of channels.

If you are going to have any high density of lines, I would ditch the analog cards completely and use a digital T1 card connected to a channel bank. A single T1 can give you 23 channels, configured as either FXO or FXS in any combination.

T1/E1, at least when used with common channel signalling (e.g. ISDN), gives much better answer and disconnect signalling (you may not get these at all on analogue lines).

T1 end to end or analog lines on a T1?

I use kewl start (loop start with loop disconnect) for my analog lines on a T1 and have no issue with it. I had an issue with FXO not disconnecting but I realized I had to enable disconnect explicitly on the channel bank as well. So some things, there are 3 places things need to be set: the channel bank, DAHDI, and chan_dahdi/Asterisk.

I think my current arrangement doesn’t support polarity, so I would actually argue the reverse. The analog card (24 something) supports polarity changes but the T1 driver does not. So the analog cards might be more capable than the digital ones, but the digital ones, if you’re willing to forgo a couple things, probably work better for most use cases.

The problems people get with analogue lines are:

Not every provider reverses battery on answer, so the default is set to assume answer immediately after sending digits.

Not every provider sends a reliable disconnect signal, and if they do, you have to set options to detect it. (The most commonly used convention in the UK is a short removal of battery.)

The analogue network doesn’t distinguish between clear and release. This is probably particularly true of single subscriber lines, but then people using PABXes generally don’t use analogue any longer on new systems. As a result if the PABX answers, the caller’s line is left up, awaiting for a re-answer. This is being mitigated as providers have moved from the original several minutes between subscriber clear and network release, to a few seconds, now that people use DECT phones, rather than multiple extensions on the same line.

I’m pretty sure I’ve seen all of these problems from people trying to do complex things on analogue lines, here.

Line reversal doesn’t mean anything on T1, and I specifically mentioned common channel signalling, and ISDN, where explicit answer, clear and release messages are sent.

I guess you’re coming at more from the perspective of POTS lines. I’m talking more about providing your own FXS lines (extensions).

As far as the PSTN goes, providers should sent a disconnect, if not, that is a major deficiency.

Not sure entirely but are you talking about Called Subscriber Held? Sounds like it at least. I thought it was more common in the UK still, here in the US there is often still a hold time of a couple minutes depending on if the call is long distance or not, etc. This is a feature, since it allows you to hang up one phone and pick up another, and not have to leave phones off the hook. I actually recently added a patch to make that possible with chan_dahdi, if you have multiple phones on the same pair, it can be very convenient.

Line reversal doesn’t mean anything on T1, yes, but it’s reasonable to want to reverse polarity on an FXS port that is signalled via a T1 back to Asterisk. But that’s not something that I can do. I would need an analog card to do that. So using a T1 rather than an analog card directly can have a few limitations.

That’s why it used to be several minutes in the UK. The reason that it has been reduced in most or all of the UK, as well as not really being needed, and not being known about by most users, was that it was exploited by scammers who would ask the mark to ring back the organisation that was being faked, and would present their own dial tone. Advice on avoiding that tactic is still prevalent, even though the risk has largely gone.

(More sophisticated potential marks, would initiate a register recall, then hang up, which did release the caller.)

I’m aware of the scammer risks, but to me the convenience is worth the downside. A minute doesn’t seem inappropriately long to me, but maybe a few minutes is excessive. If you take the time to walk from one end of a large house to another, say from the basement to the top floor, the time doesn’t need to be longer than that.

Making this configurable per line so that different subscribers could choose their own timeout value is something that always struck me as a nice compromise, but of course, the telcos are too lazy to add POTS features these days.

(More sophisticated potential marks, would initiate a register recall, then hang up, which did release the caller.)

Register recall? Like hook flash and hang up? Here that would not hang up, it would ring back immediately with the other call.

I might have misremembered the exact tactic, it might have been more one of ensuring that call wasn’t represented, so that they knew they had a clear line.

It was a feature that almost no-one knew existed. Most people had moved to cordless, and even those who hadn’t assumed that they had to leave the first phone off hook until they’d lifted the second handset.

Ah, I get it now, if you flash when there’s a spoofed dial tone, you’d get a recall dial tone. If it was the real dial tone, it would either do nothing or do a loop disconnect and give you a new dial tone.

It was a feature that almost no-one knew existed. Most people had moved to cordless, and even those who hadn’t assumed that they had to leave the first phone off hook until they’d lifted the second handset.

Yes, I’ve seen this happen “out in the wild” a few times recently :slight_smile:

I have 1 free PCI slot available.
At the moment I have no idea how many FXO and FXS lines I need. I want a card that fits the PCI port. To start a small one. And at the same time in the future it can be expanded.
My PC is: DELL Inspiron 545s PCI Express 1.1 provides 8 Gb/s bandwidth.
For reference My PCI has 11 pins and has a separation of 32 pins.

Bandwidth of your NIC is not really relevant, a single T1 span runs at 1.544 Mbps.

I don’t think there’s a right answer here. If you go with a T1 card, that’s probably more future proof. If you want to start with an analog card, you may have to buy more later or find that no analog card suits your purpose, and then get a T1 card. On the flip side, if you get a T1 card, you also need a channel bank or something else, so it’s not standalone.

So look at the supported cards and find one that meets your use case. If you want a basic setup, one of the analog cards should work fine.

There is no future for any circuit switched, analogue or digital, telephony in the UK. Switch off of the circuit switched network in favour of VoIP is scheduled for Autumn 2025. I imagine are similar consideration in T1 using areas.

That’s not relevant here. We’re talking about using these cards for extensions locally, which is the major use case for them these days. What the telco does have no bearing on a T1 between a PCI card and a channel bank sitting on my property.

The original poster did say “FXO and FXS”, which implies at least one POTS type CO line.

I do agree that putting FXO or FXS ports into the server is not really a future-proof solution, and that a T1/E1 interface paired with an external channel bank is the way to go. This setup externalizes the FXO/FXS ports allowing the channel bank to be configured for either as needed.

If I had a requirement to support any analog lines (FXO or FXS), this is absolutely the way I would do it (and have done so in the past).

  • Darrin

This topic was automatically closed 30 days after the last reply. New replies are no longer allowed.