ISDN phone lines

No. An ISDN line is completely different to a POTS line.

No, but beware, there is 96V DC between the pairs to feed the equipment!

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Reply to nico@nctdevpuntnl (punt=.)
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Sortof. You can buy an endpoint that can convert each channel to a POTS port. The ISDN service knows when a POTS call vs. a data call is being established; voice calls won't setup on a data-only endpoint & vice-versa. (An endpoint with both is smart enough to route incoming calls to POTS vs. data port.)

No. An ISDN line is not like a POTS line. It's a 2-wire always-on data circuit. The endpoint maintains a full-time heartbeat with the telco switch at the other end. "Calls" are established via setup commands over the wire. Calls are tagged as voice vs. data and get routed appropriately.

If you have a POTS port on your endpoint (to connect your phone), it will generate the ring pattern and voltage locally.

Richard

Yes, I'd expect so. There are a lot of possible scenarios, but if the system is talking ISDN to the carrier and POTS to the phone sets, then yes.

Yes. On a POTS interface, they are expected to meet the standards, and appear as a telco switch in terms of dial tone, ring patterns, voltage levels, etc.

In effect, the question of whether it's ISDN upstream should be irrelevant if the interface to you is POTS. Translating is the job of the "system", and downstream devices should neither know nor care. In practice, the upstream trunk could be any variety of analog line, channelized T1/E1, ISDN PRI/BRI, or even VoIP.

FYI, many "phone systems" use digital phone sets, and the line between the desk set and the PBX/key system is a proprietary vendor protocol, not POTS. Your POTS ring detector won't work on these lines.

You'd only care about that if you were monitoring the ISDN line, not the POTS line. That's a whole different ball game, magnitudes more complex, requiring you to monitor and decode a digital signal.

Cheers, Richard

Do most of the ISDN phone "systems" have this ability? (I would think they ALL would have this endpoint bulit in?).

Thanks kindly Richard. I wanted to detect ring with a normal POTS detector at 90vac---I would imagine they generate the same (or close).

My question was more about ISDN "systems" rather than just a single dedicated line and certainly they're going to be backwards compatible.

This "D" channel ring stuff was scaring me.

ISDN is a digital phone line. You're probably talkng about "ISDN basic rate interface" (ISDN-BRI), which has 1 signalling channel and 2 data channels, also known as "2B+D". The signalling channel (the "D") sets up the call and controls ringing function. The actual call takes place on a "B" (for "data-Bearing") channel.

The ringing voltage is generated in the ISDN "modem" (adapter) that you'd have at your house (or whereever the ISDN line is to be).

I used to have ISDN service that my job was paying for, and I used it for my office phone plus ISDN internet. Once we figured out they were being billed over $200 each month in useage changes, I got rid of it! Some areas have true flat-rate plans (Kentuckey is one), so there is woudl make more sense to use. You didn't ask about that aspect, I'm just throwing that bit out there.

Hope that helps.

Sorry, that's not true. Depending on the settings in the public network, an ISDN line is put in 'standby' and needs to be activated before use.

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Reply to nico@nctdevpuntnl (punt=.)
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I presume you're referencing the "always on" point. No doubt ISDN can be made to be a switched service; it seems to support a great many variations. I understand it can also be made multi-drop with addressable nodes (e.g., POS terminals).

However, in practice, I've never seen an installation implement other than always-on. (IIRC, blowing the cobwebs out, this is the Q.921 link signaling, which is typically kept alive regardless of upper-layer Q.931 sessions.)

In what scenario would it be beneficial to tear down the link layer? IIRC, it takes several seconds to establish Q.921, so this wouldn't be friendly to many voice applications.

Cheers, Richard

Understood. We seem to be talking about different levels within the connectivity.

When an ISDN endpoint is physically connected and powered-on, a link-level heartbeat is established with the immediately adjacent ISDN switch. This takes several seconds to establish, and is maintained independently of any calls (voice or data). This is the "always on" I'm referring to.

The minimum service here is "2B+D" BRI service, which is two 64K "bearer" channels and a 16K "data" (control signaling) channel. Usually

1 phone number per channel, and the call gets directed to the data vs. POTS port depending on whether it's a voice call type. More numbers can be setup, and some endpoints can map these to POTS distinctive ringing (how my home office is setup).

Aside from having multiple numbers / channels per line, I understand there's a way to connect multiple ISDN endpoint devices to a single physical circuit (not normal with ISDN). An example I recall is multiple POS terminals sharing the one D channel for low-volume credit-card authorizations. I've never seen this done in practice - typically, there's just one ISDN terminating device, which muxes the downstream connectivity.

There are multiple protocol stacks involved here, so multiple "layer

1...3" references. ISDN layer 2 (Q.921) is always-on between the two endpoints of the physical wire; ISDN layer 3 (Q.931) is the call setup protocol, which does establish very quickly; a call appears to IP as a Layer 2 (link layer) service connecting to a remote IP router.

Over here all public network ISDN2 links are shut down after 30 seconds. Also, 99% of the connections are point to multi-point. For instance, I have 3 telephones and a fax connected to one ISDN line each having a different phone number.

Preserve power and CPU resources. An active link needs processor power in the switch and an energy wasting signal on the S bus.

Several seconds? More like 100ms (depending on resources). Besides, on an ISDN2 link the incoming setup message is send before layer2 is actually initiated.

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That Depends [tm]

no..

Not at all...

ISDN BRI {Basic Rate Interface} is a 1 pair echo-cancelled ~200Kb/s data link. It runs 2B1Q, i.e. 4 voltage levels, so that baud rate is half the data rate.

Of that, you have two 64Kbps "B" channels, one 16Kbps "D" channel and another 16K worth of hidden overhead bits. No 90VRMS 20 Hz allowed....

In some parts of the world, but not the US, there's DC parked on the pair to run the subscriber equipment. (There may be, however, DC superimposed to run an in-line regenerator. Don't ask-- it's a tariff issue..)

As designed, the incoming 80 KBaud BRI hits a "NT-1" to turn that point-point one pair, to multi-point 2 pair 160K S/T bus that would run around your house. (w/ a third pair for -48DC power) Ma was going to provide that NT-1 and power it. But what happened in the US was She was forbidden to do so by the MFJ, and besides, the common use was dialup Internet access at 64/128Kbps, and that was the sole use, so the routers were usually built with an internal NT-1.

Note that US ISDN {"National" or NI-1, not to be confused with NT-1.} is way different from EuroISDN, at the loop level. At the S/T bus stage, ISTM things are about the same....but at the higher level, it's way different Yet Again.

In any case, you plug a router/adapter/whatever name into the BRI and depending on its features, get 2 POTS jacks to drive sets, or an Ethernet to go to your LAN, or both. {Phone calls could pre-empt existing data calls.}

You COULD have an exotic ISDN "phone set" with lots of neat features [really, "lots of button sending signals back to the CO switch to do those neat features"...] but almost no one in the US did, except of course, me.

Where ISDN *did* shine was in Centrex business installations, esp. spanning buildings. The fact that voice and data were just bits meant a Centrex set could have buttons and displays just like a local PBX offered on its fancy sets.

Is this more than you wanted to know?

Oh yes.... ISDN PRI {Primary Rate Interface vs Basic} is a horse of a different color.. It's a DS1 1.544 mb/s data stream that offers

23B's and one 64Kb D channel. It is in WIDE use to feed PBX's, and often is the ONLY product offered by CLEC's.

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