Connect with us

ISDN phone lines

Discussion in 'Electronic Design' started by Michael, Mar 25, 2005.

Scroll to continue with content
  1. Michael

    Michael Guest

    Does an ISDN line function as both a POTS and ISDN
    line. (yes--stupid?)

    The system detects the difference and switches to adapt?

    So on an ISDN line you would still see 90vac ringing voltage from ANY
    incoming call? (yes--stupid?)

  2. Nico Coesel

    Nico Coesel Guest

    No. An ISDN line is completely different to a POTS line.
    No, but beware, there is 96V DC between the pairs to feed the
  3. Richard H.

    Richard H. Guest

    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

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

  4. Michael

    Michael Guest

    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.
  5. Richard H.

    Richard H. Guest

    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.

  6. Nico Coesel

    Nico Coesel Guest

    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.
  7. Richard H.

    Richard H. Guest

    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

    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.

  8. Nico Coesel

    Nico Coesel Guest

    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.
  9. Richard H.

    Richard H. Guest

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

    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.
  10. David Lesher

    David Lesher Guest

    That Depends [tm]

    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

    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.
Ask a Question
Want to reply to this thread or ask your own question?
You'll need to choose a username for the site, which only take a couple of moments (here). After that, you can post your question and our members will help you out.
Electronics Point Logo
Continue to site
Quote of the day