Re: ISDN phone lines

From: Richard H. (rh86_at_no.spam)
Date: 03/28/05 

Date: Mon, 28 Mar 2005 07:23:45 -0700

Nico Coesel wrote:
>>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.)
>
>
> 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.

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.

>>IIRC, it takes several seconds to establish Q.921, so this wouldn't be
>>friendly to many voice applications.
>
> Several seconds? More like 100ms (depending on resources). Besides, on
> an ISDN2 link the incoming setup message is send before layer2 is
> actually initiated.

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.

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