Re: ISDN phone lines

From: Nico Coesel (nico_at_puntnl.niks)
Date: 03/28/05 

Date: Mon, 28 Mar 2005 11:10:36 GMT

"Richard H." <rh86@no.spam> wrote:

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

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.

>In what scenario would it be beneficial to tear down the link layer?

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

>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. 

-- 
Reply to nico@nctdevpuntnl (punt=.)
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