[NTLUG:Discuss] Off-Topic - ISDN Phones

[Jeff Rush]

On Wed, 18 Aug 1999 16:22:56 -0500, Greg E wrote:

>The last conversation I had with GTE I asked if this would require a 
>4-wire line and they said that it did.  They would run a new line from
>the post to my outter wall and tag the line for the $50 install.

>That's
>when I decided to go ahead and convert the whole house rather than have
>to reroute in the attic.  Apparently (according to the sales people) they
>can't run both my analog on 1 pair and ISDN on a second pair with my
>current 4-wire line.  I had  thought their ISDN was 2-wire so that's where
>I'm confused with their sales people.  My gut feeling is that they simply
>don't know.

No, the wire into your house is a U (or unified) interface, consisting of
-two- conductors.  They always run it on a four-conductor cable because
that is what they commonly have in stock.  They won't run it on the same
cable as your analog due to crosstalk interference.  Up on the pole they
run the digital lines on a separate cable bundle than the analog ones.

The reason it is called a U-interface is because it transmits and receives
over a single pair of wires.  You then plug the wire into an NT-1 (network
terminator level 1) which may or may not be built into your equipment.
In my case it is a standalone Alpha Telecom NT-1 box.  A basic NT-1
splits out the signals  to a -four- wire cable, where two wires are for
transmit and two wires are for receive.  This four-wire cable is the
S/T-interface (send/transmit).  ISDN telephones then plug into the 
S/T-interface.

The more advanced NT-1 boxes, like the Alpha Telecom, have a
audio encoder/decoder with POTS ports.  Such NT-1s are basically
an ISDN telephone (or two), but w/o the handset and dialpad.

When a call is being set up by the telephone company to your house,
many packets are exchanged, some of which specify the -type- of
call being made and others the -phone number-.  Smart NT-1 boxes
recognize voice and fax call types and ring the POTS lines, and
route the data call types thru to the S/T interface.  They can also
use the caller ID info and the quality of service to decide how to
route the call.

One note -- in the U.S. most consumer ISDN equipment comes
with the NT-1 built into the device, meaning you can only have
one per ISDN line.  Outside of the U.S. the phone company provides
the NT-1 in your phone closet and you are free to use many more
of the smarts of ISDN.  In the business ISDN equipment in the U.S.
such as PBXs, the telephones are sold w/o an NT-1 so you can
indeed get the flexibility.  Most consumers object to have any
more 'black boxes' than necessary.


>All in all it's cheaper per month to convert the whole house than have
>both services.  Yes I'll have to layout about $350 for new phones but
>I'll recover that in about 10 months.

Gotchas -- many of the POTS ports on ISDN boxes are not able to
safely drive many telephones.  Check out the ringer equivalents on
the ISDN box and then add up the ringer equivalents of all your
phones.  You may find your phones don't ring if you have many.

Other gotcha -- ISDN phones don't operate at all if the power fails
which means no 911 service.  Outside the U.S.  the phone company
is legally mandated to provide power over the phone lines to those
NT-1 boxes in the phone closet for just that reason.  Here in the
U.S. ISDN is considered a 'non-essential' service and no such
safeguards are provided.

Having said that, you can put an UPS on your NT-1/ISDN box.
Also when choosing your ISDN phones, watch out.  Some require
an external adapter, which means they too must be plugged into
an UPS.  Or you can buy ISDN phones which draw they power
from the NT-1, but then you need to be sure to buy an NT-1
that provides that power.  Some do and some don't.  My Alpha
Telecom NT-1 does.

One other item -- unless you have PBX or a really fancy setup
with the phone company (good luck!  it's hard to get their techs
to understand what you want), you won't be able to take a call
on one phone, and then have someone pick up another phone
in your house and participate in the call.  Neither will you be
able to put the person on hold and move to another phone
in your house.  ISDN in the U.S. is a point-to-point operation,
where the 'point' is your telephone, not your house.  What a
PBX does is detect this situation and conference the calls,
even though you are just conferencing between two rooms
in your house.

I've wanted for years to build a really neat consumer-level
PBX, with Linux acting as the brains using the ISDN4Linux
drivers that already exist in 2.0 kernels.  I know where to
get the hardware and firmware but I lack the capital and the
proven existence of anyone to buy it.  It'd be really cool!

-Jeff Rush