In this configuration, the wires at points S and T are point-to-point
links. Electrically, the S and T points are the same, which is why the
name S/T bus is almost always used. This makes sense; the terminal
should see the same physical interface whether it is hooked up with or
without a PBX. But, logically they are different. The telephone
company needs to know that there is a PBX between itself and the user
so that it can coordinate its efforts with the PBX. So, in cases where
the difference is important, the specifications use the S and T
terminology.
When there is no PBX in the configuration, the NT1 device is usually a
standalone device that is packaged a lot like a modem: in a small box
when there are only a few, and in a rackmount when you need a lot of
them. In the United States, the customer buys the NT1 but in most of
the rest of the world the telephone company provides the NT1. When
there is a PBX the rackmounted NT1s are quite common. Also, when
there is a PBX the use of PRI lines instead of BRI lines is common.
cherkus@unimaster.com (Dave Cherkus)
cliff@Berkeley.EDU (Cliff Frost)
curt@kcwc.com (Curt Welch)
dror@digibd.com (Dror Kessler)
Eric_Boll-RXNN70Q@email.sps.mot.com (Eric Boll)
glarson@bnr.ca (Greg Larson)
krowett@large.cisco.com (Kevin J. Rowett)
mea@intgp1.att.com (Mark Anderson)
paul@suite.sw.oz.au (Paul Antoine)
pturner@eng.auburn.edu ( Patton M. Turner)
ronnie@cisco.com (Ronnie B. Kon)
----
4) What is a NT1? Who sells them?
[ Ed Note: Some may feel that there's a bit of overlap between the
preceeding sections and this one, but the preceeding sections are
hard to write without integrating NT1 information and this one
is so informative and well-written that it can stand on its own
so I think I should leave it as is. Comments? ]
An NT1 (network terminator 1) is a device which provides an interface
between the two-wire twisted pairs used by telephone companies in
their ISDN Basic Rate (BRI) network and an end-user's four-wire
terminal equipment. The NT1 also provides power for the terminal
equipment if necessary (most ISDN phones need power from the NT1, but
most data terminal adapters--TAs--don't).
Most ISDN central office equipment (including AT&T 5ESS and Northern
Telecom DMS-100 switches) sends data to your home or office via what's
known in ITU-T lingo as a "U interface" on a single twisted pair. The
NT1 hooks up to this twisted pair, and converts the signals from the
"U interface" to the four-wire "S/T interface". Most terminal
equipment (for example, the IBM Wave Runner add-in-card TA and most
telephones) offers the "S/T interface".
In North America, you have to buy and maintain your own NT1 device.
The telephone company offers end-users a "U interface." In Europe and
Japan, the telephone company provides the NT1, owns it, and offers
end-users a "S/T interface" directly. In North America, some ISDN
equipment vendors offer devices which connect directly to the "U
interface" (for example, the Combinet CB160). If you have one of
these devices, you don't need to buy a separate NT1. T "U interface"
can't be built in to the device when it's offered for sale in Europe
or Japan. (This is unfortunate for vendors, who must build and test
separate products for the relatively small North American market if
they want to offer the convenience of a "U-interface.")
You will need to buy a power supply with your NT1. There are
typically two classes of power supplies. One class provides ten to
twelve watts--enough power for both the NT1 and for the terminal
equipment. The other class provides about two watts--enough power for
the NT1 alone. Many good power supplies offer at least a few seconds
of battery backup, to cover for glitches in line power.
Physically, the NT1 is a little plastic box with LEDs on it which can be screwed to a wall. The power supply is a typical plug-wart. If you're using a lot of BRI lines, you can buy a rack holding a dozen or so NT1s with a built in power supply.
It's a good idea to install your NT1 in a permanent fashion. If you
unplug the ISDN line (the "U interface" twisted pair) from the NT1, it
shows up as a sign of line trouble in the central office. Some
telephone companies respond to this so-called "trouble" by disabling
your ISDN line at the central office, and require you to place a
service call on your analog telephone to get your ISDN service
restored.
All the vendors shown here accept credit card orders and ship
promptly. All the vendors have well-organized telesales operations
with friendly and reasonably knowledgeable sales people. Prices are in
US dollars, as of 4/6/94, for single-unit purchases. Pricing is
becoming volatile; competition seems to be heating up.
AT&T and Northern Telecom NT1s can be ordered from Bell Atlantic
Teleproducts
Bell Atlantic Teleproducts
West Building, Suite 150
50 E. Swedesford Rd
Frazer Pa, 19355
tel +1-215-695-2300 or 800-221-0845
Maker Description Part No. Price
----- ----------- -------- ------
Northern Telecom NT1 standalone IN51000 129.00
Northern Telecom 10w power supply IN61000 86.00
Northern Telecom 2w power supply IN61005 43.00
AT&T NT1 IA51007 328.00
AT&T 10w power supply IA61000 125.00
Tone Commander offers their own NT1 for sale. Their sales literature
says "it may be used as a drop-in replacement for the AT&T NT1U-220."
Tone Commander Systems
4379 150th Ave NE, PO Box 97039
Redmond WA 98073-9739 USA
+1 206 883-3600 or 800 524 0024
fax +1 206 881 7179
Maker Description Part No. Price
----- ----------- -------- ------
Tone Commander NT1 standalone NT1U-220TC 195.00
Tone Commander Power supply 901034 30.00
Adtran offers their own NT1 products for sale.
Advanced Transmission Products, Inc.
901 Explorer Blvd Huntsville, AL 35806-2807 USA
+1 205 971 8000
fax +1 205 971 8030
Maker Description Part No. Price
----- ----------- -------- ------
Adtran NT1 NT1 ACE 395.00
Adtran Power Supply PS2 150.00
Adtran Power Kit 74.00
Adtran Standalone NT1 NT1/T400 575.00
(incl 7W supply)
Adtran Rackmount NT1 NT1/T400 395.00
AT&T issued a press release on 2/24/94, announcing a new lower-cost
NT1 device. I haven't been able to get ordering information (Bell
Atlantic doesn't carry it as of 4/6/94),
Maker Description Part No. Price
----- ----------- -------- ------
AT&T NT1 L-230 230.00
Thanks to the following people who helped uncover this information.
tynane@chdasic.sps.mot.com (Ed Tynan)
rkp@bighorn.dr.att.com (Russell Pierce)
"H.A. Kippenhan Jr." <KIPPENHAN@fndcd.fnal.gov>
The people who compiled the NIUF solutions catalog
Special thanks to oj@vivo.com (Oliver Jones) for editing this section.
--
5) Can the existing local loop lines be reused for ISDN?
The ISDN pairs are the same wires as used for regular telephone
service. If you became an ISDN user at home, the same wire pair that
now provides your telephone service would be used to provide ISDN
(assuming you no longer have the regular line).
Most of the lines do not require any special conditioning. Yes, if
a line has load coils on it they must be removed, BUT load coils
are usually only found on existing lines that are 15,000 feet or
longer. As to lines with bridge taps, the 2B1Q line transmission
scheme (not to be confused with 2B + D channelization) is tolerant
of a certain amount of bridge taps and, therefore it is only a minimal
subset of existing lines (lines with bridge taps whose total length is
greater than 3000 feet for the bridge taps) that would require
special "de-conditioning."
With those things as the criteria, (in North America) we find than
generally around 90% or so of existing telephone lines need no
"de-conditioning" in order to be used for ISDN BRI service.
whs70@cc.bellcore.com (sohl,william h)
---
6) How does this compare to regular phone lines?
The ISDN line may act like two independent phone lines with two numbers.
Depending on the CO equipment, conferencing features etc. may be available
(conferencing in the telephone switch). BRI ISDN phones can support key-set
features such as you would expect to get on an office PBX like:
- multiple directory numers per line.
- multiple lines per directory number.
- conferencing features.
- forwarding features.
- voice mail features.
- speed call.
- call park.
- call pickup.
- ring again.
- textual status displays.
curt@kcwc.com (Curt Welch)
glarson@bnr.ca (Greg Larson)
---
7) Is caller ID available on ISDN?
Caller ID (name or number display) may be supported (depending on the
CO setup). The availability of caller ID for residential phones would
depend on the capabilities of the local phone network and legislation
allowing or disallowing caller ID. The availability of Caller ID
relies on the underlying switching protocol used by the switches
that make up the telephone system (e.g. SS7).
curt@kcwc.com (Curt Welch)
glarson@bnr.ca (Greg Larson)
KUMQUAT@SMCVAX.SMCVT.EDU (Gary C. Kessler)
---
8) What do I get above and beyond plain old telephone service?
Plain old telephone service is transmitted between the central office
to your home or office telephone set (or modem, or fax) in analog
form. At the central office, the analog signal is converted to a
series of digital samples at a rate of 8000 samples per second. Each
sample is seven or eight bits in length. As the signals for a
telephone call move around the central office, or between central
offices, they are transmitted in digital form. Thus, a telephone call
consumes a transmission bandwidth of either 56 or 64 kilobits per
second. The theoretical (Nyquist) limit for the frequency response of
a signal sampled 8000 times per second is 4kHz. However, due to
various losses in the telephone system, the frequency response of an
ordinary telephone call is usually quoted as 3.1kHz. Ordinary
modem-based data transmission uses schemes for encoding data in an
analog signal so it fits in this 3.1kHz bandwidth. 14.4kbps is a
commonly available transmission rate at the high end of the scale.
With this transmission rate, over three-quarters of the bitrate handled
by the central office is wasted.
Notice that in telephony, 64kpbs means 64000 bits per second, whereas
in computer engineering 64k bytes typically means 65536 bytes.
ISDN brings the digital signal all the way to your home or desktop. With
ISDN, you can place a data call which uses all 56kbps or 64kbps, because
there is no need to convert the signal to analog in your modem and back
to digital at the central office. The availability of the full bandwidth
presents some interesting technological opportunities:
-- transmission of high-fidelity compressed audio
-- transmission of encrypted audio
-- transmission of lots of data
-- transmission of other compressed signals, such as video
Basic-rate ISDN (BRI) offers two channels of this service. In BRI, the
connection between your site and the central office offers 64kbps
bidirectionally on each channel. Each of these channels may be used
for a voice call, for circuit-switched data, or for X.25 packet
switched data. Thus, the existing POTS circuit [POTS: Plain Old
Telephone Service, i.e. traditional analog telephony] can be
conditioned to carry two calls at the same time. (Your mileage may
vary; you have to specifically order and pay for the various services
from your telephone company, just as you have to order and pay for Call
Waiting for an ordinary phone line. Also, not all services are
available everywhere; X.25 connectivity between COs is a notable
problem in the Greater Boston area as of 9/93, for example.)
Incidentally, ISDN brings another interesting service to your home or
desktop: a highly reliable 8000Hz clock signal. In most cases, the
central office switches, long-distance carriers, and ISDN terminal
equipment all operate with exactly the same clock frequency. In a
real-time communications environment (like a voice phone call) this
means that there's no need to compensate for differences between the
sampling rates at each end of the call.
One of the other features is that instead of the CO sending an AC ring
signal to activate your bell, it sends a digital packet that tells WHO
is calling (if available), WHAT TYPE of call (speech, datacomm?), the
NUMBER DIALED (maybe one of your aliases) and some other stuff. Your
equipment can then analyze this stuff and make an "intelligent" decision
what to do with it. For example, a phone (with speech-only capacity)
would completely ignore a datacomm call while a Terminal Adapter (ISDN
"modem") or a phone with built-in datacom functions would respond to it.
If you have several "aliases" tied to your line, you can program certain
phones to answer calls for certain numbers only. Datacomm calls contain
baud rate and protocol information within the setup signal so that the
connection is virtually instantaneous (no messing around with trying
different carriers until both ends match).
curt@kcwc.com (Curt Welch)
etxorst@eos.ericsson.se (Torsten Lif)
oj@vivo.com (Oliver Jones)
Helge.Oldach@Stollmann.DE (Helge Oldach)
---
9) What do ISDN phones cost?
The ISDN sets can cost between $180 for an AT&T 8503T ISDN phone from
Pacific Bell up to $1900 depending on what/how many features are needed.
A recent report states that the price is $536.90 for an AT&T 7506 with
the RS-232 port on the back and $102.70 to get the 507A adaptor to hook
analog devices to my 7506.
Recent quotes were "$200" for a Coretelco 1800 and "$600" for a Fujitsu