to the Telephone Network - U and S/T Interfaces
have ISDN service, you need to know which ISDN interface your
equipment expects. There are two ISDN interfaces. The U-Interface
carries ISDN signals over a single pair of wires between your
location and the central office. This interface is designed
to carry ISDN signals over long distances. The Subscriber/Termination
(S/T) Interface uses two pairs of wires to deliver the signal
from the wall jack to your ISDN adapter or other ISDN equipment.
your equipment supports the S/T-Interface, you need to
get a device known as a Network Termination 1 (NT-1) which
converts between the U-Interface and the S/T-Interface.
The NT-1 has a jack for the U-Interface from the wall
and one or more jacks for the S/T Interface connection
to the PC, other ISDN or analog devices, as well as an
external power supply
adapters sold in North America connect directly to a U-Interface.
If the PC is the only equipment to be connected to an ISDN
line, this type of adapter is the easiest to install. Manufacturers
may describe this feature as a "built-in NT-1" or
simply as a U-Interface ISDN adapter.
Wiring and Jacks
ISDN service from the phone company officially ends at what
is called the demarcation point ("demarc") usually
just inside the building. You are responsible for the "inside
wiring" from the demarc to your ISDN equipment, including
the wall jacks. The telephone company or an electrical contractor
will install and maintain the inside wiring for an additional
If you are just connecting your PC to the ISDN line, the
wiring requirements may be very simple. Many homes and offices
are wired with extra sets of telephone wires and one of those
sets can be used for your ISDN line. There are a number of
possible wiring pitfalls however:
- your “extra” wires may already be in use for
- your “extra” wires are being used to power
lighted phone buttons
- your “extra” wires are not connected directly
to the demarc
- the wiring may be “daisy-chain” rather than
Direct wiring between the ISDN wall jack and the demarc (also
known as a 'home run') is recommended. For more information
on wiring issues, consult your telephone company or an electrical
There are two types of jacks associated with ISDN:
- RJ11 - the standard analog phone jack. The RJ11
has 4 wires. The wire from the wall to the NT-1 will
usually have RJ11 jacks.
- RJ45 - this jack is slightly wider than the RJ11,
and has 8 wires. The wire from the NT-1 to the ISDN
adapter will usually use RJ45 jacks.
Connecting Multiple Devices to an ISDN Line
If you do not plan to connect anything except a single PC
ISDN adapter to your ISDN line, you can ignore this section.
It is possible to connect up to eight devices to a single
ISDN line. These devices can include network routers and bridges,
Group 4 ISDN fax machines, ISDN telephones as well as traditional
analog telephone devices. ISDN is intelligent enough to arbitrate
the use of the two B channels between these devices (up to
two devices can be in use simultaneously) and route incoming
calls to the appropriate device.
Instead of connecting the ISDN line to a single PC, it is
possible to connect an ISDN line to a LAN so all the PCs on
the LAN can share the ISDN line. This requires an ISDN network
bridge or router.
It is possible to connect several ISDN devices to a single
ISDN line. For example, you might wish to have an ISDN adapter
in your PC, an ISDN telephone to make voice calls and a Group
4 ISDN fax machine all connected to the same ISDN line. Incoming
data calls would go to the PC, voice calls to the telephone
and fax calls to fax machine. To support this configuration,
you need an NT-1 that supports multiple S/T Interface connections.
Each device would be connected to the NT-1. Each device would
also need its own Service Profile Identifier (SPID) to ensure
the telephone company can route calls to the appropriate device.
In addition to ISDN devices, some NT-1s or ISDN adapters
also support analog telephone devices like phones, data modems,
Group 3 fax machines and answering machines. The NT-1 or the
ISDN adapter converts the analog signal into ISDN and vice
There are two major types of National ISDN CPE. They are:
- Terminal Adapters
- ISDN Terminals (an actual ISDN telephone)
There is a 3rd major type of National ISDN CPE called a Network
Termination Interface (NT1). There must be an NT1 in the ISDN
loop before the terminal adapters or ISDN terminals will work.
The NT1 converts the ISDN from a 2-wire to an 8-wire circuit.
The NT1 can be a stand-alone unit or be built into the terminal
adapter or ISDN terminal. Most National ISDN terminal adapters
have the NT1 built into the unit.
NOTE: This document isn't intended to recommend any particular
piece of ISDN CPE. While the basic functions of the CPE are
pretty consistent (circuit switched data and circuit switched
voice) the actual capability of any particular piece of ISDN
CPE is as varied as there are manufacturers. For example,
while this document will look at terminal adapters that are
stand alone pieces of equipment, a terminal adapter can also
be a card that is placed inside a computer.
Terminal Adapters and Routers
From the most basic standpoint, a terminal adapter (TA)
could be describe as a circuit switched data device that can
do some voice, and an ISDN terminal as a voice device that
can do some circuit switched data. However, there are some
terminal adapters that are capable of some pretty sophisticated
voice, and some ISDN terminals that can do some pretty sophisticated
circuit switch data, such as bonding the 2 B channels together
for 128 K/bps of bandwidth.
ISDN routers are also available. Think of an ISDN router
as a smart terminal adapter. Routers are targeted toward the
circuit switched data users but they can also deliver voice
features. Routers require the use of a Network Interface Card
(NIC) and the NIC connects to the router as a 10BASE-T connection.
For the purposes of this document, we'll consider the router
as a type of terminal adapter.
A terminal adapter is a physical piece of equipment in which
the ISDN line is terminated. Terminal adapters allow for 64
K/bps of circuit switched data, and most terminal adapters
allow the user to bond together both B channels for 128 K/bps
of circuit switch data. By adding a high-speed serial port
to a computer and using compression, the throughput on a terminal
adapter can reach speeds of 230 K/bps.
Bonding is a function of the CPE. From a network standpoint,
there are two separate 64 K/bps data paths. The CPE at the
receiving and sending ends bonds the 2 channels together for
a 128 K/bps data stream. There are some settings in the switch
that allow the CPE to be notified of an incoming call so one
of the B channels can be dropped to accept the call, but the
actual bonding function is a function of the CPE.
To get true 128 K/bps data speed (bandwidth) a user must
be using a router. A terminal adapter, because of conversion
considerations between the computer serial ports and terminal
adapter, will provide 115.2 K/bps of data speed. With compression,
a router may generate throughput of 512 K/bps and a terminal
adapter, with a high-speed serial port, may generate throughput
of 230.4 K/bps. The actual speed received can be impacted
by other factors, such as Internet connections or number of
people accessing the server.
Automatic Service Profile Identification (AutoSPID) is a
function of the terminal adapter and the ability of the switch
to deliver the SPIDs. A SPID is the telephone number with
four 1's after it. A SPID uses one of the B channels to deliver
data or voice and normally two SPIDs are entered for each
ISDN line. Once the ISDN line is plugged into the CPE and
the power is turned on, the necessary communications between
the switch and CPE will take place. The AutoSPID function
uniquely identifies the terminal's service profile and initializes
the terminal by downloading the SPIDs into the CPE. Not having
to manually enter the SPIDs reduces the start-up problems.
Cerzán only offers this service in Lucent 5ESS switches,
Terminal adapters can only have Primary Directory Numbers.
The normal configuration is 2 PDNs (2 different SPIDs). Terminal
adapters cannot have Secondary Directory Numbers or Shared
Call Appearances. The diagram below (INSERT HERE) shows the
use of an analog phone plugged into the analog jack on the
back of the terminal adapter. Some ISDN features that can
be used with the analog phone are conference and transfer,
call forwarding, speed calling and voice messaging. Call waiting
functionality is available with the feature called Additional
Call Offering Service.
The ISDN line terminates into a jack in the back of the TA.
A cable connects the TA to the computer for 64 K/bps or 128
K/bps of digital circuit switched data. The cable may be for
a USB port or a serial port on the computer, depending on
which is shipped with the TA.
Most terminal adapters have 2 analog ports in the back. Standard
analog telephones or FAX machines can be plugged into these
ports (using standard telephone cords) and can make and receive
voice calls. Depending on the CPE, most voice features can
be assigned to these analog phones. Most terminal adapters
are smart enough to know that if someone picks up one of the
analog phones, the person wants to make a voice call, the
terminal adapter will drop one of the circuit switched data
channels to free up a B channel for voice.
Here's a picture (insert here) of a very normal configuration.
The user's terminal adapter plugs into the computer for circuit
switched data and an analog telephone plugs in the back of
the terminal adapter for voice services.
ISDN terminals are actual ISDN telephones; they have buttons
for call appearances and features. ISDN terminals support
Secondary Directory Numbers, Shared Call Appearances and Analog
Call Appearances. ISDN terminals may also be referred to as
ISDN sets or ISDN telephones.
Most ISDN terminals have a LED Display that gives Calling
Line ID, Outgoing Line ID, time / date, and displays features
and functions that have been built into the terminal by the
Some ISDN terminals have a data port in the back. The data
port can run circuit switched and/or packet switched data.
Some ISDN terminals even have an analog port in the back,
that could be used, for example, by a fax machine. ISDN terminals
can use an external or internal NT1.