One ISDN line has two completely separate bi-directional 64kbit/s data circuits.
Therefore from point X you can open one 64kbit/s circuit to point Y and at the same
time have another completely separate 64kbit/s circuit from point X to point Z.
These individual 64kbit/s bearer circuits are commonly called 'B' channels and as
there are two of them they are called B1 or B2.
On the modern and the now standard ISDN2E lines an incoming or outgoing call could
end up on either one of these B channels.
On one configuration of an old style ISDN2 line an individual B channel was specific
to a dial-in ISDN number.
Pieces of equipment can normally be set to make outgoing or accept incoming calls
on any B channel or to be forced to just using one of the B channels.
CODEC (Coder Decoder):
For broadcasting audio down ISDN lines a Codec is needed. A Codec takes an audio
signal and turns it into a digital signal; at the same time it compresses the signal
so that all the digital bits will fit down a data (ISDN) line. A codec will also
receive compressed data, uncompresses it and turn it back into an audio signal.
When compressing the audio a codec passes the data through a compression algorithm
that is a clever device that works how to make the audio signal fit down a specific
data link. It is imperative that the other end of the data link has an identical
algorithm, which can decompress the data back to an audio signal.
There are many different algorithms available, by far the most common are G722, MPEG
Layer2 and APTX. Each has different properties with advantages and disadvantages.
It is common in modern office buildings for a catagory 5 structured wiring system
to be in place for use as a local area network (LAN) for computers, telephones etc.
If a cat 5 structured wiring system is used for routing ISDN then a line adaptor
unit, containing terminating resistors to provide the correct impedance for line
termination is needed between the cat 5 wall socket and your ISDN equipment.
Multiple Subscriber Numbering 'MSN':
One ISDN line is designed to be able to handle multiple pieces of equipment attached
at the same time.
A single ISDN line may have many different 'dial in' numbers associated with it.
In normal practice the telecom providers will limit the number of different dial
in numbers to 10.
In most pieces of ISDN equipment it is possible to programme in a multiple subscriber
numbers (MSN). The number programmed in to the equipment should be the dial-in number
that you wish that piece of equipment to answer. Once a piece of equipment has had
a MSN programmed in to it, it will only answer an incoming call that has been dialled
with the matching dial in number.
A simple example would be if you had a telephone and a fax machine attached to one
ISDN line. If the ISDN line had two dial in numbers associated with it e.g., 01***
*78540 and 01*** *79049.
You would then programme the MSN in your telephone to be 01*** *78540 and the MSN
in your fax machine to be 01*** *79049 then when someone dialled into 01*** *78540
your telephone would ring and when someone dialled into 01*** *79049 you fax machine
(Be aware: your local exchange will forward the number dialled into to your ISDN
equipment. This number has to match exactly with the number that is programmed as
the MSN number in your equipment. Some exchanges will forward the number including
the area code, some may just forward the number without the area code and others
may forward the number and area code but with the leading zero removed from the area
code. The easiest way of finding out which format your exchange forwards the number
in is trial and error!).
MSN numbers do not have to be used. If no MSN number is set in a piece of equipment
then it can be plugged into any appropriate ISDN line and will be able to answer
an incoming call.
NT 'Network Terminator':
This is the name given to the box of electronics that changes a U interface into
an S interface.
Network terminators are provided as standard by ISDN providers throughout most of
the World, the main exception being North America where they have to be obtained
See also S interface and U interface.
This is the standard connector used to connect a cable from a piece of equipment's
terminal adaptor to the S interface of an ISDN line. Both the S interface of the
ISDN line & the terminal adaptor will be terminated with RJ45 sockets (female) and
the interconnecting cable will be terminated with RJ45 plugs (male). For ISDN use
the cable needs to wired 1 to 1, i.e. pin number 1 of the RJ45 plug on one end of
the cable needs to be connected to pin number 1 of the RJ45 plug on the other end,
pin 2 connected to pin 2, pin 3 connected to pin 3 etc, etc, there are 8 pins in
total on an RJ45 connector.
Be aware RJ45 cables are also used for computer networks and not all computer network
cables are wired 1 to 1.
Only 4 off the 8 pins of an RJ45 connector are used, pins 4 & 5 are data from the
network & pins 3 & 6 are data to the network.
The data signal from a 2 wire U interface is expanded to a 4-wire signal (i.e. 2
cable pairs). This 4-wire signal is called an S interface. An S interface makes it
possible to connect more than one piece of ISDN equipment to one ISDN line.
See also Network Terminator.
This is a modem for ISDN. It provides the electronic link between subscriber’s equipment
and the providers ISDN line. The terminal adaptor has to be compatible with the ISDN
line for the equipment to operate correctly, see Variations of ISDN lines.
Single twisted pair of cables carry all the data for both B channels between a telecom
providers ISDN exchange and the subscriber. The subscriber’s end of these cables
is called a U Bus.
See also Network Terminator.
Use of MSN & B Channels Together:
A piece of ISDN equipment should never need both MSN setting and B channels set at
the same time.
If using a modern ISDN2E line then B channels should never be set.
If using an old style ISDN2 line that has been configured with a particular dial-in
number associated with a particular B channel then the equipment's B channels should
be set instead of using MSN.
Typical ISDN Wiring using structured cabling
An ISDN line will typically be presented as an RJ45 socket in the ISDN linebox. The
diagrams below outline a variety of methods for extending the ISDN line either via
direct wiring, daisy-chaining or via a structured cabling system.
In each instance, the terminating resistors in the ISDN linebox. will be switched
In the first diagram, a Cat 5 outlet is used with a Cat 5 patch cord as a simple
way to connect the extension wiring to the Linebox. As there is only one socket on
the extension wiring, this may extend as far as 800m.
In the second diagram, Cat 5 outlets are daisy-chained from the Linebox. to the ISDN
termination jack. In this case the total line length must be kept below 150m.
ISDN via structured cabling
If the Linebox. is suitably located, it can be patched directly to a Category 5 Patch
Panel. An ISDN Termination Adaptor must then be used to provide the necessary termination
This method will only allow one device to access the ISDN line.
If it is necessary for several devices to be able to access the ISDN line, an ISDN
Termination Patch Panel should be used. Each Panel will support up to three ISDN
The panels allow the ISDN line to be connected to the rear of the panel either via
a Cat 5 Patch cord to an RJ45 socket or using Cat 5 UTP cable to IDC's.
The Patch cord approach shown in the diagram above is simpler and recommended where
the location of the Linebox permits.
If the Linebox is located too far from the Patch Panel or if the wire route is not
suitable for a Patch Cord, a Cat 5 outlet can be located close to the Linebox and
connected to the Patch Panel using Cat 5 UTP cabling.
The Cat 5 outlet is then connected to the Linebox using a standard Category 5 Patch