ISDN A basic Guide
ISDN ' Integrated Services Digital Network'
B Channels:
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. Unless using
equipment on old ISDN2 line then your equipment should always be set to make or accept
calls on either B1 or B2.
CODEC (Coder Decoder):
For broadcasting audio down ISDN lines a Codec is needed. A codec takes an audio
waveform 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, un compresses it and turn it back into an analogue waveform.
When compressing the audio a codec passes the data through a compression algorithm which
is a clever device that works how to make the audio waveform 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 waveform.
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 (please
contact us to find out which is best for you).
Category 5:
It is common in modern office buildings for a category 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 lots of 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 an 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. 01622 678540 and
01622 679049. You would then programme the MSN in your telephone to be 01622 678540 and the
MSN in your fax machine to be 01622 679049 then when someone dialled into 01622 678540
your telephone would ring and when someone dialled into 01622 679049 you fax machine would
ring.
(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
separately.
See also S interface and U interface.
RJ45:
This is the standard connector used to connect a cable from a piece of equipments
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.
S Interface:
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.
Terminal Adaptor:
This is a 'modem' for ISDN. It provides the electronic link between
subscribers
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.
U Interface:
A single twisted pair of cables carry all the data for both B channels between a
telecom providers ISDN exchange and the subscriber. The subscribers end of these cables is
called a U Bus.
In America & Canada ISDN lines are normally left without an NT as 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 equipments B channels should be set
instead of using MSN.
Variations of ISDN lines: (The wonderful thing about standards is that
there are so many to choose from!)
For correct operation it is important that your ISDN equipment is compatible with the ISDN
line that it is plugged into.
For the users of our equipment the most common type of ISDN line is called an ISDN2E. This
is now the standard throughout Europe, Australia and many other Countries, the biggest
exception being North America.
For very large organisations ISDN '30' are not uncommon, these have a large data link from
the organisations local exchange to the telecom provider. The local exchange takes the
large data link and splits it up into 30 off 64kbit/s channels and makes each one look
like an ISDN2E 'B' channel.
An ISDN2E line is often called 'basic rate ISDN'.
An ISDN30 line is often called 'primary rate ISDN'.
Links to Other ISDN Pages
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