Nagra Seven Operating Instructions Manual page 79

I.S.D.N. (R.N.I.S) (Hardware option part number 71 19186 000)
The Nagra Seven can be equipped with an internal I.S.D.N (R.N.I.S.) transmission codec system for
communication via a digital telephone system. The features offered are very complete and allow many
different communication possibilities which are all explained in the following pages.
I.S.D.N. BASICS.
A standard analog telephone line consists of two copper wires running from the users' telephone unit to the
external telephone exchange and is more commonly known as a Public Switched Telephone Network or
P.S.T.N. When the receiver of the telephone is lifted a relay closes and makes a circuit causing a current to flow
through the telephone line, and this current alerts the exchange to attribute a free line and waits for a series
of numbers (either pulses or dial tones) to follow. This two wire system is more commonly known as a "U"
connection. The acronym I.S.D.N. stands for Integrated Services Digital Network which is a digital telephone
network using the same two copper wires but has to be connected to a digital exchange.
The standard analog line is a two wire system and I.S.D.N. is a four wire system which is more commonly
known as an "S" connection. In order to transform the users "U" connection (2 wire) into an "S" connection (4-
wire) an interface known as an NT needs to be installed. This unit not only converts from 2 wire to 4 wire but
also serves for line equalization.
The four wire system of I.S.D.N. consists of two symmetrical pairs known as TX-, RX-, TX+ and RX+. A single
I.S.D.N. line is used to pass digital data and will permit 2 x 64 kbits signals and 1 x 16 kbit signal to be
transmitted along it. These data streams are known as channels, so one I.S.D.N. line consists of 2 x "B" (bearer)
channels and 1 x "D" (delta) channel.
If an I.S.D.N. line has been installed with the NT it can still be used in the same way as a standard telephone
line providing that the telephone unit is a special digital telephone. If this is the case then the user can dial
normally to a standard analog number. All information passing on an I.S.D.N. line is compressed therefore the
A/D and D/A conversion as well as the compression and decompression is done within the special digital
telephone unit. Such a digital telephone can be represented as follows:
If the handset is lifted on the digital phone, then immediately one of the "B" channels is used and part of the
"D" channel. The "B" channel will carry the audio data and the "D" channel is used to carry communication
control data such as the number being called, the return signal coming from the receiver with its number as
well as the tariffs for the connection etc. As this system uses only one "B" channel, it is possible to connect a
second digital telephone to the same I.S.D.N. line if required, and this will use the second "B" channel.
I.S.D.N. SYSTEMS
The format of the Euro I.S.D.N. standard is as described above. That is to say 2 x 64 kbits/sec "B" channels and
1 x 16 kbits/sec "D" channel. A second system also exists, and is known as the "Switched 56" system, which
consists of transmission of the signal at 56 kbits/second and generally found in the U.S.A.
AUDIO COMPRESSION USED IN THE I.S.D.N. DOMAIN.
Signals sent down an I.S.D.N. line are compressed and vary from country to country. There are three principal
transmission modes, G-711 or "Speech" and G 722 or "Data" and MPEG-1 Layer II. In Europe, an I.S.D.N.
telephone in "speech" mode will normally use the A-Law compression standard which allows a bandwidth of
3.5 kHz. In the United States a similar telephone will generally use the µ-Law compression standard.
The G722 and MPEG compression standards for data transmission are used on both sides of the Atlantic.
Chapter IV 78 June 2014