Technical data

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ISDN
What is ISDN
ISDN (Integrated Services Digital Network) is a digital equivalent to the standard
PSTN telephone network (Public Switched Telephone Network). The ISDN technol-
ogy is standardised according to the recommendations from the International
Telecommunications Union (ITU).
Signalling
Instead of the telephone company activating the ring signal in your telephone
(“In-Band signal”), a digital packet is sent on a separate channel (“Out-of-Band signal”).
The Out-of-Band signal does not disturb the call in progress and has a short connec-
tion time. The signal contains information about who is calling, the type of call
(data/voice), and number that is calling. Available ISDN equipment then determines
how the call should be handled.
Connections
An ISDN-connection is built up of a number of B-channels that primarily carry data,
and a D-channel mainly for control signals. The transfer rate for data on one B-channel
is 64 kbit/s. Numerous channels can be interconnected to increase the
speed. Customers are usually offered ISDN in the form of two different
subscriptions: Basic access, that comprises of two B-channels and a
16 kbit/s D-channel (2B+D). This gives a maximum speed of two times
64 kbit/s, i.e. 128 kbit/s, suitable for users that require higher data transfer
rates or wish to combine telephone, fax and data communication, or a small local-area
network. It is possible to connect up to 8 ISDN devices on the same line. This is a big
advantage if there are different types of devices on an ISDN connection.
The devices receive individual numbers exactly as if they had their own connection
to the network. Primary access, comprises of 30 B-channels and a 64 kbit/s D-channel
(30B+D). The maximum capacity will then be 2 Mbit/s, when all 30 channels are con-
nected together. Primary access ISDN is suitable for connecting computers where
there is a high data transfer rate requirement (for example, for video conferencing),
large local networks, digital switches and bridges between large regional networks.
The greatest advantages of ISDN is the transfer rate (64–128 kbit/s), connection
times of less than 2 seconds, connections that are more stable and less sensitive to
interference as well as the flexibility of being able to connect multiple devices to the
same line (for example, telephone, fax or computer).
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