Appendix: What is the ISDN?
38
MicroLink ISDN/PCC-16
Manual © 1997 ELSA GmbH
Every ISDN service has a unique identifying signal for detection by the receiving device, which can
then determine whether or not it is suitable to receiving the incoming call.
ISDN Service Features
ISDN offers many services not available to analog users, such as the caller-ID, call waiting, selection
of the terminal device at the bus, "parking" of current calls, terminal portability (cable disconnection
without call disconnection), online switching of service and/or device, call hold, three-party service,
multiple services (channels can be used simultaneously for multiple services), call forwarding within
PBXs, unconditional call forwarding (world-wide, even to mobile phones, either immediately or after a
15s wait, and when busy), connection blocking (for incoming telephone calls or all calls, for all out-
going calls, outgoing long-distance calls, outgoing overseas calls, outgoing intercontinental calls,
however with permanent availability of emergency numbers), advice of charge, capture (determining
from which number a call to the device under surveillance was made), the closed user group (max. 100
total users, max. 20 per service, multiple user groups per connection possible), semi-permanent and
leased-line connections.
Basic Access
The basic access can also be described as a multiple device connection. This is a four-wire bus
system to which can make up to 12 ISDN sockets available, from which a maximum of 8 devices (max.
4 telephones due to the current demanded) can be connected.
The multiple device connection uses a control channel (D channel) with a transfer rate of 16 bps and
two data channels (B channels) with transfer rates of 64,000 bps each. Both data channels can be
operated simultaneously and from separate devices.
The Network Terminator (NT) is the connection between the public ISDN network and the user's
installation. The transition point from the public line (to the exchange) and the NT is called the U
K0
/U
K2
interface. The physical transmission from the NT to the exchange uses a copper wire pair, which
means that the existing analog telephone infrastructure does not need to be replaced when upgrading
to ISDN. The connection point between the NT and the bus to the terminal devices is the S
0
/S
2M
user
interface. The NT is connected to the 230V power supply and offers increased protection against
current spikes (e.g. lightning strikes).
The Protocols
The D channel controls the communications to an ISDN connection. In Europe the D channel protocols
available are DSS1 (Euro-ISDN) and 1TR6 (German national ISDN).
With the DSS1 (Digital Subscriber System number 1) D channel protocol a connection can be allocated
several (phone) numbers or MSNs (Multiple Subscriber Numbers). As a rule, 3 numbers (with up to 12
digits) are allocated, though a maximum of 8 is possible. The numbers can be freely allocated to any
terminal device, meaning specific services (such as fax) can be given a specific (phone) number.
With the 1TR6 D channel protocol, the various devices attached to the S
0
bus are accessed via an EAZ
number (German for Terminal Device Selection Digit). This one-digit number is attached to the phone
number of the connection. The digit 0 has the function of a global call.
