3.3. Importance of the impedance matching adapters (ISDN Adapter)

When ISDN-BRI ports (e.g. BRI-HFC4, BRI-HFC8) are used, a line termination resistor (ISDN Adapter or impedance matching adapter) must be used to avoid signal reflections. One adapter per port must be used.

The Telecom NT1 Network-termination units and most of the PBX (switchboards) usually have already a line termination resistor. The Abilis BRI port doesn't have any.

Differences between a Point-to-Point connection and a Point-to-multiPoint connection:

Figure 3.4. ISDN Adapter

ISDN Adapter


The impedance matching adapter MUST be connected near the ISDN port of the Abilis (you can use the 10 cm cable provided with the Abilis).


The Improper connection of line terminations may cause disturbances like noisy conversations, data loss, line drops, and in general a poor and unstable service.


Interesting chapters: Section 75.1, “How to configure ISDN ports to connect PBX or ISDN lines”.