Reference piont refers to the lable used to identify individual interfaces between two elements of an ISDN installation. It defines how two elements are connected. There are four reference points generally used. They are R, S, T and U. Fig. 12.9 illustrates the use of reference point.
Fig. 12.9. Reference points.
The reference point R defines connection between a TE2 and a TA. R interface is not defined by the ISDN. A subscriber can use any of the EIA standards.
The reference point S defines the connection between a TE1 or TA and TA and an NT1 (or NT2). ITU-7 speficics the ISO standard ISO 8887 for S interface. A maximum of 8 TE's can be connected to one ISDN line throuh 'S' bus wiring, even though only two can be in use at a time. RJ45 cable called S bus having 8 wire ribbon type cable is available in electronic market.
Reference point T defines the interface between an NT2 and NT1. Reference point U defines the interface between an NT1 and the ISDN office.
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