Features
1.4
System Integration
ISDN Applications
1.4.1
The reference model for the ISDN basic access according to CCITT I series recommendations
consists of
– an exchange and trunk line termination in the central office (ET, LT)
– a remote network termination in the user area (NT)
– a two-wire loop (U interface) between NT and LT
– a four-wire link (S interface) which connects subscriber terminals and the NT in the user
area as depicted in figure 4.
ISDN User Area
TE
ISDN Central Office
S
U
NT
LT
ET
NT1
NT2
NT1
T
TE
ITS02314
Figure 4
ISDN Basic Subscriber Access Architecture
The NT equipment serves as a converter between the U interface at the exchange and the
S interface at the user premises. The NT may consist of either an NT1 only or an NT1 together
with an NT2 connected via the T interface which is physically identical to the S interface. The
NT1 is a direct transformation between layer 1 of S and layer 1 of U. NT2 may include higher
level functions like multiplexing and switching as in a PABX.
The ISAC-S is designed for the user area of the ISDN basic access, especially for subscriber
terminal equipment and for exchange equipment with S interfaces. Figure 5 illustrates the
general applications of the ISAC-S.
Semiconductor Group
25
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