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- ISDN was designed to reuse the existing customer devices as much as possible in order to
accelerate the acceptance of the technology. As a result, ITU defined several options of the
equipments required for ISDN access to provide several migration paths for the public.
Customers have the options to purchase the required equipments according to their budgets.
- Function is referred to as device or hardware.
Reference Point is referred to as demarcation or interface a function connects to.
C
TE2
D
TE1
B
TE1
R
TA
S NT2
A
NT1
S/T
T
NT1
Figure 26-4: ISDN Function Groups and Reference Points
Router Type of Interface Used
A ISDN card, U interface
B ISDN card, S/T interface
C Router with a serial interface or PC with a RS–232 connection to the R ref. point
D ISDN card, S/T interface
- Below lists the ISDN function groups:
Function Group Description
TE1 An ISDN-compliant device (eg: router). Understands ISDN standards.
Can be directly connected to the local loop. Uses an S or S/T ref. point.
TE2 Non ISDN-complaint device – a node that does not understand ISDN
protocols and specifications (PC or router). Uses an R reference point.
TA It converts TE2 non-ISDN signals to ISDN BRI signals. Uses R and S
reference points. (Note: TAs are often misinterpreted as ISDN modems)
NT1 A device that implements the ISDN physical layer (L1) specifications.
Converts between the 4-wire BRI signals from an S/T interface (from
routers) and the 2-wire signals of a U interface (to Telco). Uses a U
reference point and directly connects to the local loop (Telco), and
connects with S/T or T reference points to other CPEs.
NT2 A more complicated device (performs L2 and L3 functions) that uses a T
reference point to Telco outside North America or to an NT1 inside
North America. Connects with an S reference point to other CPEs.
NT1/NT2 A combined NT1 and NT2 in the same device. Can be directly connected
to the local loop (Telco). Very common in North America.
190
U
Service Provider Network
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