A
reference point
defines a connection point between two functions; you can also refer to it as
an interface, though it does not represent an actual physical interface. The reference point is
where data is converted between device types. Figure 26.3 shows the reference points defined
in an ISDN network.
The reference points shown in Figure 26.3 are described in detail in the following list:
R reference point
The R reference point defines the point between non-ISDN equipment and a
TA. It enables a non-ISDN device to appear on the network as an ISDN device. Unlike the others,
this is a nonstandardized reference because it is dependent on the TE2 equipment’s interface.
S reference point
The S reference point is the point between the user terminals and NT2 or, in
other words, between a TE1 or a TA and the network termination (which is either an NT1 or NT2).
ISDN reference points
U
U
TA NT1
NT1
ISDN switch
service
Router with
built-in NT1
ISDN
device (TE1)
T reference point
The T reference point defines the point between NT1 and NT2 devices.
S/T interface
As the name implies, the S/T interface combines both the S and T interfaces. This
interface is governed by the ITU I.430 standard, which defines the connection as a 4-wire connection.
The S/T interface is typically an RJ-45, with 8-pin cables using pins 3 and 6 to receive
data and pins 4 and 5 to transmit data. Service providers in Europe use this interface to deliver
ISDN BRI service.
NOTE:The International Telecommunications Union (ITU) is a United Nations–sponsored
organization formed in 1865 to promote worldwide communication systems compatibility.
It has two groups: ITU-T and ITU-R. ITU-T deals with telecommunications,
and ITU-R is responsible for radio communications. You can visit their
website at
www.itu.int
for more information.
U reference point The U reference point is also known as a U (user) interface. This is a 2-wire
connection between the NT1 and the telephone company (LE). Cisco routers are marked with
an X if the interface is a U and with a crossed-out X if the interface is an S/T. This is an ANSI
standard used in the U.S. but not in Europe.
V reference point The V reference point is the interface point in an ISDN environment between
the line termination and the exchange termination.
In practical terms, it may be important for you to remember that in the TE1 is part
of the customer premises equipment (CPE) and is installed inside the route. Thus
an American router presents a U interface to the world, whereas in the rest of the
world, the TE1 is service provider–owned and the router presents an S/T interface.
So you cannot use a U.S.-sourced router elsewhere in the world, but you can use
an elsewhere-sourced router in the U.S. if you buy an external TE1.