COMNET III CACI

FDDI
ATM
Router
ATM
Switch
Private
NNI
Public
UNI
ATM
Switch
Public
NNI
ATM
Switch
ATM
Router
Private
UNI
ATM
Switch
Public ATM
Network
Private ATM
Network
Figure 28: Typical ATM topology showing the network interfaces
Figure 28 also highlights four different network interfaces:
1. Private user-network (UNI) interface
2. Public UNI
3. Private network-node (NNI) interface
4. Public NNI
A private UNI defines the interface between the user’s equipment and a private ATM
switch. The public UNI defines the interface between a private ATM switch and a public
ATM switch. Similarly, a private NNI defines the interface between two ATM switches
within a private ATM network. A public NNI defines the interface between two ATM
switches within a public ATM network. These distinctions not only permit a more precise
discussion about the different elements of an ATM network. They also emphasize the
different protocols and cell formats which are used within an ATM network. For
example, the management functions required for the public UNI differ from those
required for the private UNI. Similarly, signaling functions across the UNI differ from
those at the NNI. The distinction between the interfaces is outlined here because some of
the explanations below are specific to particular interfaces. For example, the CAC and
UPC/NPC functions discussed in the following appendix relate particularly to the UNI.
However, to pinpoint the precise differences between these interfaces is beyond the scope
of this brochure.
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