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An Introduction to ATM and VLAN Management Basics C-3
which is typically used to interconnect two ATM switches managed as
part of the same network.
■ The UNI is the physical interface between the user and the network.
A private UNI consists of a private end-point to private switch
interface, whereas a public UNI is typically used to interconnect an
ATM user with an ATM switch deployed in a public service provider’s
network.
■ The Network-to-Network Interface (NNI) defines the ATM switch
(network) to ATM switch (network) interface. It is also known as a
private NNI (P-NNI) when the switches are not part of a WAN. NNI
allows multiple switches to be interconnected in arbitrary topology
and still work as one network.
ATM is based on a layered architecture. In the protocol stack, the ATM
layer sits directly above the physical layer. Many physical layers can be
specified, including several for 100 to 155 Mbps. The ATM adaptation
layer (AAL) sits above the ATM layer. LAN Emulation (LE) sits above the
AAL5 in the protocol hierarchy.
ATM Switching ATM switching is performed at the ATM layer by defining virtual
channels. Cells are switched through the network based on the cell
header, therefore the routing is done for every cell and hence the virtual
channel.
Virtual Channels
Virtual channels are communications channels that provide for the
sequential, unidirectional transport of ATM cells. Multiple virtual channels
can exist on a physical link. There are two types of virtual channels:
Virtual Path Connection (VPC) and Virtual Channel Connection (VCC). A
VPC is a collection of VCCs (see Figure C-1).
A VCC extends between two end-points in the network and is uniquely
identified by the Virtual Path Identifier (VPI) and Virtual Channel Identifier
(VCI) values. It is also called a point-to-point VCC. A VPC extends
between two end-points in the network and may contain multiple VCCs.
VPCs are uniquely identified by the VPI value. VPCs and VCCs can be set
up either dynamically via signalling or permanently via management.
The VPI/VCI is only meaningful in the context of the given interface. The
combination of the VPI/VCI values allows the network to associate a given