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Tech Note #22: Asynchronous Transfer Mode (ATM)  Addresses
©
1998 Bionic Buffalo Corporation; All Rights Reserved.
       14 September 1998
http://www.tatanka.com
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ATM Network
User Plane
Control Plane
Underneath the user and control planes are the same protocol stacks, although there are
differences at the adaptation layer level.
The connection between the user and the exchange or switch is used for signalling protocols,
which handle call establishment and release, as well as some other control functions relating to
switched services.
Cross-Connects, Concentrators, and VPCIs
There are two basic types of ATM switch: VP switches (often, these are also simply known as
cross-connects), and VC switches. A VP switch can terminate or re-route paths. A VC switch
can terminate and re-route both paths and channels.
Cross-connects are (obviously) simpler than VC switches. One common use of a cross-connect
is as a concentrator or multiplexor. A cross-connect can take a fast link containing more than
one virtual path, and re-route those paths into separate, slower links. In such applications, the
end-points of a network often are “aware” of the cross-connect and the way it is connected to
the network.
If an end-point uses a VPI to specify a path, the result might be considered ambiguous when
cross-connects are used. VPIs are unique only on a given link, and cross-connects have multiple
links. The VPI will not uniquely identify all of the paths entering the cross-connect. To allow
unique identification of the paths traversing or terminating at a cross-connect, the Virtual Path
Connection Identifier
 (VPCI) is used.
VPCIs are returned to the endpoints by the switching protocol when connections are
established.