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Tech Note #22: Asynchronous Transfer Mode (ATM)  Addresses
1998 Bionic Buffalo Corporation; All Rights Reserved.
       14 September 1998
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A virtual path, which passes through an intermediate node without termination, may be given
two different VPIs on the two different links. However, the VCI of a cell within a path will not
change within a node unless the containing path terminates at that node.
Allocating VPI & VCI Numbers: Permanent Assignment
Certain VCI & VPI numbers are assigned in advance, either by specification or by optional
local configuration, while others are assigned dynamically as required.
Various ATM specifications allocate VCI and VPI numbers for specific purposes. For example,
an endpoint will use VPI=0, VCI=5 to ask the network to set up a call to another endpoint.
There are also specific addresses reserved by non-ATM specifications for applications on ATM
networks. For instance, the DAVIC specification calls for VPI=0, VCI=33 as part of the MAC
layer initialization procedure for set-top boxes.
An important permanent assignment is VPI=0, VCI=16, which belongs to the Interim Local
Management Interface
 (ILMI). ILMI defines a Management Information Base (MIB) accessed
using the Simple Network Management Protocol (SNMP). The database allows end-points to
register their addresses for retrieval by other nodes. If those addresses are assigned
systematically, then a simple directory service is available to the network. (For example, the ESI
can be assigned conventionally as part of the complete address, so that addresses containing
specific ESI values can have special significance.)
System designers sometimes assign specific VPI and VCI numbers for permanent or semi-
permanent channels to specific applications or for specific connections. Uses include not only
application-level communication (such as video transport) but also management-level or system
level communication (for initialization, diagnostics, or control).
These first two assignment mechanisms can create permanent virtual circuits (PVCs). Such
PVCs require a simple ATM architecture: