Lucent Technologies CAJUN M770, User Manual

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Appendix C Routing and Signalling Concepts
240 Cajun M770 ATM Switch User’s Guide
The value of the VPI and VCI within a particular ATM cell header will change as the
ATM cell is switched through the ATM network. In a single switch configuration a
cell’s VPI and VCI are translated only once, but in a multiple switch environment a
cell’s VPI and VCI may be translated many times. VPI and VCI values are assigned
symmetrically, that is, the same values are reserved in both directions across a link.
This means that all virtual circuits are inherently bi-directional. This does not imply
that SVC traffic must be sent in both directions; it can be either bi-directional or
uni-directional. With point-to-multipoint virtual circuits, data is only sent from the
root (source) to the leaves (destination parties).
Note: On a given link, VPI/VCI identifiers are assigned in both directions of a circuit.
However if a circuit is uni-directional, one of these will not be used.
Note:
A cell’s VPI and VCI are of local significance only. They identify the cell as
being associated with a particular virtual circuit through a single link.
When the M770 ATM Switch receives a cell, it examines the ATM cell header to
determine the VPI/VCI on which the cell was transmitted. Using this information,
the M770 ATM Switch determines the destination port and appropriate VPI/VCI for
the transmission of the ATM cell.
For example, in a single switch environment the switch can be configured such that
a cell received on port A with VPI.VCI=0.35 is switched to port B with
VPI.VCI=0.72. The translation from input port VPI and VCI to output port VPI and
VCI is carried out by the switch hardware.
Figure T.3 Cell switching through a M770 ATM Switch from end-station one to end-station two