3Com CoreBuilder 7000 Operation Manual page 133

The call is transferred as follows:
1 The edge device translates the Ethernet frames into ATM cells and
forwards them to the ATM output port.
2 The ATM output port writes the VPI/VCI values for the external link for
the VC on the cell header and transmits the cells on the line that connects
the port to the switch. Cells for simultaneous calls are multiplexed on the
line.
3 The cells that are received by the switch are identified by VPI/VCI and sent
through the virtual channel internal link to the appropriate output port.
4 The output port overwrites the VPI/VCI values in the cell header with ones
for the next external link and transmits the cell over the line to the edge
device.
5 The edge device identifies the VPI/VCI values, translates the cells back into
frames, and sends them out the appropriate Ethernet port.
Figure 43 shows another view of the ATM switching process when two
switches are involved. The external link between the switch units is a NNI
link instead of a UNI link.
ATM ETHERNET
VPI = 6
VCI = 412
H D
CoreBuilder 7000
UNI
Figure 43 ATM Switching over Virtual Channel
Cells are switched and their VPI/VCI
field is translated by every ATM switch
along the path between end stations.
LS2700
VPI = 1
VCI = 117
H
D
NNI
ATM Switching
ATM
ETHERNET
CoreBuilder 7000
UNI
133
LS2700
VPI = 3
VCI = 35
D
H