3Com CoreBuilder 9000, Руководство пользователя

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350 C
HAPTER
13: E
STABLISHING
ATM C
HANNELS
Virtual Channels
A virtual channel is a communications path that can carry a single call
from the source station to the destination station. A channel is virtual
because it does not necessarily correspond to a single physical line and, in
fact, usually passes through many different network devices and cables
before reaching its destination.
Local links and
connections
A virtual channel is built from a series of local links of two types:
External links between ports of adjacent ATM devices
Internal links between input and output ports of an ATM device
A simple example of a virtual channel is shown as a dashed line in
Figure 16.
Figure 16 Virtual Channel Passing Through ATM Network Devices
ATM Edge Device
ATM Edge Device
Destination Station
Transmitting Station
ATM Switch
The virtual channel
starts at the ATM
edge device.
A VPI/VCI pair is
associated with
the virtual channel
by both the output
port of the edge
device and the
input port of the
switch.
The source station
is connected to an
Ethernet port of
the attached ATM
edge device
The ATM switch has
been setup to associate
the specific VPI/VCI pair
on the input port with
another VPI/VCI pair on
an output port.
A VPI/VCI pair is
associated with the
virtual channel by both
the output port of the
switch and the input port
of the edge device
The virtual channel
terminates at the
edge device.
The ATM edge
device associates
the VPI/VCI
information with
the destination
Ethernet output
port.
The Ethernet
output port of
the edge device
is connected to
the destination
station.
VPI=0
VCI=32
VPI=1
VCI=44