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2-10 C
HAPTER
2: N
ETWORK
L
AYER
C
ONCEPTS
A VCC is defined as spanning end-to-end, whereas a
Virtual Channel (VC)
is the name given to a section of
the VCC, refer to Figure 2-6.
Figure 2-6
Connection Terminology
Many virtual channels can exist on the same physical
link. Each virtual channel is identified by a pair of
numbers:
■
The
Virtual Path Identifier (VPI)
and
■
The
Virtual Channel Identifier (VCI)
.
Any end-system that wishes to communicate with
another end-system must first use the
Signalling
protocol to set up the VCC.
The
Signalling
protocol negotiates with each ATM
device between the end-systems to set up a series of
virtual channels. Each of these virtual channels is
identified using the VPI and VCI values.
Figure 2-7 on page 2-11 shows how ATM cells are
switched through the ATM network using the VPI/VCI
values.
Instead of containing the ATM address of the final
destination device, each cell header contains the
VPI/VCI values associated with the virtual channel it is
going to take to get to the next ATM Switch in the
connection.
Each ATM switch knows that when it receives a cell
with a particular VPI/VCI value on one port that it must
transmit the cell on another port with another VPI/VCI.
Cells are switched through the network based on
these VPI/VCI values, and switching is performed
independently for every cell. Each cell can be thought
of as taking a virtual channel connection.
The VPI/VCI values are only meaningful in the context
of that user-to-switch, or switch-to-switch, interface.
Identical VPI/VCI values can exist on different
interfaces within the network.
Connections that are established dynamically using the
Signalling protocol are known as
Switched Virtual
Circuits (SVCs)
.
Switched Virtual Circuits
are described
in “Switched Virtual Circuits (SVCs)” on page 2-12.
ATM connections can also be established via
management, and these type of connections are
known as
Permanent Virtual Circuits (PVCs)
.
Permanent Virtual Circuits
are described in
“Permanent Virtual Circuits (PVCs)” on page 2-12.