IBM ATM OC-3c - Manual (Page 22)

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.