IBM ATM OC-3c manual Virtual Path Identifier VPI Virtual Channel Identifier VCI

2-10CHAPTER 2: NETWORK LAYER CONCEPTS

AVCC 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 2-6Connection 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.