Role of VCI and VPI in ATM:

• ATM uses VPI/VCI fields of a virtual connection to forward cells. At each intermediate node, VPI/VCI values are swapped and new values filled. This process is also referred to as cell relaying, cell switching ot cell forwarding.
• The VPI/VCI pair of an incoming cell is used as an index to search the VPI/VCI translation table. In the VPI/VCI translation table, a separate entry is maintained for each established virtual circuit.
• The values in the table get filled when a connection is established. For a PVC connection, each switch between source and destination is configured using some management tool.
• For an SVC coonection, thie connection takes place automatically via an exchange of signalling messages.
• Virtual Path Identifier (VPI) refers to an 8-bit (user to network packets) or 12-bit (network-network packets) field within the header of an Asynchronous Transfer Mode packet.
• The VPI, together with the VCI (Virtual Channel Identifier) is used to identify the next destination of a cell as it passes through a series of ATM switches on its way to its destination.
• VPI is useful to reduce the switching table for some Virtual Circuits which have common path. In simple words these two numbers simply tell the DSL equipment on both the sides that the communication to be established between them.
• VPI/ VCI settings are very much important to establish the connection between your Modem and Internet Service Providers.
• These settings varies from ISP to ISP. It is important that you should enter the correct VPI/VCI settings while configuring the MODEM.