Editing Asynchronous Transfer Mode (section)

=== Motivation ===
ATM operates as a channel-based transport layer, using VCs. This is encompassed in the concept of the virtual paths (VP) and virtual channels. Every ATM cell has an 8- or 12-bit virtual path identifier (VPI) and 16-bit virtual channel identifier (VCI) pair defined in its header.<ref name="cisco-atm">Cisco Systems [https://indigothemes.com/wikipedia-contribution/techgd.pdf ''Guide to ATM Technology''] (2000). Section "Operation of an ATM Switch". Retrieved 2 June 2011.</ref> The VCI, together with the VPI, 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. The length of the VPI varies according to whether the cell is sent on a user-network interface (at the edge of the network), or if it is sent on a network-network interface (inside the network).

As these cells traverse an ATM network, switching takes place by changing the VPI/VCI values (label swapping). Although the VPI/VCI values are not necessarily consistent from one end of the connection to the other, the concept of a circuit ''is'' consistent (unlike IP, where any given packet could get to its destination by a different route than the others).<ref name="cisco-atm-cell">Cisco Systems [https://indigothemes.com/wikipedia-contribution/techgd.pdf ''Guide to ATM Technology''] (2000). Section "ATM Cell Header Formats". Retrieved 2 June 2011.</ref> ATM switches use the VPI/VCI fields to identify the virtual channel link (VCL) of the next network that a cell needs to transit on its way to its final destination. The function of the VCI is similar to that of the [[data link connection identifier]] (DLCI) in [[Frame Relay]] and the logical channel number and logical channel group number in [[X.25]].

Another advantage of the use of virtual circuits comes with the ability to use them as a multiplexing layer, allowing different services (such as voice, Frame Relay, IP). The VPI is useful for reducing the switching table of some virtual circuits which have common paths.<ref>{{cite web |title=What is VPI and VCI settings of broadband connections? |url=http://www.techlineinfo.com/what-is-vpi-and-vci-settings-of-broadband-connections/ |website=Tech Line Info |publisher=Sujith |access-date=1 July 2010}}</ref>