Editing Asynchronous Transfer Mode (section)

== Virtual circuits == <!--[[Virtual channel identifier]] and [[Virtual path identifier]] redirect here-->
An ATM network must establish a connection before two parties can send cells to each other. This is called a [[virtual circuit]] (VC). It can be a permanent virtual circuit (PVC), which is created administratively on the end points, or a switched virtual circuit (SVC), which is created as needed by the communicating parties. SVC creation is managed by [[signaling (telecommunications)|signaling]], in which the requesting party indicates the address of the receiving party, the type of service requested, and whatever traffic parameters may be applicable to the selected service. ''[[Admission control|Call admission]]'' is then performed by the network to confirm that the requested resources are available and that a route exists for the connection.

=== 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>

=== Types ===
ATM can build virtual circuits and virtual paths either statically or dynamically. Static circuits (permanent virtual circuits or PVCs) or paths (permanent virtual paths or PVPs) require that the circuit is composed of a series of segments, one for each pair of interfaces through which it passes.

PVPs and PVCs, though conceptually simple, require significant effort in large networks. They also do not support the re-routing of service in the event of a failure. Dynamically built PVPs (soft PVPs or SPVPs) and PVCs (soft PVCs or SPVCs), in contrast, are built by specifying the characteristics of the circuit (the service ''contract'') and the two endpoints.

ATM networks create and remove switched virtual circuits (SVCs) on demand when requested by an [[end station]]. One application for SVCs is to carry individual telephone calls when a network of telephone switches are interconnected using ATM. SVCs were also used in attempts to replace [[local area network]]s with ATM.

=== Routing ===
Most ATM networks supporting SPVPs, SPVCs, and SVCs use the [[Private Network-to-Network Interface]] (PNNI) protocol to share topology information between switches and select a route through a network. PNNI is a [[link-state routing protocol]] like [[OSPF]] and [[IS-IS]]. PNNI also includes a very powerful [[route summarization]] mechanism to allow construction of very large networks, as well as a [[call admission control]] (CAC) algorithm which determines the availability of sufficient bandwidth on a proposed route through a network in order to satisfy the service requirements of a VC or VP.