A virtual circuit (VC) is a means of transporting data over a data network, based on

packet switching In telecommunications, packet switching is a method of grouping Data (computing), data into ''network packet, packets'' that are transmitted over a digital Telecommunications network, network. Packets are made of a header (computing), header and ...

and in which a connection is established within the network between two endpoints. The network, rather than having a fixed data rate reservation per connection, like in

circuit switching Circuit switching is a method of implementing a telecommunications network in which two network nodes establish a dedicated communications channel (circuit) through the network before the nodes may communicate. The circuit guarantees the full b ...

, takes advantage of the statistical multiplexing on its transmission links (an intrinsic feature of packet switching, well suited to data traffic). In addition, VCs standardized by the CCITT in 1976 impose per-connection flow controls at all user-to-network and network-to-network interfaces. They thus eliminate the need for the network to lose user packets in heavily loaded network zones, an intrinsic feature of datagram networks for their congestion control. Before a connection or virtual circuit may be used, it must be established between two or more nodes or software applications by means of call setup. After that, a

bit stream A bitstream (or bit stream), also known as binary sequence, is a sequence of bits. A bytestream is a sequence of bytes. Typically, each byte is an 8-bit quantity, and so the term octet stream is sometimes used interchangeably. An octet may ...

or byte stream may be delivered between the nodes; hence, a virtual circuit protocol allows higher-level protocols to avoid dealing with the division of data into Protocol data units. Many virtual circuit protocols, but not all, provide reliable communication service through the use of data retransmissions invoked by

error detection In information theory and coding theory with applications in computer science and telecommunication, error detection and correction (EDAC) or error control are techniques that enable reliable delivery of digital data over unreliable communic ...

and automatic repeat request (ARQ). An alternative to virtual-circuit networks are

datagram A datagram is a basic transfer unit associated with a packet-switched network. Datagrams are typically structured in header and payload sections. Datagrams provide a connectionless communication service across a packet-switched network. The del ...

networks.

Comparison with circuit switching

Virtual circuit communication resembles

, since both are connection oriented, meaning that in both cases data is delivered in correct order, and signalling overhead is required during a connection establishment phase. However, circuit switching provides a constant bit rate and latency, while these may vary in a virtual circuit service due to factors such as: * varying packet queue lengths in the network nodes, * varying bit rate generated by the application, * varying load from other users sharing the same network resources by means of

statistical multiplexing Statistical multiplexing is a type of communication link sharing, very similar to dynamic bandwidth allocation (DBA). In statistical multiplexing, a communication channel is divided into an arbitrary number of variable bitrate digital channels or ...

, etc.

Virtual call capability

telecommunication Telecommunication is the transmission of information by various types of technologies over wire, radio, optical, or other electromagnetic systems. It has its origin in the desire of humans for communication over a distance greater than tha ...

, a virtual call capability, sometimes called a virtual call facility, is a service feature in which: *a call set-up procedure and a call disengagement procedure determine the period of communication between two DTEs in which user data are transferred by a

packet switched In telecommunications, packet switching is a method of grouping Data (computing), data into ''network packet, packets'' that are transmitted over a digital Telecommunications network, network. Packets are made of a header (computing), header and ...

network *end-to-end transfer control of packets within the network is required *data may be delivered to the network by the

call originator The calling party (in some contexts called the "A-Number") is a person who (or device that) initiates a telephone call. The person who, or device that, receives a telephone call is the called party (or callee or B-party). In some countries, it ...

before the call access phase is completed, but the data are not delivered to the call receiver if the call attempt is unsuccessful *the network delivers all the user data to the call receiver in the same

sequence In mathematics, a sequence is an enumerated collection of objects in which repetitions are allowed and order matters. Like a set, it contains members (also called ''elements'', or ''terms''). The number of elements (possibly infinite) is called ...

in which the data are received by the network *multi-access DTEs may have several virtual calls in progress at the same

time Time is the continued sequence of existence and events that occurs in an apparently irreversible succession from the past, through the present, into the future. It is a component quantity of various measurements used to sequence events, t ...

. An alternative approach to virtual calls is connectionless communication using

s. In the 1970s, the "virtual call" concept was used in the British EPSS and enhanced by Rémi Després as "virtual circuits" in the French RCP.

Layer 4 virtual circuits

Connection oriented

transport layer In computer networking, the transport layer is a conceptual division of methods in the layered architecture of protocols in the network stack in the Internet protocol suite and the OSI model. The protocols of this layer provide end-to-end ...

protocols such as

TCP TCP may refer to: Science and technology * Transformer coupled plasma * Tool Center Point, see Robot end effector Computing * Transmission Control Protocol, a fundamental Internet standard * Telephony control protocol, a Bluetooth communication s ...

RFC 1180 may rely on a connectionless packet switching network layer protocol such as IP, where different packets may be routed over different paths, and thus be delivered out of order. However, it is possible to use TCP as a virtual circuit,RFC 1644 since TCP includes segment numbering that allows reordering on the receiver side to accommodate out-of-order delivery.

Layer 2/3 virtual circuits

Data link layer The data link layer, or layer 2, is the second layer of the seven-layer OSI model of computer networking. This layer is the protocol layer that transfers data between nodes on a network segment across the physical layer. The data link layer ...

and

network layer In the seven-layer OSI model of computer networking, the network layer is layer 3. The network layer is responsible for packet forwarding including routing through intermediate routers. Functions The network layer provides the means of trans ...

virtual circuit protocols are based on connection-oriented

, meaning that data is always delivered along the same network path, i.e., through the same nodes. Advantages with this over connectionless packet switching are: * Bandwidth reservation during the connection establishment phase is supported, making guaranteed

quality of service Quality of service (QoS) is the description or measurement of the overall performance of a service, such as a telephony or computer network, or a cloud computing service, particularly the performance seen by the users of the network. To quantitat ...

(QoS) possible. For example, a

constant bit rate Constant bitrate (CBR) is a term used in telecommunications, relating to the quality of service. Compare with variable bitrate. When referring to codecs, constant bit rate encoding means that the rate at which a codec's output data should be con ...

QoS class may be provided, resulting in emulation of

. * Less overhead is required since the packets are not routed individually and complete addressing information is not provided in the header of each data packet. Only a small

virtual channel identifier Asynchronous Transfer Mode (ATM) is a telecommunications standard defined by American National Standards Institute (ANSI) and ITU-T (formerly CCITT) for digital transmission of multiple types of traffic. ATM was developed to meet the needs of ...

(VCI) is required in each packet. Routing information is only transferred to the network nodes during the connection establishment phase. * The network nodes are faster and have higher capacity in theory since they are switches that only perform routing during the connection establishment phase, while connectionless network nodes are routers that perform routing for each packet individually. Switching only involves looking up the virtual channel identifier in a table rather than analyzing a complete address. Switches can easily be implemented in ASIC hardware, while routing is more complex and requires software implementation. However, because of the large market of IP routers, and because advanced IP routers support

layer 3 switching A multilayer switch (MLS) is a computer networking device that switches on OSI layer 2 like an ordinary network switch and provides extra functions on higher OSI layers. The MLS was invented by engineers at Digital Equipment Corporation. Switch ...

, modern IP routers may today be faster than switches for connection-oriented protocols.

Example protocols

Examples of transport layer protocols that provide a virtual circuit: *

Transmission Control Protocol The Transmission Control Protocol (TCP) is one of the main protocols of the Internet protocol suite. It originated in the initial network implementation in which it complemented the Internet Protocol (IP). Therefore, the entire suite is common ...

(TCP), where a reliable virtual circuit is established on top of the underlying unreliable and connectionless IP protocol. The virtual circuit is identified by the source and destination

network socket A network socket is a software structure within a network node of a computer network that serves as an endpoint for sending and receiving data across the network. The structure and properties of a socket are defined by an application programmin ...

address pair, i.e. the sender and receiver IP address and port number. Guaranteed QoS is not provided. * Stream Control Transmission Protocol (SCTP), where a virtual circuit is established on top of the IP protocol. Examples of network-layer and data-link-layer virtual circuit protocols, where data always is delivered over the same path: * X.25, where the VC is identified by a

(VCI). X.25 provides reliable node-to-node communication and guaranteed QoS. *

Frame Relay Frame Relay is a standardized wide area network (WAN) technology that specifies the physical and data link layers of digital telecommunications channels using a packet switching methodology. Originally designed for transport across Integrated ...

, where the VC is identified by a DLCI. Frame Relay is unreliable, but may provide guaranteed QoS. *

Asynchronous Transfer Mode Asynchronous Transfer Mode (ATM) is a telecommunications standard defined by American National Standards Institute (ANSI) and ITU-T (formerly CCITT) for digital transmission of multiple types of traffic. ATM was developed to meet the needs o ...

(ATM), where the circuit is identified by a virtual path identifier (VPI) and

(VCI) pair. The ATM layer provides unreliable virtual circuits, but the ATM protocol provides for reliability through the ATM adaptation layer (AAL) Service Specific Convergence Sublayer (SSCS) (though it uses the terms "assured" and "non-assured" rather than "reliable" and "unreliable").ITU-T, ''B-ISDN ATM Adaptation Layer specification: Type 3/4 AAL'', Recommendation I.363.3 (08/96), International Telecommunication Union, 1996, p5.ITU-T, ''B-ISDN ATM Adaptation Layer specification: Type 5 AAL'', Recommendation I.363.5 (08/96), International Telecommunication Union, 1996, p5. * General Packet Radio Service (GPRS) *

Multiprotocol label switching Multiprotocol Label Switching (MPLS) is a routing technique in telecommunications networks that directs data from one node to the next based on labels rather than network addresses. Whereas network addresses identify endpoints the labels identi ...

(MPLS), which can be used for IP over virtual circuits. Each circuit is identified by a label. MPLS is unreliable but provides eight different QoS classes.

Permanent and switched virtual circuits in ATM, Frame Relay, and X.25

Switched virtual circuits (SVCs) are generally set up on a per- call basis and are disconnected when the call is terminated; however, a permanent virtual circuit (PVC) can be established as an option to provide a

dedicated circuit A leased line is a private telecommunications circuit between two or more locations provided according to a commercial contract. It is sometimes also known as a private circuit, and as a data line in the UK. Typically, leased lines are used by ...

link between two facilities. PVC configuration is usually preconfigured by the service provider. Unlike SVCs, PVC are usually very seldom broken/disconnected. A switched virtual circuit (SVC) is a virtual circuit that is dynamically established on demand and is torn down when transmission is complete, for example after a phone call or a file download. SVCs are used in situations where data transmission is sporadic and/or not always between the same data terminal equipment ( DTE) endpoints. A permanent virtual circuit (PVC) is a virtual circuit established for repeated/continuous use between the same DTE. In a PVC, the long-term association is identical to the data transfer phase of a

virtual call Virtual may refer to: * Virtual (horse), a thoroughbred racehorse * Virtual channel, a channel designation which differs from that of the actual radio channel (or range of frequencies) on which the signal travels * Virtual function, a programming ...

. Permanent virtual circuits eliminate the need for repeated call set-up and clearing. * Frame Relay is typically used to provide PVCs. * ATM provides both switched virtual connections and permanent virtual connections, as they are called in ATM terminology. * X.25 provides both virtual calls and PVCs, although not all X.25 service providers or DTE implementations support PVCs as their use was much less common than SVCs

References

*{{FS1037C Communication circuits Network protocols Packets (information technology) Telephone services