Virtual circuit

Last updated September 16, 2024

A virtual circuit (VC) is a means of transporting data over a data network, based on packet switching and in which a connection is first established across the network between two endpoints. The network, rather than having a fixed data rate reservation per connection as in circuit switching, takes advantage of the statistical multiplexing on its transmission links, an intrinsic feature of packet switching.

A 1978 standardization of virtual circuits by the CCITT imposes per-connection flow controls at all user-to-network and network-to-network interfaces. This permits participation in congestion control and reduces the likelihood of packet loss in a heavily loaded network.^[1]^[2] Some circuit protocols provide reliable communication service through the use of data retransmissions invoked by error detection and automatic repeat request (ARQ).

Before a virtual circuit may be used, it must be established between network nodes in the call setup phase. Once established, a bit stream or byte stream may be exchanged between the nodes, providing abstraction from low-level division into protocol data units, and enabling higher-level protocols to operate transparently.

An alternative to virtual-circuit networks are datagram networks.

Comparison with circuit switching

Virtual circuit communication resembles circuit switching, since both are connection oriented, meaning that in both cases data is delivered in correct order, and signaling 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, etc.

Virtual call capability

In telecommunications, 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 network
end-to-end transfer control of packets within the network is required
data may be delivered to the network by the call originator 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 which the data are received by the network
multi-access DTEs may have several virtual calls in progress at the same time.

An alternative approach to virtual calls is connectionless communication using datagrams.^[3]

In the early 1970s, virtual callcapability was developed by British Telecom for EPSS (building on the work of Donald Davies at the National Physical Laboratory). The concept was enhanced by Rémi Després as virtual circuits for the RCP experimental network of the French PTT.^[4]^[5]^[6]

Layer 4 virtual circuits

Connection oriented transport layer protocols such as TCP ^[7]^[8] 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,^[8]^[9]^[10] 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 and network layer virtual circuit protocols are based on connection-oriented packet switching, 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 (QoS) possible. For example, a constant bit rate QoS class may be provided, resulting in emulation of circuit switching.
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 (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, 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 (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 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 virtual channel identifier (VCI). X.25 provides reliable node-to-node communication and guaranteed QoS.
Frame Relay, where the VC is identified by a DLCI. Frame Relay is unreliable, but may provide guaranteed QoS.
Asynchronous Transfer Mode (ATM), where the circuit is identified by a virtual path identifier (VPI) and virtual channel identifier (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).^[11]^[12]
General Packet Radio Service (GPRS)
Multiprotocol Label Switching (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 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. 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

Related Research Articles

Asynchronous Transfer Mode (ATM) is a telecommunications standard defined by the American National Standards Institute and International Telecommunication Union Telecommunication Standardization Sector for digital transmission of multiple types of traffic. ATM was developed to meet the needs of the Broadband Integrated Services Digital Network as defined in the late 1980s, and designed to integrate telecommunication networks. It can handle both traditional high-throughput data traffic and real-time, low-latency content such as telephony (voice) and video. ATM provides functionality that uses features of circuit switching and packet switching networks by using asynchronous time-division multiplexing. ATM was seen in the 1990s as a competitor to Ethernet and networks carrying IP traffic as, unlike Ethernet, it was faster and designed with quality-of-service in mind, but it fell out of favor once Ethernet reached speeds of 1 gigabits per second.

The Internet Protocol (IP) is the network layer communications protocol in the Internet protocol suite for relaying datagrams across network boundaries. Its routing function enables internetworking, and essentially establishes the Internet.

The Internet protocol suite, commonly known as TCP/IP, is a framework for organizing the set of communication protocols used in the Internet and similar computer networks according to functional criteria. The foundational protocols in the suite are the Transmission Control Protocol (TCP), the User Datagram Protocol (UDP), and the Internet Protocol (IP). Early versions of this networking model were known as the Department of Defense (DoD) model because the research and development were funded by the United States Department of Defense through DARPA.

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 identify established paths between endpoints. MPLS can encapsulate packets of various network protocols, hence the multiprotocol component of the name. MPLS supports a range of access technologies, including T1/E1, ATM, Frame Relay, and DSL.

In computer networking, Point-to-Point Protocol (PPP) is a data link layer communication protocol between two routers directly without any host or any other networking in between. It can provide loop detection, authentication, transmission encryption, and data compression.

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 quantitatively measure quality of service, several related aspects of the network service are often considered, such as packet loss, bit rate, throughput, transmission delay, availability, jitter, etc.

In computer networking, the User Datagram Protocol (UDP) is one of the core communication protocols of the Internet protocol suite used to send messages to other hosts on an Internet Protocol (IP) network. Within an IP network, UDP does not require prior communication to set up communication channels or data paths.

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 Services Digital Network (ISDN) infrastructure, it may be used today in the context of many other network interfaces.

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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 delivery, arrival time, and order of arrival of datagrams need not be guaranteed by the network.

<span class="mw-page-title-main">X.25</span> Standard protocol suite for packet switched wide area network (WAN) communication

X.25 is an ITU-T standard protocol suite for packet-switched data communication in wide area networks (WAN). It was originally defined by the International Telegraph and Telephone Consultative Committee in a series of drafts and finalized in a publication known as The Orange Book in 1976.

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 communication services for applications. It provides services such as connection-oriented communication, reliability, flow control, and multiplexing.

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 provides the functional and procedural means to transfer data between network entities and may also provide the means to detect and possibly correct errors that can occur in the physical layer.

ATM Adaptation Layer 5 (AAL5) is an ATM adaptation layer used to send variable-length packets up to 65,535 octets in size across an Asynchronous Transfer Mode (ATM) network.

In telecommunications and computer networking, connection-oriented communication is a communication protocol where a communication session or a semi-permanent connection is established before any useful data can be transferred. The established connection ensures that data is delivered in the correct order to the upper communication layer. The alternative is called connectionless communication, such as the datagram mode communication used by Internet Protocol (IP) and User Datagram Protocol (UDP), where data may be delivered out of order, since different network packets are routed independently and may be delivered over different paths.

ITU-T Recommendation Q.931 is the ITU standard ISDN connection control signalling protocol, forming part of Digital Subscriber Signalling System No. 1. Unlike connectionless systems like UDP, ISDN is connection oriented and uses explicit signalling to manage call state: Q.931. Q.931 typically does not carry user data. Q.931 does not have a direct equivalent in the Internet Protocol stack, but can be compared to SIP. Q.931 does not provide flow control or perform retransmission, since the underlying layers are assumed to be reliable and the circuit-oriented nature of ISDN allocates bandwidth in fixed increments of 64 kbit/s. Amongst other things, Q.931 manages connection setup and breakdown. Like TCP, Q.931 documents both the protocol itself and a protocol state machine.

In computer networking, a reliable protocol is a communication protocol that notifies the sender whether or not the delivery of data to intended recipients was successful. Reliability is a synonym for assurance, which is the term used by the ITU and ATM Forum.

In computer networking, an edge device is a device that provides an entry point into enterprise or service provider core networks. Examples include routers, routing switches, integrated access devices (IADs), multiplexers, and a variety of metropolitan area network (MAN) and wide area network (WAN) access devices. Edge devices also provide connections into carrier and service provider networks. An edge device that connects a local area network to a high speed switch or backbone may be called an edge concentrator.

A computer network is a set of computers sharing resources located on or provided by network nodes. Computers use common communication protocols over digital interconnections to communicate with each other. These interconnections are made up of telecommunication network technologies based on physically wired, optical, and wireless radio-frequency methods that may be arranged in a variety of network topologies.

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 programming interface (API) for the networking architecture. Sockets are created only during the lifetime of a process of an application running in the node.

References

↑ "X.25 - Interface between Data Terminal Equipment (DTE) and Data Circuit-terminating Equipment (DCE) for terminals operating in the packet mode and connected to public data networks by dedicated circuit". ITU-T. October 1976.
↑ Rybczynski, A; Wessler, B; Després, R; Wedlake, J (1976-06-07). "A new communication protocol for accessing data networks: The international packet-mode interface". In AFIPS (ed.). Proceedings of the June 7-10, 1976, national computer conference and exposition on - AFIPS '76. p. 477. doi:10.1145/1499799.1499869. S2CID 8790311.
↑ Tanenbaum, Andrew S.; Wetherall, David J. (2011). Computer Networks (5th international ed.). Pearson. p. 361. ISBN 978-0-13-255317-9.
↑ Smith, Ed; Miller, Chris; Norton, Jim (2017). "Packet Switching: The first steps on the road to the information society". National Physical Laboratory.
↑ "RCP, The Experimental Packet-Switched Data Transmission Service of The French PTT". 21 January 2022.
↑ R. Despres, "A packet switching network with graceful saturated operation", in Computer Communications: Impacts and Implications, S. Winkler, Ed. Washington, D.C., 1972
↑ RFC 793
1 2 RFC 1180
↑ RFC 955
↑ RFC 1644
↑ 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.

This article incorporates public domain material from Federal Standard 1037C. General Services Administration. Archived from the original on 2022-01-22.

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[1] "X.25 - Interface between Data Terminal Equipment (DTE) and Data Circuit-terminating Equipment (DCE) for terminals operating in the packet mode and connected to public data networks by dedicated circuit". ITU-T. October 1976.

[2] Rybczynski, A; Wessler, B; Després, R; Wedlake, J (1976-06-07). "A new communication protocol for accessing data networks: The international packet-mode interface". In AFIPS (ed.). Proceedings of the June 7-10, 1976, national computer conference and exposition on - AFIPS '76. p. 477. doi:10.1145/1499799.1499869. S2CID 8790311.

[tanenbaum-3] Tanenbaum, Andrew S.; Wetherall, David J. (2011). Computer Networks (5th international ed.). Pearson. p. 361. ISBN 978-0-13-255317-9.

[4] Smith, Ed; Miller, Chris; Norton, Jim (2017). "Packet Switching: The first steps on the road to the information society". National Physical Laboratory.

[RCP-5] "RCP, The Experimental Packet-Switched Data Transmission Service of The French PTT". 21 January 2022.

[6] R. Despres, "A packet switching network with graceful saturated operation", in Computer Communications: Impacts and Implications, S. Winkler, Ed. Washington, D.C., 1972

[7] RFC 793

[rfc1180-8] 1 2 RFC 1180

[9] RFC 955

[10] RFC 1644

[I.363.3-11] ITU-T, B-ISDN ATM Adaptation Layer specification: Type 3/4 AAL, Recommendation I.363.3 (08/96), International Telecommunication Union, 1996, p5.

[I.363.5-12] ITU-T, B-ISDN ATM Adaptation Layer specification: Type 5 AAL, Recommendation I.363.5 (08/96), International Telecommunication Union, 1996, p5.

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