Virtual circuit
Encyclopedia
In telecommunications and computer networks, a virtual circuit (VC), synonymous with virtual connection and virtual channel
, is a connection oriented communication service that is delivered by means of packet mode communication. After a connection or virtual circuit is established between two nodes or application processes, a bit stream or byte stream
may be delivered between the nodes; a virtual circuit protocol allows higher level protocols to avoid dealing with the division of data into segments, packets, or frames.
Virtual circuit communication resembles circuit switching
, 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 constant bit rate and latency, while these may vary in a virtual circuit service because of reasons such as:
Many virtual circuit protocols, but not all, provide reliable
communication service, by means of data retransmissions because of error detection and automatic repeat request (ARQ).
datalink protocols such as TCP
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, a virtual circuit is possible since TCP includes segment numbering and reordering on the receiver side to prevent out-of-order delivery.
and datalink 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:
Examples of network layer and datalink layer virtual circuit protocols, where data always is delivered over the same path:
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
.
Virtual channel
In telecommunications, a logical channel number , also known as virtual channel, is a channel designation which differs from that of the actual radio channel on which the signal travels....
, is a connection oriented communication service that is delivered by means of packet mode communication. After a connection or virtual circuit is established between two nodes or application processes, a bit stream or byte stream
Byte stream
In computer science, a byte stream is a bit stream, in which data bits are grouped into units, called bytes.In computer networking the term octet stream is sometimes used to refer to the same thing; it emphasizes the use of bytes having the length of 8 bits, known as octets.Formally, a byte stream...
may be delivered between the nodes; a virtual circuit protocol allows higher level protocols to avoid dealing with the division of data into segments, packets, or frames.
Virtual circuit communication resembles circuit switching
Circuit switching
Circuit switching is a methodology of implementing a telecommunications network in which two network nodes establish a dedicated communications channel through the network before the nodes may communicate. The circuit guarantees the full bandwidth of the channel and remains connected for the...
, 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 constant bit rate and latency, while these may vary in a virtual circuit service because of reasons 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 multiplexingStatistical multiplexingStatistical multiplexing is a type of communication link sharing, very similar to dynamic bandwidth allocation . In statistical multiplexing, a communication channel is divided into an arbitrary number of variable bit-rate digital channels or data streams. The link sharing is adapted to the...
, etc.
Many virtual circuit protocols, but not all, provide reliable
Reliability (computer networking)
In computer networking, a reliable protocol is one that provides reliability properties with respect to the delivery of data to the intended recipient, as opposed to an unreliable protocol, which does not provide notifications to the sender as to the delivery of transmitted data.A reliable...
communication service, by means of data retransmissions because of error detection and automatic repeat request (ARQ).
Layer 4 virtual circuits
Connection oriented transport layerTransport layer
In computer networking, the transport layer or layer 4 provides end-to-end communication services for applications within a layered architecture of network components and protocols...
datalink protocols such as TCP
Transmission Control Protocol
The Transmission Control Protocol is one of the core protocols of the Internet Protocol Suite. TCP is one of the two original components of the suite, complementing the Internet Protocol , and therefore the entire suite is commonly referred to as TCP/IP...
may rely on a connectionless packet switching network layer protocol such as IP
Internet Protocol
The Internet Protocol is the principal communications protocol used for relaying datagrams across an internetwork using the Internet Protocol Suite...
, where different packets may be routed over different paths, and thus be delivered out of order. However, a virtual circuit is possible since TCP includes segment numbering and reordering on the receiver side to prevent out-of-order delivery.
Layer 2/3 virtual circuits
Network layerNetwork layer
The network layer is layer 3 of the seven-layer OSI model of computer networking.The network layer is responsible for packet forwarding including routing through intermediate routers, whereas the data link layer is responsible for media access control, flow control and error checking.The network...
and datalink layer virtual circuit protocols are based on connection oriented packet switching
Packet switching
Packet switching is a digital networking communications method that groups all transmitted data – regardless of content, type, or structure – into suitably sized blocks, called packets. Packet switching features delivery of variable-bit-rate data streams over a shared network...
, 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 ServiceQuality of serviceThe quality of service refers to several related aspects of telephony and computer networks that allow the transport of traffic with special requirements...
(QoS) possible. For example, a constant bit rate QoS class may be provided, resulting in emulation of circuit switchingCircuit switchingCircuit switching is a methodology of implementing a telecommunications network in which two network nodes establish a dedicated communications channel through the network before the nodes may communicate. The circuit guarantees the full bandwidth of the channel and remains connected for the...
. - 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 ASICApplication-specific integrated circuitAn application-specific integrated circuit is an integrated circuit customized for a particular use, rather than intended for general-purpose use. For example, a chip designed solely to run a cell phone is an 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.
Examples of protocols that provide virtual circuits
Examples of transport layer protocols that provide a virtual circuit:- Transmission Control ProtocolTransmission Control ProtocolThe Transmission Control Protocol is one of the core protocols of the Internet Protocol Suite. TCP is one of the two original components of the suite, complementing the Internet Protocol , and therefore the entire suite is commonly referred to as TCP/IP...
(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 ProtocolStream Control Transmission ProtocolIn computer networking, the Stream Control Transmission Protocol is a Transport Layer protocol, serving in a similar role to the popular protocols Transmission Control Protocol and User Datagram Protocol...
(SCTP), where a virtual circuit is established on top of either the IP protocol or the UDP protocol.
Examples of network layer and datalink layer virtual circuit protocols, where data always is delivered over the same path:
- X.25X.25X.25 is an ITU-T standard protocol suite for packet switched wide area network communication. An X.25 WAN consists of packet-switching exchange nodes as the networking hardware, and leased lines, Plain old telephone service connections or ISDN connections as physical links...
, where the VC is identified by a virtual channel identifier (VCI). X.25 provides reliable node-to-node communication and guaranteed QoS. - Frame relayFrame relayFrame Relay is a standardized wide area network technology that specifies the physical and logical link layers of digital telecommunications channels using a packet switching methodology...
, where the VC is identified by a VCI. Frame relay is unreliable, but may provide guaranteed QoS. - Asynchronous Transfer ModeAsynchronous Transfer ModeAsynchronous Transfer Mode is a standard switching technique designed to unify telecommunication and computer networks. It uses asynchronous time-division multiplexing, and it encodes data into small, fixed-sized cells. This differs from approaches such as the Internet Protocol or Ethernet that...
(ATM), where the circuit is identified by a virtual path identifier (VPI) and virtual channel identifier (VCI) pair. ATM is unreliable, but may provide guaranteed QoS. - General Packet Radio ServiceGeneral Packet Radio ServiceGeneral packet radio service is a packet oriented mobile data service on the 2G and 3G cellular communication system's global system for mobile communications . GPRS was originally standardized by European Telecommunications Standards Institute in response to the earlier CDPD and i-mode...
(GPRS) - Multiprotocol label switchingMultiprotocol Label SwitchingMultiprotocol Label Switching is a mechanism in high-performance telecommunications networks that directs data from one network node to the next based on short path labels rather than long network addresses, avoiding complex lookups in a routing table. The labels identify virtual links between...
(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-callTelephone call
A telephone call is a connection over a telephone network between the calling party and the called party.-Information transmission:A telephone call may carry ordinary voice transmission using a telephone, data transmission when the calling party and called party are using modems, or facsimile...
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
Data link
In telecommunication a data link is the means of connecting one location to another for the purpose of transmitting and receiving information. It can also refer to a set of electronics assemblies, consisting of a transmitter and a receiver and the interconnecting data telecommunication circuit...
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)
Data terminal equipment
Data Terminal Equipment is an end instrument that converts user information into signals or reconverts received signals. These can also be called tail circuits. A DTE device communicates with the data circuit-terminating equipment...
endpoints.
A permanent virtual circuit (PVC) is a virtual circuit established for repeated/continuous use between the same DTE
Data terminal equipment
Data Terminal Equipment is an end instrument that converts user information into signals or reconverts received signals. These can also be called tail circuits. A DTE device communicates with the data circuit-terminating equipment...
. In a PVC, the long-term association is identical to the data transfer phase
Phase (waves)
Phase in waves is the fraction of a wave cycle which has elapsed relative to an arbitrary point.-Formula:The phase of an oscillation or wave refers to a sinusoidal function such as the following:...
of a virtual call. Permanent virtual circuits eliminate the need for repeated call set-up and clearing
Clearing (telecommunications)
Clearing, in telecommunications means:* A sequence of events used to disconnect a call and return to the ready state. It is sometimes, particularly in the context of common channel signaling, called teardown....
.
- Frame relayFrame relayFrame Relay is a standardized wide area network technology that specifies the physical and logical link layers of digital telecommunications channels using a packet switching methodology...
is typically used to provide PVCs. - ATMAsynchronous Transfer ModeAsynchronous Transfer Mode is a standard switching technique designed to unify telecommunication and computer networks. It uses asynchronous time-division multiplexing, and it encodes data into small, fixed-sized cells. This differs from approaches such as the Internet Protocol or Ethernet that...
provides both switched virtual connections and permanent virtual connections, as they are called in ATM terminology. - X.25X.25X.25 is an ITU-T standard protocol suite for packet switched wide area network communication. An X.25 WAN consists of packet-switching exchange nodes as the networking hardware, and leased lines, Plain old telephone service connections or ISDN connections as physical links...
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
See also
- DLCI
- Flow (computer networking)Flow (computer networking)In packet switching networks, traffic flow, packet flow or network flow is a sequence of packets from a source computer to a destination, which may be another host, a multicast group, or a broadcast domain...
- Label SwitchingLabel switchingLabel switching is a technique of network relaying to overcome the problems perceived by traditional IP-table switching...