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 bandwidth of the channel and remains connected for the duration of the communication session. The circuit functions as if the nodes were physically connected as with an electrical circuit.

The defining example of a circuit-switched network is the early analog telephone network. When a call is made from one telephone to another, switches within the telephone exchanges create a continuous wire circuit between the two telephones, for as long as the call lasts.

Circuit switching contrasts with packet switching, which divides the data to be transmitted into packets transmitted through the network independently. In packet switching, instead of being dedicated to one communication session at a time, network links are shared by packets from multiple competing communication sessions, resulting in the loss of the quality of service guarantees that are provided by circuit switching.

In circuit switching, the bit delay is constant during a connection, as opposed to packet switching, where packet queues may cause varying and potentially indefinitely long packet transfer delays. No circuit can be degraded by competing users because it is protected from use by other callers until the circuit is released and a new connection is set up. Even if no actual communication is taking place, the channel remains reserved and protected from competing users.

Virtual circuit switching is a packet switching technology that emulates circuit switching, in the sense that the connection is established before any packets are transferred, and packets are delivered in order.

While circuit switching is commonly used for connecting voice circuits, the concept of a dedicated path persisting between two communicating parties or nodes can be extended to signal content other than voice. The advantage of using circuit switching is that it provides for continuous transfer without the overhead associated with packets, making maximal use of available bandwidth for that communication. One disadvantage is that it can be relatively inefficient, because unused capacity guaranteed to a connection cannot be used by other connections on the same network.

The call

For call setup and control (and other administrative purposes), it is possible to use a separate dedicated signalling channel from the end node to the network. ISDN is one such service that uses a separate signalling channel while plain old telephone service (POTS) does not.

The method of establishing the connection and monitoring its progress and termination through the network may also utilize a separate control channel as in the case of links between telephone exchanges which use CCS7 packet-switched signalling protocol to communicate the call setup and control information and use TDM to transport the actual circuit data.

Early telephone exchanges were a suitable example of circuit switching. The subscriber would ask the operator to connect to another subscriber, whether on the same exchange or via an inter-exchange link and another operator. In any case, the end result was a physical electrical connection between the two subscribers' telephones for the duration of the call. The copper wire used for the connection could not be used to carry other calls at the same time, even if the subscribers were in fact not talking and the line was silent.

Compared with datagram packet switching

Circuit switching contrasts with packet switching which divides the data to be transmitted into small units, called packets, transmitted through the network independently. Packet switching shares available network bandwidth between multiple communication sessions.

Multiplexing multiple telecommunications connections over the same physical conductor has been possible for a long time, but nonetheless each channel on the multiplexed link was either dedicated to one call at a time, or it was idle between calls.

In circuit switching, a route and its associated bandwidth is reserved from source to destination, making circuit switching relatively inefficient since capacity is reserved whether or not the connection is in continuous use.

In contrast, packet switching is the process of segmenting data to be transmitted into several smaller packets. Each packet is labeled with its destination and a sequence number for ordering related packets, precluding the need for a dedicated path to help the packet find its way to its destination. Each packet is dispatched independently and each may be routed via a different path. At the destination, the original message is reordered based on the packet number to reproduce the original message. As a result, datagram packet switching networks do not require a circuit to be established and allow many pairs of nodes to communicate concurrently over the same channel.

Examples of circuit-switched networks

See also

External links

Alexander G. Fraser

Alexander G. Fraser, also known as A. G. Fraser and Sandy Fraser, is a noted British-American computer scientist.

Fraser received his B.Sc. degree in Aeronautical Engineering from Bristol University in 1958, and his Ph.D. in Computing Science from Cambridge University in 1969. Between degrees he worked at Ferranti, where he was responsible for compiler development, and designed and implemented an operating system.

From 1966-1969 he was Assistant Director of Research at Cambridge, where in 1967 he designed and implemented the Titan computer's file system, and worked on file archival, privacy, and persistent names. He moved to AT&T Bell Laboratories in 1969 where he invented cell-based networks that anticipated Asynchronous Transfer Mode (ATM) and co-developed a reduced instruction set computer prototype with techniques for instruction set optimization. He subsequently became director of its Computing Science Research Center (1982), Executive Director (1987), and Associate Vice President for Information Science Research (1994). As Vice President for Research, he founded AT&T Laboratories in 1996, and in 1998 was named AT&T Chief Scientist. After his retirement in 2002 he established Fraser Research.

Fraser is a member of the National Academy of Engineering, and a Fellow of the British Computer Society and IEEE. He has received the 1989 Koji Kobayashi Computers and Communications Award "for contributions to computer communications and the invention of virtual-circuit switching", the 1992 SIGCOMM Award for "pioneering concepts, such as virtual circuit switching, space-division packet switching, and window flow control", and the 2001 IEEE Richard W. Hamming Medal "for pioneering contributions to the architecture of communication networks through the development of virtual circuit switching technology".

Best-effort delivery

Best-effort delivery describes a network service in which the network does not provide any guarantee that data is delivered or that delivery meets any quality of service. In a best-effort network, all users obtain best-effort service, meaning that they obtain unspecified variable bit rate and latency and packet loss, depending on the current traffic load. This can be contrasted with reliable delivery, which can be built on top of best-effort delivery (possibly without latency and throughput guarantees), or with circuit switching schemes which maintain a defined, continuous quality of service.


A CSU/DSU (channel service unit/data service unit) is a digital-interface device used to connect data terminal equipment (DTE), such as a router, to a digital circuit, such as a Digital Signal 1 (DS1) T1 line. The CSU/DSU implements two different functions. The channel service unit (CSU) is responsible for the connection to the telecommunication network, while the data service unit (DSU) is responsible for managing the interface with the DTE.

A CSU/DSU is the equivalent of the modem for an entire LAN.

Clos network

In the field of telecommunications, a Clos network is a kind of multistage circuit-switching network which represents a theoretical idealization of practical, multistage switching systems. It was invented by Edson Erwin in 1938 and first formalized by Charles Clos (French pronunciation: ​[ʃaʁl klo])in 1952.

By adding stages, a Clos network reduces the number of crosspoints required to compose a large crossbar switch. A Clos network topology (diagrammed below) is parameterized by three integers n, m, and r: n represents the number of sources which feed into each of r ingress stage crossbar switches; each ingress stage crossbar switch has m outlets; and there are m middle stage crossbar switches.

Circuit switching arranges a dedicated communications path for a connection between endpoints for the duration of the connection. This sacrifices total bandwidth available if the dedicated connections are poorly utilized, but makes the connection and bandwidth more predictable, and only introduces control overhead when the connections are initiated, rather than with every packet handled, as in modern packet-switched networks.

When the Clos network was first devised, the number of crosspoints was a good approximation of the total cost of the switching system. While this was important for electromechanical crossbars, it became less relevant with the advent of VLSI, wherein the interconnects could be implemented either directly in silicon, or within a relatively small cluster of boards. Upon the advent of complex data centers, with huge interconnect structures, each based on optical fiber links, Clos networks regained importance. A subtype of Clos network, the Beneš network, has also found recent application in machine learning.

Connection-oriented communication

Connection-oriented communication is a network communication mode in telecommunications and computer networking, where a communication session or a semi-permanent connection is established before any useful data can be transferred, and where a stream of data is delivered in the same order as it was sent. The alternative to connection-oriented transmission is connectionless communication, for example the datagram mode communication used by the IP and UDP protocols, where data may be delivered out of order, since different network packets are routed independently, and may be delivered over different paths.

Connection-oriented communication may be a circuit switched connection, or a packet-mode virtual circuit connection. In the latter case, it may use either a transport layer virtual circuit protocol such as the TCP protocol, allowing data to be delivered in order although the lower layer switching is connectionless, or it may be a data link layer or network layer switching mode, where all data packets belonging to the same traffic stream are delivered over the same path, and traffic flows are identified by some connection identifier rather than by complete routing information, allowing fast hardware based switching.

Connection-oriented protocol services are often, but not always, reliable network services, that provide acknowledgment after successful delivery, and automatic repeat request functions in case of missing data or detected bit-errors. ATM, Frame Relay and MPLS are examples of a connection-oriented, unreliable protocol.

Digital cross connect system

A digital cross-connect system (DCS or DXC) is a piece of circuit-switched network equipment, used in telecommunications networks, that allows lower-level TDM bit streams, such as DS0 bit streams,to be rearranged and interconnected among higher-level TDM signals, such as DS1 bit streams. DCS units are available that operate on both older T-carrier/E-carrier bit streams, as well as newer SONET/SDH bit streams.

DCS devices can be used for "grooming" telecommunications traffic, switching traffic from one circuit to another in the event of a network failure, supporting automated provisioning, and other applications. Having a DCS in a circuit-switched network provides important flexibility that can otherwise only be obtained at higher cost using manual "DSX" cross-connect patch panels.

It is important to realize that while DCS devices "switch" traffic, they are not packet switches—they switch circuits, not packets, and the circuit arrangements they are used to manage tend to persist over very long time spans, typically months or longer, as compared to packet switches, which can route every packet differently, and operate on micro- or millisecond time spans.

DCS units are also sometimes colloquially called "DACS" units, after a proprietary brand name of DCS units created and sold by AT&T's Western Electric division, now Alcatel-Lucent.

Modern digital access and cross-connect systems are not limited to the T-carrier system, and may accommodate high data rates such as those of SONET.

Dynamic synchronous transfer mode

Dynamic synchronous transfer mode (DTM) is an optical networking technology standardized by the European Telecommunications Standards Institute (ETSI) in 2001 beginning with specification ETSI ES 201 803-1. DTM is a time division multiplexing and a circuit-switching network technology that combines switching and transport. It is designed to provide a guaranteed quality of service (QoS) for streaming video services, but can be used for packet-based services as well. It is marketed for professional media networks, mobile TV networks, digital terrestrial television (DTT) networks, in content delivery networks and in consumer oriented networks, such as "triple play" networks.


The GTD-5 EAX (General Telephone Digital Number 5 Electronic Automatic Exchange) is the Class 5 telephone switch developed by GTE Automatic Electric Laboratories. This digital central office telephone circuit switching system is used in the former GTE service areas and by many smaller telecommunications service providers.

General Packet Radio Service

General Packet Radio Service (GPRS) is a packet oriented mobile data standard on the 2G and 3G cellular communication network's global system for mobile communications (GSM). GPRS was established by European Telecommunications Standards Institute (ETSI) in response to the earlier CDPD and i-mode packet-switched cellular technologies. It is now maintained by the 3rd Generation Partnership Project (3GPP).GPRS is typically sold according to the total volume of data transferred during the billing cycle, in contrast with circuit switched data, which is usually billed per minute of connection time, or sometimes by one-third minute increments. Usage above the GPRS bundled data cap may be charged per MB of data, speed limited, or disallowed.

GPRS is a best-effort service, implying variable throughput and latency that depend on the number of other users sharing the service concurrently, as opposed to circuit switching, where a certain quality of service (QoS) is guaranteed during the connection. In 2G systems, GPRS provides data rates of 56–114 kbit/sec. 2G cellular technology combined with GPRS is sometimes described as 2.5G, that is, a technology between the second (2G) and third (3G) generations of mobile telephony. It provides moderate-speed data transfer, by using unused time division multiple access (TDMA) channels in, for example, the GSM system. GPRS is integrated into GSM Release 97 and newer releases.

Network packet

A network packet is a formatted unit of data carried by a packet-switched network. A packet consists of control information and user data, which is also known as the payload. Control information provides data for delivering the payload, for example: source and destination network addresses, error detection codes, and sequencing information. Typically, control information is found in packet headers and trailers.

In packet switching, the bandwidth of the communication medium is shared between multiple communication sessions, in contrast to circuit switching, in which circuits are preallocated for the duration of one session and data is typically transmitted as a continuous bit stream.

Optical burst switching

Optical burst switching (OBS) is an optical networking technique that allows dynamic sub-wavelength switching of data. OBS is viewed as a compromise between the yet unfeasible full optical packet switching (OPS) and the mostly static optical circuit switching (OCS). It differs from these paradigms because OBS control information is sent separately in a reserved optical channel and in advance of the data payload. These control signals can then be processed electronically to allow the timely setup of an optical light path to transport the soon-to-arrive payload. This is known as delayed reservation.

Overlay network

An overlay network is a computer network that is built on top of another network.

Public switched data network

A public switched data network (PSDN) is a network for providing data services via a system of multiple wide-area networks, similar in concept to the public switched telephone network (PSTN). A PSDN may use a variety of switching technologies, including packet switching, circuit switching, and message switching. A packet-switched PSDN may also be called a packet-switched data network.Originally the term PSDN referred only to Packet Switch Stream (PSS), an X.25-based packet-switched network, mostly used to provide leased-line connections between local area networks and the Internet using permanent virtual circuits (PVCs). Today, the term may refer not only to Frame Relay and Asynchronous Transfer Mode (ATM), both providing PVCs, but also to Internet Protocol (IP), GPRS, and other packet-switching techniques.

Whilst there are several technologies that are superficially similar to the PSDN, such as Integrated Services Digital Network (ISDN) and the Digital Subscriber Line (DSL) technologies, they are not examples of it. ISDN utilizes the PSTN circuit-switched network, and DSL uses point-to-point circuit switching communications overlaid on the PSTN local loop (copper wires), usually utilized for access to a packet-switched broadband IP network.

Shared medium

In telecommunication, a shared medium is a medium or channel of information transfer that serves more than one user at the same time.Most channels only function correctly when one user is transmitting, so a channel access method is always in effect.

In circuit switching, each user typically gets a fixed share of the channel capacity. A multiplexing scheme divides up the capacity of the medium. Common multiplexing schemes include time-division multiplexing and frequency-division multiplexing. Channel access methods for circuit switching include time-division multiple access, frequency-division multiple access, etc.

In packet switching, the sharing is more dynamic — each user takes up little or none of the capacity when idle, and can utilize the entire capacity if transmitting while all other users are idle. Channel access methods for packet switching include carrier sense multiple access, token passing, etc.

Switching Control Center System

The Switching Control Center System was an operations support system developed by Bell Laboratories and deployed during the early 1970s. This computer system was first based on the PDP-11 product line from Digital Equipment Corporation and used the CB Unix operating system and custom application software and device drivers that were developed and maintained by Bell Labs in Columbus, Ohio USA. SCCS was ported to the AT&T 3B20 and 3B5 computers running UNIX System V Release 2 in the early 1980s.

Prior to the SCCS, many telephone company switching centers were staffed 24 hours a day 365 days a year. Deployment of the SCCS allowed telephone companies to significantly reduce the number of technicians by implementing a local switching control center office and dispatch technicians as required to resolve problems or perform routine maintenance operations.

During the early 1970s, telephone companies began to phase out the older electromechanical switching systems such as the Number 1 Crossbar, Number 5 Crossbar, and step-by-step circuit switching systems and replace them with newer electronic switching systems that were controlled by proprietary computers but still used analog switch fabrics such as Bell-proprietary ferreed switch devices.The SCCS system was phased out during the late 1980s and replaced by an OSS known as Network Monitoring and Analysis or NMA that was developed by Bell Communications Research now Telcordia Technologies. AT&T developed additional products based on SCCS software, such as Compulert.

The primary purpose of the SCCS system was to provide operations, administration, maintenance, and provisioning (OAMP) functions for telephone company network operations staff. The SCCS accepted as input the slow 110 baud Teletype messages from circuit switching systems such as the Number 1 ESS, Number 2 ESS, Number 3 ESS, Number 5 ESS, and Traffic Service Position System (TSPS) network switches and provide analysis, reports, troubleshooting support, and other functions using newer faster DataSpeed-40 terminals.

This system was documented in the Bell System Technical Journal and AT&T internal Bell System Practices during the 1970s.

Telecommunications network

A telecommunications network is a collection of terminal nodes in which links are connected so as to enable telecommunication between the terminals. The transmission links connect the nodes together. The nodes use circuit switching, message switching or packet switching to pass the signal through the correct links and nodes to reach the correct destination terminal.

Each terminal in the network usually has a unique address so messages or connections can be routed to the correct recipients. The collection of addresses in the network is called the address space.

Examples of telecommunications networks are:

computer networks

the Internet

the telephone network

the global Telex network

the aeronautical ACARS network

Time-driven switching

In Telecommunication and Computer networking, time-driven switching (TDS) is a node by node time variant implementation of Circuit switching, where the propagating datagram is shorter in space than the distance between source and destination. With TDS it is no longer necessary to own a complete circuit between source and destination, but only the fraction of circuit where the propagating datagram is temporarily located.

TDS adds flexibility and capacity to Circuit Switched networks but requires precise synchronization among nodes and propagating datagrams.

Datagrams are formatted according to schedules that depend on Quality of service and availability of switching nodes and physical links. Respect to Circuit switching the added time dimension introduces additional complexity to network management. Like Circuit switching, TDS operates without buffers and header processing according to the pipeline forwarding principle; therefore an all optical implementation with Optical fibers and Optical Switches is possible with low cost. The TDS concept itself pervades and is applicable with advantage to existing data switching technologies, including Packet switching, where packets, or sets of packets become the datagrams that are routed through the network.

TDS has been invented in 2002 by Prof. Mario Baldi and prof. Yoram Ofek of Synchrodyne Networks, Inc. that is the assignee of several patents issued by both the United States Patent and Trademark Office and the European Patent Office.

Virtual circuit

A virtual circuit (VC) is a means of transporting data over a packet switched computer network in such a way that it appears as though there is a dedicated physical layer link between the source and destination end systems of this data. The term virtual circuit is synonymous with virtual connection and virtual channel. Before a connection or virtual circuit may be used, it has to be established, between two or more nodes or software applications, by configuring the relevant parts of the interconnecting network. After that, a bit stream 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 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 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.Many virtual circuit protocols, but not all, provide reliable communication service through the use of data retransmissions because of error detection and automatic repeat request (ARQ).

An alternate network configuration to virtual circuit is datagram.


X.21 (sometimes referred to as X21) is an interface specification for differential communications introduced in the mid-1970s by the ITU-T. X.21 was first introduced as a means to provide a digital signaling interface for telecommunications between carriers and customers' equipment. This includes specifications for DTE/DCE physical interface elements, alignment of call control characters and error checking, elements of the call control phase for circuit switching services, and test loops.

When X.21 is used with V.11, it provides synchronous data transmission at rates from 600 bit/s to 10 Mbit/s. There is also a variant of X.21 that is only used in select legacy applications, “circuit switched X.21”. X.21 normally is found on a 15-pin D-Sub connector and is capable of running full-duplex data transmissions.

The Signal Element Timing, or clock, is provided by the carrier (your telephone company), and is responsible for correct clocking of the data. X.21 is primarily used in Europe and Japan, for example in the Scandinavian DATEX and German DATEX-L circuit switched networks during the 1980s.

Duplexing methods

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