A virtual circuit is a means of transporting data over a packet-switched network in such a way that it appears as though there is a dedicated physical link between the source and destination end systems of this data. The term virtual circuit is synonymous with virtual connection. A virtual channel is a type of virtual circuit. 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 or byte stream may be delivered between the nodes. Virtual circuit communication resembles circuit switching, since both are connection oriented, meaning that in both cases data is delivered in correct order, 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. An alternate network configuration to virtual circuit is datagram. Connection oriented transport layer 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, thus be delivered out of order. However, it is possible to use TCP as a virtual circuit, since TCP includes segment numbering that allows reordering on the receiver side to accommodate out-of-order delivery. Datalink 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 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 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 be implemented in ASIC hardware, while routing is more complex and requires software implementation. However, because of the large market of IP routers, because advanced IP routers support layer 3 switching, modern IP routers may today be faster than switches for connection oriented protocols.

Examples of transport layer protocols that provide a virtual circuit: Transmission Control Protocol, 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, where a virtual circuit is established on top of the IP protocol. Examples of network layer and datalink 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. 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, where the circuit is identified by a virtual path identifier and virtual channel identifier pair; the ATM layer provides unreliable virtual circuits, but the ATM protocol provides for reliability through the ATM adaptation layer Service Specific Convergence Sublayer.

General Packet Radio Service Multiprotocol label switching, which can be used for IP over virtual circuits. Each circuit is identified by a label. MPLS provides eight different QoS classes. Switched virtual circuits are set up on a per-call basis and are disconnected when the call is terminated. PVC configuration is preconfigured by the service provider. Unlike SVCs, PVC are very broken/disconnected. A switched virtual circuit is a virtual circuit, 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 endpoints. A permanent virtual circuit 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 nee

Vasily Vasilyevich Glagolev was a Red Army Colonel general, Hero of the Soviet Union, commander of the Soviet airborne. After serving in the Imperial Russian Army during World War I, Glagolev joined the Red Army in 1918, he rose to command the 42nd Cavalry Division on the Crimean Front in World War II, going on to command the 73rd and 176th Rifle Divisions as well as the 10th Guards Rifle Corps. Glagolev became the commander of the 9th Army in February 1943 before being transferred to command of the 46th Army, which he would lead until May 1944, he led it during the Vitebsk -- Orsha Offensive. In January 1945, Glagolev commanded the 9th Guards Army, composed of Soviet airborne divisions converted into infantry. In April 1946, he became the commander of the Soviet airborne forces and died on in 1947 during exercises. Vasily Glagolev was born on 21 February 1896 in Kaluga, his father died when Glagolev was still young. He graduated from a technical school in Kaluga. In March 1916, he joined the Imperial Russian Army.

He became a senior intelligence non-commissioned officer and gunner in the 1st Siberian Artillery Brigade of the 10th Army on the Western Front. In February 1918, Glagolev was demobilized, after. In August 1918, Glagolev joined the Red Army, he fought in the 1st Cavalry 3rd Cavalry Regiment of the Kaluga-Moscow Rifle Division. From May 1919, Glagolev fought against elements of the Ural Cossacks and the Orenburg Cossacks, but soon became sick and returned to Kaluga for treatment. From October 1919 to March 1920, he served in the 140th Internal Security Battalion, but was ill again. In June 1920, he became a sergeant in the 1st Reserve Cavalry Regiment and 68th Cavalry Regiment of the 12th Cavalry Division, fighting in the North Caucasus. In 1921, Glagolev graduated from the 3rd Baku Command Courses. Between 1921 and 1924, he was a platoon commander, deputy squadron commander and intelligence chief of the 68th Cavalry Regiment of the 12th Cavalry Division, he commanded a squadron in the same regiment and transferred to the 68th Cavalry Regiment.

Glagolev commanded a squadron of the 2nd Separate Cavalry Brigade from December 1924. In 1925, Glagolev joined the Communist Party of the Soviet Union. In 1926, he graduated from the Novocherkassk cavalry commanders refresher course. In June 1931, he became the head of cavalry tactics and the Novocherkassk cavalry commanders refresher course. In January 1934, Glagolev became the commissar of the 76th Cavalry Regiment. In July 1937, he became the divisional chief of staff. In August 1939, he commanded 42nd Cavalry Division. In 1941, Glagolev graduated from the higher academic courses at the Frunze Military Academy. In January 1942, the 42nd Cavalry Division was transferred to the Crimean Front. Glagolev became the commander of the 73rd Rifle Division in February 1942, which he led during the Battle of Voronezh. In July, the division was encircled near Millerovo but broke out in August while suffering heavy losses. After the division's disbandment in September, Glagolev became the commander of the 176th Rifle Division in October.

He led the division during the Nalckik-Ordzhonikidze Defensive Operation during the Battle of the Caucasus. In November, he became the commander of the 10th Guards Rifle Corps, which Glagolev led until February 1943. For his leadership in the Caucasus, Glagolev was awarded the Order of the Red Banner on 13 December 1942. On 27 January 1943, Glagolev was promoted to Major general and in February became the commander of the 9th Army. In March, he was transferred to command the 46th Army, which he led during the Donbass Strategic Offensive. In September, the army fought in the Battle of the Dnieper. Between 25 and 29 September, the 46th Army crossed the Dnieper, seizing a bridgehead near the village of Aula in Dnipropetrovsk Oblast. After holding the bridgehead against German counterattacks, the army, as part of the offensive, captured Dnipropetrovsk. For his leadership during the Battle of the Dnieper, Glagolev was awarded the title Hero of the Soviet Union and the Order of Lenin on 1 November, he continued to lead the army during the Nikopol–Krivoi Rog Offensive of January 1944, the Bereznegovatoye–Snigirevka Offensive in March and the Odessa Offensive.

In May 1944, Glagolev was transferred to command the 31st Army. He led the army during its Vitebsk -- Orsha and Minsk Offensives. In October, the army fought in the Gumbinnen Operation. In January 1945, Glagolev became the commander of the 9th Guards Army, composed of airborne divisions converted to infantry; the army advanced into Hungary as part of the 2nd Ukrainian Front and the 3rd Ukrainian Front. It fought in Vienna Offensive and the Prague Offensive. After the end of World War II, Glagolev continued to command the 9th Guards Army, now part of the Central Group of Forces. In April 1946, he became the commander of the Soviet airborne and served as a deputy at the 2nd congress of the Supreme Soviet of the Soviet Union in the same year. Glagolev was buried in Novodevichy Cemetery. Glagolev received the following honors and awards

The year 1778 in science and technology involved some significant events. Lagrange delivers his treatise on cometary perturbations to the Académie française. Molybdenum discovered by Carl Wilhelm Scheele. Antoine Lavoisier, considered "The father of modern chemistry", recognizes and names oxygen, recognizes its importance and role in combustion. January 18 – On his third voyage, Captain James Cook, with ships HMS Resolution and HMS Discovery, becomes the first European to view the Hawaiian Islands in the Pacific Ocean. March 6 – October 24 – Captain Cook explores and maps the Pacific Northwest coast of North America from Cape Foulweather to the Bering Strait. James Rennell publishes a chart and memoir of the Agulhas Current, one of the first contributions to scientific oceanography. John Hunter publishes The Natural History of the Human Teeth. Samuel-Auguste Tissot begins publication of Traité des nerfs et de leurs maladies, including a classical account of migraine. Samuel Thomas von Sömmerring describes the organization of the cranial nerves.

Joseph Bramah patents an improved design of flush toilet in London. The brothers Hans Ulrich and Johannes Grubenmann complete a bridge across the Limmat at Wettingen in Switzerland, a 60 m span, the first known use of a true arch in a timber bridge. Petrus Camper publishes On the Points of Similarity between the Human Species, Quadrupeds and Fish. Johan Christian Fabricius publishes his Philosophia Entomologica in Hamburg. Copley Medal: Charles Hutton February 4 – A. P. de Candolle, Swiss botanist May 18 – Andrew Ure, Scottish industrial chemist and encyclopaedist December 6 – Joseph Louis Gay-Lussac, French chemist and physicist December 17 – Humphry Davy, English chemist December 25 – Joseph Aspdin, English inventor Maria Dalle Donne, Bolognese physician Anna Maria Walker, Scottish botanist January 10 – Carl Linnaeus, Swedish botanist, first to develop standard nomenclature for naming species February 20 – Laura Bassi, Italian scientist