Circuit switching

Circuit switching is a method 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 duration of the communication session; the circuit functions. Circuit switching contrasts with message packet switching; 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. In circuit switching, the bit delay is constant during a connection. 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. If no actual communication is taking place, the channel remains reserved and protected from competing users.

While circuit switching is 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 inefficient, because unused capacity guaranteed to a connection cannot be used by other connections on the same network. In addition, calls will be dropped if the circuit is broken. For call setup and control, 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 does not; the method of establishing the connection and monitoring its progress and termination through the network may 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; the end result was a physical electrical connection between the 2 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 if the subscribers were in fact not talking and the line was silent. In circuit switching, a route and its associated bandwidth is reserved from source to destination, making circuit switching inefficient since capacity is reserved whether or not the connection is in continuous use. Circuit switching contrasts with message packet switching. Both of these methods can make better use of available network bandwidth between multiple communication sessions under typical conditions in data communication networks. Message switching routes messages in their entirety, one hop at a time, that is, store and forward of the entire message.

Packet switching divides the data to be transmitted into packets transmitted through the network independently. 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. Packet switching can be based on connection-oriented communication or connection-less communication; that is, based on virtual datagrams. Virtual circuits use packet switching technology that emulates circuit switching, in the sense that the connection is established before any packets are transferred, packets are delivered in order. Connection-less packet switching divides the data to be transmitted into packets, called datagrams, transmitted through the network independently; each datagram 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 datagram 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. Multiplexing multiple telecommunications connections over the same physical conductor has been possible for a long time, but each channel on the multiplexed link was either dedicated to one call at a time, or it was idle between calls. Public switched telephone network B channel of ISDN Circuit Switched Data and High-Speed Circuit-Switched Data service in cellular systems such as GSM Datakit X.21 Optical mesh network Clos network Switching circuit theory Time-driven switching Netheads vs Bellheads by Steve Steinberg University of Virginia RFC 3439 Some Internet Architectural Guidelines and Philopsophy

Master Bertram

Master Bertram known as Meister Bertram and Master of Minden, was a German International Gothic painter of religious art. Bertram was born in Minden, he is first recorded in Hamburg in 1367, lived there for the rest of his life, becoming a citizen and Master in 1376, achieving considerable prosperity. In 1390 he made a will in advance of an intended pilgrimage to Rome, but if he made the journey it had no detectable influence on his art, he was married. He died in Hamburg, his most famous surviving work is the large Grabow Altarpiece in the Kunsthalle Hamburg, the largest and most important North German painting of the period. There is a 45-scene altarpiece of the Apocalypse by his workshop, in the Victoria and Albert Museum in London. He, or his workshop produced sculpture in wood. A sculpture group depicting Saint Christopher by his hand is located in Sweden, his style is less emotional than that of his Hamburg near-contemporary Master Francke, but has great charm. Bertram was forgotten after the Renaissance until the end of the 19th century when, like Master Francke, he was rediscovered and published by Alfred Lichtwark, director of the Hamburg Kunsthalle.

Dube, Elizabeth Healy: "The Grabow Altar of Master Bertram von Minden", Brown Univ. Diss. 1982 Asteroid 85320 Bertram, discovered by German astronomer Freimut Börngen in 1995, was named after Master Bertram. The official naming citation was published by the Minor Planet Center on 18 September 2005. List of German painters

Ippolito Baccusi

Ippolito Baccusi was an Italian composer of the late Renaissance, active in northern Italy, including Venice and Verona. A member of the Venetian School of composers, he had a strong reputation as a master of counterpoint, wrote both sacred and secular vocal music. Little is known of his life other than the details of a few appointments, what can be inferred from the dedications he wrote for his publications, he was born in Mantua. Sometime in the late 1560s he was appointed assistant director of the choir at San Marco in Venice, but he did not hold the position for long, going to Ravenna to study. In 1572 he was maestro di cappella at the church of Sant'Eufemia in Verona, where he may have been associated with the Veronese Accademia Filarmonica. By 1583 he held the maestro di cappella position at Mantua Cathedral, where among other things he taught counterpoint to Lodovico Zacconi, who mentioned him glowingly in his Prattica de musica seconda parte of 1622 for his contrapuntal skill. In 1592 Baccusi accepted the position of maestro di cappella at Verona Cathedral, where he remained for the rest of his life.

Baccusi's music is in the Venetian style, influenced by Adrian Willaert, Giaches de Wert, Cipriano de Rore and Andrea Gabrieli. He was a prolific composer, his works published in Venice, include six books of masses, six books of motets and psalm settings, seven books of madrigals, including a complete setting as a madrigal cycle of Petrarch's 11-stanza Vergine. He composed five- and six-voice settings of poems celebrating the Venetian victory over the Turks at Lepanto. In the introductions to his 1596 and 1597 publications of masses and motets he mentioned the practice of instrumental doubling of vocal parts, something associated with the Venetian School. Alfred Einstein, The Italian Madrigal. Three volumes. Princeton, New Jersey, Princeton University Press, 1949. ISBN 0-691-09112-9 Gustave Reese, Music in the Renaissance. New York, W. W. Norton & Co. 1954. ISBN 0-393-09530-4 Iain Fenlon and Patronage in Sixteenth-Century Mantua. Cambridge University Press, Oct 30, 2008. ISBN 0-521-08833-X