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Computer Communications: Bus NetworkBus Network, in computer science, a topology (configuration) for a localarea network in which all nodes are connected to a main communications line(bus). On a bus network, each node monitors activity on the line. Messages aredetected by all nodes but are accepted only by the node(s) to which they areaddressed. Because a bus network relies on a common data "highway," amalfunctioning node simply ceases to communicate; it doesn't disrupt operationas it might on a ring network, in which messages are passed from one node to thenext. To avoid collisions that occur when two or more nodes try to use the lineat the same time, bus networks commonly rely on collision detection or TokenPassing to regulate traffic.Star NetworkStar Network, in computer science, alocal area network in which each device (node) is connected to a centralcomputer in a star-shaped configuration (topology); commonly, a networkconsisting of a central computer (the hub) surrounded by terminals. In a starnetwork, messages pass directly from a node to the central computer, whichhandles any further routing (as to another node) that might be necessary.
A starnetwork is reliable in the sense that a node can fail without affecting anyother node on the network. Its weakness, however, is that failure of the centralcomputer results in a shutdown of the entire network. And because each node isindividually wired to the hub, cabling costs can be high.Ring networkRingNetwork, in computer science, a local area network in which devices (nodes) areconnected in a closed loop, or ring. Messages in a ring network pass in onedirection, from node to node. As a message travels around the ring, each nodeexamines the destination address attached to the message. If the address is thesame as the address assigned to the node, the node accepts the message;otherwise, it regenerates the signal and passes the message along to the nextnode in the circle. Such regeneration allows a ring network to cover largerdistances than star and bus networks.
It can also be designed to bypass anymalfunctioning or failed node. Because of the closed loop, however, new nodescan be difficult to add. A ring network is diagrammed below.Asynchrous TransferModeATM is a new networking technology standard for high-speed, high-capacityvoice, data, text andvideo transmission that will soon transform the waybusinesses and all types of organizationscommunicate. It will enable themanagement of information, integration of systems andcommunications betweenindividuals in ways that, to some extent, haven't even been conceived yet. ATMcan transmit more than 10 million cells per second,resulting in higher capacity,faster delivery and greater reliability. ATM simplifies information transferand exchange by compartmentalizing information into uniformsegments called cells.These cells allow any type of information--from voice to video--to betransmittedover almost any type of digitized communications medium (fiber optics, copperwire,cable). This simplification can eliminate the need for redundant local andwide area networks anderadicate the bottlenecks that plague current networkingsystems.
Eventually, global standardizationwill enable information to move fromcountry to country, at least as fast as it now moves from officeto office, inmany cases faster.Fiber Distributed Data Interface. The Fiber Distributed DataInterface (FDDI) modules from Bay Networks are designed forhigh-performance,high-availability connectivity in support of internetwork topologies thatinclude: Campus or building backbone networks for lower speed LANsInterconnection of mainframes or minicomputers to peripherals LANinterconnection for workstations requiring high-performance networking FDDI is a100-Mbps token-passing LAN that uses highly reliable fiber-optic media andperformsautomatic fault recovery through dual counter-rotating rings. A primaryring supports normal datatransfer while a secondary ring allows for automaticrecovery. Bay Networks FDDI supportsstandards-based translation bridging andmultiprotocol routing. It is also fully compliant with ANSI,IEEE, and InternetEngineering Task Force (IETF) FDDI specifications.Bay Networks FDDI interfacefeatures a high-performance second-generation Motorola FDDI chipset in a designthat provides cost-effective high-speed communication over an FDDI network.TheFDDI chip set provides expanded functionality such as transparent andtranslation bridging as wellas many advanced performance features. Bay NetworksFDDI is available in three versions -multimode, single-mode, and hybrid.
Allversions support a Class A dual attachment or dual homingClass B singleattachment.Bay Networks FDDI provides the performance required for the mostdemanding LAN backboneand high-speed interconnect applications. Forwardingperformance over FDDI exceeds 165,000packets per second (pps) in the high-endBLN and BCN. An innovative High-Speed Filters optionfilters packets at wirespeed, enabling microprocessor resources to remain dedicated topacketforwarding.Data Compression In GraphicsMPEGMPEG is a group of people thatmeet under ISO (the International Standards Organization) to generate standardsfor digital video (sequences of images in time) and audio compression. Inparticular, they define a compressed bit stream, which implicitly defines adecompressor. However, the compression algorithms are up to the individualmanufacturers, and that is where proprietary advantage is obtained within thescope of a publicly available international standard. MPEG meets roughly fourtimes a year for roughly a week each time. In between meetings, a great deal ofwork is done by the members, so it doesn't all happen at the meetings.
The workis organized and planned at the meetings. So far (as of January 1996), MPEG havecompleted the 'Standard of MPEG phase called MPEG I. This defines a bit streamfor compressed video and audio optimized to fit into a bandwidth (data rate) of1.5 Mbits/s. This rate is special because it is the data rate of (uncompressed)audio CD's and DAT's. The standard is in three parts, video, audio, and systems,where the last part gives the integration of the audio and video streams withthe proper timestamping to allow synchronization of the two. They have alsogotten well into MPEG phase II, whose task is to define a bitstream for videoand audio coded at around 3 to 10 Mbits/s.How MPEG I worksFirst off, it startswith a relatively low resolution video sequence (possibly decimated from theoriginal) of about 352 by 240 frames by 30 frames/s, but original high (CD)quality audio. The images are in color, but converted to YUV space, and the twochrominance channels (U and V) are decimated further to 176 by 120 pixels. Itturn out that you can get away with a lot less resolution in those channels andnot notice it, at least in 'natural' (not computer generated) images.
The basicscheme is to predict motion from frame to frame in the temporal direction, andthen to use DCT's (discrete cosine transforms) to organize the redundancy in thespatial directions. The DCT's are done on 8x8 blocks, and the motion predictionis done in the luminance (Y) channel on 16x16 blocks. In other words, given the16x16 block in the current frame that you are trying to code, you look for aclose match to that block in a previous or future frame (there are backwardprediction modes where later frames are sent first to allow interpolatingbetween frames). The DCT coefficients (of either the actual data, or thedifference between this block and the close match) are 'quantized', which meansthat you divide them by some value to drop bits off the bottom end. Hopefully,many of the coefficients will then end up being zero. The quantization canchange for every 'macroblock' (a macroblock is 16x16 of Y and the corresponding8x8's in both U and V). The results of all of this, which include the DCTcoefficients, the motion vectors, and the quantization parameters (and otherstuff) is Huffman coded using fixed tables. The DCT coefficients have a specialHuffman table that is 'two-dimensional' in that one code specifies a run-lengthof zeros and the non-zero value that ended the run.
Also, the motion vectorsand the DC DCT components are DPCM (subtracted from the last one) coded..
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