Computer Communication Review – Sigcomm
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computer Communications
Article by iTech Troubleshooter
NICATIONSBus 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 busnetwork, each node monitors activity on the line. Messages are detected by all nodes but areaccepted only by the node(s) to which they are addressed. Because a bus network relies on acommon data “highway,” a malfunctioning node simply ceases to communicate; it doesn’tdisrupt operation as it might on a ring network, in which messages are passed from one nodeto the next. To avoid collisions that occur when two or more nodes try to use the line atthe same time, bus networks commonly rely on collision detection or Token Passing toregulate traffic.Star NetworkStar Network, in computer science, a local area network inwhich each device (node) is connected to a central computer in a star-shaped configuration(topology); commonly, a network consisting of a central computer (the hub) surrounded byterminals. In a star network, messages pass directly from a node to the central computer,which handles 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 any other node onthe network. Its weakness, however, is that failure of the central computer results in ashutdown of the entire network. And because each node is individually wired to the hub,cabling costs can be high.Ring networkRing Network, in computer science, a local areanetwork in which devices (nodes) are connected in a closed loop, or ring. Messages in a ringnetwork pass in one direction, from node to node. As a message travels around the ring, eachnode examines the destination address attached to the message. If the address is the same asthe address assigned to the node, the node accepts the message; otherwise, it regeneratesthe signal and passes the message along to the next node in the circle. Such regenerationallows a ring network to cover larger distances than star and bus networks. It can also bedesigned to bypass any malfunctioning or failed node. Because of the closed loop, however,new nodes can be difficult to add. A ring network is diagrammed below.Asynchrous TransferModeATM is a new networking technology standard for high-speed, high-capacity voice, data,text andvideo transmission that will soon transform the way businesses and all types oforganizationscommunicate. It will enable the management of information, integration ofsystems andcommunications between individuals in ways that, to some extent, haven’t evenbeen conceived yet. ATM can transmit more than 10 million cells per second,resulting inhigher capacity, faster delivery and greater reliability. ATM simplifies informationtransfer and exchange by compartmentalizing information into uniformsegments called cells.These cells allow any type of information–from voice to video–to betransmitted over almostany type of digitized communications medium (fiber optics, copper wire,cable). Thissimplification can eliminate the need for redundant local and wide area networksanderadicate the bottlenecks that plague current networking systems. Eventually, globalstandardizationwill enable information to move from country to country, at least as fast asit now moves from officeto office, in many cases faster.Fiber Distributed Data InterfaceTheFiber Distributed Data Interface (FDDI) modules from Bay Networks are designedforhigh-performance, high-availability connectivity in support of internetwork topologiesthat include: Campus or building backbone networks for lower speed LANs Interconnection of mainframes or minicomputers to peripherals LAN interconnection forworkstations requiring high-performance networking FDDI is a 100-Mbps token-passing LAN thatuses highly reliable fiber-optic media and performsautomatic fault recovery through dualcounter-rotating rings. A primary ring supports normal datatransfer while a secondary ringallows for automatic recovery. Bay Networks FDDI supportsstandards-based translationbridging and multiprotocol routing. It is also fully compliant with ANSI,IEEE, and InternetEngineering Task Force (IETF) FDDI specifications.Bay Networks FDDI interface features ahigh-performance second-generation Motorola FDDI chipset in a design that providescost-effective high-speed communication over an FDDI network. TheFDDI chip set providesexpanded functionality such as transparent and translation bridging as wellas many advancedperformance features. Bay Networks FDDI is available in three versions -multimode,single-mode, and hybrid. All versions support a Class A dual attachment or dual homingClassB single attachment.Bay Networks FDDI provides the performance required for the mostdemanding LAN backboneand high-speed interconnect applications. Forwarding performance overFDDI exceeds 165,000packets per second (pps) in the high-end BLN and BCN. An innovativeHigh-Speed Filters optionfilters packets at wire speed, enabling microprocessor resources toremain dedicated to packetforwarding.Data Compression In GraphicsMPEGMPEG is a group ofpeople that meet under ISO (the International Standards Organization) to generate standardsfor digital video (sequences of images in time) and audio compression. In particular, theydefine a compressed bit stream, which implicitly defines a decompressor. However, thecompression algorithms are up to the individual manufacturers, and that is where proprietaryadvantage is obtained within the scope of a publicly available international standard. MPEGmeets roughly four times a year for roughly a week each time. In between meetings, a greatdeal of work is done by the members, so it doesn’t all happen at the meetings. The work isorganized and planned at the meetings. So far (as of January 1996), MPEG have completed the”Standard of MPEG phase called MPEG I. This defines a bit stream for compressed video andaudio optimized to fit into a bandwidth (data rate) of 1.5 Mbits/s. This rate is specialbecause it is the data rate of (uncompressed) audio CD’s and DAT’s. The standard is in threeparts, video, audio, and systems, where the last part gives the integration of the audio andvideo streams with the proper timestamping to allow synchronization of the two. They havealso gotten well into MPEG phase II, whose task is to define a bitstream for video and audiocoded at around 3 to 10 Mbits/s.How MPEG I worksFirst off, it starts with a relatively lowresolution video sequence (possibly decimated from the original) of about 352 by 240 framesby 30 frames/s, but original high (CD) quality audio. The images are in color, butconverted to YUV space, and the two chrominance channels (U and V) are decimated further to176 by 120 pixels. It turn out that you can get away with a lot less resolution in thosechannels and not notice it, at least in “natural” (not computer generated) images. Thebasic scheme is to predict motion from frame to frame in the temporal direction, and then touse DCT’s (discrete cosine transforms) to organize the redundancy in the spatial directions.The DCT’s are done on 8×8 blocks, and the motion prediction is done in the luminance (Y)channel on 16×16 blocks. In other words, given the 16×16 block in the current frame thatyou are trying to code, you look for a close match to that block in a previous or futureframe (there are backward prediction modes where later frames are sent first to allowinterpolating between frames). The DCT coefficients (of either the actual data, or thedifference between this block and the close match) are “quantized”, which means that youdivide them by some value to drop bits off the bottom end. Hopefully, many of thecoefficients will then end up being zero. The quantization can change for every”macroblock” (a macroblock is 16×16 of Y and the corresponding 8×8′s in both U and V). Theresults of all of this, which include the DCT coefficients, the motion vectors, and thequantization parameters (and other stuff) is Huffman coded using fixed tables. The DCTcoefficients have a special Huffman table that is “two-dimensional” in that one codespecifies a run-length of zeros and the non-zero value that ended the run. Also, the motionvectors and the DC DCT components are DPCM (subtracted from the last one) coded.
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