Computer Communications

NICATIONSBus NetworkBus Network, in computer science, a topology (configuration) for a local area 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 are detected by all nodes but are accepted only by the node(s) to which they are addressed. Because a bus network relies on a common data "highway," a malfunctioning node simply ceases to communicate; it doesn't disrupt operation as it might on a ring network, in which messages are passed from one node to the next. To avoid collisions that occur when two or more nodes try to use the line at the same time, bus networks commonly rely on collision detection or Token Passing to regulate traffic.Star NetworkStar Network, in computer science, a local area network in which 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 by terminals. 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 star network is reliable in the sense that a node can fail without affecting any other node on the network. Its weakness, however, is that failure of the central computer results in a shutdown 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 area network in which devices (nodes) are connected in a closed loop, or ring. Messages in a ring network pass in one direction, from node to node. As a message travels around the ring, each node examines the destination address attached to the message. If the address is the same as the address assigned to the node, the node accepts the message; otherwise, it regenerates the signal and passes the message along to the next node in the circle. Such regeneration allows a ring network to cover larger distances than star and bus networks. It can also be designed 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 Transfer ModeATM 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 of organizationscommunicate. It will enable the management of information, integration of systems andcommunications between individuals in ways that, to some extent, haven't even been conceived yet. ATM can transmit more than 10 million cells per second,resulting in higher capacity, faster delivery and greater reliability. ATM simplifies information transfer and exchange by compartmentalizing information into uniformsegments called cells. These cells allow any type of information--from voice to video--to betransmitted over almost any type of digitized communications medium (fiber optics, copper wire,cable). This simplification can eliminate the need for redundant local and wide area networks anderadicate the bottlenecks that plague current networking systems. Eventually, global standardizationwill enable information to move from country to country, at least as fast as it now moves from officeto office, in many cases faster.Fiber Distributed Data InterfaceThe Fiber Distributed Data Interface (FDDI) modules from Bay Networks are designed forhigh-performance, high-availability connectivity in support of internetwork topologies that include: Campus or building backbone networks for lower speed LANs Interconnection of mainframes or minicomputers to peripherals LAN interconnection for workstations requiring high-performance networking FDDI is a 100-Mbps token-passing LAN that uses highly reliable fiber-optic media and performsautomatic fault recovery through dual counter-rotating rings. A primary ring supports normal datatransfer while a secondary ring allows for automatic recovery. Bay Networks FDDI supportsstandards-based translation bridging and multiprotocol routing. It is also fully compliant with ANSI,IEEE, and Internet Engineering Task Force (IETF) FDDI specifications.Bay Networks FDDI interface features a high-performance second-generation Motorola FDDI chipset in a design that provides cost-effective high-speed communication over an FDDI network. TheFDDI chip set provides expanded functionality such as transparent and translation bridging as wellas many advanced performance features. Bay Networks FDDI is available in three versions -multimode, single-mode, and hybrid. All versions support a Class A dual attachment or dual homingClass B single attachment.Bay Networks FDDI provides the performance required for the most demanding LAN backboneand high-speed interconnect applications. Forwarding performance over FDDI exceeds 165,000packets per second (pps) in the high-end BLN and BCN. An innovative High-Speed Filters optionfilters packets at wire speed, enabling microprocessor resources to remain dedicated to packetforwarding.Data Compression In GraphicsMPEGMPEG is a group of people that meet under ISO (the International Standards Organization) to generate standards for digital video (sequences of images in time) and audio compression. In particular, they define a compressed bit stream, which implicitly defines a decompressor. However, the compression algorithms are up to the individual manufacturers, and that is where proprietary advantage is obtained within the scope of a publicly available international standard. MPEG meets roughly four times a year for roughly a week each time. In between meetings, a great deal of work is done by the members, so it doesn't all happen at the meetings. The work is organized 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 and audio optimized to fit into a bandwidth (data rate) of 1.5 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 with the proper timestamping to allow synchronization of the two. They have also gotten well into MPEG phase II, whose task is to define a bitstream for video and audio coded at around 3 to 10 Mbits/s.How MPEG I worksFirst off, it starts with a relatively low resolution video sequence (possibly decimated from the original) 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 two chrominance channels (U and V) are decimated further to 176 by 120 pixels. It turn out that you can get away with a lot less resolution in those channels and not notice it, at least in "natural" (not computer generated) images. The basic scheme is to predict motion from frame to frame in the temporal direction, and then to use DCT's (discrete cosine transforms) to organize the redundancy in the spatial directions. The DCT's are done on 8x8 blocks, and the motion prediction is done in the luminance (Y) channel on 16x16 blocks. In other words, given the 16x16 block in the current frame that you are trying to code, you look for a close match to that block in a previous or future frame (there are backward prediction modes where later frames are sent first to allow interpolating between frames). The DCT coefficients (of either the actual data, or the difference between this block and the close match) are "quantized", which means that 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 can change for every "macroblock" (a macroblock is 16x16 of Y and the corresponding 8x8's in both U and V). The results of all of this, which include the DCT coefficients, the motion vectors, and the quantization parameters (and other stuff) is Huffman coded using fixed tables. The DCT coefficients have a special Huffman table that is "two-dimensional" in that one code specifies a run-length of zeros and the non-zero value that ended the run. Also, the motion vectors and the DC DCT components are DPCM (subtracted from the last one) coded.