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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 peripheralsLAN 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.

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