HPAVC/HPAVC CD-ROM.iso/pc/HOMEWORK.ZIP/CPUCOMM.TXT

Text File | 1998-07-25 | 8.3 KB | 95 lines

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 ôhighway,ö 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.