Network Bus Network


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 performs automatic 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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