Essay/Term paper: Computer communications: bus network

Essay, term paper, research paper:  Information Technology

Computer Communications: Bus Network


Bus 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 Interface. The 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.