| Bioinformatics |
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| INFORMATION
COMMUNICATION TECHNOLOGY (ICT) DEVELOPMENTS – ESTABLISHMENT
OF THE FIRST SWITCH BASED LOCAL AREA NETWORK AT NBRI (NBRI-LAN) |
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Introduction:
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The information
community’s insatiable appetite for timely information has led
to the development of Local Area Networks (LANs). LANs provide
a means for users to interconnect a wide range of devices together
into united geographical resource sharing systems, using a high
bandwidth communication system over relatively inexpensive transmission
medium making possible the sharing of all software and hardware
resources. By providing such access, LANs allow users to dramatically
expand their use of up-to-date information, thereby enhancing
performance with a high degree of Accuracy and timelines.
A local area network (LAN) is a communication network that provides
interconnection of a variety of data communicating devices or
computers within a small area of an office building or campus
not exceeding a few kilometers. |
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Some characteristics of a LAN
can be listed as below:
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- Station relationship: In a typical LAN, all stations
(nodes) that access the common communication facility are
peers on the network. There is generally no distinction between
primary station and secondary station.
- Message exchange: A LAN is designed to give the appearance
of supporting multiple exchanges at any given time between
various pairs of nodes, although in actual practice only a
single message can be transmitted at any given point of time.
- Transmission speed in LANs is very high, ranging
from a few Mbps to as high as 100 Mbps (1Gbps) depending on
the Ethernet/ technology used.
- Distance: A LAN is designed to support communication
over a limited geographical area, for example, within a building
or a group of related buildings in a campus. The physical
length of the network should not exceed a few kilometers.
- Transmission medium: A LAN typically uses private,
user-installed wiring as the communication medium. Wireless
communication can also be used.
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| Local area networks can be
classified according to the following criteria: |
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- Network topology
- Transmission medium
- Transmission technique
- Access protocol
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| NETWORK TOPOLOGY |
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The network topology relates to the
logical way in which stations are interconnected. The growing
complexity and scale of networks has caused some topologies to
bleed over into others. However, the topologies can be useful
starting points for describing the overall layout of a network
or network segment.
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- BUS
The bus topology uses a single common cable or link (coaxial
cable, broadcast radio frequency) to connect the nodes of
the network to one another. In the bus topology the nodes
are connected to the common media through a series of taps
located at specified distances from one another along the
common cable, but only one station can transmit on to the
common medium at any one time.
Bus topologies are voted for their simplicity, low cable lengths
and low sensitivity to node failure. However, they are sensitive
to distance and are difficult to troubleshoot. In addition,
a failure of the bus media itself due to disruption or poor
configuration causes the network to cease functioning.
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- TREE
The tree topology arranges links and nodes into distinct hierarchies
in order to allow greater control and easier troubleshooting.
Networks using tree topologies must incorporate some form
of ‘traffic control ‘, to determine when traffic is allowed
to travel up and down the branches of the tree. Similar to
a well-defined chain of command, the tree topology shields
disparate network groups from affecting each other.
The main downfall of the tree topology is its own organization.
If there is a failure on one of the branches of the tree,
every branch that fork from that point of failure becomes
unable to communicate with the rest of the network.
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- RING
The ring topology connects each node to the next one to form
a closed loop. Each station has a transmitter and a receiver,
and data is transmitted in one direction around the ring.
It is most common in the token ring and FDDI network technologies.
In this topology, nodes are connected in a definite series,
with information going from one node to the next in a predefined
order, since each station is expecting transmission from the
station before it and sending transmission only to the station
following it, ring topologies can be made to incorporate automatic
fault location and recovery procedures.
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Ring topology has decreased distance sensitivity since each station
regenerates the signal, and it is easy to implement distributed
control and checking facilities. Disadvantages with the ring topology
are susceptibility to node failures (a failed node could break
the ring), and difficulty to add or change nodes to the ring. |
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- STAR
The star topology consists of a number of individual nodes,
which communicate through a common central point. The common
central point in a star topology networks is often a concentrator
device, or hub. Many concentrators incorporate their own troubleshooting
and monitoring functions, allowing network managers to determine
faulty stations and remove them from the concentrator without
disrupting the remaining network. It requires more cable than
most other topologies, and the entire network can come down
if the hub goes bad.
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- HYBRID
Any network topology that incorporates elements of two or
more of the previously discussed topologies is a hybrid. For
example, a tree that led down to a series of buses would be
a hybrid called a ‘tree of buses’. A ring topology network
with a series of concentrators acting, as nodes on the ring
would be called a ‘ring of stars’. As networks grow to encompass
more and more of a facility’s needs, topologies tend to hybridize
in order to fulfill the individual needs of workgroups and
departments.
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TRANSMISSION MEDIUM
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The second criterion by which Local
Area Network can be classified is according to the type of transmission
medium that is used to interconnect processors (nodes). Most Local
Area Networks use some form of cable to connect the various devices
on the network, although radio transmissions can also be used.
The various types of cables being used are:
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- UNSHEILDED TWISTED PAIR
This is the ordinary twisted pair typified by conventional
telephone wiring. Its major advantages include low cost and
the fact that this type of medium has a large installation
base in many existing facilities. Its major disadvantage is
that it is typically limited to relatively low transmission
speeds (a few megabits per second). It is available under
various names, some of which are 10 BASE-T, CAT-3, 4 or 5
Ethernet wire.
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- SHIELDED MULTIPLE TWISTED PAIR
With this medium, a number of individual twisted pairs are
enclosed in a single cable with shielding. They are better
isolated from noise and other disturbances, but each twisted
pair is limited to relatively low transmission speeds. Examples
include IBM type 1 STP, type 6 STP and type 9 STP cables.
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- COAXIAL CABLE
A single central conductor is surrounded by an outer tabular
conductor. An example of coaxial cable is the cable used in
cable television system. For data communication systems, coaxial
cable is reasonable in cost and supports high transmission
speeds. Examples of this type of cables are 10 BASE-2, 10
BASE-5, BNC, thinnet/ thicknet cables, etc.
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- TWINAXIAL CABLE
This is similar to coaxial cable, but instead of a single
central conductor, two central conductors are surrounded by
an outer tabular conductor.
- FIBRE-OPTIC CABLE
Fibre-optic cable is a high performance medium constructed
of glass or plastic, which uses pulses of light as the method
of transmission. Because optical fibres do not use electrical
charges to pass data, they are free of the possibility of
interference from proximity to electrical fields. This, combined
with the extremely low rate of signal degradation and dB loss,
enables signals to traverse long distances. Fibre-optic cables
can be spread upto 50 kms distance without any amplification.
Fibre-optic links provide very large bandwidth that can support
extremely high transmission speeds.
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| TRANSMISSION TECHNIQUE |
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The third criterion for classification
of local area networks is according to the method that is used
for transmitting signals over the cable. There are basically two
methods.
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- BASEBAND
In the baseband technique of signal transmission, a digital
signal is directly applied on to the cable, and the entire
cable is used to propagate a single digital signal. Depending
on the transmission medium used, very high transmission speeds
can be achieved. An example of a base band local area network
is Ethernet, in which stations communicate using a bus-structured
network at approximately 10-mbps. The main advantage of baseband
are simple interface units and inexpensive interfaces, but
the entire cable is allocated to a single channel, which is
a disadvantage.
- BROADBAND
In broadband technique, information is transmitted over the
cable in the form of radio-frequency signals. The total bandwidth
is usually divided into a number of channels, each of which
is capable of carrying different types of information. An
example of a broadband LAN is IBM’s PCNET, where various channels
of the same physical cable can be allocated to computer data,
video signals etc.
The disadvantages of this technique are that relatively expensive
radio-frequency modems must be incorporated into the interface
units, and broadband cables may be difficult to install and
tune properly.
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