Loadstar 226/226.d81/t.networking b

Text File | 2003-01-01 | 13.0 KB | 439 lines

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          NETWORKING BASICS
          by Kevin R. Barrow


[DAVE'S PREFACE:] Kevin wrote this
paper for an English Comp class at
Lamar Junior College. It is for people
who are generally familiar with
computers, but probably know little or
nothing about networking. That is to
say, LOADSTARites! The discussion is
crisp and fast; you may need to read,
ponder, and re-read. But by the time
you are through, you will know a thing
or two about LANs, WANs, packets, and
topologies.



    There are various networks types,
topologies, and media to meet the
needs of any situation. To understand
these concepts it is important to know
that a computer network is a group of
computers joined together by
electronic means and programmed with
communication protocols. A protocol is
just a list of rules to make
communication possible.

    The two most common types of
computer networks are Local Area
Networks (LANs), and Wide Area
Networks (WANs). A LAN is a network
over a small geographical area. It may
be as small as a single workstation or
desk that has two or more networked
computers on it. However a LAN will
usually cover an entire room, floor,
building, or site.

    A WAN, on the other hand, covers a
wide geographical area, but will often
consist of only two remote points.
Dialing into an Internet Service
Provider (ISP) initiates a 'dial-up'
WAN connection. But this is just one
example of the various types of WAN
links.

    Topology, both physical and
logical, is the way the network is
setup. Physical topology is the
actual, physical configuration of the
media, i.e. how and where the cables
are run. A cable run is the path
cables follow through the 'walls' of
the building containing the network.

    The logical topology is more of an
imaginary topology -- the way the
network looks to the computers. A
computer network may use different
physical and logical topologies. For
instance, the cables may be built in a
hierarchical fashion but the computers
may use a ring or bus media access
mechanism thus having a physical bus
topology and a logical ring topology.

    The various types of network
topologies are Bus, Ring,
Hierarchical, and Mesh. Any of these
can be employed in either the physical
or the logical sense.

    A network that employs a physical
or logical ring topology is configured
so that each computer receives
packets, or network traffic, on a
unidirectional token. Think of a mail
cart moving around a track, picking up
and delivering letters. Every computer
on a ring network has two physical or
logical connections. One is for
incoming data and the other is for
outgoing data. Operating in this
manner is called simplex, because it
is 'simple'.

    This type of network would be very
similar to every member of group
passing a pencil and a notebook around
in a circle. If the token is empty a
computer may decide to fill the token,
placing data in it and passing it to
the next computer. When the packet
inside the token has reached its
destination the computer will retrieve
the data from the token, freeing it
for another computers' use by passing
on an empty token. If the token is
full or the computer has nothing to
transmit the unchanged token will be
passed on to the next computer in the
ring. A system like this works until,
for whatever reason, a node no longer
forwards the token to the next node.
Ring topology are collision free.

    Collisions occur on other types of
networks called broadcast networks. A
broadcast network is similar to a
party line or any time more than two
people are talking on the same line.
When two or more people speak
simultaneously, they talk over each
other -- creating confusion. A bus
network topology is a great example of
a broadcast network. On a broadcast
network all network nodes are
connected to a shared bi-directional
medium (wire or electromagnetic
waves). The traffic flows in both
directions on a single cable.

    Unlike on a Ring network topology,
any node on a broadcast network can
send data directly to any other node.
However this creates the potential for
collisions. Collisions occur when two
or more computers try to broadcast
their packets at the same time. This
would be like two trains headed
towards each other on the same track,
resulting in a crash when they reach
each other. Fortunately computer
engineers have created things like
Carrier Sense Multiple Access with
Collision Detect (CSMA/CD).

    CSMA/CD is a media access
mechanism were a computer wishing to
transmit first listens to the line,
and only broadcasts when it does not
detect another computer transmitting.
A broadcasting computer listens for
other computer transmissions because
of propagation delays. Propagation is
the time it takes a transmitted signal
to reach the entire length of the
media. When a collision is detected
all the devices that are transmitting
data must then cease transmission for
a random delay before attempting to
re-transmit. This ensures that the
same collision does not occur again.

    Hierarchical topology1 is similar
to a spoked wheel, with lines, or
cables radiating out from a central
node or connection point through which
all traffic must pass. An extended
hierarchical network will also have
nodes that have their own hierarchy or
spokes. This type of network is not
easy to implement because each node
has its own cable run. However,
because it does not have a single
shared cable to connect all the nodes
in a room, building or campus, this
makes the network much easier to
segment, or divide, into smaller more
manageable pieces. Most new networks
use a physical hierarchical topology
no matter what logical topology or
media access mechanism they may use.

    Mesh topology is were every node
on a network is directly connected to
every other node. This configuration
provides for multiple redundancies on
a mission critical network Mesh
topology is the most difficult network
implementation to setup because it has
so many interconnecting lines. For
this reason, it is most often only
used as a backbone or WAN topology.
Imagine a pentagram with a five-
point-star inside, each point
representing a node, each line
representing a connection. This
illustrates how mesh topology connects
all the nodes directly and indirectly
to every other node on the network. A
mesh network topology is not necessary
unless the network traffic is of such
a critical nature that a temporary
failure or delay could result in
irreparable damages, possibly even
loss of life.

    Media, the transmission means of a
computer network, is the physical
environments though which transmitted
signals pass to reach their designated
destination. All network media will
either be baseband or broadband.
Baseband media is when the entire
transmission capabilities of the media
are dedicated to carrying a single
signal; this does not mean that the
entire available frequency range of
the cable is used however.

    Broadband media is the opposite of
baseband. The transmission
capabilities of the cable are split
into different frequency ranges called
channels. The channels of broadband
media are allocated to different
signals. This may be for the purpose
of carrying signals in both directions
simultaneously, or carrying multiple
signals. A DSL uses a type of
broadband technology. It divides the
frequency transmission capabilities of
standard copper-wire phone line into
three different channels. One channel
of the DSL is dedicated to voice
communications, the other two are
allocated to bi-directional
communications between a computer or
network and the ISP.

    Several media types include --

  the "Atmosphere,"
  Unshielded/Shielded Twisted Pair
              (UTP/STP),
  Screened Twisted Pair coaxial,
  and fiber-optic cable.

    There are two coaxial cable types,
thicknet and thinnet. Thicknet is a
larger equivalent to what is used for
cable, or satellite TV. Thicknet's
primary use is as a LAN backbone, a
cable that connects network segments
which might be room-to-room,
floor-to-floor, or building- to-
building at a network site. It is more
suitable for this purpose than thinnet
because it has a longer transmission
distance.

    The smaller version of coaxial
cable called thinnet is very similar
to a TV cable. The main difference
between the cables is in the
connectors used on the ends. Thinnet
uses British Navel Connectors (BNC),
which have a quicker release than the
screw-on connectors of TV cable. Also,
they are not as prone to being pulled
off like the slip-on "quick connect"
version for the TV. These media types
are primarily used in bus and ring
topology networks.

    UTP/STP and ScTP are all basically
the same; they consist of multiple
pairs of copper wire that are twisted
together. The difference is that UTP
cable is manufactured without any
shielding from electromagnetic
interference, whereas both STP and
ScTP have some form of additional EMI
shielding. STP and ScTP are therefore
more expensive to manufacture. STP and
ScTP have additional EMI shielding and
can be used in areas were typically
Twisted Pair media could not be used
because of EMI interference. A
location where TP media might be
unacceptable would be around a
generator or radio transmission tower
because of the abnormally high EMI
emissions.

    STP cabling has a metallic shield
around each pair of copper wires and
around the whole bundle. It is then
encased in plastic. ScTP media will
only have the outer metallic shield
and plastic coating.

    UTP/STP media is now basically the
standard for in-building wiring
because the Cat5 and later
specifications are capable of at least
100 Mbps (Megabits per second)
approximately 5 MBpm (MegaBytes per
minute).

    The Cat5 specification also allows
for full-duplex operation when used in
conjunction with a network switch.
Full-duplex is where a computer can
transmit and receive data
simultaneously, as opposed to the
standard half-duplex operation where a
computer can either transmit or
receive. Cat5 and Cat5e have become
primary media implementation of new
networks since the media is much
cheaper than it used to be and allows
the creation of a hierarchical network
topology. In most cases today the
benefits of using Cat5 or other
UTP/STP cable out-weight the cost.

    Fiber-optic cables are strands of
glass or plastic carrying laser light
signals from end to end. Fiber-optic
cable is a very fast and reliable
media for signaling over long
distances when compared to most other
types of network media. Unfortunately
fiber-optic cabling is expensive and
often may require special technicians
and gear to implement. Even so it is
still a very popular backbone media.

    Fiber-optic cable can operate in
single-mode or multi-mode. The
difference between single-mode and
multi-mode is how many laser light
sources are used, and at what angle
the laser is to the media. A
Single-mode Fiber-optic cable uses one
laser light source emitted directly
into the cable end. Single-mode
Fiber-optic cable is therefore usually
thinner than its multi-mode
counterpart.

    Multi-mode Fiber-optic cable uses
different frequencies of light emitted
at slightly different angles from the
cable end, allowing the light to
bounce around inside the cable to
reach its destination.

    Wireless media networks use the
'atmosphere', Signals are transmitted
by radio or infared to their
destinations. Wireless networks are
often the easiest to setup because
they require little or no cable
installation. Unfortunately more
security risks exist with wireless
networking than any other type of
network media because anyone with the
right Network Interface Card (NIC) can
access the media and network traffic.

    The benefits of wireless operation
out-weigh the security issues for some
and wireless networks are becoming
more and more secure. All that is
necessary to implement a very basic
network are two computers with
wireless NICs Wireless communications
are relatively new to computer
networks and the specifications are
still subject to revision and
modification.

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1 Networks are classified by what
technologies they use to interconnect
the network computers and devices
also called network nodes or nodes.

2 Other types of computer networks
include the Personal Area Network
(PAN) and the Metro Area network
(MAN)

1 Depending on the technology used
this size might also be considered a
PAN

2 A wan is not limited to two end
points however, this is just the most
common configuration due largely in
part because of dial-up, DSL, ISDN,
and other similar WAN connections.

3 Other types of WAN Connections are
the Digital Subscriber Line (DSL)
Integrated Services Digital Network
(ISDN), and the T-carrier systems,
including a T-1, or fractional T-1,
and T-3.



1 Singular media, one wire or cable

1 Hierarchical topology may also be
called star or tree topology
depending on how it is implemented.

2 Extended hierarchical topology is
sometimes called daisy chain
topology.

3 When a mesh topology is used only
for a backbone or wan it is usually
considered a partial mesh topology.

1 When referring to the atmosphere as
media it is generally just called
Wireless media.

1 Megabits per second

2 Megabytes per minute

3 A switch is a device for connecting
multiple computers in a logical and
physical hierarchical topology.