2.1.2 GUIDED MEDIA
2.1.2.1 The twister pair
Twisted pair cabling is a type of wiring in which two
conductors; the forward and return conductors of a single circuit are twisted
together for the purposes of canceling out electromagnetic interference from
external sources.
Generally several pairs are gathered under an envelope called
sheath to form a cable, The signals transmitted in this type of medium can
according to their characteristics traverse several tens of kilometers without
amplification or regeneration, Beyond these distances rearrangement of the
numerical signals in baseband and the amplification of the analogical signals
in modulation are necessary.[3]
There are two types of twister pair cables such as :
> UTP (unshielded twisted pair) is most commonly used in LAN's
with maximum cable length of 100m.
> STP (shielded twisted pair) is extensively used for
telephones
2.1.2.2 Coaxial cable
Another current medium of transmission is the coaxial cable,
it has better protection than the twisted pair which enables him to offer flow
raised on long distances, Coaxial cable is the kind of copper cable used by
cable TV companies between the community antenna and user homes and businesses,
is also sometimes used by telephone companies from their central office to the
telephone poles near users.
A coaxial cable is composed by:
Copper wire: the copper wire in the middle.
Insulation: isolates the copper wire and the external copper mesh
wire.
Copper mesh: It is like a pipe or tube of copper that surrounds
the insulator which in turn surrounds the copper wire.
Outside insulation or jacket: protects the copper mesh tubular
shield.
Figure 1: Coaxial cable
The coaxial cable offers at the same time a broad bandwidth
and an excellent immunity against the noise, his bandwidth depends on the
quality of the cable, its length and the signal to noise ratio, this type of
cable was largely employed within the telephone system on the trunk lines but
it is now replaced by optical fiber, especially on the long distance. It
however still very much used for the cable television and on the underground
railway networks.
The major disadvantages of copper are a strong attenuation and
a speed transmission relatively low which limits the maximum distance between
two stations or two apparatuses of interconnection
2.1.2.3 Waveguide
Waveguides are metallic transmission lines that are used at
microwave frequencies, typically to interconnect transmitters and receivers
We distinguish several forms such as:
> Rigid Guide with rectangular section
> Guide with circular section
> Flexible semi Guide with elliptic section etc
Waveguide has a number of advantages over coax, it is
completely shielded and excellent isolation between adjacent signals can be
obtained, it can transmit extremely high peak powers and it has very low loss
often almost negligible at microwave frequencies.[4]
One disadvantage of waveguide is its high cost, Manufacturing
volumes are low and waveguide materials such as copper and silver are
relatively expensive other disadvantages include unwieldy size and mass
particularly at lower frequencies.
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