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Project of
Data Communication Teacher: Mr. Waqar BS V G II (E) |
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Project Name |
Fundamentals of
Transmission system |
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Project Description |
In this
project we are going to discuss about the application of transmission system. |
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Group Members |
Shahab
Ahmed Farooqui 1522-299-047 |
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Muhammed
Farhan 1522-200-063 |
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Bashrat
Ur Rehman 1522-299-056 |
Fundamentals of Transmission System
Information
is of considerably increased value if it can be conveyed clearly to others.
This is a basic principle that is well understood in this information age. The
conveyance, or transmission, of information across a distance necessarily
involves some form of transmission medium. The selection of physical
transmission media, which serve to transport that information, is critical to
its successful conveyance. In interactive communication, the medium can be
critical to the message.
The
project commonly used in traditional voice, data, video, and image networks,
whether analog or digital in nature. Those media can be grouped into two distinctive
categories, the first of which includes all wired media, also referred to as
conducted, guided, or bounded media. The second category includes all
traditional wireless media, also referred to as radiated, unguided, or
unbounded.
Wired
transmission systems employ physical media which are tangible. In other words,
they can be seen, felt and perhaps even smelled and tasted (the last two
options are not recommended, as the results might well be unpleasant). Also
known as conducted systems, wired media generally employ a metallic or glass
conductor which serves to conduct, or carry on, some form of electromagnetic
energy. For example, twisted pair and coaxial cable systems conduct electrical
energy, employing a copper medium; fiber optic systems conduct light, or
optical, energy, generally using a glass conductor. The term guided media
refers to the fact that the signal is contained within an enclosed physical
path. Finally, bounded media refers to the fact that some form of shield,
cladding, and/or insulation is employed to bind the signal within the core
medium, thereby improving signal strength over a distance and enhancing the
performance of the transmission system in the process. Twisted pair (both
unshielded and shielded), coaxial and fiber optic cable systems fall into this
category.
Selection Criteria
The
selection of the most effective transmission system for a given application
must be made in the context of a number of key design considerations. Such
considerations include general transmission characteristics such as bandwidth
and error performance, both of which affect throughput. Additionally, one must
consider the allowable distance between devices, as well as issues of
propagation delay, security, mechanical strength, and physical dimensions. Finally,
and perhaps most important of all, are issues of local availability and cost,
including cost of acquisition, deployment, operation and maintenance, and
upgrade or replacement.
Transmission Characteristics
The
basic transmission characteristics of a given medium are of primary importance.
Those characteristics include bandwidth, or capacity, error performance, and
distance between network elements. These three dimensions of a transmission
system, in combination, dictate effective throughput, the amount of information
that can be put through the system.
Bandwidth,
in this context, refers to the raw amount of bandwidth the medium supports.
Error performance refers to the number or percentage of errors which are
introduced in the process of transmission. Distance refers to the minimum and
maximum spatial separation between devices over a link, in the context of a
complete, end-to-end circuit. Clearly, any given transmission system increases
in attractiveness to the extent that available bandwidth is greater, introduced
errors are fewer, and the maximum distance between various network elements
(e.g., amplifiers, repeaters, and antennae) is greater.
It
should be noted that bandwidth, error performance, and distance are tightly
interrelated. In a twisted pair network, for example, more raw bandwidth
requires higher transmission frequencies. High
Propagation Delay and
Response Time
Propagation
delay refers to the length of time required for a signal to travel from
transmitter to receiver across a transmission system. While electromagnetic
energy travels at roughly the speed of light (186,000 miles per second) the
nature of the transmission system impact the level of propagation delay to a
considerable extent. In other words, the total length of the circuit directly
impacts the length of time it takes for the signal to reach the receiver. That
circuit length can vary considerably in a switched network, as the specific
circuit route will vary in length from call to call, depending on availability
of individual links and switches. Dedicated networks offer the advantage of a
reliable and consistent level of propagation delay. In either case, the level
of delay is affected by the number of network elements (devices) in the
network, as each device (e.g., amplifier, repeater, and switch) acts on the
signal to perform certain processes, each of which takes at least a small
amount of time. Clearly, the fewer devices involved in a network, the less
delay imposed on the signal.
Security
Security,
in the context of transmission systems, addresses the protection of data from
interception as it transverses the network. Clearly, increasing amounts of
sensitive data are being transmitted across wide and metropolitan area
networks, outside the protection of one’s own premises. Therefore, security is
of greater concern than ever before and will heighten as nations and commercial
enterprises seek to gain competitive advantage and as they apply ever more
sophisticated means to do so. In hearings (May 1996) before the United States
Senate, it was stated that 120 nations either have or are in the process of
developing sophisticated computer espionage capabilities.
Mechanical Strength
Mechanical
strength applies most especially to wired systems. Twisted pair, coaxial, and
fiber optic cables are manipulated physically as they are deployed and
reconfigured. Clearly, each has certain physical limits to the amount of
bending and twisting (flex strength) they can tolerate, as well as the amount
of weight or longitudinal stress they can support (tensile strength), without
breaking (break strength). Fiber optic cables are notoriously susceptible in
this regard. Cables hung from poles expand and contract with changes in ambient
temperature; while glass fiber optic cables expand and contract relatively
little, twisted pair copper wire is more expansive
Physical Dimensions
The
physical dimensions of a transmission system must be considered as well. This
is especially true, once again, in the case of wired systems. Certainly, the
sheer weight of a cable system must be considered as one attempts to deploy it
effectively. Additionally, the bulk (diameter) of the cable is of importance,
as conduit and raceway space often is at a premium. The physical dimensions of
airwave systems also must be considered, as the size and weight of the
reflective dish and mounting system (e.g., bracket and tower) may require
support.
Twisted Pair
Metallic wires were used almost exclusively in
telecommunications networks for the first 80 years, certainly until the development
of microwave and satellite radio communications systems. Initially, uninsulated
iron telegraph wires were used, although copper was soon found to be a much
more appropriate medium. The early metallic electrical circuits were one-wire,
supporting two-way communications with each telephone connected to ground in
order to complete the circuit. In 1881, John J. Carty, a young American Bell
technician and one of the original operators, suggested the use of a second
wire to complete the circuit and, thereby, to avoid the emanation of electrical
noise from the earth ground. In certain contemporary applications,
copper-covered steel, copper alloy, nickel- and/or gold-plated copper, and even
aluminum metallic conductors are employed. The most common form of copper wire
used in communications is that of twisted pair.
A
twisted pair involves two copper conductors, which generally are solid core,
although stranded wire is used occasionally in some applications. Each
conductor is separately insulated by polyethylene, polyvinyl chloride,
flouropolymer resin (Category 5, or Cat 5), or some other low-smoke, fire
retardant substance. The insulation separates the conductors, so that the
electrical circuit is not shorted. This is accomplished by virtue of the two
conductors, and serves to reduce electromagnetic emissions. Both conductors
serve for signal transmission and reception. Because each conductor carries a
similar electrical signal, twisted pair is considered to be a balanced medium.
The Twisting Process
The
separately insulated conductors are twisted 90º at routine, specified
intervals, hence the term twisted pair. This twisting process serves to improve
the performance of the medium by containing the electromagnetic field within
the pair. Thereby, the radiation of electromagnetic energy is reduced and the
strength of the signal within the wire is improved over a distance. Clearly,
this reduction of radiated energy also serves to minimize the impact on
adjacent pairs in a multipair cable configuration. This is especially important
in high-bandwidth applications, as higher frequency signals tend to lose power
more rapidly over distance. Additionally, the radiated electromagnetic field
tends to be greater at higher frequencies, impacting adjacent pairs to a greater
extent. Generally speaking, the more twists per foot, the better the
performance of the wire.
Gauge
Gauge
is a measure of the thickness of the conductor. The thicker the wire, the less
the resistance, the stronger the signal over a given distance, and the better
the performance of the medium. Thicker wires also offer the advantage of
greater break strength.
American
Wire Gauge (AWG) is a commonly used standard measurement of gauge, although
others are used outside the United States.
|
Category |
Gauge (AWG) |
Performance |
Data |
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CAT1 |
Various
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Undetermined
|
No
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CAT2 |
22
& 24 |
Undetermined
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No
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CAT3 |
22
& 24 |
16
MHz–10 Mbps |
Yes |
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CAT4 |
Various
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16
Mbps |
Yes |
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CAT51 |
Various
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100
Mbps |
Yes |
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Configuration
In
a single pair configuration, the pair of wires is enclosed in a sheath or
jacket, also of polyethylene, polyvinyl chloride or Teflon. Oftentimes,
multiple pairs are so bundled in order to minimize deployment costs associated
with connecting multiple devices (e.g., electronic PBX or KTS telephone sets,
data terminals, and modems) at a single workstation.
Larger
number of pairs are bundled into large cables to serve departments, quadrants
of a building, or floors of a high-rise office building—such cables may contain
25, 50, 100, 500 or more pairs. While twisted pair cables of up to 3,600 pairs
are still used in outside plant applications, such continuing use is decidedly
uncommon. In large cables, pairs are grouped into binder groups of 25 pairs for
ease of connectivity management. Each binder group is wrapped (bound) with some
sort of tape in order to separate it from other groups. Each pair within a
binder group is color-coded for further ease of connectivity management. with
the color codes being repeated within each binder group.
Bandwidth
The
effective capacity of twisted pair cable depends on several factors, including
the gauge of the conductor, the length of the circuit and the spacing of the
amplifiers/repeaters. One must also recognize that a high-bandwidth (high
frequency) application may cause interference with other conversations on other
pairs in close proximity.
While
a voice grade circuit over twisted pair is guaranteed at 4 kHz, standard copper
is capable of supporting much greater bandwidth. A single twisted pair, in a
typical telephone installation, is capable of providing up to 250 kHz, or 1–4
Mbps compressed, assuming amplifier or repeater spacing every 2-3 km.
Additional examples follow:
• T1 connections (1.544 Mbps) are
routinely provided over specially conditioned, four-wire twisted pair, with
repeaters spaced at approximately 6,000 ft.
• Category 5 (CAT 5) copper, in a Local
Area Network (LAN) environment, provides bandwidth of up to 100 Mbps over
twisted pair at distances of [le]20 meters.
Error Performance
Signal
quality is always important, especially relative to data transmission. Twisted
pair is especially susceptible to the impacts of outside interference, as the
lightly insulated wires act as antennae and, thereby, absorb such errant
signals. Potential sources of ElectroMagnetic Interference (EMI) include
electric motors, radio transmissions and fluorescent light boxes. As
transmission frequency increases, the error performance of copper degrades
significantly with signal attenuation increasing approximately as the square
root of frequency.
Distance
UTP
is especially distance-limited. As distance between network elements increases,
attenuation (signal loss) increases and quality decreases at a given frequency.
Even low-speed (voice grade) analog voice transmissions require amplifiers
spaced at least every 2 to 4 miles. As a result, local loops generally are
10,000 to 18,000 feet in length. As bandwidth increases, the carrier frequency
increases, attenuation becomes more of a issue, and amplifiers/repeaters must
be spaced more closely.
Security
UTP
is inherently an insecure transmission medium. It is relatively simple to place
physical taps on UTP. Additionally, the radiated energy is easily intercepted
through the use of antennae or inductive coils, without the requirement for placement
of a physical tap.
Applications
Generally
speaking, UTP no longer is deployed in long-haul outside plant transmission
systems, satellite, microwave and fiber optic cable are the media of choice in
such applications. However, its low cost, coupled with recently developed
methods of improving its performance, have increased its application in
short-haul distribution systems. UTP is still the medium of choice in most
inside wire applications. Current and continuing applications include the local
loop, inside wire and cable, and terminal-to-LAN.
Shielded
Copper
Shielded
twisted pair (STP) differs from UTP in that a metallic shield or screen
surrounds the pairs, which may or may not be twisted. the pairs can be
individually shielded. A single shield can surround a cable containing multiple
pairs or both techniques can be employed in tandem. The shield itself is made
of aluminum, steel, or copper; is in the form of a metallic foil or woven mesh;
and is electrically grounded. Although less effective, the shield sometimes is
in the form of nickel and/or gold plating of the individual conductors.
Shielded
copper offers the advantage of enhanced performance for reasons of reduced
emissions and reduction of electromagnetic interference. Reduction of emissions
offers the advantage of maintaining the strength of the signal through the
confinement of the electromagnetic field within the conductor; in other words,
signal loss is reduced. An additional benefit of this reduction of emissions is
that high-frequency signals do not cause interference in adjacent pairs or
cables. Immunity from interference is realized through the shielding process,
which reflects electromagnetic noise from outside sources, such as electric
motors, other cables and wires, and radio systems.
Shielded
twisted pair, on the other hand, has several disadvantages. First, the raw cost
of acquisition is greater as the medium is more expensive to produce. Second,
the cost of deployment is greater as the additional weight of the shield makes
it more difficult to deploy. Additionally, the electrical grounding of the
shield requires more time and effort
Applications
The
additional cost of shielded copper limits its application to inside wire
applications. Specifically, it generally is limited to application in
high-noise environments. It also is deployed where high frequency signals are
transmitted and there is concern about either distance performance or
interference with adjacent pairs. Examples include LANs and image transmission
Coaxial Cable
Coaxial cable is a very robust shielded copper wire. The
center conductor (much thicker than a twisted pair conductor) is surrounded by
an outer shield/conductor which serves to greatly improve signal strength and
integrity. The two conductors generally are separated by a layer of foam or
solid insulation; the entire cable is then protected by a layer of dielectric
(nonconductive) material, such as PVC or Teflon. The two conductors share a
common axis, hence the term coaxial. Reportedly invented by AT&T Bell
Telephone Laboratories in 1934, the first coaxial cable was placed into service
in New York City in 1936. Such a cable was used in 1940 to televise in New York
City the Republican National Convention in Philadelphia at which Wendell Wilke
was nominated.
Configuration
Coax
cables traditionally consist of a single, two-conductor wire, with a center
conductor and an outer shield/conductor, which is of solid metal. Sometimes
braided or stranded metal is used. Twinaxial cables contain two such
configurations within a single cable sheath. As the center conductor carries
the carrier signal and the outer conductor generally is used for electrical
grounding and is maintained at 0 volts, coax is described as an unbalanced
medium. Coax connectivity can be extended through the use of twisted pair, with
a BALUN (BALanced/UNbalanced) connector serving to accomplish the interface.
Gauge
The
gauge of coax is thicker than twisted pair. While this increases the available
bandwidth and increases the distance of transmission (less resistance), it also
increases the cost. Traditional coax is quite thick, heavy and bulky of which
Ethernet LAN coax (10Base5) is an example. ThinNet, or CheaperNet, (10Base2)
coax is of much lesser dimensions, but offers less in terms of performance.
Bandwidth
The
effective capacity of coaxial cable depends on several factors, including the
gauge of the center conductor, the length of the circuit, and the spacing of
amplifiers and other intermediate devices. The available bandwidth over coax is
very significant, hence its use in high capacity applications, such as data and
image transmission. As examples, the following are coax standards for Ethernet
LANs, with 10Base5 involving more substantial cable, with a thicker center
conductor than is the case with 10Base2:
• 10Base5 10 Mbps; Baseband (single
channel); [le]500m
• 10Base2 10 Mbps; Baseband (single
channel); [le]200m ([le]180 m, rounded up)
As
in the case of UTP, 100 Mbps is possible over coax, and over longer distances
with better error performance. In CATV and other applications, coax routinely
supports transmission of multiple channels at an aggregate rate of 500 MHz.
Error Performance
Coax
performs exceptionally well, due to the outer shielding. As a result, it is
often used in data applications.
Distance
Coax
is not so limited as UTP, although amplifiers or other intermediate devices
must be used to extend high frequency transmissions over distances of any
significance.
Security
Coax
is inherently quite secure. It is relatively difficult to place physical taps
on coax. Additionally, little energy is radiated.
Applications
Historically,
coax was often used in telephone company interoffice trunking applications, as
a superior option to twisted pair cables. However, that is no longer the case
as satellite, microwave, and fiber optic cable are the media of contemporary
choice in such applications. Yet, coax’s superior performance characteristics
make it the favored medium in many short haul, bandwidth-intensive data
applications. Current and continuing applications include LAN backbone,
host-to-host, cabinet-to-cabinet (PBX & computer), host-to-peripheral
(e.g., host-to-Front End Processor), and CATV.
Fiber Optics
Fiber
optic transmission systems are opto-electric in nature. In other words, a
combination of optical and electrical electromagnetic energy is involved. The
signal originates as an electrical signal, which is translated into an optical
signal, which subsequently is reconverted into an electrical signal at the
receiving end.
Configuration
Fiber optic systems consist of light sources, cables and light
detectors, In a simple configuration, one of each is used. In a more complex
configuration over longer distances, many such sets of elements are employed.
Much as is the case in other transmission systems, long haul optical
communications involves a number of regenerative repeaters. In a fiber optic
system, repeaters are optoelectric devices. On the incoming side of the
repeater, a light detector receives the optical signal, converts it into an
electrical signal, boosts it, converts it into an optical signal, and places it
onto a fiber, and so on. There may be many such optical repeaters in a long
haul transmission system, although typically far fewer than would be required
using other transmission media.
Fiber-Optic Cables
While
plastic wires are used in some specialized, low bandwidth, short haul
applications (e.g., automobiles and airplanes), glass predominates. Generally
speaking, fiber cables contain a large number of pairs of glass fibers, as the
additional cost of redundancy is relatively low. Oftentimes, only a few of the
fibers are active, with others being left dark for backup or future use. While
current technology, although more expensive, allows two-way transmission over a
single fiber, two fibers generally are used, with one transmitting in each
direction.
The
mass production of glass fiber employs several techniques, all of which take
place in a vacuum environment. First, silica is heated to the point that it
vaporizes. The ultra-pure glass vapor is then deposited on a designated surface
to create a glass cylinder. That cylinder is then reheated and collapsed into a
preform cylinder. The preform cylinder is reheated and drawn, in a process
known as broomsticking, into fibers which can be as long as 10 km in length.
The
light pulse travels down the center core of the glass fiber, which is
especially pure. Surrounding the inner core is a layer of glass cladding, with
a slightly different refractive index. The cladding serves to reflect the light
waves back into the inner core. Surrounding the cladding is a layer of
protective coating, such as Kevlar, which seals the cable and provides
mechanical protection. Typically, multiple fibers are housed in a single
sheath, which may be heavily armored. Glass optical fibers are of two basic
types, multimode and monomode (or singlemode).
Multimode fiber is less expensive to
produce, but performs less well, as the inner core is larger in diameter. As
the light rays travel down the fiber, they spread out due to a phenomenon known
as modal dispersion. Although reflected back into the inner core by the
cladding, they travel different distances and, therefore, arrive at different
times. As the distance of the circuit increases and the speed of transmission
increases, the pulses of light tend to overrun each other in a phenomenon know
as pulse dispersion. At that point, the light detector is unable to distinguish
between the individual pulses. As a result, multimode fiber is relegated to
applications involving relatively short distances and lower speeds of
transmission (e.g., LANs and campus environments).
Monomode fiber has a thinner inner
core; therefore, it performs better than does multimode fiber over longer
distances at higher transmission rates. Although more costly, monomode fiber is
used to advantage in long-haul, and especially in high bandwidth, applications.
Light Detectors
These
are of several basic types, with the most common being PhotoINtrinsic diodes
(PINs) and Avalanche PhotoDiodes (APDs). The light detectors serve to reverse
the process accomplished by the light sources, converting optical energy back
into electrical energy. APDs, although more expensive, are preferable, as they
use a strong electric field to accelerate the electrons flowing in the
semiconductor. This results in an avalanche of electrons. Therefore, a very
weak incoming light pulse will create a much stronger electrical effect.
Although more sensitive, APDs require more power and are more sensitive to
ambient temperatures.
Bandwidth
Fiber
offers by far the greatest bandwidth of any transmission system, often in
excess of 2 Gbps in long haul carrier networks. Systems of 10 Gbps and 20 Gbps
have been deployed, and 40 Gbps and 50 Gbps systems have been tested successfully
on numerous occasions in laboratory environments. AT&T Bell Laboratories
recently successfully tested 40 Gbps, error-free transmission at distances of
900 miles. The theoretical capacity of fiber is in the Terabit (Tbps) range,
with current monomode fiber capacity being expandable to that level with the
application of subsequent generations of electronics.
Error Performance
As
fiber is dielectric (a nonconductor of direct electric current), it is not
susceptible to EMI/RFI; neither does it emit EMI/RFI; The light signal will
suffer from attenuation, although less so than other media. Such optical
attenuation can be caused by scattering of the optical signal, bending in the
fiber cable, translation of light energy to heat, and splices in the cable system.
Error performance, depending on the compression scheme utilized, ranges between
10-9 and 10-14, one errored bit in every 100 trillion
Distance
Monomode
fiber optic systems routinely are capable of transmitting unrepeatered signals
over distances in excess of 200 miles (322 km.). As a result, relatively few
optical repeaters are required in a long-haul system, thereby reducing costs,
and eliminating points of potential failure or performance degradation. In
fact, tests have indicated that unrepeatered signals can travel distances of up
to 8,000 miles (12,900 km.) Such systems employ a process of chemical
amplification, achieved by chemically doping a section of the cable with
erbium, a metallic rare earth element. When the laser pulse strikes that section
of the cable, the chemical is excited and, in turn, amplifies the light pulse.
Security
Fiber
is intrinsically secure, as it is virtually impossible to place a physical tap
without detection. As no light is radiated outside the cable, physical taps are
the only means of signal interception. Additionally, the fiber system supports
such a high volume of traffic that it is difficult to intercept and distinguish
a single transmission from the tens of thousands of other transmissions that
might ride the same cable system. The digital nature of most fiber, coupled
with encryption techniques frequently used to protect transmission from
interception, make fiber highly secure. In fact, a number of U.S. government
agencies recently have lobbied Congress and the FCC to require that the
carriers place physical taps on existing fiber optic systems in order to ease
their ability to place wiretaps, assuming that an enabling court order is
issued.
Durability
While
fiber certainly does not have the break strength of copper or coax, it does
have the same tensile strength as steel of the same diameter. When covered by a
protective jacket or armored, fiber can be treated fairly roughly without
damage. Additionally, fiber is more resistant to temperature extremes and corrosion.
However, and in consideration of the huge number of conversations supported
over a typical fiber optic cable, a train derailment, earthquake or other
traumatic event can have consequences of disruptive, and even catastrophic,
proportions.
Applications: Bandwidth-Intensive
As
one would expect, cost-effective applications for fiber optic transmission
systems are those which are bandwidth-intensive. Such applications include
backbone carrier networks, international submarine cables, backbone LANs
(FDDI), interoffice trunking, computer-to-computer or cabinet-to-cabinet (e.g.,
mainframes and PBXs) distribution networks (e.g., CATV and Information
Superhighway), and fiber to the desktop (e.g., CAD or Computer Aided Design).
Hybrid Transmission
Systems
While
each transmission medium/system has its own unique properties and applications,
it is clear that digital fiber optic cable offers the most potential. As it
also is costly and fragile, it, however, is not always the ideal approach. In
fact, the most appropriate transmission medium depends of issues mentioned at
the beginning of this chapter. Namely, those considerations include bandwidth,
error performance, throughput, distance between elements, propagation delay,
security, mechanical strength, physical dimensions, and a number of cost
factors. In fact, a given long-haul conversation typically will traverse a
number of transmission systems, perhaps both wired and wireless, and typically
including twisted pair and fiber optics, at a minimum.
The true concept of a hybrid
transmission system, however, generally involves a local loop connection
deployed in a well-planned convergence scenario. Such a scenario involves a
single provider, or multiple providers, developing a communications grid
designed to deliver voice, data and entertainment information to the premise.
Hybrid systems usually are described as involving Fiber-to-the-Neighborhood
(FTTN), Fiber-to-the-Curb (FTTC), or Fiber-to-the-Home (FTTH). Although
conventional wisdom suggests that cost considerations will dictate that the
last link of such a hybrid network involve either coaxial cable or twisted
copper pair, various wireless technologies are currently challenging that
concept.
While
many of the traditional telephone carriers and CATV providers have made
dramatic announcements about their plans to install fiber optic cable to the
curb or even to the premise, their ardor has cooled as the economics of such an
approach have become apparent. It is likely that fiber will be deployed to the
neighborhood, with coaxial cable deployed to the premise. Additionally, it is
likely that the telcos will extend the life of the embedded twisted pair cable
as long as possible, through the use of new local loop technologies such as
Asymmetric Digital Subscriber Loop (ADSL). In the future, it is possible that
Wireless Local Loop (WLL) technology will seriously challenge the traditional
wired approach.