Transcript Chapter 15 Cygansky Book

Chapter 15 Cyganski Book
Monica Stoica, Boston University
[email protected]
Wire as transmission mediums
• Wire was the original medium for the electronic
transmission of information; today it is still the most
common and versatile medium.
• transmission systems can involve both guided and
unguided movement of electro magnetic waves.
• The wire-based transmission scheme guides electro
magnetic waves, either between a pair of separate
wires, or inside a coaxial arrangement.
• A coaxial cable (often called coax for short) has both
a ``center'' conductor and a second ``shield''
conductor. These conductors are separated by some
insulating material, such that the shield conductor
entirely surrounds the center conductor.
Cables
• In the case of noncoaxial transmission, the pair of wires
may be held either parallel to each other by an appropriate
stiff insulating material, or individually insulated and
twisted around each other.
• Finally, some arrangement of surrounding shield conductor
may be placed around the resulting twisted pair to form a
shielded twisted pair (STP). Implicit in this construction is
that the physical arrangement of the shield conductor is not
nearly as accurate as in the construction of coax.
• The unshielded twisted pair is called TP or often UTP to
distinguish it clearly from STP. All of the above wire-based
transmission media are called cables, not just the coaxial
cable.
Cost of cables
• The cost of a cable is a function of the cost of the
materials and the manufacturing process.
• Thus, cables with larger diameter,involving more
copper conductor and more insulation, are more
expensive than those with small diameter.
• Likewise, cables that have twisted pairs of
conductors are more expensive than those that do
not, while STP is more expensive,and coaxial is
even more so .
• This of course leads to the question:why not use the
smallest, simplest cable for all applications?
E&M
• a cable moves electro magnetic (E&M) waves by
providing a channel in which the pair of conductors
act like a pair of mirrors between which the wave
bounces back and forth until it reaches its
destination.
• To be precise, E&M waves may be confined in such
as way that they can traverse the cable moving
parallel to these conductors, that is without
bouncing, yet still interacting with the conductors.
• The E&M wave interacts with the free electrons in
the conductors, which are responsible for the
guiding of this wave.
E&M
• Think of these electrons, which are free to move
within the conductor but confined there, as ball
bearings,and think of the E&M wave as a package
of energy riding on those ball bearings, guided to its
destination by the shape of the conductor that holds
the ball bearings.
• Now, no one can make a perfectly frictionless ball
bearing! So, we would expect in this analogous
system that our package would eventually slow
down and stop on the conveyer if it offers no other
source of energy for the packages.
Speed
• The electrons in our conductor also are subject to a frictionlike energy loss mechanism that we call resistance.
• However, in our case, our E&M energy packages act a little
differently.By the theory of special relativity, our E&M
packages (which are made of the elementary particles called
photons) cannot slow down! They must travel at the speed
of light.
• But because our packages ``are'' energy, they themselves
can be consumed to provide the power needed to sustain
their speed in the face of the losses due to the electrons.
In effect, our electron ball bearings are eating away the
package as it moves along.
At the end of our trip,
• any pulses of E&M energy we have transmitted will be
found to be smaller in size. Or, in terms of the kinds of
graphs of light intensity or voltage and current we have
been using throughout the preceding chapters, the height of
all the transmitted pulses will be reduced.
• We call this loss of energy and related reduction in the size
of transmitted pulses attenuation.
• As you might suspect, the longer the cable that we are
using, the greater the attenuation.
• On the other hand, the larger the conductors we use, the
less this attenuation will become, up to some limit.
Hence, larger, more expensive cables will have less
attenuation and be more desirable if this attenuation has
negative effects on our ability to move information.
Willingness to pay
• Because we will still be dealing with the problem
of noise that is determined by the temperature of
the receiving system at the far end, the reduction
in the size of our pulses directly reduces the
rate at which we can transmit information
over a certain cable.
• In the next slide we see typical attenuation figures
for various cables. These figures alone explain
the willingness to pay more for STP and still
more for coaxial cable in applications that require
the highest information transmission rates.
Why some cables are better than others
• While the previous section explained the desirability of
some cables owing to their low attenuation characteristics, it
did not explain why they possess these different
characteristics other than with respect to behavior versus
conductor size. Here we will discuss the role of the
``geometry'' of the cable.
• In a vacuum, each of the above cables would perform
nearly as well as the others.
• However, cables tend to be routed next to each other, and
near other metallic objects and generators of E&M energies.
The ``open'' nature of untwisted cable presents a problem in
that nearby conductors can steal some of the energy that it
carries and can insert unwanted E&M noise.
UTP, STP
• UTP cable still is subject to the loss problem but is less
affected by noise pickup because the twists cause interfering
pickup signals to cancel themselves when picked up inside
of adjacent twists! STP reduces the losses by better
confining the E&M to the inside of the shield.
• The coaxial cable, because of the complete confinement of
the E&M wave, is not subject to the level of loss and noise
problems found in other wired cables. Furthermore, because
its geometry is very tightly held in position, the signal itself
undergoes less distortion in shape while traversing it.
• In a very rough extension of our analogy of the last section,
we can think of the coax as presenting very well
manufactured mirrors for the transport of our signal, versus
warped mirrors in the case of STP.
Parallel Conductor Cable
Application
• While parallel conductor cable is used extensively
for power delivery, about the only signal-carrying
application you will find this cable being used to
support is as the short extension cord leading from
your telephone wall plug to the telephone itself.
• The short distance and small bandwidth of the
signal involved allow use in this application, in
which the flat nature of the cable makes it more
attractive to the eye in its very visible role.
UTP Applications
• UTP cable is found extensively in the so-called local loop of
the telephone company.
• The local loop is that wiring that connects your house to the
telephone company's local``switch'' building. This cable is
typically under 18,000 feet in length and suffices for
transmission of telephone signals.
• New kinds of special digital modems for ISDN and XDSL
data services can sometimes (depending upon the length and
nearby interfering sources) be used to move data at higher
speeds than a telephone-signal-based modem. For example
128 kbps (ISDN, bidirectional) to 1.544 Mbps (HDSL in
one direction) can be achieved using (now) relatively low
cost and very sophisticated special connection devices.
UTP
• Rates as high as 52 Mbps (VDSL in one direction)
can be obtained if the length of the local loop is
below 3,280 feet.
• The telephone companies also make extensive use
of UTP for movement of digitized groups of voice
signals between their switching stations. The T1
signal unidirectionally carries groups of 24 voice
channels in a 1.544 Mbps digital format over 6,000
foot distances between regenerator circuits.
• is also found in the walls (in spaces called plenums)
throughout most buildings. It is used to complete the
local loop from the building entrance to the
telephone wall plates in the rooms.
More UTP
• UTP has found extensive use as a cheap medium for the
distribution of medium-speed computer network data
connectivity. Ethernet data is routinely transmitted in a
signaling system known as 10 Base-T Ethernet in which
UTP cable is used for distances up to 100 m(328 feet).
• UTP can be made with a variety of materials, sizes of
conductors,and numbers of pairs inside a single cable. A
particularly high quality UTP is called UTP-5. This
cable type has been used to support 100 Mbps Ethernet
transmissions over distances of 100 m.
STP Applications
• STP is used to some extent by telephone
companies for moving groups (96) of digitized
telephone conversations over distances of 6,000
feet between ``repeaters'' that receive and
retransmit the signal for the next such hop, to span
the distance of several miles between telephone
company switching stations.
• The so-called T2 connection involves digital data
transmission at speeds of 6.312 Mbps. Highquality STP has been applied by the telephone
companies for transmission rates as high as 8.448
Mbps in Europe.
Coaxial Cable Applications
• coaxial cable is used wherever there exists a need for
long-distance, low-attenuation, and low-noise
transmission of information.
• Probably everyone is familiar with the use of coaxial cable
for the transmission of a hundred TV channels into the
home via CATV coaxial cable, because 70% of all homes in
the United States have CATV service.
• These cables provide a bandwidth of nearly 1 GHz (that is,
1,000 MHz) into the home.
• These same cables are capable of transmitting many Gbps
of information into those same homes. In fact, the research
and test deployment of CATV-based Internet delivery
systems is currently a growth industry.
Coaxial Cable
• Until recently, coaxial cable has been the major delivery
system for 10 and 100 Mbps Ethernet computer network
data signals, for hop distances of 500 m (1,640 ft) and 185
m (607 ft), respectively, for the larger and smaller
diameter cables.
• Coaxial cable for this purpose is being rapidly supplanted
by UTP cable.
• The telephone companies also resort to coaxial cable to
bridge larger distances with higher-rate digital
connections.
• One example is the use of coaxial cable to transmit 140
Mbps data signals between telephone switch buildings
with a hop distance of up to 2 km (6,562 ft).
Fiber Optic Cable
• The means of guiding an E&M wave as in the fiber optic
cable has an immense advantage over the use of wire-based
wave guides: this system does not depend upon the quality
(low resistance) of the conductors to obtain low attenuation
propagation of the E&M wave.
• In fact, the reflecting surface of the waveguide is formed by
the surface of an insulator.In effect the boundary between
two layers of glass or plastic (the core and cladding of the
fiber) acts as an ideal (no loss) mirror.
• The mirror behavior of this boundary derives from a
property of such boundaries and E&M waves that strike
them at shallow angles.
Fiber Optic
• If an E&M wave strikes such a boundary at an angle below
the ``critical angle'' it undergoes ``total internal reflection.''
• You may be familiar with this phenomenon as it can be seen
in operation by looking at the bottom side of the water-air
boundary at the top of an aquarium from a position at the
side of the glass enclosure. If your eye is near enough to the
boundary, you see an exceedingly clear mirror view of the
scene out the other side of the aquarium.
• Thus, the attenuation of a fiber-optic cable is essentially
only dependent upon the clarity of the optical material used
in the core, and exceedingly high clarities have been
achieved.
Other benefits of the fiber optic
• The total confinement of the E&M wave means that
surrounding materials do not increase the attenuation of
the wave.
• Because there are no free electrons as would be found in a
conductor-based cable, no interference can be generated
even by large surrounding magnetic fields.
• Being an insulator, the fiber insulates the connected
systems from each other. This is a major factor in cable
systems in which atmospheric potentials and ground
potentials can cause interfering and sometimes destructive
currents to flow parasitic ally along communication cables.
• For a given attenuation, a fiber cable is exceedingly
lightweight and small in diameter. This means that many
fibers may be placed in a cable where once only one wirebased cable may have been possible.
Properties of fiber optic
• Relatively low cost fiber-optic cable available off-the-shelf
today exhibits attenuations of less than 0.5 dB/1,000 ft
while offering usable bandwidths of hundreds of MHz.
• A cable exhibiting these characteristics containing four
separate fibers, for example, could be purchased in 1999
for under $1.50/foot. Compare this attenuation and cost
with that of even high grade coaxial line.
• Higher cost fiber cables that achieve as little as 0.03
dB/1,000 ft attenuations and usable bandwidths in excess
of 1 Tbps (1 Tera bps = 1,000,000 Mbps) have also been
constructed.
Amplifiers
• A new technology is rapidly coming into commercial
application and is revolutionizing the long-distance fiberoptic transmission of data.
• By adding a small amount of Erbium additive to a fiber
during its manufacture, it is possible to turn the fiber itself
into a laser amplification system!
• By simply passing an optical signal through a short piece of
Erbium-doped fiber (typically 10 m) and pumping that
length with light from another laser, the optical signal will
be strengthened and can be returned to its original levels
without ever leaving the fiber-optic cable for separate
electronic processing. The Erbium-doped fiber amplifier
(EDFA) is being incorporated in all of the newest
transoceanic cable runs to interconnect the continents with
very high-speed, low-maintenance, data service.
Examples
• For example, a transoceanic cable from the United States to
England that uses EDFA technology was completed in
September of 1996.
• This cable holds four fibers each providing 2.5 Gbps of data
service,for a total of 10 Gbps of data. It is anticipated that
the achievable data rates can be increased by improved
methods of signaling and upgrades that will bring each
fiber's bit rate to 20 Gbps or greater.
• Prior to deployment of this cable, the connection was served
by a fiber-optic cable, deployed in 1988, which used
electronic repeaters and provided a total of 280 Mbps of
data service. Thanks to the immutable under sea electronic
repeaters, no upgrades in data rates were possible despite
significant new capabilities in fiber transmission and
reception systems since that time.
Solutions
• A remarkable property of special materials has been
discovered that promises to have an impact like that of the
EDFA on the speeds and distances achievable with fiberoptic cables.
• A soliton is a special packet of optical energy within such a
cable that does not ``disperse'' , it is immune to the lowpass phenomenon that we have previously said characterizes
all physical systems that transmit waves. Thus, a fiber using
soliton transmission can achieve fantastic data rates over
large distances.
• Nippon Telephone of Japan, has demonstrated soliton
transmission of a 10 Gbps data stream over a distance of
50,000 km (over 30,000 miles). Laboratory tests show that
soliton technology may provide data rates in excess of 1
Tbps (1012 bps, or 1 million megabits per second) in the
future over transcontinental and inter continental distances.