1c by wanghonghx


									Physical Transmission
        Physical Transmission Media
Electrical signals can be sent over a wire because wire is a good
conductor of electricity. We are not restricted to just using wire,
however. Other transmission media, such as radio and optical fibres,
can also be used. Each of these media are used in different ways but
they perform the same task: transmitting information from A to B.

We need to know about physical transmission media in computer
networks because these media form the links from one computer to
the next.
Electrical Media
Broadly speaking, electrical media means wires. Most wires are
made from copper because it is a particularly good conductor of
electricity. The reason for this is that the copper atom has a free
electrons in its outer shell that can jump easily from one atom to the
next (see figure 3.1). By applying an electromotive force (measured
in volts) along the media, the electrons will begin to flow.

                                                           Just one electron in outer shell. This is a
                                                           free electron.

Figure : Diagram of a                                             Just one
copper atom and the free                                          electron in
                                                                  outer shell.
electron in its outer shell                                       This is a free
that moves easily                                                 electron.

from one atom to the next
under the influence of an
electromotive force.
        Physical Transmission Media
The telegraph system used a single wire but you will not see single
wires being used to transmit data these days. The reason is that a
single wire is badly effected by noise. Noise can come from many
sources, such as radio interference or physical damage to the wire.
Most noise, however, is due to thermal noised (sometimes called
Gaussian noise) which causes a constant hiss on the line.

Thermal noise is due to electrons jostling around because of heat
inside the wire. The only way to remove this noise is to super-cool
the wire (i.e. bring its temperature down to near absolute zero - that
is to say 273.15C).

For most systems, this is impractical and we have to find other ways
of coping with noise.
                  Twisted Pair Cable
To improve the quality of the simple telegraph system, a second wire
was added to provide a non-ground return circuit. This second wire
improves quality because any non-thermal interference that effects
one wire usually effects the other in the same way. The signal is sent
in the form of a voltage difference between the two wires. Since the
effect of non-thermal noise on both wires is usually the same, the
interference tends to leave the voltage difference unchanged.

        Site A                                           Site B

  Figure 3.2: Twisted pair cables are less effected by non-
  thermal noise than single cables.
                Twisted Pair Cable
Figure on previous slide shows a twisted pair cable connecting two
sites. The common Category 3 twisted pair cable consists of two
insulated[1] 1mm thick copper wires gently twisted around each other.
It is important for the wires to be twisted because otherwise they
would form a simple antenna that would actually pick up noise.

Category 3 twisted pair cables were widely used for telephone
systems until 1988. In 1988, Category 5 twisted pair cables were
introduced. These cables are similar to Category 3 twisted pair
cables except that they use Teflon insulation and have more twists
per centimetre – resulting in even less interference. Category 5
cables are widely used in computer networks.

[1] By insulated we mean the wires are coated in plastic so that they
will not be short circuited by touching each other or other metal
              Twisted Pair Cable
Twisted pair cables come in shielded and unshielded varieties. The
shielded twisted pair cables (STP cables) are surrounded by a
grounded metal sheath that provides extra protection against
interference. This, however, makes them more expensive and bulkier
than the more popular unshielded twisted pair cables (UTP cables).

Twisted pair cables are typically bundled together to form 4 or 5-
pair twisted pair cables (8 or 10 wires altogether). They are suitable
for transmitting data at rates of about 1-100Mbps (a 100 million bits
per second) over short distances (less than 100m). Higher data rates
in twisted pair cables are limited by the effects of noise and a
phenomenon           called         the         skin        effect[1].
[1] As the data rate (and hence frequency) in a wire increases,
electrons tend to flow only near the surface of the wire. This
reduces the cross section of the wire carrying the signal. This means
the resistance of the wire increases at higher frequencies and this
causes signal attenuation (weakening).
                        Coaxial Cable
Coaxial cable consists of a copper core surrounded by a grounded
sheath (usually a woven braided copper mesh). The sheath gives the
copper core excellent protection from external noise and allows
coaxial able to be used near machinery and other sources of
electromagnetic radiation. It also reduces attenuation due to the skin
effect because less energy can be radiated from the outer surface of
the core.

Broadband[1] Coaxial Cable is usually used for carrying analogue
transmissions whereas Baseband[2] Coaxial Cable is commonly used
for digital transmission. The only physical difference between the
two cables is that the Baseband Coaxial Cable has a resistance of 50
Ohms/metre whereas the Broadband Coaxial Cable has a resistance of
75 Ohms/metre.

[1] Broadband means the available bandwidth of the cable is divided up into
several frequency channels.
[2] Baseband means that all the available bandwidth is made available to a
single channel.
         Physical Transmission Media
                              Protective plastic
                                                                                 The structure of
                                                                                 coaxial cable.
          Plastic insulator

                                                                 Grounded copper sheath

                                                   Copper wire

Figure above shows the structure of coaxial cable. Broadband Coaxial
Cable can carry frequencies of 350 MHz (millions of cycles per second)
or more over nearly 100 km. The Baseband Coaxial Cable used in
computer networks can be used to transmit digital data at bits rates of
10 Mbps (millions of bits per second) or more over several hundred
                       ElectroMagnetic Waves
       Rather than using electricity to transmit data, we can use
       electromagnetic radiation. Electromagnetic radiation includes light,
       microwaves and radio waves. Figure below shows the different types
       of electromagnetic radiation and their frequencies.

10 0     10 2   10 4      10 6     10 8           10 10   10 12          10 14        10 16   10 18           10 20   10 22       10 24   Hz

                       Radio              Microwave        Infrared              UV                   X-ray                   Gamma Ray

                                                             Visible Light

                                 The electromagnetic wave spectrum.
        Physical Transmission Media
This is because electromagnetic radiation travels as waves as can be
seen below.
                                               The frequency of a wave is
                                               measure in Hertz (Hz) and is the
                                               number of wave cycles every
                                               second. The wavelength of a wave
                                               is the distance between two
                                               consecutive wave crests and is
                                               measured in metres (m).        The
                                               frequency f and the wavelength 
                   Magnetic Field

                                               are related by equation (3.1).

                                               where c is the speed of light
                                               (299,792,458        m/s        or
                                               approximately 3108m/s).
        Electromagnetic waves
The amplitude of a wave is the height of its crest. In the case of an
electromagnetic wave, amplitude means its intensity. By varying the
amplitude of a wave we can send signals. This is type of signalling is
called amplitude modulation. The bit rate that we can send using an
electromagnetic wave depends heavily on its frequency. If we have a
coherent wave, such as a radio wave, then we can calculate its
bandwidth H from its frequency f using equation (3.2).
        Physical Transmission Media
The bandwidth represents the range of signal frequencies that a
medium can carry.

In the case of a radio wave, only signal frequencies up to half the
carrier frequency can be carried using amplitude modulation.

The reason for this has something to do with the wagon wheel
effect: in cowboy movies, the wagon wheels reach a certain speed
before slowing down and even going backwards. The number of movie
frames per second is not fast enough to show faster wheels.

Similarly, the frequency of a radio wave is not high enough to
represent signal frequencies more than half its own frequency. They
just look like slower frequencies.
                    Nyquists Limit
The maximum data rate for any channel can be calculated from
Nyquist’s Limit that states:


where H is the bandwidth of the channel and M is the number of
signal levels used. For example, we might use 2 signal levels to
represent 0s and 1s. In this case, max_bps=2 H since log2(2)=1.
There is no reason why 4 signal levels could not be used to
represent the bit combinations 00, 01, 10 and 11. Unfortunately, M
cannot be made arbitrarily large in practice.

Nyquist’s Limit does not take account of noise and noise is present,
to a greater or lesser extent, in all communication systems. Noise
is the second thing that limits the data rate.
                      Optical Fiber
Data can be transmitted using pulses of light. These pulses can be
conveyed to their destination via optical fibers. An optical fiber is
composed of two types of glass with different refractive indexes.
Light pulses in the core are unable to escape because they are
reflected back into the core by total internal reflection at the
interface of the two glass types.

 Figure above illustrates the structure of the optical fibre. Currently optical
 fibres can carry data at the rate of about 1Gbps (billion bits per second).
                      Optical Fiber
 The limiting factor in fiber optic transmission rates is the speed of
 the electronics used to convert light pulses to and from electrical
 signals. Transmission rates in excess of 50,000 Gbps (50 Tbps) are
 theoretically possible.

            Light pulses become distorted in long optical fibers because
            each photon takes slightly different path inside the fiber.

One problem encountered with optical fibers is light pulses
transmitted over long distances become distorted. This is due to an
effect known as dispersion. Figure above shows why this effect
occurs. Each of the photons that go to make up the light pulse takes a
slightly different path inside the optical fiber.
               Microwave Transmission
Microwaves form part of the electromagnetic spectrum, which
includes light and radio waves. At high power, microwaves can be
used for cooking (hence microwave ovens). At low power, microwaves
are harmless and can be used for communication. Microwaves cannot
travel through mountains or hills but they can travel through fog,
trees and brick walls. This means that microwaves can be used to
transmit data to locations that are in line-of-sight.
    Receiver                                             Transmitter
       Dish                                              Dish

   Microwaves are transmitted to locations that are in line of sight.
   The microwaves are send and received using parabolic dishes.
        Microwave Transmission cont.
Previous figure shows how microwaves are transmitted from site to
site using parabolic dishes. Using microwaves is cheaper than using
landlines are widely used for telecommunications.

A single microwave link can cover a distance of up to 50km and
transmit at a frequency of up to 10GHz.

One problem is that microwaves are sometimes refracted by low level
atmospheric conditions.

This creates multiple paths for the microwave signal that may arrive
at the receiver dish out of phase and cause severe interference.
             Microwave / Satellites
Microwaves are also used to communicate with satellites. Satellites
used for communication purposes are typically geostationary, meaning
that they orbit the earth once every 24 hours and appear to remain
in the same place in the sky (at a height of 35,880 km or 22,300
miles). Communication satellites have directional antenna and on-
board circuits called transponders. Each transponder receives a
particular band of microwave frequencies and relays (retransmits)
the signals it receives to another location on the Earth’s surface.
Figure below shows how a satellite can provide out of sight

                                Up link               Antenna

                                                    Down Link

                                                         Ground Station
         Radio Wave Transmission
Radio waves, like microwaves and light, are part of the
electromagnetic spectrum. Data can be transmitted using a radio
wave as the carrier wave. Longer wavelength radio waves can
penetrate hills and mountains.      Shorter radio waves must be
transmitted in line-of-sight because the ground easily absorbs them
(although they can travel through buildings).

Some high frequency (HF) radio waves, however, are able to travel
great distances by reflecting off the upper part of the Earth’s
atmosphere (the ionosphere).

All radio waves are subject to interference from machinery such as
motors or electrical devices. Also the strength of a radio waves falls
off quickly over distance (roughly at a rate proportional to 1/r3 for
low frequencies where r is the distance from the transmitter).
    Modes of serial data transfer
• Simplex communications
  – Unidirectional data path from transmitter to receiver
    in the manner of radio broadcasts
• Half Duplex
  – Unidirectional at any one time in the manner of a
    conversation over radio link with change of direction
    signaled by ‘over’.
• Full Duplex
  – two computers using two comms channels one for
    transmission and one for reception both working
                 Parallel data transfer
• Most data in the form of bytes or wider.
  – Transfer all of the bits at the same time however one conductor for each
    bit, more copper etc. suitable for short distances and very high data rates,
    used inside computer where groups of conductors are called busses .
  – synchronisation between each bit on different conductors becomes
    difficult specially as distance increases due to tiny differences between
    conductors and their environment.
Serial Transmission
        Connectors and cables
• D-type 25way used for RS232 serial links
  – consider computer- modem cable with straight
    through cable connecting DTE and DCE.

• RJ45 - telephone type connectors.

• Ribbon Cables and IDC connectors

• Network connectors and cables
Old Fashioned Coaxial
Cables for data transmission
            Benefits of both….
            • Common mode rejection.
            • Speeds of each (cat 5e
              100m bits/sec)
            • Shielding against induced
      Transatlantic Communications
It was only 150 years ago since the first Atlantic submarine cable
was completed, connecting Trinity Bay via Newfoundland to
Valentia Island in Ireland.

Although the subsea cable laying is still an arduous, expensive
and sometimes dangerous task as it was then, the key differences
in today’s cable is the quality and the sheer quantity of
information that can be sent through links underwater.

In August of 1858, the first transatlantic message was sent over
the new link from Queen Victoria of the United Kingdom to the
United States President James Buchanan, the 15th President of
the United States, who served right before President Abraham
                  Source: Dark Fibre Communications Magazine, Looking Back to Look Forward: 150 Years
                  of Transatlantic Communications by Jaymie Scotto and Ilissa Miller, April 13, 2009
       Transatlantic Communications
The message was 99 words and took 12 hours to send. The cable
that carried this message sent only 400 messages before it began to
fail, just 23 days after it initially went live.

Today, a privately held Trans-Atlantic submarine cable network
could send the entire Library of Congress in only 63 milliseconds
over one of their fiber optic submarine cable links and can carry
30 million simultaneous phone calls at a single time.
       Transatlantic Communications

• The first transatlantic submarine cable system was born from a gentleman
named Cyrus Field. He was born in 1819 in Stockbridge, Massachusetts and
to this day is considered to be the ‘father’ of the Atlantic sub-sea cable

• Initially working in the paper industry with his family, he was fascinated
with telegraphy, which is how he got the idea for a transatlantic telegraphic

• In 1854, with friends and associates, Cyrus Field formed the New York,
New Foundland and London Telegraph Company that raised $1,500,000, a
lot of money especially at that time.

• The company secured landing rights for the American side of the Ocean and
set out to install the first transatlantic sub-sea cable.
         Transatlantic Communications
One of Cyrus Fields very close friends was Samuel Morse, who in 1832
had the first ideas of the electromagnetic telegraph with his Associate Dr.
Charles T. Jackson.
Just 4 years later, Mr. Morse demonstrated his recording Telegraph. The
following year, in 1837, he successfully relayed a message through ten
miles of wire, on reels.
In 1842, after further developing telegraph communications, he began
experiments with underwater transmissions on the two- mile stretch
between the Battery in lower Manhattan and Governors Island in New
York Harbor where he was successful at sending signals.

 In 1843, Congress voted to grant $30,000 to install an experimental
telegraph line from Washington DC to Baltimore, Maryland.
Unfortunately, the lead pipes that were installed did not work so they
converted the telegraph lines to above ground poles.
        Transatlantic Communications
By 1845, two years later, the Magnetic Telegraph Company was
created to extend cables from Baltimore to Philadelphia and New

By 1849, it was estimated that there was 12,000 miles of telegraph
lines run by twenty different companies in the US.

In 1856, the Western Union Telegraph Company was formed by a
number of small companies.

This brings us back to the years of 1854-1858, during the early
times of transatlantic cabling.

Samuel Morse was an electrician for Cyrus Field during the first
attempts to lay the cable across the Atlantic.
        Transatlantic Communications
An interesting turn of events occurred when Cyrus Field went to
England to recruit assistance for the commission of the first
transatlantic submarine cable.
There he met Dr. Whitehouse, originally trained as a surgeon though
by this time, his interests were more focused on the advancements of

When planning to build the system, Samuel Morse and Dr.
Whitehouse decided together that the cable should be constructed as
thinly as possible.

This was in opposition to others on the project that included Lord
Kelvin and Charles Bright. Cyrus Field ‘broke the tie’ and set out to
install a thin wire to lay the cable.
        Transatlantic Communications
Others included Lord Kelvin and Charles Bright. Lord Kelvin
introduced ‘kinetic energy’ in 1856 and joined the Cyrus Field
project to lay the cable where he applied his analogy of heat flow to
the flow of electricity.
Lord Kelvin was heavily involved with the project. He assisted in
improving the design of the cables. He also invented the mirror
galvanometer to act as a long distance telegraph receiver and
supervised the laying of the galvanometers.
On the seas, Kelvin improved the way mariners worked with the
invention of an improved gyrocompass, a new sounding equipment,
and a tide-prediction, chart-recording machine.
In 1866, Lord Kelvin was knighted for his achievements in
submarine cable laying. When he died in 1907, he was buried next
to Sir Isaac Newton in Westminster Abbey.
         Transatlantic Communications
The other gentleman was Sir Charles Tilston Bright, who in 1856, at
the age of 26, became the youngest person knighted at the time, Sir
Bright was known in England as the man who first laid a complete
system of wires under the streets of Manchester, at age 19.

After this feat, he became the Chief Engineer of the Magnetic
Telegraph Company where he extended these lines from Manchester
throughout the United Kingdom.

He later established the first connection from Great Britain to Ireland
propelling Lord Kelvin to proclaim him as the ‘first to lay a cable in
deep water.’ This was the feat, laying the first Atlantic cable that
earned him his title as knight in 1856.
       Transatlantic Communications
Sir Bright was an inventor, and one of his earliest
inventions was as system that tested the insulated
conductors to localize faults from a distance point, by
means of a series of standard resistance coils of different
values, brought into circuit successively by turning a
connecting handle.

This became the best way to test submarine telegraphs.

After appointing Lord Kelvin as the Chairman of the
Atlantic Telegraph Co, the ability to influence the
development of transatlantic cable connectivity was
        Transatlantic Communications
As the Chief Engineer of the project, it was Charles Bright who
suggested a much thicker line, that would weigh 460 tons and would
have had 3.5 more power in conducting speed, be used; however,
this idea was passed over for Mr. Whitehouse’s much thinner copper

So back in 1857-58, with a vote of three to two, these gentlemen set
out to connect two continents with a thin cable. They began in
Vanetia Harbor, Ireland on August 5, and six days later, with limited
rate of descent and with just 3580 miles laid, the cable snapped.

After returning to shore, an extra 700 miles of cable were made and
the second attempt was made on June 25. This time two ships met
each other in mid-Atlantic where they joined their respective ends—
and the cable broke almost immediately.
       Transatlantic Communications
Again the two ships made anther splice, and after another
40 miles, the cable broke again. The fourth time they had
laid 145 miles before the cable snapped again.

The ships returned to Ireland and the men decided that
despite the loss, there was still enough cable for one more

On July 29, the crew set out for one final, fifth attempt,
starting from the midpoint.

This time, it was a success - and on August 5, 1858, the
two continents were connected.
        Transatlantic Communications
Unfortunately, this success, which garnered much celebration and
fanfare, was short-lived.
Dr. Whitehouse overloaded the system by applying very high
voltages rather than the very weak currents that had been tested
during the cable laying.
Within three weeks, the damage from the high voltages became
apparent and the cable stopped working. In addition, the dots and
dashes of Morse code ended up smearing out over such a long haul.
The failure of this cable was so catastrophic that the creation of the
Committee of Inquiry was formed in England to investigate the
cause. As a result, Dr. Whitehouse lost credibility and his career
spiraled downward. To successfully construct a second working
transatlantic cable system, Cyrus Field had to employ others instead.
        Transatlantic Communications
• After several other cable snaps and failed attempts, and an
additional $2,500,000 funding, the Great Eastern cable was pulled
ashore a tiny fishing village in Newfoundland on July 27, 1866.

• The distance was 1868 nautical miles and the Great Eastern
averaged 120 miles a day while paying out the cable.

• Perhaps the story of the first cable system would have ended
differently if Sir Charles Tilston Bright’s thicker, copper cable were
installed. But nonetheless, the enhanced communications it provided
between Europe and North America were unfounded at the time.

• The transatlantic cable was considered ‘The Great Scientific
Achievement of the Century’; the laying of cable in open sea was a
feat of strength, endurance and wonder.
• Today, there are several cable systems interconnecting North
America and Europe.

• However, there is only one company that offers a similar route to
the first transatlantic cable system, and that is Hibernia
Atlantic. This cable connects Halifax, Nova Scotia directly to
Dublin, Ireland.

• In August 2009, the company finally connected Northern Ireland at
Portrush onto the northern spur of the existing Hibernia Atlantic
submarine system, providing the first modern submarine cable link
connecting Northern Ireland directly to Europe and North America.

• The name assigned to this undertaking is Project Kelvin, named
after Lord Kelvin and his significant role in the first undertaking.
      Transatlantic Communications

Only 150 years ago, the first transatlantic submarine cable system
was painstakingly deployed.

Today billions of bits of data are transmitted every minute
between North America, New Foundland, Ireland and Europe,
connecting the world with high-speed capacity.

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