Physical Transmission

1
Physical Transmission Media
2
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.
3
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.  
               
Figure Diagram of a copper atom and the free
electron in its outer shell that moves
easilyfrom one atom to the next under the
influence of an electromotive force.
Just one electron in outer shell. This is a free
electron.
4
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.
5
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.
Figure 3.2 Twisted pair cables are less effected
by non-thermal noise than single cables.
6
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 insulated1
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 objects.
7
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
effect1. 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).
8
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.   Broadband1 Coaxial Cable is usually
used for carrying analogue transmissions whereas
Baseband2 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.
9
Physical Transmission Media
The structure of coaxial cable.
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 metres.
10
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.
The electromagnetic wave spectrum.
11
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 ?
are related by equation (3.1).   (3.1)   where
c is the speed of light (299,792,458 m/s or
approximately 3?108m/s).
12
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).
13
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.  
14
Nyquists Limit

The maximum data rate for any channel can be
calculated from Nyquists Limit that
states   (3.3)   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_bps2H 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. Nyquists 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.
15
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).
16
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.
17
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.
Microwaves are transmitted to locations that are
in line of sight.The microwaves are send and
received using parabolic dishes.
18
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.
19
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 Earths surface. Figure below shows how a
satellite can provide out of sight communication.
20
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 Earths 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).
21
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
  simultaneously.

22
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.

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Serial Transmission
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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

25
Old Fashioned Coaxial connection
26
Cables for data transmission

• Benefits of both.
• Common mode rejection.
• Speeds of each (cat 5e 100m bits/sec)
• Shielding against induced noise.

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28
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 todays 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
Lincoln. 
Source Dark Fibre Communications Magazine,
Looking Back to Look Forward 150 Years of
Transatlantic Communications by Jaymie Scotto and
Ilissa Miller, April 13, 2009
29
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. 
30
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 systems. 
• 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 cable.
• 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.

31
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.
32
Transatlantic Communications
By 1845, two years later, the Magnetic Telegraph
Company was created to extend cables from
Baltimore to Philadelphia and New York.  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.
33
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 telegraphy.   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. 
34
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.
35
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.   
36
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 realized.
37
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. Whitehouses
much thinner copper cable.   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 endsand the cable broke
almost immediately.  
38
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
attempt.  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.
39
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.
40
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 Brights 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.

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Summary

•  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. 

42
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.