Transcript Fiber Optics - WordPress.com

Physical Layer
Dr. K. Raghava Rao
Professor, Dept. of ECM
[email protected]
Transmission Media
Physical layer: Transport a raw bit stream
 Characteristics :bandwidth, delay, cost, and ease
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of installation and maintenance
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Physical media
Guided media
Information transmitted on wires by varying
some physical property such as voltage or current
Copper wire, fiber optics
Unguided media
Information transmitted wirelessly by
electromagnetic waves
Radio, lasers
Guided Media
Magnetic Media
 Twisted pairs
 Coaxial cable
 Fiber optics

Magnetic Media
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Transport data from one computer to another
by writing data onto magnetic tape or
removable media (e.g., recordable DVDs).
Physically transport the tape or disks to the
destination machine, and read them back in
again.
More cost effective, especially for applications
in which high bandwidth or cost per bit
transported is the key factor.
Magnetic Media
Bandwidth
 An industry standard Ultrium tape can hold 200
gigabytes.
 A box 60 x 60 x 60 cm can hold about 1000 of these
tapes, for a total capacity of 200 terabytes.
 A box of tapes can be delivered anywhere in the
United States in 24 hours by Federal Express and other
companies.
 The effective bandwidth of this transmission is 200
terabytes/86,400 sec, or 19 Gbps.
 If the destination is only an hour away by road, the
bandwidth is increased to over 400 Gbps
Magnetic Media
Cost
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The cost of an Ultrium tape is around $40 when
bought in bulk.
A tape can be reused at least ten times, so the tape
cost is maybe $4000 per box per usage.
Add to this another $1000 for shipping (probably
much less), and we have a cost of roughly $5000 to
ship 200 TB
Twisted Pair Cable
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Oldest, but still most common
Two twisted insulated copper wires about 1 mm thick.
The wires are twisted together in a helical form, just like
a DNA molecule.
Twisting is done because two parallel wires constitute a
fine antenna.
When the wires are twisted, the waves from different
twists cancel out, so the wire radiates less effectively.
Why twisted?
To reduce electrical interference
Twisted pair Cable
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Applications: Telephone system, Ethernet
Repeater needed for longer distances
Repeater: device that extends the distance a
signal can travel by regenerating the signal
Twisted pairs can be used for transmitting either
analog or digital signals.
The bandwidth depends on the thickness of the wire
and the distance traveled.
Adequate performance at low cost
Twisted Pair
Category 5 UTP cable with 4 twisted pairs
Twisted Pair

Category 3 and 5 .
Popular by UTP (Unshielded Twisted Pair)
Twists results in less crosstalk and a better-quality
signal over longer distances.
Up-and-coming categories are 6 And 7, which are
capable of handling signals with bandwidths of 250
MHz and 600 MHz, respectively (versus a mere 16
MHz and 100 MHz for categories 3 and 5,
Cat3
Cat 5
Coaxial Cable

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Better shielding than twisted pairs
Span longer distances at higher speeds
Lower error rate
Widely used for
Cable TV
WAN (Internet over cable)
Coaxial Cable
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Two kinds of coaxial cable are widely used.
50-ohm cable-for digital transmission .
75-ohm cable-for analog transmission .
A coaxial cable consists of a stiff copper wire as the
core, surrounded by an insulating material. The
insulator is encased by a cylindrical conductor, often
as a closely-woven braided mesh. The outer
conductor is covered in a protective plastic sheath.
Shielding gives high bandwidth and excellent noise
immunity.
The bandwidth depends on the cable quality, length,
and signal-to-noise ratio of the data signal.
Modern cables have a bandwidth of close to 1 GHz.
Fiber Optics

Transmission of light through fiber
 Including 3 components:
1. Light source: Pulse of light=1,
Absence of light=0
1. Transition medium: an ultra-thin fiber of glass
2. detector: generate an electrical pulse when
light falls on it.
Fiber Optics

Light
Electromagnetic energy traveling at 3108 m/s
Refraction
Critical angle
Reflection
Fiber Optics
(Less dense)
(More dense)
cladding
core
I
(critical
angle)
cladding
(a) Three examples of a light ray from inside a silica
fiber impinging on the air/silica boundary at
different angles.
(b) Light trapped by total internal reflection.
Reflection & Refraction
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Reflection is the change in direction of a wavefront at
an interface between two different media so that the wavefront
returns into the medium from which it originated.
Refraction is the change in direction of a wave due to a
change in its transmission media.
The critical angle is the angle of incidence above
which total internal reflection occurs.
Total internal reflection is a phenomenon that happens when
a propagating wave strikes a medium boundary at an angle
larger than a particular critical angle with respect to the normal
to the surface.
Cladding is one or more layers of materials of lower refractive
index, in intimate contact with a core material of higher
refractive index.
Reflection & Refraction
Total Internal Reflection
Fiber Optics

Thickness of core: 8~10 microns or 50 microns
 Two typically light sources:
1. LED (Light Emitting Diode)
response time=1ns  data rate = 1Gbps
2. Semiconductor laser
Fiber Optics
Properties include total internal reflection
and attenuation of particular frequencies.
 Fiber Optic Networks - can be used for
LANs and long-haul.
Fiber Optics Vs Copper Wire
Fiber
Copper
Bandwidth
Higher
Lower
Distance between repeaters
30 KM
5 Km
Interference
Low
High
Physical
Smaller/Lighter
-
Flow
Uni-directional
Bi-directional
Structure of Telephone System
Major components:
1. Local loops (analog twisted pairs going into houses and
businesses).
2. Trunks (digital fiber optics connecting the switching offices).
3. Switching offices (where calls are moved from one trunk to
another)
Modems
Computer is digital
Telephone line is analog
Need translation device called a modem
Analog
Signal
Digital
Signal
Modem
MODEM
Modulation

A Modem is a Modulator and Demodulator
 Modulation is converting outgoing digital device
signals into analog transmission line signals
 Demodulation is converting incoming analog
transmission line signals into digital device signals
Analog
Signal
Digital
Signal
Modem
Problems in Transmission Lines
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Attenuation is the loss of energy as the signal propagates
outward.
Delay distortion is caused by different Fourier components
propagating at different speeds in the wire.
Noise is unwanted energy from sources other than the transmitter.
 Thermal noise is caused by the random motion of the electrons
in a wire and is unavoidable.
 Sometimes when talking on the telephone, you can hear
another conversation in the background. That is crosstalk.
DC Signaling is subject to strong attenuation and delay distortion.
Hence on telephone lines, AC signaling is use
.
Analog signaling consist of varying a voltage with time to represent
an information stream. If transmission perfect , the receiver
receive exactly same signal, but media is not perfect. For digital
data , this difference can lead to errors.
Why modulation is necessary ?
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Signals are transmitted between a transmitter over
some form of transmission medium
But normally signals are not in the form that is suitable
for transmission and need to be transformed
 Bandwidth requirement
 Signals multiplexing
 Complexity of transmission system
 Preventing noise, interference, attenuation
Modulation is a process of impressing (applying) a low
frequency information signals to onto a relatively high
frequency carrier signal
Modulation techniques
 Amplitude modulation: Two different
amplitudes of sine wave are used to represent 1's
and 0's.
Frequency modulation: Two (or more) different
frequencies, close to the carrier frequency, are
used.
Phase modulation: The phase of the sine wave
is changed by some fixed amount.
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The number of samples per second is measured in baud. During
each baud one symbol sent. This n-baud lines send n symbols /Sec.
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If symbol consists of 0 volts for logical 0 and 1 volt for logical 1, the
bit rate 2400 bps. If voltages 0,1,2 and 3 volts are used, every
symbol consists of 2 bits, so a 2400 baud-line can transmit 2400
symbol s/sec at a data rate of 4800 bps.

For example, a 2400-baud line sends one symbol about every
416.667 microsecond. If every symbol consist of 2 bits , a 2400 –
baud line can transmit 2400 symbols/sec at a data rate of 4800
bps.

Similarly, with four possible phase shifts , there are also 2 bits
symbol , so again the bit rate twice the baud rate. This technique is
called QPSK(Quadrature Phase Shift keying)
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Bandwidth of a medium is the range of frequencies
that pass through it with minimum attenuation. It is a
physical property of the medium and measured in Hz.
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The baud rate is the number of samples/sec made.
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Each sample sends one piece of information, that is,
one symbol. The baud rate and symbol rate are thus
the same.
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All advanced modems use a combination of
modulation techniques to transmit multiple bits per
baud.
Often multiple amplitudes and multiple phase shifts
are combined to transmit several bits/symbol.
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a)With four possible phase shifts, Used to transmit 2 bits per
symbol. It is QPSK ((Quadrature Phase Shift Keying)
b)Transmit 4 bits per symbol. It is called QAM-16 (Quadrature
Amplitude Modulation).
c)Allows 64 different combinations, so 6 bits can be transmitted
per symbol. It is called QAM-64.
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Noise in detected amplitude or phase
can result in an error and potentially,
many bad bits.

To reduce the chance of an error,
standards for the higher speeds modems
do error correction by adding extra
bits(parity bit) to each sample. The
scheme known as TCM(Trellis Coded
Modulation)
Telephone Modems
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A telephone line has a bandwidth of
3000 Hz (3300 – 300) for voice
2400 Hz (3000 – 600) for data
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Modem standards
V.32: 9,600 bps
V.32bis: 14,400 bps
V.34bis: 28,800 ~ 33,600 bps
V.90: download up to 56kbps (56K modem)
V.92: adjustable speed, call waiting, etc.
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What is DSL?
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Digital Subscriber Line-New modem technology.
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Data transmission is based on digital encoding (digital).
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Use digital coding techniques to provide more capacity.
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Allows high-speed Internet access over existing twistedpair and ordinary copper telephone wires.
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Provides "always-on" connection.
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To transport high-bandwidth data.
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A special hardware attached to both the user and switch
ends of line.
Advantages of DSL
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High-speed.
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Secure connection.
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No dial-up, waiting or dropped connections.
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It's always on connection.
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Saves both money and time.
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Provides large file transfers.
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Multiple workers on a network can connect to a single
DSL.
What is ADSL?
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Asymmetric Digital Subscriber Line.
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Is a form of DSL.
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A high-speed Internet access service.
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Speed depends on the length and the diameter of the
cable and the type of the mode
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Requires a special ADSL modem and an Internet
service provider (ISP) .
What is ADSL?
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It is asymmetric since the data coming to your computer
from the Internet (download) is faster than the data
traveling from your computer to the Internet (upload).
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Uses standard telephone lines.
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Telephone can be used normally, even when surfing in
the Web with ADSL service.
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An "always on" service.
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Not available to everyone.
How does ADSL work?
ADSL
 ADSL requires a special ADSL modem and subscribers must be in
close geographical locations to the provider's central office to
receive ADSL service. Typically this distance is within a radius of
2 to 2.5 miles.
 ADSL supports data rates of from 1.5 to 9 Mbps when receiving
data (known as the downstream rate) and from 16 to 640 Kbps
when sending data (known as the upstream rate).
 ADSL divide the available 1.1 MHz spectrum on the local loop
into 256 independent channels of 4312.5 Hz each.
 Channel 0 is used for POTS. Channels 1–5 are not used, to keep
the voice signal and data signals from interfering with each
other.
 Of the remaining 250 channels, one is used for upstream control
and one is used for downstream control. The rest are available
for user data.
ADSL(Asymmetric Digital Subscriber Line)
Operation of ADSL using discrete multitone modulation.
Wireless local Loop
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Wireless local loop service called LMDS
Local Multipoint Distribution Service.
Like ADSL, LMDS uses an asymmetric bandwidth
allocation favoring the downstream channel.
With current technology, each sector can have 36 Gbps
downstream and 1 Mbps upstream, shared among all the
users in that sector.
IEEE set up a committee called 802.16 to draw up a
standard for LMDS.
The IEEE calls 802.16 a wireless MAN.
LMDS
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A tower with multiple antennas , each pointing in a
different direction.
Each antenna defines a sector, independent of the
other ones.
A single tower with four antennas could serve 100,000
people within a 5-km radius of the tower.
LMDS has a few problems
Millimeter waves propagate in straight lines, so there
must be a clear line of sight between the roof top
antennas and the tower.
Leaves absorb these waves , so the tower must be
high enough to avoid having trees in the line of sight
Trunks & Multiplexing
Trunks (digital fiber optics connecting
the switching offices).
 How to collect multiple calls together
and send them out over the same fiber.
This subject is called multiplexing.
 Telephone companies have developed
elaborate schemes for multiplexing
many conversations over a single
physical trunk.
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Multiplexing
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Multiplexing schemes can be divided into two basic
categories:
FDM (Frequency Division Multiplexing)
TDM (Time Division Multiplexing).
In FDM, the frequency spectrum is divided into
frequency bands, with each user having exclusive
possession of some band.
In TDM, the users take turns (in a round-robin
fashion), each one periodically getting the entire
bandwidth for a little burst of time.
Advanced FDM applied to fiber optics called WDM
(wavelength division multiplexing).
Advanced TDM system used for fiber optics
(SONET)Synchronous Optical NETwork.
Multiplexing
Multiplexor (MUX)
Demultiplexor (DEMUX)
Sometimes just called a MUX
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Multiplexing
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Two or more simultaneous
transmissions on a single circuit.
Transparent to end user.

Multiplexing costs less.
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Frequency Division Multiplexing
Assignment of non-overlapping frequency ranges to
each “user” or signal on a medium.
 Thus, all signals are transmitted at the same time,
each using different frequencies.
A multiplexor accepts inputs and assigns frequencies
to each device.
The multiplexor is attached to a high-speed
communications line.
A corresponding multiplexor, or demultiplexor, is on
the end of the high-speed line and separates the
multiplexed signals.
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Frequency Division Multiplexing
Analog signaling is used to transmits the signals.
Broadcast radio and television, cable television, and the
AMPS cellular phone systems use frequency division
multiplexing.
This technique is the oldest multiplexing technique.
Since it involves analog signaling, it is more susceptible to
noise.
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Time Division Multiplexing
Sharing of the signal is accomplished by dividing available
transmission time on a medium among users.
Digital signaling is used exclusively.
Time division multiplexing comes in two basic forms:
1. Synchronous time division multiplexing, and
2. Statistical, or asynchronous time division multiplexing.
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Synchronous Time Division Multiplexing
The original time division multiplexing.
The multiplexor accepts input from attached devices in a
round-robin fashion and transmit the data in a never ending
pattern.
T-1 and ISDN telephone lines are common examples of
synchronous time division multiplexing.
Drawbacks
If one device generates data at a faster rate than other devices, then
the multiplexor must either sample the incoming data stream from
that device more often than it samples the other devices, or buffer the
faster incoming stream.
If a device has nothing to transmit, the multiplexor must still insert
a piece of data from that device into the multiplexed stream.
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The T1 carrier (1.544 Mbps).
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Statistical Time Division Multiplexing
A statistical multiplexor transmits only the data from active
workstations (or why work when you don’t have to).
If a workstation is not active, no space is wasted on the
multiplexed stream.
A statistical multiplexor accepts the incoming data streams
and creates a frame containing only the data to be transmitted.
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To identify each piece of data, an address is included.
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If the data is of variable size, a length is also included.
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Wavelength Division Multiplexing

Prisms form the basis of optical multiplexing and
demultiplexing
a multiplexor accepts beams of light of various
wavelengths and uses a prism to combine them into a
single beam
a demultiplexor uses a prism to separate the
wavelengths.
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Wavelength Division Multiplexing (WDM)
Here four fibers come together at an optical combiner,
each with its energy present at a different wavelength.
The four beams are combined onto a single shared
fiber for transmission to a distant destination.
At the far end, the beam is split up over as many
fibers as there were on the input side.
Each output fiber contains a short, speciallyconstructed core that filters out all but one wavelength.
The resulting signals can be routed to their destination
or recombined in different ways for additional
multiplexed transport.
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SONET/SDH
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Synchronous Optical NETwork
Synchronous Digital Hierarchy
SONET is a synchronous system.
It is controlled by a master clock with an accuracy of
about 1 part in 109.
Bits on a SONET line are sent out at extremely
precise intervals, controlled by the master clock.
The basic SONET frame is a block of 810 bytes put
out every 125 μsec.
Since SONET is synchronous, frames are emitted
whether or not there are any useful data to send.
SONET
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The 810-byte SONET frames are best described as a
rectangle of bytes, 90 columns wide by 9 rows high.
Thus, 8 x 810 = 6480 bits are transmitted 8000 times
per second, for a gross data rate of 51.84 Mbps.
This is the basic SONET channel, called STS-1
(Synchronous Transport Signal-1).
The first three columns of each frame are reserved
for system management information.
The first three rows contain the section overhead;
the next six contain the line overhead.
The section overhead is generated and checked at
the start and end of each section, whereas the line
overhead is generated and checked at the start and
end of each line.
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A SONET transmitter sends back-to-back 810-byte
frames, without gaps between them, even when
there are no data (in which case it sends dummy
data).
The remaining 87 columns hold 87 x 9 x 8 x 8000 =
50.112 Mbps of user data.
However, the user data, called the SPE (Synchronous
Payload Envelope), do not always begin in row 1,
column 4.
The SPE can begin anywhere within the frame.
A pointer to the first byte is contained in the first row
of the line overhead. The first column of the SPE is
the path overhead (i.e., header for the end-to-end
path sublayer protocol
Two back-to-back SONET frames
Switching

The phone system is divided into two principal parts:
Outside plant (the local loops and trunks, since they
are physically outside the switching offices)
Inside plant (the switches), which are inside the
switching offices.
Types of Switching
Circuit switching
Packet switching
Circuit & packet Switching
Circuit Switching
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When you or your computer places a telephone call,
the switching equipment within the telephone system
seeks out a physical path all the way from your
telephone to the receiver's telephone. This technique
is called circuit switching.
An important property of circuit switching is the need
to set up an end-to-end path before any data can be
sent.
Message Switching
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An alternative switching strategy is message
switching.
No physical path is established in advance between
sender and receiver.
Instead, when the sender has a block of data to be
sent, it is stored in the first switching office (i.e.,
router) and then forwarded later, one hop at a time.
Each block is received in its entirety, inspected for
errors, and then retransmitted.
A network using this technique is called a store-andforward network
Packet Switching
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With message switching, there is no limit at all on block
size, which means that routers (in a modern system) must
have disks to buffer long blocks.
It also means that a single block can tie up a router-router
line for minutes, rendering message switching useless for
interactive traffic.
To get around these problems, packet switching was
invented.
Packet-switching networks place a tight upper limit on
block size, allowing packets to be buffered in router main
memory instead of on disk.
In packet Switching individual packets are sent as need
be, with no dedicated path being set up in advance. It is
up to each packet to find its way to the destination on its
own.
Timing of events in (a) circuit switching, (b) message switching, (c) packet switching