COM347J1 Networks and Data Communications L1
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Transcript COM347J1 Networks and Data Communications L1
COM347J1
Networks and Data Communications
Lecture 3: The Physical layer
Ian McCrum
Room 5D03B
Tel: 90 366364 voice mail on 6th ring
Email: [email protected]
Web site: http://www.eej.ulst.ac.uk
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Today physical media:
•
•
•
•
•
Serial and Parallel connections
Connectors
Cables Coaxial, twisted pair
Optical fibers
Radio waves
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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.
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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.
Start
End
Transmission -->
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Serial slower but cheaper
Serial-to-parallel conversion
Parallel data in
Transmit Buffer Register
Control
Signals
Serial data out
Transmit Data Register
Control
Control
Parallel data link
Unit
Signals
Serial data in
Receiv e Data Register
Control
Signals
Parallel data link
Receiv e Buffer Register
Parallel data out
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Connectors and cables
• Standards… often specify details
• D-type 25way used for RS232 serial links in old days (and in the
“official standard”) Modern usage dictated by PC design … 9 pin Dtype connector
– consider computer- modem cable with straight through cable connecting
DTE and DCE. Necessary because uni-directional line drivers all that
were available in the old days…
• RJ45
– telephone type connectors.
• Ribbon Cables and IDC connectors
• Network connectors and cables
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Cables for data transmission
Twisted pair
P VC/T eflon
Braid Foil Braid
Insulation
T in-plated solid copper core
Cent re conduct or
Dielect ric
Jacket
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Electrostatic shielding
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Typical Coaxial connection
T -piece
Segment
Connector
Network interface
T ransceiver circuit
T erminator 10base2 Cable
T erminator
Maximum cable lengt h 200m
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Benefits of coaxial and Twisted
pair
• Shielding against induced noise.
• Common mode rejection.
• Speeds of each (cat 5e 100m bits/sec)
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Twisted Pair
(a) Category 3 UTP.
(b) Category 5 UTP.
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Coaxial Cable
A coaxial cable.
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Fiber Cables
(a) Side view of a single fiber.
(b) End view of a sheath with three fibers.
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Fiber Optic Networks
A fiber optic ring with active repeaters.
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Fibre optic cable is available in three basic forms:
1. Stepped-index fibre. In this type of fibre, the core has a
uniform refractive index throughout. This generally has a
core diameter of
to
. This is a multi-mode fibre.
Stepped-index fibre
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Graded-index fibre. In this type of fibre, the
core has a refractive index that gradually
decreases as the distance from the centre of
the fibre increases. This generally has a core
diameter of
. This is a multi-mode fibre.
Graded-index fibre
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Mono-mode fibre. As the name suggests, the distinguishing
characteristic of this fibre is that allows only a single ray path. The
radius of the core of this type of fibre is much less than that of the
other two, however it does have a uniform refractive index.
From, 1 to 3, we find that the cost of production increases, the
complexity of transmitter and receiver increases, while the dispersion
decreases. This latter property change means that the mono-fibre also
has the potential to provide greater bandwidth. As it becomes cheaper to
produce mono-mode fibre technology, we will see an increased use of
this type of optical fibre
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Fiber Optics
(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.
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Transmission of Light through Fiber
Attenuation of light through fiber in the infrared region.
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Fiber Cables
A comparison of semiconductor diodes and LEDs as light
sources.
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Optical fibre is a waveguide. The fibre (in its simplest form)
consists of a core of glass of one refractive index, and a cladding
of a slightly lower refractive index (Figure ). The fibre is then
surrounded by a refractive sheath. Typical fibre dimensions
are
to
diameter.
The basic structure of a fibre optic waveguide
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In simple terms, the action of a waveguide can be partially understood
by considering the rays down the fibre. A light-wave entering the fibre
is either refracted into the cladding, and attenuated, or is totally
internally reflected at the core/cladding boundary. In this manner it
travels along the length of the fibre. The maximum angle at which it
may enter the guide and travel by total internal reflection is termed the
acceptance angle It is also possible for the wave to follow a helical
path down the guide. These rays are called skew-rays.
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However, this view is too simple to explain all
features of waveguide behaviour. In fact, it is not
possible for the wave to take any ray down the
guide. Only certain rays can be taken. These rays
are called modes. For any particular frequency,
there is a different ray. The modal action of a
waveguide is a consequence of the wave nature of
the radiation. A mono-mode fibre is a fibre that
only has one acceptable ray-path per frequency. A
multi-mode fibre has a number of possible rays
that light of a particular frequency may take.
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Snell’s Law
n1
y
1 1
n1
n2
n2
2
1
Sin
1
Sin
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2
=
2
n
n2
n1
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Total Internal Reflection
n1
y
1
1
2
n2
as
then
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1
n1
2
Sin1
n
2
Sin 2
n1
n2
1
2
n
Cos1
n
2
Cos 2
n1
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From the diagram n1 is greater than n2
so
2
decreases as
until as
1 2
1
,
for a finite value of
1
decreases
2 0
1 .
is now the critical angle
c
beyond which Total Internal Reflection
occurs and
n2
c Cos
n1
1
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Light Acceptance cone
y
n2
n2
n1
c
n1
m
1
as
then
n2
n1
c Cos 1
2
n
Sin c
1
m Sin 1
when Snell is applied therefore the light
acceptance cone is 2 m
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n
Propagation of light by total
internal refection
n2
n1
c
a
c
l
See attenuation profile Fig 2.6 A.T.
and then
Fig 2.7 for fibre construction
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Copper v F.O.
• repeaters 5km
• reactive
• E.M. R.F.
problems
• bulky
• tappable
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• repeaters 30km
• relatively inert
• no E.M. R.F.
problems
• >bandwidth in
duct
• no tapping
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Wireless Tx
c
f
• Wavelength* frequency = speed of light
• therefore Atlantic 252 where the 252
refers to the frequency in kilohertz ..
leads to the wavelength being 1190m
long where the speed of light is taken to
be 300,000,000 m/s
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Variety see Fig 2.11 for
spectrum
• Radio VLF,LW,MW 9kHz bandwidth, long dist,
earth hugging Fig 2.12
• Radio HF,VHF various bandwidths, straight lines
and ionosphere bounce up to 60MHz
• Microwave line of sight, large bandwidths
(418MHz)
• Infra Red line of sight, good for LAN in rooms
• Light - building to building good bandwidth Fig
2.13
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Communication Satellites
•
•
•
•
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Geostationary Satellites
Medium-Earth Orbit Satellites
Low-Earth Orbit Satellites
Satellites versus Fiber
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Communication Satellites
Communication satellites and some of their properties,
including altitude above the earth, round-trip delay time
and number of satellites needed for global coverage.
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Communication Satellites (2)
The principal satellite bands.
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Communication Satellites (3)
VSATs using a hub.
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Globalstar
(a) Relaying in space.
(b) Relaying on the ground.
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Telephone system for data
comms:
• Why telephone system for data
communications
• Structure of PSTN
• How it can carry digital data
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Public Switched Telephone
Network
• It exists everywhere and is relatively cheap
to establish contact
• It is slow and error prone.
• It is improving rapidly and costs are falling
• allows access for many home users to
Internet and enables home working.
• Vast investment
• Relies on Circuit switching
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PSTN Structure
• pairs of handsets therefore a conductor per pair, n
houses implied n conductors! Fig2.14a
• first manual centralised switching office with
jumpers being placed by operators Fig2.14b
• the interconnection of switching offices(cities) led
to the same problem one conductor per office pair
same problems as fig 2.14a
• hierarchy developed as in fig 2.14c
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PSTN Structure Fig 2.15
• Subscriber linked to local exchange by local loop by a pair
of copper wires, distance can be small or up to many
kilometres.
• thus a local call is switched with the local exchange.
• Local exchanges are connected by trunk lines in an
ascending hierarchy.
• medium and long distance calls are carried on multiplexed
high bandwidth links and managed through switching
higher up the hierarchy.
• International connections demand interfaces and
standardisation
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Transmission
• Local loops consist of twisted pairs and signalling is
analogue.
• trunks are higher bandwidth and employ coaxial(ageing), microwave and fibre optics. This uses
multiplexing for analogue(ageing) and digital signals.
• Amplification of an analogue signal can also amplify
the noise arising as it propagates thus noise can
predominate over a long connection.
• Amplification of a digital signal is merely the
regeneration of the original digital signal, thus only
noise is that which was originally present.
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Digital v Analogue
• Digital -Predictable attenuation therefore regenerators can be
reliably sited to restore the signal to either 0 or 1, therefore no
loss of signal even over long distances c.f. international
telephone calls.
• Analogue amplification is imperfect and cumulative over long
distances.
• Many sources can produce digital signals using the same
connections
• Data rates are increasing
• digital is cheaper
• digital more readily maintained.
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Transmission and reception
• Attenuation, loss in signal strength, increases
as a proportion to the length of conductor.
dB/km. varies with wavelength distorts wave
shape.
• delay distortion also varies with wavelength,
overlaps different bits, can limit bandwidth.
• noise, random and burst.
• crosstalk
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Modem
• MOdulator DEModulator
• Change a wave in such a manner that the
changes represent another signal
• recognise the changes in the received wave and
deduce what the modulating signal was.
• falling prices.
• high speeds.
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Modulation techniques
•
•
•
•
•
•
•
•
•
amplitude modulation
frequency modulation
phase modulation frequency shift keying
combination Quadrature amplitude modulation QAM
constellation patterns upto 64 points for 6 bits per baud
compression (more later)
echos supression and cancellation
full and half duplex
in-band signalling.
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Signal
Energy Distribution for
Human Speech
Bypass Filter
O Hz
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300 Hz
~3,400 Hz
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20 kHz
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Modems
(a) A binary signal
(c) Frequency modulation
(b) Amplitude modulation (d) Phase modulation
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Modems (2)
(a) QPSK.
(b) QAM-16.
(c) QAM-64.
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Modems (3)
(a)
(b)
(a) V.32 for 9600 bps.
(b) V32 bis for 14,400 bps.
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Trunk Line
SONET/SDH*
OC3/STM1
OC12/STM4
OC48/STM16
OC192/STM64
OC768/STM256
Speed
156 Mbps
622 Mbps
2.5 Gbps
10 Gbps
40 Gbps
speeds are multiples of 51.84 Mbps
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1. Normally, One Ring is Used in Each Ring
Telephone
Switch
SONET/SDH Ring
Telephone
Switch
2.
Rings Can Be
Wrapped if a
Trunk line
Is Broken.
Still a Complete
Loop.
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Break
Telephone
Switch
SONET/SDH Ring
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Digital Subscriber Lines
Bandwidth versus distanced over category 3
UTP for DSL.
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