L3161

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COM347J1Networks and Data Communications
Lecture 3 The Physical layer

• Ian McCrum Room 5D03B
• Tel [phone] voice mail on 6th ring
• Email IJ.McCrum_at_Ulster.ac.uk
• 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.

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Serial slower but cheaper
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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 D-type
  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
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Typical Coaxial connection
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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

• 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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Snells Law
y
1 2 n
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Total Internal Reflection
y
1 2 n
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From the diagram n1 is greater than n2
so
decreases as
decreases
,
until as
for a finite value of
.
is now the critical angle
beyond which Total Internal Reflection occurs and
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y
Light Acceptance cone
n
1 2
as
then
when Snell is applied therefore the light
acceptance cone is
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Propagation of light by total internal refection
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

• repeaters 30km
• relatively inert
• no E.M. R.F. problems
• bandwidth in duct
• no tapping

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Wireless Tx

• 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

• 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
  co-axial(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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Bypass Filter
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Modems

• (a) A binary signal
• (b) Amplitude modulation

• (c) Frequency modulation
• (d) Phase modulation

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Modems (2)

• (a) QPSK.
• (b) QAM-16.
• (c) QAM-64.

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Modems (3)
(b)
(a)

• (a) V.32 for 9600 bps.
• (b) V32 bis for 14,400 bps.

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Trunk Line
Speed
SONET/SDH
OC3/STM1
156 Mbps
OC12/STM4
622 Mbps
OC48/STM16
2.5 Gbps
OC192/STM64
10 Gbps
OC768/STM256
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.
Break
Telephone Switch
SONET/SDH Ring
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Digital Subscriber Lines

• Bandwidth versus distanced over category 3 UTP
  for DSL.