Download Tutorial on TRANSMISSION technique

Transmission
© Manzur Ashraf
Preface
• For the transfer of information, we need transport
facilities dimensioned for the maximum information
flow between a user and a network node, or between
network nodes. These transport facilities employ a
variety of what we have traditionally called transmission
techniques.
• The original technique was optimised to handle voice
transport in its basic analog form. Today, however,
development is moving in the direction of completely
digital transport systems capable of meeting the
different requirements that are imposed by voice,
data and video.
Change of the transport load from
analog to digital
Existing network digitised in stages,
coming closer to the subscriber
Bandwidth
• The usable frequency range of a connection is
called bandwidth. For telephony, the ITU-T
recommends connections that can handle
frequencies between 300 and 3,400 Hz; that is,
a bandwidth of 3.1 kHz. Normally, the ear can
detect sound with frequencies in the interval 15
to (approximately) 15,000 Hz, but
measurements show that the frequency range
300-3,400 Hz is fully adequate for speech to be
heard clearly, and for us to be able to recognise
the voice of the person speaking.
Contd..
• Therefore, the microphone in our telephone must react
to frequencies in the range 300-3,400 Hz and transform
them into electrical voltages with an acceptable
amplitude (strength) in the entire frequency band. The
same requirements for bandwidth apply to the
loudspeaker in our telephone
B/W contd.
• This analog bandwidth can (somewhat
incongruously) be said to have a corresponding
digital "bandwidth":
• PSTN
analog
300-3,400 Hz
digital
64 kbit/s (after
PCM coding)
• GSM
300-3,400 Hz
13 kbit/s (after special coding
in and between exchanges, the bit rate is converted into 64 kbit/s)
Transmission media
• copper, which is used in two main types of cable: paired
cable and coaxial cable;
• glass fibre, which is used in optical fibre cable; and
• radio waves, which are used in terrestrial point-to-point
systems or area coverage systems (such as mobile
telephony), and for point-to-point or area coverage
communication via satellite.
In principle, all media can be used for point-to-point
communication, but only radio technology can handle
communication with mobile terminals.
Contd..
Optical systems feature advantages of capacity,
quality and economy.
With respect to economy, copper-based systems
are able to keep up only over the very last
section - to the residential subscriber - where the
need for capacity is small. Copper is still used to
a large extent between subscribers and the
exchange, which is mainly due to past practices.
For the same reason, coaxial cables are still
used between exchanges - although in recent
years their main use has been for cable TV.
Carriers
Carriers are exclusively analog in nature; that is, they carry waves
of some kind: light waves or electromagnetic waves.
In a purely physical sense, light too is made up of electromagnetic
waves but - owing to the special characteristics of light - we see
optical fibres as carriers of their own type of signal.
On the other hand, the information to be transported is digital in many
cases, at least the output signal from voice coders, video coders and
computers. The GSM system is representative of the combination
of digital information on an analog (radio) carrier, in so far as the
voice coder is placed in the mobile telephone. (In ordinary fixed
telephony, the voice coder is usually located in the local exchange or
in the access node.)
Modulation and baseband
transmission
•  By allowing the transferred information to manipulate
the carrier in some way - for example, by turning on and
off light waves - the information can be perceived by a
receiving exchange or terminal. This manipulation of
the carrier is called modulation.
•  The original transmission technique for telephony
over copper pairs (baseband transmission) is still the
most commonly used technique between a fixed
telephone and the switching node. It applies the principle
of analog information without carrier between a
telephone and the voice coder.
Simplex - duplex; asymmetrical
transmission
• In TV broadcasts it is enough to send information in one
direction - a technique called simplex. Duplex means
that information is sent simultaneously in both directions
between two points. The technique of sending video
information with a large bandwidth in one direction,
and considerably less information (such as control
signals) in the opposite direction, is called
asymmetrical transmission. This technique has been
brought to the forefront for video-on-demand and for
high-quality information services, such as the wideband
Internet.
•  Half-duplex means “alternative receive/send”
Amplification
• Owing to the phenomenon of attenuation, special
equipment must be used between nodes when the
distance exceeds certain values (which are different for
copper-based transmission, optical fibre and radio link
systems). The points at which such equipment is found
are called intermediate repeaters. Repeaters can be
used purely for amplification (when the analog carrier
has become too weak) or for a combination of
amplification and regeneration (next page), when the
digital baseband signals need to be "refreshed "
Regeneration
• It means that distorted information signals are read and
interpreted, and recreated and amplified to their original
appearance before they are forwarded. Noise and other
disturbances completely disappear. This is not the case with
analog transmission where disturbances are amplified as well.
Multiplexing
• Implementing and maintaining the transmission
links in telecommunications networks is an
expensive undertaking for network operators.
Much is gained by transmitting several calls on
the same physical connection (such as a wire
pair). The technique used for multichannel
systems, both in analog and digital networks, is
called multiplexing. It can be divided into three
groups:
• A) frequency division multiplexing (FDM);
• B) time division multiplexing (TDM); and
• C) wavelength division multiplexing (WDM).
Frequency division multiplexing
• Frequency division
multiplexing (FDM) is used for
transmitting analog
information. Multiplexing is
comparable to the technique
that makes it possible to select
the desired transmitting station
on a radio. Each transmitter
has been assigned a particular
frequency on which
information is superimposed
and sent to the listener. By
turning the frequency selector
we can easily change to
another transmitter.
Time division multiplexing
Total scenario
But..
• All methods are not necessary for all types
of communication. For example, in terms
of transmission, a local telephone call can
be illustrated as shown in following figure.
Transmission media
• Copper :There are two main types of metallic cable:
paired cable and coaxial cable. In addition, open wire
(metal wire without insulation) is also used in rural areas.
Paired cable and coaxial cable are used both for analog
and digital transmission; open wire as a rule is only used
for analog 3.1 kHz connections.
• Paired cable :The simplest form of paired cable is found
in our homes. This cable has only two conductors that
connect the telephone to the wall socket. Operators have
more to choose from in their store rooms: 2-, 10-, 50-,
100- and 500-pair cables, to name a few examples.
Paired cable is used mainly in the access network
between subscribers and the exchange, and - to some
extent - in the trunk network between exchanges.
paired cable
• The paired cable was
originally developed for
analog connections. A large
part of the existing cable
network still consists of paperinsulated cables. Plastic is a
better insulation material,
because it is insensitive to
moisture and has lower
attenuation at higher
frequencies, and that is why
newly manufactured paired
cable is plastic-insulated. The
major part of the paired-cable
network is buried under
ground.
CONTD..
• Normally, the conducting
material is copper, and the
conducting wires are
manufactured in a number of
standardised diameters - the
most common diameters are
0.4, 0.5, 0.6 and 0.7 mm. The
wires in the cable are twisted
together to form pairs (two
conductors) or quads (four
conductors). The attenuation
per kilometre depends on the
diameter of the wire and on the
frequency.
Coaxial cable
• Coaxial cable is used in analog (FDM) and digital (TDM)
multichannel systems - in local data networks, cable-TV
networks, and as a feeder for radio antennas. The cable
consists of one or more coaxial tubes, each of which has an
inner conductor surrounded by a tube-shaped outer
conductor. The coaxial tube has very high transmission
capacity (10,800 voice channels in analog multichannel
systems). In the trunk network, the tubes are used in pairs,
one for each direction of transmission. Today, coaxial cable is
no longer installed in the trunk part of the telecommunications
network. Instead it is being replaced by optical fibre cable.
Nonetheless - apart from being used for cable TV - coaxial
cable may come into use in the access part of future
broadband networks.
The radio spectrum
• The radio spectrum, from 3 kHz to 300 GHz, is one range of the
electromagnetic spectrum (infrared, visible and ultraviolet light, and
X-ray frequencies are other ranges). The radio spectrum is divided
into eight frequency bands as shown by Figure from VLF (very low
frequency) to EHF (extremely high frequency).
Radio wave (type 1)
• The propagation of a radio wave depends
on its frequency. Radio waves with
frequencies below 30 MHz are reflected
against different layers of the atmosphere
and against the ground, allowing them to
be used for maritime radio, telegraphy and
telex traffic. The capacity is limited to
some tens or hundreds of bit/s.
Radio wave (type 2)
• Above 30 MHz, the frequencies are too high to be
reflected by the ionised layers in the atmosphere. The
VHF and UHF frequency bands , which are used for TV,
broadcasting and mobile telephony, belong to this group.
Frequencies above 3 GHz suffer severe attenuation
caused by objects (such as buildings) and therefore
require a free "line of sight" between the transmitter and
the receiver. Radio link systems use frequencies
between 2 and 40 GHz, and satellite systems normally
use frequencies between 2 and 14 GHz. The capacity is
in the magnitude of 10-150 Mbit/s.
Radio link
•
•
 Each radio link needs two radio channels: one for each direction. A few
MHz spacing is needed between the transmitter frequency and the receiver
frequency
 The higher the carrier frequency, the shorter the range. For example, a 2
GHz system has a range of approximately 50 kilometres, and an 18 GHz
system has a range of 5-10 km.
 At regular intervals, the signal is
received and forwarded to the next link
station. The link station may be either
active or passive. An active link station
amplifies or regenerates the signal. A
passive link station generally consists of
two directly interconnected parabolic
antennas without any amplifying
electronics between them.
Satellite
• Satellite systems are quite
similar to radio link systems;
the only real difference being
that the intermediate link
station is in orbit around earth
instead of being set up on the
ground
• Satellites describe either a
polar or a geostationary orbit.
Those with a polar orbit pass
over the poles at an altitude of
about 1,000 km and are used
for meteorological and military
purposes
Geostationary Satellites
• Satellites used for telecommunications are
placed in geostationary orbits in the equatorial
plane 35,800 km above the earth's surface.
They have an orbiting time of 24 hours which,
because of the earth's rotation, gives them the
appearance of being stationary. Approximately
one third of the earth's surface is covered by an
antenna with global radiation. Satellite links are
used in national as well as international
telecommunications networks. Intercontinental
use has decreased in favour of optical
submarine cables.
• The transmission properties of satellite links are
excellent and problems are few. However, the long
distance between terrestrial stations via the satellite
does cause a 240 ms delay, which in itself is
troublesome to voice communication and which may
give echoes with a propagation time of about 0.5
seconds.
• Intelsat (International Telecommunications Satellite
Organization) was founded with the aim of financing,
developing and running worldwide commercial
telecommunication satellite systems.
• One of Intelsat's satellites is Intelsat VI, which has
80,000 voice channels
Optical fibre
• In the 1870s, an Englishman, Tyndall, showed
that light can be conducted through a bent jet of
water. At the end of the 19th century, Graham
Bell designed an optical telephone. The difficulty
in finding appropriate light sources, however,
made it necessary to wait 100 years before this
technology could be used in practice. The first
field trials with optical cable were carried out in
1975, and in 1980, the first commercial systems
were opened for telephone traffic.
• It has an enormous transmission capacity. Today, there
are systems for several Gbit/s - 2.5 Gbit/s approximately
corresponds to 32,000 simultaneous telephone calls at
64 kbit/s. The limitations are in the terminal equipment.
• The interface between electrical and optical transmission
requires E/O converters.
advantages of optical fibre
• The advantages of optical fibre systems can be
summarised in the following points:
• very high capacity;
• long repeater spacing;
• small cable dimensions;
• low weight;
• small bending radius;
• no crosstalk; and
• immunity to electromagnetic interference
An optical fibre
optical cable - a slotted core cable
with 36 fibres
structure
• The glass fibre has a glass core with a
surrounding glass cladding. The core consists of
doped glass with a somewhat higher refractive
index than the cladding, which is made of pure
quartz glass. Normally, the diameter of the
cladding is 125µm. The diameter of the core is
different for different types of fibre - 8, 10 or 50
µm.
• The fibre has a primary coating (as a rule
consisting of cured acrylate) to provide
protection against moisture and chemicals, and
an outer - fixed or loose - secondary coating
Structure/contd..
• The optical cable is provided with a strength
member made of steel or plastic that gives the
cable the strength necessary to withstand tensile
stress and bending. In addition, the cable
contains filling that fixes the fibres and protects
them from excessive bending and moisture. The
cable cladding is made of plastic, as a rule
polyethylene. The number of fibres in a cable
varies depending on the field of application there are, in fact, cables with thousands of
fibres.
Properties of light
Optical communication
• This total reflection is utilised in optical fibre
communication. If the angle of incidence is
sufficiently large, then the light in the fibre will
reflect repeatedly in the interface between the
materials. The fibre need not be straight but can
conduct light even when bent
Wavelength division multiplexing
(WDM)
•
Wavelength division multiplexing enables a number of channels to be
sent at different wavelengths in the same fibre, in the same direction or
in both directions. Although it has been given a name of its own, WDM is
an optical fibre variant of frequency division multiplexing.
WDM/contd..
• Two WDM systems currently being developed are
coarse WDM and dense WDM . In coarse WDM, 2 to 10
wavelength channels are used with large wavelength
separation between the channels (5-50 nm). In dense
WDM, 5 to 100 wavelength channels are used with less
wavelength separation between the channels (0.1-5 nm).
• A simple variant of WDM already in use employs 1,310
and 1,500 nm at the same time in the same fibre. This
makes it possible to double the capacity, or to achieve
duplex transmission with one wavelength per direction.
Analog transmission
• Bandwidth:
The bandwidth requirements for analog transmission
equipment in access and interexchange networks vary
depending on the utilisation of the transmission link. In
the case of analog single-channel connections, a
bandwidth of 3.1 kHz (frequency range 300-3,400 kHz)
is required. Other requirements apply to analog
multichannel connections. A frequency-multiplexed
FDM connection with 2,700 channels requires a
bandwidth of 12 MHz, and a connection with 10,800
channels requires 60 MHz. Accordingly, the possible
number of channels is proportional to the bandwidth of
the connection
Analog transmission
• Attenuation:
Attenuation as well as its opposite, amplification, are measured in
decibels. By measuring the transmitted power, P1, and the received power,
P2, the attenuation, A, can be calculated. The formula is:
Attenuation: A = 10·log (P1/P2)
Example: A = 10log(400mW/10mW) = 16 dB. By using the 10-logarithm, the
total attenuation of a connection can be calculated as the sum of the
attenuation/amplification of the parts.
 Note: measurements in decibel solely indicate a relationship between two
quantities. As a rule of thumb, the doubling or halving of power corresponds
to an increase or decrease by about 3 dB. The attenuation value obtained in
no way indicates the real strength of the received signal.
Fading
• Fading, a phenomenon that
arises when radio signals are
reflected against different
atmospheric layers and the
ground, is one of the most
difficult problems to master in
radio communication
• CASE 1: The refractive index
of air for radio waves is
normally inversely proportional
to height. It affects radio
waves, which gradually bend
towards the earth and reach a
bit beyond the horizon.
Therefore, radio-optical sight is
somewhat longer than optical
sight. The refractive index is
affected by the air
temperature, pressure and
humidity
Deflection caused by refraction
Fading…
• Case2: At frequencies
below 10 GHz, fading is
mainly caused by the
interference that arises
between direct signals
and those reflected from
the earth, or between
different signal paths
through the atmosphere.
This type of multi-path
propagation is most
common during summer
nights and in early
autumn
Interference due to multipath propagation
Fading…
• Case 3: The attenuating effect of rain
becomes noticeable at frequencies above
10 GHz when the wavelength gets down
to the size of a rain drop. Signals are
heavily attenuated due to scattering and
absorption of energy. The higher the
frequency, the more importance
attenuation due to rain assumes,
compared with other causes of fading.
Noise
• Noise is generated in all types of electronic circuit. No
system can be made totally free of noise - the best we
can do is set limits to how much noise we are willing to
permit. Limit values have been recommended by the
ITU-T.
• The absolute noise level is not the most interesting
phenomenon; what determines audibility is the
relationship between the level of the transmitted signal
and the noise. That is why we use the term signal-tonoise ratio. When the attenuation on the line makes it
necessary to use amplifiers, the signal-to-noise-ratio will
decrease, because the incoming noise is amplified just
as much as the signal. In the end, the noise level may be
so high that we have to select a lower signal level (fewer
amplifiers) to maintain an acceptable signal-to-noise
ratio.
Noise (External interference)
• In metallic systems, external interference caused
by power lines and radio transmitters can lead to
a noticeable degradation of transmission quality.
Interference can also be due to lightning strikes
in the vicinity of a metallic cable. Should this
occur, an "energy-rich" pulse will propagate
along the line. In a worst-case scenario - if
adequate lightning arresters are not provided this pulse might destroy cables and electronic
equipment.
Noise (Crosstalk )
• The phenomenon of a connection being disturbed by a conversation
on another line is called crosstalk. The problem is particularly
noticeable on connections with paired-cable transmission
• As far as telephony is concerned, we distinguish between two types
of crosstalk:
• intelligible crosstalk, where we can understand the content of
another call between two subscribers; and
• unintelligible crosstalk, which means that the conversation on an
adjacent channel disturbs us, although we do not understand what is
being said. The interference may have the character of noise but is
more disturbing because of its rhythm and intonation.
• Of these two, intelligible crosstalk is most severe since it jeopardises
integrity. The ITU-T recommends a level spacing of at least 52 dB
between the received test tone level and crosstalk level.
Noise (Sidetone )
• When we talk on the phone we are supposed to
hear our own voice in the receiver. This is called
sidetone. The level of the sidetone should not be
too high or too low.
• Modern telephone sets contain functions that
balance the sidetone effect, while taking line
impedance into account. For this reason, older
telephone sets ought to be exchanged for newer
ones whenever the network is being modernised.
Noise (Echo)
Noise in Digital transmission
Frequency spectrum of Noise
• By means of the frequency spectrum of noise, we
can distinguish between two main types of noise:
white noise and 1/f noise
• Examples of white noise include thermal noise, shot
noise and partition noise.
Other noises..
• Thermal noise occurs because the charge carriers in a
material are in constant motion. This motion becomes
more vivid as temperatures increase and vice versa.
• Shot noise arises in semiconductors and is caused by
the individual charge carriers (electrons or holes) that
make up the electric current. In calculations for
semiconductors - diodes or transistors, for example - we
assume that the current is constant. However, in reality
only the time average is constant. We disregard the fact
that the individual charge carriers cause current
variations, what we call shot noise.
• for 1/f noise the power is greatest at low frequencies and
decreases with increasing frequency.
Multiplexing
Why? Advanced time division multiplexing makes it
possible to produce multiplexed signals up to 10 Gbit/s
commercially. The capacity will be further increased by
means of wavelength division multiplexing in optical
fibre cable.
• Multiplexing is used to reduce transmission
costs - several channels along the same route
share the same transmission medium, such as
optical fibre. We will now familiarise ourselves
with three standardised multiplexing hierarchies,
each of which is based on time division
multiplexing:
• PDH, plesiochronous digital hierarchy;
• SDH, synchronous digital hierarchy; and
• SONET, synchronous optical network.
Plesiochronous digital hierarchy
• The level of the first order (PCM frame )
 The 125 microseconds are the result of sampling each
voice channel 8,000 times per second (8,000 Hz); that is,
the cyclic time is 1/8,000 = 125 10-6 seconds. After firstorder multiplexing, the voice channels share this time.
 In the European variant, the first channel - time slot 0 - is
used for frame synchronisation. Time slot 16 can be
used for signalling or traffic, and all other time slots are
used for traffic, usually voice communication. Signalling
system No. 7 (SS7) can use any time slot except TS0. In
the American PCM frame, with 24 time slots, no special
time slot has been allocated for signalling. Instead, one
bit in each time slot in every sixth frame is replaced by
signalling information. The replaced bit corresponds to
the least significant bit in the sample. As a consequence,
only seven out of eight bits can be used transparently
through the network
Higher-order levels
Multiplexing and demultiplexing into
and from PDH level two
• The PDH structure is "rigid". To
rearrange and drop tributaries
(for example, 2, 8 or 34 Mbit/s)
from a main flow (such as 140
Mbit/s), the main flow must be
demultiplexed step by step
down to the desired tributary
level and then multiplexed
back to the desired main flow
level. Rearrangement is done
manually in a digital
distribution frame (DDF).
Plesiochronous tributaries
• Justification bits :