Chapter III Final

8/17/2019 Chapter III Final

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Chapter III

INTRODUCTION TO DIGITAL TELECOM TRANSMISSION

&APPLICATIONS

Transmission is the process of transporting information between end points of a system or a

network. The end-to-end communication distance is often very long and there are many

electrical systems on the line. These systems, network elements such as exchanges,

are connected to the other elements with connections provided by the transmission

systems.

The main elements of a communication system are shown in Figure 5.1. The transducers, such as a

microphone or a TV camera that we need to convert an original signal to an electrical form are

omitted; unwanted disturbances such as electromagnetic interference and noise are included. ote

that bidirectional communication re!uires another system for simultaneous transmission in the

opposite direction.

Figure 5."# Transmission system

Transmitter:The transmitter processes the input signal and produces a transmitted signal suitable to the

characteristics of a transmission channel. The signal processing for transmission often involves

encoding and modulation. $n the case of optical transmission, the conversion from an electrical

signal format to an optical one is carried out in the transmitter.

Transmission Channel:

The transmission channel is an electrical medium that bridges the distance from the source to the

destination. $t may be a pair of wires, a coa%ial cable, a radio path, or an optical fiber. &very channel

introduces some amount of transmission loss or attenuation and, therefore, the transmitted power

progressively decreases with increasing distance. The signal is also distorted in the transmission

channel because of different attenuation at different fre!uencies.

Receiver:

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The receiver operates on an output signal from the channel in preparation for delivery to the

transducer at the destination. 'eceiver operations include filtering to take away out(of(band

noise, amplification to compensate for transmission loss, e!uali)ing to compensate for distortion

and demodulation and decoding to reverse the signal processing performed at the transmitter.

Noise, Distortion, and Interference:

Various unwanted factors impact the transmission of a signal. *ttenuation is undesirable because it

reduces signal strength at the receiver. &ven more serous problems are distortion, interference, and

noise, the last of which appears as alterations of the signal shape. To decrease the influence of

noise, the receiver always includes a filter that passes through only the fre!uency band of message

fre!uencies and disables the propagation of out(of(band noise.

Transmission Medias:

Transmission systems may use copper cable, optical cable, or radio channels to interconnect far(end

and near(end e!uipment. These channels and their characteristics are introduced ne%t.

Copper (wired) Links:

+opper cable is the oldest and most common transmission media. $ts main disadvantages are high

attenuation and sensibility to electrical interference.

*ttenuation in copper cable increases with fre!uency appro%imately according to the following

formula#

A ( d- k √ f d

/here, * 0d- is attenuation in decibels, f is the fre!uency, and k is a constant specific for eachcable. This formula gives us appro%imate attenuation at other fre!uencies if the attenuation at one

fre!uency is known. ifferent types of copper links are shown in figure 5.".

Figure 5."# +opper cable as a transmission medium.

Twisted Pair

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* twisted pair consists of two insulated copper wires that are typically 2.3 to 2.4 mm thick or about

1 mm thick if insulation is included. These two wires are twisted together to reduce e%ternal

electrical interference and interference from one pair to another in the same cable. Twisted pairs are

used in the telecommunications networks in subscriber lines, in "(bps digital transmissions with

distances up to " km between repeaters, in 67s up to several megabits per second, and in short(

haul data transmissions up to 122 bps in 7*s.

Open-Wire Lines

The oldest and simplest form of a two(wire line uses bare conductors suspended at pole tops. The

wires must not touch each other, otherwise short circuit occurs in the line and communication will

be interrupted. ew open(wire lines are rarely installed today but they are still in use in rural areas

as subscriber lines or analog carrier systems with a small number of speech channels.

Coaxial Cable

$n a coa%ial cable, stiff copper wire makes up the core, which is surrounded by insulating material.The insulator is encased by a cylindrical conductor. The outer conductor is covered in protective

plastic sheath. The construction of the coa%ial cable gives a good combination of high bandwidth

and e%cellent noise immunity. +oa%ial cables are used in 7*s 0original 12(bps &thernet-, in

antenna systems for broadcast radio and TV, and in high capacity analog and digital transmission

systems in telecommunications networks and even in older generation submarine systems.

Microwave Transmission:

The most important advantage of radio transmission over cable transmission is that it does not

re!uire any physical medium. 'adio systems are !uick to install and because no digging of cable

into the ground is re!uired, the investment costs are much lower.

8ne important factor that restricts the use of radio transmissions is the shortage of fre!uency bands.

The most suitable fre!uencies are already occupied and there are many systems with a growing

demand for wider fre!uency bands. &%amples of other systems using radio waves are public cellular

systems, professional mobile radio systems, cordless telephones, broadcast radio and TV, satellite

communications, and /7*s. The use of radio fre!uencies is regulated by the $T9(' at the global

level and, for e%ample, by &T6$ at the &uropean level and the F++ in the 9nited 6tates. To

implement a radio system, permission from a national telecommunications authority is re!uired.

atellite Transmission:

$n satellite communications a microwave repeater, which is called transponder, is located in a

satellite. *n &arth station transmits to the satellite at one fre!uency band and the satellite

regenerates and transmits the signal back at another fre!uency band. The fre!uencies allocated by

$T9 for satellite communications are in the fre!uency range of 1 to :2


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a so(called >geostationary? orbit so that they seem to be in the same location all the time from the

point of view of the &arth station, as shown in Figure 5.:. The distance of this orbit is around

:4,222 km from the e!uator on the &arth@s surface and this introduces a long transmission delay that

is appro%imately "52 ms from the transmitting &arth station to the receiving &arth station. The

speaker has to wait for a response for appro%imately 2.5 seconds and this disturbs an interactive

communication.

Figure 5.:# 6atellite transmission.

8ne maAor application for satellite communications has been broadcast satellite TV. * TV program

from a single satellite may be received in any part of a continent simultaneously making distribution

cost per customer low.

!i"er links:

8ptical fiber is the most modern of the transmission media. $t offers a wide bandwidth, low

attenuation, and e%tremely high immunity to e%ternal electrical interference. The fiber optic links

are used as the maAor media for long(distance transmission in all developed countries and high(

capacity coa%ial cable systems are gradually being replaced by fiber systems.

*n optical fiber has a central core 0with a diameter around B or 42 Cm- of very pure glass

surrounded by an outer layer of less dense glass. * light ray is refracted from the surface between

these materials back to the core and it propagates in the core from end to end. The principle of

optical cable transmission is presented in Figure 5.3. +ompare the dimensions of optical fiber with

the diameter of a human hair that is appro%imately 122 Cm.



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0>almost the same data rate?-, which allows a small fre!uency difference between tributary signals

that are multiple%ed into a higher aggregate rate as it is shown in figure 5.5. For e%ample, at ",23B

Hbps the fre!uency tolerance was standardi)ed at 52 ppm, and at B,33B Hbps the allowed tolerance

is "2 ppm. This means that, for e%ample, the data rate of a ",23B(Hbps system may deviate by 122

bps.

The G< 0&uropean standard-

The basic principle of the &uropean standard for higher(order multiple%ers is that each multiple%er

stage takes four signals of a lower data rate and packs them together into a signal at a data rate that

is a little bit over four times as high, as shown in Figure. $n addition to tributaries, aggregate frames

contain frame alignment information and Austification information.

The tributary fre!uencies may differ slightly and their fre!uencies must be Austified to the higher(

order frame. This process, called Austification or stuffing, adds a number of Austification bits to each

tributary in order to make the average tributary data rates e%actly the same. $n the demultiple%er

these Austification bits are e%tracted and the original data rate for each tributary is

generated. *dditional bits are needed in the frame for frame synchroni)ation 0frame alignment- and

Austification, and therefore the ne%t level has a slightly higher rate than four times the nominal

tributary rate. Iustification bits are added to tributaries so make their data rates e!ual for fram(

ing. The frame also contains some spare bits that can be used, for e%ample,for management data

transmission for a network management system.

The standards for G< ensure compatibility in multiple%ing between systems from different

manufacturers. The management functions are not standardi)ed and they differ from manufacturer

to manufacturer. 8nly the local interfaces and the multiple%ing scheme are standardi)edin G


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line interfaces of the line terminals for copper cable, optical fiber, and radio transmission are

manufacturer specific so the vendor has to be the same at both ends.

D$ and 'NT :

The G< higher(order systems were standardi)ed more than :2 years ago. y the end of the 1EB2s,

a lot of optical fiber cable had been installed and analog networks upgraded into digital networks.

Then researchers reali)ed that new standards were re!uired to meet future re!uirements. Groblems

with the G< standards include the following#

• *ccess to a tributary rate re!uires step(by(step de(multiple%ing because of stuffing

0Austification-.

• 8ptical interfaces are not standardi)ed but vendor specific.

• To use optical cables, a separate multiple%er for each level 0e.g., multiple%ing from " to 132

bps in &uropean G< re!uires "1 pieces of multiple%ing e!uipment- and separate line

terminals are needed.

• *merican and &uropean standards are not compatible.

• etwork management features and interfaces are vendor dependent.

•


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The main advantages of 6< over G< standards are as follows#

• The data rates for optical transmission are standardi)ed 0i.e., vendor independent-.

•ifferent systems are included in standards, for e%ample, terminal, add=drop, and cross(

connection systems.

• *ccess to the tributary data rates is efficient 0no step(by(step multiple%ing is re!uired-.

• The system is tolerant against synchroni)ation and other system faults. 6tandardi)ed

redundancy functions allow operators to switch from a faulty line to an operational line.

• $n the future, network management is slated to become vendor independent, with

sophisticated management functions.

6< is replacing G< systems in the transport network. y transport network we mean the fle%ible

high(capacity transmission network that is used to carry all types of information. y fle%ible we

mean that telecommunications operators are able to easily modify the structure of the transport

network from the centrali)ed management system. This makes the delivery times for leased lines

shorter. 7eased lines are needed, for e%ample, for 7* interconnections between the offices of a

corporation.

TM:

*T defines the structure of cells, continuous transfer of cells, and cell switching. $sochronous

service is available by reserving certain fi%ed capacity of *T cells from the network. *T cells

are packed into an 6< frame, 6T(1, or into a 68&T frame and then the physical data rate may

reach 155 bps or higher. 6ignificant advantages of cell(relay technology follow from the use of

fi%ed(si)e small packets or cells instead of packets with variable lengths. The conse!uences of this

principle are as follows#

• elays in the network are much lower and more predictable. y ensuring that the cells from

a specific data stream occur at regular intervals in the cell stream, it is possible to provide

guaranteed bandwidth with low delay and Aitter Aust as in circuit(switched networks.



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• The fi%ed si)e of cells allows the switching function to be removed from software into

hardware with a dramatic increase in switching speed.

*T thus provides the benefits of circuit( and packet(switched networks, hence allowing all types

of traffic to be integrated onto a single network. any network operators use *T technology in

their core network. $n *T networks the switches are usually configured to provide semi(

permanent data connections. y semi(permanent, we mean that these connections are not dialled up

by users, but controlled from the network management center by a network operator.

Transmission *%ipment in the Network:

any different systems are needed in the telecommunications network to transmit signals via

various different channels+

• odems convert digital signal into an analog form.

• ultiple%ers combine lower rate data signals into a higher rate aggregate

signal.

• igital cross(connect e!uipment switch data streams from one time slot to

another or from one port to another.

• 'epeaters amplify and regenerate signals on the line.

• 8ptical line systems terminate optical fibers and convert a signal from

electrical to optical and vice versa.

• icrowave radio systems convert digital data into high fre!uency radio

signals.

Modems:



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* modem is a piece of e!uipment that includes a modulator and demodulator. odems are used to

transmit digital signals over an analog channel. The microwave radio systems are sometimes also

called modems because they send digital information over a microwave radio link, and in order to

do this, they also carry out modulation and demodulation processes.

oice-"and Modems:

Voice(band modems are needed when an analog voice channel of the telephone network is used for

data transmission. The fre!uency band of the voice channel is :22 to :,322


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Terminal multiple%ers 0Ts- or multiple%ers combine digital signals to make up a higher bit rate

for high(capacity transmission as shown in above figure. The digital multiple%ing hierarchies in use

are G< and 6


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+ross(connect systems are available that are able to switch high(order data rates, not Aust 43

Hbps as ordinary e%changes do. K+ may also contain redundancy functions that automatically

change configurations so as to bypass a faulty transmission section.

6< and 68&T networks often use a ring topology like that shown in Figure. For higher

reliability.These standards specify redundancy functions and a node in a ring may switch traffic

from a faulty connection to the redundant path as shown in Figure.

'ptical Line &stems:

8ptical line systems contain two terminal repeaters at each end of the fiber.They convert an

electrical digital signal into an optical one and vice versa.These systems include, as most other

transmission systems do, supervisory functions such as fault and performance monitoring. $n G<

multiple%ers, optical line systems are separate devices that are interconnected with standardi)ed

interfaces.

$n G< multiple%ers, optical line systems are separate devices that are interconnected with

standardi)ed interfaces. optical systems transmit light energy pulses to the fiber; they do not use

light as a carrier the same way as in radio communications. $n bidirectional systems two fibers, one

for each transmission direction, are needed as shown in Figure. wavelength(division multiple%ing

0/- uses an optical coupler to combine optical signals 0/ multiple%er- and optical

filters 0/ demultiple%er- to separate optical signals at the receiving end.

1DM:8ptical signals of different wavelength can propagate without interfering with each other. The

scheme of combining a number of wavelengths over a single fiber is called wavelength division

multiple%ing 0/-.

any single(mode fiber cables have been installed and technical solutions that increase fiber

capacity without installation of new cable have become very attractive as the demand for

transmission capacity increases. Garticularly in long(distance systems, / has become popular

and it can increase fiber capacity by a factor from 12 to 122. This wavelength(division

multiple%ing 0/- uses an optical coupler to combine optical signals 0/ multiple%er-

and optical filters 0/ demultiple%er- to separate optical signals at the receiving end as

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shown in Figure.

/ has the potential for e%ploiting the large bandwidth offered by optical Lbers. For e%ample,

hundreds of 12(b=s channels can be transmitted over the same Lber when channel spacing is

reduced to below 122


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the help of / technology, a pair of fibers can provide data capacity of several hundreds

gigabits per second.

ost / systems support standard 68&T=6< optical interfaces. 8ften short(haul

6T(14 0".3 bps- at the 1:12(nm wavelength is used as an input signal for / systems but

also other interfaces, such as 8+(1E" for 12(b &thernet, can be supported. The basic structure

of a / system is shown in Figure 3.:3. 8nly one transmission direction is shown in the

figure. Transponders in Figure 3.:3 convert incoming optical signals into $T9(standard

wavelengths.

/ technology has improved, and will continue to further

improve, utili)ation of fiber bandwidth close to the huge capacity of optical fibers that will be

achieved in the future by coherent radio(like optical technology.

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Chapter III Final

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