Optical fiber communication - TRANSMISSION MEDIA

Optical fiber communication

The use of visible optical carrier waves or light for communication has

been common for years. Moreover, as early as 1880 Alexander Graham Bell

reported the transmission of speech using light beam. The photo-phone

proposed by Bell just after four years of the invention of the telephone,

modulated sunlight with diaphragm giving speech over a distance of 200 m.

Although the investigations of optical communication continued, its use was

limited to mobile, low capacity communication links. This was due to both lack

of suitable light sources and the problem that light transmission in the

atmosphere is restricted to line of sight and is severely affected by the

disturbances such as rain, snow, fog, dust and the atmospheric turbulences.

Nevertheless longer wavelengths electromagnetic waves proved to be

the suitable carriers for information transfer in the atmosphere. The

information carrying capacity is directly related to their bandwidth or

frequency extent of the modulated carrier, which is generally fixed to the

fraction of the carrier frequency. Actually higher the carrier frequency, larger

the bandwidth and thus the information carrying capacity of the

communication system. The communication at optical frequencies offers an

increase in the potential usable bandwidth by a factor of around 10⁴ over high

frequency microwave transmission. The general ability of the communication

system to concentrate the power within the transmitted electromagnetic wave

increased, thus giving an improved system performance. Laser device

provided a powerful coherent light sources together with the possibility of

modulation at the higher frequency. In the addition the low beam divergence

of the laser made enhanced the free space optical transmission a practical

possibility.

The proposals for optical communication via dielectric waveguides or

optical fibers fabricated from glass to avoid degradation of the optical signal

by the atmosphere were made almost simultaneously in 1966 by Kao and

Hockham and Werts. In parallel with the development of the fiber waveguides,

attention was also focused on the other optical components. A semiconductor

optical source (i.e. injection lasers and light emitting diodes) and detectors

(i.e. photodiodes and to a certain extent photo-transistors) compatible in size

with the optical fiber were designed and fabricated to enable successful

implementation of the optical fiber system.

THE GENERAL SYSTEM

An optical fiber communication system is similar in basic concept to any

type of communication system.

A block schematic of a general communication system is as shown, the

function of which is to convey the signal from the information sources over the

transmission medium to the destination.

The communication systems consist of a transmitter or a modulator

linked to the information source, the transmission medium, and a receiver or

the demodulator at the destination point. In the electrical communications the

information source provides an electrical signal, usually derived from a

message signals which is not electrical (sound), to a transmitter containing

electrical and electronic components which converts the signal into suitable

form for propagation over the transmission medium. Modulating carrier

achieves this, which may be an electromagnetic wave. The transmission

medium can consist of a pair of wires, a coaxial cable or a radio link through

which the signal is transmitted to the receiver, where it is demodulated before

transmitting to the destination point.

In any transmission medium the signal is attenuated or suffers loss, and

is subjected to degradations. These factors necessitate the installation of

repeaters or line amplifiers at intervals, both to remove the signal distortion

and to increase signal level. The optical carriers may be modulated using

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TRANSMISSION MEDIA-OFC

either an analogue or the digital information signal. With the digital

modulation, discrete changes in light intensities are obtained. Analog

modulation in optical fiber communication system is less efficient, requiring a

far s/n ratio at the receiver than digital modulation. The linearity needed for

analog modulation is not always provided by semiconductor optical sources.

Hence, analog optical fiber communication links are limited to shorter

distances and lower bandwidths than digital links.

ADVANTAGES OF OPTICAL FIBER COMMUNICATION

Communication using an optical carrier wave guided along a glass fiber has a

number of extremely useful features, some of which are listed below.

1. Enormous potential bandwidth: The information carrying capacity of

the optical fiber systems has proved to be superior to the best copper

system. The optical carrier frequency in the range 10¹³ to 10¹⁶ Hz

(generally in the near infrared around 10¹⁴ Hz to 10⁵ GHz) yields a far

greater potential transmission bandwidth than metallic cable systems

(i.e. coaxial cable bandwidth up to around 500 MHz.).

2. Small size and weight: Optical fibers have very small diameters that

are no greater than a human hair. Hence, even when such fibers are

covered with protective coatings they are far smaller and much lighter.

3. Electrical isolation: Optical fibers that are fabricated from glass, or

sometimes a plastic polymer, are electrical insulators and therefore,

they do not exhibit earth loop and interface problems.

4. Immunity to interference and crosstalk: optical fibers form dielectric

waveguide and therefore free from EMI, RFI, or switching transients

giving electromagnetic pulses. Thus the system is unaffected by

transmission through an electrically noisy environment and requires no

shielding from EMI. The fiber cable is also not susceptible to lightning

strikes.

5. Signal security: The light from optical fibers provides a high degree of

signal security as does not radiate much.

6. Low transmission loss: A major advantage of OFC is the low

transmission loss due to which both system cost and complexity gets

reduced. There can be production of OFC cables exhibiting low losses

as compared to copper cables with losses as low as 0.2 db Km-1

7. Ruggedness and flexibility: Optical fibers with very high tensile

strengths are manufactured.

8. System reliability and ease of maintenance: The low loss property

reduces the requirement of immediate repeaters or line amplifiers, thus

system reliability is enhanced. It thus reduces the maintenance time

and the overall cost of system.

9. Potential low cost: The glass becomes the optical fiber transmission

medium that is made from sand not a scarce resource.

Thus overall system cost when utilizing optical fiber

communication on long haul links, however, are substantially less than

those for equivalent electrical line systems because of low loss and

wideband properties of the optical transmission medium.

Information

source

Electrical

transmit

Optical

source

Optical

detector

72

Optical

fiber cable

Electrical

receiver

Destination

Optical fiber communication system

Introduction

OFC-8Mbps system

Second order Digital MUX i.e., 2/8 MBPS multiplexer is a second order

multiplexer which multiplexes four stream of 2.048 MBPS into one stream of

8.448 MBPS and vice versa.

The main features of the equipment are:

Compactness

Comprehensive alarm monitoring

Local and remote loopback

Low profile consumption

The equipment can be used with 8 MBPS optical line terminal equipment, 8

MBPS digital microwave radio equipment or with third and fourth multiplexers

as shown below:

8 Mb/s

OLTE

2/8

MUX

8 Mb/s

RADIO

8 Mb/s

Typical Application OLTE

34/140

MUX

Equipment configuration

Equipment is housed in a slim subrack and two such subracks can be

mounted in one slim rack. The configuration in depicted in figure below.

Insert figure

As shown, two subracks can be mounted in a slim rack and each

subrack can be accommodate one no. of 2/8 MBPS multiplexer. Rack is

prewired for both the types of subracks and does not require any intra wiring

at site except wiring for input output ports and DC power.

2/8 MBPS mux is a single board system i.e. one complete mux along with

power supply and alarm circuit is contained in a single board called D12M.

D12M accommodates two smaller daughter boards called PSU 2 and a QUAD

E1 INF board.

If required an 8 MBPS optical line terminal equipment can also be

mounted in the same subrack for integrated operation. But in this

configuration, second subrack should not be mounted in the rack as the space

for second subrack is used for mounting of a handset for orderwire. It is also

possible to mount a DDF on the same rack. DDM-H caters to four sets of

transmit and receive tributaries of 2mbps and one set of transmit and receive

signal of 8mbps. Thus one no. of equipment can be mounted on a single rack

of 1660 mm ht and having a footprint of 120 x 237mm.

Equipment Description

General Description

2/8mbps mux is a highly integrated single card system and contains

power supply as well as alarm circuit. Extensive alarm monitoring facility and

diagnostics test facilities such as loopback are provided. All the displays and

the control switches are provided in the front of the unit. However some

switches which are required to be set once at the time of installation, are not

accessible from the front. This prevents accidental operation of such switches.

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OFC-8Mbps system

All input and output ports of multiplexers are provided on the rear of the

subrack.

The multiplexing of four tributaries of 2.048 MBPS is based on cyclic

bit interleaving technique as per ITU-T Rec. G.742. Alarm displays and

consequent actions such as generation of urgent / non-urgent alarms,

AIS injection and remote alarm generation is also as per G.742.

Quad E1 INF board and PSU 2 board is mounted on D12M card as

daughter boards. Incoming four streams of 2mbps rateHDB-3 encoded, are

fed to quad E1 INF daughter board performs bipolar HDB-3 to unipolar HDB-3

signal conversion and clock recovery for four tributaries. The unipolar HDB-3

consists of two streams of data termed as +ve data and ve data. +Ve data

signal is an NRZ signal corresponding to +ve mark pulse of bipolar HDB-3

signal. Ve data stream corresponds to ve mark pulses of bipolar HDB-3

signal. These are combined together using a logic OR gate and fed to a clock

recovery circuit which recovers the 2.048 MHz clock.

The two streams of data and clock are fed to an ASIC in D12M card. D12M

card does the multiplexing and demultiplexing functions.

D12M unit description

The unit performs multiplexing of four 2.048 MBPS signals into a single stream

of 8.448 MBPS rate multiplexing is as per ITU-T Rec. G742.

Incoming 2 MBPS signals are fed to a QUAD E1 RX INF does bipolar HDB-3 to

uni-polar hdb-3 conversion and recovers the clock. These signals are fed to

mux ASICs. Mux ASIC accepts 2.048 MBPS unipolar hdb-3 signals and four

recovers clock and converts it into one stream 8.448 MBPS.

Incoming =ve and ve data and clock from quad e1 INF goes to hdb-3

decoder block which converts unipolar hdb-3 to NRZ signal. NRZ signal is

written into an elastic buffer with WRITE CLK, which is same as 2M TX CLK.

The data is read out at 8.448 /4 =2.112 MBPS gapped clock.

Frame structure

The 8 MBPS signal frame is 848 bits long and has duration of 100.38 s. It

is divided into four sets numbered I to IV. The sets compose 212 bits each.

The bits within each set are numbered B1 to B212.

The frame alignment word 1111010000 is contained in the first 10 bits of

Set I.

B11 of set I is used for remote alarm indication, i.e. B11 = 0, means No

alarm while B11 = 1, means Remote alarm.

B12 is reserved for national use, and is set at 1. The remaining bits of set I

carry interleaved data for the four tributaries.

The first four bits of Set II, Set II and Set IV contain the three bits

Justification Control words for the four tributaries.

Positive justification is indicated by the justification control word 111.

Absence of justification is indicated by 000.

Positive justification

Positive justification is used to compensate for the slight differences in

data rate that are possible between the four 2048 KBPS inputs to the

multiplexer. By making a justification it available for each tributary in every

frame, the data rate into each tributary can be normalized by inserting dummy

bits as required into this location.

ASIC circuit description

The application specific integrated circuit (ASIC) performs most of the

functions of the second order multiplexer. The description given in this section

assumes knowledge of the general theory of operation of second order

multiplexers utilizing positive justification.

Frame alignment word detector

The frame alignment word detector searches for the presence of the Frame

Alignment word 1111010000. Frame Alignment is deemed to occur when three

connective FA words have been detected. Frame Alignment is lost when four

consecutive FA words have been incorrectly received in their expected

positions.

Receive gapped clock generator

The four 2112 KBPS tributaries extracted within the 8M INPUT block from

the 8448 KBPS interface inputs are passed to the 2M OUTPUT.

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OFC-8Mbps system

The non-data bits are the FA word, the Justification control word and, when

the tributary is justified. The four gapped clocks are passed to the 2M OUTPUT

block.

Transmit tributary generator

The tributary data, when clocked out of the 2M input block, contains gaps

in the locations of the FA word, the three JC bits and when tributary is

justified, the justification bit.

8M OUTPUT

Transmit timing generator

The transmit timing generator generates timing signals that are used by

other blocks in the transmit path of the ASIC. The sync pulses from the 8M

input block resets the transmit timing generator so that it is correctly aligned

with the received 8448 KBPS interface input data.

The FA and Service Bit generator block compiles the FA word and the

Service Bits into four sets of three bits for insertion into the four tributaries by

the FA and SB insertion block.

2M INPUT

The 2M input block consists of the blocks.

Elastic Buffer

The 24 bit elastic buffer is used to compensate for jitter and the variations

in frequency that occurs between the gapped 2112 KBPS tributary rate and

the relatively constant 2048 KBPS output rate that is set by an Integrated

Digital Phase Lock Loop (DPLL).

The data is written into the elastic buffer using the receive path Write

Gapped; and then read out of the buffer with the read clock from the

associated DPLL.

The comparative states of the fi read and write position with the elastic

buffer is monitored. If they are 12 bits apart the buffer is in equilibrium,

indicating that the frequency of the VCO output, is identical to the average

frequency of the Write Gapped Clock.

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OFC-34Mbps system

Salient features and general system description of 2-34Mb/s Optimux

The 2-34Mb/s Optimux comprises of a 2-34Mb/s digital skip mux and 34Mb/s

optical line terminating equipment, mounted in the same bay. It employs the

latest state-of-the-art design.

The salient features are.

General

a) Digital specification conforms to ITU-T G.703, G.751 and G.823.

b) FPGA devices employed for high reliability.

c) Slim rack construction.

d) Conforms to environmental QM 333 Category-B.

e) Order wire facility with Latching and Reset facility.

f) Provision for extension of status of terminal to remote centralized

supervision system.

Transmitter

a) Accepts 16 tributaries of 2Mb/s, HDB3 input.

b) Provides one 34Mb/s data stream HDB3 output.

c) In the OLTE section, this electrical 3368 Kb/s signal directly converted to

41241.6 Kb/s rate to accommodate Supervisory and Orderwire information.

Receiver

a) Accepts one 34Mb/s data stream HDB3 input.

b) Provides 16 tributaries of 2Mb/s data stream HDB3 output.

c) In the OLTE section, the optical signal is converted to digital electrical

signal using PINEET / APD with suitable line decoder.

34Mb/s OLTE section

The 34Mb/s OLTE is a single card version. It comprises of this integrated

single card in addition to the CPU card.

a) Processor card.

b) 34Mb/s OLTE card.

It also has a separate Supervisory display and control unit mounted at a

nominal height, with a LCD display.

Technical Specifications

Bit rate:

Nominal rate :

Tolerance :

Timing signal

34368 Kb/s.

+/- 20 ppm.

System capable of synchronizing on one of following clock sources.

(a) Internal oscillator at 34368 Kb/s.

(b) 34368 Kb/s recovered clock.

(c) External 34368 Kb/s signals.

Service digits

Bit 12 of set 1 in each frame is available for national use and is

accessible. Bit 11 of set 1 is used to transmit an alarm indication to the

remote multiplex equipment.

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Alarms

a) Failure of derived power supplies.

OFC-34Mbps system

b) Loss of incoming signal at any of the sixteen 2048 Kb/s tributaries

and 34 Mb/s tributary at the input of Optimux.

c) Loss of incoming optical signal.

d) AIS received at the input of demux.

e) Laser bias current out of limit.

f) Auto laser shut off function disabled.

Functions of supervisory systemt

In normal conditions, this system scans the status of each terminal

sequentially and stored data is upgraded at a regular interval.

Supervisory display unit with LCD monitors following status of each terminal

and regenerator

Laser bias

Absence of input optical signal

BER performance

Status of alarm conditions

In case of degradation in performance of terminal, it displays on master

controller.