TERMENI importanti pentru acest document
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
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
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
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.
Optical fiber communication 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:
· 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:
Typical Application OLTE
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.
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.
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.
All input and output ports of multiplexers are provided on the rear of the
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.
The 8 MBPS signal frame is 848 bits long and has duration of 100.38 µs. 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
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 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
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.
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
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.
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.
The 2M input block consists of the blocks.
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
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
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.
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.
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
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.
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.
Nominal rate :
+/- 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.
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.
a) Failure of derived power supplies.
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
· 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
Adauga cod HTML in site