Wavelenth division multiplexing

1. Q. 1. What is wavelength division Multiplexing (WDM) and what
are the advantages of WDM transmission
Ans. WDM—The .technology of combining a number of wavelengths onto
the same Fiber is known WDM.
WDM is the basic technology of optical networking. It is a technique for
using a fiber to carry many separate and independent channels. In WDM
multiple optical carriers at different wavelengths are modulated by using
independent electrical bit streams and are then transmitted over the
same fiber. It increases the information carrying capacity of a fiber.
Advantages —
(i) It has greater transmission capacity
(ii) Duplex transmission
(iii) Simultaneous transmission of various signals
(iv) Easy system expansion.
(v) Lower cost
(vi) Faster access to new channels.
Q. 2. Explain key features of WDM system.
Ans. The key system features of WDM are
(1) Capacity upgrade — WDM system can increase the capacity of a fiber
network.
(ii) Transparency — In WDM, each optical channel can carry any
transmission format with WDM, any type of information, analog or digital
can be sent simultaneously over the same fiber.
(iii) Wavelength routing—In addition to using multiple wavelengths to
increase
link capacity; the use of wavelengths sensitive optical routing devices
makes it possible to use wavelength as another dimension in designing
communication networks and switches.
Q. 3. What are the different types of WDM MUXS and DEMUXS.
Ans. Depending on the wavelength spacing, WDM, MUXS and DEMUXS
are classified into three types.

2. (i) Broad band WDMS (B WDM)—they combine and separate 1310 and
1510 nm channels or even 850nm and l3lOnm channels.
(ii) Narrow band WDMs (NWDM) — They combine and separate
wavelength channels with centre to centre spacing greater than 200 GHz.
(iii) Dense WDMs (DWDMs)—They combine and separate wavelength
channels not more than 200 GHz.
Q. 4. Explain in detail WDM systems with suitable diagrams.
Ans. In fiber optic communications, wavelength division multiplexing
(WDM) is a technology which multiplexes multiple optical carrier signals
on a single optical fiber by using different wavelengths of laser light to
carry different signals. This allows for a multiplication in capacity in
addition to enabling bidirectional communications over one strand of fiber.
WDM system uses a multiplexer at the transmitter to join the signals
together and demultiplexer at the receiver to split them apart.
A block diagram of WDM system is show in Figure.
Fig. 6.1 a shows WDM system with multiplication in capacity of system. In
WDM, several baseband modulated channels are transmitted along a
single fiber with each channel located at different wavelength. Each of N
different wavelength channels are operating at slower speeds, but the
aggregate system is transmitting at N times the individual channel speed,
providing a significant capacity enhancement. In this WDM system,
numbers of electrical signals are efficiently combined and transmitted
over a single fiber. The signals can be voice, video or data which may be
either in analog or multiplexer format. A WDM multiplexer couples light
from different sources to the transmitting fiber. At the receiver WDM
demultiplexer separates the different carriers before the photo detection
of the individual signals. The WDM channels are separated in wavelength
to avoid cross-talk when they are demultiplixed by a non ideal optical
fiber.

3. In the second case, as shown in fig. 6.1 b, we can achieve full duplex
communication using a single fiber cable. In this the information flow in
each of the two directions is discriminated by the use of two different
wavelengths. it involves sending information in one direction at a
wavelength from left to right simultaneously by sending information
from right to left at wavelength . To achieve bidirectional
communication we use MDM which performs the function of Multiplixer
and Demultiplexer.
Q. 5. Explain the classification of WUM systems depending on
channel resoultion and number of channels.
Ans.Depending on channel resolution and number of channels, there are
two types of WDM sytems.
(i) Conventional or coarse WDM
(ii) Dense WDM
Conventional WDM systems provide upto 16 channels in the 3rd
transmission window (c-band) of silica fibers around 1550 nrn. DWDM
uses the same transmission window but with denser channel spacing.
Channels plans vary but typical sytem would use 40 channels at 100 GHz
spacing or 80 channels with 50 GHz spacing. Some technologies are
capable of 25 GHz spacing called ultra dense WDM.
Q. 6. Explain the classification of WDM system depending on
sytsem architecture.
Ans. Depending on system architecture there are three types of WDM
sytems
(i) High capacity point to point links

4. (ii) Wide Area and metro area networks
(iii) Multiple Access WDM networks.
(1) High capcity point to point links—WDM increases the total bit rate in
optical communication system.
Fig. 6.2 shows point to point high capacity WDM link. The output of
different transmitters, each hving its own carrier wavelength are
combined in multiplexer. The multiplexed signal is launched into the
optical fiber for transmission to receiver side. At receiver end,
demultiplexer separates each carrier and sends each channel to its own
receiver. Suppose that each channel has bit rate B1 B2…… BN. These
channels are transmitted simultaneoulsy over fiber of length L.
then total bit rate distance product BL (B1 + B2 + BN) L.
If all channels have equal bit rates then the capacity of system will be
increased
by a factor of N.
The capacity of WDM fiber links depends on how closely channels can be
packed in the wavelength domain. The channel spacing should exceed
213 at a bit rate B but this requirement wastes Band width. The number
of channels that can be transmitted over system are limited by number of
factors which include degrdation of signal during transmission and inter
channel cross talk during demultiplexirig.
(ii) Wide Area and Metro area Netowork—There are three types of optical
networks (i) LAN (ii) MAN (iii) WAN depending on the area they cover.
Hub, star or ring topology are used for designing optical networks For
MANs and WANS, ring topology is used and for LANs, star topology is
used. Both star and ring toplogies are shown in fig. 6.3.

5. Each node in the star has a transmitter and a receiver, with the
transmitter connected to one of the central passive stars inputs and the
receiver connected to one of the stars output. WDM networks can also use
ring topology because rings are easy to implement for any network
geographical configuration. As shown in Fig. 6.3b each node in the
unidirectional ring can transmit on a specific signature wavelength and
each node can recover any other node wavelength signal by means of a
wavelength tunable receiver. In both the star and the ring topology, each
node has a signature wavelength and any two nodes can communicate
with each other by transmitting on that wavelength. This implies that we
require N wavelengths to connect N nodes. It has an advantage that data
transfer occurs with art uninterrupted optical path between the origin and
the destination known as a single hop network. The optical data start at
the originating node and reach the destination node without stopping at
any other intermediate node. A disadvantage of a single hop WDM
network is that the network and all its components must accommodate N
wavelengths which may be difficult to achieve in a large network.
(iii) Multiple Access WDM networks—Multiple access networks allows a
bidirectional access to each subscriber. At all time, each user can transmit
and receive information from any other user of the network. WDM
networks can be classified into two categories (1) single hop optical
networks (ii) Multiple hop optical networks. In single hop network, all
nodes are connected to each other while in multiple hop networks, an
optical signal sent by one node passes through number of intermediate
nodes before reaching its destination node. Consider single hop WDM
network called Lambda net while is an example of broadcast and select
network.

6. In Lambdanet, each node has one transmitter emitting a particular
wavelength and number of receivers operating at N different wavelengths.
The output of all transmitters is joined ii a passive star and equally
distributed to all receivers. Desired channel is selected with the help of
tunable optical filter. In place of a tunable filter, each node uses a number
of receivers. In Lambdanet, the capacity and connectivity can be changed
depending on the application. Data can be transmitted at different bit
rates with different modulation formats in this network. This network has
a disadvantage that the number of users is limited by the number of
available wavelengths. Each node requires number of receivers. The
disadvantages of Lambdanet can be compensated in rainbow network
with the help of tunable receiver. Rainbow network use central passive
star together with the high performance parallel interface. The main
drawback of rainbow network is that tuning of receivers is a slow process
due to which packet switching can’t be used.
The disadvantage of single hop network to accommodate N wavelength by
each component can be compensated in multichip network in which two
nodes can communicate with each other by sending through a third node,
with many such intermediate hops possible, A dual bus multihop eight
node WDM network is shown in Fig. 6.5. For which each node can
transmit on two wavelengths and receive on two other wavelengths.

7. For example, if node 1 wants to communicate with node 5, it transmits on
wavelength A1 and on.ly a single hop is required, However if node I wants
to communicate with node 2, if first must transmit to node 5, which then
transmit to node 2, including two hops. Any extra hops increase the
transmit time between two communicating nodes and decrease the
through put. However a multiple hop networks reduce the required
number of wavelengths and the wavelength tenability range of the
components.
Q. 7. With the help of a block diagram, explain the basic concept
of subcarrier multiplexing.
Ans. Subcarrier Multiplexing allows multiple broadband signals to be
transmitted over single mode fiber. In sub carrier multiplexing, instead of
directly modulating terahertz optical carrier wave with—lOOs Mbps
baseband data, the baseband data are impressed on a gigahertz
subcarrier wave that is subsequently impressed on the THz optical carrier.
In SCM, microwave subcarriers are used for multiplexing rather than
optical carrier. SCM is similar to commercial radio, in which many stations
are placed at different radio frequencies such that a radio receiver can
tune its filter to the appropriate subcarrier radio frequency. The
multiplexing and demultiplexing of the SCM channels is done
electronically not optically
In analog SCM system, analog format is used to modulate each micro
subcarrier. The output of all subcarriers is added by microwave power
combiner. The output of microwave power combiner is used to modulate
the intensity of a optical transmitter i.e. laser. Optical signal is then
transmitted through optical fiber. At the receiver photo detector detects
the optical signal. Microwave receiver detects and selects all the channels.
If the communication channel is perfectly linear, the received power will
be in the same form as transmitted power. But no communication channel
is perfect so distortion occurs in received power which is called

8. intermediation distortion (IMD). IMD occurs due to various non linear
mechanisms. The performance of SCM system depends on the CNR of
demodulated signal. CNR is the ratio of RMS value of carrier power to
RMS value of noise power at the receiver.
Advantages/Disadvantages—
Advantages
(i) Several channels can share the same expensive optical components.
(ii) Electrical components are less expensive than optical components
used in SCM systems.
Disadvantage
SCM is limited in maximum subcarrier frequencies and data rates by the
available bandwidth of the electrical and optical components. SCM must
be used in conjunction with WDM to utilize any significant fraction of the
fiber band width.
DIIM SCM system
In digital SCM system multilevel QAM format is used to modulate the light
intensity of laser. By using QAM to modulate the light intensity of laser,
no coherent detection is required at receiver. It also requires lower CNR
as compared to AM-VSB systems. The capacity of as SCM system can be
increased by employing hybrid techniques that mix analog and digital
formats. Hybrid SCM systems can transmit a large number of channels
over the same fiber simultaneously. Hybrid SCM systems can transport up
to 80 analog and 30 digital channels using a single optical transmitter.
Such systems are affected by non linear mechanisms such as SPM and
SBS, and clipping noise.
Q. 8. Explain in detail code division multiplexing will necessary
diagrams.
Ans. Code Division Multiplexing (CDM) is a technique in which each
channel transmits its bits as a coded channel specific sequence of pulse.
This coded transmission is accomplished by transmitting a unique time
dependent series of short pulses, which are placed within chip times
within the larger bit time. CMD allows signals from a series of
independent sources to be transmitted at the same time over the same
frequency band. This is accomplished by using orthogonal codes to spread
each signal over a large common frequency band. At the receiver, the
appropriate orthogonal code is than used again to recover the particular
signal intended for a particular user.
Direct Sequence Encoding

9. In CDM systems, encoder and decoders are used at transmitter and
receiver ends. The encoder spreads the signal spectrum using spread
spectrum technique. The spread spectrum technique ha& following
features
(i) Each information bearing signal is transmitted with a band width in
excess of the minimum band width necessary to send the information.
(ii) The bandwidth is increased by using a spreading code that is
independent of the information.
(iii) The receiver has advance knowledge of the spreading code and uses
this knowledge to recover the information from the received spread out
signal. Spread spectrum has a advantage that the transmitted signal is
undetectable by other receivers that do not know the spreading code.
There are different methods which can be used for data coding such as
direct sequence encoding, time hopping and frequency hopping. Direct
Sequence Encoding is a modulation technique. It phase modulates a sine
wave pseudo randomly with a continuous string of pseudo noise (PN)
code symbols called “chips”, each of which has a much shorter duration
than an information bit. That is each information bit is modulated by a
sequence of much faster chips. Therefore, the chip rate is much higher
than the information signal bit rate. Direct sequence spread spectrum
transmissions multiply the data being transmitted by a “noise” signal. This
noise signal is a pseudorandom sequence of 1 and — 1 value, at a
frequency much higher than that of the original signal, thereby spreading
the energy of the original signal into a much wider band width. This noise
signal is used to reconstruct the original data at the receiving end by
multiplying it by same pseudorandom sequence. This process is known as
de-spreading. For despreading to work correctly, the transmit and receive
sequences must be synchronized.
Spectral Encoding—spreading of spectrum can also be accomplished using
the technique of frequency hopping. Frequency hopping spread spectrum
is a method of transmitting radio signals by rapidly V switching a carrier
among many frequency channels using a pseudorandom sequence known
to both transmitter and receiver frequency hopping technique is different
from WDM as fixed frequency is not assigned to a given channel. All
channels share the entire bandwidth by using different carrier frequencies
at different times according to a code. A spectrally encoded signal can be
represented in the form of a matrix as shown in figure 6.7.

10. The matrix columns correspond to time slots and matrix rows correspond
be assigned frequencies. The matrix element mix equals I if and only if
the frequency w1 is transmitted in the interval t. Different users are
assigned different frequency-hop patterns (or codes) to ensure that two
users do not transmit at the same frequency during V the same time slot.
So orthogonal codes are used. Pseudo-orthogonal codes are used in the
case of asynchronous transmission.
The overall bandwidth required for frequency hopping is much wider than
that required to transmit the same information using only one carrier
frequency. However, because transmission occurs only on a small portion
of this bandwidth at any given time, the effective interference bandwidth
is really the same. The frequency hopping approach reduce the
degradation caused by narrowband interferes.
Q. 9. With the help of block diagram briefly explain optical TDM
systems.
Ans. Time division multiplexing is a type of analog or digital multiplexing
in which two or more signals or bit streams are transferred
simultaneously as sub channels in one communication channel. The time
domain is divided into several recurrent timeslots of fixed length, one for
each sub channel. The electrical TDM becomes difficult to implement at bit
rates above 10 Tb/s. To implement TOM at more than 10 Tb/s, optical
TOM is used. OTDM can increase the bit rate of a signal optical carrier to
above I Tb/s. In OTDM, the number of optical signals at a bit rate B share
the same carrier frequency and are optically multiplexed to generate a bit
stream at the bit rate NB where N is the number of channels.
Optical TDM Principles

11. OTDM system is shown in figure 6.8. In this system four bit streams are
merged into one. Rz coding modulation techniques is used. The system
operates as follows.
Each time slot is subdivided into 4 bit times. Each bit time is further
divided into two halves. For a 1 bit the first half of the bit time will be
occupied by an optical pulse. For a 0 bit the whole bit time will be dark. A
laser produces a short pulse at the beginning of each time slot. The laser
signal is split 4 ways. Each signal is then delayed by a fixed amount. Then
each signal is separately modulated to carry its own unique information
stream. The signals. are then recombined to form a single data stream.
During all this the original signal has lost a very large amount of power.
So whole stream must be amplified to reach strength suitable for
transmission on the link.
To demultiplex the OTDM data stream, we must extract each slow speed
data stream based on timing. Data for a single channel is extracted by
using NOLM (Non linear op6c op Minor).
NOLM is used as an AND gate to select every nth pulse from the TDM
stream.
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