A

Seminar Report
On

Solar Power satellites

By
G.Pradeepika
07691A0471
Pradeepika789@gmail.com

DEPARTMENT OF ELECTRONICS AND COMMUNICATION

ENGINEERING
MADANAPALLE INSTITUTE OF TECHNOLOGY AND
SCIENCE, ANGALLU
JNTU ANANTAPUR,
2007 – 2011
Abstract
This paper deals with the twin concepts of optical networking and dense wavelength
division multiplexing. The paper talks about the various optical network architectures and the
various components of an all-optical network like Optical Amplifiers, Optical Add/Drop
Multiplexers, Optical Splitters etc. Important optical networking concepts like wavelength
routing and wavelength conversion are explained in detail. Finally this paper deals with
industry related issues like the gap between research and the industry, current and projected
market for optical networking & DWDM equipment and future direction of research in this
field.
 Network architecture
 Network management
 Network restoration and protection
 Packet switching
 Wavelength routing
 Wavelength conversion

1. INTRODUCTION

One of the major issues in the networking industry today is tremendous demand for
more and more bandwidth. Before the introduction of optical networks, the reduced
availability of fibers became a big problem for the network providers. However, with the
development of optical networks and the use of Dense Wavelength Division Multiplexing
(DWDM) technology, a new and probably, a very crucial milestone is being reached in
network evolution. The existing SONET/SDH network architecture is best suited for voice
traffic rather than today’s high-speed data traffic. To upgrade the system to handle this kind
of traffic is very expensive and hence the need for the development of an intelligent all-
optical network. Such a network will bring intelligence and scalability to the optical domain
by combining the intelligence and functional capability of SONET/SDH, the tremendous
bandwidth of DWDM and innovative networking software to spawn a variety of optical
transport, switching and management related products.
1.1 Optical Networking

Optical networks are high-capacity telecommunications networks based on optical

technologies and component that provide routing, grooming, and restoration at the
wavelength level as well as wavelength-based services. The origin of optical networks is
linked to Wavelength Division Multiplexing (WDM) which arose to provide additional
capacity on existing fibers. The optical layer, whose standards are being developed, will
ideally be transparent to the SONET layer, providing restoration, performance monitoring,
and provisioning of individual wavelengths instead of electrical SONET signals. So in
essence a lot of network elements will be eliminated and there will be a reduction of electrical
equipment.
It is possible to classify networks into three generations depending on the
physicallevel technology employed. First generation networks use copper-based or
microwave technologies e.g Ethernet, satellites etc. In second generation networks, these
copper links or microwave links with optical fibers. However, these networks still perform
the switching of data in the electronic domain though the transmission of data is done in the
optical domain. Finally we have the third generation networks that employ Wavelength
Division Multiplexing technology. They do both the transmission and the switching of data in
the optical domain. This has resulted in the onset of tremendous amount of bandwidth
availability. Further the use of non-overlapping channels allows each channel to operate at
peak speeds.

Evolution of DWDM

Developments in fiber optics are closely tied to the use of the specific region on the
optical spectrum where optical attenuation is low. The regions, called windows, lie between
areas of high absorption. The earliest systems were developed to operate around 850n, the
first window in silica based optical fiber. A second windows (S BAND at 1310 nm, soon
proved to be superior because of its low attenuation followed by a third windows (C-BAND)
at 1550nm with even lower optical loss.

Now a day, a fourth window (L- BAND) near 1625 nm is under development.
(Appendix A) Early WDM, in late 1980s [4], were using the two widely spaced wavelength
in the 1310 nm and 1550 nm regions. The early 1990s saw a second generation of WDM in
which two to eight channel were used. These channels were now spaced at an interval at
about 400GHZ in the 1550 nm window .By the mid-1990s DWDM system emerged with 16
to 40 channels and spacing from 100 to 200 GHZ. By late 1990s DWDM system had evolved
to the point where they were capable of 64 to 160 parallel channels, densely packed at 50 or
even 25 GHZ interval.

1.2 Dense Wavelength Division Multiplexing (DWDM)

Dense Wavelength Division Multiplexing (DWDM) is a fiber-optic transmission technique. It

involves the process of multiplexing many different wavelength signals onto a single fiber.
So each fiber has a set of parallel optical channels each using slightly different light
wavelengths. It employs light wavelengths to transmit data parallel-by-bit or serial-by-
character. DWDM is a very crucial component of optical networks that will allow the
transmission of data: voice, video-IP, ATM and SONET/SDH respectively, over the optical
layer.

Hence with the development of WDM technology, optical layer provides the only
means for carriers to integrate the diverse technologies of their existing networks into one
physical infrastructure. For example, though a carrier might be operating both ATM and
SONET networks, with the use of DWDM it is not necessary for the ATM signal to be
multiplexed up to the SONET rate to be carried on the DWDM network.Hence carriers can
quickly introduce ATM or IP without having to deploy an overlay network for multiplexing.

2. DWDM SYSTEM

As mentioned earlier, optical networks use Dense Wavelength Multiplexing as the

underlying carrier. The most important components of any DWDM system are transmitters,
receivers, Erbium-doped fiber Amplifiers, DWDM multiplexers and DWDM demultiplexers.
Fig 1 gives the structure of a typical DWDM system.
 Fig.1 Block Diagram of a DWDM System

The concepts of optical fiber transmission, amplifiers, loss control, all optical header
replacement, network topology, synchronization and physical layer security play a major role
in deciding the throughput of the network. These factors have been discussed briefly in this
sections that follow.

Versatility of DWDM
DWDM is a fiber optic transmission technique that employs light wavelength to transmit data
parallel by bit or serial by character. It is the optical network that allows the transmission of e-
mail, video, multimedia data voice carried in Internet protocol (IP), asynchronous transfer mode
(ATM) and synchronous optical network’s (SDH). DWDM increases the capacity of embedded
fiber by first assigning incoming optical signal to specific frequencies within a designated
frequency band and multiplexing the resulting signal out onto one fiber.

Fig. Versatility of DWDM

Salient Features of DWDM
Apart from having better Bandwidth characteristics DWDM system are more adaptable
under faulty conditions as it can be used in Rings. In case of optical fiber cable cut, the ring goes
in protection mode and thus avoids any failure. This fault tolerant capability is described below.
DWDM in Ring
The DWDM system only provides “virtual” optical fiber. The protection for each
wavelength on SDH layer is independent to the protection mode of other wave length .This ring
can be two fiber or 4 fiber. To employ OADM with the add/ drop multiplexing capability to form
rings is another application mode of DWDM technology in ring networks. At present network
formed by OADM s can be classified into two mode.
One is wave length path protection based on single wave length protection. ie. 1+1
protection of single wave length which is similar to path protection of SDH system. The other is
line protection ring which protect the single of multiplexed wave length. when fiber is cut off, the
“ loop back” function can be implemented in two nodes near the fiber cut off. Thus all the ervices
are protected .This is similar to MSP of SDH. From the aspect of specific forms, the line
protection ring can be divided into two fiber bi directional ring and two fiber unidirectional ring,
half of the wave length operate as working and other as protection wave length.
Other main features supported by DWDM are as follows:
􀀹 DWDM networks is smooth upgradeable network
􀀹 DWDM network is Combination of Integrated and open system
􀀹 Channel equalization technology is used for better performance
􀀹 Main channel and supervisory channel are independent
􀀹 Forward error correction (FEC) technology is used in DWDM
􀀹 DWDM network has Automatic level control (ALC) function
􀀹 DWDM has gradable optical ADD/DROP multiplexing technology
􀀹 Unified and intelligent Network Management
􀀹 DWDM can be used up to 640 Km

Research Issues & Future Directions in DWDM

Transmission Impairment:

Developing network-layer solutions to counter physical-layer impairments, such as laser

shift, dispersion in fiber, and also impairment that affects optical components such as amplifiers,
switches, and wavelength converters, is another important issue. A signal degrades in quality due
to physical-layer impairment as it travels through switches (picking up crosstalk) and EDFAs
(picking up noise). This may cause a high bit error rate (BER) at the receiving end of a light path.
The work is going for estimates the online BER on candidate routes and wavelengths before
establishing a connection between a source–destination pair.

One approach is to establish a connection with minimum BER. Another is to establish a

connection with BER lower than a certain threshold if no such connection is found, the
connection request is rejected. Another networking study which considers physical-layer device
characteristics while attempting to solve a network-layer problem is amplifier placement in
WDM optical networks.

Optimization of Location for Amplifier Placements:

In wavelength routed networks, optical amplification is required to combat various power

losses such as fiber attenuation and coupling loss in wavelength routers .Since optical amplifiers
are costly, their total number in the network should be minimized ,apart from determining their
exact placements in the network. However, optical amplifiers have constraints on the maximum
gain and the maximum output power they can supply. When optical signals on different
wavelengths originating at various nodes at locations separated by large distances arrive at an
amplifier, their power levels may be very different. This phenomenon, known as near–far effect,
can limit the amount of amplification available since the higher-powered wave lengths could
saturate the amplifier and limit the gain seen by the lower-powered wave lengths .The amplifier
placement problem considering the limitations of the devices(for example, maximum power of a
transmitter, fiber attenuation, minimum power required on a wavelength for detection [this
represents both the receiver sensitivity level and the amplifier sensitivity level], maximum power
available from an amplifier, and maximum [small-signal] amplifier gain. The general problem of
minimizing the total amplifier count is a mixed-integer nonlinear optimization problem.

Virtual Private Networks over WDM Optical Networks:

A virtual private network (VPN) is a communication network between two or more

machines or networks, built for the private use of an organization, over a shared public
infrastructure such as the Internet. In other words, a VPN turns the public network (Internet) into
a simulated WAN by letting an organization securely extend its network services to remote users,
branch offices, and partner companies. VPNs require strong security protocols, such as IP Sec (IP
Security), to be used for data transfer, as they consist of several machines not under the control of
the organization—IP routers and the Internet that carries the traffic .VPNs can make use of the
concept of a light path offered by WDM, to create secure tunnels (channels) of bandwidth across
the WDM backbone network.
NEXT-GENERATION Optical Internet Networks

WDM-based optical networks are becoming the right choice for the next-generation
Internet networks to transport high-speed IP traffic. In the first phase, light path based circuit
switching WDM networks are deployed as a means of carrying IP traffic. SONET and ATM
networks have been widely deployed in the transport networks. SONET systems have several
attractive features such as high-speed transmission and network survivability. ATM networks
have several attractive features such as flexible bandwidth allocation and QoS support.
Therefore, ATM and/or SONET layers can be used in between the IP layer and the WDM
optical layer for transporting IP packets. A major drawback of this multilayer approach is that it
incurs increased control and management overhead. WDM technology is evolving from circuit
switching technology to burst switching and packet switching technologies. The granularity of the
basic switching entity is large in circuit switching networks, medium in burst switching networks,
and small in packet switching networks. While circuit switching technology is mature ,the other
technologies are not. In a circuit switching network, a wavelength channel on a link is used by a
circuit (light path) for a long time, until it is torn down. In a burst (packet) switching network, a
wavelength channel on a link is reserved only for the duration of the burst (packet). The
bandwidth utilization in burst and packet switching networks is higher when compared to that in
circuit switching networks. This is because, the former networks use statistical multiplexing
while the latter does not. In a burst switching network, the basic switching entity is a burst. A number of IP
packets which are destined for the same egress router are assembled into a burst at the ingress
router. The major challenges in burst switching networks include the design of cost-effective and
fast switches, burst switching protocols, and wavelength channel scheduling. In a packet
switching network, the basic switching entity is a packet. The major challenges in packet
switching networks include the design of cost-effective and fast switches, packet synchronization,
and contention resolution.
Since optical processing is technologically and economically infeasible, the packet/burst
header is processed electronically while the payload is switched optically. Since optical random
access memory (RAM) is not available, a packet or burst cannot be buffered optically for a long
time. A possible way is to use fiber delay lines (FDLs) to buffer (delay) a packet or burst for a
short time. A multi protocol label switching (MPLS) framework has several advantages, such as
traffic engineering, explicit path routing, fast packet forwarding, and network survivability. Due
to the above advantages, future Internet networks employing circuit/burst/packet switching are
likely to use the MPLS approach.

2.1. Optical Transmission Principles

The DWDM system has an important photonic layer, which is responsible for
transmission of the optical data through the network. Some basic principles, concerning the
optical transmission, are explained in this section. These are necessary for the proper
operation of the system.
Channel Spacing

The minimum frequency separation between two different signals multiplexed in

known as the Channel spacing. Since the wavelength of operation is inversely proportional
to the frequency, a corresponding difference is introduced in the wavelength of each signal.
The factors controlling channel spacing are the optical amplifier’s bandwidth and the
capability of the receiver in identifying two close wavelengths sets the lower bound on the
channel spacing. Both factors ultimately restrict the number of unique wavelengths passing
through the amplifier.

Signal Direction

An optical fiber helps transmit signal in both directions. Based on this feature, a
DWDM system can be implemented in two ways:
Unidirectional: All wavelengths travel in the same direction within the fiber. It is similar to a
simplex case. This calls in for laying one another parallel fiber for supporting transmission on
the other side.

Bi-directional: The channels in the DWDM fiber are split into two separate bands, one for
each direction. This removes the need for the second fiber, but, in turn reduces the capacity or
transmission bandwidth.

Signal Trace

The procedure of detecting if a signal reaches the correct destination at the other end.
This helps follow the light signal through the whole network. It can be achieved by plugging
in extra information on a wavelength, using an electrical receiver to extract if from the
network and inspecting for errors.

The receiver the reports the signal trace to the transmitter. Taking into consideration
the above two factors, the international bodies have established a spacing of 100GHz to be
the worldwide standard for DWDM. This means that the frequency of each signal is less than
the rest by atleast 0.1THz.

2.2 Network classification

A network can be physically structured in the form of either a ring, a mesh, star based
or linear bus based on the connection between the various nodes. Although the physical
topology of a DWDM system might be that of a ring, the logical traffic distribution topology
can be arbitrary. This is done through the use of different wavelengths to interconnect each
node.

Ring Topology vs Mesh Topology

Until the development of EDFAs the passive star configuration was the most popular
configuration due to its superior power budget. However, with the advent of EDFAs, the ring
network works out much better after overcoming its power budget problems. What makes the
ring network better is its superior resilience. The Optical Cross Connect (OXC) help pass on
traffic between each of the rings. A Path-in-Lambda architecture for connecting all-optical
networks is under development.
A ring topology is preferable owing to many of its capabilities. Unlike a mesh
network, the expense of laying out the links is reduced in the ring, because the number of
links increases only as a linear progression. The rings also have better resilience and
restoration than meshes. The ring topology besides serving as a standby link helps share the
load. The working segment (Refer to Fig.2) and the protection segment of the fiber together
handle the large data burst of the computer network. This reduces the load on the router and
removes the need for buffering
 Fig.2 Ring Topology Connecting Nodes A & B

Single-Hop Networks vs Multi-hop Networks

Multi-wavelength networks can be also classified as single-hop networks and
multihop networks. In single-hop networks, the data stream travels from source to destination
as a light stream. There is no conversion to electronic form in any of the intermediate nodes.
Two examples of a single-hop network are the broadcast-andselect and the wavelength-
routed architecture.
Broadcast-and-select networks:
It is based on a passive star coupler device connected to several nodes in a star
topology. Basically a signal received on one port is split and broadcast to all ports. Networks
are simple and have natural multicasting capabilities. Generally used in high speed LANs or
MANs. Other elements in this type of network are tunable receivers and fixed transmitters or
fixed receivers and tunable transmitters.
Wavelength routed networks:
The key element here is the wavelength-selective switching subsystem. There are
again two types of wavelength switching. Wavelength path switching involves dynamic
signal switching from one path to another by changing WDM routing while wavelength
conversion the reuse of the same wavelength in some other part of the network as long as
both light paths don’t use it on the same fiber. Wavelength routing is explained in more detail
in section In multi-hop networks, each node has access to only a small number of the

wavelength channels used in the network. Fixed wavelength transmitters and receivers are
used for this purpose with a minimum of at least a single wavelength transmitter and a single
wavelength receiver tuned to different wavelengths. This type of network requires at least one
intermediate node for a packet to reach the destination. Also, at each intermediate node
electronic switching of packets take place. Two examples of actual multihop systems on
which packet switching has been implemented experimentally are Starnet (developed by
Optical Communication Research Laboratory at Stanford University) and Teranet (developed
by Columbia University).

2.3 Optical Amplifiers

Researchers are working on managing traffic optically rather than first converting it to
electronic signals. However, it has been noticed that in long-haul networks, the effects of
dispersion and attenuation are significant. What this means is that a signal cannot maintain
its integrity over really long distances without having to be amplified. Towards that end, the
production of optical amplifiers became important, which would help in amplifying signals at
regular intervals. This led to development of the Erbium-doped fiber Amplifiers (EDFA).
EDFAs are as the name says, are silica based optical fibers that are doped with erbium. It is
this doping that achieves the conversion of a passive fiber to an active one. Traditionally it
has been used for terrestrial and under-sea purposes. With the development of EDFA we have
basically almost rendered ‘Wavelength Regenerators’ redundant. The element erbium (Er68)
boosts the power of wavelengths and eliminates the need for regeneration. It is the optical
amplifier that has made WDM economically feasible. The usable bandwidth by using EDFAs
is about 30nm (1530nm-1560nm).
However, attenuation is minimum in the range of 1500nm –1600nm. Hence that
implies very less utilizations. Also typically what happens is that with the need to place as
many wavelengths (channels) as possible in a single fiber, the distance between two channels
is very small (0.8-1.6nm). This results in the Inter-channel crosstalk becoming a very
important issue at this point.
It became imperative that the amplifier's bandwidth had to be increased while
eliminating crosstalk. So this led to the development of Silica Erbium fiber-based Dual-
band fiber amplifier (DBFA). These fibers are similar to the EDFAs and have been able to
generate terabit transmission successfully. However, the most important feature of the DBFA
is its bandwidth => 1528nm-1610nm. The DBFA has two sub-band amplifiers. The first is in
the range of the EDFA and the second one is what is known as extended band fiber
amplifier (EBFA). It has been shown that this EBFA has several attractive features compared
to the traditional EDFA.
 Flat Gain: EBFAs achieve a flat gain over a range of wide range (35nm) as compared
to the EDFAs
  Slow Saturation: EBFAs reach saturation slower than the EDFAs. Saturation is the
state where output remains constant even though input level keeps increasing.
 Low Noise: EBFAs exhibit lower noise than EDFAs Therefore, the 1590-nm EBFA
represents a huge leap in meeting the ever-increasing demands of high-capacity fiber-
optic transmission systems. A similar product is Lucent’s Bell Labs of an "Ultra-
Wideband Optical Amplifier (UWOA) that can amplify up to 100 wavelength
channels as they travel along a single optical fiber and has a usable bandwidth of
80nm. This bandwidth spans the 1530-1565nm channel (Cband) and also the long
wavelength channels beyond 1565-1620nm (L-band).

Up to December 1998 industrial capability is such that, wavelength systems were

developed that could carry a maximum of 40 wavelengths per fiber. The various stages of this
development included 4, 8, 16, 32 wavelengths per fiber.

2.4 Synchronization

The SONET networks currently support the multiplexing of lower Time Division
Multiplexing (TDM) rates onto higher rates. The Add/Drop Multiplexers (ADM) and
transponder en route provide the much-needed synchronization. This ensures the quality and
guarantees proper delivery of data. But, since DWDM systems support the multiplexing of
different wavelengths, no timing relation exists for the system. The need for a clocking
system, similar to one used in SONET, is absent. Nevertheless, synchronization may still be
used for assuring good quality. The numerous regenerators/transponders and other devices in
the path of a signal introduces jitter. Synchronization can be used to ensure quality by
cleaning up the signals transmitted at each node. SONET terminals and ADMs have a special
timing output port, which provides timing to customers. It is sometimes referred to as the
Derived DS1. It is a true DS1 signal, but carries no traffic. All data bits are set to logic 1 to
minimize timing jitter. A clock distribution amplifier may be used to split the Derived DS1
signal, to synchronize many network elements. In a network, each distribution amplifier
output may be routed to a different network element.
2.5 Security
Optical fibers too facilitate secure connections. Quantum cryptography is one such
operation, which exploits the fundamental properties of quantum complimentarily (The
concept that particle and wave behavior are mutually exclusive, but, are together necessary
for the complete description of any phenomena) to allow two remote parties to generate a
shared random bit sequence. Users can safely use their shared bit sequence as a key for
subsequent encrypted communications. In conventional complexity-based approaches to
security, privacy is achieved by posing a difficulty mathematical problem to the interceptor,
which is computationally intensive. In contrast, Quantum Key Distribution (QKD), as it is
called, provides a new paradigm for the protection of sensitive information in which security
is based on fundamental physical laws.
3. DWDM COMPONENTS
Important components of a DWDM system are the Add/Drop Multiplexer (ADM), the
Optical Cross Connect (OXC), Optical Splitter. The Add/Drop Multiplexer as the name
suggests, selectively adds/drops wavelengths without having to use any SONET/SDH
terminal equipment. We require the ADM to add new wavelengths to the network or to drop
some wavelengths at their terminating points. There are two types of implementations of the
ADM, the Fixed WADM and the Reconfigurable WDM.

Fig.3 Block Diagram of the WADM

The Optical Cross Connect acts across connect between n-input ports and n-output
ports. It allows the efficient network management of wavelengths at the optical layer. The
varieties of functions that it provides are signal monitoring, restoration, provisioning and
grooming.
 Fig.4 Block Diagram of the OXC

Optical Splitters are being suggested for use in multicast-capable wavelength-routing

switches to provide optical multicasting. It is a passive device that will help in replicating
optical signals. This is explained in detail in a later section. Optical Gateways are devices that
will allow the smooth transition of traffic to the optical layer. We can have high-speed ATM
networks or a mix of SONET and ATM services with such a gateway. They provide the
maximum benefits of optical networks.

4. OPTICAL NETWORK ARCHITECTURE

Just like every other layer defined in networking, a layer architecture has to be
defined for the optical layer. A multi-wavelength mesh-connected optical network is used to
define the architecture of the optic layer.

A light path is defined as the path between two nodes and is equivalent to a
wavelength on each link on that path. Two aspects of the network topology have been
described: physical topology and virtual topology.

The physical topology has WDM cross-connect nodes interconnected by pairs of

point-to-point fiber links in an arbitrary mesh topology as shown in the following figure.
Fig.5 A WDM network consisting of crossconnect nodes interconnected by pairs of point-to-
point fiberoptic links(i.e physcial topology)
5. DWDM ARCHITECTURE
Using some of the basic concepts of DWDM systems, it is possible to form an All-
Optical layer. Transport of Gigabit Ethernet , ATM, SONET, IP on different channels is
feasible. By achieving this, the system becomes more flexible and any signal format can be
connected to, without the addition of any extra equipment that acts as a translator between the
formats. In this section we will talk about the various types of technologies that can be used
over DWDM systems. In particular, we will discuss ATM over DWDM and IP over DWDM.
5.1 ATM over DWDM
As bandwidth requirements increase, Telcos are faced with huge investments in order
to fulfill the capacity demands. Along with this the demand for QoS has increased. There
seems to be a general move towards providing QoS while still maintaining the same capacity.
ATM over DWDM solves the bandwidth and Quality of Service issues in a cost-effective
way. In DWDM networks, if there is a carrier that operates both ATM and SONET networks
there is no need for the ATM signal to be multiplexed up to the SONET rate. This is because
the optical layer can carry any type of signal without any additional multiplexing. This results
in the reduction of a lot of overlay network.
While there are a lot of advantages of running ATM over DWDM, there are certain
issues that are of importance that need to be considered. They are channel spacing (four
Wave Mixing) and optical attenuation. Hence, we need good wavelength
conditioning techniques to solve this problem. The techniques used are Forward Error
Correction Technique and the pilot light technique. By using the latter technique network
management systems are able to ensure connectivity, signal on each channel and also identify
faults. This network management is similar to the way test cells are used on specific Virtual
Channels in ATM.
Testing ATM over DWDM
Testing of ATM over DWDM consists of similar concepts to those provided in ATM
over SONET. However, with DWDM it is more complex because we now have multiple
parallel links on a single fiber. So besides the need of taking into account the connectivity
and the conformance to QoS agreements, we need to make sure that these parallel links are
all mutually exclusive. Hence, the following parameters need to be measured:
Signal-to-noise ratio

Channel power

Channel center wavelength and spacing

Crosstalk

Total Optical Power

Chromatic dispersion

Polarization Mode Dispersion

5.2 IP over DWDM (or IP over lambda)

The ultimate solution would be to take IP directly over DWDM. This will bring about
scalability and cost-effectiveness. Now we have industry products that actually implement IP
over DWDM for example Monterey Networks (bought by Cisco in August '99) have their
Monterey 20000 Series Wavelength Router & trade. They claim that by using their product,
"service providers can traffic-engineer and rapidly scale up survivable mesh optical cores
without introducing intermediate ATM switches or proliferating legacy SONET multiplexers
and cross-connects". In effect we are totally eliminating ATM and SONET layers from the
networks. The proponents of IP over DWDM say that SONET’s reliability is due to a lot of
redundancy. This overkill prevents the network from using a large portion of its resources.
The real test is whether it would be possible to create an end-to-end optical Internet operating
from OC-3 to OC-48 and build systems around an optical Internet backbone. Compare that
with the news that SONET handles OC-192 smoothly and can touch OC-768. As of
March’99, all the IP over DWDM systems that were operational were all SONET frame
based. With the development of erbium-doped fiber amplifiers most systems that use IP over

DWD using SONET frames have removed the SONET multiplexers. GTS Carrier Service in
March launched the first high capacity transport platform in Europe that uses IP over DWDM
technology. Furthermore, major carriers such as AT&T, Sprint, Enron, Frontier, Canaries,
have all begun to realize the huge economic potential of IP over DWDM and there is no
longer any skepticism about this technology.

6. WAVELENGTH ROUTING IN OPTICAL NETWORKS

An optical network consists of wavelength routers and end nodes that are connected
by links in pairs. The wavelength-routing switches or routing nodes are interconnected by
optical fibers. Although each link can support many signals, it is required that the signals be
of distinct wavelengths.
Routers transmit signals on the same wavelength on which they are received. An All-

Optical wavelength –routed network that wavelength-routed network that carries data across
from one access station to another without any O/E (Optical/Electronic) conversions.

Categories of Wavelength Switches (or routers as the authors call them):

 Non-reconfigurable switch: These types of switches, for each input port and each
wavelength, transmit onto a fixed set output ports at the same wavelength. These cannot
be changed once the switch is built. Networks that contain only such switches are called
non-reconfigurable networks.
 Wavelength-Independent Reconfigurable switch: These types of switches have
input-output pattern that can be dynamically reconfigured. However, the input-output
pattern is independent of the wavelength of the signal i.e. there are only fixed sets of
output ports onto which an incoming signal can be transmitted.
 Wavelength-Selective Reconfigurable Switch: These types of switches combine the
features of the first two categories.
Also known as generalized switch, they basically have both the properties of dynamic
reconfiguration and the routing pattern being a function of the wavelength of the incoming
signal.
 Reconfigurable routers are of bounded degree, while no reconfigurable routers may
not be. That is, the complexity of non-reconfigurable networks can be ignored as it is not of a
fixed degree. However, the complexity of reconfigurable networks is strongly dependent on
its degree and it is bounded.
7. WAVELENGTH CONVERSION IN OPTICAL NETWORKS
The networks that we have been discussing about until now can be said to be
wavelength-continuity constraint networks. In such networks, to establish any light path, we
require that the same wavelength be allocated on all of the links in the path. Suppose we have
the following portion of a network. The wavelengths 1 and 2 that are shown in dotted
arrows are the free wavelengths between nodes 1, nodes 2 and node3 respectively. There are
2 wavelength converters, one in node 2 and another in node 3. Here it is not possible to
establish a light path from 1 to 4 without a wavelength converter because the available
wavelengths are different on the link.

Fig.8 Wavelength Conversion

So, we could eliminate this problem by converting data that is arriving on the link
from node 1 to node 2 on 1 to 2 on the link between node 2 and node 3. Such a
technique is called wavelength conversion. Functionally, such a network is similar to a
circuit-switched network. For any model of optical routing, we need to make as efficient use
of the given optical bandwidth that we have as possible. Wavelength converters have been
proposed as a solution to this problem. Wavelength converters have been defined earlier in
this report as those devices that convert an incomingsignal's wavelength to a different
outgoing wavelength thereby increasing the reuse factor. Wavelength converters offer a 10%-
40% increase in reuse values when wavelengths availability is small.
Categories of Wavelength Conversion
No conversion: No wavelength shifting
Full conversion: Any wavelength shifting is possible and so channels can be
connected regardless of their wavelengths.
Limited conversion: Wavelength shifting is restricted so that not all combination of
channels may be connected.
Fixed conversion: Restricted form of limited conversion that has for each node, each
channel maybe connected to exactly one predetermined channel on all other links.
Sparse Wavelength Conversion: Networks are comprised of a mix of nodes having
full and no wavelength conversion.

CONCLUSION:

Here various concepts that are integral to the development of all-optical network,
various new technologies available in DWDM system are discussed. In near future it is
possible that only two optical layers will exist: WDM layer and IP layer.

However SONET equipment has two features: restoration and troubleshooting

capabilities. For this reason that a lot of investment into SONET has taken place, SONET
will survive. As routers become faster, it will be difficult to convert every wavelength to add
or drop off bandwidth. Thus managing hundred plus wavelength systems is probably
difficult. Research is being done to achieve a high speed optical network. New concepts such
as all-optical switching are coming up.

References
1. “Advance Level Telecom Training Center (ALTTC)” training manual
2. Kieser , “Optical Fiber Communication”, McGarw- HLL
3. “Emerging Optical Networks” KM Sivalingam & S Subramaniunm, Kluwer
Academic Publisher , Boston.
4. R K Pankaj & R G Galler , “ Wave length Requirement of all Optical N/W”
IEEE/ACM/Trng N/W VO -3 NO -3
5. “WDM N/W Economics Sensitive” in PROC NFOEC VOL-1 (APRIL 1995).