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This paper is about implementation of wavelength division multiplexing in system and network design of wavelength division multiplexing. High quality content from application servers, such as web servers, video servers and e-commerce sites, in short time has driven the need for speed for individual and corporate users alike. Residential customers, small-sized business and even large business are demanding high speed. Applications like (HDTV) and (SDTV) requires a sheer bandwidth, so for this purpose WDM is used. The paper elaborate about WDM network design that how optical line terminal (OLT), optical add/drop multiplexers and optical cross connects are used in network design. The paper enlightened how WDM is implemented in a network system.
Wavelength division multiplexing has enjoyed quick and very commercial acceptance due to its simplicity and cost compared to Time division multiplexing solutions. The networks like GPON(gigabit passive optical network) uses WDM that can support 10Gbps.In WDM several wavelengths are multiplexed over a single fiber, from  four kinds of WDM System are available: Metro WDM(<200km),long haul WDM(200 to 800km),extended long haul(800km to 2000km),ultra long haul WDM(>2000km).WDM is an additional option where there is a need of additional bandwidth, without the requirement of additional fiber along the way. It is the only multiplexing technique which allows the full use of the low-attenuation bandwidth regions of an optical fiber . Wavelength Division Multiplexing (WDM) technology is increasing rapidly from an economic alternative to upgrade point-to-point transmission links towards a true optical networking technology. Dense wavelength multiplexing is basically used for long haul network and it can multiplex up to 240 wavelengths. It can support up to 10 Gbps of data. WDM optical network consist of five components: transmitter, optical multiplexer and de-multiplexer, optical fiber media and receivers that is been integrated to give us the complete WDM system. Dispersion and compensation in WDM is taken into account while designing the WDM system. The dispersion is of different characteristic like linear or non-linear. Linear characteristic include chromatic dispersion, attenuation and polarized mode dispersion while no-linear characteristic include cross-phase modulation, four wave mixing, stimulated Raman scattering and stimulated Brillouin scattering. In WDM, SDH architecture is used that have different stream line on which data is transmitted. SONET is also supported by WDM system but it's outdated.
Overview of the Paper
The paper is structured as follows. In section 2 we give a brief introduction to SDH architecture for an internal optical interface standard for carriers and feature for improved network managements. SDH standardizes the rates, framing format, signaling and termination between SDH equipment. In section 3 we explain the network elements that is used form WDM network design. Network elements include add drop multiplexers, optical cross connects. In section 4 we check the general design goal which enable us to provide analytical results and reasonable .We summarize the paper in section 5.
2) SDH network architectures
In SDH architecture there are four different layers.1) Path layer.2) Multiplex section layer.3) Regeneration section layer.4) Photonic layer. The path is a logical connection between the points at which the standard SDH frame format for the signal at given rate is assembled and the point at which the standard SDH format for the signal is dissembled. Path terminating equipment acts as a demultiplexer. The regenerator section is a portion of transmission facility including termination points between terminating NE and a regenerator, or two regenerators. The regenerator section overhead bytes are added or stripped at this layer. The photonic layer mainly deals with the transport of bits across the physical medium. Its main function is the conversion between STM-N signals and light pulses on the fiber media. It includes pulse shaping, launching and modulation of power levels. As from  SDH container C->from high perspective a low level PDH is mapped into SDH entity to form a container. Low order VC (virtual container) =The C is mapped along with POH bytes to form another entity knows as virtual container. Tributary units=Low order VCs are aligned with tributary unit pointers to form entities known as tributary units(TUs).The TUs are multiplexed to form tributary unit groups(TUG. The Tug's are further multiplexed to form high-order VCs. These high order VCs are aligned with fixed byte-stuffing and administration unit (AU) to form administration units. The AUs are further multiplexed to form AUGS.
Byte interleaving multiplexing is used in SDH architecture.Low rate tributary signals: 2Mbps, 4Mbps contain containers, virtual containers and tributary signals. Payload=data and VC=payload mapped with POH byte to form virtual container. POH=monitor the tribute signals. A tu-11 constitutes SDH=3 columns*9rows can be used to transport a DS2.Four consecutive VC-4 constitutes 500Us called a Tu multiframe.SDH pointers=provides payload pointers to permit differences in the phase and frequency with respect to STM-N frames. VC payload pointer, that indicates where actual payload pointer starts. For a VC-4 payload this pointer is located in 1 and 4 of the fourth row of the transport overhead. If there is difference in phase and frequency, the pointer value is adjusted. A process is known as byte stiffing.SDH pointer=total STM-1 bytes - transport overhead bytes=pointer position. terminal multiplexer form an end to end link.
From  we get the complete understanding of SDH architecture graphically.
Sometimes certain signal needs to be passed and some needs to be dropped. Unwanted signals are dropped and wanted signals are passed through the ADMs. Frames are sent byte by byte.
3) Network Elements used for WDM network design
A WDM network is designed using network elements. A light path consists of an optical channel or wavelength between two network nodes that is routed through multiple intermediate nodes. Intermediate may switch and convert wavelengths. Network may be thought of as wavelength routing networks. Light paths are set up and taken down as dictated by the users of this network. Network consist of optical line terminals (OLT) and Optical ADM(Add drop multiplexer).OADMS and OXCS may themselves use optical amplifiers to make up for losses. Architecture supports a variety of topologies including ring and mesh topologies. OLTS multiplex multiple wavelengths into a single fiber and also demultiplex a composite WDM signal into individual wavelengths.
This is taken from  that explain the complete architecture of WDM network design.
OLT (optical line terminal) are used at either end of a point-to-point link. OADM's(optical add drop multiplexer) are used at locations where some fraction of wavelengths need to be transmitted locally and other needs to be routed to other destinations.As by  Light path is path is carried on a wavelength between its source and destination and may get converted from one wavelength to another along the way. They are practically developed in Ring topologies. Optical cross connect perform a similar function but on a larger scale in terms of number of ports and wavelengths involved, and are deployed in mesh topologies. The users of this network are connected to the OLT, OADM, or OXC's. Network support a variety of client types, such as routers, SONET terminals and add drop multiplexer. Each link can support a number of wavelengths. Numbers of wavelengths that can be supported depend on the component and transmission imposed limitations. Light paths provided by the optical layer can be set up and taken down demand. These light paths once set up remain in the network for months to years and it is to be note that packet switching is not provided within the optical layer. The technology for optical packet switching is still fairly immature. It is left to the higher layer to perform any packet switching needed. Networks can be configured such that in event of failures, light paths can be routed over alternative path automatically. This provides a high degree of resilience in the network.
A) OPTICAL LINE TERMINAL
The OLT has wavelength multiplexers and de-multiplexers and adaptation devices called transmission over the WDM link and an incoming signal from the WDM link to a suitable signal toward the client. Transponders are not needed if the client can directly send and receive signal with the WDM link. OLT also terminates a separate optical supervisory link used on the fiber link. The functional element inside an OLT: transponders, wavelength multiplexers and optionally optical amplifiers. A transponder adapts the signal coming in the form a client of optical network into a signal suitable for use inside the optical network. The most common interface is SDH short reach. wavelength multiplexers multiplex the signals coming and optical amplifiers are used where signal is degraded .
It is taken from .
B. Optical Line Amplifiers.
The optical amplifier is deployed in the middle of optical fiber link at periodic interval typically 80-120km.Basic element is erbium doped fiber gain block. The amplifiers contain automatic gain control and built in performance monitoring of the signal.
C.Optical ADD/Drop multiplexer
Optical add drop multiplexer provide a cost effective means for handling pass-through traffic in both metro and Long-haul networks. The figure taken from  illustrate the role of add/drop multiplexer. Three wavelengths are needed between A and C, and wavelength each between node A and B and between nodes B and C.
4. Design Goals
The design goals that we achieve from writing this paper are:
1) Understanding of SDH architectures.
2) Implementation of WDM network.
3) Reducing the cost by implementing SDH architecture as they are cost effective.
4) The set of light paths that are routed through the network.
5) SDH interconnection of WDM systems.
As numbers of users are increasing day by day. The basic problem is the data rate and bandwidth required for optical systems. So for this we took a step to write on WDM systems. It provides the highest bandwidth compared to other network systems. In this paper we discussed a complete WDM network design and architecture of SDH, that is widely used nowadays and integration of SDH with WDM. We discussed the network elements that are the essential part of WDM network design, including OLT, OADM and OXCs. IN short WDM give us a flexible network to implement and to accommodate greater traffic.
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