It is two stage (EDFA) amplifier consisting of pre-amplifier and booster amplifier. With out two stages it is not possible to amplify the signal up to 33dB on EDFA principle avoiding large ASE (amplifier spontaneous emission) noise.
PRE-AMPLIFIERS
This amplifier along is used at the terminal to interface the DEMUX and line for receiving coming from the distant station. Hence the attenuated line signal is amplified to a level of 3dBm to 10dBm before entering into DEMUX unit.
OPTICAL SUPERVISORY CHANNEL (OSC)
The function of transmission of additional data at a separate wavelength of lower optical power with out any optical safety provision, accompanied with and independent of the main optical traffic signal, is performed by this OSC.The OSC helps management to control and monitor the optical line devices. The management for fault location, configuration, performance and security.
OPTICAL ADD/DROP MULTIPLEXER (OADM)
Adding or dropping of channels at optical level is possible by using optical add/drop multiplexer module. It is a unidirectional module with facility for dropping or adding optical channel of specific wavelength. The dropping and adding of the optical wavelength this performed with fixed optical filters. With the help of OADM module it is possible to insert or drop maximum for optical wave lengths at any intermediate stations.
Fig(9 ):optical add\drop MUX
Depending upon the design, pre and post optical amplifiers may or may not be present in OADM.There are two types of OADM.The first type is fixed device that is physically configured to add/drop pre defined wavelengths. The second type is reconfigurable and capable of dynamically selecting the wave length to be added or dropped.
ADVANTAGES OF DWDM TECHNOLOGY
· The capacity of transmission media can be upgraded easily by using DWDM technology. The capacity of existing DWDM system can be upgraded by deploying higher channel capacity system.Thus, The need of laying new fibers for increasing capacity of transmission media is avoided.
· Bit rate transparency: in DWDM system, optical channels can carry any transmission format. thus the different wavelengths from different systems can be transmitted simultaneously and independently over the same fiber without need for a common ATM,Gigabit Ethernet etc over a common layer. Thus DWDM system can transport any type of optical signal.
· Quick deployment: The DWDM technology is, generally, deployed using existing fibers. The time required for laying new fiber is much more as compared to equipment deployment time.hence, the deployment of dwdm systems can be done quickly.
· Economical: The DWDM system is cheaper as compared to overall cost of laying new fiber for increasing transmission capacity. In DWDM system, one optical amplifier is used for amplification of all the channels, hence per channel cost is drastically reduced as compared to providing regenerator for individual channels in SDH network.
· Wavelength routing: In DWDM system, by using wavelength sensitive optical routing devices, it is possible to route any wavelength to any station. Thus it is possible to use wavelength as other dimension, in addition to time and space in designing transmission network.
· Wavelength switching: In DWDM system, wavelength switching can be accomplished by using OADM, optical cross connect and wavelength converters.thus, it is possible to reconfigure the optical layer using wavelength switched architecture.
DISADVANTAGE
PROTECTION INDWDM SYSTEM: DWDM link can be designed to provide either path switched protection (two fibre working) or bi-directional line switched protection (four fibre working). The equipment protection can also be provided by using additional set of equipment .the protection facility is not available in the equipment being deployed in telecom network. In case of falure, the protection system of SDH ring will take care of the fault.
REQUIREMENT OF FIBRE
There are two categories of optical fibres namely mono mode and multi mode. The mono mode fibre is used for long haul transmission and it is of following three types.
1. Non-dispersion shifted fibre(NDSF)
2. Dispersion shifted fibre(DSF)
3. Non-zero dispersion shifted fibre(NZDSF)
To optimize the performance of fibre in L (1625nm) and C (1550nm) bands, it was designed in such a way that dispersion was very low at 1310nm(S band) and this type of fibre is called NDSF.Later anew type of fibre was developed in which the zero dispersion was shifted to 1550nm region called DSF.But due to the non linear effect the DSF is not suitable for DWDM. NZDSFis designed in such away that the dispersion is low at 1550nm but not zero.
CONCLUSION
The demand of bandwidth is increasing day by day, especially for data traffic. Service providers are required to provide the bandwidth dynamically and in shortest possible time. This can only be done by DWDM. In future advanced DWDM components will be available. Thus, it will be possible to manage the optical signal dynamically, which will allow more flexibility to the service providers.
Bibliography
1. Shri H. Saha, Shri Nural Anowar, DWDM System & Testing Telecommunication March –April 2002
2. P.K. Pandy, Dense Wave Length Division Multiplexing Telecommunication November –December 2002
3. www.bsnl.co.in.
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