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  COMPARISON AND PERFORMANCE ANALYSIS OF SMF AND MMF in LAN Ayush Jalan 1 , Piyush Chowdhury 2 , S. Sugumaran 3   1, 2 B.Tech ECE, VIT University, Vellore. 3 Faculty, SENSE, VIT University, Vellore. Abstract   —    Optical fiber technology employs networks which have high capacity and are used for providing high bandwidth services. They have become an indispensable medium of communication for the transmission of data with minimum possible errors. On the basis of number of modes optical fibers can be classified as single mode and multimode fibers. The objective of this paper is to study the performance analysis of these two fibers in LAN (Local Area Network) environment. LAN is a type of communication network which links and provides connectivity to various devices such as computers, terminals, and peripheral devices within a small geographical area. Simulations are being carried out with the help of OptSim simulator in which observations based on eye diagram including attenuation, pulse spreading in optical spectrum are studied using variation in the distance as the parameter . Index Terms   —   Single mode, multimode, pulse spreading, eye diagram & attenuation. I.   I  NTRODUCTION  Optical fiber refers to the medium and technology related to the transmission of signals as light pulses along a plastic wire or fiber. The light forms an electromagnetic carrier wave which is modulated to carry the information from one place to another with fewest possible errors. As, fiber transmission is an dispersive medium which leads to the phenomena of pulse spreading in which the pulses spread as they move along the fiber. Dispersion reduces the bandwidth of fiber as the spreading pulses limits the rate that pulses can follow one another and be distinguishable at the receiver end. Hence, some kind of dispersion mitigation technique is employed at successive repeater stage. Dispersion such as chromatic dispersion can be reduced by using a dispersion compensator. Dispersion compensator involves the use of fixed length fiber which builds up a dispersion opposite to that induced by the transmitting fiber. As, a result of which the spreading of pulse is minimized and in turn the pulse is sharpened. In a single mode fiber only one mode can propagate through the fiber. It has a smaller core diameter (10 µm) and the difference between refractive index of the inner layer core and outer layer cladding is very small. It is very difficult to fabricate single mode fiber due to which it is quite expensive. Moreover, the toughest job in single mode fiber is the launching of light in it. This mode of fiber is suitable for long distance communications as it involves very low amount of degradation in signal and the effects of dispersion are also very less. On the other hand in multimode fibers, a number of modes propagate through the fiber. It has a larger core diameter and greater difference in refractive index of core and cladding. Fabrication of multimode fiber is easy and light can easily be launched into the fiber. Multimode fibers are associated with „multipath effect‟. Due to this effect, multimode fibers are  being used for applications which involve lesser bandwidth such as in local area networks. A comparison between single mode and multimode fibers is shown in table:- SINGLE MODE FIBER MULTIMODE FIBER Core diameter of 8-10 µm. Core diameter of 62.5 µm. Attenuation (db/km) is maximum 1. Attenuation ( db/km) is at least  2. Bandwidth distance  product is large. Bandwidth  distance  product is low. Used for transmission over large distances. Used for transmission over smaller distances. Easier to couple Difficult to couple II.   R  ELATED WORKS  Till today, a lot of research work is being carried out in comparison and analysis of performance of the two fibers. Much of the research work is being carried out based on the various modulation schemes, source, types of filters, amplifiers used in the process. While many of the enterprises through research have already concluded Optical LANs, based on  passive optical network (PON) technology are the lower-cost, energy-efficient alternatives to copper-based active Ethernet LANs. Optical LANs deliver the cost-effective, low-energy, small form-factor infrastructure that directly impacts eco-friendly, high-performance building initiatives. III.   SIMULATION We have used OptSim4 simulator from Rsoft Design Group for the performance analysis of the two modes of fibers in LAN environment. OptSim is an innovative, rapidly evolving, and powerful software design tool that enables users to plan, test, and simulate almost every type of optical link. It offers transmission layer optical communication system design and  planning from component to system level, and visually   presents analysis and scenarios. OptSim consists of more than 400 component models which are often used in optical communication systems. These components range from signal generators, transmitters, waveguides, and optical filters to various electrical components such as electrical filters and amplifiers. OptSim utilizes these components to calculate  parameters such as eye diagrams, BER and Q-Factor using numerical analysis or semi-analytical techniques for systems limited by inter symbol interference and noise. The simulation is carried out for both the modes of fiber  –   single mode as well as multimode. The simulation in each case is done by varying the distance of data transmission. In other words, we have used different lengths of fibers and observed the response of each fiber. The block diagrams for single mode and multimode fiber are shown in Fig.1 and Fig.2. Fig.1 Block diagram for single mode fiber Fig.2 Block diagram for multimode fiber. As per the block diagram, at the transmitter side we have used  pn sequence generator in order to transmit the information. We have assumed NRZ modulating scheme. Vertical cavity laser is used for each fiber. The working is as follows:-Modulator is used for conversion of Electrical data signals to optical signals. After this, the signal is amplified using an optical amplifier  before being fed into an optical fiber. Erbium doped fiber amplifier (EDFA) is used for amplification purpose. Amplifiers are not being used for multimode fibers as they are mostly employed for shorter distances. As, the signal is still in optical form ,so at the receiver end, a sensitivity optical receiver and a  photodiode is used to detect the optical signal and convert it  back to electrical signal. As usual Filter is being deployed to remove all the amplifier noise. The optical spectrum of the transmission is gathered at the Optical Spectrum Analyzer (OSA). We have placed the analyzer both before and after the fiber medium in order to monitor its performance after certain distance. At last, we have analyzed OSA in line with EDFA along with the eye diagram. Results are being observed after simulation, parameters such as pulse spreading and attenuation is being studied and comparison of optical spectrum before and after transmission is being done. IV.   RESULT   AND   ANALYSIS The objective is to study the behavioral pattern of both the modes of fibers by variation in length of fiber over a wide range. Firstly, the process of spreading of pulses is studied followed by the attenuation caused in each fiber mode with respect to the transmission distance. Fig. 3 and Fig .4 show the optical spectrum before and after transmission. Fig.3 Optical spectrum before transmission Fig.4 Optical spectrum after transmission By comparing the two graphs, one can clearly observe a decrease in the value of OSNR (Optical Signal-To-Noise Ratio). Hence, it can be inferred that the pulse is spreading. It can also be seen that the spectrum from before entering fiber transmission is smoother in comparison to the one received after transmitting along the fiber as expected. The figures below depict the eye diagrams with different transmission distances for both single mode and multimode  fiber at a transmission rate of 10 Gb/s. For multimode fibers(Fig. 5,6,7) it is observed that the attenuation increases with increase in distance and we get distorted pattern. After a certain distance, in range of km the eye opening becomes very narrow due to superimposition of various signals. So, if the eye opening is larger as in the case with smaller distance, the attenuation caused is limited and better transmission is ensured. The decrease in eye opening is related to various factors such as bending, absorption and scattering. Also the bit error is  being reduced if the size of eye opening is wide enough. This implies Bit error rate is inversely proportional to eye opening. Fig. 5 Eye diagrams after 100m   Fig. 6 Eye diagrams after 500m   Fig. 7 Eye diagrams after 1km For, single mode fibers from (Fig. 8, 9) it is observed that the eye opening is wider for greater distances justifying the fact that for single mode fibers attenuation decreases with increase in distance. By comparing the eye diagram pattern shown in figures 8 and 9, one can easily understand why single mode fibers are being deployed for large distances. The Bit error rate is also decreased with increase in distance. Fig. 8 Eye diagrams after 5km   Fig. 9 Eye diagrams after 50km V.   CONCLUSION  The paper is basically a comparative study on the  performance of both, single mode and multimode fiber in LAN environment. The performance analysis is achieved with the help of Optsim simulation tool which provides us with the necessary and required information through the use of graph. By studying the graph one can easily conclude the effect of spreading of pulses as they move along the fiber. The comparison of the two modes is done using the eye diagram. In single mode fibers, eye opening is direct in proportion with fiber length. Eye Opening grows in size as distance increases over a certain range, in case of single mode fiber proving the fact that eye diagram is larger when there is less attenuation. For multimode fibers the eye distance is larger at a shorter distance only. Thus, one can easily conclude that single mode fibers get less attenuated over larger distances and this is the reason why they are used in long distance communication networks. However, for short distance communications multimode fibers are preferred as they are cheaper and launching of light can be easily done. As, technological developments are taking at a rapid pace, Every day we come across certain innovations. So, In case of LAN environments future developments of optical networks can be extended by use of a combination of DWDM with EDFA boosters. It will result in better flexibility, reduced interference and high reliability of the system. Also, with the increasing success of PON (Passive Optical Networks) deployments in world and their capacity to cost effectively support the converged network ,efforts and commitments are being made to employ it in Enterprise LAN environment. The basic reason for this the fact that PON technology offers sharing of total available  bandwidth among all the users. ACKNOWLEDGEMENT We would like to thank Dr. G. Ramachandra Reddy, Dean, SENSE, Prof. P. Arulmozhivarman, Program Chair, B.Tech, ECE and Prof S. Sugumaran for their kind guidance and support in compilation of the paper.    R  EFERENCES  [1]Monic a Bhuatani, “ Simulation and Performance Analysis of SMF and MMF using Dispersion Compensation”,IJCA  [2]Farah Diyana Abdul Rehman,  "Performance Evaluation of Single-mode and Multimode fiber in LAN Environment”. World academy of science, 2007. [3]J.M Senior, Optical fiber Communications  –   principles and practice, prentice Hall, Second edition ,1992 [4]Anjali Singh, Modulation Format for High Speed, Long Haul Fiber Optic Communication System. Inphi Corporation. [5]M.sieben etal,J.of LightwaveTech,Vol.17(1999),p1742 [6]Kong R and Zhou H.  "End to end availability analysis of physical network”.IEEE 2004.  [7]OptSim Models Reference [8]D.M Rothnie,1996,   “ Improved standard fiber  performance by positioning The dispersion compensating fiber.” Electronics Lett ers,Volume 32. [9]G.P. Agrawal:Fiber-Optic Communication Systems. John Wiley & Sons Inc., Second Edition,1997 [10]OptSim 4.5 Models Reference Volume 2 Block Mode,Rsoft Design Group. Inc.2006. [11]P.M Watts et al,London Communications Symposium(2003) [12]R.I.Killey et al, "Optimisation of WDM Transmission of Multi-10   Gbits,50 GHz-Spaced Channels over Standard Fiber.” IEEE   [13]H.Bulow et al,OFC 2001,WDD34-1  [14]J.Morikuni P.Mena,B.K Whitlock and R.Scarmpozzino.Measurements Requirements for optical and optoelectronic model Verification, Validation & Calibration.Technical Digest: NIST Special Publication 988, Boulder,CO,2002. [15] Fiber Optics Rotary Joints. Available at: 
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