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Investigation of passive optical network based fiber to the home architecture for downstream traffic.

INTRODUCTION

Fiber optics is made of flexible glass as thin as human hair that is used for telecommunications. Fiber strands carry digital signals along with light. The outer protective coating is called cladding. To protect the special glass cable from damage and moisture buffer is used which is the final outer layer. Fiber optics work using the principle of Total Internal Reflection. Light travelling from one material to another changes its speed, which results in light changing its direction of travel. This deflection of light is called Refraction. As the angle of incidence increases, the angle of refraction approaches 90[degrees] to the normal. The angle of incidence that yields an angle of refraction of 90[degrees] is the critical angle. If the angle of incidence increases more than the critical angle, the light is totally reflected back into the first material so that it does not enter the second material. The angle of incidence and reflection are equal and it is called Total Internal Reflection

The optical Fiber has two concentric layers called the core and the cladding. The inner core is the light carrying part. Typical values for example are a core refractive index of 1.47 and a cladding index of 1.46. This buffer coating is a shock absorber. The specific characteristics of light propagation through a Fiber depend on many factors, including the size of the Fiber, the composition of the Fiber, the light injected into the Fiber. The idea of light travelling through a Fiber is first the light gets injected into the Fiber and strike the core to cladding interface at an angle greater than the critical angle, reflects back into core, since the angle of incidence and reflection are equal. The light will continue zigzagging down the length of the Fiber.

2. Overview of DSL Technology:

The range of DSL technologies is quite broad, and this breadth can be somewhat confusing to the uninitiated. This section briefly describes DSL technology that has been developed or is currently under development. The existing Digital Subscriber Line (DSL) provides internet access by transmitting digital data over the wires of a local telephone network. On the customer premises, a filter on each outlet removes the high frequency interference, to enable simultaneous use of telephone and data. The data rate of consumer DSL services ranges from 256 kbps to 40 Mbps in the direction to the customer.

FTTH (Fiber To The Home):

FTTH (Fiber-To-The-Home), the future proof access technology incorporating voice, data and video in single connection, However, for providing multi-play services (voice, video, data etc.) and other futuristic services fiber in the local loop is must. The subscriber market for multi-play is large and growing and includes both residences and businesses. Businesses need more bandwidth and many of the advanced services that only fiber can deliver. The customer on the FTTH will get more than 1.0 Gbps bandwidth which is a virtually unlimited bandwidth on today's scenario. The growing use of the Internet by businesses and general households in recent years, coupled with demands for increased capacity, the need for optical fiber cable for the last mile has increased. A primary consideration for providers is to decide whether to deploy an active (Point-To-Point) or passive (Point-To-Multipoint) fiber network.

3. PON (Passive Optical Network) Elements: OLT (Optical Line Terminal):

The OLT provides management functions for the subtended Optical Distribution Network (ODN) and Optical Network Unit (ONU). It provides two main functions

* To perform conversion between the electrical signals used by the service provider's equipment and the fiber optic signals used by Passive Optical Network.

* Between the conversion devices it will coordinate multiplexing on the other end of ONT/ONU

[FIGURE 1 OMITTED]

ONU/ONT (Optical Network Unit /Optical Network Terminal):

A single subscriber device that terminates any one of the distributed end points of an ODN, implements a PON protocol, and adapts PON PDU to subscriber service interfaces. An ONT is a special case of an ONU. The ONT converts fiber optic light signals to electric signals. ONU is used for multiple user's and ONT is used for single user.

Splitters:

Splitters are passive power dividers. It allows communication between the OLT and their respective ONT. Splitters not only multiplex or demultiplex signals, but also combine power. Splitters are bidirectional optical distribution devices with one input and multiple outputs. These are available in various options like 1:4, 1:8, 1:16, 1:32 and 1:64.

NMS (NetworkManagement System):

Management of the complete PON system shown in Fig. 1 from OLT.

* One OLT can serve multiple ONU/ONTs through PON

* TDM/TDMA protocol is used between OLT & ONT

* Single Fiber/ Dual Fiber is used for upstream & downstream

* Provision to support protection for taking care of fiber cuts, card failure etc.

* It can provide a maximum Split Ratio of 1:64

* The distance between OLT & ONT can be greater than 15Km.

* The Downstream transmission i.e. from OLT to ONU/ONT is generally TDM

* The Upstream traffic i.e. from ONU/ONT to OLT is usually TDMA.

MATERIAL AND METHODS

Design of WDM-PON:

In this chapter the parameters and scenarios of simulation are described. The purpose is to show the behavior of links of optical fiber when the signal goes through all the elements such as optical fiber, splitters, multiplexers and the goal is to find a good quality of signal in all receivers. The final goal pursued with this paper is to evaluate the performance of the downstream system. The parameters used to evaluate this behavior are the BER and the Eye Diagram. The first condition used to describe the performance of the link is generally BER (Bit Error Rate) at the receiver. Simulations are done trying to approach at the receiver with a minimum BER. Any simulation with a BER < [10.sup.-4] results in inconsistent and unreliable link. The FEC techniques are used for correcting errors and bringing the BER from 10-4 to 10-13. The second condition corresponds to the eye diagram. The eye diagram describes the quality of the received signal. In the presence of ISI, when the pulse does not satisfy the Nyquist criterion, the diagram tends to close vertically. For error-free transmission in the absence of noise, the eye must be kept some vertical opening, or otherwise there exist interference between symbols that would cause errors. When the eye is not fully closed, the interference between symbols reduces the value of allowable additive noise. Therefore, the higher vertical opening the greater immunity to noise. In this chapter the various network has been described and the most important parameters are also analyzed. The comprehensive description of the entire network and the detailed analysis of each of the different areas that compose the design are described in the following sections. Finally the results of each ONU is obtained, which means that the network is viable and therefore its practical implementation would work.

[FIGURE 2 OMITTED]

WDM PON: Complete Design:

The complete design of WDM-PON is shown in Fig. 2. The Steps in evaluation of network deployment planning has been studied. To facilitate understanding, the design from the OLT to the different ONT's for downstream is described. On the left side of the design in Fig. 2 you can see the OLT which transmits information to different users and also receives information from the ONT's. All this information is transmitted multiplexed at different wavelengths through a single optical fiber, and then demultiplexed to spread to different areas in downstream. The simulation has been divided into five areas.

WDM PON in Downstream:

In downstream, the optical signal is transmitted from the OLT to the end users (ONT's). Returning to Fig. 2, the first network element is the optical transmitter located at the OLT. This laser broadcast's five different wavelengths from 1450 nm to 1530 nm with a frequency spacing of 20nm but in this paper 3 wavelengths were shown. The transmission power is set to 0dBm and NRZ modulation is used. The next element is an optical multiplexer, which multiplexes these five wavelengths to transmit on a single fiber. This multiplexer 1:5 (which actually correspond to a multiplexer 1:8) has an insertion loss of 2 dB. Since the residential area has many users the signal suffers in that area so EDFA amplifier of about 5 dB of gain is placed in front of OLT to ensure good quality signal in the case of major losses. In addition, the OLT has to handle the downstream traffic, so an optical circulator with insertion losses of 3 dB and return losses of 65 dB has been inserted. Being described the elements of the OLT, the optical signal is then inserted in the optical fiber which reaches the multiplexer located at 5 km from the central office.

The SPLITTER contains a multiplexer and demultiplexer (splitters). Each is used in one direction of traffic. For downstream traffic, the signal from optical fiber is demultiplexed into 5 different branches with each of them carries a different wavelength but for understanding purpose only three wavelengths shown in (Fig. 4, 5, 6). This 1:5 splitter has a starting frequency of 1450 nm and a frequency spacing of 20 nm. As explained in the previous multiplexer, the splitter is also implemented in reality as 1:8 splitter, therefore it has insertion losses of 2 dB.

[FIGURE 3 OMITTED]

The first branch of the splitter is directed to the block corresponding to the HOSPITAL area, the second to the BUSINESS area and third to residential area. The Fig. 3 shown is hospital block. The hospital block consists of one ONU. The business block consists of two ONU's but wavelength differs for each block. The residential block consists of multiple ONU's. Performance of all the blocks has to be checked. For understanding purpose only the hospital block is taken as shown in Fig. 3 which carries information in both directions. Note that optical delays that occur around the optical fiber are required to perform the simulation, since all network elements need to sign all inputs for the simulation to run. At the initial time, the value of the input connected to the optical delay is set to zero, and it has a delay of 1 time interval for the next moments.

The downstream spectrum for all the blocks are shown in (Fig. 4, 5, 6, 7).

As expected, the peak is centered in 1450 nm as shown in Fig. 4.

Visualizing the Optical Spectrum Analyzer for Business Block the carrier centered in 1470nm as shown in Fig. 5.

Visualizing the Optical Spectrum Analyzer for Residential Block the carrier centered in 1490nm as shown in Fig. 6.

[FIGURE 4 OMITTED]

[FIGURE 5 OMITTED]

[FIGURE 6 OMITTED]

RESULTS:

Let's see downstream results. In the case of the ONU located in the HOSPITAL Block as shown in Fig. 7. In the BUSINESS block as shown in Fig. 8, the path that the signal covers to two ONU's is exactly the same since the last fiber has the same dimensions. So the results are identical as shown in Fig. 8. The RESIDENTIAL Block has 20 ONU's. As expose the results of the 20 ONU's is required. The results of the closest ONU as shown in Fig. 9 (located at 50 meters from the splitter) and the farthest ONU as shown in Fig. 10(situated at 185 meters from the splitter).

The results are very similar, although as it expected, the results of the ONU placed on the shortest fiber results of the ONU is better than the longest fiber.

Table 1 shows the downstream parameters (wavelength and splitting ratio) which vary for each block

From the Table 3, the existing and proposed technologies has been compared and the results show's that the DSL and PON technologies gives BER (Bit Error Rate) of [10.sup.-4] this makes the link of the network inconsistent and unreliable so by using forward error correction techniques in the proposed system of WDM-PON the bit error rate for downstream has been improved to 10-13, thus it makes the network reliable and improve its performance.

Conclusion and Future Work:

This paper concludes that the designed network be the lines that can be followed for future network improvement. Throughout the paper a number of general guidelines for the development had been undertaken and had resulted in a thorough understanding of a passive optical network. They basically are: Theoretical detail and physical basis of the operation of optical networks, Planning and design of a PON. Simulation and results obtained by the designed network. A comparison is made with point to point/multipoint networks and active optical networks (AON), concluding PON design optimal for its exploit of bandwidth and its lower implementation costs. Finally a comparison between these technologies is done, choosing WDM-PON as the best option because of its greater speed, reach and number of users supported. The simulation results show that the network works. The images show what has the design been and an explanation of all the important elements and parameters that have taken into account to make it work. To make more understandable of the design, it has been separated into two parts, downstream work has been divided in 5 areas that define the entire design. Finally, the obtained results in each of the ONT's for downstream alone have been attached, being these the eye diagram and the BER decision threshold for the minimum BER. These results have showed a BER of about [10.sup.-13] for the worst case for the farthest house in the residential area. This shows the high reliability in each of its points on this network. The choice of optical fiber as transmission medium allows guaranteeing longevity of the network during the coming years. In addition, the specified equipment will be valid for migration to future standards as 10GPON and other technologies that improve the performance of optical fiber and provide the best possible performance. Other features of the design is the high degree of availability and network capacity, the ability to integrate multiple systems and services and the possibility of interconnecting these networks with other wide area networks and adding local networks by creating LAN.

ARTICLE INFO

Article history:

Received 12 March 2015

Accepted 28 April 2015

Available online 1 June 2015

Corresponding Author: Vaishnavi, R., Sri Ramakrishna Institute of Technology affiliated to Anna University, Department of Electronics and Communication Engineering, Box.3030. City. Country.

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(1) Vaishnavi, R., (2) Dr. Mary Praveena, S., (3) Dr. Ila Vennila

(1) Anna university, Department of Electronics and Communication Engineering, Box.3030.coimbatore. India.

(2) Anna university, Department of Electronics and Communication Engineering, Box.3030.coimbatore. India.

(3) Anna university, Department of Electrical and Electronics Engineering, Box.3030.coimbatore. India.
Table 1: Downstream Parameters.

BLOCKS        WAVELENGTH   SPLITTER RATIO

Hospital        1450nm          1:1
Business        1470nm          1:2
Residential     1490nm          1:20

Table 2: Comparing Existing and Proposed Technologies.

TECHNOLOGIES                 BIT ERROR RATE     PERFORMANCE
                             For DOWNSTREAM

Existing       DSL and PON    [10.sup.-4]       The network
                                              shows that the
                                                link is not
                                              consistent and
                                                 reliable
Proposed         WDM-PON      [10.sup.-13]     Reliable and
                                                 Efficient
                                                performance

Fig. 7: WDM PON--HOSPITAL ONU Result.

Max. Q Factor         174.009
Mm. BER                     0
Eye Height        0.000300333
Threshold        3.72232e-005
Decision Inst.       0.515625

Fig. 8: WDM PON--BUSINESS ONU.

Max. Q Factor         109.352
Min. BER                    0
Eye Height        0.000141453
Threshold        2.80688e-005
Decision Inst.            0.5

Fig. 9: WDM PON--RESIDENTIAL ONU1 Results.

Max. Q factor         7.09078
Min. BER         6.65572e-013
Eye Height       4.28723e-006
Threshold        3.99957e-005
Decision Inst.          0.625

Fig. 10: WDM PON--RESIDENTIAL ONU2 Results.

Max. Q factor         0.77201
Min. BER         6.29865e-012
Eye Height       4.07834e-005
Threshold        4.07975e-005
Decision Inst.         0.4375
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Author:Vaishnavi, R.; Praveena, Mary; Vennila, Ila
Publication:Advances in Natural and Applied Sciences
Article Type:Report
Date:Jun 15, 2015
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