Migration to the Next Generation Optical Access Networks Using ...

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JOURNAL OF NETWORKS, VOL. 6, NO. 1, JANUARY 2011

Migration to the Next Generation Optical Access Networks Using Hybrid WDM/TDM-PON Md. Shamim Ahsan

Korea Advanced Institute of Science and Technology, Daejeon, South Korea Email: [email protected]

Man Seop Lee, S. H. Shah Newaz

Korea Advanced Institute of Science and Technology, Daejeon, South Korea Email: {leems1502, newaz}@kaist.ac.kr And Syed Md. Asif Sylhet International University, Sylhet, Bangladesh Email: [email protected] Abstract—Due to the rapid growth of Internet and multimedia services, broadband fiber access technologies, such as Passive Optical Networks (PONs), come to the forefront of the research field. Network service providers mostly deploy Time Division Multiplexed (TDM)-PON everywhere in the world. In order to mitigate the future demand, researchers have investigated some nextgeneration PON systems, such as Wavelength Division Multiplexed (WDM)-PON and hybrid WDM/TDM-PONs. In this report, we propose the architecture of a self-restored hybrid WDM/TDM-PON. Due to the restorable capacity of the architecture, the availability of the PON system increases. Furthermore, the proposed architecture is a cost effective solution compared to the existing restorable PON architectures. Finally, we analyzed the power budget and signal quality of the proposed architecture, which proves the applicability of the proposed architecture to the access networks.

However, from a topological point of view, we can classify FTTx networks as either Point-to-Point (P2P) or Point-to-Multi-Point (P2MP) networks. Fig. 1 shows a simple architecture of the P2MP based TDM-PON and AON architectures. For signal distribution, AONs require some electrically powered equipment, such as Ethernet switch, router, or multiplexer, whereas PONs require passive equipment, such as optical splitter or arrayed waveguide grating. In both cases, the feeder fiber runs from the Optical Line Terminal (OLT) of the Central Office (CO) up to a splitting point in the path. From this point, distribution fibers extend to several Optical Network Units (ONUs). In the rest of the paper, we focus on PONs as essential FTTx architectures.

Index Terms—FTTXs, PON, TDM, WDM, BSWDM

Filter. I. INTRODUCTION Due to the expansion of multimedia services, such as video on demand, video-conferencing, High-Definition Television (HDTV), e-learning, interactive games, and VOIP, each Internet user will need a guaranteed bandwidth of more than 100 Mb/s by the year of 2012 [1]. In order to provide broadband services to the users, there is a strong competition between several kinds of technologies: digital subscriber loop, coaxial cable, wireless, and FTTx (Fiber to the x, where x stands for node, curb, building, home, and premise). However, the bandwidth of the copper wire and wireless access technologies is limited due to physical media constraints. As a result, to satisfy the demand for bandwidth in the future, network service providers will have to deploy optical access networks. Depending on the use of passive or active devices, we can classify FTTx networks as either Passive Optical Network (PON) or Active Optical Network (AON). Manuscript received: January 31, 2010; accepted September 1, 2010.

© 2011 ACADEMY PUBLISHER doi:10.4304/jnw.6.1.18-25

revised: July 23, 2010;

Figure 1. Optical networks. (a) Time Division Multiplexed Passive Optical Network (TDM-PON); (b) Active Optical Network (AON).

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We categorize PONs into current generation and next generation PONs. TDM-PONs, such as Asynchronous transfer mode PON (A-PON), Broadband PON (B-PON), Gigabit capable PON (G-PON), and Ethernet PON (EPON) are considered to be the current generations of PONs. The next generation of PONs are sub-categorized into short-term and long-term future generation PONs: 10GE-PON, XG-PON1, and XG-PON-2 as short-term future generation PONs, while WDM-PON and hybrid WDM/TDM-PONs are long-term future generation PONs. The details of the different kind of PON architectures are discussed in the next section. Many researchers have demonstrated the effectiveness of the different architectures of WDM-PON and hybrid WDM/TDM-PONs. In [2], the researchers proposed a shared tunable laser based hybrid WDM/TDM PON architecture. Each ONU was assigned a wavelength for a particular time slot that connects to the OLT. A long reach hybrid Dense WDM and TDM (DWDM-TDM) PON architecture was presented in [3], where the architecture provided support to TDM-PONs only. In [4], the authors proposed a hybrid WDM/TDM-PON that used tunable lasers in the OLT. A coarse AWG was used to support multiple TDM and WDM-PONs by using a single OLT. In [5], the authors proposed a multiple star WDM-PON that would provide services to several subscriber groups of wider dispensed areas using Band Splitting WDM (BSWDM) filters. The authors in all cases proposed two-stage remote nodes for increasing the total number of users [2–5]. Due to the low operating speed, the tunable laser based PON architectures proposed in [2–4] were found to be not a good option in providing support to many users at the same time. On the other hand, the architecture proposed in [5] did not provide support to both TDM-PON and WDM-PON that were using a single OLT. In all the cases mentioned, the authors did not provide any protection against fiber fault, which may have hampered the uninterrupted network connectivity of many users. In this paper, we propose the architecture of a selfrestored hybrid WDM/TDM-PON with a duplex fiber system. The proposed model has a hybrid OLT that supports both TDM and WDM-PONs. We used BSWDM filters that provide network connectivity to several groups located at different areas by a single OLT. The proposed structure provides protection to both feeder fiber and distribution fibers. We analyzed the performance of the proposed architecture by calculating the system availability. We found that the proposed architecture provides more system availability compared to existing non-restorable PONs due to the protection scheme. Furthermore, we performed the cost analysis for different PONs and found the proposed architecture to be more cost-effective. Finally, we calculated the power budget and the signal quality (Signal-to-Noise Ratio (SNR), Bit Error Rate (BER), and Quality factor (Q)) of the proposed model. Our analyses proved the applicability of the proposed architecture in the access networks that support multiple TDM and WDM-PONs, whose architecture will play important roles in the migration process from

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TDM-PONs to WDM-PONs. II. BACKGROUND In 1995, the Full Service Access Network (FSAN) group defined the initial PON specifications, which used Asynchronous Transfer Mode (ATM) as their layer 2 signaling protocol, which became known as A-PON (ITU-T G.983.1). In order to provide a full set of telecommunication services, FSAN produced the initial recommendations for Broadband-PON (or B-PON) (ITUT G.983.x) in 1998. In 2001, the IEEE 802.3 working group took initiative in deploying Ethernet in the First Mile (EFM), where the existing Ethernet devices were applied to PONs with little modifications known as Ethernet-PON or E-PON (IEEE 802.3ah). In 2003, the FSAN proposed the first recommendation for Gigabit capable PON or G-PON (ITU-T G.984.x) [6]. In 2007, the IEEE 802.3av task force was established to develop 10 GE-PON. Other TDM-PONs under the attention of FSAN and IEEE are XG-PON1 and XG-PON-2. Although TDM-PONs are cost effective, they suffer from the data speed as the total bandwidth is divided among the users. As a result, WDM-PON has come to the forefront of the research field. WDM-PONs have been reported with up to 32 users at 1.25 Gb/s to 2.5 Gb/s per user, offering both security and protocol transparency [2,7]. WDM-PON provides an optical P2P link by allocating a pair of bidirectional wavelengths between each ONU and OLT connected to the PON, as shown in Fig. 2.

Figure 2.

Basic WDM-PON architecture.

The main limitation of WDM-PON is its higher cost due to the requirement of a wavelength specific laser for each ONU. Several approaches have been used in implementing cost-effective colorless ONUs in WDMPONs. The most important enabling technologies for WDM-PON are based on injection-locked Fabry-Perot (FP) lasers [7], tunable components [8], spectral slicing [9], and Centralized Light Sources (CLSs) [10]. A comparison between different PONs is shown in Table I. TDM-PONs have been deployed all over the world due to their cost effectiveness. However, in order to mitigate the future bandwidth demand, we need to deploy WDMPONs at the same time as TDM-PONs. In order to make TDM-PONs and WDM-PONs interoperable, hybrid WDM/TDM-PONs are needed. In order for the TDM-

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TABLE I.

COMPARISON OF DIFFERENT PONS TDM-PONs

Long-term Future Generation PON

Current Generation PONs Short-term Future Generation PONs A-PON B-PON E-PON G-PON 10 GE-PON XG-PON1 XG-PON2 WDM-PON Standards ITU-T G.983.1 ITU-T G.983.x IEEE 802.3ah ITU-T G.984.x IEEE 802.3av FSAN FSAN No (draft) standard Framing ATM ATM Ethernet GEM Ethernet GEM GEM Protocol independent Maximum 155 Mb/s (↓↑) 622 Mb/s (↓↑) 1.25 Gb/s (↓↑) 2.5 Gb/s (↓) 10 Gb/s (↓↑) 10 Gb/s (↓) 10 Gb/s (↓↑) 1-10 Gb/s per channel bandwidth 1.5 Gb/s (↑) 2.5 Gb/s (↑) User per PON 16-32 16-32 16-32 32-64  64  64  64 16-32 Bandwidth per 10-20 Mb/s 20-40 Mb/s 30-60 Mb/s 40-80 Mb/s  100 Mb/s  100 Mb/s  100 Mb/s 1-10 Gb/s user Line coding Scrambled Scrambled 8b10b Scrambled 64b66b Scrambled 64b66b NRZ NRZ NRZ NRZ Video RF/IP RF/IP RF/IP RF/IP RF/IP RF/IP RF/IP Cost Low Low Low Medium High High High Very high

PONs to migrate to the WDM-PONs, and for both of them to be interoperable, researchers have proposed two types of proposals for hybrid WDM/TDM-PONs [11]: an OLT add scenario and an OLT replace scenario. These two scenarios were discussed from both the WDM and TDM approaches. In the OLT add scenario, the addition of Next Generation OLTs (NG-OLTs) with the present OLTs is proposed. In this model, the NG-OLTs and present OLTs will work at the same time as each other. In the OLT replace scenario, the replacement of present OLTs with NG-OLTs is proposed, as shown in Fig. 3 [11]. With the OLT add and WDM approach, only the C/L bands are available for NG-PON wavelength allocation. With the OLT replace and TDMA approach, the O band is also available with the C/L band for NG-PON wavelength allocation. Both these approaches are good for migration and interoperability purposes, but the OLT add and WDM approach is superior to the OLT replace and TDMA approach in terms of bandwidth allocation to an individual user. On the other hand, in the OLT add and TDMA approach, individual E/G-OLTs and NG-OLTs need to be synchronized with each other, which is technically difficult. With the OLT replace and WDM approach, NG-OLTs require transmitters and receivers for current PONs, which is more expensive than the OLT replace and TDMA approach [11]. The hybrid WDM/TDM-PONs proposed in [2–4] are based on one of the four approaches discussed here. However, all of the proposed hybrid architectures suffer from the poor speed of tunable lasers and reliability (i.e. system availability). In this report, we propose the architecture of a selfrestored hybrid WDM/TDM-PON with a duplex fiber system based on the OLT add and WDM approach. We believe that our proposed model will be able to provide interoperability among the TDM-PONs and WDM-PONs with a higher system availability, higher bandwidth, and lower cost. III. PROPOSED ARCHITECTURE Due to the expected higher bandwidth requirement,

© 2011 ACADEMY PUBLISHER

(a)

(b) Figure 3. Migration scenario from the current generation PONs to the next generation PONs systems. (a) OLT add scenario; (b) OLT replace scenario [11].

migration from TDM-PON to WDM-PON will be essential for the long term future. Many researchers have reported different ways of migration using hybrid WDM/TDM-PON architectures [2–4]. In this section we

JOURNAL OF NETWORKS, VOL. 6, NO. 1, JANUARY 2011

propose the architecture of a self-restored hybrid WDM/TDM-PON. The architecture of the proposed model is shown in Fig. 4. The proposed architecture could serve multiple TDM-PONs and WDM-PONs using a single OLT. We used 1×16 AWGs in the OLT and in the second stage Remote Node (RN) for WDM-PONs. Each output from AWGs were separated by an Optical Circulator (OC), which were connected to the Transmitters (Txs) and Receivers (Rxs), to separate upstream and downstream transmissions. We used 1×4 Band Splitting Wavelength Division Multiplexing (BSWDM) filters in the OLT, as well as in the first stage RN, which de-multiplexes and multiplexes downstream and upstream signals. The advantages of using the BSWDM filter are the independency of the insertion loss with the splitting ratio and the wider bandwidth of the transmission pass-band of each port [5]. The BSWDM filter used in the proposed architecture consisted of one input and four outputs for four channels. The BSWDM filters were separated into several channels, whose channels were assigned to different subscriber groups. A pair of 1:16 Passive Splitters (SPs) was used at the second stage RN for TDMPON 1 to de-multiplex and multiplex the signals. For our proposed architecture, we used the four ITU-T channels 1 - 4 (1 = 1530 nm, 2 = 1550 nm, 3 = 1570 nm and 4 = 1590 nm), each of which could incorporate up to 16 dense wavelengths to address all ONUs of a PON. In order to provide downstream services to TDMPON 1, the Tx in the OLT utilized the dense wavelength positioned at the center of the channel 1 to address all ONUs connected to the TDM-PON 1 in a broadcasting manner, although it could use any wavelength within the same pass-band. For upstream transmission, the ONUs of

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TDM-PON-1 shared the same wavelength (1) based on TDM approach. In order to provide downstream services to WDM-PONs, each Tx of the WDM-PONs employed one of the 16 dense wavelengths for each channel 2 - 4. To avoid the need for laser sources at the ONUs, CLSs with Reflective Semiconductor Optical Amplifiers (RSOAs) were used to implement colorless transceivers. 3dB couplers were used to separate the upstream and downstream signals in the ONUs. WDM filters in each ONU separated the appropriate downstream information. One Broadband Light Source (BLS) was placed in the OLT. It provided optical signals to ONUs, where the optical signals were modulated with the upstream data and sent back to the OLT. The Coupling Device (CD) used in the OLT consisted of one Optical Circulator (OC) and one WDM filter. Single Mode Fibers (SMFs) were used as feeder and distribution fibers. To provide protection against fiber fault, one 1×2 OC was used at OLT to split the signals into two paths: one of which would follow the working fiber and the other would follow the restoring fiber. Each ONU had a 1×2 Optical Switch (OS) that selects the connection from working or restoring fibers. In a normal state, the ONU received the downstream signal from both fibers ( working and restoring fibers). The OS s automatically blocked the signals from the restoring fiber path. When any fiber fault occurred in the working feeder fiber (from OLT to second stage RN), all ONUs switched to connect to the restoring feeder fiber. However, when fiber fault occurred in any distributed fiber (from second stage RN to each ONU), the OS of that particular ONU switched only to connect to the restoring fiber. To provide protection inside the OLT, we added 1×2 OSs in

Figure 4. Architecture of the proposed self-restored hybrid WDM/TDM-PON with duplex fiber. © 2011 ACADEMY PUBLISHER

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OLT. In this way, we achieved the protection against all kinds of faults.

UOverall-WDM = UOLT + UFF + UAWG + UDF + UONU = 765×10 – 7 Therefore, the availability of non-restorable WDMPON is as follows:

IV. PERFORMANCE ANALYSIS In this section, we analyzed the performance of the proposed architecture by determining the system availability, cost, power budget, and signal quality.

AOverall-WDM = 1 – UWDM-Overall = 0.9999235 TABLE II.

A. System Availability Analysis The performance of the proposed system was quantified by calculating the system availability. The system Un-availability (U) can be obtained as follows: U = MTTR / MTBF

MTTR, MTBF, AND UN-AVAILABILITIES OF DIFFERENT ELEMENTS Elements Tx Rx Optical switch (OS) AWG/BSWDM Optical circulator (OC) 3-dB coupler WDM filter BLS Feeder Fiber (20+5 km) Distribution Fiber (5 km)

(1)

The system Availability A can be obtained as follows: A=1–U

(2)

where MTTR and MTBF represent the mean-time-torepair and mean-time-between-failure. The model for calculating the system availability is shown in Fig. 5. The values for calculating MTTR and MTBF are provided in Table II [12,13]. For non-restorable WDM-PON (Fig. 5(a)), the overall un-availability of a channel was calculated by the sum of the un-availabilities of an OLT, a feeder fiber, an AWG, a distribution fiber, and an ONU. The un-availability of OLT was calculated by the sum of the un-availabilities of a transmitter, a receiver, an optical circulator, an AWG, one BLS, and a bidirectional Coupling Device (CD), which consisted of one Optical Circulator (OC) and one WDM filter. Therefore, the un-availabilities of nonrestorable WDM-PON are as follows:

MTTR (hours) 2 2 2 2 2 2 2 2 24 24

MTBF (Years) 630 1630 570 570 570 570 229 57 50 200

Un-availability 3.62×10 – 7 1.4×10 – 7 4×10 – 7 4×10 – 7 4×10 – 7 4×10 – 7 9.97×10 – 7 40×10 – 7 547.9×10 – 7 137×10 – 7

For non-restorable hybrid WDM/TDM-PON (Fig. 5(b)), the un-availabilities are as follows: UOLT- WDM/TDM = UTx + URx+ UOC + UAWG + UBSWDM + UBLS + UCD = 71×10–7 UOverall-WDM/TDM = UOLT + UFF + UBSWDM + UAWG + UDF + UONU = 777×10 – 7 Therefore, the availability of non-restorable hybrid WDM/ TDM-PON is as follows:

UOLT-WDM = UTx + URx + UOC + UAWG + UBLS + UCD = 67×10– 7

A Overall-WDM/TDM = 1 – UOverall-WDM/TDM = 0.9999223

(a)

(b)

(c) Figure 5. The schematic architectures for system availability calculation. (a) The non-restorable WDM-PON; (b) The non-restorable hybrid WDM/ TDM-PON; (c) The proposed self-restored hybrid WDM/TDM-PON. © 2011 ACADEMY PUBLISHER

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For the proposed self-restored hybrid WDM/TDMPON (Fig. 5(c)), the un-availabilities are as follows: UOLT-Proposed = UTx + URx + U2AWG + UBSWDM + 2UOS + UBLS + UCD + 2UOC = 79×10 – 7 UOverall-Proposed = UOLT + (UFF + UBSWDM + UAWG + UDF) 2 + UOS + UONU = 92×10 – 7 Therefore, the availability of the proposed architecture is as follows: AOverall-Proposed = 1 – UOverall-Proposed = 0.9999908 B. Cost Analysis We calculated the cost of the proposed architecture and existing WDM-PON & TDM-PON based architectures (with and without protection) that would provide services to the same number of users. For this purpose, we considered that there were 16 users in each individual PON. We considered the price of ONUs of WDM-PONs as X for all cases. The cost of the equipments and other related items are shown in Table III, whose costs were collected from [13]. The cost for 64 users with existing non-restorable architectures is calculated as follows: CENR = 3×CWDM-PON + CTDM-PON =X + 3×[16×(CTx + CRx + COC) +2×CAWG+ CBLS&CD + CF + CBF +CCO] + [CTx +CRx+ COC + CSP-16 + CCO + 16×(CTx + CRx + CFil) + CF + CBF] = X + $ 399,155 TABLE III.

TENTATIVE COSTS OF DIFFERENT EQUIPMENTS AND OTHER COSTS Name Transmitter (DFB laser) Receiver (APD receiver) Filter AWG (1×16) BLS and CD Passive splitter (1: 16) Optical circulator (1×2) Optical switch BSWDM filter (Estimated) Optical fiber Burying cost of fiber (Estimated) Office for OLT (CO)

Symbol CTx CRx CFil CAWG CBLS&CD CSP-16 COC COS CBSWDM CF CBF CCO

Cost (US $) 110 70 80 3,000 5,000 650 525 100 5,000 40/km 500/km 25,000

The cost for 64 users with existing architectures with restorable capability is calculated as follows:

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+ 3×CBSWDM + CBLS & CD + COC + 2×CSP-16 + 16×(CTx + CRx + CFil) + CF + CBF + CCO  CP = X + $ 521,830 C. Power Budget Analysis In order to calculate the power budget for the proposed architecture, we considered that the proposed system operates at 1.25 Gb/s per channel (i.e. per user) for both upstream and downstream transmission with a Non Return to Zero (NRZ) data format. The estimated power budgets for both upstream and downstream transmission of the proposed hybrid WDM/TDM-PON architecture are shown in Table IV. D. Signal Quality Analysis In order to determine the quality of the signal, we estimated the Signal-to-Noise Ratio (SNR) at the output of an optical receiver using the equation as follows [14]: SNR 

Signal power from photocurrent (3) Photodetector noise power + Amplifier noise power

The noise sources of the photodetector noise power are the quantum noise current, bulk dark current, and surface dark current. By substituting the appropriate equations for each term (signal power from photocurrent, photodetector noise power, and amplifier noise power), (3) becomes as follows [14]: SNR 

I P2 M 2 (4) 2q  I P  I D  M F  M  B  2qI L B  4k BTB RL 2

The values of each term of (4) are collected from [14] and various vendor data sheets. Let us consider the values of all the terms of (4) as follows: Photo current (IP) = 2 μA; bulk dark current (ID) = 10 nA; surface dark current (IL) = 2 mA; charge of electron (q) = 1.6×10 – 19 C; gain of APD (M) = 10; noise figure (F(M) = Mx), where x = 0.7; receiver bandwidth (B) = 2.5 GHz; Boltzmann’s constant (kB) = 1.38×10 – 23 J/K; temperature (T) = 293 K; and load resistance (RL) = 19 KΩ. By putting these values in (4) we obtain the value of the expected SNR for the proposed hybrid WDM/TDM-PON architecture as follows: SNR  166.24  22.2 dB

The relation between the received SNR and the Quality factor (Q) are defined as follows [15]:

SNR  20log 2Q

(5)

CER = 3×CWDM-PON (Restorable) + CTDM-PON (Restorable) = X +3×[16×(CTx + CRx + COC) + 4×CAWG + 33×COS + CBLS&CD +COC + CF + CBF + CCO] + [CTx + CRx + 2×COC + 2×CSP-16 + 16×COS + CCO + 16×(CTx + CRx + CFil) + CF + CBF]  CER = X + $ 658,205

On the other hand, the relation between the Quality factor (Q) and the Bit Error Rate (BER) are defined as follows [15]:

Currently, the cost for 64 users with the proposed restorable hybrid WDM/TDM-PON architecture is calculated as follows:

From (5) we found that for SNR of 22.2 dB, the expected value of the Q factor was  6.44. From (6) we found that for Q = 6.44, the expected value of the BER was  310 – 10.

CP = X + 49×(CTx + CRx+ COC) + 115×COS + 12×CAWG © 2011 ACADEMY PUBLISHER

BER 

2 Q erfc 3 2

(6)

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TABLE IV.

UPSTREAM AND DOWNSTREAM POWER BUDGET ANALYSES OF THE PROPOSED ARCHITECTURE Component/ loss parameter

Laser output R-SOA output Receiver sensitivity (APD) Allowed loss Source connector loss Circulator/3- dB coupler loss (Three) Optical switch loss (Three) AWG loss (Two) BSWDM filter (Two) BLS & CD loss Cable attenuation & other loss (30 km) Insertion loss of APD

Downstream Output/ sensitivity/ loss Power margin (dB /dBm) (dB) 7 dBm

V. Results and Discussions From the system availability analysis, we found that the availability of the non-restorable WDM-PON was 0.9999235 (i.e.  99.99 %). On the other hand, the availability of the non-restorable hybrid WDM/TDMPON was 0.9999223 (i.e.  99.99 %), whose value was almost equal to the availability of non-restorable WDMPON. However, the availability of the proposed architecture shown in Fig. 4 was 0.9999908 (i.e.  99.999 %), whose availability was the desired value for any network. From this analysis, we discovered that the availability of the proposed architecture had increased by a factor of 8 compared to the non-restorable WDM-PON and existing hybrid WDM/TDM-PON. From the cost analysis we found that the cost for 64 subscribers with existing non-restorable architectures (WDM -P ONs and TDM -P ONs) was $ 3 99,1 55 , excluding the cost of the ONUs of WDM-PONs. On the other hand, the cost for 64 subscribers with the restorable existing architectures (WDM-PONs and TDM-PONs) was $ 658,205, excluding the cost of the ONUs of WDMPONs. However, the cost for 64 subscribers with the proposed restorable hybrid WDM/TDM-PON was $ 521, 830, excluding the cost of the ONUs of WDM- PONs. The cost analysis showed that the proposed architecture is more cost effective compared to the existing restorable PON architectures. Also, the proposed architecture will use less space for the central office compared to other PON architectures. From the power budget analysis of Table IV, we found that the proposed architecture has a sufficient power margin in both upstream (10.6 dB) and downstream (13.6 dB) transmission. Therefore, the proposed architecture has the flexibility to add more components in the transmission path for future purposes. From the signal quality analysis, the estimated value of the SNR at the output of the optical receiver was 22.2 dB. The corresponding value of the Q factor was  6.44 and the value of BER was  3×10 – 10. These values are highly acceptable for data communication. Due to the un-availability of the commercialized

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Power margin (dB)

4 dBm – 34 dBm

– 34 dBm 0.5 dB 3 dB 4.2 dB 2 dB 3 dB 4 dB 10 dB 0.7 dB

Upstream Output/ sensitivity/ loss (dB /dBm)

41 40.5 37.5 33.3 31.3 28.3 24.3 14.3 13.6 (Excess power)

0.5 dB 3 dB 4.2 dB 2 dB 3 dB 4 dB 10 dB 0.7 dB

38 37.5 34.5 30.3 28.3 25.3 21.3 11.3 10.6 (Excess power)

BSWDM filter, it was not possible for us to do the experiment or carry out any simulations for the proposed architecture. However, the calculated results proved the applicability of the proposed architecture for optical access networks to provide support to multiple TDM and WDM-PONs simultaneously. VI. CONCLUSION In order to mitigate the present and future bandwidth requirements, PON has become a promising solution for access networks. Although the current generation PONs, such as E-PON and G-PON, are adequate for current services, their bandwidth is insufficient for future access networks. As a result, WDM-PON is considered to be one of the ultimate future generation PONs. In order to change from TDM-PONs to WDM-PONs, and provide interoperability between these two PONs, hybrid WDM/TDM-PONs were found to be the most suitable solution. The proposed self-restored hybrid WDM/TDMPON provides higher system availability compared to other PON architectures. In addition, the proposed architecture is more cost effective compared to the existing restorable PON architectures with desirable signal quality for data communication. We believe that the proposed architecture is one of the most suitable solutions for migration from TDM-PONs to WDMPONs. REFERENCES [1] C. H. Lee, W. V. Sorin, and B. Y. Kim, “Fiber to the home using a PON infrastructure,” J. Lightw. Technol., vol. 24, no. 12, pp. 4568–4573, December 2006. [2] C. Bock, J. Prat, and S. D. Walker, “Hybrid WDM/TDM PON using the AWG FSR and featuring centralized light generation and dynamic bandwidth allocation,” J. Lightw. Technol., vol. 23, no. 12, pp. 3981–3988, December 2005. [3] G. Talli and P. D. Townsend, “Hybrid DWDM-TDM longreach PON for next-generation optical access,” J. Lightw. Technol., vol. 24, no. 7, pp. 2827–2834, July 2006. [4] Y. Shachaf, C. H. Chang, P. Kourtessis, and J. M. Senior, “Multi-PON access network using a coarse AWG for smooth migration from TDM to WDM PON,” Opt. Express, vol. 15, no. 12, pp. 7840–7844. June 2007.

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[5] S. G. Mun, S. M. Lee, K. Okamoto, and C. H. Lee, “A multiple star WDM-PON using a band splitting WDM filter,” Optics Express, vol. 16, no. 9, pp. 6260–6266, April 2008. [6] L. D. Lamb, Overview of EPON-Based Fiber Networks: Current Status and Future Directions, Teknovus, USA, June 2008 [Online]. Available: http:// www.sigtech.com.m y/trainingnotes/teknovustelekom_malaysia_17_june_2008. pdf [7] S. G. Mun, J. H. Moon, H. K. Lee, J. Y. Kim, and C. H. Lee, “A WDM-PON with a 40 Gb/s (32×1.25 Gb/s) capacity based on wavelength-locked Fabry-perot Laser diodes,” Opt. Express, vol. 16, no. 15, pp. 11361–11368, July 2008. [8] H. Suzuki, M. Fujiwara, T. Suzuki, N. Yoshimoto, H. Kimura, and M. Tsubokawa, “Demonstration and performance of colorless ONU for coexistence-type WDM-PON using a wavelength-tunable L-Band DWDMSFP transceiver,” IEEE Photon. Technol. Lett., vol. 20, no. 19, pp. 1603–1605, October 2008. [9] D. K. Jung, S. K. Shin, C.-H. Lee, and Y. C. Chung, “Wavelength-division-multiplexed passive optical network based on spectrum-slicing techniques,” IEEE Photon. Technol. Lett., vol. 10, no. 9, pp. 1334–1336, September 1998. [10] H. Takesue and T. Sugie, “Wavelength channel data rewrite using saturated SOA modulator for WDM networks with centralized sources,” J. Lightw. Technol., vol. 21, no. 11, pp. 2546–2556, November 2003. [11] H. Mukai, “Physical layer requirements for smooth migration from the current FTTH,” Joint ITU-T/IEEE Workshop on Next Generation Optical Access Systems, June 2008 [Online]. Available: http://www.itu.int/dms_pub /itu-t/oth/06/13/T06130000300002PDFE.pdf, Accessed: January 2010. [12] S. Verbrugge et al. “Methodology and input availability parameters for calculating OpEx and CapEx costs for realistic network scenarios,” J. Opt. Netw., vol. 5, no. 6, pp. 509–520, June 2006. [13] J. Chen and L. Wosinska, “Analysis of protection schemes in PON compatible with smooth migration from TDMPON to hybrid WDM/TDM-PON,” J. Opt. Netw., vol. 6, no. 5, pp. 514–526, May 2007. [14] G. Keiser, Optical fiber communications, 3rd ed., McGrawHill, 2000, pp. 243–268. [15] R. J. Hoss, Fiber optic communications design handbook, International ed., Prentice-Hall, 1990, pp.154–162. Md. Shamim Ahsan received his B.Sc. Engg. degree in Electronics and Communic ation Engineerin g fro m Khulna University, Khulna, Bangladesh, in 2003. He is currently working toward an integrated M.S./Ph.D. degree under the supervision of Prof. Man Seop Lee in the Photonics Application Lab at the Korea Advanced Institute of Science and Technology (KAIST), Daejeon, South Korea. Mr. Ahsan served as a lecturer in Khulna University, Khulna, Bangladesh from December 2003 to February 2007. He has been serving as an assistant professor at the same university since February 2007. His current research focuses on laser micro & nanomachining and surface treatment of different materials, such as metals, glasses, crystals, and polymars.

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Prof. Man Seop Lee received his B.Sc. and M.Sc. degrees from Busan National University, Busan, South Korea, in 1976 and 1978, respectively, and his Ph.D. degree from the Korea Advanced Institute of Science and Technology (KAIST), Daejeon, South Korea, in 1991, all in electrical engineering. Prof. Lee served as the Director of Transmission Technology Department in Electronics and Telecommunications Research Institute (ETRI), South Korea from 1979 to 1997. He served as an Adjunct Professor at the Chun-Buk University, South Korea from 1993 to 1997. He served as a professor in the Information and Communications University (ICU), South Korea from 1998 to 2009. He also served as the director of the ICU Joint Research Center and ICU Venture from 1998 to 2005. He was the Dean of the ICU Office of Planning Affairs from 2008 to 2009. Since 2009, he has been serving as a professor at KAIST, South Korea. The main research interests of Prof. Lee include analysis and design of high-speed optical transmission systems, optical Internet, optical access networks and systems, and micro & nano-machining using lasers. S. H. Shah Newaz received his B.Sc degree in Information and Communication Engineering from East West University (EWU), Dhaka, Bangladesh, in 2008. He received his M.Sc degree in Information and Communication Engineering from the Korea Advanced Institute of Science and Technology (KAIST) in 2010. Currently, he is working in Broadband Network Lab (BNLab) at KAIST, South Korea as a researcher. His research interests include: Mobile IP, Energy Efficient Networks, Optical wireless converged networks, and Call admission control for wireless access networks. Syed Md. Asif received his B. Sc. Engg. Degree in Electronics and Communication Engineering from Khulna University, Khulna, Bangladesh, in 2008. He has been currently serving as a lecturer in Sylhet International University, Sylhet, Bangladesh, since 2009. His current research focuses on optoelectronic devices and optical networks.