《信息网络协议基础》课程教学资源（学习资料）The next generation of passive optical networks：A review Huda Saleh Abbas n , Mark A. Gregory（ng-pon）

Journal of Network and Computer Applications 67(2016)53-74 Contents lists available at ScienceDirect Journal of Network and Computer Applications ELSEVIER journal homepage:www.elsevier.com/locate/jnca Review The next generation of passive optical networks:A review Huda Saleh Abbas*,Mark A.Gregory RMIT University.Melbourne,Australia ARTICLE INFO ABSTRACT Article history. Passive Optical Networks (PONs)have become a popular fiber access network solution because of its Received 8 November 2015 service transparency,cost effectiveness,energy savings,and higher security over other access networks. Received in revised form PON utilizes passive low-power components which removes the need for power-feeding in the fiber 5 February 2016 Accepted 21 February 2016 distribution network.This paper presents three different generations of PON that are based on the Available online 2 March 2016 Ethernet PON and Gigabit PON standards.This article showcases the first generation of PON in terms of physical and data link layers and forms the basis for discussion about the different approaches being Keywords: pursued for the next generation stage 1 PON(NG-PON1).Additionally,the main objective of this study is EPON GPON to review the technologies proposed for the next generation stage 2 PON(NG-PON2):highlighting the XG-EPON important contributions and limitations of the corresponding technologies.Hybrid approaches that XG-GPON1 combine multiple technologies are introduced as a solution to eliminate major limitations and to XG-GPON2 improve overall system-wise performance.However,NG-PON2 is still suffering from a number of chal- TDM-PON lenges include cost,reach.capacity and power consumption are discussed at the end of this paper WDM-PON Another purpose of this paper is to identify potential remedies that can be investigated in the future to TWDM-PON improve the performance of the NG-PON2. OCDM-PON 2016 Elsevier Ltd.All rights reserved. OFDM-PON Physical layer Data link layer Hybrid technology Contents 1. Introduction............. 2. Deployed EPON and GPON................................. 55 2.1 Physical layer.......................... 444444444444444444444 2.2. Data link layer...“ 56 3. NG-P0N1.: 57 3.1. From EPON to XG-EPON 3.2. From GPON to XG-GPON.. 3.3. Mixed scenario. 58 4.ING-PoN2 pure technologies..·.. 4.1. High speed TDM-PON............................ 42. WDM-P0N.++………… 5g 4.3 OCDM-PON 6 4.4. OFDM-PON 1 4.5. UN-P0N.…· 61 4.6 pDM-P0N.· 5.TU-TNG-PON2 technology....·,..· 5.1. TWDM-PON...... 62 5.2. Point-to-Point WDM Overlay 6. ITU-T Standards for NG-PON2........... 6 6.1. Wavelength band........................................ 63 *Corresponding author. E-mail addresses:Huda.s.abbas@gmaiLcom (H.S.Abbas),markgregory@rmit.edu.au (M.A.Gregory). htp:/dx.doi.org/10.1016 j.jnca.2016.02.015 1084-8045/2016 Elsevier Ltd.All rights reserved

Review The next generation of passive optical networks: A review Huda Saleh Abbas n , Mark A. Gregory RMIT University, Melbourne, Australia article info Article history: Received 8 November 2015 Received in revised form 5 February 2016 Accepted 21 February 2016 Available online 2 March 2016 Keywords: EPON GPON XG-EPON XG-GPON1 XG-GPON2 TDM-PON WDM-PON TWDM-PON OCDM-PON OFDM-PON Physical layer Data link layer Hybrid technology abstract Passive Optical Networks (PONs) have become a popular fiber access network solution because of its service transparency, cost effectiveness, energy savings, and higher security over other access networks. PON utilizes passive low-power components which removes the need for power-feeding in the fiber distribution network. This paper presents three different generations of PON that are based on the Ethernet PON and Gigabit PON standards. This article showcases the first generation of PON in terms of physical and data link layers and forms the basis for discussion about the different approaches being pursued for the next generation stage 1 PON (NG-PON1). Additionally, the main objective of this study is to review the technologies proposed for the next generation stage 2 PON (NG-PON2); highlighting the important contributions and limitations of the corresponding technologies. Hybrid approaches that combine multiple technologies are introduced as a solution to eliminate major limitations and to improve overall system-wise performance. However, NG-PON2 is still suffering from a number of chal￾lenges include cost, reach, capacity and power consumption are discussed at the end of this paper. Another purpose of this paper is to identify potential remedies that can be investigated in the future to improve the performance of the NG-PON2. & 2016 Elsevier Ltd. All rights reserved. Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54 2. Deployed EPON and GPON . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55 2.1. Physical layer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55 2.2. Data link layer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56 3. NG-PON 1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57 3.1. From EPON to XG-EPON . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57 3.2. From GPON to XG-GPON. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57 3.3. Mixed scenario. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58 4. ING-PON2 pure technologies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58 4.1. High speed TDM-PON . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58 4.2. WDM-PON . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59 4.3. OCDM-PON . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60 4.4. OFDM-PON. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61 4.5. UNI-PON. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61 4.6. PDM-PON . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62 5. ITU-T NG-PON2 technology. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62 5.1. TWDM-PON . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62 5.2. Point-to-Point WDM Overlay . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63 6. ITU-T Standards for NG-PON2. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63 6.1. Wavelength band. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63 Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/jnca Journal of Network and Computer Applications http://dx.doi.org/10.1016/j.jnca.2016.02.015 1084-8045/& 2016 Elsevier Ltd. All rights reserved. n Corresponding author. E-mail addresses: Huda.s.abbas@gmail.com (H.S. Abbas), mark.gregory@rmit.edu.au (M.A. Gregory). Journal of Network and Computer Applications 67 (2016) 53–74

54 HS.Abbas,M.A.Gregory Journal of Network and Computer Applications 67(2016)53-74 6.2 Spectral flexibility..................................63 6.3 C0-existence..,………………… 63 6.4 ODN re-use 63 6.5 4 6.6. Additional components 64 7. Precents implementation of TWDM. .64 8 XDM/WDM hybrid technologies................................ 65 8.1. OCDM/WDM-PON 5 8.2. OFDM/WDM-PON 66 9. XDM/TDM hybrid technologies.......... 66 9.1. OCDM/TDM-PON 66 9.2. OFDM/TDM-PON 67 10. Hybrid XDM/TDM/WDM................ 6 11. NG-PON2 challenges...... 67 11.1. Increase the capacity. 67 11.2. Extend the reach. 6 11.3.Power saving,...·..··· 68 12. PON reliability aspects..··. 9 12.1. PON protection mechanisms...... 69 122. PON security..,·· 69 12.3. P0 N monitoring.… 69 13. Future aspects of PON........·· 70 14. Discussion and future works...,....·.,,·.··· 15. Conclusion.++…+ 71 References …….72 1.Introduction splitters by fiber.The optical splitters connect to customer pre- mises making PON a point to multi-point architecture(P2MP) Passive Optical Networks (PONs)are a series of promising (Ragheb and Fathallah,2012). broadband access network technologies that offer enormous The EPON and the GPON standards have the same general advantages when deployed in fiber to the home(FTTH)scenarios. principle in terms of framework and applications but their The advantages include a point to multi-point architecture,high operation is different due to the implementation of the physical quality triple play service capabilities for data,voice and video, and data link layers (Olmos et al.,2011).EPON is defined by IEEE high speed internet access,and other services in a cost-effective 802.3 and it is widely deployed in Asia whilst GPON is deployed in manner(Ragheb and Fathallah,2012). a number of other regions.GPON's requirements were defined by Over the past decade several PON architectures have been the Full Service Access Network (FSAN)group that was ratified as developed by the International Telecommunications Union (ITU) ITU-T G.984 and is implemented in North America,Europe,Middle and the Institute of Electrical and Electronic Engineers(IEEE).The East,and Australasia (Van Veen et al.,2011:Skubic et al.,2009). four main PON variations developed by the ITU and IEEE can be In this paper the advancement of PON technology is classified categorized into two groups.The first kind of architecture is based into three generations:the first generation(deployed PON),next on Asynchronous Transfer Mode (ATM)and includes ATM PON generation stage 1(NG-PON1),and next generation stage 2(NG- (APON),Broadband PON(BPON)and Gigabit PON(GPON)and the PON2).The evolution of the PON architectures and their corre- second group consists of Ethernet PON(EPON).EPON and GPON sponding capacity features are shown in Fig.2. are the most popular PON variations found in use today.A con- The first generation of PON is based on Time Division Multiple ventional PON architecture is presented in Fig.1 (Ragheb and Access(TDMA)and provides an EPON downstream rate of 1 Gbps Fathallah,2012).In the figure,it can be seen that the PON archi- and a GPON downstream rate of 2.4 Gbps.The NG-PON1 increases tecture consists of an Optical Line Terminal (OLT).Optical Dis- the data rate up to 10 Gbps for both standards(Biswas and Adak, tribution Network(ODN),and Optical Network Units(ONU).The 2011).There are two main scenarios to achieve an upgrade that are OLT is placed at the Central Office (CO)and connected to the the upgrade from deployed EPON to XG-EPON and from deployed GPON to XG-GPON.An upgrade from deployed GPON to XG-EPON Central office Optical Distribution Network Customer Side Feeder Fiber】 Distribution Fiber ONU1 NG-PON3 1o0/40 Gb/s XG-EPON NG-PONZ Splitte ONU 2 OLT M0/10D/ 10/10Gb/ N EPON 1Gb/s ONU 3 XG-GPON XG-PON 1o/2.4Gb/ GPON -XG-PON2 2.5/1Gb/ 10/10Gb/s ONU N 2004 2010 Years 2015 2020 Fig.1.PON architecture Fig.2.PON generations

6.2. Spectral flexibility . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63 6.3. Co-existence. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63 6.4. ODN re-use . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63 6.5. Pay as you grow. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64 6.6. Additional components . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64 7. Precents implementation of TWDM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64 8. XDM/WDM hybrid technologies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65 8.1. OCDM/WDM-PON . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65 8.2. OFDM/WDM-PON . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66 9. XDM/TDM hybrid technologies. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66 9.1. OCDM/TDM-PON . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66 9.2. OFDM/TDM-PON . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67 10. Hybrid XDM/TDM/WDM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67 11. NG-PON2 challenges . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67 11.1. Increase the capacity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67 11.2. Extend the reach . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67 11.3. Power saving . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68 12. PON reliability aspects. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69 12.1. PON protection mechanisms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69 12.2. PON security . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69 12.3. PON monitoring. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69 13. Future aspects of PON . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70 14. Discussion and future works. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71 15. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72 1. Introduction Passive Optical Networks (PONs) are a series of promising broadband access network technologies that offer enormous advantages when deployed in fiber to the home (FTTH) scenarios. The advantages include a point to multi-point architecture, high quality triple play service capabilities for data, voice and video, high speed internet access, and other services in a cost-effective manner (Ragheb and Fathallah, 2012). Over the past decade several PON architectures have been developed by the International Telecommunications Union (ITU) and the Institute of Electrical and Electronic Engineers (IEEE). The four main PON variations developed by the ITU and IEEE can be categorized into two groups. The first kind of architecture is based on Asynchronous Transfer Mode (ATM) and includes ATM PON (APON), Broadband PON (BPON) and Gigabit PON (GPON) and the second group consists of Ethernet PON (EPON). EPON and GPON are the most popular PON variations found in use today. A con￾ventional PON architecture is presented in Fig. 1 (Ragheb and Fathallah, 2012). In the figure, it can be seen that the PON archi￾tecture consists of an Optical Line Terminal (OLT), Optical Dis￾tribution Network (ODN), and Optical Network Units (ONU). The OLT is placed at the Central Office (CO) and connected to the splitters by fiber. The optical splitters connect to customer pre￾mises making PON a point to multi-point architecture (P2MP) (Ragheb and Fathallah, 2012). The EPON and the GPON standards have the same general principle in terms of framework and applications but their operation is different due to the implementation of the physical and data link layers (Olmos et al., 2011). EPON is defined by IEEE 802.3 and it is widely deployed in Asia whilst GPON is deployed in a number of other regions. GPON's requirements were defined by the Full Service Access Network (FSAN) group that was ratified as ITU-T G.984 and is implemented in North America, Europe, Middle East, and Australasia (Van Veen et al., 2011; Skubic et al., 2009). In this paper the advancement of PON technology is classified into three generations: the first generation (deployed PON), next generation stage 1 (NG-PON1), and next generation stage 2 (NG￾PON2). The evolution of the PON architectures and their corre￾sponding capacity features are shown in Fig. 2. The first generation of PON is based on Time Division Multiple Access (TDMA) and provides an EPON downstream rate of 1 Gbps and a GPON downstream rate of 2.4 Gbps. The NG-PON1 increases the data rate up to 10 Gbps for both standards (Biswas and Adak, 2011). There are two main scenarios to achieve an upgrade that are the upgrade from deployed EPON to XG-EPON and from deployed GPON to XG-GPON. An upgrade from deployed GPON to XG-EPON Fig. 1. PON architecture. Fig. 2. PON generations. 54 H.S. Abbas, M.A. Gregory / Journal of Network and Computer Applications 67 (2016) 53–74

H.S.Abbas.MA.Gregory Joumal of Network and Computer Applications 67 (2016)53-74 55 is another potential pathway that can be considered.However, (Tanaka et al.,2010:Ricciardi et al.,2012).Fig.3(a)and (b)shows with the rapid increase in high bandwidth applications and the structure of EPON and GPON respectively.The differences at Internet services the NG-PON1 would not be able to meet the the physical and data link layers are discussed in this section and future demand for bandwidth and Quality of Service (QoS) summarized in Table 1(Olmos et al.,2011). requirements.To find an acceptable future upgrade pathway.the research community is investigating the options for NG-PON2 and several technologies that might be used in NG-PON2 have been 2.1.Physical layer studied extensively in order to meet the future requirements of users and network operators(Olmos et al.,2011;Ling et al..2010) The variations between both standards in the physical layer Four multiplexing technologies are being considered for NG- include:bit rate,wavelength and splitter ratio. PON2 to provide a downstream transmission of 40 Gbps and In terms of bit rate,the deployed EPON offers a bit rate of upstream transmission of 10 Gbps.The technologies indlude high 1.2 Gbps for both downstream and upstream transmissions. speed Time Division Multiplexing PON (TDM-PON).Wavelength However,as a result of 8B/10B line coding,the actual available bit Division Multiplexing PON(WDM-PON).Optical Code Division Mul- rate is 1 Gbps (Skubic et al,2009).In contrast,GPON supports tiplexing PON (OCDM-PON),and Orthogonal Frequency Division different downstream and upstream transmission rates.For Multiplexing PON (OFDM-PON).The multiplexing techniques that downstream transmission,GPON defines rates of 1.2 Gbps or have been identified to provide a P2MP connection between a single 2.4 Gbps.Whereas for upstream transmission it offers 1.5 Gbps. OLT and multiple ONUs.However,each technology has its own pros 6.2 Gbps,1.2 Gbps or 2.4 Gbps.GPON typically operates using and cons (Cvijetic et al.,2010).To eradicate the multiplexing-specific 1.2 Gbps for upstream transmission and 2.4 Gbps for downstream limitations,hybrid approaches that combine the advantages of transmission (Selmanovic and Skaljo,2010). multiple technologies have been introduced as a dominant option for EPON and GPON define the same wavelength bands for the NG-PON2.In the literature,several hybrid technologies have been downstream transmission which are 1480-1500 nm and both studied including TDM/WDM-PON,OCDM/WDM-PON,OCDM/TDM- provide a separate wavelength band for a video signal which is PON,OFDM/WDM-PON,and OFDM/TDM-PON.Among them,hybrid 1550 nm.For the upstream wavelength bands EPON uses a TDM/WDM PON (TWDM-PON)has been selected as the base ele- wavelength band of 1260-1360 nm and GPON uses a wavelength ment for the NG-PON2 by the FSAN community (Luo et al.,2013).The band of 1290-1330 nm (Erzen and Batagelj.2015). decision was made based on several factors including technology The fiber spilt ratio supported by EPON is 16 users.while,GPON maturity,system performance,power consumption,and cost effec- supports a higher spilt ratio up to 64 users.The high split ratio tiveness (Luo et al.,2012,2013). supported by GPON is obtained as a result of deploying a Reach Despite the efforts to adapt these technologies to meet the Extender(RE)at the ODN.The RE is an important concept in GPON requirements of NG-PON2,challenges like increasing the capacity. that is utilized to increase the power budget and consequently reducing the cost,extending the reach and power saving still increase the reach and the split ratio.This can be achieved by persist and required to be investigated further. implementing technologies such as amplifiers and regenerators Several reviews have been published addressing PONs and its requirements.The possible solutions and prospective technologies (Tanaka et al,2010;Erzen and Batagelj.2015). for the NG-PONs are also suggested in(Orphanoudakis et al.2008; Kani et al,2009;Effenberger et al 2009a,2009b;Nesset,2015; Laver 5 Shaddad et al.,2014;Mohamed and Ab-Rahman,2015).However. this study reviews the different generations of PONs and focuses on the potential enabling technologies for NG-PON2.In addition. the paper outlines the major limitations and challenges of NG- Layer 3 PON2 technologies.This paper also studies the relevant contribu- tions in field for the past three years that tried to accomplish the requirements of NG-PON2. The rest of the paper is organized as follows;Section 2 presents Layer 2 Ethernet Frame the deployed EPON and GPON and discusses the key differences in MAC Layer terms of the physical and data link layers.Section 3 provides a description of NG-PON1 and outlines approaches for the Layer I Physical Layer improvements of the system.In Section 4,the pure technologies of PONs are discussed.Section 5 showcases the ITU-T NG-PON2 (a)EPON layer structure. technologies including TWDM-PON and PtP WDM.In Section 6. the requirements of ITU-T standards for NG-PON2 are reviewed. Section 7 briefly reviews the recent implementation of TWDM- PON.The hybrid technologies based on XDM/WDM,XDM/TDM, and XDM/TDM/WDM are discussed in Sections 8,10,and 9 Layer 4 respectively.In Section 11,major challenges of NG-PON2 are pre- Laver 3 sented.Section 12 outlines reliability aspects and Section 13 out- lines some of the future aspects of NG-PON2.A general discussion Ethernet and several suggestions for future work are given in Section 15. Laver 2 ATM cell GEM Frame GrC sub-laye GTC TC Frame 2.Deployed EPON and GPON aver Physical Layer Although EPON and GPON provide the same services to the customers,there are some differences in the physical and data link (b)GPON layer structure. layers,leading to some variations in the features of each standard Fig.3.Layer 2 structure (a)EPON.(b)GPON

is another potential pathway that can be considered. However, with the rapid increase in high bandwidth applications and Internet services the NG-PON1 would not be able to meet the future demand for bandwidth and Quality of Service (QoS) requirements. To find an acceptable future upgrade pathway, the research community is investigating the options for NG-PON2 and several technologies that might be used in NG-PON2 have been studied extensively in order to meet the future requirements of users and network operators (Olmos et al., 2011; Ling et al., 2010). Four multiplexing technologies are being considered for NG￾PON2 to provide a downstream transmission of 40 Gbps and upstream transmission of 10 Gbps. The technologies include high speed Time Division Multiplexing PON (TDM-PON), Wavelength Division Multiplexing PON (WDM-PON), Optical Code Division Mul￾tiplexing PON (OCDM-PON), and Orthogonal Frequency Division Multiplexing PON (OFDM-PON). The multiplexing techniques that have been identified to provide a P2MP connection between a single OLT and multiple ONUs. However, each technology has its own pros and cons (Cvijetic et al., 2010). To eradicate the multiplexing-specific limitations, hybrid approaches that combine the advantages of multiple technologies have been introduced as a dominant option for the NG-PON2. In the literature, several hybrid technologies have been studied including TDM/WDM-PON, OCDM/WDM-PON, OCDM/TDM￾PON, OFDM/WDM-PON, and OFDM/TDM-PON. Among them, hybrid TDM/WDM PON (TWDM-PON) has been selected as the base ele￾ment for the NG-PON2 by the FSAN community (Luo et al., 2013). The decision was made based on several factors including technology maturity, system performance, power consumption, and cost effec￾tiveness (Luo et al., 2012, 2013). Despite the efforts to adapt these technologies to meet the requirements of NG-PON2, challenges like increasing the capacity, reducing the cost, extending the reach and power saving still persist and required to be investigated further. Several reviews have been published addressing PONs and its requirements. The possible solutions and prospective technologies for the NG-PONs are also suggested in (Orphanoudakis et al., 2008; Kani et al., 2009; Effenberger et al., 2009a, 2009b; Nesset, 2015; Shaddad et al., 2014; Mohamed and Ab-Rahman, 2015). However, this study reviews the different generations of PONs and focuses on the potential enabling technologies for NG-PON2. In addition, the paper outlines the major limitations and challenges of NG￾PON2 technologies. This paper also studies the relevant contribu￾tions in field for the past three years that tried to accomplish the requirements of NG-PON2. The rest of the paper is organized as follows; Section 2 presents the deployed EPON and GPON and discusses the key differences in terms of the physical and data link layers. Section 3 provides a description of NG-PON1 and outlines approaches for the improvements of the system. In Section 4, the pure technologies of PONs are discussed. Section 5 showcases the ITU-T NG-PON2 technologies including TWDM-PON and PtP WDM. In Section 6, the requirements of ITU-T standards for NG-PON2 are reviewed. Section 7 briefly reviews the recent implementation of TWDM￾PON. The hybrid technologies based on XDM/WDM, XDM/TDM, and XDM/TDM/WDM are discussed in Sections 8, 10, and 9 respectively. In Section 11, major challenges of NG-PON2 are pre￾sented. Section 12 outlines reliability aspects and Section 13 out￾lines some of the future aspects of NG-PON2. A general discussion and several suggestions for future work are given in Section 15. 2. Deployed EPON and GPON Although EPON and GPON provide the same services to the customers, there are some differences in the physical and data link layers, leading to some variations in the features of each standard (Tanaka et al., 2010; Ricciardi et al., 2012). Fig. 3(a) and (b) shows the structure of EPON and GPON respectively. The differences at the physical and data link layers are discussed in this section and summarized in Table 1 (Olmos et al., 2011). 2.1. Physical layer The variations between both standards in the physical layer include: bit rate, wavelength and splitter ratio. In terms of bit rate, the deployed EPON offers a bit rate of 1.2 Gbps for both downstream and upstream transmissions. However, as a result of 8B/10B line coding, the actual available bit rate is 1 Gbps (Skubic et al., 2009). In contrast, GPON supports different downstream and upstream transmission rates. For downstream transmission, GPON defines rates of 1.2 Gbps or 2.4 Gbps. Whereas for upstream transmission it offers 1.5 Gbps, 6.2 Gbps, 1.2 Gbps or 2.4 Gbps. GPON typically operates using 1.2 Gbps for upstream transmission and 2.4 Gbps for downstream transmission (Selmanovic and Skaljo, 2010). EPON and GPON define the same wavelength bands for downstream transmission which are 1480–1500 nm and both provide a separate wavelength band for a video signal which is 1550 nm. For the upstream wavelength bands EPON uses a wavelength band of 1260–1360 nm and GPON uses a wavelength band of 1290–1330 nm (Eržen and Batagelj, 2015). The fiber spilt ratio supported by EPON is 16 users, while, GPON supports a higher spilt ratio up to 64 users. The high split ratio supported by GPON is obtained as a result of deploying a Reach Extender (RE) at the ODN. The RE is an important concept in GPON that is utilized to increase the power budget and consequently increase the reach and the split ratio. This can be achieved by implementing technologies such as amplifiers and regenerators (Tanaka et al., 2010; Eržen and Batagelj, 2015). Fig. 3. Layer 2 structure (a) EPON, (b) GPON. H.S. Abbas, M.A. Gregory / Journal of Network and Computer Applications 67 (2016) 53–74 55

56 HS.Abbas,M.A.Gregory Journal of Network and Computer Applications 67(2016)53-74 Table 1 available bandwidth among the users,whilst the upstream MAC EPON versus GPON layer is based on TDMA. Features EPON GPON GPON supports two layers of encapsulation where the Ethernet frame is encapsulated into a GPON Encapsulation Method (GEM) Standard IEEE nu-T frame which is encapsulated again into a GPON Transmission Transmission speed DS:1.2 Gbps DS:1.2/2.4 Gbps Convergence(GTC)frame.The GTC frame also includes pure ATM US:1.2 Gbps US:1.5/6.2/1.22.4Gbps cells and TDM traffic.The downstream frame is broadcast to every Split ratio 1:16 1:64 Line code 8B/10B NRZ ONU and the ONUs use the information in the Physical Control Protocol Ethernet ATM Block downstream (PCBd)field to extract its own data.In case Security Not guaranteed AES there is no data to be transmitted,the downstream frame will be Qos Not supported Supported transmitted continuously and utilized for time synchronization FEC Optional RS(255,239) Optional RS(255,239) (Ricciardi et al 2012).The upstream frame contains multiple transmission bursts arriving from the ONUs.Along with the pay- Downstream load,each of the upstream burst frames consists of the Physical Layer Overhead (PLOu),a bandwidth allocation interval which LLID Start Length LUD Start Length contains the Dynamic Bandwidth Report upstream (DBRu),and allocation identifiers (Alloc-IDs).When traffic reaches the OLT, ONU traffic is queued based on Classes of Service (CoS)with a 100 200 400 100 diverse QoS dependent on the type of the Traffic Containers (T- CONTs)that is specified in the Alloc-ID(Segarra et al,2013).GPON introduces five types of T-CONTs that provide QoS in the upstream direction.The T-CONT frame is used in GPON to establish a virtual connection between ONU and OLT as well as to manage fragment transmission. Upstream LUID 2 (ONU 1) (ONU 2) 1)T-CONT type 1 Supports fixed bandwidth that is sensitive to time.The jitter of (a)EPON frame structure. T-CONT type-1 is 0 which enhances the suitability it has for Constant Bit Rate (CBR)traffic. 2)T-CONT type 2 Frame Header (PBCd) Downstream UP BW map Payload This type supports Assured bandwidth where it has a higher delay than T-CONT 1.It is used with Committed Information Rate (CIR)traffic. Alloc-ID Start End Alloc-ID Start End 3)T-CONT type 3 Supports assured and non-assured bandwidths providing a 200 400 500 600 guaranteed minimum CIR and surplus Excess Information Rate (EIR).This type is appropriate for Variable Bit Rate(VBR)traffic that does not guarantee delay. 4)T-CONT type 4 Supports Best-Effort services such as Internet browsing.SMTP and FTP Upstream T-CONT 1 T-CONT 2 (ONU 1) (ONU 2) 5)T-CONT type5 This type is mix of all the above T-CONT types.It is appropriate (b)GPON frame structure. for general traffic flows(Begovic et al.,2011:Tanaka et al.,2010: Ricciardi et al,2012:Selmanovic and Skaljo,2010). Fig.4.Frame structure (a)EPON.(b)GPON. ONUs are located at different distances from the OLT as shown 2.2.Data link layer in Fig.5(a).When each ONU transmits its upstream traffic during the assigned time slot,there is a possibility that frames from dif- Fig.4(a)presents the EPON frame structure which uses the ferent ONUs collide at some point due to the difference in pro- native Ethernet frame to transmit traffic.The downstream MAC pagation delay.This scenario is illustrated in Fig.5(b).In order to layer has the same operation as a standard Gigabit Ethernet MAC guarantee that the upstream transmissions do not collide,a ran- (GbE MAC).where the traffic is broadcast to all users.In the ging process is performed by the OLT during the activation and downstream frame,the preamble field contains a logical link registration of the ONUs.The ranging process is based on calcu- identifier(LLID)which is a unique identifier assigned by the OLT to lating a specific delay time for each ONU according to its distance each ONU.The ONUs identify received traffic by matching the LLID from the OLT to equalize its transmission delay with other ONUs. of the received frame with its own LLID and if there is a match This delay is called Equalization Delay(ED).Each ONU will store then it will accept the received frame,otherwise it is discarded. and apply its ED to all the upstream transmissions.The ED values For upstream traffic,the MAC layer has been modified by the IEEE are broadcast to other ONUs using Physical Layer Operations and to operate using a TDMA approach,where the OLT assigns a spe- Maintenance (PLOAM)messages and each ONU resumes its cific time slot to every ONU taking into account the distance transmission based on the ED.Fig.6 shows an ONU in a ranging state.While one ONU is active and sending traffic,transmissions between each ONU and the OLT(Chen.2012). from other ONUs must be suspended (Kramer,1999). Fig.4(b)shows the frame structure of GPON.The downstream Multipoint control protocol (MPCP)has been introduced to MAC layer operates in the same manner as a GFP-framed SONET.It facilitate dynamic bandwidth allocation process.This is executed supports a frame of 125 us long that uses TDM to divide the at the MAC layer(Chochliouros,2009).For EPON,MPCP can be run

2.2. Data link layer Fig. 4(a) presents the EPON frame structure which uses the native Ethernet frame to transmit traffic. The downstream MAC layer has the same operation as a standard Gigabit Ethernet MAC (GbE MAC), where the traffic is broadcast to all users. In the downstream frame, the preamble field contains a logical link identifier (LLID) which is a unique identifier assigned by the OLT to each ONU. The ONUs identify received traffic by matching the LLID of the received frame with its own LLID and if there is a match then it will accept the received frame, otherwise it is discarded. For upstream traffic, the MAC layer has been modified by the IEEE to operate using a TDMA approach, where the OLT assigns a spe￾cific time slot to every ONU taking into account the distance between each ONU and the OLT (Chen, 2012). Fig. 4(b) shows the frame structure of GPON. The downstream MAC layer operates in the same manner as a GFP-framed SONET. It supports a frame of 125 ms long that uses TDM to divide the available bandwidth among the users, whilst the upstream MAC layer is based on TDMA. GPON supports two layers of encapsulation where the Ethernet frame is encapsulated into a GPON Encapsulation Method (GEM) frame which is encapsulated again into a GPON Transmission Convergence (GTC) frame. The GTC frame also includes pure ATM cells and TDM traffic. The downstream frame is broadcast to every ONU and the ONUs use the information in the Physical Control Block downstream (PCBd) field to extract its own data. In case there is no data to be transmitted, the downstream frame will be transmitted continuously and utilized for time synchronization (Ricciardi et al., 2012). The upstream frame contains multiple transmission bursts arriving from the ONUs. Along with the pay￾load, each of the upstream burst frames consists of the Physical Layer Overhead (PLOu), a bandwidth allocation interval which contains the Dynamic Bandwidth Report upstream (DBRu), and allocation identifiers (Alloc-IDs). When traffic reaches the OLT, ONU traffic is queued based on Classes of Service (CoS) with a diverse QoS dependent on the type of the Traffic Containers (T￾CONTs) that is specified in the Alloc-ID (Segarra et al., 2013). GPON introduces five types of T-CONTs that provide QoS in the upstream direction. The T-CONT frame is used in GPON to establish a virtual connection between ONU and OLT as well as to manage fragment transmission. 1) T-CONT type 1 Supports fixed bandwidth that is sensitive to time. The jitter of T-CONT type-1 is 0 which enhances the suitability it has for Constant Bit Rate (CBR) traffic. 2) T-CONT type 2 This type supports Assured bandwidth where it has a higher delay than T-CONT 1. It is used with Committed Information Rate (CIR) traffic. 3) T-CONT type 3 Supports assured and non-assured bandwidths providing a guaranteed minimum CIR and surplus Excess Information Rate (EIR). This type is appropriate for Variable Bit Rate (VBR) traffic that does not guarantee delay. 4) T-CONT type 4 Supports Best-Effort services such as Internet browsing, SMTP and FTP. 5) T-CONT type5 This type is mix of all the above T-CONT types. It is appropriate for general traffic flows (Begovic et al., 2011; Tanaka et al., 2010; Ricciardi et al., 2012; Selmanovic and Skaljo, 2010). ONUs are located at different distances from the OLT as shown in Fig. 5(a). When each ONU transmits its upstream traffic during the assigned time slot, there is a possibility that frames from dif￾ferent ONUs collide at some point due to the difference in pro￾pagation delay. This scenario is illustrated in Fig. 5(b). In order to guarantee that the upstream transmissions do not collide, a ran￾ging process is performed by the OLT during the activation and registration of the ONUs. The ranging process is based on calcu￾lating a specific delay time for each ONU according to its distance from the OLT to equalize its transmission delay with other ONUs. This delay is called Equalization Delay (ED). Each ONU will store and apply its ED to all the upstream transmissions. The ED values are broadcast to other ONUs using Physical Layer Operations and Maintenance (PLOAM) messages and each ONU resumes its transmission based on the ED. Fig. 6 shows an ONU in a ranging state. While one ONU is active and sending traffic, transmissions from other ONUs must be suspended (Kramer, 1999). Multipoint control protocol (MPCP) has been introduced to facilitate dynamic bandwidth allocation process. This is executed at the MAC layer (Chochliouros, 2009). For EPON, MPCP can be run Table 1 EPON versus GPON. Features EPON GPON Standard IEEE ITU-T Transmission speed DS: 1.2 Gbps DS: 1.2/2.4 Gbps US: 1.2 Gbps US: 1.5/6.2/1.2/2.4 Gbps Split ratio 1:16 1:64 Line code 8B/10B NRZ Protocol Ethernet ATM Security Not guaranteed AES QoS Not supported Supported FEC Optional RS (255,239) Optional RS (255,239) Fig. 4. Frame structure (a) EPON, (b) GPON. 56 H.S. Abbas, M.A. Gregory / Journal of Network and Computer Applications 67 (2016) 53–74

H.S.Abbas.MA.Gregory Joumal of Network and Computer Applications 67 (2016)53-74 57 occupancy of the buffer status of each T-CONT which are status- reporting Dynamic Bandwidth Allocation (DBA)and traffic- monitoring DBA.In the case of status-reporting DBA,each ONU directly sends status report information to the OLT.Whereas,in the traffic monitoring DBA,the inference of the T-CONT's buffer 10k OLT status at the OLT is reliant on the historical information of band- width use and the amount of defined bandwidth.The header in the downstream frame includes the upstream bandwidth map (BW map)field that depicts the start and end time for upstream transmission for each ONU(Skubic et al..2009;Ansari and Zhang. 2013). (a)ONUs at different location from OLT 125s 1254 3.NG-PON 1 NG-PON1 has been introduced to attain bit rate up to 10 Gbps. OLT The possible scenarios for the upgrade are discussed in this section (Begovic et al.,2011). 3.1.From EPON to XG-EPON ONUI 5 km from OLT XG-EPON inherits many features from the deployed EPON. However,some modifications at the physical layer are required. These modifications are summarized in Table 2 (Gorshe and Mandin,2009). ONU2 10 km from OLT In terms of bit rate,XG-EPON supports two physical layer modes.The first one is symmetric transmission with 10 Gbps.The (b)ONUs upstream collision [24] second mode is asymmetric transmission with 10 Gbps for Fig.5.(a)ONUs at different location from OLT.(b)ONUs upstream collision(Kra downstream transmission and 1 Gbps for upstream transmission mer,1999). (Gorshe and Mandin,2009).The XG-EPON uses the wavelength band 1260-1280 nm for upstream traffic and the wavelength band start 1575-1580 nm for downstream traffic.The line coding applied in XG-EPON is 64B/66B,which is an improved version of 8B/10B. Pre-assigned ET Thus,it reduces the bit-to-baud overhead from 20%to 3%.More- OLT over,FEC was optional in deployed EPON but has become a com- pulsory requirement for XG-EPON with the use of RS(255,223). The supported XG-EPON split ratios are 1:16 with a distance of at least 10 km and a split ratio of 1:32 with a distance of at least ONU response Pre time assigned ET start 20 km (Tanaka et aL,2010). ONU U5书Wm■o The TDM technique used in EPON enables the deployed EPON state and the XG-EPON to coexist.However,a multi-rate OLT is required to provide pre-amplification by utilizing semiconductor optical amplifiers (SOA)(Olmos et al,2011). 3.2.From GPON to XG-GPON Fig.6.Ranging state (Kramer.1999). XG-GPON has similar characteristics to the deployed GPON in one of the two modes.Firstly,in the normal mode,it makes use with some variations in the physical layer that lead to considerable of the two control messages to control the allocation of band- performance improvements.These include split ratio,power width,which are GATE and REPORT messages.In the downstream budget,and reachability(see Table 3).The data link layer framing direction,the GATE messages travel from the OLT to ONUs and and management process have not changed which results in carry the allocated bandwidth information(Chochliouros,2009). reduced migration complexity. In the upstream direction,the REPORT messages that contain bandwidth request information are sent by ONUs to the OLT.A Table 2 specific algorithm is used to determine the grant allocation for G-EPON VS XG-EPON. each of the ONU (Chen,2012).The second mode is the auto- Feature GPON discovery.It is based on three control messages that are REGISTER. XG-GPON REGISTER_REQUEST,and REGISTER_ACK.These messages are used Bit rate 2.4/1.2 Gbps XG-GPON1: to discover and register a new ONU.In addition,it reports infor- Asymmetric 10/2.5 Gbps XG-GPON2: mation about the ONU including MAC address and round trip Symmetric 10 Gbps delays(Chochliouros,2009).In the GPON scenario,grant messages Wavelength(nm) US:1290-1330 US:1260-1280 are sent based on T-CONT.Like EPON,MPCP protocol is imple- DS:1480-1500 DS:1575-1580 mented to facilitate the dynamic bandwidth allocation in GPON. FEC Optional SR(255,239) RS(255.223) Two main approaches supported in GPON to deduce the

in one of the two modes. Firstly, in the normal mode, it makes use of the two control messages to control the allocation of band￾width, which are GATE and REPORT messages. In the downstream direction, the GATE messages travel from the OLT to ONUs and carry the allocated bandwidth information (Chochliouros, 2009). In the upstream direction, the REPORT messages that contain bandwidth request information are sent by ONUs to the OLT. A specific algorithm is used to determine the grant allocation for each of the ONU (Chen, 2012). The second mode is the auto￾discovery. It is based on three control messages that are REGISTER, REGISTER_REQUEST, and REGISTER_ACK. These messages are used to discover and register a new ONU. In addition, it reports infor￾mation about the ONU including MAC address and round trip delays (Chochliouros, 2009). In the GPON scenario, grant messages are sent based on T-CONT. Like EPON, MPCP protocol is imple￾mented to facilitate the dynamic bandwidth allocation in GPON. Two main approaches supported in GPON to deduce the occupancy of the buffer status of each T-CONT which are status￾reporting Dynamic Bandwidth Allocation (DBA) and traffic￾monitoring DBA. In the case of status-reporting DBA, each ONU directly sends status report information to the OLT. Whereas, in the traffic monitoring DBA, the inference of the T-CONT’s buffer status at the OLT is reliant on the historical information of band￾width use and the amount of defined bandwidth. The header in the downstream frame includes the upstream bandwidth map (BW map) field that depicts the start and end time for upstream transmission for each ONU (Skubic et al., 2009; Ansari and Zhang, 2013). 3. NG-PON 1 NG-PON1 has been introduced to attain bit rate up to 10 Gbps. The possible scenarios for the upgrade are discussed in this section (Begovic et al., 2011). 3.1. From EPON to XG-EPON XG-EPON inherits many features from the deployed EPON. However, some modifications at the physical layer are required. These modifications are summarized in Table 2 (Gorshe and Mandin, 2009). In terms of bit rate, XG-EPON supports two physical layer modes. The first one is symmetric transmission with 10 Gbps. The second mode is asymmetric transmission with 10 Gbps for downstream transmission and 1 Gbps for upstream transmission (Gorshe and Mandin, 2009). The XG-EPON uses the wavelength band 1260–1280 nm for upstream traffic and the wavelength band 1575-1580 nm for downstream traffic. The line coding applied in XG-EPON is 64B/66B, which is an improved version of 8B/10B. Thus, it reduces the bit-to-baud overhead from 20% to 3%. More￾over, FEC was optional in deployed EPON but has become a com￾pulsory requirement for XG-EPON with the use of RS (255, 223). The supported XG-EPON split ratios are 1:16 with a distance of at least 10 km and a split ratio of 1:32 with a distance of at least 20 km (Tanaka et al., 2010). The TDM technique used in EPON enables the deployed EPON and the XG-EPON to coexist. However, a multi-rate OLT is required to provide pre-amplification by utilizing semiconductor optical amplifiers (SOA) (Olmos et al., 2011). 3.2. From GPON to XG-GPON XG-GPON has similar characteristics to the deployed GPON with some variations in the physical layer that lead to considerable performance improvements. These include split ratio, power budget, and reachability (see Table 3). The data link layer framing and management process have not changed which results in reduced migration complexity. Fig. 5. (a) ONUs at different location from OLT. (b) ONUs upstream collision (Kra￾mer, 1999). Fig. 6. Ranging state (Kramer, 1999). Table 2 G-EPON VS XG-EPON. Feature GPON XG-GPON Bit rate 2.4/1.2 Gbps XG-GPON1: Asymmetric 10/2.5 Gbps XG-GPON2: Symmetric 10 Gbps Wavelength (nm) US: 1290–1330 US: 1260–1280 DS: 1480–1500 DS: 1575–1580 FEC Optional SR (255,239) RS (255, 223) H.S. Abbas, M.A. Gregory / Journal of Network and Computer Applications 67 (2016) 53–74 57

58 HS.Abbas,M.A.Gregory Journal of Network and Computer Applications 67(2016)53-74 Table 3 (WBF)is required in order to differentiate the PON data traffic 1G-EPON VS XG-EPON (Erzen and Batagelj,2015;Galveias,2012). 2)XG-GPON2 Feature 1G-EPON XG-EPON The major aim of XG-GPON2 is to offer symmetrical transmis- Bit rate Symmetric 1Gbps Symmetric 10Gbps Asymmetric 10/ sion by increasing the upstream transmission up to 10 Gbps. 1Gbps The expectation of spontaneous movement from GPON to the Wavelength (nm)US:1260-1360 US:1260-1280 XG-GPON1/XG-GPON2 has been discussed in the literature. DS:1480-1500 DS:1575-1580 Line code 8B/10B 64B/66B However,a number of drawbacks associated with the coex- FEC Optional Mandatory istence of these technologies have appeared.This approach SR(255,239) RS(255.223) requires different receivers at the OLT in order to receive the upstream data at different transmission speeds.In addition,it is not certain that the fragmentation process will be supported at The XG-GPON is divided into two classes.The first class called a higher transmission rate in the upstream transmission XG-GPON1 provides asymmetrical transmission with 10 Gbps (Kataoka et al.,2011). downstream and 2.5 Gbps upstream.The second class is XG- GPON2 which provides 10 Gbps symmetrical transmission(Erzen 3.3.Mixed scenario and Batagelj.2015:Leng et al..2013).Details about the XG-GPON1 physical layer have been described in ITU-T G.987.2.Whereas,the The mixed scenario is another possible upgrade to NG-PON1.In XG-GPON2 physical layer standard is still to be finalized. this platform,GPON and XG-EPON coexist with each other and operate on the same infrastructure.However,this scenario 1)XG-GPON1 requires suitable wavelength band separation with the help of a According to the G.987.1 recommendation for XG-GPON1,two WDM filter at the OLT in order to eliminate interference (Kataoka scenarios have been proposed to enable migration from GPON etal,2011). to XG-GPON1.The first scenario is a green-field migration which is the replacement of the copper connection into premises with an optical connection.The other option is the PON brown-field 4.ING-PON2 pure technologies migration scenario which is an upgrade of the existing GPON system and this includes replacing or upgrading some of the Studies have been conducted for several NG-PON2 technologies network components such as ONU units or OLT modules if necessary(Erzen and Batagelj.2015). that offer up to 100 Gbps.This includes high speed TDM-PON The downstream wavelength band selected for XG-GPONI is WDM-PON,OCDM-PON,OFDM-PON,and hybrid technologies between 1575 and 1580 nm and the upstream wavelength band (Cvijetic et al.,2010:Luo et al..2012).The pure technologies will be reviewed in this section. is between 1260 and 1280 nm.The wavelength bands were selected to enlarge the guard band between the wavelengths which reduces signal interference (Erzen and Batagelj.2015). 4.1.High speed TDM-PON The coexistence of XG-GPON1 with the deployed GPON is an important criterion when an upgrade is considered.Even TDM-PON allows multiple users to share the same bandwidth though this approach decreases the overall cost,there is an using a single wavelength.A typical TDM-PON structure is shown additional cost associated with wavelength filtering that is in Fig.8.The downstream traffic is broadcast to all users and a required at the ONUs.Fig.7 shows the coexistence scenario, specific time is assigned by the OLT to every ONU to control where the Co consists of two OLTs,one to carry the GPON upstream transmissions.These time slots are allocated in down- connection and the other to carry the XG-GPON1 connection. stream and upstream frames where a complex algorithm is New equipment named as WDMr1 is installed at the CO. required to arrange and assign the bandwidth in order to avoid Multiplexing/demultiplexing the signal of both OLTs and RF is collisions (Esmail and Fathallah,2013:Muciaccia et al.2014). its functionality.On the user's side,a Wavelength Blocking Filter TDM-PON is a simple and cost effective technology,however;it has limited scalability due to the fact that ONUs share bandwidth ONU XG-PON1 ONU1 OLT ONU ONU WDM r1 123 OLT Splitter ONU 1☑3 GPON OLT ONU ◆Downstream 2 Upstream ONU3 ONU Fig.7.Coexistence of GPON and XG-PON1 Fig 8.TDM architecture

The XG-GPON is divided into two classes. The first class called XG-GPON1 provides asymmetrical transmission with 10 Gbps downstream and 2.5 Gbps upstream. The second class is XG￾GPON2 which provides 10 Gbps symmetrical transmission (Eržen and Batagelj, 2015; Leng et al., 2013). Details about the XG-GPON1 physical layer have been described in ITU-T G.987.2. Whereas, the XG-GPON2 physical layer standard is still to be finalized. 1) XG-GPON1 According to the G.987.1 recommendation for XG-GPON1, two scenarios have been proposed to enable migration from GPON to XG-GPON1. The first scenario is a green-field migration which is the replacement of the copper connection into premises with an optical connection. The other option is the PON brown-field migration scenario which is an upgrade of the existing GPON system and this includes replacing or upgrading some of the network components such as ONU units or OLT modules if necessary (Eržen and Batagelj, 2015). The downstream wavelength band selected for XG-GPON1 is between 1575 and 1580 nm and the upstream wavelength band is between 1260 and 1280 nm. The wavelength bands were selected to enlarge the guard band between the wavelengths which reduces signal interference (Eržen and Batagelj, 2015). The coexistence of XG-GPON1 with the deployed GPON is an important criterion when an upgrade is considered. Even though this approach decreases the overall cost, there is an additional cost associated with wavelength filtering that is required at the ONUs. Fig. 7 shows the coexistence scenario, where the CO consists of two OLTs, one to carry the GPON connection and the other to carry the XG-GPON1 connection. New equipment named as WDMr1 is installed at the CO. Multiplexing/demultiplexing the signal of both OLTs and RF is its functionality. On the user’s side, a Wavelength Blocking Filter (WBF) is required in order to differentiate the PON data traffic (Eržen and Batagelj, 2015; Galveias, 2012). 2) XG-GPON2 The major aim of XG-GPON2 is to offer symmetrical transmis￾sion by increasing the upstream transmission up to 10 Gbps. The expectation of spontaneous movement from GPON to the XG-GPON1/XG-GPON2 has been discussed in the literature. However, a number of drawbacks associated with the coex￾istence of these technologies have appeared. This approach requires different receivers at the OLT in order to receive the upstream data at different transmission speeds. In addition, it is not certain that the fragmentation process will be supported at a higher transmission rate in the upstream transmission (Kataoka et al., 2011). 3.3. Mixed scenario The mixed scenario is another possible upgrade to NG-PON1. In this platform, GPON and XG-EPON coexist with each other and operate on the same infrastructure. However, this scenario requires suitable wavelength band separation with the help of a WDM filter at the OLT in order to eliminate interference (Kataoka et al., 2011). 4. ING-PON2 pure technologies Studies have been conducted for several NG-PON2 technologies that offer up to 100 Gbps. This includes high speed TDM-PON, WDM-PON, OCDM-PON, OFDM-PON, and hybrid technologies (Cvijetic et al., 2010; Luo et al., 2012). The pure technologies will be reviewed in this section. 4.1. High speed TDM-PON TDM-PON allows multiple users to share the same bandwidth using a single wavelength. A typical TDM-PON structure is shown in Fig. 8. The downstream traffic is broadcast to all users and a specific time is assigned by the OLT to every ONU to control upstream transmissions. These time slots are allocated in down￾stream and upstream frames where a complex algorithm is required to arrange and assign the bandwidth in order to avoid collisions (Esmail and Fathallah, 2013; Muciaccia et al., 2014). TDM-PON is a simple and cost effective technology, however; it has limited scalability due to the fact that ONUs share bandwidth. Table 3 1G-EPON VS XG-EPON. Feature 1G-EPON XG-EPON Bit rate Symmetric 1Gbps Symmetric 10Gbps Asymmetric 10/ 1Gbps Wavelength (nm) US: 1260–1360 US: 1260–1280 DS: 1480–1500 DS: 1575–1580 Line code 8B/10B 64B/66B FEC Optional Mandatory SR (255,239) RS (255, 223) Fig. 7. Coexistence of GPON and XG-PON1. Fig. 8. TDM architecture. 58 H.S. Abbas, M.A. Gregory / Journal of Network and Computer Applications 67 (2016) 53–74

H.S.Abbas.MA.Gregory Joumal of Network and Computer Applications 67 (2016)53-74 59 Increasing the bit rate for all of the users will be a challenging task ONU1 because every ONU receiver operates at a bit rate that is higher than the bit rate assigned per ONU.Utilizing a high speed digital BS signal processor and field-programmable gate array to increase the OLT bitrate to higher than 10 Gbps increases cost and complexity ONU Z (Muciaccia et al.,2014;Sotiropoulos et al.,2013).In addition,TDM- TX PON is not very secure due to the shared infrastructure which array opens the possibility of eavesdropping and other attacks.More- 1,2,入3，入4 9 over,the variation in the distance between ONUs and the OLT is G ONU 3 another drawback that causes variation in the optical power and RX consequently,the OLT receiver operates in burst mode (Hara et al.. array 2010;Yoshima et al.,2012). In order to upgrade the current TDM-PON to meet the NG- PON2 requirements,a number of approaches have been investi- gated to increase the capacity of TDM-PON,including: d BS TX .Conventional ON OFF Key (OOK)systems:Applying OOK is the easiest way to increase the capacity of TDM-PON.However,this Fig.9.WDM-PON. solution is not favorable for future PONs because it requires a 40 Gbps burst-mode receiver,high cost 40 GHz electronics and transmissions (Duan et al.,2013)and consequently degrades the photonics as well as it requires highly sensitive receivers transmission distance and the receiver sensitivity (Feng et al., (Sotiropoulos et al.,2013). 2014). Due-binary modulation:this scheme is similar to the deployed There are several schemes that can be used to mitigate RB PON system that uses one wavelength for downstream and noise,for example: another one for upstream.Invest such modulation in the downstream grants the ONUs with 20 GHz bandwidth and reduce the disruption(Nesset,2015). Using phase modulation.In (Chow and Yeh,2013)the authors Bit interleaving:This approach employs two wavelengths,one claim that the RB noise can be reduced by using Wavelength- for downstream that supports a 40 Gbps signal and another Shifted amplitude-shift keying (WS-ASK)modulation.In addi- tion,the role of phase modulation non return to zero(PM-NRZ) wavelength for upstream transmission that supports 10 Gbps. modulation format has been investigated in (Talli et al.,2008)to Bit interleaving is introduced in the downstream frame where each ONU is pre-assigned an offset and an interval.This reduce BR noise which can be farther reduced by utilizing an optical filter. technique requires the ONU receiver operating at a rate lower than 40 Gbps.It simplifies the transmission process,reduces Using dual parallel Mach-Zehnder modulator(DP-MZM) power consumption,and reduces the electronic circuitry of the Four-wave mixing(FWM). ONU receiver (Luo et al,2012). Serial 40G NRZ-40G serial Non-Return-To-Zero (NRZ):is A key advantage of WDM-PON is that it allows every ONU to another approach that has been investigated to increase the transmit at the peak speed as the OLT bandwidth is not shared capacity of legacy TDM-PON.However,it has a transmission Thus,it is capable of supporting a higher data rate(Yoshima et al.. distance limitation due to chromatic dispersion and the asso- 2012:Srivastava,2013).Another type of WDM-PON is based on ciated optical power requirement at the receiver (Srivastava splitter and known as WDM-PON wavelength switched in which 2013)】 the power splitter is implemented to distribute incoming signals equally into all ONUs.However,each ONU is required to equip 4.2.WDM-PON with a wavelength filter to select specific wavelength.Although wavelength switched PON considers simple and distributed WDM-PON has been considered as an alternative technology to structure,its signal loss is higher than wavelength routed PON TDM-PON.A typical WDM-PON structure is shown in Fig.9.It (Banerjee et al.,2005).WDM-PON is classified into two classes provides a virtual point-to-point connection between the OLT and based on the number of wavelengths supported and the wave- several ONUs;where,each ONU is assigned a different wavelength length spacing between the individual wavelengths transmitted for transmission. over a single fiber.The first class is Dense WDM(DWDM)and its The major difference between the implementation of WDM- wavelength plan is defined by ITU-T G.694.1 and the second class PON and TDM-PON is that WDM-PON employs a WDM device in is Coarse WDM(CWDM)and its wavelength plan is defined by the ODN such as an Array Wavelength Gratings(AWG)instead of a ITU-T G.694.2.The main objective of DWDM is to increase the power splitter.This leads to dramatic reduction in the power loss network capacity by minimizing the wavelength spacing:CWDM and consequently supports a large number of ONUs(Nesset,2015). aims to reduce the cost where the wavelength spacing is suffi- This type of WDM is called Wavelength routed.Each port of the ciently high to permit the transmitters to be more accurately AWG is assigned to a specific wavelength;each transmitter at the controlled (Muciaccia et al.,2014;Ragheb and Fathallah,2011). ONU transmits a signal on the wavelength that is specified by the In the literature,there are number of approaches that have port.This architecture offers lower insertion loss and a simple been proposed to be implemented in WDM-PON.The approaches ONU receiver structure.However,the OLT is required to install a are discussed below. standard receiver and a wavelength de-multiplexing device. Upstream transmission in a WDM loop back structure is 1)Externally seeded WDM-PON (Sotiropoulos et al.,2013):In a achieved by utilizing a single or two fiber link.In the case of a wavelength-splitter based ODN,a light source is splitted spec- single fiber link,bidirectional transmission of the light and the trally and distributed to reflective ONUs.This approach is modulated signal leads to Rayleigh Backscattering(RB)noise.This mature and available with the commercially existing systems. issue affects the performance of downstream and upstream However,the commercially available systems require that the

Increasing the bit rate for all of the users will be a challenging task because every ONU receiver operates at a bit rate that is higher than the bit rate assigned per ONU. Utilizing a high speed digital signal processor and field-programmable gate array to increase the bitrate to higher than 10 Gbps increases cost and complexity (Muciaccia et al., 2014; Sotiropoulos et al., 2013). In addition, TDM￾PON is not very secure due to the shared infrastructure which opens the possibility of eavesdropping and other attacks. More￾over, the variation in the distance between ONUs and the OLT is another drawback that causes variation in the optical power and consequently, the OLT receiver operates in burst mode (Hara et al., 2010; Yoshima et al., 2012). In order to upgrade the current TDM-PON to meet the NG￾PON2 requirements, a number of approaches have been investi￾gated to increase the capacity of TDM-PON, including:
Conventional ON OFF Key (OOK) systems: Applying OOK is the easiest way to increase the capacity of TDM-PON. However, this solution is not favorable for future PONs because it requires a 40 Gbps burst-mode receiver, high cost 40 GHz electronics and photonics as well as it requires highly sensitive receivers (Sotiropoulos et al., 2013).
Due-binary modulation: this scheme is similar to the deployed PON system that uses one wavelength for downstream and another one for upstream. Invest such modulation in the downstream grants the ONUs with 20 GHz bandwidth and reduce the disruption (Nesset, 2015).
Bit interleaving: This approach employs two wavelengths, one for downstream that supports a 40 Gbps signal and another wavelength for upstream transmission that supports 10 Gbps. Bit interleaving is introduced in the downstream frame where each ONU is pre-assigned an offset and an interval. This technique requires the ONU receiver operating at a rate lower than 40 Gbps. It simplifies the transmission process, reduces power consumption, and reduces the electronic circuitry of the ONU receiver (Luo et al., 2012).
Serial 40G NRZ- 40G serial Non-Return-To-Zero (NRZ): is another approach that has been investigated to increase the capacity of legacy TDM-PON. However, it has a transmission distance limitation due to chromatic dispersion and the asso￾ciated optical power requirement at the receiver (Srivastava, 2013). 4.2. WDM-PON WDM-PON has been considered as an alternative technology to TDM-PON. A typical WDM-PON structure is shown in Fig. 9. It provides a virtual point–to-point connection between the OLT and several ONUs; where, each ONU is assigned a different wavelength for transmission. The major difference between the implementation of WDM￾PON and TDM-PON is that WDM-PON employs a WDM device in the ODN such as an Array Wavelength Gratings (AWG) instead of a power splitter. This leads to dramatic reduction in the power loss and consequently supports a large number of ONUs (Nesset, 2015). This type of WDM is called Wavelength routed. Each port of the AWG is assigned to a specific wavelength; each transmitter at the ONU transmits a signal on the wavelength that is specified by the port. This architecture offers lower insertion loss and a simple ONU receiver structure. However, the OLT is required to install a standard receiver and a wavelength de-multiplexing device. Upstream transmission in a WDM loop back structure is achieved by utilizing a single or two fiber link. In the case of a single fiber link, bidirectional transmission of the light and the modulated signal leads to Rayleigh Backscattering (RB) noise. This issue affects the performance of downstream and upstream transmissions (Duan et al., 2013) and consequently degrades the transmission distance and the receiver sensitivity (Feng et al., 2014). There are several schemes that can be used to mitigate RB noise, for example:
Using phase modulation. In (Chow and Yeh, 2013) the authors claim that the RB noise can be reduced by using Wavelength￾Shifted amplitude-shift keying (WS-ASK) modulation. In addi￾tion, the role of phase modulation non return to zero (PM-NRZ) modulation format has been investigated in (Talli et al., 2008) to reduce BR noise which can be farther reduced by utilizing an optical filter.
Using dual parallel Mach-Zehnder modulator (DP-MZM)
Four-wave mixing (FWM). A key advantage of WDM-PON is that it allows every ONU to transmit at the peak speed as the OLT bandwidth is not shared. Thus, it is capable of supporting a higher data rate (Yoshima et al., 2012; Srivastava, 2013). Another type of WDM-PON is based on splitter and known as WDM-PON wavelength switched in which the power splitter is implemented to distribute incoming signals equally into all ONUs. However, each ONU is required to equip with a wavelength filter to select specific wavelength. Although wavelength switched PON considers simple and distributed structure, its signal loss is higher than wavelength routed PON (Banerjee et al., 2005).WDM-PON is classified into two classes based on the number of wavelengths supported and the wave￾length spacing between the individual wavelengths transmitted over a single fiber. The first class is Dense WDM (DWDM) and its wavelength plan is defined by ITU-T G.694.1 and the second class is Coarse WDM (CWDM) and its wavelength plan is defined by ITU-T G.694.2. The main objective of DWDM is to increase the network capacity by minimizing the wavelength spacing; CWDM aims to reduce the cost where the wavelength spacing is suffi- ciently high to permit the transmitters to be more accurately controlled (Muciaccia et al., 2014; Ragheb and Fathallah, 2011). In the literature, there are number of approaches that have been proposed to be implemented in WDM-PON. The approaches are discussed below. 1) Externally seeded WDM-PON (Sotiropoulos et al., 2013): In a wavelength-splitter based ODN, a light source is splitted spec￾trally and distributed to reflective ONUs. This approach is mature and available with the commercially existing systems. However, the commercially available systems require that the Fig. 9. WDM-PON. H.S. Abbas, M.A. Gregory / Journal of Network and Computer Applications 67 (2016) 53–74 59

60 HS.Abbas,M.A.Gregory Journal of Network and Computer Applications 67(2016)53-74 wavelength splitter operate over the power splitter,which improved and the potential eavesdropping issue is eliminated imposes the major challenge in terms of link budget.Addition- (Srivastava,2013;Urata et al.,2012). ally,the possibility of attaining more than 1 Gbps of data rate is Despite these features,a number of restrictions make WDM- not clear as it exceeds the capability of the current system PON an inappropriate technology for NG-PON2.With the limita- (Nesset,2015). tion of the number of wavelengths allowed in the system and with 2)Wavelength re-use WDM-PON (Nesset,2015):This approach the large bandwidth requirement,it leads to inefficient utilization assigns a wavelength to each user for downstream and of the bandwidth (Hernandez et al.,2012).Additionally,the cost is upstream transmission.The re-use of the wavelength is enabled a prominent issue in WDM-PON where it increases due to the by the transmitter based on semiconductor amplifier.This need for extra equipment such as colored ONUs and a transceiver amplifier modulates the downstream signal in inverse Return- for every wavelength at the OLT(Sotiropoulos et al.,2013;Urata to-Zero format and the upstream signal in Return-to-Zero etal,2012). format (Nesset,2015). 3)Tunable WDM-PON (Begovic et al.2011):This approach is 4.3.OCDM-PON based on a low cost tunable transmitter module instead of the conventional module.The reduction of the cost is achieved by Introducing OCDM-PON technology leads to considerable removing thermoelectric coolers and the wave-lockers from the improvements for NG-PON2.The advantages include highly effi- conventional modules.Tuning at the upstream is performed cient use of bandwidth,good correlation performance,asynchro- utilizing the shared OLT based wave-locker.However,tunable nous transmission,flexibility of user allocation,low signal pro- receivers are needed at each ONU to perform colorless function cessing latency as well as improving network security (Yoshima (Nesset,2015). et al.,2013:Kataoka et al.,2010). 4)Ultra-dense Coherent WDM-PON (Begovic et al.2011):This OCDM can be classified into two main categories:coherent approach is based on coherent detection where the channels are system and incoherent system.In coherent system,OCDM is tightly spaced(around 3 GHz )1 Gbps data rate is allocated to implemented through a bipolar approach that requires informa- every user utilizing dedicated Quadrature Phase Shift Keying tion about the phase of the carriers.On the other hand,the inco- (QPSK)modulated wavelength.However,the transmitters and herent system is implemented through a unipolar approach. the receivers are very complex systems and expensive.Thus, Owing to the simplicity of incoherent hardware as well as its non- more improvements in photonic integration are essential to be reliance on phase synchronization detection,incoherent system used in practical implementation(Nesset,2015). has emerged as the preferred detection scheme.Fig.10 shows the 5)Self-seeded WDM-PON (Tanaka et al,2010):In this scheme,the basic structure of the OCDM network,which has four main com- seed light of the ONU is self-generated by a reflector at the ponents including transmitter,encoder,decoder,and the receiver. common port of the wavelength splitter.However,the length of At the transmitter,an information source provides a data bit for a the drop fiber(the fiber between the splitter and the ONU)is laser at every T second.The encoder then multiplies the data bit limited (Nesset,2015). "when it equals 1"by a code-word.The code-word can be formed by one-dimensional encoding using the time or wavelength Several schemes have been proposed to allow migration from domain or by a two-dimensional encoding scheme,which is a TDM-PON to WDM-PON.Hybrid TDM/WDM PON or SUCCESS- combination of both domains.Yet,recent studies have shown HPON (The Stanford University aCCESS)provides a cost effective advantages of three dimensional codes(Yen and Chen,2015:Wang and smooth migration path from TDM to WDM.SUCCESS-HPON is and Chang.2015;Jindal and Gupta,2012;Garg and Kaler,2013; based on the lasers at ONUs and shares tunable WDM components Shum,2015).The pulses generated are referred to as chips and at the OLT.Hence,it achieves bandwidth equivalent to the pure have a duration of Tc=T/n,where I donates the duration of each WDM-PON bandwidth with lower costs (Gutierrez et al..2005). bit and n denotes the code length. In (Chow and Yeh,2013),another migration scheme has been The multiplexed signal is broadcast to all of the users.The proposed.In this scheme,the differential phase-shift keying (DPSK)technique is used for the downstream signal.The signal arrives at the receiver and passes through the decoder.The decoder matches the code and accepts only the intended user's wavelength-shifted amplitude-shift keying (WS-ASK)is used for the upstream signal.At the ONU,an optical filter is implemented signal.Then the output of the decoder passes through photo- to select the intended downstream wavelength and to demodulate detection and integration.Later,the output power is sampled for each bit interval and compared to the threshold value to provide the downstream signal.The upstream signal is generated by signal demodulation that is based on reusing the downstream wave- length.Another benefit of this scheme,beside the smooth OLT migration,is that it does not require any changes to the existing r】 ONU1 fiber infrastructure. In (Shachaf et al.,2007),a multi-PON architecture based on a Modulator Decoder rmation Receive Encoder 1 coarse AWG at the OLT has been introduced to allow smooth migration path from TDM-PON to WDM-PON.The AWG is designed to support several TDM-PON and WDM-PON by employing tunable laser at the OLT.In addition,the splitter in the distribution side is replaced by a multiplexing unit that works to justify parallel processes of TDM-PON and WDM-PON.This pro- ONU N aser N vides the required bandwidth for the ONUs.At the ONU side, RSOAs is required to implement colorless transceivers,hence,no Decoder Recelve N change is needed at the customers'side. N The multiple-wavelength characteristic in WDM-PON offers several unique features.Firstly,each user can upgrade its capacity without the need for pre-designing a new fiber.Furthermore,the upgrade will not impact other users.Secondly.security is Fig.10.OCDM architecture

wavelength splitter operate over the power splitter, which imposes the major challenge in terms of link budget. Addition￾ally, the possibility of attaining more than 1 Gbps of data rate is not clear as it exceeds the capability of the current system (Nesset, 2015). 2) Wavelength re-use WDM-PON (Nesset, 2015): This approach assigns a wavelength to each user for downstream and upstream transmission. The re-use of the wavelength is enabled by the transmitter based on semiconductor amplifier. This amplifier modulates the downstream signal in inverse Return￾to-Zero format and the upstream signal in Return-to-Zero format (Nesset, 2015). 3) Tunable WDM-PON (Begovic et al., 2011): This approach is based on a low cost tunable transmitter module instead of the conventional module. The reduction of the cost is achieved by removing thermoelectric coolers and the wave-lockers from the conventional modules. Tuning at the upstream is performed utilizing the shared OLT based wave-locker. However, tunable receivers are needed at each ONU to perform colorless function (Nesset, 2015). 4) Ultra-dense Coherent WDM-PON (Begovic et al., 2011): This approach is based on coherent detection where the channels are tightly spaced (around 3 GHz ). 1 Gbps data rate is allocated to every user utilizing dedicated Quadrature Phase Shift Keying (QPSK) modulated wavelength. However, the transmitters and the receivers are very complex systems and expensive. Thus, more improvements in photonic integration are essential to be used in practical implementation (Nesset, 2015). 5) Self-seeded WDM-PON (Tanaka et al., 2010): In this scheme, the seed light of the ONU is self-generated by a reflector at the common port of the wavelength splitter. However, the length of the drop fiber (the fiber between the splitter and the ONU) is limited (Nesset, 2015). Several schemes have been proposed to allow migration from TDM-PON to WDM-PON. Hybrid TDM/WDM PON or SUCCESS￾HPON (The Stanford University aCCESS) provides a cost effective and smooth migration path from TDM to WDM. SUCCESS-HPON is based on the lasers at ONUs and shares tunable WDM components at the OLT. Hence, it achieves bandwidth equivalent to the pure WDM-PON bandwidth with lower costs (Gutierrez et al., 2005). In (Chow and Yeh, 2013), another migration scheme has been proposed. In this scheme, the differential phase-shift keying (DPSK) technique is used for the downstream signal. The wavelength-shifted amplitude-shift keying (WS-ASK) is used for the upstream signal. At the ONU, an optical filter is implemented to select the intended downstream wavelength and to demodulate the downstream signal. The upstream signal is generated by signal demodulation that is based on reusing the downstream wave￾length. Another benefit of this scheme, beside the smooth migration, is that it does not require any changes to the existing fiber infrastructure. In (Shachaf et al., 2007), a multi-PON architecture based on a coarse AWG at the OLT has been introduced to allow smooth migration path from TDM-PON to WDM-PON. The AWG is designed to support several TDM-PON and WDM-PON by employing tunable laser at the OLT. In addition, the splitter in the distribution side is replaced by a multiplexing unit that works to justify parallel processes of TDM-PON and WDM-PON. This pro￾vides the required bandwidth for the ONUs. At the ONU side, RSOAs is required to implement colorless transceivers, hence, no change is needed at the customers' side. The multiple-wavelength characteristic in WDM-PON offers several unique features. Firstly, each user can upgrade its capacity without the need for pre-designing a new fiber. Furthermore, the upgrade will not impact other users. Secondly, security is improved and the potential eavesdropping issue is eliminated (Srivastava, 2013; Urata et al., 2012). Despite these features, a number of restrictions make WDM￾PON an inappropriate technology for NG-PON2. With the limita￾tion of the number of wavelengths allowed in the system and with the large bandwidth requirement, it leads to inefficient utilization of the bandwidth (Hernandez et al., 2012). Additionally, the cost is a prominent issue in WDM-PON where it increases due to the need for extra equipment such as colored ONUs and a transceiver for every wavelength at the OLT (Sotiropoulos et al., 2013; Urata et al., 2012). 4.3. OCDM-PON Introducing OCDM-PON technology leads to considerable improvements for NG-PON2. The advantages include highly effi- cient use of bandwidth, good correlation performance, asynchro￾nous transmission, flexibility of user allocation, low signal pro￾cessing latency as well as improving network security (Yoshima et al., 2013; Kataoka et al., 2010). OCDM can be classified into two main categories: coherent system and incoherent system. In coherent system, OCDM is implemented through a bipolar approach that requires informa￾tion about the phase of the carriers. On the other hand, the inco￾herent system is implemented through a unipolar approach. Owing to the simplicity of incoherent hardware as well as its non￾reliance on phase synchronization detection, incoherent system has emerged as the preferred detection scheme. Fig. 10 shows the basic structure of the OCDM network, which has four main com￾ponents including transmitter, encoder, decoder, and the receiver. At the transmitter, an information source provides a data bit for a laser at every T second. The encoder then multiplies the data bit “when it equals 1” by a code-word. The code-word can be formed by one-dimensional encoding using the time or wavelength domain or by a two-dimensional encoding scheme, which is a combination of both domains. Yet, recent studies have shown advantages of three dimensional codes (Yen and Chen, 2015; Wang and Chang, 2015; Jindal and Gupta, 2012; Garg and Kaler, 2013; Shum, 2015). The pulses generated are referred to as chips and have a duration of Tc¼T/n, where T donates the duration of each bit and n denotes the code length. The multiplexed signal is broadcast to all of the users. The signal arrives at the receiver and passes through the decoder. The decoder matches the code and accepts only the intended user's signal. Then the output of the decoder passes through photo￾detection and integration. Later, the output power is sampled for each bit interval and compared to the threshold value to provide Fig. 10. OCDM architecture. 60 H.S. Abbas, M.A. Gregory / Journal of Network and Computer Applications 67 (2016) 53–74

H.S.Abbas.MA.Gregory Joumal of Network and Computer Applications 67 (2016)53-74 61 Table 4 the partitioning process and distribute the total bandwidth over Types of OCDM codes. the subcarriers,over the timeslots or on both to different ONUs Name of Bipolar/ Coherent Codes according to their demand.For downstream transmission,each encoding Unipolar incoherent ONU recognizes its own OFDM subcarriers and/or time slots based on information obtained by the OLT's schedule.For upstream 1D Code Pulse amplitude Unipolar Incoherent -00C PC transmission,the OLT works to assemble the sub-frames coming from different ONUs to generate a complete OFDMA frame(Cvi- -HCC jetic,2012.2010). Pulse phase Bipolar Coherent -M-sequence Various benefits can be achieved by applying the OFDM mul- Spectral Unipolar Incoherent Gold code amplitude tiplexing technique.Firstly,the total cost is reduced because of the Spectral Phase Bipolar Coherent -Walsh- cost of the complex optical modulation at the OLT can be shared Hadamard Codes between the users.In addition,the ONU implements a simple and 2D Code Wavelength- Unipolar Incoherent 2-D WH/TS OOC Hopping Time inexpensive optical modulation in order to identify data for that spreading ONU.Moreover,OFDM-PON technology helps to reduce the cost by Space encoding Unipolar Incoherent 2-D Space Codes using cost-effective electronic devices instead of optical devices. 3D Code Three dimension Unipolar Incoherent Space/time/wave- encoding The overlapping characteristic of OFDM produces no interference codes length or Polarization/time/ which results in the effective utilization of the spectral resources. wavelength codes Furthermore,in comparison with other technologies,OFDM-PON provides a two dimensional bandwidth map with finer granularity. offering flexibility for assigning the bandwidth at different levels. Despite the enormous advantages of OFDM,some limitations have been identified.OFDM-PON requires complex receivers that are reliant on high speed DSP and FPGAs.Furthermore,OFDM-PON an estimation of the transmitted bit (Zahedi and Salehi,2000: is disadvantaged by noise and a high Peak Average Power Ratio Anaman and Prince,2012). (PAPR).The PAPR issue appears as a result of sinusoidal signals The performance of the OCDM network is reliant on the per- from multiple OFDM subcarriers that interfere constructively in formance of the address codes that have been designed to be orthogonal in order to reduce Multiple-Access Interference(MAl) the time domain.This generates a higher amplitude value than the and performance of the receiver structure that must successfully average amplitude value of the signal.The noise is generated as a operate in an environment including various noise sources(Sri and result of interference when multiple signals from multiple users Sundararajan,2013;Zahedi and Salehi,2000). are detected on the photodiode at the OLT.Such interference leads Various types of codes have been investigated and the codes to performance degradation (Cvijetic,2012:Cano et al..2013) and corresponding coding devices are shown in Table 4(Yin and D. Frequency offset is also a disadvantage of OFDM technique which 2007).In Table 5 a comparison of different OCDM receiver struc- occurs due to mismatch of carrier frequencies (Bindhaiq et al.. tures is presented(Zahedi and Salehi.2000). 2015) 4.4.OFDM-PON 4.5.UNI-PON OFDM-PON is considered as the most attractive system because High costs,wastage of resources are the main limitations in the of its scalability and ability to provide bit rate up to 40 Gbps per user.OFDM for NG-PON2 is used as multiplexing technique as it is existing multiplexing techniques insist researchers to think about more appropriate and effective methods.Some researchers came- spectrally efficient modulation method.OFDM technique offers flexibility on dynamic bandwidth allocation,enables multiple up with the idea of UNI-PON (Cloud-Radio Access Network services,and attains high spectral efficiency.OFDM utilizes a large Onlinel). number of orthogonal subcarriers that are closely-spaced in order In UNI-PON data manipulation is done at OLT using cloud to carry traffic.These subcarriers are modulated at a low symbol computing.The advantages of UNI-PON include access of all ser- rate utilizing conventional or advanced modulation techniques vices for all users,lower cost,and connectivity of radio remote (Muciaccia et al.,2014). units,multi-rate adjustment,and dynamic bandwidth allocation. OFDM-PON architecture is similar to the conventional PON.It In(Liu et al.2012).a physical layer adaptive algorithm is used to utilizes two different wavelengths for downstream and upstream attain multi-rate and dynamic bandwidth allocation.With the transmissions (Shaddad et al.,2014).The OLT generates multiple rapid advancement in technology the systems should be resilient orthogonal subcarriers that are assigned to different ONUs.Each to adopt future changes.Therefore,UNI-PON can be a suitable subcarrier is divided into different time slots.The OLT performs choice for future networks. Table 5 comparison of several reciever structures. Receiver structure Characteristics Passive correlation receiver Cheap.not suitable for high speed applications.high power loss Active correlation receiver Expensive,supports high speed applications. Optical hard limiter and passive correlation receiver Not suitable for high speed applications,relies on the availability of optical hard limiter Optical hard limiter and active correlation receiver Supports high speed applications,relies on the availability of an optical hard limiter Double optical hard limiter and passive correlation receiver Not suitable for high speed applications,expensive,good performance. Double optical hard limiter and active correlation receiver Supports medium to high speed applications,high power loss High speed chip detector Not suitable for high speed applications.low power loss

an estimation of the transmitted bit (Zahedi and Salehi, 2000; Anaman and Prince, 2012). The performance of the OCDM network is reliant on the per￾formance of the address codes that have been designed to be orthogonal in order to reduce Multiple-Access Interference (MAI) and performance of the receiver structure that must successfully operate in an environment including various noise sources (Sri and Sundararajan, 2013; Zahedi and Salehi, 2000). Various types of codes have been investigated and the codes and corresponding coding devices are shown in Table 4 (Yin and D, 2007). In Table 5 a comparison of different OCDM receiver struc￾tures is presented (Zahedi and Salehi, 2000). 4.4. OFDM-PON OFDM-PON is considered as the most attractive system because of its scalability and ability to provide bit rate up to 40 Gbps per user. OFDM for NG-PON2 is used as multiplexing technique as it is spectrally efficient modulation method. OFDM technique offers flexibility on dynamic bandwidth allocation, enables multiple services, and attains high spectral efficiency. OFDM utilizes a large number of orthogonal subcarriers that are closely–spaced in order to carry traffic. These subcarriers are modulated at a low symbol rate utilizing conventional or advanced modulation techniques (Muciaccia et al., 2014). OFDM-PON architecture is similar to the conventional PON. It utilizes two different wavelengths for downstream and upstream transmissions (Shaddad et al., 2014). The OLT generates multiple orthogonal subcarriers that are assigned to different ONUs. Each subcarrier is divided into different time slots. The OLT performs the partitioning process and distribute the total bandwidth over the subcarriers, over the timeslots or on both to different ONUs according to their demand. For downstream transmission, each ONU recognizes its own OFDM subcarriers and/or time slots based on information obtained by the OLT's schedule. For upstream transmission, the OLT works to assemble the sub-frames coming from different ONUs to generate a complete OFDMA frame (Cvi￾jetic, 2012, 2010). Various benefits can be achieved by applying the OFDM mul￾tiplexing technique. Firstly, the total cost is reduced because of the cost of the complex optical modulation at the OLT can be shared between the users. In addition, the ONU implements a simple and inexpensive optical modulation in order to identify data for that ONU. Moreover, OFDM-PON technology helps to reduce the cost by using cost-effective electronic devices instead of optical devices. The overlapping characteristic of OFDM produces no interference which results in the effective utilization of the spectral resources. Furthermore, in comparison with other technologies, OFDM-PON provides a two dimensional bandwidth map with finer granularity, offering flexibility for assigning the bandwidth at different levels. Despite the enormous advantages of OFDM, some limitations have been identified. OFDM-PON requires complex receivers that are reliant on high speed DSP and FPGAs. Furthermore, OFDM-PON is disadvantaged by noise and a high Peak Average Power Ratio (PAPR). The PAPR issue appears as a result of sinusoidal signals from multiple OFDM subcarriers that interfere constructively in the time domain. This generates a higher amplitude value than the average amplitude value of the signal. The noise is generated as a result of interference when multiple signals from multiple users are detected on the photodiode at the OLT. Such interference leads to performance degradation (Cvijetic, 2012; Cano et al., 2013). Frequency offset is also a disadvantage of OFDM technique which occurs due to mismatch of carrier frequencies (Bindhaiq et al., 2015). 4.5. UNI-PON High costs, wastage of resources are the main limitations in the existing multiplexing techniques insist researchers to think about more appropriate and effective methods. Some researchers came￾up with the idea of UNI-PON (Cloud-Radio Access Network [Online]). In UNI-PON data manipulation is done at OLT using cloud computing. The advantages of UNI-PON include access of all ser￾vices for all users, lower cost, and connectivity of radio remote units, multi-rate adjustment, and dynamic bandwidth allocation. In (Liu et al., 2012), a physical layer adaptive algorithm is used to attain multi-rate and dynamic bandwidth allocation. With the rapid advancement in technology the systems should be resilient to adopt future changes. Therefore, UNI-PON can be a suitable choice for future networks. Table 4 Types of OCDM codes. Name of encoding Bipolar/ Unipolar Coherent/ incoherent Codes 1D Code Pulse amplitude Unipolar Incoherent – OOC – PC – QCC – HCC Pulse phase Bipolar Coherent – M-sequence Spectral amplitude Unipolar Incoherent – Gold code Spectral Phase Bipolar Coherent – Walsh￾Hadamard Codes 2D Code Wavelength￾Hopping Time spreading Unipolar Incoherent 2-D WH/TS OOC Space encoding Unipolar Incoherent 2-D Space Codes 3D Code Three dimension encoding Unipolar codes Incoherent Space/time/ wave￾length or Polarization/time/ wavelength codes Table 5 comparison of several reciever structures. Receiver structure Characteristics Passive correlation receiver Cheap, not suitable for high speed applications , high power loss Active correlation receiver Expensive, supports high speed applications. Optical hard limiter and passive correlation receiver Not suitable for high speed applications, relies on the availability of optical hard limiter Optical hard limiter and active correlation receiver Supports high speed applications, relies on the availability of an optical hard limiter Double optical hard limiter and passive correlation receiver Not suitable for high speed applications, expensive, good performance. Double optical hard limiter and active correlation receiver Supports medium to high speed applications, high power loss High speed chip detector Not suitable for high speed applications, low power loss H.S. Abbas, M.A. Gregory / Journal of Network and Computer Applications 67 (2016) 53–74 61

62 H.S.Abbas,M.A.Gregory Journal of Network and Computer Applications 67(2016)53-74 4.6.PDM-PON ONU PDM-PON technology uses orthogonal polarizations at the same wavelength.It is capacity efficient but due to the polarization behavior in the fiber,it would be very hard to separate the signals at the receiving end (Muciaccia et al.,2014). AE A1,A2,A3,4 ONU OLT 5.ITU-T NG-PON2 technology 5.1.TWDM-PON In April 2012,hybrid TDM and WDM(TWDM-PON)technology was selected as the multiplexing technique for NG-PON2 by the ONU FSAN community (Luo et al.,2013).The decision was made based ONU on several factors including:technology maturity,system perfor- mance,power consumption and cost (Luo et al.,2012).In July Fig.12.Wavelength routed hybrid PON. 2013,the selection of TWDM-PON was confirmed by ITU-T"under the G.989 series"and it was named as NG-PON2 (Murano.2014). use of identical colorless ONUs and supports a higher number of TWDM-PON combines the advantages of the high capacity wavelengths than the stacked PON(Kramer et al.,2012). provided by TDM and the large number of wavelengths provided TWDM-PON is broadly classified into static and dynamic by WDM into one architecture by transmitting TDM frames to approaches.For the static approach,the downstream and several users over several wavelengths (Ragheb and Fathallah, upstream wavelengths specified for ONUs are static and do not 2011:Hernandez et al.,2012). change during the process.On the other hand,in the dynamic The basic structure of TWDM-PON consists of four techniques approach the wavelength is able to change dynamically based on of XG-GPON1s.They are stacked by utilizing four pairs of wave- operation and communication needs.As a result of frequent lengths.Fig.11 shows TWDM-PON and the wavelength pairs that changes in wavelengths,ONUs are required to deploy burst mode are“{u1,5.{2,.6.{03.7}and{4,8}”(TU-T,G.989.1.2014). receivers.However,the dynamic approach has advantages over Each XG-GPON1 provides 10 Gbps and 2.5Gbps of data rate in the static counterpart because it allows load balancing,power downstream and upstream transmissions respectively.Thus, saving,and resilience(Ragheb and Fathallah,2011). TWDM-PON increases the bit rate up to 40 Gbps for downstream A major limitation of the TWDM-PON is the Crosstalk issue that transmission and 10 Gbps for upstream transmission (Luo et al., rises up due to the rival power from the multiple ONUs.A sig- 2013). nificant crosstalk occurs at OLT receiver due to staking of a mul- Implementing a simple network requires that each ONU is tiple wavelength channels and the presence of dynamic power equipped with programmable transmitter and receiver that can be range at the upstream transmission (Bonk et al,2015). tuned to any wavelengths (ITU-T.G.989.1,2014).Additionally,such Several studies have been conducted addressing this issue.In a network requires an optical amplifier at the OLT in order to (Poehlmann et al.,2014),the sources of crosstalk in the upstream promote the downstream signal and to pre-amplify the upstream transmission have been analyzed and a number of requirements at signals.Therefore,TWDM-PON obtains a higher power budget the OLT receiver have been introduced.The paper analyzes three than XG-GPON1.The ODN is still passive where OLT is equipped cases of crosstalk in TWDM-PON,each with specific requirements. with the amplifier,multiplexor,and the de-multiplexor(Luo et al., The cases are discussed below. 2013). Another implementation of TWDM-PON is referred to as Case 1:When ONUs not-transmitting (WNT). wavelength routed hybrid PON that works by combining the Case 2:Insufficient isolation of WDM channels in the wave- power splitters and AWG (see Fig.12).This configuration makes length demultiplexer (WM)of the OLT receiver(IWM). Case 3:out-of channel optical power from neighboring channels (OCP). In addition,the paper proposes mitigations in case of the requirements are difficult to realize. In (Han Hyub et al.,2014).two methods of ONU power leaving to mitigate the inter channel crosstalk of TWDM-PON in the upstream transmission have been proposed.The first method is based on transmitter bias current and/or modulation current that are low cost method.The other method is based on implementing SOA or variable optical attenuator(VOA)in the transmitter. In TWDM-PON,ONUs required to be colorless to enable re-use ONU3 of the wavelength.RSOA ONU optical transceiver is considered as the most preferable option amongst other colorless ONU due to its simplicity and colorlessness.It helps to eliminate the volume provisioning problem of the ONUs in the WDM-PON.However, RSOA ONU leads to impairments when operating in full-duplex mode.Numbers of approaches to address the optical modulation formats and compensating techniques have been proposed to Fig.11.TWDM-PON overcome the bandwidth noise and crosstalk challenges (Schrenk

4.6. PDM-PON PDM-PON technology uses orthogonal polarizations at the same wavelength. It is capacity efficient but due to the polarization behavior in the fiber, it would be very hard to separate the signals at the receiving end (Muciaccia et al., 2014). 5. ITU-T NG-PON2 technology 5.1. TWDM-PON In April 2012, hybrid TDM and WDM (TWDM-PON) technology was selected as the multiplexing technique for NG-PON2 by the FSAN community (Luo et al., 2013). The decision was made based on several factors including; technology maturity, system perfor￾mance, power consumption and cost (Luo et al., 2012). In July 2013, the selection of TWDM-PON was confirmed by ITU-T “under the G.989 series” and it was named as NG-PON2 (Murano, 2014). TWDM-PON combines the advantages of the high capacity provided by TDM and the large number of wavelengths provided by WDM into one architecture by transmitting TDM frames to several users over several wavelengths (Ragheb and Fathallah, 2011; Hernandez et al., 2012). The basic structure of TWDM-PON consists of four techniques of XG-GPON1s. They are stacked by utilizing four pairs of wave￾lengths. Fig. 11 shows TWDM-PON and the wavelength pairs that are “{λ1, λ5}, {λ2, λ6}, {λ3, λ7} and {λ4, λ8}” (ITU-T, G.989.1, 2014). Each XG-GPON1 provides 10 Gbps and 2.5Gbps of data rate in downstream and upstream transmissions respectively. Thus, TWDM-PON increases the bit rate up to 40 Gbps for downstream transmission and 10 Gbps for upstream transmission (Luo et al., 2013). Implementing a simple network requires that each ONU is equipped with programmable transmitter and receiver that can be tuned to any wavelengths (ITU-T, G.989.1, 2014). Additionally, such a network requires an optical amplifier at the OLT in order to promote the downstream signal and to pre-amplify the upstream signals. Therefore, TWDM-PON obtains a higher power budget than XG-GPON1. The ODN is still passive where OLT is equipped with the amplifier, multiplexor, and the de-multiplexor (Luo et al., 2013). Another implementation of TWDM-PON is referred to as wavelength routed hybrid PON that works by combining the power splitters and AWG (see Fig. 12). This configuration makes use of identical colorless ONUs and supports a higher number of wavelengths than the stacked PON (Kramer et al., 2012). TWDM-PON is broadly classified into static and dynamic approaches. For the static approach, the downstream and upstream wavelengths specified for ONUs are static and do not change during the process. On the other hand, in the dynamic approach the wavelength is able to change dynamically based on operation and communication needs. As a result of frequent changes in wavelengths, ONUs are required to deploy burst mode receivers. However, the dynamic approach has advantages over the static counterpart because it allows load balancing, power saving, and resilience (Ragheb and Fathallah, 2011). A major limitation of the TWDM-PON is the Crosstalk issue that rises up due to the rival power from the multiple ONUs. A sig￾nificant crosstalk occurs at OLT receiver due to staking of a mul￾tiple wavelength channels and the presence of dynamic power range at the upstream transmission (Bonk et al., 2015). Several studies have been conducted addressing this issue. In (Poehlmann et al., 2014), the sources of crosstalk in the upstream transmission have been analyzed and a number of requirements at the OLT receiver have been introduced. The paper analyzes three cases of crosstalk in TWDM-PON, each with specific requirements. The cases are discussed below. ● Case 1: When ONUs not-transmitting (WNT). ● Case 2: Insufficient isolation of WDM channels in the wave￾length demultiplexer (WM) of the OLT receiver (IWM). ● Case 3: out-of channel optical power from neighboring channels (OCP). In addition, the paper proposes mitigations in case of the requirements are difficult to realize. In (Han Hyub et al., 2014), two methods of ONU power leaving to mitigate the inter channel crosstalk of TWDM-PON in the upstream transmission have been proposed. The first method is based on transmitter bias current and/or modulation current that are low cost method. The other method is based on implementing SOA or variable optical attenuator (VOA) in the transmitter. In TWDM-PON, ONUs required to be colorless to enable re-use of the wavelength. RSOA ONU optical transceiver is considered as the most preferable option amongst other colorless ONU due to its simplicity and colorlessness. It helps to eliminate the volume provisioning problem of the ONUs in the WDM-PON. However, RSOA ONU leads to impairments when operating in full-duplex mode. Numbers of approaches to address the optical modulation formats and compensating techniques have been proposed to Fig. 11. TWDM-PON. overcome the bandwidth noise and crosstalk challenges (Schrenk Fig. 12. Wavelength routed hybrid PON. 62 H.S. Abbas, M.A. Gregory / Journal of Network and Computer Applications 67 (2016) 53–74