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 challenges 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 conventional PON architecture is presented in Fig. 1 (Ragheb and Fathallah, 2012). In the figure, it can be seen that the PON architecture consists of an Optical Line Terminal (OLT), Optical Distribution 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 premises 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 (NGPON2). The evolution of the PON architectures and their corresponding 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 NGPON2 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 Multiplexing 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/TDMPON, OFDM/WDM-PON, and OFDM/TDM-PON. Among them, hybrid TDM/WDM PON (TWDM-PON) has been selected as the base element 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 effectiveness (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 NGPON2 technologies. This paper also studies the relevant contributions 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 TWDMPON. 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 presented. Section 12 outlines reliability aspects and Section 13 outlines 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 specific 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 payload, 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 (TCONTs) 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 different ONUs collide at some point due to the difference in propagation delay. This scenario is illustrated in Fig. 5(b). In order to guarantee that the upstream transmissions do not collide, a ranging process is performed by the OLT during the activation and registration of the ONUs. The ranging process is based on calculating 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 bandwidth, 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 autodiscovery. 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 information 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 implemented 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 statusreporting Dynamic Bandwidth Allocation (DBA) and trafficmonitoring 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 bandwidth 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%. Moreover, FEC was optional in deployed EPON but has become a compulsory 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 (Kramer, 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