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)GPONis 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