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 (Schrenk4.6. 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