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 architectureThe 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