Figure 1. Generalized Next Generation Passive Optical Network Architecture
Optical fiber is one of the effective modes of transmission to access the network. They are widely used in telecommunications since they allow sending large amount of data at a greater distance and have larger bandwidth than other forms of communication. The main objective of an optical access network is to provide access to the user even in long-haul distance. A fiber link from the central office provides services to multiple users using transmitters over a single fiber link. This paper reviewed the technical options and comprehensive study on the next generation of optical access solutions. To upgrade the optical access networks, it should satisfy the bandwidth demand co-existing with previous technology requirements. There are lot of technologies used to access the NG-PON, but Full Service Access Network (FSAN) selects the TWDM as the primary solution for second generation Passive Optical Network (NG-PON2). This TWDM is the combination of both Time Division Multiplexing (TDM) and Wavelength Division Multiplexing (WDM). TWDM is largely deployed in NG-PON2 access networks because of its high data rates. In addition, this paper previews the various models in TWDM networks.
Optical communication uses fiber as the medium and the light as the carrier for communication between the transmitters and receivers. This optical communication is widely deployed in all geographical locations including oceans, forests, deserts, etc. It forms the network infrastructure among all countries. There are more advantages in optical communication like low transmission loss, less susceptible to electromagnetic interferences, higher data rates, wide bandwidth, etc. To transmit the data over long-haul, network operators have to slightly change the legacy network infrastructures. The changes are based on including some components like multiplexers, isolators, couplers, filters, amplifiers, etc. By including these components, the optical communication is used over the long distance (ojo, 2016). Multiplexers are used to transfer the data at high rates over a single fiber than to transmit the data at lower rates among multiple fibers.
Based on its multiplexing technique, the transmit data rate and its capacity can raise. There are two main multiplexing techniques are widely used in optical networks, namely Time Division Multiplexing and Wavelength Division Multiplexing. TDM is used to multiplex the data streams based on time slot basis. Beyond 40 Gbps, TDM is done optically named, Optical TDM (OTDM). WDM is a complement to TDM, which uses the wavelength instead of time slot basis. WDM forms the virtual fibers in the transmission that single fiber looks like multiple fibers (Yi, Li, Bi, Wei, & Hu, 2013). This methodology is deployed in undersea applications.
TWDM uses both the advantages of TDM and WDM. A Passive Optical Network is fiber optical network with no active elements in the signal path, which connects Optical Line Terminal (OLT) with several Optical Network Units (ONU). OLT consists of transponders, wavelength multiplexers, and optionally optical amplifiers. It is used to multiplex and demultiplex the wavelength at the terminals. The P2MP architecture becomes most popular solution for FTTx deployment among operators. ONU are the users in the terminals (Li et al., 2014). PON allows sharing of a single fiber link among many users. It provides a link without any active component, which in effect reduces installation cost of the fiber link. There are previous PON generations available named Ethernet PON (EPON), Gigabit PON (GPON), 10G-PON (XG-PON) ,etc. Next Generation Optical Access Networks (Figure 1) overcomes the existing technologies, such as EPON, GPON, etc. Inspite of all features available in the PON generations, ITU and FSAN accepted TWDM is the solution for second generation PON. It can achieve 40 Gbps downstream and 10 Gbps upstream for high speed data transmission (Sharma, Bajpai, & Prajapati, 2017). It can extend upto 40 km distance and support the services upto 128 customers. The purpose of the paper is to review the TWDM-PON in all possible configurations (Bindhaiq et al., 2014).
Figure 1. Generalized Next Generation Passive Optical Network Architecture
The OLT controls the bidirectional data flow across the ODN (Dubey & Mishra, 2016). An OLT must be able to support transmission distance upto 20 km. It performs two operations in downstream and upstream directions. In the upstream direction, it accepts and distributes all the network traffic from the users. In the downstream direction, it takes voice, data, and video traffic from a long-haul network and broadcasts it to the ONT modules. Each OLT uses different wavelengths for uplink and downlink channel to avoid interferences (Gaudino et al., 2011)
The ONU is placed at the destination's premises. It is used to provide an optical connection to the PON on the upstream side and to interface electrically to the customer equipment on the other side (Bakarman, Shaari, & Ismail, 2010). It is capable of filtering the data associated with a particular user from the OLT.
Passive power divider allows the communication between OLT and their corresponding ONU. The splitter of each stage is used for collecting the information from all corresponding ONTs and multiplexed into a single output towards the OLT.
First, this paper analyses how the NG-PON is similar to previous generations of PON strategies.
This paper discusses the G/E PON to upgrade their performance nearer to NG-PON.
EPON and XG-PON can co-exist in the same ODN by using co-existing element. With the help of co-existing element, operators can provide multiple services and reuse the ODN in order to reduce the network migration cost (Hsu et al., 2015). Co-existing element is a CWDM element that combines GPON, XG-PON, and NG-PON2 on the same ODN. To improve classical PON to GPON, OLT can be connected to WDM filter to convert it to the NG-PON (Skaljo, Hodzic, & Bektas, 2009). To improve the GPON performance, the square root module is used as the MATLAB code to interface the ONU. By using Raman amplification and square root module, we increase the distance up to 60 km at data rate of 2.5 Gbps (Kumar, 2014). This model acquires better Q-factor using DRZ (Duo-binary return zero), CSRZ (Carrier suppressed return zero) modulation schemes, and also get better OSNR (Optical SNR) by using the RZ, NRZ schemes (Kaur & Singh, 2017). A bit error rate of 10-10 with Q- values between 6 to7 is obtained, then it should be a good communication. Using RoF (Radio over Fiber) system and OFDM scheme in GPON architecture, it results in a very high data rate, efficient utilization of bandwidth, and also increased Q point (Raj & Mascreen, 2015).
RoF GPON using 20 users, BPSK is the most appropriate modulation scheme. It provides maximum power, negligible jitter, minimum BER for satisfactory receiver performance (Dane & Kaushal, 2013). CSRZ- DQPSK with narrow spectrum is the best dispersion tolerance for PMD and it suppresses the non-linear effects in fiber (Li, Zhang, Duan, & Yin, 2012). To convert TDM PON to WDM PON, replacing the splitter with thermal AWG is used as multiplexer/ demultiplexer atremotenode (Satyanarayana & Balagoni, 2013). Hybrid optical amplifier Raman- EDFA provides the highest output power and least BER for 100 km. Raman amplifiers are mostly used for enhancing OSNR and improving the repeaterless transmission with low receiver penalty. If Raman amplifier is cascaded with EDFA, it is known as hybrid amplifier. Crosstalk is reduced by using the Bessel filter (Kaler, 2012; El-Nahal & Husein, 2011).
In WDM-PON scheme, the most suitable modulations are RZ (50%), CRZ, CSRZ (67%), which has the maximum Qfactor (Kaur & Sarangal, 2013; Latal et al., 2014). In TDMPON scheme, RZ provides the optimum performance than NRZ. Bandwidth Scalable OFDM (BS-OFDM) PON divides the total OFDM bandwidth into N sub bands; each consists of the quantity of subcarriers required by each ONU. This architecture allows to assigning any subscriber to any subcarrier. OFDM- PON and Coherent PON are the suitable solutions to satisfy the higher splitting ratio and high capacity (Nunes, Coelho, Silva, & Segatto, 2014; Chen, Zhang, & Hu, 2013). By implementing Adaptively Modulated Optical OFDM (AMO-OFDM), it provides 10 Gbps for 60 km distance without any amplification (Gharba et al., 2011). Polarization Modulated QPSK (PM-QPSK) provides 40 Gbps downstream transmission without any optical amplification or filtering at ODN and ONU. PM-BPSK needs less launching power to achieve the same receiver sensitivity of PM-QPSK. 40 Gbps RZ-DQPSK provides high data rate transmission system for 20 km with 3.3 dB power penalty (Memon, Khan, Musavi, Kumar, & Memon (2017).
Compared to GPON, XG-PON increases the bit rate of 10 Gbps, the split ratio to 1:128 and it can reach 60 km (Eržen & Batagelj, 2012). While the channel spacing is greater, the performance of network is better. While channel spacing decreases, Optical path penalty value increases, which causes the decrease in receiver sensitivity. Thermally tuned DML is used to achieve a high loss budget in TWDM-PON. EDFA, SOA, and C-band Raman/EDFA are used to increase the system loss budget to 53.3 dB. In TWDM, Tunable Optical Filter is used at ONU to select the downstream wavelength and Semiconductor Optical Amplifier as a pre-amplifier to enhance the receiver sensitivity (María, Yacelga-Pinto, & Arévalo, 2018; Bindhaiq, Supa'at, & Zulkifli, 2014). In TWDM-PON, Thermally tuned DFB laser is used at transmitter and APD at receiver. It can reach 20 km and support 64 ONUs (Luo, Gao, & Effenberger, 2014). To implement wavelength allocation plans, thin fim tunable filters are used, which can be used as wavelength blocking filters for the receivers in ONU (Scholtz, Korcek, Ladányi, & Müllerová, 2014). TWDM PON can support 52 -9 Gbps at 10 km distance and achieves 10 BER. Wavelength selective switches are used at the remote node to improve flexibility, data security, and power budget in TWDM–PON (Dixit, Lannoo, Colle, Pickavet, & Demeester, 2012). Using RSOA as a pre-amplifier in the downstream, sensitivity is enhanced by 12 dB.
This review article analyses to improve the parameters of TWDM-PON from its previous generations. There are various schemes available to enhance the each parameter in the PON. To increase the network capacity, the authors have to deployed CWDM at upstream and DWDM at downstream in TWDM. To enhance the sensitivity, the RSOA has been deployed at receiver. Avoiding the optical amplification in ODN and ONU, PM-QPSK is used and to avoid dispersion, the AMOOFDM scheme can be utilized. To provide the high loss budget, thermally tuned DML can be used. Various line coding formats were analysed to get better OSNR and BER.