Optical Line Terminal Research Articles

Experimental demonstration of wavelength-division-multiplexing passive optical network employing probabilistic shaping 4-level pulse amplitude modulation

A 5 × 25-Gbaud wavelength-division-multiplexing passive optical network (WDM-PON) employing probabilistic shaping 4-level pulse amplitude modulation (PS-PAM4) with direct detection is proposed and experimentally demonstrated. Each optical carrier at the optical line terminal (OLT) carries a 25-Gbaud PS-PAM4 signal based on Maxwell–Boltzmann distribution. By employing PS-PAM4, the receiver power sensitivity (RPS) and fiber nonlinear effect tolerance of the system can be significantly improved and enhanced compared with the unshaped one, which can provide abundant system loss budget for the legacy optical distribution networks. The experimental results show that, compared with the WDM-PON employing uniform PAM4, the superior RPS of 1.44 dB and the fiber nonlinear effect tolerance improvement of 3.07 dB over 20-km standard single-mode fiber transmission are obtained for the proposed system. In addition, the system performance by introducing a blind adaptive pre-equalization based on the joint applications of constant modulus algorithm and decision-directed least-mean-square error algorithm, which can accurately compensate for the signal high-frequency loss caused by the electro-optical components, such as digital-to-analog converter, optical modulators, and electrical drivers at the OLT, is also experimentally investigated.

Nyquist four-level pulse amplitude modulation scheme (PAM-4) based on hierarchical modulation in IM/DD-TDM PON with hybrid equalization

In order to achieve higher power margin in the intensity-modulation and direct detection (IM/DD) based time division multiplexed passive optical network (TDM-PON) system, at the optical line terminal (OLT) side, we propose to use a Nyquist pulse shaped four-level pulse amplitude modulation (PAM-4) by using the electric low pass filter(LPF). In addition, we apply the hierarchical modulation (HM) scheme by assigning of most-significant bits (MSBs) and the less-significant bits (LSBs) of the PAM-4 symbol to optical network units (ONUs) located at different link positions. In addition, an overlap frequency domain equalizer (OFDE) and a vestigial-side band (VSB) filter are used to mitigate the linear inter symbol interference (ISI) and power fading (both of which are incurred by chromatic dispersion (CD)), respectively. The system performance, i.e., bit error rate (BER) and receiver sensitivity for both MSB and LSB, are analyzed by varying different system configurations. Simulation results show that increasing the hierarchical power ratio, which determines the significance difference between the points in the Nyquist pulse shaped HM PAM-4 constellation diagram, can increase the MSB’s receiver sensitivity and reduce the LSB’s receiver sensitivity. Furthermore, it has been demonstrated that the electric low pass filter can reduce the system ISI. In addition, the effect of linear ISI and power fading can be migrated by utilizing OFDE and VSB filter, respectively.

Optics-Simplified DSP for 50 Gb/s PON Downstream Transmission using 10 Gb/s Optical Devices

Directly-modulated laser (DML) is widely employed in intensity modulation and direct detection (IMDD) system due to its low cost and high output power. However, the corresponding frequency chirp is regarded as one of the main disadvantages for its application in passive optical networks (PONs). In this paper, we theoretically analyze the frequency response evolution of DML based system under different chirp and dispersion conditions, proving that the system bandwidth can be improved by interactions between negative dispersion and DML chirp. Based on this concept, we experimentally demonstrated downstream 50 Gb/s PAM4 signal transmission over 20 km single-mode fiber (SMF) access based on the 10 Gb/s DML operating at 1310 nm and avalanche photodiode (APD). A dispersion-shifted fiber (DSF) providing −150 ps/nm dispersion at 1310 nm in the optical line terminal (OLT) is used to pre-equalize the frequency response of bandwidth-limited directly modulated signals in the optical domain. Thanks to our proposed dispersion-supported equalization (DSE) technique, the system bandwidth can be improved by 5 GHz. Feed-forward equalization (FFE), decision feedback equalization (DFE) and Volterra filter are employed to evaluate the signal performance improvement, respectively. By evaluating the receiver sensitivity, the DSE combined with FFE scheme shows 2 dB improvement than the complex Volterra algorithm, indicating its potential to reduce the complexity of digital signal processing (DSP) and therefore a lower cost and power consumption in PON.

Attack-Aware Dynamic Upstream Bandwidth Assignment Scheme for Passive Optical Network

Abstract Network security is an important component of today’s networks to combat the security attacks. The passive optical network (PON) works at the medium access layer (MAC). A distributed denial of service (DDOS) attack may be launched from the network and transport layers of an Optical Network unit (ONU). Although there are various security techniques to mitigate its impact, however, these techniques cannot mitigate the impact on the MAC Layer of the PON and can cause an ONU to continuously drain too much bandwidth. This will result in reduced bandwidth availability to other ONUs and, thus, causing an increase in US delays and delay variance. In this work we argue that the impact of a DDOS attack can be mitigated by improving the Dynamic bandwidth assignment (DBA) scheme which is used in PON to manage the US bandwidth at the optical line terminal (OLT). The present DBA schemes do not have the capability to combat a security attack. Thus, this study, uses a machine learning approach to learn the ONU traffic demand patterns and presents a security aware DBA (SA-DBA) scheme that detects a rogue (attacker) ONU from its traffic demand pattern and limits its illegitimate bandwidth demand and only allows it the bandwidth assignment to it as per the agreed service level agreement (SLA). The simulation results show that the SA-DBA scheme results in up to 53%, 55% and 90% reduced US delays and up to 84%, 76% and 95% reduced US delay variance of T2, T3 and T4 traffic classes compared to existing insecure DBA schemes.

5G RAN architecture based on analog radio-over-fiber fronthaul over UDWDM-PON and phased array fed reflector antennas

This manuscript introduces a 5G radio access network architecture concept based on ultra-dense wavelength division multiplexing (UDWDM) and incorporating an optical fronthaul network that uses a novel wireless antenna system for radio frequency transmission and reception. A ring topology is proposed where optical signals travel within the 5G UDWDM passive optical networks and millimeter waves are generated in the optical line terminals by optical heterodyning. The wireless transmission of the millimeter waves is conducted by an innovative phased array fed reflector antenna approach for mobile communications that grants high antenna gain due to highly focused radiation characteristics, as well as multiplexing gain by multiple beam generation. Furthermore, beam steering is provided by a radio frequency analog beamformer network. Finally, implementation options synthesizing the total system are discussed.

HMM-Based Secure Framework for Optical Fog Devices in the Optical Fog/Cloud Network

Abstract In the present optical fog/cloud computing environment, optical line terminals and optical network units are used as the most promising optical fog devices (OFDs). The inherent characteristics of fog computing provide certain granted privileges to the attacker to hack devices and make them malicious. Also, existing security solutions generate false alarms that affect the performance of the underlying network. In this paper, a secure framework is proposed that not only predicts the malicious OFDs but also reduces false alarms. Hidden Markov model and intrusion detection system are used to detect the malicious one by computing the probability of shifting (POS) and then shift it to the virtual honeypot which is kept hidden by deploying it at the optical fog layer. In addition, it also reduces the generation of false alarm and logs all malicious activities for further analysis. In the experiment section, Python is used to validate the proposed framework. Further, HMM is simulated and tested in the MATLAB to reduce the false alarm rate. Results show that the proposed framework effectively reduces the false alarms and detects the malicious one and then shifts it onto the virtual honeypot efficiently.

Experimental Demonstration of a Real-Time Digital Filter Multiple Access PON With Low Complexity DSP-Based Interference Cancellation

A four-channel, upstream, digital filter multiple access (DFMA) passive optical network (PON) is experimentally demonstrated for the first time in a 26 km single mode fiber (SMF) intensity modulation and direct detection system employing real-time optical network units (ONUs) and optical line terminal (OLT), which are dynamically reconfigurable in terms of channel spectral allocation and signal modulation parameters. To combat physical channel frequency response-induced cross channel interference (CCI) between spectrally overlaid orthogonal channels, a real-time, reconfigurable CCI cancellation (CCIC) function implemented in the OLT receiver digital signal processing (DSP), is demonstrated to significantly improve the DFMA PON performance and robustness in terms of significant reductions in total channel bit error rates (BERs) and individual subcarrier BERs, substantial increases in channel capacities, considerably reduced sensitivity to ONU synchronization, and excellent transparency to the interfering channel's signal modulation format. The tradeoff between the complexity of the CCIC function's filter, in terms of tap count (multiplier elements) and the DFMA PONs performance is optimized to determine the optimum tradeoff. It is shown that the CCIC filter only requires as few as 42 taps to achieve excellent CCIC performance. We also fully analyze the DSP complexity of the transmit and receive DFMA filters and the CCIC filters for varying DFMA channel counts and varying ratios of AD/DA converter sample rate to digital logic clock rate (parallelization factor). It is shown that DFMA filter complexity is independent of channel count and furthermore that for channel counts ≥ 4 the CCIC filter only needs to process ~10 interference-inducing samples per wanted signal sample, resulting in only ~10 multipliers per parallel filter in the CCIC filter. We have therefore successfully demonstrated an ultra-low complexity, real-time CCIC function, which considerably enhances the performance and robustness of DFMA PONs.

Demonstration of symmetrical 50-Gb/s TDM-PON in O-band supporting over 33-dB link budget with OLT-side amplification.

Single wavelength 50 Gb/s passive optical network (PON) is an excellent candidate for meeting high capacity requirements. In this paper, we experimentally investigate a symmetrical 50 Gb/s time division multiplexed passive optical network (TDM-PON) system in the O-band based on 25G optics. Semiconductor optical amplifier (SOA) is used in optical line terminal (OLT) side to improve link power budget. We initially investigate the performances of SOA as a booster amplifier with different gain in the downstream and make a trade-off between receiver sensitivity and power budget. The performances of 50 Gb/s non-return-to-zero (NRZ) in the downstream with avalanche photodiode (APD) receiver and upstream with SOA-PIN receiver with different equalization schemes are evaluated. Experimental results show that up to 34.97 dB link power budget is achieved in the downstream direction with 7-tap feed forward equalization (FFE), and 33.76 dB link power budget is achieved in the upstream direction with only 3-tap FFE filtering.

Dedicated and broadcasting downstream transmission with energy-efficient and latency-aware ONU interconnection in WDM-PON for smart cities

Abstract Currently, Internet of Things (IoT) and use of Information & Communication Technology (ICT) have gained a lot of attraction for developing the smart cities. Wavelength division multiplexed passive optical networks (WDM-PON) are popularly used for supporting the high data rate IoT and ICT services. With the exponential growth of users and traffic demand in access networks, large energy consumption, OPEX, service reliability has become the most challenging bottleneck for smart cities. This paper proposes a flexible, wavelength division multiplexed passive optical network (WDM-PON) architecture which provides a dedicated point to point (P2P), broadcasting downstream transmission and latency-aware direct optical network units (ONUs) interconnection. Broadcasting downstream transmission is achieved by diverting a particular wavelength from the main path towards (N + 1) × (N + 1) star coupler (SC). Direct ONU interconnection is accomplished by separating the inter-ONU signal from the upstream path by using 50 GHz interleaver at the remote node (RN) and redirecting it towards ONUs. Direct ONU intercommunication avoids the complex re-routing, re-scheduling and re-modulation of the inter-ONU signal at optical line terminal (OLT) and thus increases the reliability and energy efficiency of the network. Broadcasting enables the best resource utilization, and direct ONU interconnection reduces the significant transmission latency among different communicating users. Proposed architecture reduces the operational cost and energy consumption of the network by reducing the processing by some components. By reducing the extra burden of the inter-ONU signal on downstream bandwidth, proposed architecture also provides the efficient bandwidth utilization.

AN ENHANCED G-PON FAULT MONITORING TECHNIQUE USING OPTICAL SENSOR

In this study a technique for a centralized fault monitoring and detection in Gigabit-capable Passive Optical Network (G-PON) using fiber Bragg grating (FBG) sensor is proposed. The technique detects fault once it occurs at the vulnerable locations in the fiber optic distribution link by the proposed FBG devices. Monitoring signal in the C-band is reflected by a uniform FBG with different Bragg wavelengths and reflectivities. The FBGs serve as branch identifiers in the network. The reflected signal from the FBGs is analyzed at the Optical Line Terminal (OLT) in the Central Office (CO) by an Optical Spectrum Analyzer (OSA), to identify the branch with rupture in the network. The simulated result obtained shows that the system can monitor, and detect a fault in the physical layer of the optical distribution network with negligible effects on data signal transmission.

Software-defined converged access network with cross-layer intelligent control architecture

Abstract The optical and wireless convergence technology is one of the principal means for the problem of last mile access. The software-defined network (SDN) supported by OpenFlow protocol is a promising centralized control architecture. Combining these two aspects, we propose a software-defined access network control architecture for the optical and wireless convergence technology, and design its functional modules and the overall interaction process. The proposed architecture adopts the idea of hierarchical control, introduces the OpenFlow agent, dynamically decentralizes some of the controller's authorities to the optical line terminal (OLT), and enhances its overall disaster response capability by extending the control depth of the controller to the optical network unit (ONU). The controller obtains relevant information in real time from a global perspective to achieve more comprehensive dynamic and flexible control, which effectively improves the network resource utilization and meets different service requirements of different users.

An Internet of Things Traffic-Based Power Saving Scheme in Cloud-Radio Access Network

With the proliferation of Internet of Things (IoT) devices, the load caused by network demands has been increasing. This has direct consequences in terms of increased network requests and thus significantly higher energy consumption. To this end, this paper focuses on network power saving. In a cloud-radio access network with fiber-wireless, we propose a power saving method based on the optical line terminals (OLTs) and optical network units (ONUs), the equipment responsible for data transmission to and from the devices. Our proposed method is based on the novel idea of dividing the sleep phase of the ONUs into time slots, which facilitates synchronization and coordination in the whole system, thus increasing resource efficiency and bringing power economy not only to the ONUs, but also to the OLTs and the network as a whole. This is in contrast to the existing literature, which considers only ONUs or OLTs separately, without focusing on the cooperation within the system. Additionally, our proposed method was created with IoT traffic in mind. and it was designed to guarantee a maximum delay in order to make it applicable to mobile and real-time IoT. Finally, our proposed method allows the network to operate without external power sources for the ONUs by using power over fiber in conjunction with the OLT. Mathematical analyses show that our proposed method can successfully decrease the system’s energy consumption.

An (adjustable RN)-based multi-OLT PON and the dynamic programming algorithm for downstream channel

In this paper, a multiple optical line terminal (OLT) passive optical network (PON) and the dynamic programming algorithm for downstream channel is proposed. By wavelength-selectable module, the adjustable remote node (RN) is constructed. By this adjustable RN, a simple and efficient multi-OLT PON is designed. By the way of first-plan and then-load, optical network unit (ONU) groups are established in order to realize the fast loading of ONUs. Specially, by introducing dynamic programming algorithm, the wavelength bandwidth utilization is maximized. Finally, by the simulation and analysis, the effectiveness of the proposed scheme is demonstrated.

Dynamic bandwidth allocation algorithm with demand forecasting mechanism for bandwidth allocations in 10-gigabit-capable passive optical network

10G-passive optical network (XG-PON) provides an emerging potential technology to carry multiple services for the first - mile access network. However, it offers 0.3 ms propagation delay between the optical network Unit (ONU) and optical line terminal (OLT) resulting in inevitable increased queuing delay for the bandwidth grants to the ONUs. We present a reasonable and effective uplink dynamic bandwidth allocation method - demand forecasting DBA (DF-DBA) that predicts ONU’s future demands by statistical modelling of the demand patterns and tends to fulfil the predicted demands just in time, which results in reduced delay. Results show that use of simple normal distribution in the DBA engine offers more appropriate bandwidth allocation with 14% decreased packet drop ratio (PDR) values than standard GigaPON access network (GIANT) DBA following request-grant cycle procedure. Proposed DBA when tested for all the T-CONTs, reduced 50% delay and offered 45% throughput improvement at higher traffic load. The unassigned bandwidth margin (UBM) is nearly ideal along with better QoS parameters results which validates that DF-DBA can practically improve the OLT performance. Further, this predictive DBA opens up a new horizon of research for researcher to use it for longer distances in PON (LR-PON) network with different statistical distributions for getting the possible best results in the future.

Mechanism Design and Performance Analysis of Coordinated Registration Protocol for NG-EPON

The IEEE 802.3ca task force is studying the next generation Ethernet passive optical network (NG-EPON), which introduces channel bonding to enable optical network units (ONUs) to bundle multiple wavelength channels to achieve higher peak rates. In order to register these multi-channel ONUs, the optical line terminal (OLT) needs to acquire ONUs’ device information, assign working parameters, and verify the connectivity of each channel. Traditional ONU registration protocol, designed for single-channel ONUs, is not suitable for multi-channel ONUs because it cannot guarantee the channel connectivities of all channels. Therefore, it is essential to extend the traditional registration protocol or design a new registration protocol for NG-EPON multi-channel ONUs. In this paper, we first extend the traditional registration protocol directly to satisfy the requirements of registering multi-channel ONUs. Besides, we propose a novel registration protocol based on the coordination of channel terminals. In order to compare the performances of two registration protocols, we model their registration processes, analyze their registration delays, and find their corresponding influence factors. Simulation results show that our proposed registration protocol has a smaller registration delay as well as better horizontal scalability.

Cost effective OLT arrangements in 40 Gbps WDM-PON using AWG with a single laser

In this paper, we upgraded the optical line terminal (OLT) of the optical access network for 40 Gbps by proposing a cost effective OLT based on cascaded model of phase modulator and arrayed waveguide grating for wavelength division multiplexed passive optical network (WDM-PON). The conventional setup at OLT side of WDM-PON system is replaced with a new arrangement containing single laser source in cascade with phase modulator and AWG, whereas the phase modulator is tailored with RF source. The phase modulator generates multicarrier signal and AWG splits the signal into multiple single wavelengths based on its routing configurations. AWG is actually used as a passing band periodical filter that routs the input wavelengths to four outputs depending on its free spectral range. Across each routed wavelength, differential phase shift keying is used in downlink transmission for 10 Gbps, while intensity modulation is employed in the uplink transmission based on off-keying. 40 Gbps full duplex transmission via 20 km fibre span is successfully achieved with minimum power loss and effective results. Transmission performances are examined with negligible power penalties, better Q-factors, handsome eye openings and good eye heights, which confirm the deployment of proposed arrangement in the next generation access network.

Demand Forecasting DBA Algorithm for Reducing Packet Delay with Efficient Bandwidth Allocation in XG-PON

In a typical 10G-Passive Optical Network (XG-PON), the propagation delay between the Optical Network Unit (ONU) and Optical Line Terminal (OLT) is about 0.3 ms. With a frame size of 125 μs, this amounts to three frames of data in the upstream and three frames of data in the downstream. Assuming no processing delays, the grants for any bandwidth requests reach the ONU after six frames in this request-grant cycle. Often, during this six-frame delay, the queue situation is changed drastically, as much, more data would arrive in the queue. As a result, the queued data that is delayed loses its significance due to its real-time nature. Unfortunately, almost all dynamic bandwidth allocation (DBA) algorithms follow this request-grant cycle and hence lacking in their performance. This paper introduces a novel approach for bandwidth allocation, called Demand Forecasting DBA (DF-DBA), which predicts ONU’s future demands by statistical modelling of the demand patterns and tends to fulfil the predicted demands just in time, which results in reduced delay. Simulation results indicate that the proposed technique out-performs previous DBAs, such as GigaPON access network (GIANT) and round robin (RR) employing the request-grant cycle in terms of Throughput and Packet delivery ratio (PDR). Circular buffers are introduced in statistical predictions, which produce the least delay for this novel DF-DBA. This paper, hence, opens up a new horizon of research in which researchers may come up with better statistical models to brew better and better results for Passive optical networks.

Performance Enhancement in Colorless WDM–PON Using SSB-SC

Abstract In this manuscript, an attempt has been made to enhance the performance of colorless wavelength division multiplexed-passive optical network (WDM–PON) system using a combination of dual-drive Mach–Zehnder modulator and single sideband-suppressed carrier (SSB-SC) technique along with splicing. The proposed design makes use of SSB-SC technique for optical line terminal (OLT) as well as for optical network unit (ONU) that is generated by using dual-drive modulator which in turn reduces the expense for complete communication process. Suppressed RF signals with four channels at a bit rate of 10 Gbps for each channel are multiplexed before injecting into fiber span of 25 km at OLT side. Half of the downlink power is consumed for re-modulating the data signal at ONU side. The performance is further improved by using signal splicing method. Due to suppression in the carrier in this technique, there is no carrier Rayleigh scattering problem. The proposed model is compared with the existing one for error-free transmission of signal. The results revealed that the proposed model possess less value of bit error rate with the same value of received optical power as well as channel.

Comprehensive Polling and Scheduling Mechanism for Long Reach Gigabit Passive Optical Network

Abstract Dynamic bandwidth assignment (DBA) schemes for long reach PONs face a suffer from higher upstream channel idle time due to long round trip time (RTT). In ITU PONs, the DBA schemes; Immediate allocation with colorless grant (IACG), Efficient bandwidth utilization (EBU) and GPON redundancy eraser algorithm (GREAL) minimize idle time by sending bandwidth grants to the optical network units (ONUs) every downstream frame (DF). EBU further improves IACG by utilizing unused bandwidth (UBW) of other traffic classes. Sending the grant results every DF requires optical line terminal (OLT) to remember all previous grants sent to ONU during RTT and subtract them from the received queue reports. Since, both IACG and EBU assign the excess bandwidth equally to ONUs. Therefore, the OLT is actually not aware of the complete grant to each traffic class and thus do not subtract these completely from receiving reports. This leads to wastage of bandwidth and higher US delays due to over granting. GREAL resolves this problem by not utilizing the excess bandwidth which also leads to increased US delays. The proposed scheme in this study eliminates this shortcoming by allocating excess bandwidth to each traffic class completely at the OLT. Moreover, the UBW assignment mechanism of EBU is also improved. Simulation results show a 50–85 % reduction in delays of type-2 (T2) and type-3 (T3) traffic classes versus GREAL and IACG and up to 40 % reduction in delays for type-4 (T4) versus EBU.

(Overlap and non-overlap)-based DBA algorithm with wavelength optimal length for upstream channel in multi-OLT PON

Abstract In this paper, an (overlap and non-overlap)-based dynamic bandwidth allocation (DBA) algorithm with wavelength optimal length (WOL) for upstream channel in multi-OLT passive optical network (PON) is proposed. By introducing optical line terminal (OLT) pool, the problem of OLT services imbalance is solved. Based on the service and its weight factor, proportion algorithm is constructed to allocate OLTs and wavelengths to OLT pool and wavelength group respectively. By proposing a new concept of WOL, the bandwidth wastage is reduced. By presenting overlap and non-overlap algorithms, the bandwidth utilization is improved further. By the simulation and analysis, the effectiveness of the proposed algorithms is demonstrated.