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Abstract
For the accommodation of mobile, business, and residential service in the same optical distribution network, we experimentally demonstrate 50 Gb/s (25 Gb/s × 2 wavelengths) wireless and wired service converged optical access network with 64-way power split over 20 km of single mode fiber in 1300 nm band. Applying simple Reed-Solomon based forward-error-correction and a cost-effective avalanche photodiode receiver without using an optical amplifier realize the 64-way power split. Accommodating dynamic bandwidth allocation and open interface control with OpenDaylight (ODL) controller via network configuration protocol (NETCONF) interface are demonstrated. Furthermore, error-free packet transmission of 50 Gb/s with low latency and guaranteed bandwidth are successfully demonstrated.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Increasing 5G mobile traffics induced by smart mobile devices, virtual reality service, augmented reality services, and internet of things (IoT) devices require a continuous capacity expansion of wireless networks [1,2]. The higher the mobile speed we have, the shorter the wireless distance and the more the fiber penetrates closer to the user. As a result, cost-effective optical access solutions gain much attentions to accommodate explosive 5G mobile traffic and to alleviate the capacity burden of transmission link for wireless networks.
Passive optical network (PON) could be used for mobile fronthaul/backhaul, business services, and traditional residential services in the same optical distribution network (ODN). To accommodate 5G mobile services requiring massive connectivity, capacity enhancement, and ultra-reliability as well as low latency, PON should have several functionalities [3,4]. First of all, the optical link between optical line terminal (OLT) and optical network unit (ONU) should provide guaranteed bandwidth and low latency. Conventional OLT has a dynamic bandwidth allocation (DBA) function, and it is mostly focused on ensuring the fairness of internet service. To use PON for mobile services, different quality of service (QoS) dependent on the type of services with minimizing latency of mobile traffic is required. The second issue would be the simple accommodation of high speed signal over 25 Gb/s per wavelength cost-effectively in the power budget challenged PON. In particular, ODN with high splitting ratio would be favorable because sharing the OLT optics and electronics, sharing feeder fiber, and easier fiber management at OLT reduce the cost of optical access network. Original band (O-band) with low chromatic dispersion is a good candidate for 25 Gb/s transmission in PON, and IEEE P802.3ca 50G-Ethernet PON task force decided to use O-band for downstream to avoid chromatic dispersion compensation. However, high loss of the optical fiber in O-band is a drawback for high split ratio with 20 km reach. Therefore, it is required a low-cost PON solution to archive a high power budget in O-band. Finally, open interface based control should be also provided. In the case of multi-vendors and mixed kinds of PON equipment, complex operation and vendor lock-in issues would be problem. Recently, there have been substantial efforts to find a practical solution for the accommodation of mobile traffics in PON [5–8]. These include beyond 10-Gb/s optical transmission in PON with various modulation formats such as non- return-to-zero (NRZ) in O-band, optical duo-binary and electrical duo-binary in C-band, and pulse-amplitude modulation (PAM)-4 in C-band [5–7]. Even 25 Gb/s based demonstrations were reported for high-speed PON, there is few report on system demonstration of multi-wavelength PON based on 25 Gb/s per wavelength, low latency DBA, high-sensitivity receiver module, and software defined networking (SDN) control.
In this paper, for the first time to our knowledge, we experimentally demonstrate 50 Gb/s (25 Gb/s × 2 wavelengths) wireless and wired service converged optical access network with 64-way power split over 20 km of SMF in O-band accommodating DBA enable by SDN control. Simple Reed-Solomon (RS) based forward-error-correction (FEC) and a cost-effective avalanche photodiode APD receiver without using an optical amplifier realize the 64-way power split operating at 25 Gb/s per wavelength over 20 km of SMF in O-band. For the accommodation of mobile, business, and residential service in the same ODN, DBA implemented by hardware description language (HDL) in the field programmable gate array (FPGA) provides low latency and fairness simultaneously. The control and management of optical access network is enabled by SDN with an OpenDaylight (ODL) controller via network configuration protocol (NETCONF) interface at OLT. Error-free packet transmission of 50 Gb/s with low latency and guaranteed bandwidth are successfully demonstrated.
2. Experimental setup
Figure 1 shows an experiment setup for demonstration of 50 Gb/s time division multiplexing / wavelength division multiplexed (TDM/WDM)-PON with DBA and SDN control. PON medium access control (MAC)/ physical (PHY) included DBA, quality of service (QoS), operations- administration and management (OAM), FEC and multi-point control protocol (MPCP) functions to manage multiple optical network units (ONUs) with operating multi–wavelengths downstream and upstream signals. We implemented PON MAC/PHY by extending IEEE 10G-EPON standard. OLT line card is composed of a FPGA, a switch interface chip, four PON transceiver ports and ten 10GbE UNI ports, which could be expandable up to four 25-Gb/s PON MAC/PHY as shown in Fig. 1(i). For real-time demonstration, each ONU is automatically discovered and registered to the PON which is driven by the OLT. Then, service level adjustment and DBA are performed by PON protocol. Control and management of PON is processed by SDN based open interface. The implemented SDN controller was composed of yet another next generation (YANG) data models, topology manager, inventory manager, service manager, model driven-service abstraction layer (MD-SAL) module, and NETCONF plugin for south-bound interface (SBI). YANG models on SDN controller is designed for WDM/TDM-PON. SDN controller connects to OLT by using NETCONF client and NETCONF server to configure the QoS and service level agreement (SLA) of PON. To accommodate 2 × 25-Gb/s downstream and 2 × 10-Gb/s upstream in legacy ODN (PR30, > 29-dB power budget) cost effectively, we implemented pluggable PON transceivers based on APD-ROSAs as shown in Fig. 1(ii) [9]. OLT and ONU transceivers have same packages with QSFP28 for ensuring high speed transmission up to 25 Gb/s. OLT transceivers used O-band electro-absorption modulated lasers as a low-cost downstream solution in which downstream wavelengths are 1295.4 nm and 1309.08 nm, respectively. 2 × 25 Gb/s downstream signals from OLT are launched to 5-km feeder fiber after multiplexing at a wavelength mux (WM). The output powers and extinction ratios of downstream signals are measured to be 4.5 dBm and 8 dB after the OLT transceivers, respectively. Transmission distances between OLT and each ONU is intentionally set differently to emulate PON under real deployment environment of optical access networks. The lengths from OLT to ONU1, ONU2 and ONU3 are 15 km, 6 km, and 20 km, respectively. The PON will support legacy internet service and mobile service as well. With channel bonding, multiple wavelength pairs would be coordinated by the MAC as a single logic channel to provide services to the customers, and the ONU speed could be reached to peak service rate [10]. The channel bonding could be implemented by sharing the wavelength-1 between 25G-ONUs and 50G-ONUs in time domain and by dedicating the wavelength-2 to 50G-ONUs, as discussed in the IEEE 802.3ca. We assumed a service scenario that ONU1 was operating for mobile backhaul service and ONU2 and ONU3 were operating for residential or business service. To ensure mobile backhaul service, ONU1 had 2 × 25-Gb/s downstream and 2 × 10-Gb/s upstream capacities by using WDM signals. Both ONU2 and ONU3 had 25-Gb/s downstream and 10-Gb/s upstream capacities by using a single wavelength pair. After passing through 1 × 64 splitter and distribution fibers, 2 × 25 Gb/s downstream signals are received with the ONU transceivers based on 25-Gb/s APD receivers. ONU transceivers employ 10-Gb/s burst mode (BM) directly modulated laser operating at 1270 nm or 1330 nm for complying IEEE 10G-EPON upstream transmission. At OLT, 10-Gb/s BM receivers are used to detect upstream burst packets from ONU1, ONU2, and ONU3. To investigate the performance of Ethernet packet transmission and low-latency DBA function, we used a packet generator & analyzer.
Fig. 1 Experiments setup of real-time SDN controlled 2 × 25 Gb/s WDM/TDM-PON demonstration (i) OLT linecard (ii) PON transceiver.
3. Results and discussions
Open interface based on SDN control could resolve complex operation issues of multi-vendor PON equipment. Figure 2(a) shows a sequence of SDN control for setting QoS parameters. First, graphic user interface (GUI) application sends QoS parameter to controller, and then the controller communicates with agents to change the configurations. Next, the agent send CLI message to OLT regarding QoS parameter, and then OLT replies with CLI response message after changing QoS parameters. The agent sends a notification message to the controller to inform particular data being changed, then the controller sends a GET message to the agent. Finally, the agent replies with RPC-REPLY message containing YANG data which lists all the parameters describing QoS of each ONU. Figure 2(b) show the YANG data models have a hierarchical structure with a tree type. OLT node contains a list of multiple ONUs, and the SLA parameters and transmission bandwidth were modelled for each ONU node. The application layer of SDN controller provides GUI of the PON control and management, and also provides real-time traffic monitoring functions as shown in Fig. 2(c). The configuration data can be inquired and set by the controller through GET, GET-CONFIG, and EDIT-CONFIG messages. The status data can be updated by the PON device or monitored by the controller through notification or polling methods. Cost-effective accommodation of high speed signal over 25 Gb/s per wavelength in the power budget challenged PON is important to accommodate mobile traffic with PON. High sensitivity APD-ROSA and simple FEC would be possible solutions.
Fig. 2 Control of DBA for each ONU through SDN (a) sequence diagram of QoS control (b) part of YANG model for bandwidth control (c) GUI of SDN controller and captured upstream traffic.
Figure 3(a) represents top view of high sensitivity 25-Gb/s APD-ROSA over transistor outlook (TO) stem. It consists of a lensed APD chip, a trans-impedance amplifier (TIA) chip, a sub-mount, and 8 single layer capacitors (SLCs). To implement the 25-Gb/s APD-ROSA cost-effectively, we used a TO-46 package, which could be available off-the-shelf component. Also, the lensed APD with an integrated backside lens enables optical coupling easily. Figure 3(b) plots the measured optical coupling efficiency of APD ROSA according to X-axial and Y-axial displacement between the lensed APD and the focusing lens. The black square represents measured data with respect to x-axis and the red circle represents measured data with respect to y-axis. The result shows that the alignment tolerance is about ± 15 μm for coupling efficiency of more than 80%. It can simplify alignment process and is also helpful for optical sensitivity enhancement.
Fig. 3 TO-46 packaged 25 Gb/s APD ROSA (a) top view (b) measured coupling efficiency according to X- and Y-axial displacements between APD and focusing lens.
Figure 4(a) represent measured bit-error rate (BER) performances of 2 × 25 Gb/s downstream and 2 × 10 Gb/s upstream, respectively. In all cases, the BER performances are well behaved with no indication of error floors due to high chromatic dispersion tolerance in O-band transmission. The receiver sensitivities of 25-Gb/s downstream are measured to be below –24.8 dBm at BER of 10−3 after 20-km transmission. For the 10-Gb/s upstream, the receiver sensitivities are below –30.5 dBm. Figure 4(b) shows the measured eye diagrams for downstream and upstream, respectively. The eye-diagram degradation after 20 km transmission over O-band downstream was negligible. The BM electrical signal wassuccessfully recovered even when the input power difference between BM input signals was 22 dB. To ensure error free packet transmission at BER 10−3, PON should have FEC function at MAC/PHY layer. Since RS FEC code has a low complexity among various FEC codes, we implemented downstream and upstream FEC with RS (255, 223) code. Figure 4(c) shows measured BER curves of 25 Gb/s downstream signal with FEC or without FEC. When FEC was not used, the receiver sensitivity was –18.8 dBm at the BER of 10−12. However, the use of FEC enhanced the received sensitivity at the BER 10−12 from –18.8 dBm to –24.8 dBm, which increased the link budget from 23.2 dB to 29.2 dB in downstream direction. Consequently, the 50-Gb/s WDM/TDM-PON supports 20-km transmission, 64-split remote node and wavelength multiplexer and de-multiplexer OLT or ONU.
Fig. 4 Performances of optical links (a) BER performance (b) Eye diagrams (i) downstream: back-to-back (ii) downstream: after 20 km transmission (iii) upstream measured at the OLT (c) BER performances with FEC or without FEC.
To use PON for accommodating mobile traffics, the optical link between OLT and ONU should provide different QoS dependent on the type of services with minimizing latency of mobile traffic is required. Generally, ONU requests a required bandwidth to OLT, and ONU could transmit a packet only for the time granted by the OLT. Therefore, when traffic is congested, the DBA scheme of OLT plays an important role to determine the transmission time considering differentiated service among ONUs. Considering fairness and latency, fast class-of-service oriented packet scheduling (FCOPS) could provide different classes of service in the intra-ONU as well as inter-ONU in traffic-congested situations [11]. Because the FCOPS scheme manages independently credit pools for each class of service (CoS), it allocates transmission grants for each ONU instantaneously upon receiving the report message from the ONU. The credit is charged at the beginning of a transmission cycle in which all ONUs can transmit once, which mean that duration of credit recharging cycle (CRC) is same with that of transmission cycle. Even if packet is in the higher priority, however, it might not be serviced due to credit exhaustion, in situation where traffic occurs unevenly on a cycle-by-cycle basis [12]. Thus, we intentionally increased the CRC to N × transmission cycle (TC), where N is positive integer, in order to improve unfairness among the ONUs. Figure 5 shows a conceptual scheme of FCOPS operation based on report-grant mechanism. In this method, the CRC is variable since the duration of the TC is flexible according to the allocated bandwidth to all ONUs. We have implemented modified FCOPS-based DBA algorithm by increasing the duration of CRC is seven time longer than that of TC. In other words, the credit for each CoS is recharged at the beginning of TC #1, #8, #15, and so on.
We have implemented multiple queues in the ONU to accommodate differentially residential, business, and mobile fronthaul/backhaul services at the same ODN. The packets are classified into four classes: gold, silver, bronze, and best effort (BE) according to the priority of the service type. We assigned different services types, service ratio, and granted bandwidth for each ONU, as shown in Table 1. The ONU1 was assumed for mobile fronthaul/backhaul services, ONU2 and ONU3 are for business, residential services each. Strict priority scheduling is used to maximize the effectiveness of differentiated services. Total offered load comes from traffic generator was fixed at 11 Gb/s and 6 Gb/s for ONU1 and ONU2, whereas total offered load for ONU3 was varied from 0 to 6 Gb/s. For ONU1 which we assumed to serve mobile traffic, gold class service with 100% ratio was assigned. On the other hand, ONU2 and ONU3 were set to be silver, bronze, and BE class service with 20:40:40 percent ratio. The maximum upstream bandwidth is around 16 Gb/s when the two channels are operating at 10 Gb/s per wavelength. This is because burst mode overhead and FEC overhead 8.7% were needed for upstream transmission. In this experiment, the granted bandwidth for ONU1 was set to be fixed at 11 Gb/s for both static bandwidth allocation (SBA) and DBA. In the case of SBA, the remaining 5 Gb/s was equally assigned to ONU2 and ONU3. On the other hand, in the case of DBA, ONU2 and ONU3 could use maximally 5 Gb/s for upstream transmission. All these parameters were set to the MAC in the OLT by SDN controller with NETCONF interface.
Table 1. Assignment of services types and granted bandwidth at each ONU for the test of QoS scenarios.
Figures 6(a) and 6(b) represent measured throughput and latency of upstream traffic with SBA and DBA control. The closed and open bar represent measured throughput and packet loss for each ONU, respectively. The gold, silver, bronze, and green color shows the results for gold, silver, bronze, and BE service, respectively. The open circle show the measured total throughput. Total upstream throughput is measured to be 16 Gb/s, as shown in Fig. 6(a). The results show that the packet in BE and/or bronze class was dropped first rather than the packet in the silver class as we increased the offered load higher than the upstream bandwidth (i.e. traffic congestion condition). However, the packet throughput measured at ONU1 with gold class service was always guaranteed with 11 Gb/s regardless of traffic conditions for both SBA and DBA control. In this case, the upstream wavelength 1 and 2 support 3 Gb/s and 8 Gb/s, respectively. In addition, the total throughput of DBA was higher than that of SBA when offered load was slightly higher than upstream bandwidth (i.e. the total offered load was 17 Gb/s and 19 Gb/s. light traffic congestion condition). This is because ONU2 with heavily offered load utilizes unused bandwidth of ONU1 with lightly offered load in the case of total offered load. The latency of each ONU and service class were also measured for all ONUs and the results of each ONU are represented by different symbols, as shown in Fig. 6(b). The latency of higher priority service is always better compared to that of lower priority service regardless of the load offering as shown in the ONU2 and ONU3. The overall latency with DBA was shorter than that of SBA. This is because enhanced utilization of bandwidth reduces packet loss in the case of DBA, which reduces the waiting time of packet in the queue. For example, when the total offered load was 19 Gb/s with DBA control, ONU2 maximally utilized the unused bandwidth width up to 5 Gb/s and reduced packet loss of ONU2. As a result, the latency of bronze service was reduced from 33 ms to 0.38 ms. The latency of ONU1 with gold class service was always less than 0.3 ms regardless of traffic conditions. All these confirmed that the optical link between implemented OLT and ONU provides guaranteed bandwidth and low latency for the accommodation of 5G mobile fronthaul/backhaul services, and support different QoS for the accommodation of mobile, business, and traditional residential services in the same ODN.
Fig. 6 Performances of upstream access control with four different service classes (a) measured throughput (a-i) SBA control (a-ii) DBA control (b) measured latency (b-i) SBA control (b-ii) DBA control.
4. Summary
We have successfully demonstrated, for the first time to our knowledge, 50 Gb/s (25 Gb/s × 2 wavelengths) wireless and wired service converged optical access network. Without using an optical amplifier, cost-effective accommodation of high speed signal over 25 Gb/s per wavelength in the power budget challenged PON was achieved RS based FEC and the high-sensitivity APD receiver realized 64-way power split with 20 km transmission in O-band. High splitting ratio at the remote node would induce cost-effectiveness of optical equipment due to sharing the OLT optics and electronics and sharing feeder fiber. The guaranteed bandwidth and low latency of the optical link between OLT and ONU was also demonstrated to accommodate mobile, business, and residential service in the same ODN. DBA implemented by HDL in FPGA provides low latency for mobile services and effectively use uplink bandwidth. The control and management enabled by SDN with ODL controller via NETCONF interface was successfully demonstrated.
Funding
IT R & D program of MSIT/IITP [2015-0-00040, SDN based wired and wireless converged optical access networking].
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