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Third-Generation Optical Distribution Networks Are Emerging

Views: 0     Author: Site Editor     Publish Time: 2022-11-29      Origin: Site

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A guest post by John Lively, Principal Analyst at LightCounting.

The residential optical distribution network (ODN) is the final connection between telecom operators’ Internet, cable, and telephone services and customers.  Over the past decade, and often out of the spotlight, ODNs have played a critical role in the widespread adoption and deployment of Passive Optical Networks, and development efforts have focused on reducing upfront costs rather than increasing functionality.  Now however, there is a push by industry to introduce modern technology to the ODN to reduce operating expenses and increase the performance of access networks. This research note introduces this topic.

Figure 1: 20th Century access network cabling technologies

L. Twisted pairs of copper wire / R. Coaxial cable

Starting early in the 21st century, the deployment of Passive Optical Networks began in earnest, in support of triple play service bundles, in which faster Internet speeds, lower latency, and more video bandwidth were all key selling points. The first wave of deployment used BPON, followed by GPON/EPON, and we are now in the third generation of PON deployment with NG-PON2 and XGS-PON, offering 10 Gbit/s transmission speeds and 1G services.

Unlike earlier access networks, the last mile of PON networks utilize point-to-multipoint optical fiber, with a single or pair of fibers originating at an Optical Line Terminal (OLT), terminating at a passive optical splitter located somewhere in the outside plant, with multiple fibers exiting the splitter and connecting to or near individual residences in a device called an Optical Networking Terminal (ONT) or Optical Networking Unit (ONU).  

Figure 2: Common PON network elements

The most common split ratios for GPON and EPON are 1:32 and 1:64, which can be implemented in single-stage (monolithic splitter) or two-stage (cascaded splitters) topologies. The fiber and splitter connecting an OLT with its subtending ONUs is called the Optical Distribution Network, or ODN.

Figure 3: Schematic of Optical Distribution Networks

ODN technology evolution

Starting around 2018, a second generation of ODN (ODN2) started deployment, using various pre-connectorized components made available by several vendors, including Corning, CommScope, Huber+Suhner, Huawei, Fiberhome, and Furukawa. These products, architectures, and use cases are described in detail in ETSI TR 103 775, published in August of 2021. The ETSI technical paper also introduces the term ‘QuickODN’ to describe ODNs built with pre-connectorized components.

The main advantage of ODN2 is that no fiber splicing is required in the field, as all fusion splicing and subsequent testing is done in a vendor’s factory setting. This means field installation can be done more quickly and less expensively, with more predictable results. Some products are even designed to allow subscribers to connect their homes to a FTTH Q-ODN junction box via a supplied pre-connectorized optical cable, without any involvement of the service provider.

Figure 4: Pre-terminated optical cables and pre-connectorized products for QuickODN

Along with pre-connectorization, another major innovation in ODN2 is the use of digital labels (bar codes or QR codes) for each fiber and port that can be easily entered into a smart database creating a digitalized Optical Distribution Network. This ‘Digital Quick ODN’ uses the unique identities of ODN passive elements to create intelligent management functions like automatic storage of optical fiber location information, automatic identification of optical fiber connections, optical fiber calibration 

Figure 5:information and a visual guide for onsite operations

The advent of pre-connectorized and digitally labelled fiber, splitters, and fiber handling trays, cross-connects, and boxes greatly reduced deployment time and expense for operators, but did little to address operating expense. Today a third generation ODN (ODN3) is being developed which aims to address the operational expense of ODNs by introducing active, automated monitoring and intelligence.

Using an optical monitoring system of some sort (based on reflections, introduced delay, or other), will allow an intelligent management system to automatically identify and locate impairments and failures down to the level of specific fibers and ports in an individual network element. This information is then be provided to a centralized network operations center and to handheld devices in the hands of field technicians.

Huawei has developed one such system which it markets under the trade name ‘Fiber Iris’.

Figure 6: Huawei’s Fiber Iris Quick Digital ODN

The key to Huawei’s Fiber Iris is the clever use of optical micro-structures in the 1xN splitter of the ODN to introduce unique differential phase changes in the upstream signal originating at each ONU or ONT. The combined optical signal arriving at the OLT is split via a filter and a small fraction is diverted to a highly sensitive receiver (located on what Huawei calls the OAI board), which can distinguish the phase changes from one another and thereby identify each ONU/ONT individually. No additional optics are required in the ONU or ONT, and by cascading two 1×8 splitters, up to 64 ONUs can be monitored with one OAI board, and making the added cost manageable.

The benefits of being able to ‘see through’ the 1xN splitter in the ODN are significant. Fiber breaks can be accurately located down to individual fibers, and unused ports and full ports can be individually identified ahead of a service call. Also, service uptime/downtime can be monitored on the level of individual ONU/ONT.

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