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Turbocharge your Single-fiber Access/PON Network

March 17, 2021
By Fady Masoud
Director, Solutions Marketing

Flexibly add capacity with multipoint XR optics

Infinera recently announced that American Tower completed a live network demonstration of XR optics – the first point-to-multipoint coherent optical transmission in Latin America. American Tower is a global provider of communications infrastructure, with over 186,000 sites in 22 countries across six continents. It offers solutions and services to deploy and support wireless networks.

XR optics is a superset coherent optical module that can work in point-to-point or point-to-multipoint applications over single- or dual-fiber architectures.  Additional unique characteristics of XR optics include remote management capability and full topology awareness.

This trial specifically highlights the ability to transmit coherent capacity levels over an existing PON infrastructure, preparing the network for 5G expansion and beyond. The trial also highlights one of the key capabilities of XR optics – its flexible deployment options, including deployment over a fiber pair or a single fiber, also known as single fiber working (SFW). Passive optical networks (PON) and often wireless towers (4G/5G) are connected over a single fiber. In these cases, transmit and receive traffic flows in both directions from each endpoint over a single fiber.

The trial underscored XR optics’ ability to be inserted into existing single-fiber networks like PONs used for wireless backhaul by leveraging current building blocks like PON filters and splitters. Existing PON traffic and new traffic carried over XR optics can coexist on the same infrastructure, providing network operators with the ability to maintain the current operational model while enabling a whole new level of scalability and flexibility in the network.

One of XR optics’ key attributes is its dynamic capacity assignment, where capacity at remote sites (e.g., cell towers) can be easily increased in 25G increments without truck rolls by simply assigning additional digital subcarriers. Figure 1 depicts a typical deployment of XR optics over existing Gigabit PON (GPON) or even XGS-PON infrastructure

PON NetworkFigure 1: Typical deployment of XR optics over an existing PON infrastructure

How does XR optics operate over a single fiber?

When deploying over a fiber pair, an XR optics coherent pluggable at the hub site generates 400 Gb/s of capacity consisting of 16 x 25 Gb/s digital subcarriers. A passive splitter sends the digital subcarriers to each remote site or leaf. Through the management system, each XR optics coherent pluggable at the remote site (e.g., cell tower) tunes into the one or multiple subcarriers it has been assigned – for example, digital subcarriers #5, 6, 7, and 8. Once locked into the assigned digital subcarriers, the XR optics coherent pluggable at the remote site sends back to the hub, on the second fiber, digital subcarriers that match the same line-up as the received subcarriers, #5, 6, 7, and 8 in our example, illustrated in Figure 2.

PON Network XR

Figure 2: XR optics over a fiber pair

If only one fiber is used for both transmit and receive (i.e., single fiber working), a Y-cable or an optical circulator can be used at each end of the single fiber link, enabling upstream (remote sites to hub) and downstream (hub to remote sites) optical signals being transmitted over different frequencies to use the same single fiber, resulting in bidirectional traffic.  Typically, an optical circulator provides approximately 10% better performance than a Y-cable.

The XR optics coherent pluggable at the hub site frees up the specific part of the spectrum by not transmitting the digital subcarriers that are dedicated to traffic received from the remote site, or upstream traffic. As a result, downstream and upstream digital subcarriers can flow in both directions on the same single fiber, as depicted in Figure 3. Traffic can be symmetrical, where the same number of digital subcarriers is used for downstream and upstream traffic flows, or asymmetrical, where more digital subcarriers are used in one direction to meet the demand of specific traffic profiles such as video broadcast. Both fiber pairs and single fibers can be simultaneously connected to a single XR optics coherent pluggable at the hub.

PON Network XR
Figure 3: XR optics over single fiber

The benefits of overlaying XR optics over an existing PON infrastructure can be summarized as follows:

  • Significantly more capacity and enhanced scalability: Whether the current generation of PON, such as XGS-PON with 10G of total capacity, or the next generation such as NG-PON2 with 40G of total capacity, PON solutions have limited total capacity and reach. Total capacity is defined by the bundling of 10 Gb/s wavelengths, and reach is limited to the performance of the non-return-to-zero (NRZ) modulation. XR optics brings coherent-level performance to PON networks, providing significantly more capacity – 40x compared to XGS-PON and 10x compared to NG-PON2 – with a total of 400 Gb/s that can be assigned to different endpoints in 25G increments. Remote site capacity can be increased easily and remotely, without site visits. This enhanced scalability is key for 5G deployments as it is difficult to forecast true traffic growth. Hub capacity is no longer burdened by the bundling of numerous 10 Gb/s wavelengths. XR optics’ scalability from 25 Gb/s to 400 Gb/s enables network operators to cope with unpredictable traffic demand without network disruption or significant spend in CapEx.
  • Smooth evolution path: XR optics’ ability to coexist with existing PON networks to enable higher capacity services allows network operators to smoothly scale their networks by migrating traffic to XR optics based on their own business and operational needs.
  • Increased ROI: Adding XR optics to an existing PON infrastructure leverages existing equipment such as filters, amplifiers, and splitters, allowing a maximized return on investment. Traffic carried over existing PON can coexist with traffic carried over XR optics, enabling network operators to smoothly shift the current traffic to XR at their own pace and based on their own business and operational requirements. XR optics also provides a disaggregated growth model where the hub side can be upgraded from 400G to 800G+ in the future with no changes to the leaf optics on the other side of the network.  It is the industry’s first multigenerational optics between high-speed and low-speed transceivers.
  • Expanded coverage: The ability to reach remote sites using PON is dictated by the maximum distance between the splitter and the optical network terminal (ONT), which is around 25 km. XR optics uses a 16QAM modulation scheme that enables far better reach – hundreds of kilometers. This enables network operators to tap into new addressable markets beyond their current geographical presence.

XR optics brings coherent-level capacity, reach, and management performance to existing networks with flexible deployment over point-to-point or point-to-multipoint and single- or dual-fiber architectures.  PON network operators now have a cost-effective, flexible, and highly scalable solution to cope with the relentless demand for bandwidth.