When Will Access Networks Go Coherent?

When Will Access Networks Go Coherent?

Access networks everywhere are scaling. 5G and IoT promise to interconnect exponentially more devices than before, with higher speed and latencies. This puts more pressure than ever on fixed and mobile access networks.

In fixed access, there will be a significant expansion in the deployment of fiber-to-the-home (FTTH) links and passive optical networks (PONs). Meanwhile, power-hungry business customers require capacity expansions to 100G and beyond. In the mobile sector, carriers must deliver on their 5G promises without the expensive deployment of new fiber infrastructure. The ever-increasing Internet traffic combined with the flat or declining revenue margins makes scaling up more difficult.

Coherent solutions can help cope with some of these requirements, but their size and expense made them impractical to implement in access networks. In the last few years, the miniaturization of coherent transceivers enables cost and size reductions that made this technology more accessible.

Coherent solutions are still a decade away from becoming mainstream in mobile access networks, but at least they will soon have an impact in cable networks, business services, and edge data centers. Given the continuing advances in standardization and the focus on more affordable components for shorter links, a future with coherent optics in the access network domain is upon us.

Mobile x-haul is not moving into Coherent anytime soon

The initial rollout of 5G has already happened in most developed countries, with many operators upgrading their 1G SFP to 10G SFP+ devices and deploying more wavelength division multiplexing (WDM). Mobile networks must now move to the next phase of 5G deployments: this will require installing more and smaller base stations to increase the number of devices connected to the network exponentially.

These more mature phases of 5G deployment will require operators to scale fiber capacity cost-effectively. The 25G tunable transceivers used in this new deployment phase support a typical reach of 10 km, reaching up to 15 or even 20 km with extra amplification and compensation. For now, that capacity of 25G and 10-20km distances seems to be the sweet spot for radio access network transport.

As more and more 5G antennas are deployed and more 5G users are connected, traffic in radio access networks will keep growing. This increase in traffic demand will translate up through successive network layers to the core, passing through backhaul and sometimes mid-haul stages. This will require an increase in transport capacity beyond 25G, but for now 50G and 100G transceivers are being used in limited quantities by operators in support of massive MIMO and packet fronthaul. Both will remain niche products with volumes far smaller than the mainstream 25G devices.

Fixed Access is Moving into Coherent PON Networks

As data demands continue to grow in cable networks, direct detect optical technology used in prior passive optical networks (PON) will not be enough. PON technology needs to move into the domain of 50G and 100G link capacity, and such progress will require coherent technology.

Operators want to upgrade their 10G PON networks, and the industry seems to be converging into a consensus pick of 50G over lower-capacity 25G PON, as reported by LightCounting research. The moves into 50G and a potential 100G later will benefit from the broader adoption of coherent technology. 50G PON already hits the limits of direct detect technology, and even at those speeds, the devices will require additional complexity compared to typical direct detect devices. For reasons like these, Nokia predicted in a recent white paper that 50G-PON would be “more of a quantum leap than an evolution”.

Anticipating such needs, the non-profit R&D organization CableLabs is pushing to develop a 100G Coherent PON (C-PON) standard. According to CableLabs, several applications justify the development of 100G PON standards and technology:

  • Aggregation of 10G PON and DOCSIS 4.0
  • 5G back- and mid-haul for some macro-cell sites
  • Fiber-to-the-building
  • Long-reach rural scenarios
  • High density/high split ratio urban scenarios, such as distributed access networks (DAA)

The CableLabs C-PON proposal offers 100 Gbps per wavelength, at a maximum reach of 80 km and up to a 1:512 split ratio. CableLabs anticipates C-PON and its 100G capabilities will play a significant role in future access networks.

The main challenge of C-PON is the need for sophisticated modulation and detection. Coherent technology requires more complex and expensive optics and digital signal processing technology. An essential first step toward making these optics more accessible is the 100ZR standard, which we will discuss further in the next section.

Business Services are Moving to 100ZR.

Almost every organization uses the cloud in some capacity, whether for development and test resources or software-as-a-service applications. While the cost and flexibility of the cloud are compelling, many IT executives overlook the importance of fast, high-bandwidth wide-area connectivity to make cloud-based applications work as they should.

These needs might require businesses with huge traffic loads to upgrade to 25G, 100G, or even 400G speeds. These capacity needs would require coherent technology. Fortunately, advances in electronic and photonic integration have miniaturized coherent line card transponders into pluggable modules the size of a large USB stick.


Many of these business applications will require links between 25 Gbps and 100 Gbps that span several tens of kilometers to connect to the network provider’s headend. For these sites, the 400ZR pluggables that have become mainstream in datacom applications are not cost-effective when utilization is so low. This is where 100ZR technology comes into play.

100ZR is currently a marketing term for a short-reach (~80 km) coherent 100Gbps in a QSFP pluggable. Targeted at the metro edge and enterprise applications that do not require 400Gbps, 100ZR provides a lower-cost, lower-power pluggable that also benefits from compatibility with the large installed base of 50 GHz and legacy 100 GHz DWDM/ROADM line systems.

Table 1: Summary of different use cases for 100ZR products.


Mobile access networks are still comfortable with direct detect technology, but coherent is already starting to impact cable networks and business services. Furthermore, coherent is already established as a solution to interconnect edge data centers.

Favorable coherent component cost-reduction trends are expected to continue, technological advancements will enable higher performance, and simpler implementations will make coherent technology more pervasive in the access network to achieve exponential capacity growth.

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