Today, we see the benefit of the accelerating deployment of Fiber-to-the-Home. Gigabit services are rapidly becoming available to more subscribers. Increasingly, these high-capacity services are priced within the normal elastic spend of households for their broadband services. This has favorably impacted service adoption rates, especially over the last two years, where a substantial percentage of the population has been working from home. Faster broadband speeds have been essential in the residential market for the consumption of services for leisure, education, and teleworking, where video conferencing and remote collaboration have become part of everyday life across the globe.
With cost-effective, 10G-capable XGS-PON technology reaching widespread adoption among all types and sizes of broadband service providers, the available speed and capacity have enabled those service providers to offer attractively-priced multi-gigabit services. Marketers are quickly pushing the envelope with service offerings beyond 1 Gbps to differentiate themselves from their competitors. From the United States to Italy, France, and the United Kingdom, XGS-PON enabled, multi-gigabit service offerings are resetting the broadband value point across the developed world's markets in France.
As discussed in previous CTO Insights blogs, these new 10G PON-based offerings are helping to accelerate the market’s abandonment of gigabit limited point-to-point FTTH technologies. Additionally, they are calling into question the role of GPON as a technology with which to light new fiber networks. When services of 5 Gbps and more are available for less than €20 in the European market, it is difficult to successfully argue for new fiber deployments with these legacy FTTH technologies when most new entrant FTTH operators are leveraging the 10 Gbps symmetric capacity of XGS-PON.
Innovative multi-gigabit PON technology available today
A great way to meet the growing needs of residential and small businesses on existing GPON networks is to deploy Combo PON. Combo PON combines XGS-PON and GPON on the same OLT port simultaneously. This provides the ability to augment mature GPON networks with next-generation XGS-PON technology – giving you the best of both worlds. Combo PON is a critical tool for all operators with acquisition plans as the developed world’s fiber markets approach the consolidation phase. There will be GPON footprints that are acquired, wherein replacing entire installed bases of GPON ONTs will not be feasible. For these situations, Combo PON will enable acquiring operators to deploy XGS-PON on the acquired networks while, via ONT interop, permitting the existing GPON ONTs to continue to operate. You have XGS-PON for gigabit and multi-gigabit customers and low-cost GPON to serve existing subscribers. Combo PON is a perfect recipe to extend the usable lifespan of low-cost GPON assets while delivering the least disruptive migration to multi-gigabit XGS-PON.
The future beyond 10G PON
With new fiber networks being built with a PON architecture, it is vital that these networks to be able to play a role in society’s connectivity needs, not just broadband access. XGS-PON will deliver the bandwidth needs for most residential and business broadband use cases throughout the coming decade, but what about the other applications that demand fiber connectivity with capacity, QoS, timing, and latency needs that exceed what XGS-PON can deliver? What is the technology standards roadmap for moving beyond 10 Gbps on PON architectures, and what considerations are needed to introduce them alongside existing deployments?
Each technology presented below has the ability to reuse a service provider’s existing PON infrastructure to ensure a smooth migration path to the next PON standards. With next-generation PON equipment forecasted to grow rapidly in the near future, service providers will begin to make the leap to the adoption of new, upcoming PON technologies beyond 10G PON.
IEEE 25G/50G EPON: The Institute of Electrical and Electronics Engineers (IEEE) standardizes the EPON family of technologies. Among these technologies is 25G/50G EPON which can deliver up to two channels of 25G. A benefit of the two-channel approach is that service providers can initially deploy a single channel of 25 Gbps and then bond both channels later when needed to increase the total capacity to 50 Gbps.
With IEEE EPON standards traditionally adopted by the Cable/MSO segment of the industry, we anticipate the adoption of 25G/50G EPON to be limited vs. the ITU-T based equivalent standards. While very present in the US and some European broadband markets, the cable operators' actual fiber footprints are modest compared to those of the global telco community and fiber-only providers. This modest adoption of 25G/50G EPON is being further compounded by the transition of some of the largest cable operators over to ITU standards with XGS-PON better positioning them to capitalize on future market consolidation.
For those cable operators who wish to stay on the IEEE track, the good news is that IEEE 25G EPON and 50G EPON can coexist alongside 10G EPON. This provides a smooth and gradual upgrade path to address capacity needs on their existing and future PON networks. The impact of scale economies will be an important factor to watch in this industry segment, as the next step may face challenges getting down the cost curve if volumes fail to materialize.
25GS-PON Multi-Source Agreement (MSA): A working group of vendors and some operators have published a specification for an interim 25G PON technology that combines some elements of XGS-PON and 25G EPON. This comes after the largest global operators rejected this approach in the ITU-T standards, instead choosing a 50G G.hsp standard to focus on as the next step for telco standards.
There has been much excitement generated about 25GS-PON, with many operators making press announcements about trialing this approach; however, the discrepancy between the list of operators who have trialed the technology vs. the list who have joined 25GS-PON MSA indicates some headwinds in actual adoption plans.
One of the benefits highlighted about 25GS-PON is its ability to support coexistence with GPON and XGS-PON simultaneously. This is an attractive proposition, with 37.5 Gbps of PON capacity per port, addressing the needs of multiple generations of ONTs and multiple user types. It does however require careful consideration of the proposed use cases, not to mention the more sophisticated (i.e., expensive) optical filtering required for this triple coexistence.
The use cases that are frequently cited for the impending need for 25GS-PON are for large enterprise services that require 10 Gbps or 15 Gbps symmetric bandwidth or for similar capacities for cellular backhaul. XGS-PON with Forward Error Correction (FEC) enabled is limited to 8.5 Gbps, so it cannot deliver these requirements in many scenarios. At face value, 25GS-PON looks like a good candidate; however, given that these types of services must compete with traditional, uncontended point-to-point fiber alternatives, they must be delivered with a comparable level of contention. This is where limitations in today’s chassis-based solutions may prevent them from delivering the potential for 25GS-PON. At 200 Gbps per slot, the leading chassis solutions, at their current 16-port density per slot, can offer Combo PON (GPON and XGS-PON) without oversubscription to the backplane. However, when we layer 25GS-PON on top, the oversubscription level per slot climbs to 3 to 1. With 400 Gbps of uplink capacity, most leading chassis systems run an 8:1 oversubscription rate when utilizing Combo PON across all slots. This climbs to a substantial 24:1 oversubscription level when utilizing 25GS-PON on top of GPON and XGS-PON. A potential workaround is to populate fewer ports per chassis, but this means we need three or more times the number of chassis, space, and common components per subscriber.
Ultimately if 25GS-PON is to coexist with GPON and XGS-PON and be capable of delivering demanding enterprise and backhaul services, we must wait for a future generation of OLT chassis or wait to see if it is implemented in the higher-capacity, disaggregated OLT solution.
G.hsp ITU-T 50G PON: The International Telecommunication Union (ITU), the organization that specified the GPON and XGS-PON family of standards, has developed the G.hsp (High Speed PON) standard, which delivers 50G downstream and 12.5G, 25G, or 50G upstream. The standardization work has been completed for 50G downstream and up to 25G upstream, with work ongoing to complete the 50G upstream portion of the specification. Many supporting service providers identify this technology path as the next step beyond XGS-PON in their PON technology evolution. This includes the largest global operators, whose support is key to driving the volumes required to deliver the scale economies needed for any new technology to become economically viable to deploy.
Like 25GS-PON, the leading chassis-based OLT systems lack the backplane capability to offer G.hsp-based services with any meaningful port density per slot.
With G.hsp coexisting with XGS-PON, 60 Gbps per port or 960 Gbps per slot is required to deliver uncontended services. With over 15 Tbps of capacity emanating from the typical 16 slots, the current max uplink capacities of 400 Gbps would be so heavily oversubscribed at 38:1. This calls into question the solution’s ability to deliver the types of services these next-generation PON technologies are supposedly being developed to enable.
Once again, OLT systems that can cost-effectively utilize optical interconnection of line cards will be required to deliver this vision of converged consumer, enterprise, and backhaul applications.
By utilizing the latest generations of Ethernet interfaces, such as QSFP-DD400 and QSFP-DD800, in future generations of disaggregated OLTs, operators will be able to preserve the necessary non-blocking capabilities to realize the converged vision and tap into these new revenue streams along with the associated operational savings convergence delivers.
A further consideration beyond the need for optical interconnect of line cards is the power and heat dissipation demands that 25GS-PON and G.hsp will place on today’s systems. To achieve higher data rates, forward error correction must shift from the Reed Solomon encoding used in GPON and XGS-PON over to low-density parity-check (LDPC) encoding. Due to the complexity of the LDCP line coding and FEC implementations, it is not practical to achieve port densities that are comparable to today’s Combo PON solutions with the current 16nm chipsets. As an industry we must await the next generation of 7nm components to achieve reasonable densities of these next-generation PON technologies, and likely must wait until 3nm OLT chipsets are available before we can connect comparable numbers of subscribers with a similar power footprint to that of today’s Combo PON solutions.
Future-proofed PON
The fantastic news is that PON networks being deployed across the globe have a very bright future in front of them. Work underway within the standards bodies continues as early standardization efforts have begun on Coherent PON, where speeds in the realm of 200 Gbps are being explored.
There is no question that PON architectures will be able to adopt technology innovations to ensure they can remain ahead of consumer bandwidth demands for many decades to come.
Meanwhile, the industry will continue to innovate to economically unlock the inherent capacity of PON networks, delivering service providers a long roadmap of advancements in speed, capacity, and efficiency, ensuring the longevity of their investment in FTTH networks for decades to come.
