The bandwidth wars continue. The current buzz in the FTTx space is about NG-PON2. This is the latest family of Passive Optical Network (PON) standards developed by the Full-Service Access Network (FSAN) consortium and formalized by ITU-T.
NG-PON2 departs from earlier PON standards by branching into the wavelength domain. It presently supports up to four 10 Gbps symmetric TDM/TDMA lightwave pairs, in an architecture called Time/Wavelength Division Multiplexing (TWDM). In addition, its band plan provides for four wavelength pairs of point-to-point WDM channels, and coexistence on the same fiber with older PON specifications.
With TWDM, Optical Network Termination units (ONTs, also called ONUs) include tunable downstream receivers and tunable upstream transmitters. The network s Optical Line Terminator (OLT) has up to four separate fixed wavelength transmitter/receiver pairs. It manages wavelength channels by directing the tuning of each ONT. Each channel pair operates in TDM/TDMA mode using an extension of the GPON Transmission Convergence Protocol (GTC).
The concept of TWDM has been around in the research literature since at least 2004 (pdf, the author worked on a prototype in 2009). The idea is to harvest as much of a fiber s bandwidth as is practical in the time domain, then look to the wavelength/frequency domain for more. The practical limit for on-off keyed modulation (OOK) in a PON has been considered to be about 10 Gbps. More sophisticated schemes are possible; however, when FSAN started planning for XG-PON2, they were too expensive for typical PON applications.
Slow Moving Building Blocks
Cost effective N*10/10 Gbps TWDM PON has faced three major technical challenges:
• 10 Gbps upstream burst receiver electronics for the OLT, capable of handling high dynamic range, with short burst overhead and rapid settling.
• Low cost tunable DWDM receivers for the ONT, preferably with tuning times of no more than a few microseconds.
• Most challenging, low cost tunable DWDM lasers for the ONT, with stringent optical specifications, low power consumption and (again) short tuning times.
It also faced a major business hurdle. Launch of new hardware technologies relies on development of a robust supplier ecosystem. Past customer pressure on vendors at all levels of the food chain to reduce prices of GPON components -- especially for ONTs -- left little or no profit margin. Having been burned on GPON, component and sub-system manufacturers hesitated to invest in the R&D necessary to develop the next generation. This was compounded by internal competition for resources with the rapidly growing high-speed data center optics market.
FSAN and ITU-T completed work on the NG-PON2 standards in late 2015. From then until the second half of 2016, the industry showed little public sign of a robust NG-PON2 ecosystem.
• Photonics industry players traditionally make product announcements at the annual Optical Fiber Communications (OFC) conference in March, or the European Conference on Optical Communications (ECOC) in September. In 2016 crickets. Developments in FTTx were overshadowed by data center and long-haul at rates above 100 Gbps.
• Major equipment vendors, including Adtran, Alcatel-Lucent (Nokia), Calix, Cisco, Huawei, and ZTE, made more-or-less splashy NG-PON product announcements. If read carefully, they were for NGPON-2 ready platforms, for PONs with fixed wavelength point-to-point links, unable to meet some of the NG-PON2 specs, and/or lab prototypes. Some vendors privately admitted that NG-PON2 was still vaporware, and would be for some time.
• FSAN and ITU-T retrofitted the NG-PON2 10 Gbps upstream PHY to the fixed wavelength 10/2.5 Gbps X-GPON1, to create a 10/10 Gbps XGS-PON standard. Subsequently, the major vendors -- led by Adtran - announced and started shipping XGS-PON equipment as an evolutionary step toward NG-PON2. This relieves some market pressure, while tunable ONTs continue to cook.
• Measurement of live traffic on residential GPON deployments continues to show very light utilization. There is no evidence that these networks will run out of capacity anytime soon, and thus no immediate pressure to upgrade.
• Tier-1 operators in North America and Europe deemphasized new PON infrastructure deployments in favor of other investments.
• Industry estimates placed the cost per customer of NG-PON2 at 8 to 10 times the cost of GPON. It was believed that ONT optoelectronics would have to be implemented in low-cost photonic integrated circuits (PICs), in order to achieve acceptable manufacturing cost. This kind of integration is at the cutting edge of photonic technology.
Getting Ducks in a Row
A lot has happened in the past several months. It turned out that the placidly swimming duck was paddling madly just under the surface. Starting at the bottom of the food chain:
• There has been a modest bump in the numbers of patents and journal and conference papers related to NG-PON2, particularly low-cost tunable lasers.
• Tunable lasers, tunable filters, and sub-assemblies, designed to meet the NG-PON2 standard, are sampling and/or shipping. Somewhat surprisingly, component vendors have kept a low profile, discussing these parts only with selected customers under NDA. Few of these vendors even mention NG-PON2 on their websites.
• Ligent Photonics introduced NG-PON2 OLT and ONT transceiver modules in the widely-used XFP form factor. This breaks down a key barrier to entry by second-tier equipment vendors, who lack the resources and sales volume to invest in board-level optical integration.
• Broadcom and Cortina Access introduced NG-PON2 System-on-a-Chip (SOC) devices and reference designs. These provide all of the necessary digital functions for ONTs and OLTs. They are a critical building block for cost reduction and for market entry by commodity players.
• Demos, lab trials and field trials have become more convincing.
• Verizon, CenturyLink and other operators have publicly discussed network strategies that rely on NG-PON2.
• Verizon -- the proverbial 800-pound gorilla in the PON food chain -- has conspicuously pushed very hard on equipment vendors to ramp up their NG-PON2 offerings. They selected Adtran and Calix as their primary vendors. Interestingly, they were able to drivehttps://www.dslreports.com/shownews/Verizon-NGPON-2-Upgrade-Could-Deliver-Symmertical-10-Gbps-138699 interoperability testing between these vendors respective ONTs and OLTs, as well as with Broadcom and Cortina reference designs. This is a first for the FSAN/ITU PON family, and strengthens Verizon s formidable buying power.
• Verizon has stated an expectation that prices for NG-PON2 ONTs will converge with those for GPON ONTs. This is to be taken seriously: it echoes their having driven steep declines in GPON ONT pricing and component costs in 2006-2010.
Things Finally Begin to Accelerate
All the tea leaves seem to indicate that commercial NG-PON2 is ramping up for take-off in mid- to late 2017. In the next few months, we can expect to see:
• Announcements and samples of low-cost tunable lasers and receivers, sub-assemblies and modules.
• Restricted general availability for NG-PON2 OLT line cards and ONTs. Vendors development teams will initially work closely with a few sophisticated customers to be able to manage the inevitable teething pains, while ramping up their field engineering operations.
• Larger field trials, addressing a variety of use cases.
• Verizon plans its NG-PON2 First Office Application (FOA) for June, 2017, for business Ethernet services. Expect them to roll out in selected cities in the following months. Also expect a visible marketing campaign, and counter-campaigns by the competition.
• Initial focus by operators on medium-to-large business services
• Product announcements by second-tier players, like Cambridge Industries, Zhone and Accton.
• Convergence of NG-PON2 with Software Defined Networks (SDN), Network Function Virtualization (NFV), virtual OLT, virtual CPE, open source and related network transformation initiatives.
• Perhaps a bit less secretiveness on the part of equipment, module and component vendors.
Keep in mind that this is the beginning of a long adoption curve. Optimistic estimates indicate NG-PON2 will be ramping up through 2020. Large-scale residential deployment is unlikely until at least then, and depends on ONT price reductions to near-parity with GPON.
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