The success of fiber to the user (FTTx) installations over the last 15 years has been astounding. End users’ insatiable hunger for reliable and higher-speeds has forced legacy FTTx systems to evolve to today’s next-generation (NG) systems. Of course, it’s not as seamless as it might sound. To do so requires ever increasing amounts of bandwidth and thorough migration and coexistence to provide higher speed systems. In this article, I review the International Telecommunications Union (ITU) standards, key terms such as coexistence, and new products required for higher speed and greater bandwidth requirements.
The ITU has addressed the key optical issues required for addressing migration and coexistence from legacy to next-generation FTTx networks. By assigning optical spectrums (wavelengths) for future FTTx PON standards, the ITU provided manufacturers the roadmap needed to address migration to faster FTTx systems using the same physical plant.
With a penetration rate of approximately 5% in 2019, the crossover point for G-PON to 10G-PON (known as XGS-PON) is expected to occur around 2022. One advantage XGS-PON provides over G-PON is that many more 1 gigabit services can be delivered for users along with higher tier services for businesses. XGS-PON, NG-PON2 (WDM-PON) both allow for further migration paths while providing even higher speed transmission for residential and business clients. It will also help with 5G backhaul.
Legacy Must Evolve
The term “legacy” passive optical networks (PON) includes the ITU’s B-PON and G-PON along with the IEEE’s EPON systems. The newest of these was released in 2004. Most dominant in North America is the ITU-T G.984 standard for Gigabit Passive Optical Networks (G-PON). It accommodates speeds up to 2.5 Gb/s, and is mostly used in a 1 to 32 split ratio with distance spans of up to 20 km (12 miles). While G-PON is the most popular of the various PON standards, even it needs to migrate to faster speeds due to changes in bandwidth demands such as symmetrical transmission.
10 Gb/s systems can be asymmetrical 10 Gb/s downstream and 2.5 Gb/s upstream (XG-PON) or 10/10 Gb/s symmetrical (XGS-PON). In most cases, the symmetrical systems are required from business customers.
Greenfields account for the majority of the higher-speed upgrades and are a no-brainer to build at higher speed levels. They also provide the option to increase the splitter count to 1:64. The ITU-T G.9807.1 standard, more commonly called XGS-PON, is a symmetrical 10 Gb/s upstream (US) and downstream (DS) standard providing higher bandwidth bi-directionally.
Key to implementing this migration using the same physical plant is to use the optical spectrum as assigned by the ITU. Legacy PON systems all use 1490 nm for downstream digital content and 1550 nm for analog video content from an optical line terminal (OLT). This OLT is located at the service providers central office, hub, or head-end. Upstream from the subscribers uses the traditional 1310 nm wavelength from the optical network terminal (ONT) located at the subscriber’s facility,
To efficiently migrate from a legacy PON system transmitting downstream at 1490 nm and upstream at 1310 nm to a next-generation 10 Gb/s system, the ITU specified downstream transmission at 1577 nm and upstream transmission at 1270 nm. These wavelengths allow a legacy PON system to coexist with the newer 10 Gb/s system over the same optical distribution network (OSP) as they are “out of band” from the legacy wavelengths designated by ITU and IEEE standards.
*All wavelengths listed are the center wavelengths of the spectrum. For example, 1490 nm is technically between 1480 – 1500 nm, with 1490 nm being the center wavelength.
The most common legacy standards are the IEEE 802.3ah EPON operating a 1 Gb/s and the ITU’s G.984 G-PON. To increase the bandwidth available to users, service providers can decrease the split count or the number of subscribers attached to each optical line terminal. Options include using dynamic bandwidth allocation (DBA) which can provide specific users higher bandwidth levels or use a blended approach by replacing legacy ONTs with next-generation symmetrical (XGS) ONTs. This frees up more bandwidth for legacy users while providing much higher transmission rates for high-speed clients.
Installing a 10 Gb/s XGS-PON1 system would provide 4 times the symmetrical bandwidth over legacy G-PON systems. In these cases, a new OLT would be required and a passive coexistence filter added.
Consumers’ ONTs would need to be upgraded since a new upstream 1270 laser would be required. Fortunately, most G-PON ONTs are designed for coexistence so those not requiring the higher speeds can continue to use the legacy systems without equipment upgrades.
Whether integrating both legacy and 10G PON systems together or increasing the splitter count, designers and planners must account for the 1.6 dB additional attenuation for the passive coexistence filter. Should the splitter count be increased from 1:32 to 1:64, an additional 3.4 dB is required, bringing the total up to 5dB. This 5 dB difference may require upgrades to N1 and/or N2 optical power levels as specified in the ITU-T G.987 standard which are equivalent to G-PON’s B+ and C+ optical power levels.
This technique of increasing splitter count to 1:64 is being used by Verizon by adding a 1:2 splitter at the central office while addressing the higher optical power level provided by N2 optics. Verizon has been very active in planning a migration plan for brownfield applications while also implementing the NG-PON2 (WDM-PON) standard for business and high-end users.
Following XG/XGS-PON, is the ITU-T G.989 time and wavelength division multiplexing (TWDM) standard which uses TWDM in the downstream and upstream transmissions. As with legacy and 10 Gb/s PON systems wavelength assignments have been assigned for migration and coexistence. These include 1524 nm to 1544 nm in the upstream direction, and 1596 nm to 1602 nm in the downstream direction.
TWDM transmission occurs in the downstream direction by the use of 4 fixed lasers at the OLT which are then optically multiplexed over a G.652 single-mode fiber. The upstream transmission occurs by the use of a tunable laser at the ONT. Transmission rates can be up to 10 Gb/s in both downstream and upstream directions using either symmetrical or asymmetrical transmission. Transmission rates of up to 40 Gb/s are defined within the G.989 standard with options for 10/10 Gb/s, 10/2.5 Gb/s, and 2.5/2.5 Gb/s, downstream and upstream transmission rates
This standard specifies the use of tunable lasers at the ONT which provides greater flexibility for implementation and maintenance. Verizon has stated that NG-PON2 is more flexible and has easier engineering, and provides 4 to 10 times the bandwidth over legacy PON systems. Key benefits include the ability to address load balancing of users by the use of tunable lasers used in the NG-PON2 ONTs, which is key for addressing business client needs. These allow changes to occur within 15 milliseconds to the desired wavelength. This in particular allows for problems such as “rogue” ONTs to be isolated, and specific ONTs identified for replacement. (You can my earlier column “Beware of Rogue ONTs!”, published in the April 2019 issue, at https://www.isemag.com/2019/04/beware-of-rogue-onts/.)
FTTx is evolving with the newer XG/XGS systems being installed. We’re also seeing the development and implementation of NG-PON2 by Verizon which is leapfrogging XGS-PON implementation in favor of TWDM. This addresses even greater bandwidth requirements.
The foresight by the standards groups to address wavelength assignments for migration and coexistence is commended as FTTx service providers continue to migrate to higher speeds to address the demands of a communications society.