Next Generation PON Technologies Explained
We are hearing the term Next Generation PON more and more, as service providers in various parts of the world are announcing trials and pilot deployments along with their vendors. What is next generation PON, which technologies hide behind these words, and what are the benefits that they provide to customers?
Let's start with history.
GPON which was first standardized by the ITU in 2004 and is still the most widely deployed PON technology in the world today. It is used by the vast majority of telecom operators and can deliver up to 2.5 Gbps downstream and up to 1.25 Gbps upstream over a shared network in FTTH or FTTx topology.
EPON is a parallel technology which uses a different protocol on a similar PON topology. It originated from Ethernet and was standardized by IEEE, and today it has limited deployments which are concentrated in a number of Asian countries.
New bandwidth-hungry applications are creating the demand for higher and higher throughput rates, and this demand is further fueled by competition among service providers. With fiber infrastructure becoming more widely available, these needs can be addressed by PON technologies that can exceed the throughput delivered by GPON and the traditional 1G EPON.
XG-PON1 can deliver 10 Gbps in the downstream and 2.5 Gbps in the upstream. Its principles of operation are similar to those of GPON, and in addition to the higher throughput rates it also includes several enhancements in the security and power saving modes. It was standardized in 2011 but had limited market acceptance, since many service providers felt that the increase in throughput compared to GPON is not significant enough to justify the shift. Some of the vendors reacted by putting their XG-PON1 development efforts on hold.
Then came NG-PON2, which was a much bigger promise, since it could deliver up to 40 Gbps in both directions. It is based on combining the traditional PON technology with WDM, so that 4 independent systems share the same optical infrastructure using 4 separate sets of wavelengths. Each of those systems supports 10 Gbps downstream and 2.5 Gbps or 10 Gbps upstream – both flavors are available – so the total on the PON is 4 times as much as on each individual one.
The systems on each set of wavelengths are independent with one exception. Every ONU can use any of the 4 sets of wavelengths – one set at a given time – and this requires a mechanism for initial wavelength assignment as well as for moving from one set of wavelengths to another, a process called Handover.
The principles of operation of NG-PON2 were defined by FSAN in 2012 and many service providers were glad to adopt it, only to realize that vendors were not able to deliver it, at least not at reasonable prices. The challenge was in the optical devices:
- The ONU needs a tunable optical transceiver, to allow it to select any of the 4 wavelengths in each direction, as well as switch between wavelengths.
- The OLT needs optics with high accuracy of the transmission wavelength and good separation of the received signals, since the 4 wavelengths are located closely on the spectrum.
As a result, for a long period of time services providers were stuck with no real next generation PON – very limited range of solutions for XG-PON1 with unclear future on the one hand, and no NG-PON2 on the other hand.
To bridge the gap vendors came out with some creative ideas. A number of vendors proposed a "fixed-wavelength NG-PON2"; the idea was to provide a symmetrical 10 Gbps PON system, which made use of the NG-PON2 protocol but had a fixed set of wavelengths. On the one hand, thanks to the fixed wavelengths it allowed the vendors to deliver a solution at reasonable prices; while on the other hand it had a clear migration path to NG-PON2, since once the optics became available they could be replaced quite easily and a WDM mux could be added to support true NG-PON2.
The ITU adopted this idea and in 2016 it was standardized under the name XGS-PON. It had good market acceptance – shortly after it was standardized several vendors introduced products, and service providers who had been waiting anxiously for next generation PON solutions began testing these products quite immediately. Nowadays many of these service providers are already in advanced phases of lab testing and pilot deployments.
In parallel to this the NG-PON2 optical devices are beginning to become commercially available and their prices are going down. As a result, some service providers have already begun testing this technology, while others are planning to do so in the near future.
Selecting the right equipment for the next generation PON technologies is a big challenge for service providers, especially in light of the variety of technologies and standards out there. It requires careful testing to ensure standard-compliance and interoperability, and in today's products also future readiness for NG-PON2. For those who are already testing NG-PON2, the tests must also cover the wavelength selection and handover mechanisms, which are critical factors in this technology.
Our NG-PON Xpert analyzer and OLT emulator was designed with these needs in mind, and it thus supports standards-based testing for XGS-PON and XG-PON1, as well as NG-PON2 (TWDM-PON) with a software-based wavelength switching mechanism.