Standardization Trends for Next-Generation
Passive Optical Network Stage 2 (NG-PON2)

1. Introduction

To meet the large demand for high capacity transmission in optical access systems, a 10-Gigabit-capable PON (passive optical network) has already been standardized by IEEE (Institute of Electrical and Electronics Engineers) and ITU-T (International Telecommunication Union-Telecommunication Standardization Sector). To enable cutting-edge standardization of future optical access systems, the FSAN (Full Service Access Network) Group and ITU-T SG15/Q2 (Study Group15/Question 2) are currently discussing the specifications of a 40-Gigabit-capable PON, which employs wavelength-division multiplexing (WDM) technology, for the purpose of enabling cost-effective 40-Gigabit-capable transmission capacity and multiple service capability. The FSAN Group is a forum for discussing future optical fiber access networks, and FSAN member companies offer their contributions to ITU-T SG15/Q2 in order to promote the development of international standards for PON systems.

2. NG-PON2

In 2010, FSAN initiated discussions on a 40-Gigabit-capable PON known as NG-PON2 (Next-Generation Passive Optical Network Stage 2), which is going to be standardized in the ITU-T Recommendation G.989 series. Table 1 lists the framework and standardization status of the G.989 series recommendations.

Table 1. Framework and status of ITU-T G.989 series recommendations.

As the table indicates, the G.989 series consists of recommendations G.989, G.989.1, G.989.2, and G.989.3. The G.989 recommendation specifies NG-PON2 definitions, abbreviations, and acronyms and will be consented in July 2015. The G.989.1 recommendation, which covers the general requirements for NG-PON2, was approved in March 2013. The G.989.2 recommendation, which specifies the physical media dependent (PMD) layer including the wavelength plan and power budget of NG-PON2, was consented in December 2013 and approved in December 2014. The G.989.3 recommendation specifies particulars for the transmission convergence (TC) layer of NG-PON2 such as the wavelength protocol and bandwidth allocation, and is targeted for consent in July 2015. These achievements so far suggest that the standardization of the G.989 series will be completed in 2015 or 2016.

3. Expected network function by in-service wavelength tuning

In developing colorless ONUs for NG-PON2, it is essential for the ONU transmitter and receiver to have wavelength tunability. Therefore, it is expected that using the in-service wavelength tuning technique in ONUs will enable NG-PON2 systems to have advanced network functions. In addition, cost-effective tunable components are important for practical deployment. The term in-service tuning means that an ONU can tune its wavelength during service operation in addition to the ONU’s initialization process. An example of an advanced network function called OLT (optical line terminal) protection is illustrated in Fig. 3. As shown at the top right, the OLT is composed of three OSUs (optical subscriber units), and different wavelengths are assigned to each one. In a normal state, OSU #1, OSU #2, and OSU #3 communicate with ONU 1 (λ_1u), ONU 2 and ONU 3 (λ_2u), and ONU 4 (λ_3u), respectively. Upon assuming a failure in OSU #2, ONU 2 and ONU 3 immediately tune their wavelengths to maintain communications with other OSUs. As shown at the bottom left, ONU 2 changes its wavelength from λ_2u to λ_1u in order to be connected to OSU #1, while ONU 3 changes its wavelength from λ_2u to λ_3u to be connected to OSU #3. One of the advantages of the OLT protection is that ONUs with the in-service tuning technique can easily provide a protection function without additional equipment, such as a backup OLT and an optical switch. Another example of in-service tuning is an OSU sleep function, which is also attractive as a means of reducing power consumption in the OLT. With this function, all ONUs are connected to one (or fewer) OSU(s), and the other OSUs can be forced to sleep. This function is also implemented by using wavelength tuning during service operation. For these advanced functions, a tunable component is required to have a short tuning time in order to avoid any signal frame loss even during a tuning process. On the other hand, such a fast tunable component is not needed if an operator requires wavelength tuning only for ONU initialization. In accordance with the aforementioned background, G.989.2 specifies tuning time classes, which are categorized into class 1 (<10 µs), class 2 (10 µs–25 ms), and class 3 (25 ms–1 s). The categorization is specified by taking into account the requirements from expected network functions in NG-PON2 and the current technologies of tunable components.

Fig. 3. Example of OLT protection technology using wavelength tuning in ONUs.

4. Future prospects

This article reviewed NG-PON2 as the latest standardization trend for future optical access networks. In the near future, FSAN and ITU-T will focus on discussing PtP WDM overlay specifications as an amendment to the G.989 series. It is also expected that further evolution of optical access network technologies will be discussed in FSAN and ITU-T.

References

Kota Asaka Senior Research Engineer, Optical Access System Project, NTT Access Network Service Systems Laboratories. He received the B.S. and M.S. in electrical engineering from Waseda University, Tokyo, in 1996 and 1999, respectively, and the Ph.D. in physics from Kitasato University, Tokyo, in 2008. In 1999, he joined NTT Photonics Laboratories, where he researched photonic integrated circuits and long-wavelength VCSELs (vertical cavity surface emitting lasers). From 2005–2008, he was engaged in research on OCT (optical coherence tomography) using SSG-DBR-LDs (super-structure-grating distributed Bragg reflector laser diodes) in collaboration with Kitasato University. From 2009–2012, he worked on the development of low-cost and small optical subassemblies for optical access networks. He has served as the working group secretary of TC (Technical Committee) 86/SC (Subcommittee) 86C/WG (Working Group) 4 of the IEC (International Electrotechnical Commission), and has worked on the international standardization of fiber optic systems and active devices. Since 2012, he has been with NTT Access Network Service Systems Laboratories and has been engaged in international standardization efforts for next-generation optical access systems in ITU-T SG15/Q2 and FSAN. He is a member of the IEEE Communications Society, the Institute of Electronics, Information and Communication Engineers (IEICE), and the Japan Society of Applied Physics.

Jun-ichi Kani Senior Research Engineer, Supervisor / Distinguished Researcher, Optical Access System Project, NTT Access Network Service Systems Laboratories. He received the B.E., M.E., and Ph.D. in applied physics from Waseda University, Tokyo, in 1994, 1996, and 2005, respectively. In 1996, he joined the NTT Optical Network Systems Laboratories, where he was engaged in research on optical multiplexing and transmission technologies. Since 2003, he has been with NTT Access Network Service Systems Laboratories, where he is involved in the research and development of optical communications systems for metro and access applications. He received the Leonard G. Abraham Prize from the IEEE Communications Society, the Best Paper Award from the 18th Optoelectronics and Communications Conference (OECC) in 2013, and 7 other academic/industry awards. He has been participating in ITU-T and working on the FSAN initiative since 2003. He has served as Associate Rapporteur of ITU-T Q2/15 (optical systems for fibre access networks) since 2009. He is a member of IEEE and IEICE.