New telecommunications services (high-definition 3D television and video on demand, unified communications and videoconferencing, online gaming, cloud computing, etc.) demand more advanced telecommunications networks than those currently in place. Furthermore, it is estimated that there will be more than 50 billion devices connected to the InternetGPON (Gigabit Passive Optical Network) [1], standardized in 2004 in the ITU-T G.984 series of recommendations and with commercial systems in operation since 2006, has been the technology chosen by most operators to offer residential broadband services over fiber optics. However, GPON deployments have not met expectations in many countries, mainly due to the economic crisis and the lack of a clear and stable regulatory framework to encourage investment.
For a telecommunications technology to be successful, it must adapt to future requirements and be compatible with its predecessor technologies. For example, this has been the case with ADSL, where backward-compatible technologies have emerged that improved upon previous ones, such as ADSL2, ADSL2+, and VDSL2 [2]. Similarly, there are several proposals and evolutionary paths from GPON to the new generations of PON technologies, known as NG-PON (Next Generation PON). The main requirements of NG-PON are to increase the bandwidth and reach of GPON, maximizing the reuse of the existing passive optical network (ODN) installed from the central office to the subscribers, as its cost represents approximately 75-85% of the cost of providing fiber optic broadband to users.
Within NG-PON, a distinction is made between XG-PON (NG-PON1) and WDM-PON (NG-PON2). NG-PON1 operates on the GPON ODN, while NG-PON2 will require certain modifications. There is significant industry interest in both technologies, although GPON will likely be the predominant access technology for residential users with FTTH (Fiber-To-The-Home) over the next 3-5 years. During this period, XG-PON1 is likely to be used for residential users with FTTB/C (Fiber-To-The-Building/Curv) [3], and XG-PON2 for businesses and mobile backhaul or FTTT/CS (Fiber-To-The-Tower/Cell Site). In other words, GPON and NG-PON deployments are expected to coexist, with the technology chosen depending on the operator's budget, expected return on investment and payback period, and the applications they wish to offer.
Several suppliers are researching and developing NG-PON systems, with Alcatel-Lucent, Ericsson and its joint venture LG-Ericsson, Huawei, and ZTE being the clear leaders. Other notable suppliers include Adtran, ADVA, Allied Data, Alphion, Calix, ECI, Enablence, Fujitsu, Hitachi, NEC, NSN, Motorola, Tellabs, and Zhone. To date, several XG-PON pilot tests have been conducted (Portugal Telecom, Verizon, etc.), and there are even small commercial deployments of WDM-PON (Agder, UNET, Korea Telecom, etc.). These experiences have demonstrated that these promising technologies are not yet mature enough for large-scale residential deployments and that their costs are significantly higher than GPON. However, as the cost of optical components decreases and their interoperability improves, these technologies, and especially WDM-PON, will gain attention among operators due to their remarkable advantages over other PON technologies, demonstrated in various research projects such as GigaWAM, Sardana, and Pieman.
XG-PON
In the medium term, XG-PON (NG-PON1) will begin to be deployed. This technology again uses Time Division Multiplexing (TDM) but offers higher line speeds than GPON. XG-PON1 supports 10 Gbps downstream (from the central office to the users) and 2.5 Gbps upstream (from the users to the central office), while XG-PON2 supports symmetrical 10 Gbps. This contrasts with GPON's 2.5 Gbps downstream and 1.25 Gbps upstream. XG-PON1 is the preferred technology for ONTs (Optional Network Units) in FTTH, while XG-PON2 is preferred for MDUs (Multi-Dwelling Units) in FTTB/C.
The XG-PON1 standard, which is based on the same principles as GPON (downstream optical broadcast, upstream TDMA, OMCI for ONU control and management, etc.), is defined in the ITU-T G.987 and G.988 recommendations, finalized in 2010. In addition to bandwidth, XG-PON1 differs from GPON in terms of wavelengths, optical power budgets (29 dB, 31 dB, and 33 and 35 dB), and new improvements in security mechanisms and energy-saving methods [4]. Unlike XG-PON1, the XG-PON2 standards are not yet finalized due to the complexity of implementing 10 Gbps upstream TDMA, and their completion is not expected until 2012.Several ODN architectures exist that allow coexistence between GPON and XG-PON [5]. To achieve this, the wavelength bands used by XG-PON, as defined in G.987.1, differ from those of GPON in both the uplink and downlink directions, as shown in Table 1. The wavelength range for RF video broadcast services (1480–1560 nm) is inherited from GPON. On the OLT side, the two systems, GPON and XG-PON, are combined with a coexistence optical filter (also known as “WDM1”). On the other hand, the ONUs incorporate wavelength blocking filters (WBFs), as specified in ITU-T G.984.5. GPON deployments using these ONUs will allow operators to gradually migrate to XG-PON (i.e., on a customer-by-customer basis) by replacing the customer's GPON ONT with an XG-PON1 ONT without interrupting or affecting service for customers who are not being migrated.
During these years, significant improvements have also been made to the range of GPON systems. While these improvements haven't yet materialized in current GPON deployments, they will be present in future deployments of both GPON and XG-PON. The 28 dB optical budget of GPON technology using Class B+ optics allows for a range of 30 km when the split ratio is limited to 1:16. Deployments typically consider a split ratio of 32-64 over 20 km. The much newer Class C+ optics are based on higher-power transmitters and, optionally, more sensitive receivers; this allows for an additional 4 dB to be added to the link budget, thus achieving a higher split ratio or greater range. With Class C+ optics, distances of up to 30 km can be achieved with a split factor of 64. GPON extenders, with a PON regenerator or an active optical amplifier between the OLT and the splitter, allow for distances of up to 60 km with a split factor of 128. Several architectures and interfaces are defined in ITU-T G.984.6 for GPON extenders, making them a very interesting option for providing fixed broadband in rural or remote areas, thus minimizing the number of exchanges required by the operator.
XG-PON1 is the natural next step in the evolution of PON technologies. Although the standards are ready and pre-commercial system pilots have been conducted, commercial deployments will not begin until 2012. This is because the cost of this technology is higher than GPON (especially in the case of XG-PON2, whose deployments are expected to be very limited), interoperability is lower due to the shorter standards refinement time, its energy consumption is significantly higher, and short- and medium-term bandwidth needs can be met with GPON. However, GPON and XG-PON will coexist for several years, thanks to the gradual migration process defined by the ITU-T.
WDM-PON
In the longer term, WDM-PON will be deployed, which uses Wavelength Division Multiplexing (WDM), meaning that each ONU (Optical Network Unit) receives a specific wavelength (λ). NG-PON2 is also exploring new modulation formats such as OFDM (Orthogonal Frequency Division Multiplexing) and CDM (Code Division Multiplexing), as well as 40G TDM-PON, hybrid TDM-WDM-PON, and others; however, WDM-PON is the most promising technology in the short term.
The technologies required for WDM-PON are available today, and LG Ericsson has already implemented small-scale commercial deployments using proprietary systems. Therefore, further standardization and cost reductions for optical components are necessary to make these technologies suitable for large-scale deployments. The FSAN NGA2 group has already begun the WDM-PON standardization process, although it is not expected to be fully standardized until 2013 and cost-optimized until 2014-2015, allowing for the start of large-scale commercial deployments.
WDM-PON is actually much simpler than other PON technologies because, although it maintains the same point-to-multipoint architecture as TDM-PON at the physical level, at the virtual level each ONU (Optical Network Unit) has a dedicated bandwidth. Thus, we can logically view each bandwidth as a point-to-point channel capable of carrying dedicated and symmetrical speeds to each user, ranging from 100 Mbps to 10 Gbps. For interference-free transmission over a single fiber, different bandwidth bands are used for the upstream and downstream directions. In WDM-PON, the upstream and downstream bandwidths can be dedicated to the subscriber or FTTH business customer via an ONT (Optical Network Terminal), but they can also be shared by multiple FTTB/C subscribers through an MDU (Multi-Drewling Unit).
The use of WDM-PON in the access network offers significant advantages over TDM-PON techniques:
- Since there is no time sharing of the l, it is much simpler to offer different, symmetrical or asymmetrical, dedicated, and contention-free high bandwidths to each subscriber.
- High bandwidth scalability due to bit rate transparency and ease in adding or removing channels.
- Greater distances and division factors, due to lower optical losses.
- Network management, operation and maintenance made simpler.
- Greater security, due to the separation of traffic between subscribers.
- Greater ease in creating open optical networks with "unbundling" of ls, which allows the sharing of the same physical access network by several operators as occurs in current xDSL networks over copper.
- Lower latency, which together with high bandwidth is very important for applications such as LTE mobile backhaul and will also improve the user experience in online games, cloud computing services and unified communications, etc.
In WDM-PON, the ODN (Optical Network Design) of TDM-PON technologies is not maintained intact, and at the very least, it is necessary to replace the splitter/combiner used by GPON and XG-PON with an Arrayed Wavelength Grating (AWG) multiplexer/demultiplexer. Like the splitter, the AWG is a passive component that can operate over a wide temperature range, making it suitable for integration into street-level cabinets outside of operator central offices. While the splitter replicates the optical signal across all its outputs from the central office to the end users, dividing the power among them, the AWG directs each wavelength to its corresponding ONU (Operating Unit) with very low losses. For example, while a 1:64 splitter introduces losses of around 20 dB, an AWG introduces only about 8 dB. In this way, the extra optical budget can be used to reduce the specifications and, therefore, the cost of the optical components. or to increase the split ratio or the distance. Thus, WDM-PON can support distances of up to 85 km without the need for extenders, allowing operators to consolidate the necessary active equipment in the access network and significantly reduce the number of exchanges.
The main challenge of WDM-PON is how to enable a single ONU to operate on different I-lines (colorless ONUs). The ability for ONUs to be tuned to any I-line is absolutely essential for improving manufacturing and logistics efficiency, as well as reducing installation costs and complexity, and providing ongoing support. Several WDM-PON technologies exist to achieve "colorless ONUs," depending on how the uplink (the one transmitted by the ONU) is configured [6]:
- Remote preselection. This is the solution used by LG-Ericsson. A broadband light source (BLS) is generated remotely at the OLT and filtered by the AWG, reaching the ONU, where it powers a modulating device (RSOA, REAM, IL-FPLD). It is the most mature and economical technology and allows up to 32 ONUs per PON (i.e., a division factor of 32), distances of up to 40 km, and speeds of up to 1 Gbps.
- Reuse. A portion of the energy from each downlink is reused to modulate the uplink, eliminating the costs associated with BLS and improving carrier quality and spectral efficiency. This technology still requires further research and will allow up to 96 ONUs per PON, distances of up to 50 km, and speeds of up to 2.5 Gbps.
- Tuning. This is the technology used in current long-distance and metropolitan DWDM systems, but it is currently very expensive for access. In this case, the uplink is generated locally at the ONU. It is the highest-performing technology: it allows up to 96 ONUs per PON, distances of up to 85 km, and capacities of up to 10 Gbps.
Literature
[1] “What is… GPON (Gigabit Passive Optical Network)”. Ramón Jesús Millán Tejedor, BIT nº 166, COIT & AEIT, December 2007, pp. 63-67. [www.coit.es/publicaciones/bit/bit166/63-67.pdf].
[2] “What are ADSL, ADSL2, ADSL2+ and VDSL2?”. Ramón Jesús Millán Tejedor, Monografias.com, no. 62, 2007. [http://
www.ramonmillan.com/tutoriales/vdsl2.
php]
[3] “FTTB & VDSL2... copper has a long life ahead of it.” Ramón Jesús Millán Tejedor, DINTEL Bulletin on Security and Information Technologies, March 24, 2009. [www.
ramonmillan.com/documentos/fttbvdsl2.
[pdf]
[4] “Technology to the rescue of next-generation 10G PON networks.” Allard van der Horst, Ligthwave, April 2010, p. 17-21. [http://online.qmags.com/LW0410/Default.aspx]
[5] “Shaping the future of broadband via next-generation fixed access networks”. Michael Gronovius, Lightwave, September/October 2011, p. 17-22. [http://online.qmags.com/
/LW0911/Default.aspx].
[6] “Ensuring the future of your fiber access”. Ericsson Paper, May 2011. [archive.ericsson.net/service/internet/picov/get?DocNo=44/28701-FGD101040&Lang=
=EN&HighestFree=Y].
