FSAN adds WDM for next-generation PON standard
The Full Service Access Network (FSAN) group has chosen wavelength division multiplexing (WDM) to complement PON's traditional time-sharing scheme for the NG-PON2 standard.
"The technology choice allows us to have a single platform supporting both business and residential services"
Vincent O'Byrne, Verizon
The TWDM-PON scheme for NG-PON2 will enable operators to run several services over one network: residential broadband access, business services and mobile back-hauling. In addition, NG-PON2 will support dedicated point-to-point links – via a WDM overlay - to meet more demanding service requirements.
FSAN will work through the International Telecommunication Union (ITU) to turn NG-PON2 into a standard. Standards-compliant NG-PON2 equipment is expected to become available by 2014 and be deployed by operators from 2015. But much work remains to flush out the many details and ensure that the standard meets the operators’ varied requirements
Significance
The choice of TWDM-PON represents a pragmatic approach by FSAN. TWDM-PON has been chosen to avoid having to make changes to the operators' outside plant. Instead, changes will be confined to the PON's end equipment: the central office's optical line terminal (OLT) and the home or building's optical networking unit (ONU).
Operators yet to adopt PON technology may use NG-PON2's extended reach to consolidate their network by reducing the number of central offices they manage. Other operators already having deployed PON may use NG-PON2 to boost broadband capacity while consolidating business and residential services onto the one network.
US operator Verizon has deployed GPON and says the adoption of NG-PON2 will enable it to avoid the intermediate upgrade stage of XGPON (10Gbps GPON).
"The [NG-PON2] technology choice allows us to have a single platform supporting both business and residential services," says Vincent O'Byrne, director of technology, wireline access at Verizon. "With the TWDM wavelengths, we can split them: We could have a 10G/10G service or ten individual 1G/1G services and, in time, have also residential customers."
The technology choice for NG-PON2 is also good news for system vendors such as Huawei and Alcatel-Lucent that have already done detailed work on TWDM-PON systems.
Specification
NG-PON2's basic configuration will use four wavelengths, resulting in a 40Gbps PON. Support for eight (80G) and 16 wavelengths (160G) are also being considered.
Each wavelength will support 10Gbps downstream (from the central office to the end users) and 2.5Gbps upstream (XGPON) or 10Gbps symmetrical services for business users.
"The idea is to reuse as much as possible the XGPON protocol in TWDM-PON, and carry that protocol on multiple wavelengths," says Derek Nesset, co-chair of FSAN's NGPON task group.
The PON's OLT will support the 4, 8 or 16 wavelengths using lasers and photo-detectors as well as optical multiplexing, while the ONU will require a tunable laser and a tunable filter, to set the ONU to the PON's particular wavelengths.
Other NG-PON2 specifications include the support of at least 1Gbps services per ONU and a target reach of 40km. NG-PON2 will also support 60-100km links but that will require technologies such as optical amplification.
"The [NG-PON2] ONUs should be something like the cost of a VDSL or a GPON modem, so there is a challenge there for the [tunable] laser manufacturers"
Derek Nesset, co-chair of FSAN's NGPON task group
What next?
"The big challenge and the first challenge is the wavelength plan [for NG-PON2]," says O'Byrne.
One proposal is for TWDM-PON's wavelengths to replace XGPON's. Alternatively, new unallocated spectrum could be assigned to ensure co-existence with existing GPON, RF video and XGPON. However, such a scheme will leave little spectrum available for NG-PON2. Some element of spectral flexibility will be required to accommodate the various co-existence scenarios in operator networks. That said, Verizon expects that FSAN will look for fresh wavelengths for NG-PON2.
"FSAN is a sum of operators opinions and requirements, and it is getting hard," says O'Byrne. "Our preference would be to reuse XGPON wavelengths but, at the last meeting, some operators want to use XGPON in the coming years and aren't too favourable to recharacterising that band."
Another factor regarding spectrum is how widely the wavelengths will be spaced; 50GHz, 100GHz or the most relaxed 200GHz spacing are all being considered. The tradeoff here is hardware design complexity and cost versus spectral efficiency.
There is still work to be done to define the 10Gbps symmetrical rate. "Some folks are also looking for slightly different rates and these are also under discussion," says O'Byrne.
Another challenge is that TWDM-PON will also require the development of tunable optical components. "The ONUs should be something like the cost of a VDSL or a GPON modem, so there is a challenge there for the [tunable] laser manufacturers," says Nesset.
Tunable laser technology is widely used in optical transport, and high access volumes will help the economics, but this is not the case for tunable filters, he says.
The size and power consumption of PON silicon pose further challenges. NG-PON2 will have at least four times the capacity, yet operators will want the OLT to be the same size as for GPON.
Meanwhile, FSAN has several documents in preparation to help progress ITU activities relating to NG-PON2's standardisation.
FSAN has an established record of working effectively through the ITU to define PON standards, starting with Broadband PON (BPON) and Gigabit PON (GPON) to XGPON that operators are now planning to deploy.
FSAN members have already submitted a NG-PON2 requirements document to the ITU. "This sets the framework: what is it this system needs to do?" says Nesset. "This includes what client services it needs to support - Gigabit Ethernet and 10 Gigabit Ethernet, mobile backhaul latency requirements - high level things that the specification will then meet."
In June 2012 a detailed requirements document was submitted as was a preliminary specification for the physical layer. These will be followed by documents covering the NG-PON2 protocol and how the management of the PON end points will be implemented.
If rapid progress continues to be made, the standard could be ratified as early as 2013, says O'Byrne.
ZTE takes PON optical line terminal lead
ZTE shipped 1.8 million passive optical network (PON) optical line terminals (OLTs) in 2011 to become the leading supplier with 41 percent of the global market, according to Ovum.
"ZTE is co-operating with some Tier 1 operators in Europe and the US for 10GEPON and XGPON1 testing"
Song Shi Jie, ZTE
The market research firm also ranks the Chinese equipment maker as the second largest supplier of PON optical network terminals (ONT), with 28 per cent global market share in 2011.
China now accounts for over half the total fibre-to-the-x (FTTx) deployments worldwide. ZTE says 1.05 million of its OLTs were deploy in China, with 70 percent for the EPON standard and the rest GPON. Overall EPON accounts for 85% of deployments in China. However GPON deployments are growing and ZTE expects the technology to gain market share in China.
There are some 300 million broadband users in China, made up of DSL, fibre-to-the-building (FTTB) and -curb (FTTC), says Song Shi Jie, director of fixed network product line at ZTE.
Of the three main operators, China Telecom is the largest. It is deploying FTTB and is moving to fibre-to-the-home (FTTH) deployments using GPON. China Unicom has a similar strategy. China Mobile is focussed on FTTB and LAN technology; because it is a mobile operator and has no copper line assets it uses LAN cabling for networking within the building.
The split ratio - the number of PON ONTs connected to each OLT - varies depending on the deployment. "In the fibre-to-the-building scenario, the typical ratio is 1:8 or 1:16; for fibre-to-the-home the typical ratio is 1:64," says Song.
ZTE has also deployed 200,000 10 Gigabit EPON (10GEPON) lines in China but none elsewhere, either 10GEPON or XGPON1 (10 Gigabit GPON). "ZTE is co-operating with some Tier 1 operators in Europe and the US for 10GEPON and XGPON1 testing," says Song.
Song attributes ZTE's success to such factors as reduced power consumption of its PON systems and its strong R&D in access.
The vendor says its PON platforms consume a quarter less power than the industry average. Its systems use such techniques as shutting down those OLT ports that are not connected to ONTs. It also employs port idle and sleep modes to save power when there is no traffic. Meanwhile, ZTE has 3,000 engineers engaged in fixed access product R&D.
As for the next-generation NGPON2 being development by industry body FSAN, Song says there are a variety of technologies being proposed but that the picture is still unclear.
ZTE is focussing on three main next-generation PON technologies: wavelength division multiplexing PON (WDM-PON), hybrid time division multiplexing (TDM)/ WDM-PON (or TWDM-PON) and orthogonal frequency division multiplexing (OFDM) PON. "We think OFDM PON can provide high security, high bandwidth and easy network maintenance," says Song.
ZTE says that the NGPON2 standard will be mature in 2015 but that commercial deployments will only start in 2018.
FSAN close to choosing the next generation of PON
Briefing: Next-gen PON
Part 1: NG-PON2
The next-generation passive optical network (PON) will mark a departure from existing PON technologies. Some operators want systems based on the emerging standard for deployment by 2015.
“One of the goals in FSAN is to converge on one solution that can serve all the markets"
Derek Nesset, co-chair of FSAN's NGPON task group
The Full Service Access Network (FSAN) industry group is close to finalising the next optical access technology that will follow on from 10 Gigabit GPON.
FSAN - the pre-standards forum consisting of telecommunications service providers, testing labs and equipment manufacturers - crafted what became the International Telecommunication Union's (ITU) standards for GPON (Gigabit PON) and 10 Gigabit GPON (XGPON1). In the past year FSAN has been working on NG-PON2, the PON technology that comes next.
“One of the goals in FSAN is to converge on one solution that can serve all the markets - residential users, enterprise and mobile backhaul," says Derek Nesset, co-chair of FSAN's NGPON task group.
Some mobile operators are talking about backhaul demands that will require multiple 10 Gigabit-per-second (Gbps) links to carry the common public radio interface (CPRI), for example. The key design goal, however, is that NG-PON2 retains the capability to serve residential users cost-effectively, stresses Nesset.
FSAN says it has a good description of each of the candidate technologies: what each system looks like and its associated power consumption. "We are trying to narrow down the solutions and the ideal is to get down to one,” says Nesset.
The power consumption of the proposed access scheme is of key interest for many operators, he says. Another consideration is the risk associated with moving to a novel architecture rather than adopting an approach that builds on existing PON schemes.
Operators such as NTT of Japan and Verizon in the USA have a huge installed base of PON and want to avoid having to amend their infrastructure for any next-generation PON scheme unable to re-use power splitters. Other operators such as former European incumbents are in the early phases of their rollout of PON and have Greenfield sites that could deploy other passive infrastructure technologies such as arrayed waveguide gratings (AWG).
"The ideal is we select a system that operates with both types of infrastructure," says Nesset. "Certain flavours of WDM-PON (wavelength division multiplexing PON) don't need the wavelength splitting device at the splitter node; some form of wavelength-tuning can be installed at the customer premises." That said, the power loss of existing optical splitters is higher than AWGs which impacts PON reach – one of several trade-offs that need to be considered.
Once FSAN has concluded its studies, member companies will generate 'contributions' for the ITU, intended for standardisation. The ITU has started work on defining high-level requirements for NG-PON2 through contributions from FSAN operators. Once the NG-PON2 technology is chosen, more contributions that describe the physical layer, the media access controller and the customer premise equipment's management requirements will follow.
Nesset says the target is to get such documents into the ITU by September 2012 but achieving wide consensus is the priority rather than meeting this deadline. "Once we select something in FSAN, we expect to see the industry ramp up its contributions based on that selected technology to the ITU," says Nesset. FSAN will select the NG-PON2 technology before September.
NG-PON2 technologies
Candidate technologies include an extension to the existing GPON and XGPON1 based on time-division multiplexing (TDM). Already vendors such as Huawei have demonstrated prototype 40 Gigabit capacity PON systems that also support hybrid TDM and WDM-PON (TWDM-PON). Other schemes include WDM-PON, ultra-dense WDM-PON and orthogonal frequency division multiplexing (OFDM).
Nesset says there are several OFDM variants being proposed. He views OFDM as 'DSL in the optical domain’: sub-carriers finely spaced in the frequency domain, each carrying low-bit-rate signals.
One advantage of OFDM technology, says Nesset, includes taking a narrowband component to achieve a broadband signal: a narrowband 10Gbps transmitter and receiver can achieve 40Gbps using sub-carriers, each carrying quadrature amplitude modulation (QAM). "All the clever work is done in CMOS - the digital signal processing and the analogue-to-digital conversion," he says. The DSP executes the fast Fourier transform (FFT) and the inverse FFT.
"We are trying to narrow down the solutions and the ideal is to get down to one"
Another technology candidate is WDM-PON including an ultra-dense variant that promises a reach of up to 100km and 1,000 wavelengths. Such a technology uses a coherent receiver to tune to the finely spaced wavelengths.
In addition to being compatible with existing infrastructure, another FSAN consideration is compatibility with existing PON standards. This is to avoid having to do a wholesale upgrade of users. For example, with XGPON1, the optical line terminal (OLT) using an additional pair of wavelengths - a wavelength overlay - sits alongside the existing GPON OLT. ”The same principle is desirable for NG-PON2,” says Nesset.
However, an issue is that spectrum is being gobbled up with each generation of PON. PON systems have been designed to be low cost and the transmit lasers used are not wavelength-locked and drift with ambient temperature. As such they consume spectrum similar to coarse WDM wavelength bands. Some operators such as Verizon and NTT also have a large installed base of analogue video overlay at 1550nm.
”So in the 1500 band you've got 1490nm for GPON, 1550nm for RF (radio frequency) video, and 1577nm for XGPON; there are only a few small gaps,” says Nesset. A technology that can exploit such gaps is both desirable and a challenge. “This is where ultra-dense WDM-PON could come into play,” he says. This technology could fit tens of channels in the small remaining spectrum gaps.
The technological challenges implementing advanced WDM-PON systems that will likely require photonic integration is also a concern for the operators. "The message from the vendors is that ’when you tell us what to do, we have got the technology to do it’,” says Nesset. ”But they need the see the volume applications to justify the investment.” However, operators need to weigh up the technological risks in developing these new technologies and the potential for not realising the expected cost reductions.
Timetable
Nesset points out that each generation of PON has built on previous generations: GPON built on BPON and XGPON on GPON. But NG-PON2 will inevitably be based on new approaches. These include TWDM-PON which is an evolution of XG-PON into the wavelength domain, virtual point-to-point approaches such as WDM-PON that may also use an AWG, and the use of digital signal processing with OFDM or coherent ultra dense WDM-PON. ”It is quite a challenge to weigh up such diverse technological approaches,” says Nesset.
If all goes smoothly it will take two ITU plenary meetings, held every nine months, to finalise the bulk of the NG-PON2 standard. That could mean mid-2013 at the earliest.
FSAN's timetable is based on operators wanting systems deployable in 2015. That requires systems to be ready for testing in 2014.
“[Once deployed] we want NG-PON2 to last quite a while and be scalable and flexible enough to meet future applications and markets as they emerge,” says Nesset.