Business services and mobile revive WDM-PON interest
"WDM-PON is many things to many people" - Jon Baldry
It was in 2005 that Novera Optics, a pioneer of WDM-PON (wavelength-division multiplexing, passive optical networking), was working with Korea Telecom in a trial involving 50,000 residential lines. Yet, one decade later, WDM-PON remains an emerging technology. And when a WDM-PON deployment does occur, it is for business services and mobile backhaul rather than residential broadband.
WDM-PON delivers high-capacity, symmetrical links using a dedicated wavelength. The links are also secure, an important consideration for businesses, and in contrast to PON where data is shared between all the end points, each selecting its addressed data.
One issue hindering the uptake of WDM-PON is the lack of a common specification. "WDM-PON is many things to many people," says Jon Baldry, technical marketing director at Transmode.
One view of WDM-PON is as the ultimate broadband technology; this was Novera's vision. Other vendors, such as Transmode, emphasise the WDM component of the technology, seeing it as a way to push metro-style networking towards the network edge, to increase bandwidth and for operational simplicity.
WDM-PON's uptake for residential access has not yet happened because the high bandwidth it offers is still not needed, while the system economics do not match those of PON.
Gigabit PON (GPON) and Ethernet PON (EPON) are now deployed in the tens of millions worldwide. And operators can turn to 10G-EPON and XG-PON when the bandwidth of GPON and EPON are insufficient. Beyond that, TWDM-PON (Time and Wavelength Division Multiplexing PON) is an emerging approach, promoted by the likes of Alcatel-Lucent and Huawei. TWDM-PON uses wavelength-division multiplexing as a way to scale PON, effectively supporting multiple 10 Gigabit PONs, each riding on a wavelength.
Carriers like the reassurance a technology roadmap such as PON's provides, but their broadband priority is wireless rather than wireline. The bigger portion of their spending is on rolling out LTE since wireless is their revenue earner.
As for fixed broadband, operators are being creative.
G.fast is one fixed broadband example. G.fast is the latest DSL standard that supports gigabit speeds over telephone wire. Using G.fast, operators can combine fibre and DSL to achieve gigabit rates and avoid the expense of taking fibre all the way to the home. BT is one operator backing G.fast, with pilot schemes scheduled for the summer. And if the trials are successful, G.fast deployments could start next year.
Deutsche Telekom is promoting a hybrid router to customers that combines fixed and wireless broadband, with LTE broadband kicking in when the DSL line becomes loaded.
Meanwhile, vendors with a WDM background see WDM-PON as a promising way to deliver high-volume business services, while also benefiting from the operator's cellular push by supporting mobile backhaul and mobile fronthaul. They don't dismiss WDM-PON for residential broadband but accept that the technology must first mature.
Transmode announced recently its first public customer, US operator RST Global Communications, which is using the vendor's iWDM-PON platform for business services.
"Our primary focus is business and mobile backhaul, and we are pushing WDM deeper into access networks," says Baldry. "We don't want a closed network where we treat WDM-PON differently to the way we treat the rest of the network." This means using the C-band wavelength grid for metro and WDM-PON. This avoids having to use optical-electrical-optical translation, as required between PON and WDM networks, says Baldry.
The iWDM-PON system showing the seeder light source at the central office (CO) optical line terminal (OLT), and the multiplexer (MDU) that selects the individual light band for the end point customer premise equipment (CPE). Source: Transmode.
Transmode's iWDM-PON
Several schemes are being pursued to implement WDM-PON. One approach is seeded or self-tuning, where a broadband light source is transmitted down the fibre from the central office. An optical multiplexer is then used to pick off narrow bands of the light, each a seeder source to set the individual wavelength of each end point optical transceiver. An alternative approach is to use a tunable laser transceiver to set the upstream wavelength. A third scheme combines the broadband light source concept with coherent technology that picks off each transceiver's wavelength. The coherent approach promises extremely dense, 1,000 wavelength WDM-PONs.
Transmode has chosen the seeded scheme for the iWDM-PON platform. The system delivers 40, 1 Gigabit-per-second (Gbps) wavelengths spaced 50 GHz apart. The reach between the WDM-PON optical line terminal (OLT) and the optical network unit (ONU) end-points is 20 km without dispersion compensation fibre, or 30 km using such fibre. The platform uses WDM-PON SFP pluggable modules. The SFPs are MSA-compliant and use a fabry-perot laser and an avalanche photo-detector optimised for the injection-locked signal.
"We use the C-band and pluggable optics, so the choice of using WDM-PON optics or not is up to the customer," says Baldry. "It should not be a complicated decision, and the system should work seamlessly with everything else you do, enabling a mix of WDM-PON and regular higher speed or longer reach WDM over the same access network, as needed."
Baldry claims the approach has economic advantages as well as operational benefits. While there is a need for a broadband light source, the end point SFP WDM-PON transceivers are cheaper compared to fixed or tunable optics. Also setting the wavelengths is automated; the engineers do not need to set and lock the wavelength as they do using a tunable laser.
"The real advantage is operational simplicity," says Baldry, especially when an operator needs to scale optically connected end-points as they grow business and mobile backhaul services. "That is the intention of a PON-like network; if you are ramping up the end points then you have to think of the skill levels of the installation crews as you move to higher service volumes," he says.
RST Global Communications uses Transmode's Carrier Ethernet 2.0 as the service layer between the demarcation device (network interface device or NID) at the customer's premises, while using Transmode's packet-optical cards in the central office. WDM-PON provides the optical layer linking the two.
An early customer application for RST was upgrading a hotel's business connection from a few megabits to 1Gbps to carry Wi-Fi traffic in advance of a major conference it was hosting.
Overall, Transmode has a small number of operators deploying the iWDM-PON, with more testing or trialing it, says Baldry. The operators are interested in using the WDM-PON platform for mobile backhaul, mobile fronthaul and business services.
There are also operators that use installed access/ customer premise equipment from other vendors, exploring whether Transmode's WDM-PON platform can simplify the optical layer in their access networks.
Further developments
Transmode's iWDM-PON upgrade plans include moving the system from a two fibre design - one for the downstream traffic and one for the upstream traffic - to a single fibre one. To do this, the vendor will segment the C-band into two: half the C-band for the uplink and half for the downlink.
Another system requirement is to increase the data rate carried by each wavelength beyond a gigabit. Mobile fronthaul uses the Common Public Radio Interface (CPRI) standard to connect the remote radio head unit that typically resides on the antenna and the baseband unit.
CPRI data rates are multiples of the basic rate of 614.4 Mbps. As such 3 Gbps, 6 Gbps and rates over 10 Gbps are used. Baldry says the current iWDM-PON system can be extended beyond 1 Gbps to 2.5 Gbps and potentially 3 Gbps but because the system in noise-limited, the seeder light scheme will not stretch to 10 Gbps. A different optical scheme will be needed for 10 Gigabit. The iWDM-PON's passive infrastructure will allow for an in-service upgrade to 10 Gigabit WDM-PON technology once it becomes technically and economically viable.
Transmode has already conducted mobile fronthaul field trials in Russia and in Asia, and lab trials in Europe, using standard active and passive WDM and covering the necessary CPRI rates. "We are not mixing it with WDM-PON just yet; that is the next step," says Baldry.
Further information
WDM-PON Forum, click here
Lightwave Magazine: WDM-PON is a key component in next generation access
ECOC 2013 review - Part 2
- Oclaro's Raman and hybrid amplifier platform for new networks
- MxN wavelength-selective switch from JDSU
- 200 Gigabit multi-vendor coherent demonstration
- Tunable SFP+ designs proliferate
- Finisar extends 40 Gigabit QSFP+ to 40km
- Oclaro’s tackles wireless backhaul with 2km SFP+ module
Finisar's 40km 40 Gig QSFP+ demo. Source: Finisar
Amplifier market heats up
Oclaro detailed its high performance Raman and hybrid Raman/ Erbium-doped fibre amplifier platform. "The need for this platform is the high-capacity, high channel rates being installed [by operators] and the desire to be scalable - to support 400 Gig and Terabit super-channels in future," says Per Hansen, vice president of product marketing, optical networks solutions at Oclaro.
"Amplifiers are 'hot' again," says Daryl Inniss, vice president and practice leader components at market research firm, Ovum. For the last decade, amplifier vendors have been tasked with reducing the cost of their amplifier designs. "Now there is a need for new solutions that are more expensive," says Inniss. "It is no longer just cost-cutting."
Amplifiers are used in the network backbone to boost the optical signal-to-noise ratio (OSNR). Raman amplification provides significant noise improvement but it is not power efficient so a Raman amplifier is nearly always matched with an Erbium one. "You can think of the Raman as often working as a pre-amp, and the Erbium-doped fibre as the booster stage of the hybrid amplifier," says Hansen. System houses have different amplifier approaches and how they connect them in the field, while others build them on one card, but Raman/ Erbium-doped fibre are almost always used in tandem, says Hansen.
Oclaro provides Raman units and hybrid units that combine Raman with Erbium-doped fibre. "We can deliver both as a super-module that vendors integrate on their line cards or we can build the whole line card for them" says Hansen.
The Raman amplifier market is way bigger than people have forecast
Since Raman launches a lot of pump power into the fibre, it is vital to have low-loss connections that avoid attenuating the gain. "Raman is a little more sensitive to the quality of the connections and the fibre," says Hansen. Oclaro offers scan diagnostic features that characterise the fibre and determine whether it is safe to turn up the amplification.
"It can analyse the fibre and depending on how much customers want us to do, we can take this to the point that it [the design] can tell you what fibre it is and optimise the pump situation for the fibre," says Hansen. In other cases, the system vendors adopt their own amplifier control.
Oclaro says it is in discussion with customers about implementations. "We are shipping the first products based on this platform," says Hansen.
"[The] Raman [amplifier market] is way bigger than people have forecast," says Inniss. This is due to operators building long distance networks that are scalable to higher data rates. "Coherent transmission is the focal point here, as coherent provides the mechanism to go long distance at high data rates," says Ovum analyst, Inniss.
Wavelength-selective switches
JDSU discussed its wavelength-selective switch (WSS) products at ECOC. The company has previously detailed its twin 1x20 port WSS, which has moved from development to volume production.
At ECOC, JDSU detailed its work on a twin MxN WSS design. "It is a WSS that instead of being a 1xN - 1x20 or a 1x9 - it is an MxN," says Brandon Collings, chief technology officer, communications and commercial optical products at JDSU. "So it has multiple input and output ports on both sides." Such a design is used for the add and drop multiplexer for colourless and directionless reconfigurable optical add/ drop multiplexers (ROADMs).
"People have been able to build colourless and directionless architectures using conventional 1xN WSSes," says Collings. The MxN serves the same functionality but in a single integrated unit, halving the volume and cost for colourless and directionless compared to the current approach.
JDSU says it is also completing the development of a twin multicast switch, the add and drop multiplexer suited to colourless, directionless and contentionless ROADM designs.
200 Gigabit coherent demonstration
ClariPhy Communications, working with NeoPhotonics, Fujitsu Optical Components, u2t Photonics and Inphi, showcased a reference-design demonstration of 200 Gig coherent optical transmission using 16 quadrature amplitude modulation (16-QAM).
For the demonstration, ClariPhy provided the coherent silicon: the digital-to-analogue converter for transmission and the receiver analogue-to digital and digital signal processing (DSP) used to counter channel transmission impairments. NeoPhotonics provided the lasers, for transmission and at the receiver, u2t Photonics supplied the integrated coherent receiver, Fujistu Optical Components the lithium niobate nested modulator while Inphi provided the quad-modulator driver IC.
ClariPhy is developing a 28nm CMOS Lightspeed chip suited for metro and long-haul coherent transmission. The chip will support 100 and 200 Gigabit-per-second (Gbps) data rates and have an adjustable power consumption tailored to the application. The chip will also be suited for use within a coherent CFP module.
"All the components that we are talking about for 100 Gig are either ready or will soon be ready for 200 and 400 Gig," says Ferris Lipscomb, vice president of marketing at NeoPhotonics. To achieve 400Gbps, two 16-QAM channels can be used.
The DWDM market for 10 Gig is now starting to plateau
Tunable SFPs
JDSU first released a 10Gbps SFP+ optical module tunable across the C-band in 2012, a design that dissipates up to 2W. The SFP+ MSA agreement, however, calls for no greater than a 1.5W power consumption. "Our customers had to deal with that higher power dissipation which, in a lot of cases, was doable," says JDSU Collings.
Robert Blum, Oclaro
JDSU's latest tunable SFP+ design now meets the 1.5W power specification. "This gets into the MSA standard's power dissipation envelop and can now go into every SFP+ socket that is deployed," says Collings. To achieve the power target involved a redesign of the tunable laser. The tunable SFP+ is now sampling and will be generally available one or two quarters hence.
Oclaro and Finisar also unveiled tunable SFP+ modules at ECOC 2013. "The design is using the integrated tunable laser and Mach-Zehnder modulator, all on the same chip," says Robert Blum, director of product marketing for Oclaro's photonic components.
Neither Oclaro nor Finisar detailed their SFP+'s power consumption. "The 1.5W is the standard people are trying to achieve and we are quite close to that," says Blum.
Both Oclaro's and Finisar's tunable SFP+ designs are sampling now.
Reducing a 10Gbps tunable transceiver to a SFP+ in effect is the end destination on the module roadmap. "The DWDM market for 10 Gig is now starting to plateau," says Rafik Ward, vice president of marketing at Finisar. "From an industry perspective, you will see more and more effort on higher data rates in future."
40G QSFP+ with a 40km reach
Finisar demonstrated a 40Gbps QSFP+ with a reach of 40km. "The QSFP has embedded itself as the form-factor of choice at 40 Gig," says Ward.
Until now there has been the 850nm 40GBASE-SR4 with a 100m reach and the 1310nm 40GBASE-LR4 at 10km. To achieve a 40km QSFP+, Finisar is using four uncooled distributed feedback (DFB) lasers and an avalanche photo-detector (APD) operating using coarse WDM (CWDM) wavelengths spaced around 1310nm. The QSFP+ is being used on client side cards for enterprise and telecom equipment, says Finisar.
Module for wireless backhaul
Oclaro announced an SFP+ that supports the wireless Common Public Radio Interface (CPRI) and Open Base Station Architecture Initiative (OBSAI) standards used to link equipment in a wireless cell's tower and the base station controller.
Until now, optical modules for CPRI have been the 10km 10GBASE-LR4 modules. "You have a relatively expensive device for the last mile which is the most cost sensitive [part of the network]," says Oclaro's Hansen.
Oclaro's 1W SFP+ reduces module cost by using a simpler Fabry-Perot laser but at the expense of a 2km reach only. However, this is sufficient for a majority of requirements, says Hansen. The SFP supports 2.5G, 3Gbps, 6Gbps and 10Gbps rates. "CPRI has been used mostly at 3 Gig and 6 Gig but there is interest in 10 Gig due to growing mobile data traffic and the adoption of LTE," says Hansen.
The SFP+ module is sampling and will be in volume production by year end.
For Part 1, click here
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.