FSAN unveils roadmap plans
Part 2: Next-generation passive optical networks
The Full Service Access Network (FSAN) has outlined its vision for fibre access networks for the coming decade.
FSAN is an industry forum that includes over 20 operators and 70 members overall. The group identifies service requirements and develops optical access technologies that are passed to the International Telecommunication Union (ITU) for standardisation.
Source: FSAN
“One of the messages of the roadmap is that, in the immediate future, what FSAN wants to do is evolve the existing standards,” says Peter Dawes, FSAN NGPON co-chair.
The latest FSAN technologies to become standards are XGS-PON (10 gigabits symmetrical passive optical network) and the multiple wavelength TWDM-PON (time wavelength-division multiplexing passive optical network), also known as NG-PON2 (see chart).
PON status
XGS-PON is a single-wavelength PON standard that supports two rates: a 10-gigabit symmetrical rate and the asymmetrical 10 gigabits downstream (to the user) and 2.5 gigabits upstream originally introduced by XG-PON.
Peter Dawes
TWDM-PON uses four wavelengths to deliver up to 40 gigabits of symmetrical bandwidth and has an option for eight wavelengths overall. TWDM-PON also uses tuneable lasers enabling operators to move subscribers between wavelengths.
“FSAN operators see continued growth in PON deployment,” says Dawes. “There is still strong deployment of GPON and we are on the verge of needing 10-gigabit symmetrical services.” Other operators may delay and go straight to TWDM-PON, he says.
According to Dawes, operators are seeing a variety of applications that are driving the need for 10-gigabit access rates. One is the growing use of video and video conferencing. Another bandwidth driver for access networks is mobile applications such as connecting mobile antennas and mobile backhaul. In addition, there are digital home trends such as social networking and the moving of content to the cloud.
Mobile fronthaul can eat as much bandwidth as you can supply once you start to aggregate [radio] antennas
Operators are also keen to attach the labels ‘gigabit’ and ‘gigabit services’ to their broadband offerings as a marketing differentiator.
Other drivers for the move to the newer PON technologies include peer-to-peer services and business IP services, says Dawes.
Roadmap
FSAN’s plan to evolve the existing standards in the near term will take the group to 2021.
One obvious way the existing PONs can be evolved is to adopt 25-gigabit wavelengths. This would enable a 25-gigabit symmetrical extension to XGS-PON and a future TWDM-PON variant with up to 200 gigabits of capacity if the full eight wavelengths are used. “It is a case of looking for logical evolutions of these technologies,” says Dawes.
One application that could use such high capacities is mobile fronthaul, says Dawes: “It can eat as much bandwidth as you can supply once you start to aggregate [radio] antennas.”
After 2020, FSAN will investigate disruptive technologies as it defines future optical access schemes. R&D work, new modulation schemes and component developments including silicon photonics will all be assessed as to their suitability for future optical access schemes.
Meanwhile, FSAN says it will review its roadmap on a yearly basis and amend it as required.
See Part 1: XGS and TWDM passive optical networks, click here
Telefonica tests XGS-PON
Part 1: XGS and TWDM passive optical networks
Telefonica is the latest operator to test XGS-PON, the 10-gigabit passive optical networking standard.
“Operators want to show they are taking the maximum from their fibre investment,” says Ana Pesovic, marketing manager for fibre at Nokia, the supplier of the XGS-PON equipment used for the operator’s lab tests. “Telefonica has been really aggressive in their fibre deployments in the last couple of years.”
Ana Pesovic
XGS-PON
Approved by the ITU-T in 2016, XGS-PON supports two rates: 10-gigabit symmetrical and the asymmetrical rate of 10 gigabits downstream (to the user) and 2.5 gigabits upstream.
XGS-PON has largely superseded the earlier XG-PON standard which supports the 10-gigabit asymmetrical rate only. “It is fair to say there is no traction for XG-PON,” says Pesovic. “Even in China [an early adopter of XG-PON], we see the interest slowly moving to XGS-PON.”
Nokia says it has now been involved in 40 XGS-PON trials and nine customers have deployed the technology. “These have just started and they are not massive deployments,” says Pesovic.
Nokia’s XGS-PON customers include China Telecom and SK Broadband. SK Broadband has deployed XGS-PON alongside the more advanced TWDM-PON (time wavelength division multiplexing, passive optical network), the ITU-T NG-PON2 standard.
XGS-PON uses a fixed wavelength to deliver either the 10-gigabit symmetrical or asymmetrical service. The standard supports a distance of 20km and a split ratio of up to 1:128 - one XGS-PON optical line terminal (OLT) serving up to 128 optical network units (ONUs). In contrast, TWDM-PON supports four wavelengths enabling up to 40-gigabit symmetrical rates. And unlike XGS-PON, TWDM-PON supports flexible wavelengths using tuneable lasers.
The wavelengths used by XGS-PON and TWDM-PON have been specified such that the two standards can operate alongside GPON on the same fibre. Accordingly, with SK Broadband’s deployment, the two PON standards along with GPON support an aggregate capacity of 52.5 gigabits-per-second.
As well as testing XGS-PON's performance, Telefonica has tested that XGS-PON works without disturbing existing broadband services over its GPON networks, says Pesovic.
For the test, Telefonica used an 8-port line card where each port can be configured for XGS-PON or as a wavelength of a TWDM-PON. The line card fits within Nokia’s 7360 Intelligent Services Access Manager (ISAM) FX platform.
5G will require the deployment of many more small cells. With XGS-PON, multiple small cells can be served using a single PON
Applications
XGS-PON with its symmetrical 10-gigabit rate is suited to business services. "Operators can use one network to converge business and residential; today they are two overlay networks,” says Pesovic. Many businesses require 1-gigabit connectivity or less but by having a 10-gigabit link, multiple enterprises can be aggregated on one PON.
Nokia says that in countries such as South Korea as well as in Europe and North America there is also interest in a 10-gigabit PON for residential services. “People are taking the downstream bandwidth for granted and now the upstream is becoming a differentiator, making the quality of experience much better,” says Pesovic.
The bulk of traffic is still predominately downstream but increasingly users want to upload large files and video. Even if these uploads are of shorter duration, the network must deliver, says Pesovic.
Operators are also eyeing XGS-PON for the emerging 5G cellular standard. Nokia points out that 5G will require the deployment of many more small cells. With XGS-PON, multiple small cells can be served using a single PON.
Nokia expects XGS-PON will be deployed for years to come. Broadband is advancing by adding more wavelengths. To GPON, which uses one wavelength, can be added a second wavelength supporting 10-gigabit XGS-PON. Using TWDM-PON adds four and potentially eight more wavelengths - 40 gigabits and 80 gigabits of bandwidth, respectively. “It really doesn’t matter what the technology is called,” says Pesovic.
One North American operator is looking at TWDM-PON as a way to save power. During the night when there is less broadband usage, the operator wants to use wavelength mobility to migrate users onto a single wavelength.
TWDM-PON
Besides wavelength count, TWDM-PON differs from XGS-PON in its use of tuneable lasers.
Having tuneable wavelengths delivers several benefits to the operators. One is load balancing. If users on one wavelength start to exhaust its capacity, several users can be moved to a second wavelength that is less heavily loaded.
TWDM-PON also benefits network sharing and wavelength unbundling. A third-party operator can offer its fibre to interested operators. “Each operator could then operate on a single wavelength,” says Pesovic. If a user changes operator, they can simply be moved from one wavelength to another.
There are also operational benefits. If a fault develops on a board, users can be migrated to a second card without service interruption and the faulty board replaced.
One North American operator is looking at TWDM-PON as a way to save power, says Pesovic. During the night when there is less broadband usage, the operator wants to use wavelength mobility to migrate users onto a single wavelength. This would deliver sufficient bandwidth to those users that are active while allowing the remaining wavelengths to be powered down, saving power.
The issue impeding the uptake of TWDM-PON remains the high cost of tuneable lasers. Nokia expects it to be at least another year before the cost of tuneable lasers becomes more economical for PON. That said, service providers delivering businesses services may still be tempted to adopt TWDM-PON despite the higher cost of tuneable lasers given that the average revenue per user (ARPU) of business users is 5x that of residential users, says Pesovic.
See Part 2: FSAN unveils roadmap plans, click here
100 gigabit the next stop on PON's roadmap
Source: Huawei
Q. What are the various ways the industry is considering implementing 100 Gigabit?
FE: The work happening now is to do a 25 gigabit-per-second wavelength, and then multiple wavelengths will be combined in some way to create 50-gigabit or 100-gigabit speed offerings.
Q. The IEEE 802.3ca is developing 100 gigabit EPON whereas the ITU-T/FSAN has defined NG-PON2. What does the development of 100 gigabit EPON mean for the future of NG-PON2?
The ITU NG-PON2 was a future PON project that was started in 2011 with the goal of reaching at least 40 Gigabit per PON system. This was achieved in 2015 when the whole series of standards was completed. During this time, the highest economical speed was 10 gigabit.
Now that time has passed, 25 gigabit-per-lambda is becoming more reasonable, and so the ITU plans to study 25 gigabit in both the multi-channel setting - that is NG-PON2, and in a single-channel setting that would be a follow-on to the XGS-PON system. These PON systems should not be seen as a single-point development but part of an evolution.
The IEEE 802.3ca work is complementary to this work, and many of the same people go to both standards groups - certainly Huawei does. It is in everybody’s interest to reuse components and technologies, and I’m confident that this will happen for the 25-gigabit generation just as it has in previous PON generations.
Q. In a press release, Huawei mentioned it is demonstrating a 100-Gigabit PON prototype. What is Huawei demonstrating and how has it been implemented?
The 100 gigabit PON prototype is an early look at the 25 gigabit-per-wavelength technology. Given that the standardisation is only beginning, we had to make many assumptions, but such prototypes give us a chance to find any issues early.
The details of the prototype will be more fully explained at the OFC conference, but in brief, what we have is a system that implements 25 gigabit downstream and 10 gigabit upstream per wavelength pair. A real PON MAC is operating in this system, and full PON operations: activation, ranging, dynamic bandwidth allocation etc. are taking place. The 25-gigabit transmission is done using either conventional non-return-to-zero (NRZ) or optical duobinary line coding.
Q. How is 100 gigabit PON envisaged to work with existing systems?
100 gigabit PON will co-exist with previous PON generations, most likely using a WDM-style of interworking. This is the same method used to allow G-PON, XG-PON, and TWDM-PON to co-exist on a single-fibre network.
Q. What in Huawei’s opinion are the main challenges to be overcome regarding 100 gigabit PON?
Frank EffenbergerThere are two technical challenges to this system.
First, 25 gigabit-per-wavelength transmission over 20km of standard fibre that meets the approximate 30dB loss budget and the aggressive cost targets of access is difficult. This will take time to solve.
Second, combining multiple PON channels into a single virtual channel is not trivial because it requires some level of coordination between the channels. There are solutions, but they need more study to confirm their operation.
Q. What will the advent of Nx25 gigabit do to reduce the cost of PON?
That is a loaded question, because you presuppose that 25 gigabit will lower the cost. To first order, our hope is to keep the cost-per-endpoint increase to a reasonable level. Obviously, the cost-per-bit will decline significantly, but the access business is not so tied to this factor. The key factor in higher-speed PON is to increase the system and user speed while trying to hold costs steady.
The industrial readiness of TWDM-PON is not really at a mass deployment stage; furthermore, the 'killer app' that just absolutely demands TWDM’s capacity is not immediately at hand
Q. Besides showing operators an extended roadmap beyond 40 gigabit, what applications will require 100 gigabit PON?
In reality, while 100 gigabit PON makes for a nice headline, the story is more complex. There are two separate applications where we see the technology.
The first is a single-channel 25-gigabit system that would be useful for residential applications. This would serve as a follow-on to the XG-PON or 10GEPON deployments, and it could be at a price point that allows that.
The second is the full 100 gigabit PON with four channels. This would be useful for business services, for mobile backhaul and fronthaul, and fibre-deep architectures like G.fast (fibre-to-the-distribution point) and DOCSIS3.1 (D-CMTS).
Q. Huawei has also detailed a 10 and 40 gigabit NG-PON. Is this a combination of time- and wavelength-division multiplexing that in effect supports a traditional time-division multiplexed PON per wavelength i.e. TWDM-PON with rates that include 10 gigabit symmetrical, 10 gigabit downstream and 2.5 gigabit upstream, and 2.5 gigabit symmetrical?
You describe it correctly. The NG-PON2 system’s major solution is the TWDM-PON, which defines 4- or 8-wavelength pairs that can be used simultaneously, with 10 gigabit- or 2.5 gigabit-per-wavelength. So, it can reach a total capacity of 80 gigabit per PON.
In addition, NG-PON2 also defines a WDM overlay component, which supports eight or more channels of 10 gigabit per wavelength (not TDMed). All in all, the system capacity could reach over 160 gigabit.
Q. Is this system commercially deployed?
So far, there have been only small trial deployments. Huawei has deployed its early TWDM systems in many operators' networks in Europe and in Asia, but we are not at liberty to disclose these collaborations other than what has been announced. The industrial readiness of TWDM-PON is not really at a mass deployment stage; furthermore, the 'killer app' that just absolutely demands TWDM’s capacity is not immediately at hand.
When will it be deployed is a hard question.
Keep in mind that 10 gigabit PONs were standardised and prototyped by 2010, and yet it is 2016 and we still don’t see huge deployment; the first small-scale movements are starting to happen in China. That’s a six-year lag!
If NG-PON2 is true to that, then it predicts 2021 as the start of significant deployment. Barring the introduction of some super bandwidth-hungry service, I think it will still be some time before we deploy any next-generation system.
Q. What will be the first applications for 10G-40G NG-PON systems?
The envisioned applications for TWDM-PON was primarily residential and small business, with the simple assumption that bandwidth increase will continue to grow to 1 gigabit-per-second. A 40-gigabit PON, serving 32 customers typically, fits this Gigabit FTTH picture well. But that seems far away for now. Early deployments will likely be business services and wireless backhaul.
Verizon prepares its next-gen PON request for proposal
Vincent O'Byrne
The NG-PON2 request for proposal (RFP) is being issued after the US operator completed a field test that showed a 40 gigabit NG-PON2 system working alongside Verizon’s existing GPON customer traffic.
The field test involved installing a NG-PON2 optical line terminal (OLT) at a Verizon central office and linking it to a FiOS customer’s home 5 km away. A nearby business location was also included in the trial.
Cisco and PT Inovação, an IT and research company owned by Portugal Telecom, worked with Verizon on the trial and provided the NG-PON2 equipment.
NG-PON2 is the follow-on development to XG-PON1, the 10 gigabit GPON standard. NG-PON2 supports both point-to-point links and a combination of time- and wavelength-division multiplexing that in effect supports a traditional time-division multiplexed PON per wavelength, known as TWDM-PON. The rates TWDM-PON supports include 10 gigabit symmetrical, 10 gigabit downstream and 2.5 gigabit upstream, and 2.5 gigabit symmetrical.
Verizon field-tested the transmission of NG-PON2 signals over a fibre already carrying GPON traffic to show that the two technologies can co-exist without interference, including Verizon’s analogue RF video signal. Another test demonstrated how, in the event of a OLT card fault at the central office, the customer’s optical network terminal (ONT) equipment can detect the fault and retune to a new wavelength, restoring the service within seconds.
Now we know we can deploy this technology on the same fibre without interference and upgrade the customer when the market demands such speed
Verizon is not saying when it will deploy the next-generation access technology. “We have not said as the technology has to become mature and the costs to reduce sufficiently,” says Vincent O'Byrne, director of access technology for Verizon.
It will also be several years before such speeds are needed, he says. “But now we know we can deploy this technology on the same fibre without interference and upgrade the customer when the market demands such speed.”
Verizon expects first NG-PON2 services will be for businesses, while residential customers will be offered the service once the technology is mature and cost-effective, says O’Byrne.
Vodafone is another operator conducting a TWDM-PON field trial based on four 10 gigabit wavelengths, using equipment from Alcatel-Lucent. Overall, Alcatel-Lucent says it has been involved in 16 customer TWDM-PON trials, half in Asia Pacific and the rest split between North America and EMEA.
Further reading
For an update on the NG-PON2 standard, click here
Europe gets its first TWDM-PON field trial
Vodafone is conducting what is claimed to be the first European field trial of a multi-wavelength passive optical networking system using access equipment from Alcatel-Lucent.
Source: Alcatel-Lucent
The time- and wavelength-division multiplexed passive optical network (TWDM-PON) technology being used is a next-generation access scheme that follows on from 10 gigabit GPON (XG-PON1) and 10 gigabit EPON.
“There appears to be much more 'real' interest in TWDM-PON than in 10G GPON,” says Julie Kunstler, principal analyst, components at Ovum.
The TWDM-PON standard is close to completion in the Full Service Access Network (FSAN) Group and ITU and supports up to eight wavelengths, each capable of 10 gigabit symmetrical or 10/ 2.5 gigabit asymmetrical speeds.
“You can start building hardware solutions that are fully [standard] compliant,” says Stefaan Vanhastel, director of fixed access marketing at Alcatel-Lucent.
TWDM-PON’s support for additional functionality such as dynamic wavelength management, whereby subscribers could be moved between wavelengths, is still being standardised.
The combination of time and wavelength division multiplexing, allows TWDM-PON to support multiple PONs, each sharing its capacity among 16, 32, 64 or even 128 end points depending on the operator’s chosen split ratio.
There appears to be much more 'real' interest in TWDM PON than in 10G GPON
Alcatel-Lucent first detailed its TWDM-PON technology last year. The system vendor introduced a four-wavelength TWDM-PON based on a 4-port line-card, each port supporting a 10 gigabit PON. The line card is used with Alcatel-Lucent’s 7360 Intelligent Services Access Manager FX platform, and supports fixed and tunable SFP optical modules.
“Several vendors also offer the possibility to use fixed wavelength - XG-PON1 or 10G EPON optics," says Vanhastel. "This reduces the initial cost of a TWDM-PON deployment while allowing you to add tunable optics later."
Operators can thus start with a 10 gigabit PON using fixed-wavelength optics and move to TWDM-PON and tunable modules as their capacity needs grow. “You won’t have to swap out legacy XG-PON1 hardware two years from now,” says Vanhastel.
Alcatel-Lucent has been involved in 16 customer TWDM-PON trials overall, half in Asia Pacific and the rest split between North America and EMEA. Besides Vodafone, Alcatel-Lucent has named two other TWDM-PON triallists: Telefonica and Energia, an energy utility in Japan.
You won’t have to swap out legacy XG-PON1 hardware two years from now
Vanhastel says the company has been surprised that operators are also eyeing the technology for residential access. The high capacity and relative expense of tunable optics made the vendor think that early demand would be for business services and mobile backhaul only.
Source: Gazettabyte
There are several reasons for the operator interest in TWDM-PON, says Vanhastel. One is its ample bandwidth - 40 gigabit symmetrical in a four-wavelength implementation - and that wavelengths can be assigned to different aggregation tasks such as backhaul, business and residential. Operators can also pay for wavelengths as needed.
TWDM-PON also allows wavelengths to be shared between operators as part of wholesale agreements. Operators deploying TWDM-PON can lease a wavelength to each other in their respective regions.
Vodafone, for example, is building its own fibre network but is also expanding its overall fixed broadband coverage by developing wholesale agreements across Europe. Vodafone's European broadband network covers 62 million households: 26 million premises covered with its own network and 36 million through wholesale agreements.
First operator TWDM-PON pilot deployments will occur in 2016, says Alcatel-Lucent.
Further reading:
White Paper: TWDM PON is on the horizon: facilitating fast FTTx network monetization, click here
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
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.