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