Judging by the flurry of news announcements in the last six months, wavelength-division multiplexed passive optical network (WDM-PON) activity is hotting up. Nortel, through its joint venture with Korean manufacturer LG, acquired trail-blazing WDM-PON equipment maker Novera Optics in August 2008. This was followed in September by German metro specialist ADVA Optical Networking entering the market with a product that exploits coarse wavelength-division multiplexing (CWDM). Then in October, Chinese equipment vendor ZTE unveiled a WDM-PON prototype that combines traditional PON technology with WDM and wavelength conversion, which it expects to commercialize before the end of 2009.
To these developments can be added two much-trumpeted European-funded WDM-PON research projects: GigaWAM, which is a collaboration involving vendors Ericsson, Ignis Photonyx and Syntune, and SARDANA, a consortium that includes France Telecom and Tellabs.
But is WDM-PON red hot or a red herring? The technology is of course not new, having already been deployed by KT. The South Korean incumbent has added more than 150,000 subscribers since 2006, using equipment from Novera to deliver fibre-to-the-building to residential and business customers. However, in mid-2007 KT said that it was curtailing WDM-PON installations because it was too expensive, according to Lynn Hutcheson, vice-president, communication components, at Ovum. The high cost of WDM-PON equipment is still a major stumbling block to widespread deployment, he says.
Coming of age
On the plus side, however, the business case for WDM-PON has matured in the last few years. Operators and vendors have become aware of WDM-PON’s potential for backhaul applications and network simplification, rather than just for blistering bandwidth to the home.
Although incumbents, including France Telecom and Telecom Italia, say they have not issued any specific request for WDM-PON technology, WDM-PON has started to appear in vendors’ RFP (request for proposal) responses.
“We have responded to several RFIs [request for information] and RFPs requiring xPON and/or [Ethernet] point-to-point during the last six months,” said Giovanni Manto, business leader, Ethernet fibre access at Nortel. “We were provided the latitude to respond to the requirements with a WDM-PON solution, giving us an opportunity to re-engage the operators solely based on WDM-PON.”
There is another explanation for the buzz surrounding WDM-PON. “The recent vendor announcements are essentially to do with GPON being closed to all these vendors,” said Teresa Mastrangelo, principal analyst at market-research firm broadbandtrends.com. “They are not winning any GPON business and are looking at the next [market] entry point.”
Nokia Siemens Networks is a case in point. The vendor announced earlier this year that it was eschewing existing PON technology in favour of next-generation schemes including WDM-PON. Similarly Tellabs is concentrating on WDM-PON through its involvement in the SARDANA research project. The US vendor has not abandoned GPON entirely, but decided earlier this year to terminate GPON product development focused on market leader Verizon.
And while Nortel announced in September that UNET will deliver high-speed internet for homes and businesses in the Netherlands using its WDM-PON solution, analysts still believe widespread deployments are at least five years away. Operator interest is genuine, they say, but WDM-PON is still not standardized, and that puts it at a disadvantage compared with the established PON technologies.
The status quo
Traditional time-division multiplexed (TDM) PONs use a shared-fibre architecture in which a single fibre from an operator’s central office connects to individual fibres via an optical splitter. Each end-fibre links the broadband connection via an optical network termination (ONT) unit located in the home or building. A broadcast scheme is used for transmitting downstream data to the home. All the data on the PON are seen by all of the homes, but each ONT picks off its data based on a unique packet header. For upstream communication, each ONT has a time slot in which to communicate to the optical line termination (OLT) unit that sits within the central office, in effect a point-to-point scheme. In a typical PON a single OLT serves up to 32 ONTs.
Two main PON technologies are currently being deployed: GPON, the International Telecommunication Union (ITU) standard backed by the Full Service Access Network (FSAN) group, and the Institute of Electrical & Electronics Engineers’ (IEEE) Ethernet PON (EPON) standard. GPON offers aggregate data rates of 2.5 Gbit/s downstream and 1.25 Gbit/s upstream, with the bandwidth typically being shared between 16 or 32 end-users. EPON, in contrast, is a 1 Gbit/s technology. However, EPON is the more mature standard, with GPON only becoming ready for deployment in the last 12 months.
Operators in the Far East have largely settled on EPON, whereas North American and European operators have tended to favour GPON. US operator Verizon started GPON deployments with its fibre-based triple-play service FiOS in 2008, and has now passed more than 9 million homes, half the FiOS rollout target. Europe’s five main operators – BT, Deutsche Telekom, France Telecom, Telefónica and Telecom Italia – have all backed GPON.
Both EPON and GPON standards are being developed that will offer higher bandwidth to the end-user. The IEEE 802.3av Task Force is well advanced in defining 10 Gbit/s EPON, with the standard expected to be completed in 2009. 10G-EPON will deliver 10 Gbit/s downstream, and initially 1 Gbit/s and then 10 Gbit/s upstream. Japan is expected to be the first main market for 10G-EPON, enabling operators to increase the bandwidth delivered to apartment buildings.
The Full Service Access Network (FSAN), an organization made up of operators and vendors, has two groups working on standards for Next-Generation Access (NGA). Working group NGA1 is looking at 10 Gbit/s PON and how it will co-exist with GPON. For example, bonding four 2.5 Gbit/s wavelengths in a GPON could be used to create a 10 Gbit/s PON scheme. The second group, NGA2, has a more open brief and is looking longer term, including WDM-PON schemes.
“They are working out what next-generation PON should look like, but this is ongoing and still at an early stage,” said Russell Davey, BT’s head of optical design, and until recently co-chair of the FSAN NGA Task Group.
WDM-PON may share the same point-to-multipoint fibre plant as a TDM-PON, but logically it is a point-to-point scheme, with each ONT having a dedicated wavelength. There is no time sharing of bandwidth, so an end-user could receive 100 Mbit/s, 1 Gbit/s or even 10 Gbit/s – whatever makes economic sense.
By adding WDM, PON’s three main performance metrics are improved: bandwidth, split ratio and reach. The reason these parameters are improved is because the power splitter at the branch of the PON is replaced by an arrayed-waveguide grating (AWG), which routes each wavelength to its destination ONT with very low loss. For example, a 1:64 GPON splitter introduces 20 dB of insertion loss, whereas with WDM-PON and AWGs the loss is as little as 8 dB. The extra signal margin can be used to either relax the optical component specifications or to improve the distance and split ratio.
Metro meets access
WDM-PONs can serve distances up to 80–100 km without the need for optical amplification. Such a reach changes how PONs should be viewed. Instead of simply enhancing speed, WDM-PON technology blurs the traditional boundary between access and metro networks.
Deutsche Telekom has detailed plans to redesign its first and second mile network using what it calls “passive CWDM” in combination with fibre-to-the-cabinet/building. At IIR’s WDM and Next-Generation Optical Networking conference in June 2008, the German incumbent detailed plans to reduce its 8000 local telephone exchanges to less than 900, using the infrastructure to carry business, residential and mobile backhaul services on one network (see FibreSystems Europe September 2008, p15).
In such a system the passive CWDM network connects to each outdoor DSLAM with a 1 Gbit/s link using a coloured (wavelength-specific) SFP module. The operator plans to use 8–16 CWDM wavelengths to serve distances up to 80 km from the central office (see figure 1). A simple optical multiplexer at the remote node replaces the powered local exchange, which saves on cooling and power consumption, and hence on operational expenditure.
Bruno Orth, Deutsche Telekom’s vice-president of technology and production strategy, networks, is careful to distinguish between Deutsche Telekom’s passive CWDM scheme and what he calls “customer-direct access”. “Passive CWDM is not primarily designed to be used in the first mile, but to backhaul aggregated traffic and reduce parallel fibre use,” he said. In contrast, if WDM-PON were to go all the way to residences, it would need to be cheaper than passive CWDM, he says.
According to ADVA, Deutsche Telekom’s embrace of passive CWDM is, in fact, a significant step towards WDM-PON. Although it has not been officially named as the supplier of the equipment, ADVA describes its announced WDM-PON system as being aligned with the German national carrier’s approach.
“Operators’ interest [in WDM-PON] is for next-generation access and traffic backhauling,” confirmed Klaus Grobe, principal engineer at ADVA Optical Networking. “There is also pressure to reduce cost, and one way to achieve that is to consolidate the network into a single network for metro and access.” Operators are looking to reduce operation expenses by as much as 80%, he says, and WDM-PON is a possible solution.
France Telecom is interested in WDM-PON for similar reasons. Philippe Chanclou, senior expert, research access networks at R&D division Orange Labs, gave an update on the SARDANA project at the 2008 European Conference on Optical Communications (ECOC) in Brussels. The French incumbent is investigating the network implications of a topology that has only passive components between the metro network and end-users. Such a move would save the operator a lot of money because it would need far fewer local exchanges. For example, in Brittany in Northwest France, France Telecom currently has 820 local exchanges to house its broadband and voice equipment, but believes that a WDM-PON deployment in the same region would require only 45 local exchanges.
Hybrid schemes
The other advantage of WDM-PON is a huge increase in the potential number of end-users served. Instead of just 32 or 64 in an EPON/GPON scenario, many thousands of subscribers become possible. A single wavelength can be shared among many end-users in a multi-dwelling unit via digital subscriber line (DSL) connections – as KT has done with Novera’s WDM-PON equipment.
Novera’s original system achieves 100 Mbit/s per wavelength and delivers 16 or 32 wavelengths. Launched in 2006, the upgraded TurboLight 16 system boosts the data rate to 1.25 Gbit/s per wavelength, and Nortel is planning to take development further. “We are taking their [Novera’s] field-hardened technology and starting technical and platform improvements,” explained Nortel’s Manto. This includes normalizing the WDM-PON technology to achieve a minimum 20 km reach using 32 wavelengths with 100 Mbit/s and 1 Gbit/s co-existing on the same PON branch. Nortel also plans to enhance the optical link budget to extend the reach and increase the system’s wavelength count.
Alternatively, a hybrid TDM/WDM system can be implemented, with each wavelength being used to feed a GPON or EPON. Using 32 wavelengths and 1:64 split-ratio GPONs it would be possible to serve more than 2000 end-users.
Chinese vendor ZTE’s prototype is a combination TDM/WDM PON that uses between 4 and 32 channels and a split ratio of 1:64 or more. Each ONT uses common wavelengths, with all-optical wavelength conversion being used to convert between the common wavelengths and DWDM-compatible ones.
EU-funded project PIEMAN (Photonic Integrated Extended Metro and Access Network), which is nearing completion, is also based on a hybrid TDM/WDM PON that delivers 10 Gbit/s of bandwidth both upstream and downstream. The system specification includes a 100 km reach and 32 wavelengths, with each 10 Gbit/s wavelength being shared by up to 512 customers. “On average that is 20 Mbit/s each, but using dynamic bandwidth allocation there are statistical benefits,” said BT’s Davey, who is also PIEMAN project coordinator. That means that if a user is on holiday, for example, the unused portion of bandwidth can be allocated to the remaining users – something that “pure” WDM-PONs can’t do.
Davey says that PIEMAN’s greatest achievement is the general acceptance of 10 Gbit/s data rates and 100 km reach for PON. “Four years ago we were considered mad, but not now,” he said. There was also what he calls a “trapped mindset” among operators about PON access technology being simply the more expensive optical equivalent of a DSLAM, but all that has changed. The idea of fibre’s reach being far longer than copper has been firmly taken on board, he says.
Deployment schedules
Nortel believes that WDM-PON will arrive sooner rather than later. “There is significant traction from our customer base – top-tier customers in North America and Europe,” said Manto. “In the next 18 months we will see networks being built.”
But other vendors, like Nokia Siemens Networks, are more circumspect. “We won’t fix a date,” said Richard Gröber, head of strategy and portfolio development at the company’s broadband access business unit. “By 2011 to 2013 we expect the new technology [WDM-PON] to be available and cost-optimized.”
For WDM-PON to see widespread adoption, Nokia Siemens believes three factors must be aligned: a wide-scale deployment of fibre, WDM-PON cost reduction, and services that require greater bandwidth to the end-user.
Given the recent problems in the global financial community, operators and their investors may baulk at the huge investments needed to deploy fibre. This could result in two possible scenarios. One is a slowdown in GPON deployments, so that the technology misses the “deployment window”, and when large-scale deployments resume WDM-PON has become the preferred approach. Alternatively, optical access deployments in general – including both GPON and WDM-PON – will be pushed out.
For BT the dominant issue is indeed the business case of fibre rather than technology. “The debate about WDM-PON versus GPON is secondary,” said Davey. “Until you can make a business case to put fibre in the ground, either [technology] will remain limited.”