I was in Munich last week for the FTTH Council Europe conference and engaged in a number of interesting conversations with vendors and operators on the issue of NGPON 2, a technology that could be deployed over fiber networks.
Operators are currently discussing in the FSAN pre standards body which technology will be chosen for NGPON 2. Following the FSAN decision, expected in April, the technology can then go into the standardization process in the ITU and commercial systems can be developed. NGPON 2 will be the most advanced technology for fiber networks beyond today’s widely deployed GPON and EPON systems and the XGPON 1 and 10GEPON systems that are already commercially available.
The choice of technology will have a big impact on vendors who are backing a variety of options and will also have consequences for operators looking to upgrade their networks in future years. The three basic candidates for NGPON 2 are pure WDM PON systems (of which there are a number of variants), hybrid TWDM PON and UDWDM PON. But it is the hybrid TWDM PON and UDWDM PON systems which are leading the race.
Pure WDM PON solutions appear out of the game
Most pure WDM PON systems require alterations to the outside plant, which many operators are loathe to change when it is already providing working services. Generally speaking, they are constrained in reach to around 25km, not much more than today’s GPON systems. WDM PON systems might also suffer in terms of coexistence with existing technologies. For example, the WDM PON systems may use the C band which a number of important FTTH operators such as Verizon, NTT and Portugal Telecom are already using for RF video overlay.
Hybrid TWDM PON and UDWDM PON lead the way
The two leading candidates for NGPON 2 are NSN’s NGOA UDWDM PON solution, using coherent technology from the transport network and seeking to merge the access and aggregation networks and the hybrid TWDM PON system backed by Alcatel Lucent and Huawei. The hybrid system stacks four XGPON 1s onto the same physical fiber. Each subscriber receives all four wavelengths and the technology does not offer a dedicated wavelength to each subscriber, unlike the UDWDM PON system.
Reach and OPEX are pluses for UDWDM PON
The NSN system’s potential reach of up to 100km offers significant possibilities for exchange consolidation which can net revenues for operators from exchange sales and also OPEX savings in terms of lower maintenance costs. The hybrid TWDM PON solution, however, only has a reach of 40km. Nevertheless one can question how strict these reach requirements really are given that even GPON can stretch to 20km, itself offering possibilities for exchange consolidation.
This difference in reach between UDWDM PON and the hybrid system could be important for an operator such as Deutsche Telekom which has the long term idea to reduce the number of central offices from 8,000 to 900. To do this the German incumbent believes it would need a maximum of 50km between the aggregated central office and the subscriber.
CAPEX and certainty are two boons for hybrid TWDM PON
The big advantage for the hybrid solution is that it is based on existing XGPON 1 systems, the UDWDM PON solution on the other hand is based on a new technology and so as yet there is less to show the members of FSAN. The existing nature of XGPON1 from which hybrid systems can be built provides added certainty to an operator such as Verizon which might look to deploy the technology chosen for NGPON 2 in the next five years. The fact that the hybrid solution is based on XGPON 1 also has the important consequence of reducing cost when compared to the use of coherent technology in the access network. However, the UDWDM PON solution has the advantage of using a more pay as you grow model meaning it is not necessary to light the whole PON tree when one subscriber becomes active.
Speeds not really the issue
Each of the four XGPON 1s in the hybrid system would be capable of supporting 10Gbps downstream and 2.5Gbps upstream. UDWDM PON could offer 1Gbps symmetrical on each wavelength, although it could be available with lower speed options. Both of these solutions seem to offer far greater speeds than would be required in residential broadband access for the foreseeable future. For mobile backhaul, even with LTE Advanced, a backhaul requirement of only 1Gbps per sector in a cell might be required so for a cell of three sectors, only 3Gbps could be needed. It is true, however, that UDWDM PON offers symmetrical bandwidth which could become increasingly important. For example, Portugal Telecom states that upstream traffic from GPON subscribers is 4 times greater than from DSL subscribers.
Openness is important
The currently commercially available pure WDM PON solutions cannot support wavelength unbundling, which allows multiple operators to share the spectrum on one physical fiber and install their own active equipment. Both UDWDM PON and hybrid TWDM PON would be capable of supporting wavelength unbundling, which is already included in the NGA recommendations of the European Commission. The openness of the NGPON 2 system is likely to be important for an operator such as Vodafone which could pursue a co investment model for FTTH.
Operators have different requirements
Each operator will have different requirements for what they need from NGPON 2. For example, in a country like Japan where FTTH subscribers form 60% of the total, there might be more possibility for NTT to engage in central office consolidation more quickly. Whilst it is understandable that some operators prefer the tangible existing aspect of the hybrid solution there is a danger of standardising the technology in this direction and then seeing a further need for a new standard further down the track, for example, because operators want longer reach. Indeed this case of a technology being standardised and then not being of interest to operators is applicable to XGPON 1 whose higher bandwidths have not been enough to drive any widespread rollouts. Nevertheless certain operators may have a shorter term view as competitive pressures, for example, from cable move their investment horizons to a shorter and shorter time frame making the certainties of hybrid TWDM PON a safer bet.
A more detailed version of this analysis will appear on Informa TM’s Intelligence Centre.
Stephen Wilson is a senior analyst at Informa Telecoms and Media. For more information visit www.informatandm.com/