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
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.