The Next Generation of FTTH

Oct. 16, 2017
DOCSIS 3.1 and Full Duplex rightly have received significant attention among cable operators as approaches to gigabit service delivery now and multi-gigabit in the future. However, work continues on all-fiber options that could not only match but exceed the capabilities of Full Duplex. These efforts complement a drive among telcos to implement software-defined networking and network functions virtualization (SDN/NFV) principles in the access network – an effort that cable operators could leverage as well.

DOCSIS 3.1 and Full Duplex rightly have received significant attention among cable operators as approaches to gigabit service delivery now and multi-gigabit in the future. However, work continues on all-fiber options that could not only match but exceed the capabilities of Full Duplex. These efforts complement a drive among telcos to implement software-defined networking and network functions virtualization (SDN/NFV) principles in the access network – an effort that cable operators could leverage as well.

Matching Full Duplex

Variants of EPON and GPON that support symmetrical 10-Gbps transmission rates predate DOCSIS 3.1. EPON's version arrived first, along with an option for 10 Gbps downstream/1 Gbps upstream where reduced requirements or cost considerations warranted. Several vendors have offered commercial systems for cable operators, starting with Sumitomo Electric and Alcatel-Lucent (now Nokia) in 2014. Other vendors soon followed, with Fujitsu Network Communications and CommScope (a line subsequently sold to ADTRAN) showing platforms in 2015 and Calix jumping into the market last year and following up this year.

The platforms all leveraged DOCSIS Provisioning of EPON (DPoE). As the name implies, DPoE enables operators to provision and operate EPON infrastructure in the same manner as HFC networks. The specifications also promote multivendor interoperability. DPoE supports the provision of business services in accordance with the MEF's Ethernet Private Line (EPL) service definitions.

Certainly high-speed business services were considered a major application of 10G EPON. However, the technology has yet to achieve implementation in a wide range of cable operator networks. Bright House Networks (now part of Charter/Spectrum) is known to have deployed 10G-EPON, but news of other deployments among U.S. operators has been scarce.

Where operators already have fiber in place, symmetrical 10-Gbps
approaches are already available.

The GPON community also offers a symmetrical 10-Gbps option, XGS-PON. A replacement for NG-PON1 (now more commonly known as XG-PON), which proved unpopular because it only supported 10 Gbps in the downstream, XGS-PON is considered an economical transition technology on the path toward multi-wavelength NG-PON2 architectures (more on these below). Its wavelength plan was specified to avoid interference with existing GPON traffic (so it can be added to subscriber-serving GPON networks alongside current service streams) as well as upcoming NG-PON2 capabilities.

Like EPON, CableLabs created a DOCSIS-friendly method of operation. DOCSIS Provisioning of GPON (DPoG) specifications were ratified in August of last year. To date, no cable operators have publicly announced their use of DPoG-enabled 10-Gbps GPON.

The lack of 10G PON deployments so far, whether EPON or GPON, likely derives from the ability of operators to accommodate gigabit broadband on their HFC networks, whether via DOCSIS 3.0 or 3.1. However, Altice has announced that it plans to move to an all-fiber access architecture capable of handling 10 Gbps. Altice's choice of approach could give either 10G EPON or GPON a shot in the arm.

Exceeding Full Duplex

While cable operators generally have ignored the first wave of 10G PON technologies, their telco competition has not. Such encouragement has led the ITU-T and IEEE, for GPON and EPON respectively, to pursue capabilities beyond symmetrical 10 Gbps. In the case of GPON, the focus has been on supporting multiple wavelengths of 10G; in the IEEE realm, standards developers are considering both multiple wavelengths and higher transmission rates.

The GPON work began earlier and therefore has shown the most progress. NG-PON2 specifies the support of eight 10-Gbps wavelengths per fiber. Initial NG-PON2 systems have focused on four wavelengths per fiber, plus an XGS-PON transmission stream.

Operators, with Verizon taking the lead in the U.S., see several advantages to the multi-wavelength approach. Business and residential services (and, potentially, 5G fronthaul and backhaul traffic) can occupy separate wavelengths, which makes supporting different service classes easier. Fault restoration or service changes become easier as well through the ability to switch customers from one wavelength to another. And the support of a wavelength-based version of channel bonding enables NG-PON2 to deliver greater than 10 Gbps to a customer or service area.

The multi-wavelength treat comes with a trick, however – the optical network terminal (ONT) at the customer location needs to be able to discern which wavelength(s) it should be receiving, the wavelength at which it should transmit, and what to do if network requirements demand that either change. The key to such multi-wavelength capabilities is a tunable transceiver. Network operators have used tunable transceivers for years – but not at the wavelengths NG-PON2 requires, and certainly not at the cost points access network economics demand.

NG-PON2 requires development of a new generation of tunable transceivers, which has proven challenging for optical module vendors, particularly at full symmetrical 10-Gbps transmission rates. Network planners at Verizon have expressed interest in deploying NG-PON2 in the first quarter of 2018; whether an adequate supply of ONT tunable transceivers will be available to support this timeline remains an open question.

Meanwhile, the IEEE plans to go one better than NG-PON2, to include transmission rates higher than 10 Gbps. The IEEE P802.3ca 100G-EPON Task Force is expected to tackle transmission rates of at least 25 Gbps on the way to specifying 25-Gbps, 50-Gbps and 100-Gbps versions of EPON – the latter two enabled via multiple wavelengths. The Task Force expects to establish baseline proposals for the variants by the end of the first quarter of 2018, with full ratification of the specifications targeted for 2020. As Curtis Knittle of CableLabs serves as the Task Force Chair, the requirements of cable operators should receive full consideration within the effort. However, implementation of the multi-wavelength specifications will face the same tunable transceiver challenges now faced by NG-PON2. It is currently unclear how if at all the Task Force will take this into account in developing its specifications.

Software-Defined Access Networks

The work on next-generation PON-based FTTH has played out against a backdrop of growing interest in applying SDN/NFV concepts to the access network. The emergence of software-defined access has seen collaboration among operators and technology developers in the form of public forums as well as operator-specific initiatives.

A prime example of the former is the Central Office Re-Architected as a Datacenter (CORD) initiative. The project, jointly led by the Open Networking Lab (ON.Lab) and The Linux Foundation, has attracted participation from several telcos as well as Comcast, which became a member last November. The project seeks to promote the use of cloud- and datacenter-inspired open source virtualization technology for residential, mobile and business access network applications. As envisioned by AT&T, CORD also could enable the use of whitebox optical access platforms; the telco has an XGS-PON trial planned that likely will leverage whitebox platforms based on specifications AT&T has submitted to the Open Compute Project's Telco Project.

Verizon, meanwhile, has made software-enabled multi-vendor interoperability a prime requirement in its ongoing NG-PON2 acquisition run off. Both finalists, ADTRAN and a Calix/Ericsson team, have used software-defined access principles and open system designs to achieve Verizon's objectives.

PON systems that can support Distributed Access Architecture have been announced.

The software-defined access trend dovetails with the growing interest among cable operators in Distributed Access Architecture (DAA). Both Verizon NG-PON2 finalists have leveraged their software-defined access experiences to create DAA-friendly next-generation PON offers for the cable market. Calix was the first to unveil such capabilities, with an announcement at last year's SCTE Cable-Tec Expo that its AXOS E3-2 Intelligent PON Node would support DAA by moving the OLT, routing, and IP policy and accounting functions from the headend into the network.

ADTRAN followed suit right before SCTE Cable-Tec Expo 2017 with a line of 10G EPON virtual remote OLTs, including one (the SDX 6210-4) specifically for DAA applications. It is likely other PON systems vendors will offer similar visions, if not specifically for DAA then at least in terms of applying SDN/NFV to access networks.

Ready When You Are

As already mentioned, the advent of Full Duplex DOCSIS 3.1 obviates the need for cable operators to replace their HFC infrastructure with FTTH to support symmetrical 10-Gbps transmission. However, where fiber to the home, business or building is in place today, technology is on the way that will support multi-gigabit services for residential and business customers well into the future. HFC networks won't go away anytime soon – but optical transmission technology will continue to provide operators an alternative where all-fiber makes sense.

Stephen Hardy is editorial director of Broadband Technology Report.

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