CableLabs, the innovation lab and specification developer for the cable industry, has worked on coherent transmission since at least 2017, when it announced at its Winter Conference in February of that year an initiative to develop specifications for point-to-point coherent optics. That effort has led to the development of specifications for both coherent 100- and 200-Gbps applications. Meanwhile, the organization also has tackled the application of coherent transmission to the last mile in the form of coherent 100G PON.
Both avenues of research have progressed over the past year and, alongside CableLabs’ work on DOCSIS 4.0 as an avenue for 10-Gbps capacity on hybrid fiber/coax (HFC) networks, the work highlights the desire of the organization’s operator members to develop an answer to their telco competitors’ 10G PON networks now and to what their customers may require in the future.
The point of point-to-point
CableLabs began work on point-to-point coherent specifications to help its operator members keep pace with rising bandwidth demands, particularly over aging fiber between the headend and fielded nodes and hubs within a distributed access architecture (DAA). The “P2P Coherent Optics Physical Layer 1.0 Specification,” the first of now three iterations of which CableLabs published in June 2018, describes a cost-optimized approach to coherent at 100 Gbps based on a symbol rate of 27.95 Gbaud and a DP-DQPSK modulation scheme.
At around the same time, CableLabs also created a full-duplex transmission approach that would operate over a single fiber with a single laser on each end of the connection. Development of a single-fiber approach was important; a survey cited in a 2018 CableLabs blog revealed that 20% of cable access networks used a single-fiber topology, with the expectation that this percentage would grow to 60% in five years. The full-duplex approach, dubbed “direction division multiplexing” in that blog (and sometimes referred to as “directional duplexing”), uses optical circulators on each end of the fiber connection to enable bidirectional transmission on the same wavelength. Obviating the need for multiple lasers transmitting at different wavelengths reduces laser count, power requirements, and related costs.
CableLabs added to the cable industry’s coherent capabilities in 2019 with the publication of the “P2P Coherent Optics Physical Layer 2.0 Specification,” which prescribed parameters for 200-Gbps transmission using either DP-QPSK at a symbol rate of 63.139467923 Gbaud or DP-16-QAM with a symbol rate of 31.5697339615 Gbaud.
To help ensure both lowered costs as well as a vibrant supplier ecosystem for modules that meet its specifications, CableLabs participated in the IEEE 802.3ct-2021 amendment effort, which focused on developing specifications for coherent transmission based on 100G wavelengths. As a result, module vendors should be able to meet its requirements as well as those for IEEE 802.3ct, ITU, and Open ROADM MSA applications with a single device.
With the path thus clear for technology vendors to develop 200G coherent transceivers for cable MSO use, an interoperability demonstration was a logical next step – until COVID arrived. The pandemic delayed the interop until the middle of last year. Fortunately, it looks like the participants – who included Acacia, now a part of Cisco; Ciena; Fujitsu Optical Components; Lumentum; and Marvell – used the extra time well.
“It went surprisingly well, just with the flexibility and the DSPs that were integrated into these transceivers,” recalled Curtis Knittle, vice president – wired technologies at CableLabs, who is overseeing the organization’s coherent transmission efforts. “I almost want to say we were pretty much done with our interop goals by maybe a little past midweek, it went so well.”
CableLabs has augmented the 100G and 200G point-to-point transmission work with publication of specifications for a Coherent Termination Device (CTD) in June 2021. The CTD would sit on the field end of the transmission, typically within an aggregation node, and help convert the 100G or 200C coherent input to lower-rate segments for distribution to destinations closer to the endpoint using 10G or 25G Ethernet grey optics. Knittle said he is aware of work among the systems development community to build such a platform, which could help enable the use of coherent links to support such applications as business services delivery, mobile xHaul, and more alongside broadband services delivery.
The pieces would therefore seem to be in place for cable MSOs to adopt coherent transmission in their DAAs. While there had been some thought given to creating specifications for 400G links, Knittle says that the work of such organizations as OIF on 400ZR, various MSAs, and the IEEE has removed the need for CableLabs to create something along these lines itself.
See 100G PON in CPON
As operators begin to evaluate 25G PON while work continues on 50G PON, CableLabs has embarked on development of specifications for coherent-based 100G PON. The CPON effort launched last year with a goal of enabling single-wavelength 100G transmission over 80 km or with a split ratio as high as 1:512.
The work remains in its initial stages. Topics under discussion include how to split that 100 Gbps of capacity into smaller segments. Approaches under consideration include time and frequency division multiplexing (TDFM). To help make this determination, CableLabs has launched an assessment of use cases and business requirements in collaboration with an Operator Advisory Group. As the name implies, the group comprises representatives of cable MSOs who wish to contribute to the development of the specification.
The media access control (MAC) function is another area where options are under consideration. Knittle notes that organizations such as the ITU-T and IEEE have developed MAC approaches for high-speed PON operation that CableLabs could leverage. “I'm reasonably confident that we don't want to define a different one – a third MAC. But that is an option,” Knittle notes. CableLabs is looking into joining the ITU-T’s specification development efforts on 50G PON, which may increase the odds that it won’t have to develop its own MAC.
Other elements that await discussion include the wavelength plan and whether a single-fiber full-duplex approach will be required. Knittle notes that operation through a splitter of the single-wavelength full-duplex mode developed for point-to-point networking has not yet been validated.
Knittle admits that the CPON effort isn’t as far along as he originally had hoped. “We had hoped to have our specifications done in the 2024 timeframe, and then have some interoperability events and be ready for deployment around 2025, 2026,” Knittle states. “We would need certain things to fall into place and then there's a possibility [to hit that 2024 goal]. But that was always an aggressive timeline when we started the project. So, it depends on how the cards fall in terms of some upcoming discussions that we're having with operators.”
The future is coherent
In addition to point-to-point and 100G PON, CableLabs has a few other research efforts potentially related to coherent networking on the to-do list. Knittle states the organization is looking at such areas as flexible wavelength switching and comb lasers, the latter for their ability to generate multiple wavelengths from a single device.
“We do believe that the future in the access network is coherent,” Knittle says in summary. “At the rate capacity [demand] is increasing, as are the distances that cable operators need to transmit that optical signal, it's going be coherent.”
And CableLabs is working hard to ensure that when that future arrives, cable operators will have the technology needed to meet such capacity and distance requirements.
STEPHEN HARDY is editorial director of Lightwave.